Assessment of estuarine impairment in the Virginian province: Benthic indicators and watershed inclusion

Estuaries are dynamic transition zones linking freshwater and oceanic habitats. These aquatic systems are some of the most productive ecosystems in the world, supporting a rich diversity of fauna including benthic macroinvertebrates. These invertebrate communities are important food sources for birds and fish, and are important contributors to benthic-pelagic coupling of the estuary. These communities have also been used a biomonitors to assess the condition of the estuary. They are abundant, easy to collect and their communities are very diverse, with representatives from many different phyla utilizing many different habitats and feeding strategies. Macroinvertebrate assemblages also respond predictably to pollution, are relatively sedentary, and act as integrators of stress over months to years. Because of this sensitivity to pollution, aquatic benthic invertebrates have commonly been used as indicator organisms, either individually or when summarized as an index. These indices can then be related to watershed land cover, since management actions commonly center on land and water management in the watershed. In the first chapter, estuarine benthic invertebrates that could be used as indicator species to detect presence or absence of pollution in the Virginian Biogeographic Province were identified using available monitoring data from the US Environmental Protection Agency‟s Environmental Monitoring and Assessment Program. Data were subdivided by habitat, then summarized using principal coordinates analysis and projected onto a pollution gradient. The medians and corrected interquartile ranges were to identify indicator species that were sensitive to pollution (or lack thereof) and showed low variation in response. The pollution tolerant taxa were dominated by deposit feeders while the pollution sensitive taxa were not dominated by animals using any one feeding strategy. The pollution sensitive taxa included many crustaceans, known to be sensitive to pollution, while the pollution tolerant taxa included many tubificid oligochaetes, which are common indicators of pollution. Our empirically-based results corresponded well with other studies that identified tolerance values or indicator species. In the second chapter, multiple methods of index assembly that had been utilized to develop freshwater and estuarine indices were compared to explore the indices relative strengths and weaknesses. Three different approaches were utilized -two multimetric indices (Chesapeake Bay approach and the Mebane approach) and a logistic regression technique. The data were subdivided by habitat (salinity and grain size), and indices compiled using the same initial group of benthic metrics. Each approach was examined for its classification accuracy for both reference and impaired sites for the entire Virginian Province. The Chesapeake Bay IBI approach did not perform well in this study. In contrast, another multimetric approach, the Mebane IBI approach, performed well, as did the logistic regression approach. In the third chapter, potential linkages between estuarine condition and the watershed were explored by developing regression models between landscape condition indicators and benthic invertebrate communities. The importance of the spatial arrangement of watershed variables for predicting benthic invertebrate condition was explored. Variables spread out through the landscape as well as those associated with riparian areas were examined as well as whether variables closer to the estuary were more strongly related to benthic invertebrate condition. Since riparian and watershed variables were highly correlated at this scale, either riparian or watershed variables were adequate for assessing estuarine invertebrate condition. Modeling estuarine condition indicated that inherent landscape structure (estuarine area, watershed/estuary ratio) is important to predicting benthic invertebrate condition. As shown in other studies, more natural land cover features (wetland, pasture/hay) help to improve estuarine condition while anthropogenic impacts (development, sewage treatment plants) can have adverse impacts. These results emphasize the importance of considering the distribution of natural land as well as the minimization of the effects of development through best management practices. This study did not indicate that land use closer to the estuary was more important for predicting benthic invertebrate condition than land use in the entire watershed, because land use was unequally distributed throughout the watershed rather than being concentrated around the estuary.