Two decades of satellite surface chlorophyll a concentration (1998–2019) in the Spanish Mediterranean marine waters (Western Mediterranean Sea): Trends, phenology and eutrophication assessment

Time series of satellite surface chlorophyll of more than 20 years were built for the Spanish Mediterranean waters. Chlorophyll data from SeaWiFS, MODIS-Aqua and VIIRS-Suomi-NPP were generated based on the locally developed SMED algorithms. Nevertheless, it was necessary to harmonize the trends between the three different sensors before merging the data in the composite time series. Cluster analysis of the monthly chlorophyll averages allowed distinguishing four main areas with distinctive seasonal and phenological features in the study region. The western Alboran Sea showed high productivity during all the year. In the other areas, the average duration of the productive period was also extensive (about 180 days) starting at the beginning of November, but coastal waters in general showed a belated ending. These results suggest that nutrients are the main limiting factor of the primary production in the region. During the last 20 years, a general decreasing trend in the surface chlorophyll concentration was observed in the Spanish Mediterranean waters; nevertheless, a strong variability in the intensity of the productive annual period probably related with long scale climatic oscillations was observed. The surface chlorophyll concentration reduced in a decade from 10% in coastal waters to 20% in offshore waters. Some patches in the region showed positive trends, including offshore areas around the Balearic Islands in winter and the coastal areas of the Gulf of Valencia in summer, but in the southern coasts of the Gulf of Alicante the positive trends were persistent during all the year. The performed analysis can be used globally for eutrophication assessment as well as for the evaluation of changes on the phytoplankton phenology probably driven by the global change.