Download Estuarine Ecosystems - Fort Hays State University

Estuarine
Ecosystems
South Slough National
Estuarine Research
Reserve, Oregon
Department of Biological Sciences, Fort Hays State University
Instructor: Mark Eberle
Course Homepage
To gain an understanding of estuaries, we hike through the Hidden Creek drainage in
South Slough National Estuarine Research Reserve, which is part of the Coos Bay watershed.
Estuaries form where freshwater flowing from the land mixes with tidal flows of saltwater
from the ocean. A bay is the main body of water along a river channel in an estuary. Sloughs
are tidal areas along tributary creeks flowing into the bay. The general attributes of estuaries
are 1) protection from waves, 2) variable salinity, and 3) high productivity (information from
summary by Schultz, 1990:153‒157). In fact, estuaries have a higher productivity than most
other ecosystems.
The Coos Estuary, including South Slough, is a coastal river mouth that was “drowned”
when sea levels rose following the retreat of continental glaciers. In drowned-mouth estuaries
of the Pacific Northwest, the relatively greater precipitation during winter results in a
seasonally higher discharge of freshwater and sediment. During the drier summer, saline tidal
waters dominate while stream flows are lower (summary by Schultz, 1990:153‒157).
In general, an estuary dominated by saline tidal flows is more productive than an estuary
dominated by freshwater (riverine) flows. The tidal flows flush the estuary and redistribute
nutrients. Also, tidewater regularly inundates broader areas of the estuary than streamflow,
and it varies less in both temperature and seasonal volumes (summary by Schultz,
1990:153‒160). However, precipitation and runoff are important in reducing soil salinity;
high soil salinity can impair nutrient uptake.
The gradient of an estuary also affects its productivity. Estuaries with a steep gradient
have faster flows that create narrow channels and scour both fine materials (i.e., silt, clay) and
plants. Low-gradient estuaries with moderate streamflows tend to have broad beds inundated
by tidewaters, where silt, clay, and detritus accumulate (summary by Schultz, 1990:153‒160).
The importance of these sediments in estuarine ecology is summarized below.
At South Slough, the higher gradient of the upper Hidden Creek drainage directs water
over and through the ground toward the narrow creek, which flows through second-growth
coniferous forest. Near the bottom of the hill, the slope of the land decreases and the creek
expands into a freshwater wetland of Red Alder (Alnus rubra) and Skunk Cabbage
(Lysichitum americanum), which are relatively intolerant of saltwater (photograph below).
Boardwalk through Freshwater Wetland
South Slough National Estuarine Research Reserve, Oregon
(Erin Bogan, Janet Robertson, and others being educated on the boardwalk)
Photograph by Mark Eberle, August 1999
As the creek flows closer to the bay, it mixes with tidal currents to become a saltwater
wetland (photograph below) dominated by salt-tolerant grasses, sedges, and forbs, such as the
succulent Pickleweed (Salicornia virginica). Salt marsh plants deal with high salt levels by
secreting excess salt through special structures in their epidermis or by diluting the salt in
special water-storing cells (Pickleweed has a salty taste when you bite into it). Habitats within
the South Slough Reserve are dominated by upland watersheds (~1,560 hectares = 3,855
acres), with smaller areas of freshwater habitats (riparian zones, freshwater wetlands, and
ponds; ~63 ha = 155 acres) and tidelands (salt marshes, mudflats, sandflats, and rocky
bottoms; ~324 ha = 800 acres). However, it is important to keep in mind that all of these
habitats and their ecological processes are connected through the flow of water.
Saltwater Wetland (foreground) and Freshwater Wetland (tress in the background)
South Slough National Estuarine Research Reserve, Oregon
Photograph by Mark Eberle, August 1999
Salt marshes form through sediment deposition that raises the bed of the estuary to a point
at which it is less frequently inundated by high tides. Vascular plants become established on
these deposits and spread through rhizomes (underground stems) to form hummocks that slow
currents and cause silt to accumulate around them, expanding the hummock (photograph
below). Hummocks can merge into larger islands separated by braided channels through
which freshwater from streams and saltwater from tides flow among them (summary by
Schultz, 1990:166‒171). Humans initially increased the area of salt marshes by increasing the
amount of silt eroded from areas that were logged or plowed, increasing deposition in the
estuaries. However, many salt marshes have been diked and drained for pastureland, resulting
in a net loss of salt marshes along the coast (summary by Schultz, 1990:167‒169, 316‒317).
Salt Marsh Hummocks, South Slough National Estuarine Research Reserve, Oregon
Photograph by Mark Eberle, August 2000
Beyond the marshes are tidal mudflats (photograph below), which are rich in bacteria,
fungi, algae, and invertebrates. Some primary productivity in an estuary results from
photosynthesis by unicellular algae and cyanobacteria. Although their productivity per square
meter is relatively low, the surface area they occupy is extensive; thus, their total productivity
over the entire bay can exceed the total productivity of vascular plants. They are also an
important food for many estuarine organisms, such as the zooplankton, which are, in turn, fed
on by young fish (summary by Schultz, 1990:161). Animals living within the mud include a
variety of protozoans, foraminiferans, nematodes, polychaetes, shrimps, and clams; crabs and
gobies use burrows of worms and shrimps (summary by Schultz, 1990:182‒188).
