Ecological Indicators (Sep 2024)
Performance of a multimetric index based on phytoplankton to evaluate the ecological quality of French large rivers: The IPHYGE index
Abstract
Large rivers around the world are impacted by human activities. Such activities can lead to the eutrophication of these water bodies, and consequently to potentially harmful algal blooms. In this study, we describe the development of a phytoplankton-based multimetric index used for the biomonitoring of the ecological quality of large rivers located in metropolitan France, as advocated by the European Water Framework Directive. Using a dataset containing phytoplanktonic surveys sampled over the 2010–2021 period for a total of 963 station-years, we calculated biomass-based and trophic metrics. We then selected the best biomass-based metric and several trophic metrics based on their ability to highlight situations impaired by eutrophication and/or algal blooms and well correlated to pressure gradients (i.e. using coefficients of determination). The best biomass-based metric was calculated from both the seasonal average and maximal concentrations of chlorophyll-a. Three trophic metrics, created using the Weighted Average – Partial Least-Squares method, were included in the multimetric index. Each metric is correlated with one parameter of interest, namely with total phosphorus, Kjeldahl nitrogen and nitrates. The values of the biomass metric and the three trophic metrics were aggregated to calculate the final value of the IPHYGE index (“Indice Phytoplancton pour les Grands cours d’Eau”; phytoplankton index for large rivers) in order to obtain the best relationship with the eutrophication pressure. Ecological quality classes derived from the index values were well related to gradients of parameters linked to eutrophication, for example total phosphorus and nitrogen compounds.
