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Seasonal and Latitudinal Patterns in Rocky Intertidal Communities of Ecuador Running head: Intertidal communities of Ecuador Lamb, R.W. ¹*, Franco, A.², Vinueza, L.R.¹ ¹Maestría en Ecología, Universidad San Francisco de Quito, Cumbayá, Ecuador ¹Pontificia Universidad Católica del Ecuador, Quito, Ecuador *Email: [email protected] 9 Abstract Ecuador is located at the confluence of two major, opposing marine currents: the 13 Humboldt Current and the North Equatorial Counter current (NECC). This creates a 14 unique mix of nutrient levels, water temperatures, and larval supply that may influence 15 intertidal community structures. However, the nature of these effects is largely unknown, 16 in part because the intensity and location of the convergence zone is strongly affected by 17 seasonal changes and by the strength of El Niño-Southern Oscillation. To examine how 18 communities vary along a gradient of productivity and temperature, we surveyed rocky 19 intertidal shores at each of 10 sites ranging 360 km along the Ecuadorian coast. We 20 sampled each site during both warm and cold phases of the seasonal cycle. Community 21 structure was significantly different between sites, and this variation was explained by 22 both biogeographic region (North vs. South) and phase of the local seasonal cycle (warm vs. cold). Biomass increased gradually from southern to northern sites, as did diversity, evenness, species richness, and abundance of mobile invertebrates. Temperature readings over the course of the study supported the hypothesis that the southern coast was dominated by the Humboldt Current and the northern coast was dominated by the NECC. This study provides baseline data for rocky shores along the continental coast of Ecuador, and illustrates the seasonal interplay between opposing current systems and its impacts on intertidal communities. Key Words: intertidal, community structure, Humboldt Current, productivity, temperature, Ecuador Introduction Ecuador is located directly on the equator at the confluence of two major marine current systems: the North Equatorial Counter current (NECC), which brings warm, waters of relatively low productivity southwards, and the Humboldt Current, which brings cold, productive waters northwards (Strub et al. 1998). As a result, the Ecuadorian coast 51 harbours marine life that is characteristic of both tropical and sub-tropical regions (Cruz et al. 2003). However, we lack basic information regarding biodiversity patterns and community structure in Ecuador’s mainland coast and how these patterns relate to oceanographic processes. Such understanding is essential for enabling scientists to discern natural variation from that caused by human or environmental perturbations (Harley et al. 2006), and to provide information for those managing and conserving marine systems (Tundi 2000, Edgar et al. 2004-B). In addition, the potentially stark contrast in temperature combined with the mixing of propagules from cold- and warm-adapted species from these two current systems presents an ideal setting for ecological research. Our study aimed to assess patterns of community structure and marine productivity in rocky intertidal communities of Ecuador, and how these patterns vary in time and space. Marine conditions on the western coast of equatorial South America vary as the relative strengths of the NECC and Humboldt Current affect the position of the Intertropical Convergence Zone, creating annual warm and cold phases, with particularly strong El Niño or La Niña years occurring on a semi-decadal basis (Wang & Fiedler 2006). This marine seasonality, commonly referred to as the El Niño-Southern Oscillation (ENSO), can produce substantial local variation in water temperatures and nutrient supply, with corresponding impacts on marine ecosystems (Alheit & Niquen 2004, Vinueza et al. 2006). The variation in these abiotic conditions plays a key role in shaping intertidal communities, whether by directly limiting species distributions through physiological restraints and larval recruitment (Dayton 1971, Sousa 1979, Menge & Sutherland 1987, Scrosati & Heaven 2007), or by affecting the outcomes of biological interactions (Dayton 1975, Posey et al. 1995, Kraufvelin et al. 2010). In addition, previously inferior competitors or invading species newly entering the system can become dominant as habitats become more favourable (Gilman et al. 2010). Under most global climate change models, ENSO events are predicted to become more frequent and stronger (IPCC 2001), causing more extreme temperature changes for longer durations, suggesting that we need to understand ENSO-related effects in much more detail at different spatial and temporal scales. Within the Humboldt current bioregion, strong El Niño years are characterized by a heavy influx of warm water from the equator (Fleischbein et al. 1987), which creates stagnant water conditions with decreased upwelling