Mudflats at Low Tide, South Slough National Estuarine Research Reserve, Oregon
Photograph by Mark Eberle, August 1999
Also important to estuarine productivity is the process of decomposition within the
extensive estuarine sediments. Large amounts of detritus accumulate in the mudflats because
the currents are relatively slow and there is little wave action in the protected estuary. Fungi
and bacteria decompose the dead material, and burrowing worms, shrimps, and other
organisms feed on this detritus. These organisms are fed upon by larger animals, such as
shorebirds, waterfowl, fishes, and Raccoons (Procyon lotor). Initially, aerobic bacteria
consume oxygen through cellular respiration as they decompose the detritus within the
sediments, and this helps create anoxic (oxygen depleted) conditions about 25 cm (1 foot)
below the surface of the sediments. The anoxic conditions are maintained below this level
because the fine particles of silt that comprise the sediments restrict the depth to which
oxygenated water from the surface is able to effectively penetrate (summary by Schultz,
1990:177‒188).
In these oxygen-depleted sediments, anaerobic bacteria must use chemicals other than O2
as electron acceptors in cellular respiration as they decompose organic material. Although
other compounds, such as nitrite (NO2‒) and nitrate (NO3‒), are more readily used as electron
acceptors, sulfate (SO42‒) is relatively more abundant in seawater, so it serves as the principal
electron acceptor in marsh sediments and is converted by bacteria into some form of sulfide,
such as hydrogen sulfide (H2S) (summary by Bagarinao, 1992). The presence of the sulfide
gives the deeper sediments the characteristic smell of “rotten eggs.” Although sulfide is a
highly reduced (energy rich) compound that can be used by some bacteria and other
organisms (if oxygen is available), it inhibits the functions of a variety of enzymes and other
proteins, making it toxic to many organisms (summary by Bagarinao, 1992). Thus, both the
presence of sulfides and the absence of oxygen in the sediments limit the depth to which some
organisms can survive in the mudflats.
In addition to decomposition, bacteria in the sediments also benefit the ecosystem through
2 important processes involving nitrogen. In nitrogen fixation, N2 is converted into NH3
(ammonia), a process also performed by cyanobacteria. In nitrification, bacteria convert NH3
into NO2‒ (nitrite) and NO3‒ (nitrate). As in terrestrial ecosystems, nitrogen is often a limiting
nutrient in estuaries, and these microbial processes make most of this essential nutrient
available to plants, which cannot use N2 directly.
Running through the mudflats are open channels that always contain water, even at low
tides (photograph next page). In some areas, this water supports the growth of eelgrass and a
variety of organisms that inhabit its blades or the soil held by its roots. There are 2 species of
eelgrass in Oregon, both occurring in the South Slough: native Common Eelgrass (Z. marina)
and the invasive exotic Dwarf Eelgrass (Z. japonica) (Dudoit, 2006). In the Pacific Northwest,
Dwarf Eelgrass has colonized normally unvegetated tidal mudflats, altering physical habitat
structure and processes (perhaps removing nitrogen and phosphorus from the water column),
as well as altering invertebrate community structure, possibly reducing shorebird foraging
areas, among other effects (summary in GISD, 2006). The principal human threat to native
eelgrass beds is dredging intended to keep deeper channels open.
Despite its appearance and name, eelgrass is not a true grass, but, like grasses, it is a
flowering plant. Eelgrass distributes its pollen and seeds in the currents; however, its most
effective method of reproduction is through its rhizomes. Eelgrass is largely indigestible, so
the primary means through which its nutrients are passed through the food web is by
decomposition. Animals that consume decomposing bits of eelgrass actually derive most of
their nutrients by digesting the fungi and bacteria living on the dead plant material. The
eelgrass is largely undigested and returned to the ecosystem, where bacteria and fungi again
colonize and further decompose the plant material (summary by Schultz, 1990:163‒165).
Thus, the high productivity of the estuary is driven by the critical ecological process of
decomposition of organic materials (as in the temperate rain forest), along with the regular
inputs and mixing of nutrients by saline tidal flows and freshwater streams.
Narrow Open Channel through Mudflats at Low Tide
South Slough National Estuarine Research Reserve, Oregon
Photograph by Mark Eberle, August 2000
Literature Cited
Bagarinao, T. 1992. Sulfide as an environmental factor and toxicant: tolerance and adaptations in
aquatic organisms. Aquatic Toxicology 24:21‒62.
Dudoit, C. M. 2006. The distribution and abundance of a non-native eelgrass, Zostera japonica, in
Oregon estuaries. Senior thesis, Oregon State University, Corvallis.
Global Invasive Species Database (GISD). 2006. Zostera japonica Aschers. & Graebn. Invasive
Species Specialist Group, IUCN Species Survival Commission.
http://www.issg.org/database/species/ecology.asp?si=859 (accessed July 2013).
Schultz, S. T. 1990. The Northwest Coast: A Natural History. Timber Press, Portland, Oregon.
Next Stop: Redwood Forest | Species Checklists | Return to Trip Summary Homepage