In aquatic ecosystems, environmental monitoring has typically been restricted to water-quality parameters. Water-quality measures, however, often do not adequately predict or reflect the condition of all aquatic resources and this has led to the development of biological monitoring tools and indicators. Aquatic biota integrate the effect of a range of environmental factors such as water quality, physical habitat, and biological interactions and is therefore considered a good indicator of aquatic ecosystem health. Furthermore, the biota may be the only practical means of evaluating certain impacts that are difficult to measure, for example, diffuse source impacts, overexploitation, or habitat degradation.
Biological monitoring studies should ideally determine stress at the ecosystem, community, population, individual, and cellular levels (Figure 1) using techniques that examine aspects such as community structure, toxi-city bioassays, behavioral bioassays, bioaccumulation studies, and pathology. Pathology deals with causes, processes, and effects of disease and includes postmortems, histological examinations, parasitological examinations, and liver enzyme assays. Bioaccumulation monitors the uptake and retention of potential toxicants (such as trace metals and pesticides) in the body of an organism. Toxicity bioassays are generally used to quantify acute (short-term) and chronic (long-term) effects of single or multiple substances on organisms. Behavioral bioassays monitor the behavior of organisms when exposed to contaminants; some behavioral responses include avoidance, change in swimming activity, respiratory or heart rate changes, valve movement (in bivalve mollusks), and feeding activity. Ecological surveys assess the functional and/or structural aspects of biological communities.
Bioindicators, as currently used in field bioassessment studies, tend to operate at the community level and have a high ecological relevance. It should be noted, however, that although bioassessment studies integrate the effects of environmental changes and provide a good measure of overall ecosystem health, they often are not good at identifying particular causes of degradation. Bioassessment studies therefore should comprise one component in a holistic approach, which also uses water quality and physical studies as well as other components to monitor the condition of an ecosystem.
Considerable progress has been made with identifying bioindicators for use in a wide variety of aquatic systems, with the methodology and protocol development having reached an advanced stage. Indeed biological assessments (bioassessments) and bioindicators are often used to meet legal requirements in a number of countries. In the United States, for example, the Clean Water Act requires federal
Anthropogenic stressors e.g.,
Contaminants Eutrophication Dredging
Excessive siltation Overfishing Freshwater abstraction Habitat destruction
Management Aim is to reduce the impact of anthropogenic stressors on the ecology of the estuary
Cellular level (e.g., histology)
Individual level (e.g., bioaccumulation)
Population level (e.g., stock assessments)
Community level (e.g., EFCI)
Figure 1 Diagram showing how fish biomonitoring at different levels can ultimately contribute to an identification and reduction, through active management, of anthropogenic stressors within an estuary.
and state governments to ''restore and maintain the chemical, physical, and biological integrity of the nation's waters''. In Europe, the Water Framework Directive was developed to ensure the protection of inland surface waters, transitional waters (estuaries), coastal waters, and groundwater; it requires that ''Member States shall protect, enhance, and restore all bodies of surface water ...''
Biomonitoring is the repeated assessment of a system to detect changes in its state from a particular reference condition. In this way corrective actions can be taken if the trend of the changes is in an undesirable direction. Bioassessments and biomonitoring are now used on a regular basis in the streams, rivers, and lakes of North America to assess environmental change. The measurements gathered during biomonitoring are usually designed to answer important questions such as 'is the system healthy or degraded?'; in other words, does it exhibit the characteristics indicating that all components are present at the expected levels and interacting as they should?
Bioindicators are useful tools in any biomonitoring of aquatic ecosystems, including estuaries; unfortunately, their use in estuaries is still in its infancy and it is only recently that efforts have been directed towards rectifying this imbalance. Part of the reason for the delay is that estuaries, by their very nature, are highly variable physical environments and the biota associated with these systems have, to a large extent, adapted to these fluctuating conditions. Overlying this natural variability is the fact that estuaries tend to be individually unique, thus making the identification of general estuarine ecosystem responses to stressors difficult. Despite the above difficulties, considerable progress has been made, some of which will be described in this article.
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