that can produce local extinctions and regime shifts (Glynn 1998, Harley et al. 2006). Conversely, La Niña years produce intense, persistent upwelling, although the baseline levels of productivity in this 86 bioregion are generally high, which can preclude significant ecological changes during strong La Niña events (Firstater et al 2010). Even so, such variations in temperature and upwelling regimes can affect predation rates, feeding efficiency and recruitment patterns (Menge 1992, Witman et al. 2010). Less is known regarding how ENSO variation affects marine habitats of the eastern tropical Pacific north of the equator, but the impacts in this region are somewhat less pronounced (Glynn 1990). While a given year may involve a strong La Niña, El Niño, or neither, some level of seasonal variation can be expected to occur on an annual basis. In addition, inter-annual variation in the ENSO has been shown to produce community turnover as the growth and survival of certain species are favoured over others (Vinueza et al. 2006). Although seasonal variation in a given year may not be sufficient to have this same effect, differences in recruitment from propagules carried by one dominating current or the other could potentially affect species assemblages. For these reasons, the continental coast of Ecuador presents a unique opportunity for examining the potential effects of global climate change on marine ecosystems, since substantial variation in water temperatures and nutrient availability could be expected over relatively short periods of time. Marine conditions and their impacts on ecological relationships can generally be characterized based on latitude (Menge & Lubchenco 1981, Broitman et al. 2001). In more distinctly tropical areas of the eastern Pacific (located north of the equator, with usually warm water and low nutrient levels), intertidal communities tend to be dominated by crustose algae (SibajaCordero & Cortés 2010), and species that could potentially be more competitively dominant (e.g., foliose algae, sessile invertebrates) are intensely predated upon by a diverse assemblage of consumers (Lubchenco et al. 1984, Menge et al. 1986). Conversely, studies in areas of strong subtropical upwelling off the coast of Peru indicate that in these colder, nutrient-rich waters, bottom-up processes such as nutrient availability and structural habitat play a more important role in determining intertidal community structure (Firstater et al. 2010, Firstater et al. 2011). In the Galápagos Islands, which are located 1000 km directly west of Ecuador, intertidal communities representative of both regions can be found within a very small area, due to a confluence of warmand cold-water currents (Vinueza 2009). In addition, dramatic shifts in community structure can be produced at a single site between different phases of the ENSO (Vinueza et al. 2006). We sought to determine whether similar differences in intertidal community structure could be observed within relatively small spatial and temporal scales in mainland Ecuador. Our study describes patterns of intertidal community structure along the Ecuadorian coastline, which stretches approximately 650 km from north to south. Understanding the similarities and differences between northern and southern coasts is very important, as any disparities in abiotic conditions between phases of the ENSO and between biogeographical regions may create differences in patterns of diversity and community structure. The continental coast of Ecuador has long been overlooked as a potential research site for intertidal community ecology, with most studies focusing on qualitative assessments of diversity (Cruz et al. 2003). This has left the area virtually unstudied, creating a large knowledge gap regarding intertidal community composition and the ecological processes and oceanographic phenomena that affect local marine habitats. With this in mind, we sought to investigate how latitudinal differences in oceanographic conditions and ENSO-driven seasonality affects intertidal communities along the coast of Ecuador, whether distinct biogeographical regions exist based on closer proximity to warm or cold water currents, and if so, how these regions might react differently to warm and cold phases. Methods Study Sites We sampled rocky intertidal communities at 10 different sites stretching 361 km from North to South along the Ecuadorian coastline. We took visual quadrat surveys and physical samples of algae and associated invertebrates from intertidal communities at each site during a peak cold La Niña event (August-October) and a normal warm phase (February-April) of the seasonal cycle. Our choice of sites was determined by the presence of relatively flat rocky benches with similar abiotic conditions at the landscape level and with semi-regular spacing throughout the entire study area. We measured temperature using HOBO Pendant® data loggers at 15-minute intervals from December 2010 to April 2011 (warm