Major Estuarine Subsystems or Habitats

The landscape approach to estuarine ecosystems focuses on subsystems or habitats as major components within estuaries. Because organisms respond to the amount of change in the physical (abiotic) environment, their reaction to their environment results in subsystems or habitats composed of specific groups of species that are adapted to that particular set of abiotic factors. In estuarine ecosystems, the major abiotic factors are salinity, water velocity, intertidal exposure, and depth.

Water Column

Water is the primary medium for the transport of matter and information in estuarine ecosystems. Freshwater enters the estuary either as precipitation or as an accumulation driven by gravity down-slope through streams and rivers to the estuary. Salt water enters the estuary from the sea via tidal forcing. The gradient of increasing salt concentration from freshwater to marine divides the estuary into zones of salt stress and subsequently into different pelagic subsystems (Figure 2).

Phytoplankton are small chlorophytic eukaryotes that drift as single cells or chains of cells in estuarine currents. Diatoms and dinoflagellates are the dominant groups while species composition of a specific system is usually determined by salinity, nutrients, and light. They are a major component of the estuarine water column and provide food for many suspension-feeding animals. Planktonic primary production is seasonal and varies from distinct peaks in the arctic to spring and autumn blooms in temperate systems and almost no peaks in tropical estuaries. Average annual planktonic primary production in estuaries is about 200-300 gC m"2 yr"1 and is mainly a function of light, nutrient availability, and herbivore grazing.

There are two major categories of zooplankton: holo-plankton that in most estuaries are dominated by calanoid copepods which spend their entire life in the planktonic state and the diverse meroplankton that only spend their larval state in the plankton. Most estuarine zooplankton are believed to be herbivores and play a major role in connecting carnivores to phytoplankton. They are also thought to be major sources of inorganic nutrients that are available to phytoplankton.

The microbial loop in estuaries is composed of micro-and nano-planktonic bacteria, protozoans, and flagellates. Initially, the microbial loop was thought to play a major role in recycling nutrients with dissolved organic matter (DOM) a major product. However, the recent finding that a sizable proportion of DOM is made up of viruses has forced a major change in the microbial loop model (Figure 3). The current paradigm of the microbial-viral loop envisions the viruses (1010l_1) as 10 times more abundant than bacteria (1091_1) and controllers of bacterial diversity and abundance. The viruses are small (20-200 nm), ubiquitous particles that use the process of cell lysis to attack and kill bacteria. As a result, more bacterial biomass is shunted into DOM and away from the macroplankton and suspension-feeding macro-benthos. The much more rapid viral recycling of nutrients also has the potential to generate more stability in the system.

Large mobile animals, birds, terrestrial and aquatic mammals, and fish, shrimps and crabs, are common residents as well as transients in estuarine systems. These animals transform and translocate materials both within the estuary and between the estuary and other systems. The nekton organisms, in particular, use the tidally forced water column as a pathway between deeper channels and intertidal habitats where they seek refuge, feed, and develop.

Figure 3 A simple microbial-viral loop food web for an estuarine system. D, dissolved organic matter; G, grazers; H, heterotrophs; N, nutrients; P.P., primary producers; V, viruses.

Marshes and Mangroves

Emergent vascular plant-dominated intertidal wetlands are major subsystems in most estuaries. The two most common habitats are geographically zoned latitudinally with marshes dominating the temperate zone and mangroves the frost free subtropical and tropical zones. Both are found in low-energy wave-protected, sedimentary, high-salinity, and intertidal environments near the mouth of the estuary. While wetlands in the high-salinity portion of estuaries are low in species diversity (almost monocultures) of vascular plants, diversity is much higher in the freshwater reaches.

Salt marshes reach their greatest extent and productivity along the Gulf and southeast Atlantic coast of North America where the cord grass Spartina alterniflora dominates. This high production is the result ofnear ideal conditions of temperature, salinity, light, sediment texture, nutrients, and tidal range. Marsh grasses produce large quantities of both above- and belowground biomass that accumulates in the surrounding sediments (Table 1). The stems and leaves of the grasses also provide a structural base for an epiphytic community that further increases production. Decomposition processes in the organically rich sediments generate a strongly anaerobic reducing environment making the salt marsh a major center for nutrient cycling. The nutrient uptake mechanisms of vascular plants are poisoned by the reducing environment; however, air passages in the roots, rhizomes, and stems of these grasses aerate the surrounding sediments so that nutrient uptake can be maintained. The vertical stems and leaves of Spartina also serve as a passive filter that slows water flow, can remove via deposition suspended sediments from the water column, and allows many marshes to maintain their elevation with respect to rising sea level. This same environment provides food and refuge for many economically important nekton.

