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Fogarty et al. 2007 • Climate Change and Marine Resources Impacts Potential Climate Change Impacts on Marine Resources of the Northeastern United States Michael Fogarty1, Lewis Incze2, Richard Wahle3 , David Mountain1, Allan Robinson4, Andrew Pershing5, Katherine Hayhoe6, Anne Richards1, and James Manning1 1 National Oceanic and Atmospheric Administration, Woods Hole, MA 02543 University of Southern Maine, Portland, ME 04101 3 Bigelow Laboratory for Ocean Sciences, West Boothbay Harbor, ME 04575 4 Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138 5 University of Maine and Gulf of Maine Research Institute, Portland ME, 04101 6 Department of Geosciences, Texas Tech University, Lubbock, TX 79409 2 Executive Summary Projected changes in the marine environment off the northeastern United States as a result of climate forcing include changes in water temperatures, salinity, wind stress, local precipitation patterns, and cloud cover, with potential ramifications for water column structure and circulation. These, in turn, can affect many aspects of marine systems, beginning with the transport, production and dynamics of planktonic communities and extending through various parts of the food web. Energy flow has many possible pathways in marine systems, which makes it difficult to predict system responses to the multiple types of forcing that can attend climate change, for example, changes in the degree and timing of winter cooling as well as summer warming, and changes in stratification, nutrient supplies, surface mixing, and photosynthetically active radiation. Because of the dominant role of temperature on the physiology and ecology of marine organisms, the potential effects of changing temperature regimes is a logical place to begin any assessment of potential climate change impacts. Increased water temperatures predicted by climate change models will result in a poleward shift in the distribution of many subtropical and temperate water species in the northeast, including economically important fish and shellfish species; changes in productivity of these species; and possibly increased uncertainty for the fishing industry and resource managers during periods of ecological transition. Indeed, ecological transitions add to the already difficult job of ocean prediction and managing integrated human impacts on all parts of the ecosystem. In this report, we highlight potential changes in two exploited species that have strongly shaped the identity, character and incomes of New England fishing communities: the American lobster, Homarus americanus, and Atlantic cod, Gadus morhua. These are good selections because both species can be predicted to respond to increasing temperature. We stress that numerous other concerns warrant attention in the near future but are more complicated, such as the spread of marine invasive species, occurrences of harmful algal species, and the conservation of biodiversity in general. The ocean components of GCMs currently in use exhibit significant biases in simulations of the western North Atlantic. This bias results from the inability of the models to simulate the separation of the Gulf Stream from the eastern U.S. coast correctly, resulting in an eastward displacement of the North Atlantic Current (the northern extension of the Gulf Stream). This results in a reduction in Gulf Stream water flowing into the far North Atlantic and circulating into the Labrador Sea, inducing a cold and fresh bias in that region. In recognition of these biases, we calibrated GCM model outputs against observed surface and bottom temperature fields and used predictive relationships to make projections of bottom water temperatures from surface temperature NECIA • 1 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts anomalies for three coupled ocean-atmosphere GCMs under high and low IPCC emission scenarios. We developed projections for four major regions of our study area: Northern Mid-Atlantic Bight, Georges Bank, Western Gulf of Maine, and Eastern Gulf of Maine. Dramatic differences are evident for the low and high emission scenarios for projected bottom temperature levels. For the low emission scenario in particular, substantial differences are noted among models and over the time course of the simulations. Under the low emission scenario, the average bottom temperature anomalies did not exceed 1.5oC in either season by the year 2085. For the high emission scenario, the projected bottom temperature anomalies exceeded 3.5oC in some areas/seasons by 2085. We explored the feasibility of linking a coupled ocean-atmosphere general circulation model [the Parallel Ocean Climate Model (PCM)] with the Harvard Ocean Prediction System (HOPS) in an attempt to account for the effects of complex topographic features and finer-scale hydrographic features in the Gulf of Maine. HOPS has been used for realistic simulations and real-time forecasts in the Gulf of Maine in other applications. We employed a detailed Gulf of Maine Feature Model which links together sub-basin scale circulation features (such as the Maine Coastal Current, Northeast Channel inflow, and Georges Bank gyre) and allows realistic high-resolution initialization of HOPS in the Gulf of Maine. Resources were not available at this time for a full set of model runs under varying emission scenarios, but we provide initial results to demonstrate proof of concept in anticipation of a more extensive effort to follow. Increased water temperatures over the last decade have been associated with sharp declines in lobster landings in southern New England and in Long Island Sound. A critical threshold temperature of 20oC has been exceeded frequently during summer over the last decade, and there is a high probability that this level will consistently be exceeded by mid-century, resulting in a possible loss of the lobster fishery in these areas. The dockside value of the lobster fishery in southern New England and the Mid-Atlantic states is small relative to that in northern New England. Nonetheless, the sharp decline in the southern portion of the lobster fishery over the last decade has created local hardships in the fishing industry and has elicited substantial concern farther to the north. New research programs have been implemented to understand the factors underlying the decline and whether other lobster-producing areas are at risk. In contrast, increased temperatures in the Gulf of Maine within the ranges predicted under alternative climate change scenarios hold the potential for increased lobster productivity. This would result from possible increases in the amount of thermally suitable habitat for settlement (initial recruitment to the bottom from the planktonic stages) as well as enhanced individual growth rates, assuming that other climate-induced changes do not negatively affect basic productivity states of the system. To the extent that disease and hypoxic conditions experienced in the southern areas are linked to changing temperature patterns, lesser impacts are expected in the Gulf of Maine, where model projections indicate more moderate temperature increase. Atlantic cod landings in the Gulf of Maine and on Georges Bank have declined substantially over the last decade. These declines have been clearly linked to intensive exploitation, but the role of environmental conditions in concert with over-harvesting cannot be ignored. Results of a metaanalysis of North Atlantic cod stocks suggest that increases in mean annual water temperatures above 8oC will result in a decline in recruitment (the number of young surviving to enter the fishery). Georges Bank, historically the major cod producing region in the northeastern United States, had a mean annual temperature of 9oC during the period 1978-2002. This suggests declining recruitment and reduced potential for recovery of cod populations over a substantial part of the northeastern shelf if further increases in bottom temperatures are realized. The 8oC threshold is also currently exceeded in the near shore southern New England area, which has historically supported small spawning aggregations of cod. Consideration of cod distribution patterns throughout the North Atlantic suggests that a 12oC mean annual bottom temperature describes the practical limit of cod NECIA • 2 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts distributions. If the bottom temperature projections under the high emission scenario are realized, it is likely that this level will be surpassed in the near-shore southern New England region and Georges Bank regions by the end of this century. The mean bottom water temperature in the Gulf of Maine as a whole during the period 1978-2002 was 7.1oC, and if the bottom temperature anomaly projections for this region hold, the 12oC threshold will not be exceeded. Unknown at this time is how other climate-related changes in the system would affect cod, such as through changes in prey and predator communities. Comparison of the projected impacts on cod and lobster indicates that both are potentially vulnerable to declines and loss of habitat in the southern portion of their range if projected bottom temperature increases occur, particularly under the high emission scenario. Cod is at the extreme southern extent of its range in the Northern Mid-Atlantic Bight. Lobster reaches the southern extent of its range in the vicinity of Cape Hatteras, but is largely confined to deeper waters in the southern Mid-Atlantic Bight. As noted, cod is potentially vulnerable to loss in thermal habitat on Georges Bank under the high emission scenario. Highest densities of lobster on Georges Bank are concentrated in the submarine canyons on the continental slope and are less vulnerable to temperature change in this region. Within the Gulf of Maine, cod are expected to exhibit a neutral to slightly negative change if projected bottom temperature anomalies are realized by the end of this century. In contrast, bottom temperature anomalies within the range predicted for the Gulf of Maine are potentially favorable for this species. These differences highlight the fact that, in general, the potential impacts of climate change in the ocean will exert differential effects on different species and both declines and increases in productivity are likely to occur for different components of marine communities within the region. An economic impact assessment of the effects of projected climate change on the target species is beyond the scope of this report. The potential loss of thermal habitat with resulting loss of productivity for both lobster and cod in the southern portion of the study area is substantial, particularly under the high emission scenario. Although the loss of access to both