Assessing Benthic Community Condition in Chesapeake Bay: Does the Use of Different Benthic Indices Matter?
Roberto J. Llansó 1, Jon H. Vølstad 1 , Daniel M. Dauer 2 and Jodi Dew 1
1 Versar, Inc., Columbia, Maryland
2 Department of Biological Sciences, Old Dominion University, Norfolk, Virginia
Federal and state environmental agencies conduct several programs to characterize the environmental condition of Chesapeake Bay. These programs use different benthic indices and survey designs, and have produced assessments that differ in the estimate of the extent and severity of benthic community degradation in Chesapeake Bay. Provided that the survey designs are unbiased, differences may exist in the ability of these indices to identify environmental degradation. In this study we compared the results of three indices calculated on the same data, and the assessments of three programs: Mid-Atlantic Integrated Assessment (MAIA), Chesapeake Bay Program, and NOAA’s National Status & Trends. We examined the level of agreement of index results using site-based measures of agreement, evaluated sampling designs and statistical estimation methods, and tested for significant differences in assessments. Comparison of ratings of individual sites was done within separate categories of water and sediment quality to identify which indices summarize best nutrient enrichment and sediment contaminant problems in Chesapeake Bay. The use of different benthic indices by these programs produced assessments that differed significantly in the estimate of degradation. A larger fraction of poor sites were classified as good by the EMAP-VP and MAIA indices compared to the B-IBI, although overall classification efficiencies were similar for all indices. Biases were identified in the MAIA program estimates due to post-stratification of data outside the program. The relative difference between the indices remained the same when applied to independent datasets, suggesting that the indices can be calibrated to produce consistent results.