Biological Criteria, Environmental Health and Estuarine Macrobenthic Community Structure

Biological criteria for defining water quality and the presence of acceptable levels of benthic resources are evaluated for estuarine macrobenthic communities of the lower Chesapeake Bay, USA. Models of expected community values as a function of salinity are presented for community biomass, numbers of individuals, species richness, percent biomass of deep-dwelling species, percent biomass of equilibrium species, and percent biomass of opportunistic species. The models presented may serve as, or be used to develop, biological criteria for estuaries. Tidal freshwater and oligohaline regions had the highest variability in model parameters due to patchily distributed, large-sized bivalve species. In the absence of data from pristine habitats, the models were developed from a 5 year data set (1985-1989) for stations considered to be minimally impacted. The models produced were used to evaluate benthic communities of two regions of the Chesapeake Bay-one exposed to summer low dissolved oxygen events (hypoxia/anoxia) and the other characterized by sediments contaminated with heavy metals and polynuclear aromatic hydrocarbons. Stations exposed to stress from either low dissolved oxygen events or contaminated sediments were characterized by 1. reduced community biomass, 2. reduced species richness, 3. less biomass consisting of deep-dwelling species and equilibrium species and 4. more biomass consisting of opportunistic species. Some unstressed habitats can be highly dominated by shallow-dwelling long-lived species, thus dominance of deep-dwelling species in biomass must be used with caution as a biological criterion. The number of individuals per m e was highly variable for some stressed stations and this parameter is probably of limited value as a biological criterion characterizing the quality of estuarine habitats. No single method or analysis is likely to produce stress classifications without unacceptable misclassifications. Ecological stress, from any source, is best measured by multiple methods or analyses with different assumptions. The consistency of classification between different approaches would provide the robustness necessary to judge the reliability of a stress classification. The evaluation, restoration, and maintenance of water quality and associated living resources are major goals of environmental management. Recently the U.S. Environmental Protection Agency (1990) has directed the development of biological water quality criteria as part of water quality standards for each state in the USA. Biological criteria are considered important components of water quality standards programmes because L they are direct measures of the condition of the biota, 2. they may uncover problems undetected or underestimated by other methods, and 3. such criteria provide measurements of progress of restoration efforts. Biological criteria are intended to supplement toxicity and chemical assessment methods and serve as independent evaluations of the quality of marine and estuarine ecosystems. Implementation of biological criteria to assess environmental health is dependent upon the definition of reference conditions that define attainable or desirable biological or habitat conditions. Habitats that are unaffected by anthropogenic stresses may not exist in many estuaries; therefore, habitats characterized as least impacted are used to define reference conditions. Estimates of the benthic macrofaunal community (here defined as organisms retained on a 0.5 mm screen) are often used to indicate environmental health because benthic animals 1. are relatively sedentary (cannot avoid deteriorating water/sediment quality conditions), 2. have relatively long life spans (indicate and integrate water/sediment quality conditions), 3. consist of different species that exhibit different tolerances to stress (can be classified into functional groups), 4. are commercially important or are important food sources for economically or recreationally important species, and 5. have an important role in cycling nutrients and other chemicals between the sediments and the water column (Berner, 1976; Virnstein, 1979; Aller, 1978, 1982; Holland et al. 1980: Swartz and Lee, 1980; Boesch and Rosenberg, 1981; Dauer et al. 1982a,b; Hartley, 1982; Hargrave and Theil, 1983; Philips and Segar, 1986; Bilyard, 1987; Gray et al. 1988; Warwick et al. 1990; Weston, 1990). This study presents graphical models of expected values