these species would be potentially economically significant in these areas, we can anticipate some adaptation on the part of the fishing industry, principally through switching to alternative species. For example, an increase in water temperatures in southern New England is potentially favorable for an increase in blue crab (Callinectes sapidus, Fogarty and Lipcius, in press) and it is conceivable that some lobster fishers could switch to the lucrative blue crab fishery if such an increase occurs. Similarly, possible increases in abundance and productivity of sub-tropical finfish species in the Northern Mid-Atlantic Bight (e.g., Sciaenids such as weakfish, spot, and drum) could potentially offset some losses of cod in the southern part of its range. Overall, the relative contribution of the landings of both lobster and cod in the southern portions of this study area are relatively low in relation to the total. Should large-scale losses of cod on Georges Bank occur or should adverse impacts on lobster (not currently anticipated) occur in the Gulf of Maine, however, the economic impacts would be substantially more serious. NECIA • 3 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts 1. Introduction Future climate predicted under increasing anthropogenic emissions of greenhouse gases will potentially strongly alter the physical structure of the oceans, with direct implications for marine ecosystems and human societies (IPCC 2001). These changes need to be considered in the context of impacts resulting from other human activities including fishing, pollution, and habitat loss due to coastal development. Climate change can interact with other human-induced changes to alter the fundamental production characteristics of marine systems. Living marine resources have sustained human cultures for millennia as an essential source of protein and as a cornerstone of maritime commerce and trade. However, continuously increasing fishing pressure and demand related to the burgeoning human population has resulted in significant declines in many previously abundant fish and shellfish populations. It is now clear that human harvest of these fishes can exceed their production potential in the world’s oceans (Worm et al. 2006). Climate change will potentially exacerbate the stress on some living marine resources imposed by harvesting and other anthropogenic activities in many regions, but will potentially enhance production for other populations in some areas. Considerable emphasis is now being placed on understanding the causes and consequences of climate change in temperate marine systems (e.g. Harvell et al., 2002; Helmuth et al., 2002; Barnett et al., 2005; Drinkwater, 2005; Sutton & Hodson, 2005; Altieri & Witman, 2006) to prepare for anticipated alterations in ecosystem structure and function. Fishing has played a vital role in the cultural and economic fabric of the northeastern United States. Indeed, the identity and current-day economy of many northeastern coastal communities is deeply tied to fishing and to iconic species such as cod and lobster. Combined dockside revenues from oceanic and estuarine commercial fisheries in the northeastern U.S. in 2004 exceeded $1.2 billion. In 2004, shellfish landed revenue was just over $900 million, dominated by lobster and sea scallop (Placopecten magellanicus) returns. Dockside revenue from lobsters alone in 2004 was $366 million. Overall, lobster landings have undergone dramatic increases over the last three decades, but landings in the southern part of the range have declined sharply from recent peaks in the mid-1990s. The landings of cod in the northeast have declined substantially since 1995, and were worth an estimated $21.7 million dollars in 2004. Hoagland et al. (2005) estimated the economic contribution of the seafood industry to the overall economy in the Northeast as $10.4 billion in 2003 as a result of the direct and indirect effects of commercial fishing activity and associated industries. Approximately 76,530 jobs in the Northeast in 2004 were dependent on the production of seafood by commercial harvesters, wholesalers, and processors (Hoagland et al. 2005). In that year there were 2,508 vessels actively fishing under federal fishing permits in the region. Many smaller vessels not requiring federal permits also operate in the region, typically fishing exclusively within state territorial waters. In Maine, for example, there were more than 7,000 lobstermen and approximately 4,000 boats involved in lobster fishing in 2004. Most of these fished within state waters. Here, we review aspects of the potential changes in marine ecosystems off the northeastern United States in response to climate-induced forcing, with a focus on temperature effects on living marine resources. We provide an overview of anticipated general alterations in marine ecosystems in response to climate change, review likely impacts on the northeast continental shelf, and highlight potential effects on the distribution and abundance of the American lobster and Atlantic cod to illustrate the range of possible impacts throughout the region. NECIA • 4 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts 2. Background Climate Change Impacts on the Marine Environment of the Northeastern U.S. The region covered by this paper includes the continental shelf of the northeastern United States. Major sub-regions include the northern mid-Atlantic Bight and southern New England shelf (south of Cape Cod), Georges Bank, and the Gulf of Maine (Fig. 1). In some areas the continental shelf extends more than 200 km from shore. Over-all, the shelf is characterized by southward (equatorward) flow of cold and comparatively fresh water from the sub-Arctic North Atlantic (Labrador Sea and beyond: Belkin et al. 2004) and the Gulf of St. Lawrence, but the region, especially the southern part, is substantially influenced by across-shelf exchanges with the neighboring slope sea which is conditioned by the northward-flowing Gulf Stream (Figs. 2, 3; see review by Townsend et al. 2006). At middle depths along the continental margin, slope water can be of northern or southern origin, depending on latitude and variable forcing of the slope water systems. The Gulf of Maine is particularly complex in this regard, because it contains several deep basins with a single deep-water connection to the slope, the Northeast Channel (Fig. 1). The dominant slope water at this latitude is strongly influenced by remote forcing, some of it linked to shifts in atmospheric pressure distributions over the North Atlantic [for example, the North Atlantic Oscillation (NAO) index: Rogers 1984, Pershing et al. 2001, Greene and Pershing 2003 ]. Temperature, salinity, nutrients and plankton all vary as a function of slope-water sources (Thomas et al. 2003, Pershing et al. 2005). Fig. 1. Map of the study area with selected geographical features labeled. NECIA • 5 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts Continental shelf waters experience cooling and mixing during winter months, when nutrients are replenished in the surface layers, and stratification during the warm months. Over much of the continental shelf, seasonal stratification isolates the surface from lower depths, leading to enhanced warming, plankton blooms and eventual nutrient limitation of the surface wind-mixed layer. A pool of cold bottom water typifies mid-shelf areas during summer months. Stratification decays in the fall as the surface layer cools (thus becoming less buoyant) and the frequency and intensity of strong winds causes increased mixing. Late fall is generally the warmest time of year on the bottom. An exception to the above scenario exists where tidal mixing is particularly strong, such as in the northern and eastern portions of the Gulf of Maine and over shallow Fig. 2. Schematic of the dominant circulation features that affect regions of Georges Bank and shelf and slope water systems in the northeastern U.S. (after Nantucket Shoals. In the Gulf of Townsend et al. 2006). Maine, comparatively cold, nutrientrich waters that are mixed upward by tidal mixing persist as a comparatively cold feature through summer and are transported via the cyclonic (counter-clockwise) coastal current system. The combination of tidal mixing, cold-water currents and stratified regions in the Gulf of Maine produces exceptionally steep horizontal temperature gradients in this region during warm months of the year (Fig. 3). Also see later modeling results, this paper). Winter-time cooling and convection can vary substantially from year to year, affecting minimum winter temperature and the depth of mixing. Impacts extend into the spring and summer in the form of temperature of the cold bottom water pool and, in deep areas such as the Gulf of Maine, concentrations of nutrients introduced to the surface. Throughout much of this region, however, bottom water characteristics are affected not only by winter conditioning, but also by advection into the Gulf from the slope sea. Vernal stratification and autumnal de-stratification can be shifted earlier or later by weather, including cloud cover (because of its impact on radiative heating), wind, air temperature, and freshwater input. Even winter conditions can vary by factors as subtle as cloud cover (which affects the supply of light for photosynthesis) and wind; together, they affect late winter primary production in shallow, near-shore waters. Thus, conditions on the northeast continental shelf are subject to interannual variability in weather as well as longer-term changes in weather patterns and climate, and local and remote forcing. Temperature, salinity, nutrients, light, the timing of seasonal transitions, and advection of planktonic biota, including propagules, are all factors that can bring change to a region. These changes, in turn, can affect higher trophic levels through temperature effects on physiology, migrations, reproduction, alterations in predator and prey communities, and diseases. NECIA • 6 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts Fig. 3. Monthly climatologies (1974-2004) of satellite-derived sea surface temperature over the northeastern U.S. continental shelf and adjacent ocean (cf. schematic circulation shown in Fig. 2). Temperature History Sea surface temperature records extend back over a hundred years at shore stations in Woods Hole, Massachusetts (south side of Cape Cod, Nixon et al. 2004) and in Boothbay Harbor, Maine (Fig. 1 for locations). Time series exist of shorter duration for numerous other locations, including Steel Pier, New Jersey, Georges Basin in the central Gulf of Maine (bottom temperature, see Townsend et al. 2006), Boston Harbor and Newport, Rhode Island (Nixon et al. 2004) and the southwestern Bay of Fundy (Prince 5, Fig. 1). Most of these show a period of cold temperatures during the mid-1960s which probably reflect enhanced southward advection of cold northwest Atlantic water associated with a period of negative NAO anomalies (see later discussion). Data from the two longest time-series in the Gulf of Maine and Woods Hole also show a period of very warm temperatures during the 1950s. The Boothbay time-series, based on daily observations with few significant interruptions since 1905, can be used to look at the long-term temperature change (Fig. 4) and the time of year that those changes occurred (Fig. 5). Compared to the “twentieth century” mean (1905-1999), positive anomalies of 1-2°C occurred primarily during summer months prior to 1950. During the warm period NECIA • 7 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts Winter, Summer and Annual Temperature 18 of the early 1950s, positive anomalies occurred throughout the 14 year, but were strongest (3-4°C) in 12 fall, winter and early spring. 