phase) at one northern and one southern site. We also measured the physical characteristics of each study site, including sand burial (mean percent cover of sand in the low zone at each site) and wave height (visual observations at each site at the same time of day during the same tide series) to account for confounding environmental variables between sites. Intertidal Community Surveys Within each site, we sampled two locations that were separated by at least 100 meters. At each location, we defined the low intertidal zone based on natural zonation patterns of major primary space occupiers and the relative positioning of each area with regard to tidal height. We then laid out a 100 meter transect tape parallel to shore that followed the contour of the shoreline at a consistent tidal height. Along this transect we sampled 10 quadrats of 50 x 50 cm placed horizontally on the substrate at 10 m intervals. Within each quadrat, we identified organisms down to the family, genus, or species level, and quantified the presence of each taxonomic group. Mobile species were counted individually, and percent cover was determined for primary space-occupying organisms (e.g., barnacles, algae, etc.). Additionally, we removed all algae from a 10 x 10 cm square at the centre of each quadrat and froze it in a plastic bag for weighing. We took samples and quadrats during both cold (August-October 2010) and warm (February-April 2011) ENSO phases at each site. Sample Processing We separated out each algal biomass sample in water in a plastic container to remove the sediment. We then removed all fauna from within the blades of algae and identified and recorded their abundance. We placed the algae in individual tin foil cups and placed them in a drying oven at 70° C for 48 hours. We then measured dry mass for each sample. Statistical Analyses We assessed differences in community structure in low zone quadrats between phases (warm/cold) and between biogeographical zones (North/South). We performed a multivariate analysis using PRIMER statistical software, version 6.0, of differences between these groups using two different types of data: percent cover of algae and sessile invertebrates, and diversity of algal species. We first calculated mean percent cover for each species observed across all 10 quadrats from both transects taken at each site during each seasonal phase. Algal species were grouped into functional categories following the classification system proposed by Steneck and Dethier (1994). We performed a Bray-Curtis similarity analysis using a square root transformation of the mean percent cover data for each functional group of algae and sessile invertebrates. We then performed two non-metric multi-dimensional scaling (MDS) analyses, one for the North region and another for the South. We added vectors leading from the cold phase data point for each site to the corresponding warm phase data point to show the overall trend in the transition of percent cover data between phases. Based on our a priori study design, we performed a crossed analysis of similarity (ANOSIM) with replicates to test for significant differences in community composition between phases at each site, and a nested ANOSIM (sites within regions) to test for significant differences between sites from different regions. We then repeated this same analysis using diversity of algal species, with a 1 denoting the presence of a species in each quadrat, and a 0 denoting absence. Instead of individual MDS plots for each region, we grouped all transects from both phases, producing one data point for each site. We analyzed mean dry algal biomass between sites with R statistical software, using a generalized linear model with a gamma distribution correction to test for differences between regions and/or between phases. We also assessed the differences in the invertebrate community living in algal biomass samples by quantifying evenness (J’), species richness and abundance, and diversity (H’) at each site during each phase. Finally, we assessed the difference in mean daily water temperature between site N1 and site S7 using a paired two-sample t-test. Results Mean distance between sites was 41.63 km, with a range of 0.50 – 132.32 km. For ease of interpretation of the results, we labelled the sites 1-10 from North to South. Our 208 results also led us to classify these sites into two regions: five northern and five southern 209 sites, denoted from here forward as N1-5 and S6-10. There were no significant differences between regions in terms of sedimentation, wave height, or exposure (data not shown). Our analysis of percent cover of sessile organisms grouped into functional classes demonstrated that sites were significantly grouped by phase (p < .05). The vector arrows connecting cold phase to warm phase data points in the North and South MDS plots 215 (Figure 1 – A and B) demonstrate that the direction of the shift in percent cover data 216 between phases was fairly