Estuarine
Ecosystems
South Slough NERR Oregon |
Estuarine
Ecosystems
South Slough NERR Oregon |
The Coos Estuary, including South Slough, is a coastal river mouth that was "drowned" when sea levels rose following the retreat of the continental glaciers. In Coos Bay and similar drowned-mouth estuaries in the Northwest, the relatively greater precipitation during the winter results in a seasonally higher discharge of freshwater and sediment. During the relatively drier summer, the saline waters of the tides dominate while stream flows are lower (summary by Schultz, 1990:153-157).
An estuary dominated by saline tidal flows is generally 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 reduce soil salinity, and high soil salinity can impair nutrient uptake.
The gradient of an estuary also impacts its productivity. Estuaries with a steep gradient have faster flows that create narrow channels and scour fine materials (i.e., silt, clay) and plants from the bottom. 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 channel of the 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 are being
educated along the boardwalk)
Photograph by Mark Eberle, August 1999
As the water moves closer to the bay, it begins to mix more and more with the tidal currents to become a saltwater wetland (photograph below), which lacks trees and is dominated by salt-tolerant grasses, sedges, and forbs, such as the succulent pickleweed (Salicornia virginica). The plants of the salt marshes deal with the 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 the upland watersheds (~1,560 hectares = 3,855 acres), with smaller areas of freshwater habitats (riparian zones, freshwater wetlands, and ponds; ~63 hectares = 155 acres) and tidelands (salt marshes, mudflats and channels, sandflats, and rocky bottoms; ~324 hectares = 800 acres). However, it is important to keep in mind that all of these habitats and their processes are connected through the flow of water.
Boardwalk through Saltwater Wetland (Red Alders 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 floor 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 above the tides (photograph below). The hummocks can join into larger islands separated by small channels through which freshwater from streams and saltwater from the tides flow among the islands and hummocks (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, which increased deposition in the estuaries. However, many salt marshes have been diked and drained for pastureland, so there has been 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 the tidal mudflats (photograph below), which are rich in bacteria, fungi, algae, and invertebrates. Some of the primary productivity in the estuary is from photosynthesis by unicellular algae and cyanobacteria. Although their productivity per m2 is relatively low, the surface area they occupy is extensive, so their total productivity over the entire bay is large and can exceed the total productivity of other plants. They also are an important food for many estuarine organisms, such as the zooplankton that are, in turn, fed on by young fish (summary by Schultz, 1990:161). Animals living within the mudflats include a variety of protozoans, foraminiferans, nematodes, polychaetes, shrimps, and clams. Crabs and gobies often 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 to create anoxic (oxygen depleted) conditions about 25 cm 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 continue the process of decomposing the 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, which is also a prokaryotic organism. 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.
Narrow Open Channel through Mudflats at Low Tide
South Slough National Estuarine Research Reserve, Oregon
Photograph by Mark Eberle, August 2000
Running through the mudflats are the open channels that always contain water, even at low tides (photograph above). In some areas, this water supports the growth of eelgrass (Zostera) and a variety of organisms that inhabit its blades or the soil held by its roots. 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. The principal human threat to eelgrass beds is the dredging activity intended to keep deeper channels open. 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 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.
Literature
