The alteration of marine rhythms by a large Mediterranean coastal city

Abstract

The seasonal cycle explains a large part of the variability in Mediterranean chlorophyll time series. This seasonality is fueled by the solar and mechanical energy that determine the nutrient availability to phytoplankton in the upper mixed layer waters. The Seasonality Index (SI), as the relative importance of the variance owing to annual recurrent trends with respect to total time series variance, is as high as 77% in the open Mediterranean. Areas where additional nutrient inputs may fuel the planktonic community are less related to the oceanic-astronomic rhythms and show a lower seasonality. This happens in the Rhone, Po and Nile adjacent areas, in the Alboran Sea, the northern Aegean and the Gulf of Gabes. In coastal areas, the SI decreases owing to the variability introduced by riverine inputs, resuspension and groundwater discharge, among others. However, it is in the coastal areas of large cities, where the seasonality index becomes even smaller. In the Barcelona area, freshwater input, loaded with nutrients, is largely uncoupled from meteorological conditions and depends on water use in the drainage area. Also, ammonium and phosphorus are in relatively large concentrations, unlike in the less anthropogenically impacted areas. The unbalanced nutrient stoichiometry and the uncoupling between nutrient load and natural mechanical energy inputs favors the growth of heterotrophic bacteria and other microorganisms over large phytoplankton. This alters the typical phytoplankton peaks and the succession dynamics after nutrient input events in a relatively oligotrophic Mediterranean. Consequently, the temporal predictability of chlorophyll decreases