``Extreme Highest�� and ``Extreme Anomalous��: Proposed indices for chlorophyll-a extreme events in European seas between 2003 and 2021

Abstract

This study addresses the occurrence of extreme values of chlorophyll-a concentrations in European seas, using the satellite's OBGP MODIS AQUA v2018 dataset, for the 2003-2021 period. Two novel and complementary statistical indices based on the combination of the overall period's and the monthly 90th percentiles (P90 and mP90, respectively) are proposed: (i) the Extreme Highest exceedances (EH) include the observations with highest chlorophyll-a magnitudes in each location (larger than P90 and mP90); (ii) the Extreme Anomalous exceedances (EA) address the observations larger than mP90 but lower than P90, thus representing relevant anomalies during low phytoplankton growing seasons. Given the important differences of available observations per pixel, the EH and EA exceedances are normalized and also presented in percentage units.Although the occurrence of these indices greatly varies in time and space, the aggregated statistics and maps reveal some clear patterns: EH and EA have very distinct (almost complementary) seasonal and spatial distribution, EH prevail in mesotrophic and euphotic waters during the main interannual bloom season whilst EA are more abundant in oligotrophic waters out of the main seasonal bloom in each area. Both significant (p < 0.05) increasing and decreasing annual trends have been found in different European Seas. Overall, these EH and EA trends are reflecting the climate-driven physical and ecological changes in European Seas. Although these results and the conceptual and computational simplicity of these indices are encouraging, further ground truth testing is required to account for their uncertainties, mostly related to data representativeness and the performance of the chlorophyll-a estimation algorithms.