consistent among sites located within each region. However, using this same multivariate metric of percent cover for each functional class, sites were not significantly grouped by region or by region crossed with phase. Southern sites had higher percent cover of articulated calcareous algae (p = .0002) and corticated foliose algae (p = .0051) during the cold phase (14% vs. 5%, 15% vs. 7%, respectively), and higher cover of corticated macrophytes (p > .0001) during the warm phase (17% vs. 3%). Conversely, northern sites had higher percent cover of filamentous algae (p = .0054) and sessile invertebrates (p = .0001) during the cold phase (36% vs. 25% and 11% vs. 4%, respectively), and higher cover of corticated foliose algae (p = .0021) and crustose algae (p = .032) during the warm phase (12% vs. 4% and 13% vs. 9%, respectively) (Figure 2). Our assessment of the diversity of algal species assemblages also indicated that sites might be grouped by phase and/or region. The two-way crossed analysis of these presence/absence values confirmed that sites were significantly grouped by phase (<.006). Again, the analysis of sites nested within region did not produce a significant result. However, we performed a second analysis by crossing region with phase, which demonstrated that, when multivariate diversity values were averaged across both phases, sites were significantly grouped by region (p <.006). Figure 3 illustrates this relationship, in which southern sites cluster into a distinct group in multivariate space. The generalized linear model indicated that dry algal biomass did not vary significantly between phases, but did vary significantly by region (Figure 4), with higher biomass values in the South. This trend was evident both when grouping sites into North vs South regions (p < .01) and when assessing sites by latitudinal position in a linear relationship (p < .01). The community of mobile invertebrates living within each algal sample also varied between sites, with patterns similar to those seen in biomass results. Mean values were higher in southern sites than in northern sites during both warm and cold phases for Shannon-Weaver diversity index (p = .0029), evenness index (p = .0034), species richness (p = .0051), and invertebrate abundance (p = .019) (Figure 5). Temperature was significantly higher in site N1 (mean: 27.26 ± 0.89 °C) than site S7 (mean: 26.18 ± 0.93 °) during the warm phase (p < .0001; Figure 6). Discussion Our study offers the first quantitative assessment of intertidal community structure along the coastline of continental Ecuador. The oceanographic patterns of the eastern tropical Pacific have long been studied in correlation with the long-lasting and far-reaching impacts of ENSO events (Strub et al. 1998, Wang & Fiedler 2006). However, it has not previously been shown how the dynamic convergence zone of the Humboldt Current and the NECC impacts local intertidal ecological processes in mainland Ecuador. Our temperature readings demonstrated that the southern coast of Ecuador experienced cooler water temperatures than the northern region during the study period (>1°C mean difference). While this difference may be relatively small, it does show that environmental conditions can vary significantly along this latitudinal gradient within a very limited geographic area. The Humboldt Current extends well into Ecuador during strong La Niña years such as this one (Strub et al. 1998, Wang & Fiedler 2006), but the differences in temperature that we observed could indicate that the influence of the NECC was still felt in northern sites. Conversely, during El Niño years, when a mass of warm surface seawater can extend as far as 7°-10° S (Fleischbein et al. 1987), such disparities between temperatures on the northern and southern Ecuadorian coast would probably disappear. Coastal areas affected by the Humboldt Current are generally very productive (Alheit & Niquen 2004, Firstater et al. 2011), which is characteristic of areas with high nutrient concentrations and frequent upwelling (Blanchette et al. 2009). While we were unable to directly evaluate nutrient levels, these are positively correlated with primary production (Cebrian et al. 2009), which we measured using dry algal biomass. Our results show that a gradient does exist in standing algal biomass along the coast of Ecuador, suggesting that southern sites, with their closer proximity to the cold water Humboldt Current, are more productive than northern sites. Our results also indicate a dividing point in marine productivity between sites S6 and N5 (corresponding to the towns of Puerto Cayo and Manta), which could signify that, at least during the course of our study, a mixing zone existed in this area between the Humboldt Current and the NECC. This evidence for the location of the mixing zone coincides with satellite images measuring marine productivity in this area (Saba et al. 2008). Conversely, we did not observe the hypothesized trend of higher