10 Positive anomalies generally favored 8 non-summer months in the years that 6 followed until the 1990s, when they again included most months of the 4 year. In recent years, the strongest 2 positive anomalies (3-4.5°C) have 0 been from late summer into fall and -2 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 early winter. These recent anomalies coincided with a period of Fig. 4. One hundred years (1905-2005) of sea surface great increase in the abundance of temperature in Boothbay Harbor, Maine . Data are annual small copepods whose populations values of winter (DJF, blue), summer (JJA, red) and 12-month would benefit from the extended (black) means. season of warm/stratified conditions (see later discussion). Possible explanations include greater radiative input from clearer skies, less vertical mixing from wind events, decreased heat losses to the atmosphere, increased stability from salinity changes, and increased coastal convergence/decreased divergence caused by changes in wind forcing. During winter, decreased heat loss to the atmosphere probably dominates the trend and results in decreased depth of winter mixing. The annual temperature means at Prince 5 show similar patterns as at Boothbay, except that the temperature difference between the two gets larger as Boothbay temperatures become warmer. Satellite and buoy data for 1 offshore regions of the Gulf of 4 2 Maine, Georges Bank and 3 3 southern New England/Mid2 4 Atlantic Bight waters have shorter 5 1 histories and are useful for a 6 0 7 variety of purposes over shorter -1 8 periods of time. There is an 9 -2 extensive data base of 10 -3 temperature, salinity and density 11 -4 profiles and other water column 12 05 15 25 35 45 55 65 75 85 95 05 properties, but these also are best Year: 1905 - 2005 suited to focused questions concerning spatial or shorter temporal patterns, and are not Harbor, computed against the “20th century” (1905-1999) means. Note the shifting patterns of positive temperature anomalies from considered further in this paper. 16 Month Temperature °C summer to non-summer and year-round. White boxes are missing values. NECIA • 8 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts Remote Forcing The North Atlantic Oscillation (NAO) is the dominant climate mode in the North Atlantic sector (Hurrell, 1995). NAO refers to a characteristic pattern of variability in atmospheric sea-level pressure over the North Atlantic. The strength of the two main pressure centers, the Icelandic (subpolar) Low and the Azores (sub-tropical) High, are correlated: when the Icelandic Low deepens, the Azores High typically becomes higher, and vice versa. This pattern can be summarized by a simple index by differencing the atmospheric pressures at stations located within the two pressure centers (typically Lisbon, the Azores or Gibralter for the High, and Reykjavik for the Low). The NAO Index is correlated with a wide range of atmospheric and oceanographic phenomena in the North Atlantic sector. When the NAO is positive, westerly winds shift northwards and intensify, bringing warmer and wetter conditions to northern Europe. The area over the Labrador Sea tends to be colder, leading to increased formation of Labrador Sea Deep Water (LSDW). Conversely, negative NAO conditions lead to a southward shift in the westerlies and colder conditions in northern Europe. The cold weather leads to increased formation of Norwegian Sea Deep Water, while LSDW production is diminished. Over the Gulf of Maine, weather conditions have little correlation with the NAO. Rather, the NAO influences the NW Atlantic through its association with the regional oceanography. Changes in the NAO index have been linked to shifts in the position of the Gulf Stream (northward following positive NAO conditions) and to the volume transport in the Labrador Current (increased following negative NAO conditions: Rossby 2000, Taylor 1998). Shifts in the Gulf Stream and the appearance of colder Labrador Slope Water in the Gulf of Maine occur 1-2 years after a shift in the NAO (MERCINA 2001; Greene and Pershing 2003; Drinkwater 2006). At this point, physical oceanographers are debating the degree to which the Gulf Stream and Slope Water changes are linked, and whether they are caused primarily by shifts in the wind field or hydrographic changes originating in the Labrador Sea and beyond. Changes in the physical oceanography of the NW Atlantic related to the NAO have been related to changes in biology. Variations in the abundance of Calanus finmarchicus, the dominant zooplankton (copepod) species during spring and summer, have been linked to NAO- associated changes in the physical oceanography (Greene 2001; Conversi 2001; MERCINA 2001). Shifts in the abundance of C. finmarchicus have, in turn, been linked to reproductive success in the endangered northern right whale, Eubalaena glacialis (Greene 2003, 2004). It is likely that changes in C. finmarchicus have other biological consequences as well, although these have not been quantified. Popular reference to the NAO as the “Atlantic’s El Niño” is a misleading analogy. Unlike the El Niño-Southern Oscillation (ENSO), which is a well-understood series of events in the ocean and atmosphere, the NAO is a pattern, and the mechanisms that cause this pattern are not well understood. Two examples illustrate this point. Cassou et al. (2004) reanalyzed surface pressure fields over the North Atlantic using cluster analysis and found four characteristic states in the pressure distribution. One resembled the positive NAO with enhanced pressure centers, another resembled the negative NAO, and two others showed little resemblance to the NAO. In a separate study, Joyce (2002) considered the correlation between the NAO index and weather conditions over North America. He found that the association with the NAO waxed and waned: during some periods, the NAO was a good predictor of weather, while at other times it provided no useful information. NECIA • 9 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts Despite its limitations, the NAO Index is a useful summary of physical conditions in the North Atlantic. If we knew how climate change would affect the NAO, we could exploit established correlations with the NAO to gauge the impact of climate change on the NW Atlantic. Interestingly, many GCMs, including those used in the Northeast Climate Impact Assessment (this study), predict a dominance or strong presence of positive NAOs over the next 50-100 years, especially the winter index and especially in the higher carbon emissions scenarios. The basis for these predictions requires further analysis, but these results are promising. One of the key features of the response of the NW Atlantic to the NAO is that the forcing comes from upstream, specifically, the Labrador Sea. In addition to the NAO changes, the Labrador Sea seems to be a key area in other interannual changes in the Gulf of Maine. During the 1990s, the NW Atlantic experienced widespread freshening (Smith 2001; Mountain 2003, 2004). This freshwater was not associated with increased precipitation or river input; rather, the freshwater originated beyond the Gulf of St. Lawrence (Frank 2003). During this period, the salinity in the Labrador Sea, especially on the shelf, was much lower, and it seems likely that the two events are related. Freshening in the Labrador Sea, and at higher latitudes in general, through a speed-up in the hydrologic cycle (Curry, 2003, 2005) or increased ice melt is a hallmark of most climate change scenarios. Thus, it seems likely that conditions in the Gulf of Maine during the 1990s could provide a window into future physical and biological changes that could be linked to global climate change models. In addition to the freshening, biological conditions in the 1990s were much different than previous decades. Pershing et al. (2005) documented an increase in most of the dominant copepod taxa around 1990, one that persisted throughout the decade. The notable exception to this pattern was a decrease in Calanus finmarchicus. Changes in the zooplankton community were the result of greatly enhanced fall and winter populations of smaller zooplankton, and were associated with increased fall phytoplankton abundance and warmer temperatures, presumably due to increased stratification during this season (see above discussion of SST anomalies in the Boothbay Harbor record). 