Comparison of five methods for assessing impacts of nutrient enrichment using estuarine case studies

There are several approaches for assessing nutrient enrichment impacts in marine waters including the OSPAR* Comprehensive Procedure, the TRIX* ranking process, the WFD, ASSETS* and EPA NCA*. They differ in definitions and application, but all use key indicators for evaluating eutrophication status. Data from two estuaries in the United Kingdom (UK) were used to test the hypothesis that these five methods would result in the same outcome. The intent is to provide managers with information for selecting an appropriate method, insight about how each method works, how results from different methods compare and a basis for improvement of methods. Results from all approaches indicate that both estuaries required management intervention, but detailed results differed. Methods using more than one biological indicator show that secondary biological impacts were minimal, with the exception of moderate macroalgal problems in the Medway. Comparison of final results was difficult due to differences in: timeframes of data analysis (seasonal versus annual), characteristics included in indicator metrics (concentration, spatial coverage, frequency of occurrence), and methods for combining indicators to determine eutrophication status. This comparison suggests that use of annual data, inclusion frequency of occurrence, spatial coverage and, secondary biological indicators in the index, and a multi-category rating scale results in a more representative assessment.