Nutrients enter an estuarine system via the same point and non-point source pathways as contaminants but 

 may also come from atmospheric deposition. Excess nutrients that lead to eutrophication can cause shifts in 

 species composition and abundance. Eutrophication can deplete the oxygen in the bottom waters or climatic 

 conditions may drive hypoxia because they influence the stratification of the water column. Oxygen depletion 

 causes acute stress to the benthic community resulting in die-offs or chronic stress that may lead to shifts in 

 species composition. Habitat disturbance includes physical scouring of the bottom as a result of storms, 

 trawling, or dredging, as well as salinity and temperature changes brought about by climatic events. Because 

 any of these stressors may induce alterations of the benthic community, monitoring community changes 

 reflects the environmental conditions to which the benthos are exposed. 



The benthic index represents the response of the benthic community to stressors like contaminants and 

 hypoxia (Fig. 3-2). As a multimetric indicator, the benthic index is composed of the community measures that 

 best discriminate between stressed and unstressed areas. Although these components (diversity, proportional 

 abundance of capitellids, bivalves, and amphipods, and the abundance of tubificids) were chosen empirically 

 by statistical analyses, they have biological relevance to the function of the benthic community. Diversity is 

 directly related to the relative stability of a community. Sites that have been affected by contamination or 

 hypoxia exhibit lower diversity than sites that have not been so adversely affected. Capitellid worms are 

 often regarded as opportunists because of their high reproductive rate, small body size, and short life span. 

 They are often found in great abundance in organically enriched areas and are usually the first to colonize 

 disturbed sediments. Tubificid worms have life histories similar to capitellids but may extend the range of 

 habitats in which capitellids are commonly found. Bivalves are usually indicative of stable environments 

 because most bivalves are large-bodied, have slower reproductive rates, and longer life spans than worms. 

 As filter feeders, most bivalves are the first to show signs of stress when water quality becomes unsuitable. 

 Amphipods have long been used as test organisms in toxicity tests because of their demonstrated sensitivity 

 to contaminants. 



Source 



Stress 



Effect 



Point Soureas 



Industrial 



Municipal 



Power Plant 



Ship Discharge 



Non Point Sourc* 



Agriculure 



Lrvestock 



Urban Runoff 



Dradglng/Dlsposal 



Storm Evanis 



T Rainfall 



T Contain lnat»d/Toxlc S«dlm«its 



Organic chemicals 

 Heavy metals 



\ 



t Organic Enriehmant 



Nitrogen 

 Carbon 



io. 



Habitat Disturbance 

 Physical 

 T Temperature 

 A Salinity 



Figure 3-2. Conceptual model of the indicator, showing linkages between sources of stress, types 

 of stress, and effect on the benthic community. 



3-6 