3. Climate Models and Emission Scenarios Our analysis is based on simulations from three atmosphere-ocean general circulation models (AOGCMs): NOAA/GFDL CM2.1 (Delworth et al., 2005), UKMO HadCM3 (Pope et al., 2000), and DOE/NCAR PCM (Washington et al., 2000). These are three of the latest generation of numerical models that couple atmospheric, ocean, sea-ice, and land-surface components to represent historical climate variability and estimate projected long-term increases in global temperatures due to human emissions. As shown in Table 1, atmospheric processes are simulated at a horizontal resolution of 2.5 by 2 degrees (GFDL), 3.75 by 2.5 degrees (HadCM3) and T42 or ~2.8 by 2.8 degrees (PCM). Ocean variables including salinity, potential temperature, and three-dimensional currents, simulated on a tripolar grid by GFDL and PCM, were interpolated onto a regular 1x1 degree grid using depthdependent areal weights. HadCM3 ocean simulations were performed on a regular 1.25x1.25 degree grid so required no interpolation. Climate sensitivity, a metric that captures the magnitude of the model-simulated increase in global temperature in response to a doubling of atmospheric CO2 concentration, is 1.5oC for GFDL, 1.3oC for PCM and 3.3oC for HadCM3, covering the low to medium part of the IPCC range of 1.5-4.5oC. NECIA • 10 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts Table 1. AOGCM model resolution. HadCM3 GFDL PCM Atmospheric resolution 2.5o x 3.75o 2o x 2.5o T42 (~2.8o x 2.8o) Horizontal resolution 1.125o x 1.125o 1o x 1o (interpolated tri-polar) 1o x 1o (interpolated tri-polar) Vertical resolution 20 levels 50 levels 32 levels Top 6 layers (depth, m) 5, 15, 25, 35, 48, 67 5, 15, 25, 35, 45, 55 13, 38, 64, 90, 118, 147 Emission Scenarios Future simulations are forced by the IPCC Special Report on Emission Scenarios (SRES, Nakićenović et al. 2000) higher (A1fi), mid-high (A2) and lower (B1) emissions scenarios. These scenarios describe internally consistent pathways of future societal development and corresponding greenhouse gas emissions, and cover a wide range of alternative futures based on projections of economic growth, technology, energy intensity, and population. The SRES scenarios are not assigned probabilities, but rather can be viewed as possible futures, with the actual path depending on technology, economic development and political will. The A1fi and B1 scenarios used in this study bracket the range of SRES scenarios, and can be thought of as lower and higher bounds that encompass most, but not all, potential non-intervention emissions futures. At the higher end, rapid introduction of new technologies, extensive economic globalization, and a fossil-intensive energy path causes A1fi GHG emissions to grow steadily throughout the century. In the A1fi scenario, CO2 emissions climb throughout the century, reaching almost 30 Gt/yr or 6 times 1990 levels by 2100. A2 emissions also reach 30 GtC/yr by 2100, although cumulative emissions over the century are slightly lower than A1fi. Emissions under the B1 scenario are even lower, based on a world that transitions relatively rapidly to service and information economies. CO2 emissions in the B1 scenario peak at just below 10 Gt/yr - around two times 1990 levels – at mid-century and decline slowly to below current-day levels. Together, these scenarios represent the range of IPCC non-intervention emissions futures with atmospheric CO2 concentrations reaching approximately double and triple pre-industrial levels, at 550 ppm (B1) and 970 ppm (A1fi), by 2100. Projections for the A1fi and B1 scenarios were used for the GFDL and PCM models, but as HadCM3 did not save any ocean output for its A1fi simulation, we used A2 and B1 instead. Bias in Oceanographic Projections GCMs currently in use exhibit significant biases in simulations of the western North Atlantic (e.g., Dai et al. 2005). This bias results from the inability of the ocean component of the model to simulate the separation of the Gulf Stream from the eastern US coast correctly, resulting in an eastward displacement of the North Atlantic Current (the northern extension of the Gulf Stream). This results in a reduction in Gulf Stream water flowing into the far North Atlantic and circulating NECIA • 11 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts into Labrador Sea, inducing a cold and fresh bias in that region. This has important implications for simulating temperature and salinity regimes over the northeastern U.S. continental shelf because of the importance of far-field forcing. We made a detailed examination of the biases for the Gulf of Maine by comparing model predictions of temperature and salinity against direct observations for this region (See Appendix 1). In all subsequent analyses for all areas, we used only projected anomalies in an attempt to capture the range of potential changes in oceanographic variables while recognizing the bias in absolute projections (see below). Bottom Water Temperature Projections Because of the overriding influence of bottom water temperature on the physiology of our target species, we focused on developing projections of this variable for the major geographical regions of interest. We developed predictive relationships between surface and bottom temperatures for the Northern Mid-Atlantic Bight, Georges Bank, the Western Gulf of Maine and the Eastern Gulf of Maine for spring and autumn using hydrographic observations during Northeast Fisheries Science Center surveys throughout the region. Although observations are made throughout the year, the most intensive sampling is in spring and autumn in conjunction with bottom trawl surveys. Predictive relationships were developed by regressing surface temperature on bottom temperature in the season of interest and then checking for improvement in fit by adding the surface temperature for the other season. In general, spring bottom temperature was closely related to spring surface temperature, particularly for the MAB and GB regions (accounting for 63% to 89% of the variability) while fall bottom temperature was best predicted by a combination of spring and fall surface temperature (accounting for 33% to 60% of the variance). Correlation between bottom and surface temperatures was much lower for GOM, with surface temperatures accounting for only ~20-50% of the year-toyear variance. Accordingly, projections for the Gulf of Maine in particular require care in interpretation. The seasonal and region-specific linear fits were applied to model-simulated spring and fall surface temperature anomalies for each region (relative to 1970-2000) in order to produce bottom temperature anomalies covering the full period of model simulations, from 1900 through 2099. Projected water temperature patterns by region for spring and autumn are provided in Figures 6 and 7 under the low and high emission scenarios. Dramatic differences are evident for the low and high emission scenarios for projected bottom temperature anomalies (Figures 6 and 7). For the low emission scenario in particular, substantial differences are noted among models and over the time course of the simulations. Ensemble averages over the three models for projections of bottom water temperature anomalies by region for the period 2080-2084 are provided in Table 2 Under the low emission scenario, the average spring bottom temperature anomalies (oC) range from 0.89 (northern Mid-Atlantic) to 1.33 (western Gulf of Maine) and, in autumn, from 1.17 (Eastern Gulf of Maine) to 1.48 (Western Gulf of Maine). For the high emission scenario, the projected spring bottom temperature anomalies range from 2.044 (eastern Gulf of Maine) to 3.64 (northern MidAtlantic) and, during autumn, from 1.96 (eastern Gulf of Maine to 3.48 (Georges Bank). For the low emission scenario therefore, the spring and autumn projected temperature anomalies are less than 1.5 oC and are relatively consistent among areas in comparison with the high emission scenario. For the latter, wider regional differences are evident with substantially higher projected bottom temperature increases in the southern areas. NECIA • 12 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts Spring N. MidAtlantic 2 2 1 1 0 0 -1 Autumn -1 3 2 3 Georges Bank 2 Bottom Temperature Anomaly 1 1 0 0 -1 -1 -2 -2 2 W. Gulf of Maine 2 1 1 0 2 0 E. Gulf of Maine 1 1 0 0 2020 2040 2060 GFDL HAD-1 2 2080 2020 2040 HAD-2 PCM 2060 2080 Time Period Figure 6. Projected average bottom water temperature anomalies (oC) for four time periods (2020-2024, 2040-2044, 2060-2064, and 2080-2084) in spring and autumn, by region, under IPCC low emission scenarios for the GFDL, Hadley (HAD, 2 scenarios) and Parallel Climate (PCM) models. NECIA • 13 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts 5 4 5 Spring N. MidAtlantic Autumn 4 3 3 2 2 1 1 0 0 5 Bottom Temperature Anomaly 4 -1 5 Georges Bank 4 3 3 2 2 1 1 0 3 0 3 W. Gulf of Maine 2 2 1 1 0 0 2 E. Gulf of Maine 1 1 0 0 2020 2040 2060 GFDL HAD PCM 2 2080 2020 2040 2060 2080 Time Period Figure 7. Projected average bottom water temperature anomalies (oC) for four time periods (2020-2024, 2040-2044, 2060-2064, and 2080-2084) in spring and autumn by region under IPCC high emission scenarios for the GFDL, Hadley (HAD, 2 scenarios combined) and Parallel Climate (PCM) models. NECIA • 14 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts Table 2. Ensemble averages over all models for projections of bottom water temperature anomalies by region and season for Low and High emission forecasts for the period 2080-2084. Spring Low N. Mid-Atlantic 0.890 Georges Bank 1.241 W. Gulf of Maine E. Gulf of Maine 1.330 1.052 Spring High 3.640 3.367 2.137 2.044 Autumn Low 1.249 1.254 1.481 1.172 Autumn High 2.770 3.482 2.160 1.958 Linking AOGCMs with Finer-Scale Hydrodynamic Models To more fully evaluate options for developing projections of water mass characteristics, we explored the feasibility of linking a coupled ocean-atmosphere general circulation model [the Parallel Ocean Climate model (PCM)] at 1o resolution with the Harvard Ocean Prediction System (HOPS), which has been used for realistic simulations and real-time forecasts in the Gulf of Maine in previous applications. We employed a detailed Gulf of Maine Feature Model (5 km resolution) that links together sub-basin scale circulation features (e.g. the Maine Coastal Current, Northeast Channel inflow, and the Georges Bank gyre: Gangopadhyay et al. 2002) and allows realistic high-resolution initialization of HOPS in the Gulf of Maine. To address the bias in mean conditions predicted by the GCM, we employed a perturbation approach in which the model-based projected future changes in temperature and salinity were superimposed onto observed climatological fields. A full description of the methods employed in linking the PCM and HOPS models is provided at: http://oceans.deas.harvard.edu/UCS/. Resources were not available at this time to conduct a full set of model runs under varying emission scenarios. Accordingly, we provide initial results to demonstrate proof of concept in anticipation of a more extensive effort to follow. Representations of the dynamically adjusted surface temperature and salinity fields for the reference year (2000) and in 2085 are provided in Figure 8. 