Space and Time Occurrence of Algal Blooms in the Mediterranean: their Significance for the Trophic Regime of the Basin

D'Ortenzio, Fabrizio (2004). Space and Time Occurrence of Algal Blooms in the Mediterranean: their Significance for the Trophic Regime of the Basin. PhD thesis The Open University.



The Mediterranean Sea is generally classified as an oligotrophic basin with a dominant subtropical regime. The coarse temporal resolution of the Coastal Zone Color Scanner data did not allow to properly reconstruct the dynamics of the regional events which could support or falsify that view. In this thesis, five years of the SeaWiFS Mediterranean data were analyzed to depict the trophic regime of the basin and to quantify the impact of periodic or episodic phytoplankton blooms occurring there. The retrieval of chlorophyll a concentration was based on a new empirical regional algorithm (DORMA) developed and tested in the frame of this study. While confirming the low standing stock of phytoplankton biomass evidenced by the CZCS time series, the SeaWiFS imageries allowed to discriminate among two distinct regimes, which were classified as the ‘bloom areas’ and ‘non-bloom areas’. The former are mostly driven by the coupling of local atmospheric forcing and cyclonic circulation and, therefore, are often sites of dense water formation whereas the latter do follow a typical sub-tropical seasonal cycle without a pronounced maximum of biomass over the year. Surprisingly, the second regime not only pertains to most of the Eastern basin, as previously assumed, but also to the southern part of the Western one. The ‘bloom areas’ contribute to no more than 10-15% of the primary production in the basin, but account for the relevant part of the interannual variability in the standing stock of autotrophic biomass and display a seasonal cycle close to the North Atlantic Drift Region (sensu Longhurst). The merge of atmospheric data, 1D modeling and remote sensed SST as well as color water leaving radiances also allowed a detailed reconstruction of the dynamics of each bloom site and of the impact of the climatic fluctuation, usually referred as the eastern Mediterranean Transect, which took place at the beginning of the nineties in the basin.

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