Seasonality of chlorophyll in the Mediterranean

Abstract

Annual phytoplankton dynamics in the Mediterranean Sea are strongly driven by the seasonal overturning in winter‐spring when deep waters rich in essential nutrients are mixed with upper sunlit waters. Being in the subtropical highs zone, with relatively low wind energy and variability, only enhances the seasonal component. We have analyzed the seasonality of chlorophyll using 8‐d averaged 1 x 1 degree satellite data for the whole basin and higher spatial resolution in certain areas. We calculated the Seasonality Index (SI) as the proportion of the variance owing to annual recurrent trends with respect to total time series variance. The index is calculated either from spectral analysis or using a statistical cubic spline adjustment for multiyear data. The SI is high in general of 75% or more. Some areas influenced by large rivers (Rhone, Po, Nile) and other areas such as the Alboran Sea where Atlantic waters enter the Mediterranean, the northern Aegean Sea with the influence from the Black Sea through the Sea of Marmara or the Gulf of Gabes with a shallow shelf and coastal enrichment all show decreased seasonality and a larger annual chlorophyll signal. If the variability for frequencies larger than the annual can be interpreted as a results of disturbances to the system that force nutrients into the euphotic zone, there is room for addressing ecological hypotheses that relate disturbance to system diversity and production, and to use this information in the frame of global change scenarios