4. Lobster Case Study The American lobster (Homarus americanus) is one of the most highly prized fishery resources of the Atlantic coast. The state of Maine produces more than half of the annual U.S. landings, and the lobster industry is a mainstay of the state’s economy. In addition to the monetary value of landings, lobstering supports a unique way of life in fishing villages all along the New England coast. The lobster industry and related support facilities and activities help define the heritage and character of these communities and provide a focal point for tourism, another major economic activity. NECIA • 15 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts Dynamically adjusted fields for September 2000 Dynamically adjusted fields for September 2085 Temperature Salinity Figure 8. Projected changes in surface temperature (oC) and salinity (PSU) in the Gulf of Maine between September 2000 and September 2085. NECIA • 16 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts The geographic range of the American lobster encompasses one of the steepest latitudinal gradients in sea surface temperature in the North Atlantic. Over the distance of a few hundred kilometers, summer maximum temperatures have historically ranged from the mid-20’s ºC in southern New England to only about 12ºC at the mouth of the Bay of Fundy. The southern limit of the coastal distribution of the lobster is Long Island and northern New Jersey. South of that boundary, lobsters become increasingly restricted to deeper cooler water at the edge of the continental shelf off Virginia and North Carolina. At the northern limit of its range in the northern Gulf of St. Lawrence, lobsters are largely restricted to waters less than 50m deep that warm during the summer. Lobster landings in the United States nearly tripled over the past two decades. Explanations for the rapid increase in landings have focused on a combination of sharp increases in fishing effort and in the total area fished, enhancements in fishing technology, favorable water temperatures (particularly in the Gulf of Maine), and decreases in predatory fish populations (including cod) as a result of over-exploitation (Fogarty 1995). In the state of Maine, landings exhibited an increase during the warm period 1945-1955 with a time delay consistent with the time required to reach recruitment to the fishery (Fogarty 1988). The recent increase in landings also has occurred during a period of rapidly increasing temperatures, but with temperature slightly lagging landings (Figure 9). Temperature alone, therefore, does not explain the increase in landings, but may impact the largerscale distribution patterns. Figure 10 shows results from standardized research vessel surveys during the relatively cold period 1965-69 and the recent warm period during 2000-2004. The surveys do not include rocky substrate areas, a preferred lobster habitat. However, there are striking increases in the abundance of lobsters in soft substrate habitats, and an apparent shift in the center of density toward higher latitudes within the substrate types sampled by the survey gear. Incze et al. (2006) show that the planktonic and first settled stages of lobsters in the Gulf of Maine show large interannual variations that appear to be due to environmental forcing (planktonic survival and transport). Series of good and bad years during the earliest life history stages then set the potential for subsequent population size and fishery yields (Wahle et al. 2004). 30 12 Temp 25 11 20 10 15 9 10 8 5 7 0 6 1905 1915 1925 1935 1945 1955 1965 1975 1985 1995 Year Figure 9. Landings of American lobster in Maine over the last century and mean annual sea water temperature at Boothbay Harbor. NECIA • 17 Temperature (oC) Landings ('000 mt) Landings Fogarty et al. 2007 • Climate Change and Marine Resources Impacts Figure 10. Abundance of American lobster in research vessel surveys (kg/standard tow) conducted by the National Marine Fisheries Service during 1965-69 (left panel) and 2000-2004 (right panel). Temperature Effects on Lobster Biology Physiological Responses: While the American lobster is generally described as living in environments where water temperature ranges from 5-20ºC (Aiken & Waddy, 1986), the reality is more complex. Early work on temperature tolerance of the American lobster was motivated by the need to maximize survival during storage and shipping of the commercial catch. It therefore focused on lobsters of marketable size, most of which are adults. The maximum and minimum lethal temperature limits for lobsters varies somewhat with the oxygen content and salinity of the seawater. McLeese’s (1956) classic laboratory experiments illustrate the interactive effects of temperature, dissolved oxygen, and salinity on the survivability of lobsters (Figure 11). Life at high temperatures is made precarious by the fact that the biological oxygen demand of ectothermic organisms increases with temperature just as oxygen solubility in water diminishes. In general, in sea water at temperatures below about 20ºC, lobsters are not stressed as long as oxygen concentrations are above 2 mg O2 l-1. Recent work on Long Island Sound lobsters confirmed that as water temperature rises above 20.5ºC, the respiration rate of lobsters increases significantly and the animals come under stress (Chang 2004, Powers et al. 2004, Dove et al. 2005). As a result, 20ºC has been used by the Connecticut Department of Environmental Protection as a physiological stress threshold for lobsters in Long Island Sound. Prior acclimation can enhance survival under extreme conditions. McLeese found the lower lethal limit of lobster to be 1.8ºC for lobsters acclimated at 17ºC, but 5ºC for lobsters acclimated at 27.5ºC. A lobster’s ability to live in diluted seawater also is temperature-dependent. At temperatures below 20ºC they can survive quite well in half-strength seawater (approximately 15 PSU), but survival at this salinity decreases rapidly above 20ºC (McLeese 1956). NECIA • 18 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts O Toler an Zone ce T 24 16 8 30 Leth al Zo ne 20 S 2 Salinity Finally, as molting (part of the growth cycle) occurs during the warmer months, resistance to physiologically stressful conditions may rise and fall with the molt cycle. Molting lobsters have been found to be less resistant to high temperature and low dissolved oxygen and salinity than lobsters during intermolt periods (Waddy et al., 1995). 4 Ox 6 yg en Movements and migrations: Temp Thermal tolerances strongly eratu 10 influence lobster movements and re seasonal migrations. Lobster movements in estuaries and Figure 11. Diagram of the boundary of lethal conditions for embayments are known to be lobsters in various combinations of temperature (oC), salinity linked to seasonal changes in (ppt) and oxygen (mg/l). T – region in which temperature temperature. During spring, alone acts as a lethal factor, S- region in which salinity alone lobsters become more active as acts as a lethal factor, O- region in which oxygen alone acts waters warm, and they move into as a lethal factor. (Figure and caption after McLeese 1956). the warming shallows of bays and estuaries to the extent that they can tolerate reduced salinities (Jury 1999). In southern New England, fishermen commonly observe a mid- to late summer movement into deeper channels and out of estuaries that reach temperatures greater than about 20ºC, followed by a return in the fall as temperatures cool for a time before the winter emigration. Laboratory experiments indicate that lobster activity does not vary linearly with temperature; rather, there is a threshold at ~10ºC above which lobsters become quite active and below which they are more quiescent (Jury 1999). In experiments conducted in laboratory tanks providing a horizontal gradient from 12 to 22ºC over a 1.8 m distance, adult lobsters that were acclimated at a summer ambient temperature of 15.5ºC in the Gulf of Maine, avoided temperatures warmer than 19ºC and colder than 13ºC, and spent most of their time at 16.5ºC, slightly warmer than the ambient conditions from which they came. Growth and maturity: Regional differences in the growth rate of juvenile and adult benthic stages are readily attributable to temperature. Below 5oC metabolism is slowed to a point where molting does not occur. Under stressfully warm conditions lobsters may not molt or may die in the process (Waddy et al. 1995). The southern Gulf of St. Lawrence and southern New England, where summer temperatures rise near or above 20oC, have some of the fastest growing lobsters, whereas in the colder waters of the northern Gulf of St. Lawrence and the Bay of Fundy, lobsters grow more slowly. Under constant 20oC, H. americanus in captivity has been reared to marketable size (83 mm carapace length, CL) in two years (Hughes, 1972), but lobsters are not likely ever to experience that level of growth in the wild, where it is estimated to take 5-10 years to become harvestable, depending on ambient temperatures. Year-to-year differences in temperature substantially alter the timing and frequency of molting (Templeman, 1936; Munro & Therriault, 1983; Comeau & Savoie, 2001, 2002). NECIA • 19 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts Laboratory manipulation of the seasonal temperature cycle alters the molt cycle. If lobsters are held at 10oC or higher in the spring they quickly enter pre-molt, but if they are held at the same temperature into autumn, the molt is inhibited even the next summer (Waddy et al. 1995). Moreover, artificially warm temperatures of 15-20oC experienced during the winter can over-ride seasonal molt inhibition so that the molt may occur at any time of year (Waddy et al. 1995). Still, lobsters reared under constant temperature retain some seasonality in their growth cycle. American lobsters reared from hatching at a constant 20oC in the absence of a seasonal temperature cycle nonetheless exhibit a seasonal pattern of molting with the greatest frequency in spring and autumn and lowest in midsummer and winter. This indicates that other factors, such as photoperiod or an endogenous rhythm, also affect seasonal patterns of growth (Aiken & Waddy, 1989). Warm temperatures hasten the onset of maturity. Lobsters in warmer regions not only grow faster, but they mature at a smaller size than conspecifics living in cooler water. Females mature between 68 and 76mm carapace length (CL) in the warmer parts of their range, but between 87 and 97 mm CL in the cooler parts (Estrella & McKiernan, 1989; Comeau & Savoie 2001). Finally, seasonality in the temperature cycle influences the timing of the hatch. In hatchery settings, hatching has been observed when temperatures rise to between 12.2 and 15ºC. In the field, the temperature at which newly hatched stage I larvae first appear in the water column is generally consistent with the hatchery findings: 11.0-13.6ºC in southern New England, 9.0-12.7ºC in Cape Cod Bay, 7.9-13.9ºC in the Gulf of Maine, 4.2-10.6ºC in Northumberland Strait, and 10.0-13.8ºC in Newfoundland (Ennis 1995). Thus, the hatch generally occurs in southern New England in early summer (late-June to early July) and only begins in August in the northern Gulf of Maine. Larval and Postlarval Responses to Temperature: Development time from hatching to the postlarva (last planktonic stage) is strongly influenced by temperature (MacKenzie 1988). The absolute times projected by MacKenzie’s equations appear to overestimate the time required in the Gulf of Maine (Incze et al. 2006), probably because of a strong seasonal (endogenous) component to the