biomass during the cold phase than during the warm phase across all sites. We expected to observe this trend since, in the nearby Galápagos Islands; seasonally higher nutrient levels produce greater levels of biomass along a range of baseline productivity rates (Vinueza 2009). However, we probably would need to examine biomass values over longer periods of time in order to better assess local variation in intertidal productivity between phases of the ENSO. In our study, diversity, evenness, species richness, and abundance of the mobile invertebrates found in algal samples all followed similar trends to that of dry algal biomass, with greater values in southern sites, evidence that the impacts of higher productivity levels on the southern Ecuadorian coast are also felt in consumer groups. This was to be expected, since the larger quantities of algae in samples from southern sites would provide more habitat for the small marine organisms evaluated in this analysis. Despite this bias, differences in marine productivity have been shown to propagate up through higher trophic levels in previous studies from both tropical and temperate systems (Menge 1992, Vinueza et al. 2006, Cloern et al. 2007). Our results also provide further evidence that nutrient supply and diversity in marine ecosystems are closely related (Worm et al. 2002). Several studies have observed this type of variation along geographical gradients (Broitman 2001, Schoch et al. 2006, Konar et al. 2010), but not at such a small scale. One striking exception has been described in the Galápagos, where different marine a mixing zone existed in this area between the Humboldt Current and the NECC. This evidence for the location of the mixing zone coincides with satellite images measuring marine productivity in this area (Saba et al. 2008). Conversely, we did not observe the hypothesized trend of higher biomass during the cold phase than during the warm phase across all sites. We expected to observe this trend since, in the nearby Galápagos Islands; seasonally higher nutrient levels produce greater levels of biomass along a range of baseline productivity rates (Vinueza 2009). As is the case for the Galápagos, the continental coast of Ecuador is located in a very dynamic oceanographic setting with dramatic differences in temperature and productivity, which produces distinctly tropical and subtropical biogeographic regions (Jennings et al. 1994, Edgar et al. 2004). While studies from Peru (Firstater et al. 2010) are representative of subtropical communities, other locations such as Panama (Lubchenco et al. 1984, Menge et al. 1986) and Costa Rica (SibajaCordero & Cortés 2010), are more in line with what has been described for tropical rocky shores. There, communities are dominated by encrusting algae, mostly as a result of a strong top down effect of a diverse assemblage of consumers. In contrast, the low shore in our study sites was dominated by more complex forms of algae, not only crustose species, which exemplifies the predominance of the Humboldt Current in this region during the study period. Our visual analyses using MDS plots for percent cover and diversity values pointed out phase and region as possible grouping factors for the differences observed in community structure. Both analyses showed that transects carried out at all sites during the months of the strong La Niña event from August to October 2010 were significantly different in community composition from transects carried out during the normal warm phase during February-April 2011. These trends coincide with the results of previous studies that demonstrated how significant differences in water temperature (Sanford 1999, Yamane & Gilman 2009, Meager et al. 2011) and nutrient or food supply (Bustamante et al. 1995, Vinueza et al. 2006, Witman et al. 2010) can change both the amount of biomass present and the dominance patterns of primary space occupiers. While Figure 1 demonstrates that the composition of intertidal communities along the coast of Ecuador changes with variation caused by the seasonal cycle, Figure 3 shows how sites within northern and southern regions group differently, once the interaction with the seasonal phase is accounted for. We did not observe a clear pattern of typically tropical communities in northern Ecuador and subtropical communities in the South, but when the interaction between region and phase was taken into account, region did serve as a significant predictor of community structure. This would indicate that cold and warm phases of the ENSO cycle impact the northern and southern coasts of Ecuador differently. For instance, during a strong El Niño year, we might expect to observe recruitment of more tropical species, as occurred in the Galápagos (Vinueza et al 2006), which could be expected to bring about more conspicuous changes in southern sites. A comparison of community structure values between strong El Niño and La Niña years would facilitate the elucidation of