Mangroves are intertidal, tropical, and subtropical woody vascular plants that fill a niche similar to that of Spartina. In the high-salinity portions of the estuary, the red mangrove, Rhizophora, dominates. Red mangroves have prop roots that lift the plants above the reducing environment of the surrounding sediments. There is a gradient from high production in riverine swamps to low production in high-salinity scrub areas. On a global

Table 1 Primary production in estuaries

Primary producer

Annual primary production (gCm 2)

Macrophytes

Spartina

400-2480

Rhizophora

696-2100

Microphytobenthos

50-200

Epiphytes

12-260

Phytoplankton

25-150

scale of increasing light with decreasing latitude, the closer a system to the equator, the higher the mangrove productivity. Nutrients have also been implicated as a major limiting factor on mangrove productivity. There is evidence that mangrove production is enhanced by flushing action of storms. In addition to being a nursery for many fish, shrimps, and crabs, the structural mass of mangroves may form a protective buffer to the impacts of storm surges and tsunamis on coastal and estuarine systems.

Seagrasses

Seagrasses are submerged vascular plants that are found in aerobic, clear-water, high-salinity systems with moderate water flow. Cold water systems are dominated by eel grass, Zostera, and in the tropics turtle grass, Thalassia, is the major group. These grasses are not found in estuaries with high suspended sediment loads, that is, Georgia and South Carolina where there is insufficient light penetration to support their growth. They are also limited to the upper 20 m of water because water pressure compresses their vascular tissues. Maximum seagrass production can approach 15-20 gC m~2 d_1. The high productivity of the seagrass is almost equaled by the productivity of the epiphytes on their leaves; however, the sediment trapping abilities of seagrasses give them an advantage over phytoplankton and epiphytes in nutrient limiting conditions. The structure of the seagrasses provides feeding habitat for many mobile animals as well as deposit feeding and suspension-feeding benthos.

Invertebrate Reefs and Beds

Suspension-feeding benthic animals are common in most estuaries because of the high availability of suspended phytoplankton. A number of bivalves and a few worms can aggregate in very dense, high biomass beds or reefs. These structures are found both intertidally and subtid-ally in high to moderate salinities. The eastern oyster, Crassostrea virginica, in its intertidal form builds some of the most extensive aclonal reefs known. Intertidal beds of Crassostrea and Mytilus can have biomass densities exceeding 1000 gdbm~ . Depending on the estuary, suspension feeders such as oysters and mussels have been shown to control phytoplankton populations in some systems and influence nutrient cycling by short-circuiting planktonic food webs and reducing the recycle time for essential nutrients. There is evidence that the presence of a significant bivalve suspension-feeder component in estuarine ecosystems enhances system stability.

Mud and Sand Flats

Mud and sand flats are common to the intertidal zone of most estuaries. The major biotic components of tidal flats are bacteria, microbenthic algae, small crustaceans, and burrowing deposit feeders. As in the water column, the microbial-viral loop is thought to play a major role in the decomposition of organic matter in tidal flat sediments. In some estuaries, the microbial-viral loop utilizing a variety of electron acceptors may represent a significant sink for matter and energy. Thus, the prevailing processes on these flats can potentially redirect the fluxes of matter and energy away from macrofaunal food webs to those dominated by microbial processes. The occurrence of tidal flats was originally attributed to the hydrodynamics and sediment sources in tidal creeks; however, with the application of complexity theory to ecological systems, these flats are also being described as alternative states of salt marshes and bivalve beds.

Figure 4 A plot of water volume residence time versus bivalve clearance time showing areas of potential control by suspersion-feeders.

Residence time (days)

Figure 4 A plot of water volume residence time versus bivalve clearance time showing areas of potential control by suspersion-feeders.

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