temperature-dependent rates (Waddy et al. 1995). Temperatures above 24ºC are lethal to larvae and postlarvae, and development is severely inhibited at temperatures below 12ºC (MacKenzie 1988). Postlarval vertical movements and settlement behavior are temperature dependent as well. This could have important implications for whether postlarvae ultimately settle in suitable benthic habitat. Laboratory experiments have demonstrated that postlarval lobsters are deterred by steep temperature gradients, and will terminate the dive and return to the surface if they encounter a strong thermocline (Boudreau et al. 1992). In the field, Annis (2005) observed individual postlarvae to consistently remain above the 12ºC isotherm during the stratified summer season. Avoidance of temperatures <12ºC is consistent with the temperature requirements to complete larval development (McKenzie 1988). The limited data available on the vertical distribution of newly settled lobsters substantiates this pattern in that few if any newly settled lobsters have been found deeper than 20-30 m in the western coastal Gulf of Maine (Wilson 1999, Incze et al. 2006). The implication of this behavior is that if temperatures warmed sufficiently above 12ºC in the decades to come, new nursery grounds may become available that have historically been too cold for settlement. Implication of Model Results for Future Lobster Distributions Deteriorating conditions in the south: The observed time series of bottom temperatures at Millstone Power Station in eastern Long Island Sound from 1979-2004 indicates that August temperatures have been increasing on average over the period, and as a result have been rising more frequently above the 20ºC stress threshold (Figure 12). NECIA • 20 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts Bottom Temperature (oC) 24 22 20 18 16 1975 1980 1985 1990 1995 2000 2005 Year Figure 12. August bottom water temperatures (oC) at Millstone Power Station in eastern Long Island Sound . Twenty-degree threshold indicated as horizontal line. Increases in air temperatures are consistently predicted over the next century for each of the GCMs under both high and low emission scenarios. The frequency of extreme summer temperature events is also predicted to increase. Although a direct translation to projected water temperatures is not possible for reasons described above, it is possible to infer general trends and qualitative outcomes. Given the high frequency of events in which the 20ºC stress threshold has been exceeded in the last decade, it can be anticipated that this level will be consistently exceeded by mid-century in Long Island Sound and likely in other near shore waters of the northern mid-Atlantic Bight. In short, for eastern Long Island Sound the average bottom conditions in August, which are currently marginally stressful, are expected to become substantially more so by the second half of the century, with more frequent episodes of temperatures in the very stressful range of 25ºC. Anatomy of a die-off: It is instructive to go into some detail on the sequence of events during the Long Island Sound mortality event of 1999 because of its relevance to the climate assessment for the lobster fishery and other coastal marine resources. The following summary represents the scientific consensus regarding the factors contributing to the die-off as presented by Pearce and Balcom (2005) in their synopsis of research focused on the episode. Historical records indicate that during the months of July and August 1999, basin-average temperatures in Long Island Sound were at decade highs for that time of year (Wilson & Swanson, 2005). Bottom water temperatures in August were greater than 21ºC and in some locations exceeded 23.5ºC. Temperatures above 20ºC continued into October. During that time, lobsters in the western Sound were found to be infected with the parasitic Neoparamoeba pemaquidensis (Mullen et al. 2005). Lobster abundance in the Sound was estimated to be near historic highs as it was throughout the species range. As lobsters moved to deeper, cooler water during the summer to escape the warm hypoxic zones in the shallows, they became increasingly crowded and subject to further oxygen stress and disease exposure. In late August, winds from a cold front moving through the region mixed the NECIA • 21 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts water column, bringing warm surface water to the bottom and raising bottom water temperature several degrees. Although summer hypoxic conditions were beginning to dissipate in early September, the warmer conditions at the sediment-water interface facilitated the release of toxic sulfides and ammonium from the sediments, further weakening or killing lobsters. In early September, the remnants of Tropical Storm Floyd passed over the region with more winds and substantial rainfall that mixed warm water to the bottom and lowered the salinity. Many lobsters were afflicted with paramoebiasis by this time and were unable to recover from an already weakened state. There is debate as to whether exposure to pesticides entering the Sound by way of run-off could exacerbate the problem. Other afflictions of lobster at the southern end of their geographic range appear to be correlated with chronic exposure to warm temperatures. Dove (2005) described a condition called calcinosis in which lobsters living in stressful warm-water conditions have severe changes in blood ion balance, pH, protein levels, and blood cell counts. Epizootic shell disease is another pathological condition that can be lethal and appears to be temperature related, although it has not been found to be associated with the die-off in western Long Island Sound. It has occurred with highest prevalence from eastern Long Island Sound to Buzzards Bay, MA and has been recorded in Cape Cod and north of Boston more recently (Castro et al. 2006, Glenn and Pugh 2006). Smolowitz et al. (2005) proposed that elevated temperature could promote shell disease in two ways: (1) by compromising lobsters’ ability to remove bacteria because of physiological stress and compromised cleaning behavior; and (2) increasing the growth rate of pathogens. Beneficial effects in cooler regions: Warming in the cooler northern reaches of the species range may have positive impacts on lobster populations. Lobster landings are correlated with temperature (over the acceptable temperature range), reflecting immediate effects on vulnerability to capture and delayed effects from events in early life history (Fogarty 1995). Possible positive effects of climate warming are a prolonged growing season, more rapid growth, an earlier hatching season, faster planktonic development (which may increase survival and settlement), and a smaller size at sexual maturity. Warming may also make new areas of sea bed suitable for larval settlement. While shallow estuaries and embayments at the southern extreme of the species’ range may become less hospitable to lobsters as the climate warms, areas previously too cold for lobsters to thrive may become more habitable. This includes deeper waters below the thermocline as well as regions that do not become thermally stratified during summer months. An example of the latter in the northeastern U.S. is the eastern coast of Maine. This segment of the Gulf of Maine coast is strongly subject to the influence of the cold Eastern Maine Coastal Current which carries tidally mixed water from the Bay of Fundy southward along the Maine coast before it turns offshore near Penobscot Bay. Summer bottom water temperatures in this region typically do not get much higher than 12ºC, except in protected shallow bays and inlets. Lobster larval settlement, juvenile abundance, and harvests per unit of coastline along this segment of coast have on average been substantially lower than in western regions (Wahle & Steneck, 1991; Steneck & Wilson, 2001). Detectable densities of newly settled lobsters in this region have only been reported in shallow bays and inlets (Wahle unpublished). While there is no direct evidence to date that above-average temperatures have resulted in settlement in previously unpopulated nursery habitat, it is hypothesized that, provided a larval supply, settlement will be more likely where temperatures rise above 12ºC in the future. 5. Cod Case Study Cod has been a mainstay of the commercial fishery in New England since the 17th century, and a carved representation of the ‘sacred cod’ adorns the State House of Massachusetts. NECIA • 22 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts Populations of Atlantic cod off the northeastern United States are at the southern extent of the species range in the Northwest Atlantic. Accordingly, climate change can be expected to affect their distribution and abundance. For this reason, we focus on potential temperature effects on cod dynamics, with supplemental information on the role of other environmental drivers. Landings of cod have undergone large-scale and coherent fluctuations over the last century in the Gulf of Maine and on Georges Bank and are currently at low levels in both regions (Figure 13). Periods of low cod landings have corresponded with periods of high water temperature, although many fishery-related factors affect landings in addition to environmental variables. A comparison of the distribution and abundance of cod derived from Northeast Fisheries Science Center research vessel surveys during the relatively cold period 1965-1969 and the recent warm period during 2000-2004 suggests that cod were more broadly distributed in the earlier period than at present, including the southern New England region (Figure 14). Temperature Effects on Cod 55 20 Georges Bank GoM Cod 45 15 40 35 30 10 25 20 15 5 10 5 0 1900 0 1920 1940 1960 1980 2000 Year 12 20 GoM Cod Temp 18 11 16 10 14 12 9 10 8 8 6 7 Temperature (C) Landings ('000 mt) Georges Bank Landings ('000 mt) 50 Temperature (C) Gulf of Maine Throughout the North Atlantic, cod populations inhabit a broad spectrum of seasonal temperature regimes, ranging from less than -1oC to over 20oC, with annual mean temperatures from 2-12oC (Dutil and Brander 2003; Drinkwater 2005). The optimal physiological temperature for a population largely reflects adaptive responses to environmental conditions (Jobling 1988). Examining the response of cod populations to historical changes within this broad spectrum of temperature regimes can provide important insights into the potential change in distribution and abundance under alternative climatic conditions. In the northeastern U.S., cod spawning locations range from the coastal Gulf of Maine and Georges Bank to southern New England (Lough 1984, Lough and Bolz 1989, Serchuk et al. 1994). Eastern and western spawning contingents have