BENTHIC ECOLOGY BASICS
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Types of Benthic Habitats and Associated Biota.
The word "benthos" is a Greek term for bottom (or the seafloor). There are two basic types of benthic habitat: hard bottom and soft bottom. The seafloor of GRNMS consists of both types with a discontinuous series of rock outcrops surrounded by vast expanses of sandy bottom. The hard-bottom rock substrate provides a firm base for a variety of sessile invertebrates such as sponges, barnacles, hard corals, anemones, tunicates, tube-dwelling worms, bryozoans, and sea fans to attach and grow. Other more motile invertebrates (e.g., crabs, lobsters, shrimp, snails, starfish, echinoderms) also are dominant on hard-bottom substrates. The scientific term for these surface dwellers, including both the attached and more motile members, is "epifauna." Attached epifauna are primarily filter feeders (obtaining nutrition by straining particles of food from the water column), while the more motile epifauna consist mostly of active predators and surface browsers. The hard-bottom rocky outcrops with their carpet of associated biota are referred to locally as "live-bottom habitat."
Hard bottom @ GRNMS
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Unconsolidated sandy substrates.
They fill the spaces between live-bottom habitat and extend beyond to cover the majority of the surrounding seafloor. Living buried within these sediments are assemblages of relatively sedentary worms, crustaceans, molluscs (snails and clams), echinoderms, and other invertebrate species commonly referred to as "infauna." Benthic infauna are predominantly deposit feeders, obtaining nutrition by ingesting organically enriched sediment particles and associated detrital material that settles onto the seafloor. However, the infauna may consist of filter feeders and active predators as well. Motile epifaunal species (e.g., crustaceans, echinoderms) and more sessile forms attached to small pieces of rock or shell (e.g., barnacles, corals, anemones, sea fans, sea pansies) also can be found living at the surface of these soft-bottom substrates.
Soft bottom @ GRNMS -
High Biodiversity.
The vast stretches of featureless sandy bottom within the sanctuary may appear at first glance as being a "biological desert," especially in comparison to the more visually impressive "live-bottom" assemblages of invertebrates and fishes associated with rocky outcrops. However, these soft-bottom substrates can be teeming with life. Recent research has shown that measures of species diversity at GRNMS are about twice as high as those observed for the benthos in neighboring unpolluted estuaries of comparable high salinity (Hyland et al. 2001, 2002). At GRNMS, Hyland et al. (2002) found up to 89 different species in a single 0.04-m2 grab sample of sediment, which is a very high diversity for the relatively small sampling area (a footprint about the size of a sheet of notebook paper). A total of 480 species, representing 13 different phyla, have been identified thus far from a total of 96 sediment samples (3.8 m2 combined). The sanctuary appears to be a valuable reservoir of marine biodiversity.
Data from GRNMS 2000 study -
Ecological Value.
The soft-bottom benthos is a key component of coastal ecosystems, playing vital roles in detrital decomposition, nutrient cycling, and energy flow to higher trophic levels. These sediment-dwelling fauna can be especially important as food to fishes that forage over the open sandy substrates. For example, the bivalve Ervilia and the chordate Branchiostoma (or lancelets), which are both among the dominant infaunal species at GRNMS (Hyland et al. 2001), have been shown to represent the largest percentage by number and volume, respectively, of prey consumed by tomtate Haemulon aurolineatum in samples collected throughout the South Atlantic Bight (Sedberry 1985). Benthic fauna affect their surrounding environment in other ways as well. For example, the physical turnover of sediment caused by the feeding and burrowing activities of these organisms (referred to as bioturbation) also can have significant influences on the physical and chemical characteristics of sediments.
Ervilia
Haemulon aurolineatum
Branchiostoma -
Signals of Anthropogenic Stress.
The benthos is a good place to look for signals of human-induced stress in coastal ecosystems. Benthic fauna live in close association with bottom substrata, where organic pollutants and chemical contaminants tend to accumulate and where low-oxygen conditions are typically the most severe. Because of their relatively stationary existence, it is difficult for these organisms to avoid exposure to pollutants and other adverse conditions in their immediate surroundings. Recent surveys of GRNMS (Hyland et al. 2001, 2002) have showed no major evidence of impaired benthic condition coupled to adverse levels of chemical contaminants in sediments. However, the presence of trace concentrations of pesticides, PCBs, and PAHs in both sediments and biota demonstrate that chemical substances originating from human activities are capable of reaching the offshore sanctuary environment (albeit at low concentrations not likely to be causing significant bioeffects) and thus should be monitored to ensure that future problems do not develop. A general pattern of decreasing trace concentrations of these materials with increasing distance from shore suggests possible inputs from inland sources through coastal sounds. Atmospheric deposition is another possible source.
Gray's Reef 2000 study
All toxic chemicals in tissue samples (10 black sea bass, 9 ark shell composites) were below human-health guidelines values. -
References.
- Hyland, J.L., C. Cooksey, L. Balthis, G. Scott, and D. Bearden. 2001. Survey of Benthic Macroinfauna and Levels of Chemical Contaminants in Sediments and Biota at Gray's Reef National Marine Sanctuary. FY01 Submitted to Gray's Reef National Marine Sanctuary Office, National Oceanic and Atmospheric Administration, National Ocean Service, Office of Marine Sanctuaries, Savannah, GA. 14 pp. + app. A-C.
- Hyland J.L., C. Cooksey, L. Balthis, M. Fulton, and D. Bearden. 2002. Survey of Benthic Macroinfauna and Levels of Chemical Contaminants in Sediments and Biota at Gray's Reef National Marine Sanctuary. FY01 Submitted to Gray's Reef National Marine Sanctuary Office, National Oceanic and Atmospheric Administration, National Ocean Service, Office of Marine Sanctuaries, Savannah, GA. 22 pp. + app. A-D.
- Sedberry, G.R. 1985. Food and feeding of the Tomtate, Haemulon aurolineatum (pisces, Haemulidae), in the South Atlantic Bight. Fishery Bulletin 83(3): 461-466.
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