Factors Affecting Biodiversity

Numerous abiotic environmental factors influence species diversity (Levin et al. 2001b) and potentially affect processes, goods, and services provided by marine sediments. Salinity, soil texture, organic content, nutrients, waves, currents, and oxygen are abiotic factors that control species composition, densities, and diversity. All of these factors are affected by natural and human-altered regional control of sediment supply, nutrient input, water depth, exposure to disturbance, and hydrologic environment (Diaz & Rosenberg 1995; Parsons et al. 1999; Gray 2002).

Sediment resuspension and motility in shelf and coastal regions is dictated by hydro-dynamic processes such as currents, tides, and wave action (Boudreau 1997). This disturbance affects recycling services, the maintenance of sediment oxygenation (e.g.,

Ziebis et al. 1996), and potentially the detoxification of pollutants (Bunge et al. 2003) and rates of biogeochemical cycles (Turner & Millward 2002). It also significantly affects species composition (Ysebaert & Herman 2002).

Hydrodynamic processes primarily determine sediment granulometry and therefore substrate type. This is important to food production, as substrate or habitat availability affects survival of food species of fish and invertebrates (Snelgrove & Butman 1994). Oxygen availability and temperature influence the survival of organisms, reproduction, and function (Garlo 1982), and hence the provision of goods and services by shelf biota. Oxygen availability is particularly important in maintaining sediment redox chemistry (Rhoads et al. 1978; Fenchel & Finlay 1995).

The perception of the deep sea as a species-depauperate and homogeneous habitat has been debunked in the last few decades by evidence of strong regional and temporal variation in the abundance and diversity of deep-sea sediment biota (Levin et al. 2001a; Snelgrove & Smith 2002). The density and biomass of deep-sea infauna are most strongly influenced by organic matter availability (Rowe 1983). Input of organic carbon to the seabed mirrors (but is only a fraction of) surface primary production; it is also influenced strongly by circulation and local flow conditions. Where particulate organic input is high, infaunal species are abundant, animals live deeper in the sediments, and bioturbation rates are greater (Schaff et al. 1992). The continental margins and the north Atlantic are areas of particularly high organic matter input. Topographic features such as seamounts, ridges, canyons, and gullies have accelerated flows where particulate flux is elevated. Because the benthos provides critical trophic support for larger fish and invertebrates, production of harvested species is greatest in these areas, as are rates of carbon processing, burial, and sequestration.

In some estuarine and shelf areas, excess production from surface waters can lead to hypoxia in bottom waters (see discussion of nutrient loading in Chapter 7). An intriguing parallel occurs in some deep-sea areas when high production from surface waters sinks to bottom areas with sluggish circulation, leading to the formation of mid-water oxygen minimum zones (OMZs) at depths of 100 to 1,000 meters. Within OMZs, there is reduced productivity, less remineralization of carbon, and lowered functional and species diversity of the sediment biota. These effects occur over huge areas (>106 km2) of the sea floor (Levin 2003). Temporal changes in the boundaries of OMZs exert tremendous control on seabed productivity and diversity over ecological time (e.g., with El NiƱo events; Arntz et al. 1988) and over geological time (Rogers 2000).

The structure and function of deep-sea sediment biota is also influenced by benthic storms (Hollister & McCave 1984) and turbidity flows or mass wasting (Masson et al. 1996). Microbial function and activity are greatly influenced by availability of oxygen, organic matter, and reduced compounds such as methane and sulfide. Amazing discoveries of microbial syntrophy (symbioses involving microbes of different metabolic func-

Figure 4.1. Schematic depiction of interrelated nature of soil, freshwater, and coastal marine sedimentary ecosystems. The top diagram depicts a functioning ecosystem prior to deforestation. The lower diagram illustrates the cascade of changes that may occur from disturbance to soils. Deep-sea ecosystems are not shown because their linkages with terrestrial and freshwater domains are indirect and expressed only at long temporal and large spatial scales. Arrows indicate flow of materials (water, nutrients, organic matter), and circles indicate biological filters. POC is particulate organic carbon, C is carbon, P is phosphorus, and N is nitrogen.

Figure 4.1. Schematic depiction of interrelated nature of soil, freshwater, and coastal marine sedimentary ecosystems. The top diagram depicts a functioning ecosystem prior to deforestation. The lower diagram illustrates the cascade of changes that may occur from disturbance to soils. Deep-sea ecosystems are not shown because their linkages with terrestrial and freshwater domains are indirect and expressed only at long temporal and large spatial scales. Arrows indicate flow of materials (water, nutrients, organic matter), and circles indicate biological filters. POC is particulate organic carbon, C is carbon, P is phosphorus, and N is nitrogen.

tions), multiple bacterial symbioses within invertebrates, and sediment ecosystems reliant on methane for carbon have come from highly reduced sediments in the deep sea.

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