been reported on Georges Bank, (Garrison et al. 2002, Lough et al. 2002). Spawning progresses from south to north, with spawning 60 4 6 2 0 1900 5 1920 1940 1960 1980 2000 Year Figure 13. Landings of Atlantic cod on Georges Bank and in the Gulf of Maine (upper panel), and cod landings in the Gulf of Maine in relation to annual mean water temperature at Boothbay Harbor (lower panel). NECIA • 23 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts commencing earlier in the south (Lough et al. 2002). Although the southern New England spawning components are combined with the Georges Bank population for management purposes, the southern groups may experience more immediate effects of increasing water temperatures because they inhabit the southernmost spawning habitat areas of the Northwest Atlantic. Figure 14. Distribution and abundance (kg/standard tow) of cod in Northeast Fisheries Science Center surveys during 1965-1969 and 2000-2004. Interannual variations in water temperature do not seem to affect the location of spawning, but do result in variations in the timing of spawning. The relationship is complicated, however, because temperature alone does not act as a trigger for the initiation of spawning (Hutchings and Myers, 1994). Location and timing of spawning may be of considerable consequence for the subsequent growth and survival of the early life stages. In a meta-analysis of nine cod stocks throughout the North Atlantic, including Georges Bank, Planque and Frédou (1999) examined the relationship between recruitment and temperature. Increases in temperature were shown to have a positive effect on recruitment in areas with temperatures close to the lower tolerance limit of cod, and negative effects in stocks inhabiting areas at the upper part of the temperature range. Stocks inhabiting regions characterized by intermediate temperature regimes showed no detectable effect of temperature. Recruitment of the Georges Bank stock was classified as neutral with respect to temperature change within the range of temperatures included in the analysis (see also Drinkwater 2005, Brodziak and O’Brien 2005). Planque and Frédou (1999) and Drinkwater (2005) determined the rate of change of recruitment with respect to change in temperature across stocks and suggested that further increases in temperature beyond a mean annual bottom temperature of 8.5°C would result in decreases in recruitment on Georges Bank. Brander (2000) examined the abundance of one-year-old fish in relation to average surface temperature (April-June) for five Northeast Atlantic cod stocks and concluded that the optimal mean annual bottom temperature for recruitment lies in the range 5-7°C for these populations. NECIA • 24 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts Temperature has also been shown to exert important influences on cod growth (Brander 1995). Brander (1995) showed a strong effect of temperature on the mean weight-at-age of cod for 17 stocks throughout the North Atlantic. Similarly, Rätz and Lloret (2003) discovered a close relationship between fish condition and average bottom temperature. These differences in growth have important implications for reproductive potential of the stock and the overall resilience of the population. Condition was highest for those stocks inhabiting warmer-water habitats within the range of temperatures examined. A key question is whether further increases in temperature for cod stocks off the northeastern United States would negatively affect growth and reproduction. McKenzie (1934, 1938) suggested that cod ceased feeding entirely at temperatures above 17°C. Rose (2005) reported ranges for cod feeding and spawning of 1-10°C and -0.5 to 6°C, respectively. Temperature also significantly affects the development rates of eggs and yolk-sac larvae (Pepin et al. 1997). Egg mortality is also significantly correlated with temperature (Pepin et al. 1997), where lower temperature leads to higher mortality rates. Highest egg survival occurred at 6oC in laboratory studies (Pepin et al. 1997). On Georges Bank, bottom water temperature is positively related to both egg and larval survival, acting in combination with age diversity of repeat spawners and egg distribution patterns within the range of observed temperatures (L. O’Brien, NMFS, Woods Hole, MA, pers. comm.). Atlantic cod landings in the Gulf of Maine and on Georges Bank have declined substantially over the last decade. These declines have been clearly linked to intensive exploitation, but the role of environmental conditions in concert with over-harvesting cannot be ignored. Results of a metaanalysis of North Atlantic cod stocks suggest that increases in mean annual bottom water temperatures above 8.5oC will result in a decline in recruitment (the number of young surviving to enter the fishery). Georges Bank, historically the major cod producing region in the northeastern United States, had a mean annual temperature of 9oC during the period 1978-2002. This suggests declining recruitment and reduced potential for recovery of cod populations over a substantial part of the northeastern shelf if further increases in bottom temperatures are realized. The 8.5oC threshold is also currently exceeded in the near-shore southern New England area, which has historically supported small spawning aggregations of cod. Consideration of cod distribution patterns throughout the North Atlantic suggests that a 12oC mean annual bottom temperature describes the practical limit of cod distributions. If the bottom temperature projections under the high emission scenario are realized, it is likely that this level will be surpassed in the near-shore southern New England region and Georges Bank regions by the end of this century. The mean bottom water temperature in the Gulf of Maine as a whole during the period 1978-2002 was 7.1oC and if the bottom temperature anomaly projections for this region hold, the 12oC threshold will not be exceeded. Unknown at this time is how other climate-related changes in the system would affect cod, such as through changes in prey and predator communities. North Atlantic Oscillation Brander and Mohn (2004) examined the relationship between NAO and recruitment for nine Northeast Atlantic and four Northwest Atlantic cod stocks. Their results indicate that the influence of NAO is subject to a pronounced geographical pattern. A positive NAO has a negative effect on cod recruitment in the North Sea, Baltic Sea, Irish Sea, Georges Bank and Eastern Scotian (Nova Scotia) Shelf, and a positive effect on the stocks of Iceland and Labrador/Newfoundland. The magnitude of the NAO’s effect is also linked to geographical location, depending on the relationship between the NAO index and the underlying physical factors, such as wind speed and direction, cloud cover, and temperature. Brander (2005) re-examined the NAO’s influence on cod recruitment and showed that high recruitment is observed more often when spawning stock biomass is medium or high, while low spawning stock invariably leads to a lower frequency of high recruitment. In years with a low NECIA • 25 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts spawning stock biomass, a high NAO index resulted in a much higher frequency of low recruitment levels. The consequence of this relationship between NAO, recruitment and spawning stock biomass is that stock recovery from low biomass levels will be highly dependent upon environmental conditions. Presently, spawning stock biomasses of most cod stocks are at sharply reduced levels (Brander 2005), and it is interesting that efforts to restore the severely reduced cod stocks in the northeastern U.S. over the past decade of positive NAOs have met with marginal success. Prey Availability The availability of suitable food items at critical life history stages can exert considerable influence on survival during the early life history of cod. Cod spawning is relatively constant between years with respect to both location and timing. However, the spawning of Calanus finmarchicus, a principal prey item of larval and pelagic juvenile cod, is highly temperature-dependent and may vary by as much as six weeks between cold and warm years (Ellertsen et al. 1989). The result of this temperature-dependent copepod production and advection is a mis-match in timing of peak larval production in relation to prey production in years with warm and cold environmental temperatures. Lynch et al. (2001) modeled larval cod growth in relation to larval fish trophodynamics, zooplankton population dynamics and hydrodynamic turbulence. The smallest larvae are most susceptible to variations in prey availability and their feeding rates are negatively influenced by turbulence. In contrast, larger larvae benefit from turbulence-enhanced encounter rates and their growth rates depend strongly on the distribution of both C. finmarchicus and copepods of the genus Pseudocalanus. Retention Successful recruitment is dependent on retention of early life stages within productive areas characterized by appropriate environmental and feeding regimes. On Georges Bank, the location and depth of spawning together with wind-driven advection have important impacts on the distribution of the larvae (Lough et al. 1994, Werner et al. 1993). Eggs and larvae from the spawning grounds on the northeastern peak of Georges Bank are advected along the southern flank of Georges Bank in the general clockwise circulation, concentrating between the shallow, permanently mixed area over the bank and the seasonally stratified area over the shelf-slope (Lough 1984, Lough and Bolz 1989, Lough et al. 1994, Mountain et al. 2003, Serchuk et al. 1994). Larvae remaining in the surface Eckman layer (< 25 m) have a high risk of being advected off the Bank, particularly in years where there is a strong along-shelf, east-northeastward wind stress. In contrast, larvae below the surface layer appear to be advected along the southern flank of the Bank. Larvae shoal-ward of the 60-70 m isobath have a higher probability of being retained on the Bank during spring than those further offshore (Lough et al. 1994, Werner et al. 1993). The distribution and drift patterns within the water column are essentially driven by convergence/divergence mechanisms in relation to stratification and frontal activity (Lough and Bolz, 1989; Lough and Manning, 2001). Thus, climate-related changes in wind and other factors affecting circulation on Georges Bank have the potential to affect cod recruitment. 