whether these patterns are more heavily dependent upon which current provides the majority of propagules for settlement (high turnover), or if species are able to resist temperature fluctuations between ENSO phases (low turnover). Certainly, intertidal communities on the Ecuadorian coast exhibit substantial temporal and spatial variation within a very small area (<400 km) and time frame (<1 348 year). These dramatic differences could have important implications in the management of marine fisheries and reserves on the Ecuadorian coast. For example, the Galera-San Francisco marine reserve, located near sites N1-N3, might require different management tactics than the Machililla National Park, located near sites S6-S8. In addition, the management of Ecuador’s major near shore artisanal fisheries, namely billfishes, lobster, and demersal fishes, must take into account seasonal fluctuations in marine currents (Castleberry & Riebensahm 2011). A large portion of Ecuador’s coastal rural population work in these fisheries, which have few regulations in place or supporting technology to reinforce long-term stability (Guest 2003). As current patterns fluctuate, they bring unique nutrient and temperature conditions as well as larval supply, which may favour certain species over others (Alheit & Niquen 2004). By referencing this information, policy-makers can better predict the response of each species to harvesting and make management decisions accordingly. We also observed an extremely high level of removal of intertidal organisms at our study sites, including predatory snails, chitons, limpets, octopus, and fish. This removal was higher at sites close to human populations, and as many as 500 individuals were observed to be collected per site during a single low tide (pers. obs.). This could produce significant changes in community structure, which would be expected to vary based on location. At northern sites especially, where consumer-driven processes could be predicted to have a stronger impact in governing community structure (Menge et al. 1986, Vinueza 2009) due to the predominance of the NECC, the absence of these consumers could affect species dominance patterns. This could have important implications, since the demersal habitat in intertidal communities is effectively bio-engineered by primary space occupiers such as algae and sessile invertebrates (Jones et al. 1994, de Juan & Hewitt 2011). Ocean temperatures and productivity levels along the western coast of South America may experience even greater levels of variation as the global climate continues to change. Globally, sea surface temperatures have risen by 0.6°C over the past 100 years (Pachauri 2007). In addition, most climate change models predict stronger and more frequent ENSO events in coming decades (IPCC 2001). Oceanographic conditions are very strong drivers of community structure (Broitman et al. 2001), and are susceptible to large-scale variation driven by forces such as ENSO and climate change (Wang and Fiedler 2006). Climate change may also affect basic water chemistry components such as dissolved oxygen concentrations and pH (Harley et al. 2006), which can have profound impacts on the structure and functioning of local marine communities (Przeslawski et al. 2008). Long-term monitoring of oceanographic conditions and ecological responses along the coast of Ecuador may serve as an indicator for how climate change affects ENSO phenomena and marine ecological relationships, and how these impacts differ between tropical and sub-tropical communities. Our study has produced many new questions that need answering in order to better understand the ecological processes at work along the continental coast of Ecuador. For instance, given the complex and often site-specific interactions between herbivores and nutrients in determining algal community structure (Burkepile & Hay 2006), what role are herbivores playing in the patterns we have described here? How do bottom-up and top-down processes control community structure as compared to the patterns observed in Peru, Galápagos, and Panama? How does the removal of molluscs and fish through collection by local inhabitants affect intertidal community structure? Certainly, the continental coast of Ecuador presents a number of intriguing opportunities for ecological research, and merits increased attention from the scientific community. Acknowledgements We would like to thank the Rufford Small Grants Foundation, the Universidad San Francisco de Quito, and the Pontificia Universidad Católica for funding this study, and A. Encalada and the Laboratorio de Ecología Acuático (Universidad San Francisco de Quito) for the use of their laboratory space. We would also like to thank W. Goodell, M. Hirschfeld, J. Montalvo, F. Ordoñez , and GAIAS students from the Methods of Marine Research Module for assistance in the field and laboratory, and D. Johnson, M. Frenock, and A. 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