Estuarine and Continental Shelf Sediments

Approximately 39 percent of the global human population, or approximately 2.2 billion people, lived within 100 km of the coast in 1995, most within estuarine watersheds (Burke et al. 2001). In countries such as the United States, coastal populations have increased faster than the overall population (Beach 2002). Historically, human populations have depended on estuaries for food (e.g., fish and shellfish), transportation, trade (e.g., waterways, sheltered ports), and recreation. Ancient civilizations in the Fertile Crescent (area around the rivers Tigris, Euphrates, Nile, and on the western slopes of the Mediterranean coast) are now recognized to have had a culture and society that were based on utilization of wetlands and estuaries (Pournelle 2003). This dependence on estuaries has arisen because these sedimentary environments harbor abundant fishes and shellfishes, are habitats for many invertebrates that are also integral parts of estuarine and oceanic food webs, and are essential for the long-term sustainability of coastal ecosystems.

Wherever they occur, vascular plants contribute to virtually every ecosystem service associated with estuaries. Although restricted to intertidal (e.g., marshes and mangroves) and shallow subtidal (seagrass beds) portions of temperate and tropical estuaries, the contribution of these plant communities to estuarine production can be greater than suggested by their modest areal extent (Heymans & Baird 1995). Above-ground plant structures (e.g., stems and leaves of marsh plants or prop roots of mangroves) trap and retain sediments, and provide substrata, refugia, and food for estua-rine biota (Thayer et al. 1987; Covi & Kneib 1995). Plant roots help to stabilize sediments and promote the structural integrity of tidal channels, and mediate biological activity in the sediments by transporting oxygen to the root zone and detoxifying sediments (Lee et al. 1999). Benthic plants and animals also maintain environmental quality by binding and removing particulates and contaminants from the water column and sediments and are an integral part of the aesthetic vistas of coastal landscapes that enrich the human spirit.

Although estuarine sediments contain few species relative to most other sedimentary habitats, they nonetheless represent hotspots for ecosystem processes that can extend well beyond the estuarine sediments. Of the ecosystem goods and services associated with shelf and nearshore ocean areas, people are most aware of provisioning of food (e.g., fish and shellfish), which has huge commercial and cultural importance in coastal societies worldwide. Even aquaculture businesses often rely on wild (natural) fisheries (e.g., for fishmeal) or natural supply of food (e.g., phytoplankton) for aquaculture species and, in some cases, for provision of brood and juvenile stocks. Marine plants are used as food, particularly in Asia, and seaweed extracts such as alginates and other phycocolloids are used in many industrial and food applications (e.g., manufacture of films, rubber, linoleum, cosmetics, paints, cheeses, lotions). The living components of estuarine systems provide not only the primary and secondary production that supports commercial, recreational, and subsistence fishing and other extractable resources, but also much of the structure that stabilizes sediments to provide flood and erosion control, and maintains the integrity of wetlands and coastal waterways (Levin et al. 2001a; Tables 4.2a-4.2b).

Sedimentary fauna are a critical part of the diet for many estuarine and shelf species that feed near or on the bottom, such as cod and flatfish (Feder & Pearson 1988; Carlson et al. 1997). Some pelagic fish feed directly on benthic invertebrates at the seafloor-water interface during various phases in their life cycles. Many benthic fauna spend the early parts of their life cycle in the plankton and, in some cases, are extremely abundant and potentially important for pelagic food chains (Lindley et al. 1995). Structure-rich sedimentary habitats, particularly marshes, mangrove swamps, and seagrass beds, create refuges for juveniles of commercially exploited pelagic fish and invertebrates (Laurel et al. 2003).

Nutrient cycling and sediment oxygenation (redox) processes are interlinked to lesser known, but key, services of detoxification and disposal of waste by shelf and estu-arine sediment biota. These processes are regulated directly by microbial organisms and indirectly by larger, bioturbating organisms (Henriksen et al. 1983; Pelegri & Blackburn 1995). Detoxification and immobilization of contaminants may represent a service or a disservice, depending on the circumstances. Detoxification is performed primarily by microbes (Geiselbrecht et al. 1996) and may be facilitated by bioturbation, which strongly influences oxygenation and physical movement of contaminants. Bioturbating organisms such as polychaete worms relocate sediment particles and water as they feed, and amalgamate fine particles into fecal pellets (Levinton 1995). Microbes process organic wastes and organic compounds into less hazardous breakdown products (Boyd & Carlucci 1996; Lee & Page 1997), which can be recirculated back into the water column through bioturbation. Microbial processing of toxic waste such as organometallic compounds can produce harmful breakdown products that can be biomagnified through the food web (Srinivasan & Mahajan 1989). Bioturbation activity by large invertebrates can also accelerate pollutant burial by feeding and removing material at the sediment surface and defecating deeper in the sediment, but feeding at depth by other species that defecate at the surface can also remobilize buried contaminants (Gallagher & Keay 1998).

Sediment-dwelling organisms contribute to sediment formation through their skeletal remains (e.g., the shells and calcareous structures of mollusks, foraminifera, and lithothamnia [algae]). More importantly, particularly in nearshore, shallow-subtidal habitats, sedimentary organisms directly affect sediment stability and erodability (Lev-inton 1995; Paterson & Black 1999). Sediment particles are bound together by extracellular polymeric substances (mucus) within diatom and microbial films (Grant & Gust 1987), and within meiofaunal and macrofaunal secretions. Macrofaunal fecal and pseudofecal production also binds sediments (Rhoads 1963). Although biological adhesion (Grant et al. 1982) and biological structures above the sediment (such as sea-grass, Fonseca & Fisher 1986), can stabilize sediment, biologically generated bottom roughness (Wright et al. 1997) and increased water content of sediments as a result of bioturbation (Rhoads & Young 1970) can also increase erodability.

Shelf and estuarine sediments are habitats for many fishes and invertebrates, and are valued for recreation, sport and subsistence fishing. Sandy beaches, for example, are of particular importance as recreational areas (Weslawski et al. 2000). Sediments provide educational value because of their role in the ecosystem and can have spiritual importance for humans as a source of food, ornaments, and even currency (shells).

Estuaries are the most accessible marine sedimentary habitats for humans, and they are also the most productive. The value of ecological services from estuaries can be substantial, an observation that can be attributed to the service of nutrient cycling defined as the storage, internal cycling, processing, and acquisition of nutrients (Costanza et al. 1997; Ewel et al. 2001). In open estuaries, much of the nutrient cycling occurs in the water column, but the benthic component in shallow subtidal and intertidal systems is also important. As with estuaries, depending on the local communities' values and willingness to pay (Daily et al. 2000; Dasgupta et al. 2000), the value of ecosystem services for intertidal wetlands could be substantial. Intertidal wetlands provide critical services such as waste treatment, environmental buffering/flood control, recreation, and food production. Many service categories (e.g., nutrient cycling) must be considered based on their value at local levels; thus, total economic value of these systems may be underestimated at regional and global levels. It is also important to recognize that many methods have been applied in placing monetary values on estuarine habitats, including the substantial cost of restoration to recover lost functionality (Kruczynski 1999). There is insufficient evidence available to know whether estuaries can be restored to all previous functions, although partial restoration of some functions has been achieved in some cases (see Snelgrove et al. Chapter 7).

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