Simple tools for assessing water quality and trophic status in transitional water ecosystems

In this study we have developed an index for assessing trophic status and water quality in transitional aquatic ecosystems of Southern Europe. The index has been developed from the water quality index of the U.S. National Sanitation Foundation and integrates the main causal factors (inorganic nutrients), the key biological elements (primary producers) and indicator of effects (oxygen) of eutrophication. Six main variables have been used: relative coverage of benthic phanerogams and opportunistic macroalgae species, and concentrations of dissolved oxygen, phytoplankton chlorophyll-a, dissolved inorganic nitrogen and phosphorus. Non-linear functions are used to transform each measured variable into its quality value. Each quality value is then multiplied by a weighting factor, to take into account the relative contribution of each variable to the overall water quality. Finally, the index value is calculated as the sum of the weighted quality values, ranging from 0 (poorest state) to 100 (best condition). The index has been tested and validated in six transitional water ecosystems which differ in anthropogenic pressures and eutrophication levels, for which data sets were available from 1989 to 2004: Sacca di Goro (Northern Adriatic Sea, Italy), Lesina Lagoon (Southern Adriatic Sea, Italy), Ria Formosa (Algarve, Southern Portugal), Mar Menor (Murcia, Southern Spain), Etang de Thau (Herault, Southern France) and Gulf of Gera (Lesvos Island, Greece). The index assessments have been compared with evaluations from the IFREMER (French Research Institute for the Exploitation of the Sea) classification scheme (France) and the trophic index TRIX (Italy), which are currently used for national monitoring of coastal waters and lagoons. Based on the conclusions of this study we suggest to use the index for monitoring water quality in shallow coastal transitional waters, where benthic vegetation controls primary productivity, which makes indices based on phytoplankton only (e.g., TRIX) unsuitable.