The Environmental Monitoring and Assessment Program (EMAP) focused on providing much needed 

 information about the condition of the Nation's ecological resources. EMAP was designed to answer the 

 following questions (Summers etal. 1995); 



1. What is the status, extent, and geographical distribution of our ecological resources? 



2. What proportions of these resources are declining or improving? Where? At what rate? 



3. What factors are likely to be contributing to declining conditions? 



4. Are pollution control, reduction, mitigation, and prevention programs achieving overall 

 improvement in ecological condition? 



To accomplish these management objectives, EMAP sought to develop a suite of indicators that would 

 represent the response of biota to environmental perturbations. These indicators were categorized as response, 

 exposure, habitat, or stressor. Our indicator, the benthic index, was classified as a response indicator because 

 it represents the response of the estuarine benthic community to environmental stressors (e.g., sediment 

 contaminants and hypoxia). A good response indicator should demonstrate the ability to associate responses 

 with well-defined exposures. EMAP-Estuaries (EMAP-E) sought to apply these management objectives to 

 the development of indicators to represent the condition of estuaries. 



The specific assessment question addressed by the benthic index emerged from a hierarchy of assessment 

 questions that were relevant to EMAP-E management goals. The broad assessment question for EMAP-E 

 is: What is the condition of estuaries? Our project was geographically limited to estuaries in the Gulf of 

 Mexico; therefore, our regional assessment question became: What percent of estuarine area in the Gulf of 

 Mexico is in good (or degraded) ecological condition? Because biological integrity is one component of 

 ecological condition, the next logical assessment question was: What percent of estuarine area in the Gulf of 

 Mexico exhibited acceptable (or unacceptable) biological integrity? The condition of benthic biota is one 

 measure of biological integrity. This tenet led to the specific assessment question addressed by the benthic 

 index: What percent of estuarine area has degraded benthic communities? As a response indicator for 

 estuaries, the benthic index was intended to contribute information to the broad assessment question above 

 and to be used in conjunction with a suite of indicators to evaluate the overall condition of estuaries. 



Macroinvertebrates provide an ideal measure of the response of the benthic community to environmental 

 perturbations for many reasons (e.g., see Boesch and Rosenberg 1981, Reish 1986). Benthos are primarily 

 sedentary and, thus, have limited escape mechanisms to avoid disturbances (Bilyard 1987). Benthic 

 invertebrates are relatively easy to monitor and tend to reflect the cumulative impacts of environmental 

 perturbations, thereby providing good indications of the changes in an ecosystem over time. They have been 

 used extensively as indicators of the impacts of both pollution and natural fluctuations in the estuarine 

 environment (Gaston etal. 1985, Bilyard 1987, Holland et al. 1987, Boesch and Rabalais 1991). Benthic 

 assemblages are often comprised of a variety of species (across multiple phyla) that represent a range of 

 biotic responses to potential pollutant impacts. 



The concept behind development of our benthic index begins with the assumption that adverse environmental 

 conditions (e.g., hypoxia and sediment contamination) affect benthic communities in predictable ways. The 

 basic tenets of Pearson and Rosenberg (1978) for organic pollution provide a good example of the biological 

 principles that operate in benthic communities. Pollution induces a decrease in diversity in favor of (sometimes 

 high) abundances of relatively few species labeled as pollution-tolerant or opportunist. In pristine areas or 

 areas unaffected by pollution, benthic communities exhibit higher diversity and stable populations of species 

 labeled as pollution-sensitive or equilibrium. In general, although pollution-tolerant species may thrive in 

 relatively undegraded areas, the converse is almost never true - pollution-sensitive species do not normally 



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