6. General Conclusions Substantial uncertainty exists in projections of the effects of climate change in the marine environment off the northeastern United States. Existing coupled Ocean-Atmosphere General Circulation Models do not resolve circulation and other important dynamical physical processes on NECIA • 26 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts continental shelves, and they exhibit known biases with respect to predicted water mass properties in the northwestern Atlantic Ocean. We have attempted to reduce the effects of these biases by calibrating model output against direct observation for the period of instrumental record, and then considering only relative changes in predicted properties. That is, we used GCM model output from present-day and future runs to estimate temperature change over time, and then added the predicted difference to the current-day observations. The projections examined in this report should be viewed as provisional estimates, and caution must be exercised in interpreting results. Previous attempts to examine the effects of global climate change on marine resources in this region have utilized projections of atmospheric temperatures (e.g., Drinkwater 2005) or sea surface temperature estimates derived from GCMs (e.g., WWF 2005). We have attempted to refine these approaches by examining potential changes in bottom water characteristics as a small step forward. A full assessment of potential climate impacts on the marine environment of continental shelves ultimately requires coupling of the results of Ocean General Circulation Models with finerresolution hydrodynamic models in a fully nested structure. Such coupling properly accounts for responses of the shelf system to the combined effects of forcing at the boundaries with local forcing and adjustments. In the northeastern U.S., such a nested approach is necessary to capture features such as topography and vertical mixing which affect circulation and the distributions of temperature, salinity, nutrients and plankton. This need is most pronounced in topographically complex areas such as the Gulf of Maine. Our work using a perturbation approach to developing dynamically adjusted fields for a fine-scale hydrodynamic model (HOPS) forced by anomalies derived from a GCM is an initial step in this direction. Development of a fully nested model, however, will require much further effort. Regional models can incorporate other factors that we have not discussed here, such as changes in precipitation patterns (both seasonal patterns and extreme events), cloud cover, wind forcing, and advective exchanges with the neighboring ocean. To make further progress and incorporate some of these other important variables, we recommend that NPZ (nutrientphytoplankton-zoolankton) or NPZD (add detritus) models should be coupled to the regional circulation models. Such models would link climate change and remote forcing to the internal plankton dynamics of shelf systems. More advanced models will be of immense value as we try to predict, monitor and evaluate environmental change over the next century, and try to understand and manage anthropogenic influences on the coupled natural and human ecosystem. In examining cod and lobster as our examples, we neglected many other aspects of the marine environment that we should mention briefly. Of practical and pressing concern is the question of how climate forcing would affect marine invasive species that can substantially alter ecosystems. A second, significant question relates to the temporal and geographical extent of harmful algal blooms that can stop shellfish harvests (because the shellfish cannot be consumed) and affect human health through non-consumptive means (for example, through aerosols). A third question is the effect on endangered or threatened species and what, if any, action can be taken to mitigate the climate impacts. Indeed, a warming climate might be expected to alter many parts of the marine biosphere, with direct and indirect effects on the ecosystem and human interests, including commercial, recreational, and aesthetic values. Effective conservation and management require that we understand (and preferably anticipate) the factors driving system change. Comparison of the projected impacts of climate change on cod and lobster indicates that both are potentially vulnerable to population declines and losses of habitat in the southern portion of their range if projected bottom temperature increases are realized, particularly under the high emission scenario. Cod is at the extreme southern extent of its range in the northern Mid-Atlantic Bight. Under the high emission scenario, cod is potentially vulnerable to a loss of productivity and thermal habitat on Georges Bank, historically the dominant production region of the northeastern U.S. shelf. Within the Gulf of Maine, cod are expected to exhibit a neutral to slightly negative change if the high NECIA • 27 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts end of projected bottom temperature anomalies are realized by the end of this century. While lobster reaches the southern extent of its range in the vicinity of Cape Hatteras, farther south than cod, it is largely confined to deeper waters in the southern Mid-Atlantic Bight. In the northern Mid-Atlantic Bight, temperature is already a problem in some areas, and continued warming will pose an increasing threat to lobster populations over the southern New England shelf. On Georges Bank, the highest densities of lobsters are concentrated in the submarine canyons on the continental slope, where they are less vulnerable to temperature change in this region. In contrast, bottom temperature anomalies within the range predicted for the Gulf of Maine, the dominant production region for lobster, are potentially favorable for this species. These differences highlight the fact that, in general, the potential impacts of climate change in the ocean will exert differential effects on different species, and both declines and increases in productivity are likely to occur for different components of marine communities within the region. Although the bottom water temperature predictions are tentative, we note that differences between low and high emission scenarios are largest for the northern MidAtlantic Bight and Georges Bank. An economic impact assessment of the effects of projected climate change on the target species is beyond the scope of this report. The potential loss of thermal habitat with resulting loss of productivity for both lobster and cod in the southern portion of the study area is substantial, particularly under the high emission scenario. Although the loss of access to both these species would be potentially economically significant in these areas, it is likely that some adaptationin the fishing industry can be anticipated, principally though switching to alternative species. For example, an increase in water temperatures in southern New England is potentially favorable for an increase in blue crab (Fogarty and Lipcius, in press), and it is conceivable that some lobster fishers would switch to the lucrative blue crab fishery if such an increase occurs. Similarly, possible increases in abundance and productivity of sub-tropical finfish species in the northern Mid-Atlantic Bight (e.g., Sciaenids such as weakfish, spot, and drum) could potentially offset some losses of cod in the southern extent of the range. Overall, the contribution of the landings of both lobster and cod in the southern reaches of this study area are relatively low in relation to the total. Should large-scale losses of cod on Georges Bank occur or should adverse impacts on lobster (not currently anticipated) occur in the Gulf of Maine, the economic impacts would be substantially more serious. Acknowledgments We express our appreciation to Nick Wolff and Michelle Traver for data analysis and Adrienne Adamek for GIS work. We are grateful for editorial reviews by James McCarthy and three anonymous referees. The support and encouragement of Erika Spanger-Siegfried and the Union of Concerned Scientists are also gratefully acknowledged. NECIA • 28 Fogarty et al. 2007 • Climate Change and Marine Resources Impacts References Aiken, D. E. & Waddy, S. L. 1986. Oocyte maturation and spawning in wild American lobsters (Homarus americanus): lack of evidence for significant regulation by photoperiod. Can. J. Fish. Aquat. Sci., 43(7), 1451-1453. Altieri, A. H. & Witman, J. D. 2006. Local extinction of a foundation species in a hypoxic estuary: intergrating individuals to ecosystem. 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For each cruise in the NEFSC database an average temperature and salinity was calculated for the water column and for 0-25 m and 104-133 m depth layers, to correspond with model output. Only cruises since 1991 were used, since that is when an electronic conductivity/temperature/depth (CTD) instrument began to be routinely used for all hydrographic data collection. The average date of the observations from each cruise also was determined. The model values for the month containing the date of the observations were matched to the observed temperature and salinity values for both layers. For temperature in the surface layer over the ~15 year period, the model and observed series had approximately the same annual means. However, model values were about 4°C too warm in summer and about 4°C too cold in winter compared to the observations. In the deep layer over the same time period, modeled temperatures were always unrealistically cold. In contrast to other (broader geographical scale) analyses that suggest a fresh bias in prediction of salinity fields, model surface and deep layers in the Gulf of Maine were about 1 PSU too high compared to observations during the period of instrumental record. The overly large seasonal cycle in the model surface layer temperature suggests that the seasonal heating and cooling are not appropriately mixed into the deep parts of the water column. This suggests that the model (or downscaling of the original model results to the interior of the Gulf) does not properly represent the vertical physical processes in the Gulf. This could explain, at least in part, why the model deep layer salinities are too high. The model values of 34-34.5 PSU are appropriate for oceanic water outside the Gulf, but within the Gulf, vertical mixing with the lower salinity upper layers reduces the deep salinity. The cause of the very low deep layer temperatures is not obvious, as nowhere in the region, including outside the Gulf, are such cold temperatures observed at depth. All other comparisons of model outputs for the surface layer with a hydrographic data base revealed similar over-estimation of surface temperatures and a lack of resolution of the horizontal temperature gradients that characterize this region. This was not surprising given the importance of regional scale processes (tidal mixing and currents) in controlling water movements and temperature distributions. NECIA • 33