A satellite view of the space-time variability of phytoplankton biomass in the Mediterranean Sea

Abstract

The general thesis objective is to determine, with sufficient accuracy (i.e. using the most reliable datasets and techniques) the phytoplankton space time variability in the Mediterranean Sea (MED) using satellite data (SeaWiFS), at different timescales, and their relationship with the physical environment. A validation exercise was performed over SeaWiFS data in the context of the reliability of both oceanic and atmospheric remote sensing data, and using the most comprehensive in situ bio-optical dataset over the MED basin. This exercise led to the development of an ad hoc regional ocean colour algorithm, which has then been implemented as standard algorithm within the operational satellite data processing chain at the Group for Satellite Oceanography (GOS) at the Istituto di Scienze dell’Atmosfera e del Clima of the Italian National Research Council, Rome. Moreover, within this context, the MED basin bio-optical signature has been found to be significantly different than the global ocean, ultimately justifying the adopted approach. The derived product (i.e., chlorophyll concentration, CHL) has then been used in the rest of the thesis to answer relevant questions, such as how the phytoplankton dynamics is influenced by its physical environment, from the water column stratification to the atmospheric input of nutrients, at different space and time scales: from daily to seasonal and interannual, and from hundreds of km to the basin scale. The basin scale interannual variability of phytoplankton has been found to be very sensitive to circulation patterns in both the western and eastern sub-basins. A phytoplankton biomass decrease, at basin scale, is significantly correlated with the longterm reduction of the cyclonic circulation in the eastern basin. Similarly, the transport variability associated with the Algerian Current system has been found to play an important role in affecting the cyclonic circulation of the Ionian Sea, which in turn determines a phytoplankton decrease on a multi-year time scale in the area. Seasonally, localized to the northwestern MED and in the southern Adriatic Sea, where deep water formation processes are active during autumn-winter, the phytoplankton spring bloom dynamics is found to be significantly correlated to the surface thermal field of the previous season: a time lag of six months identifies the coupling between the preconditioning phase to deep water formation and the spring bloom. A debated and still open question concerns with the role of atmospheric dust in the regulation of the biogeochemistry of oligotrophic gyres, so that one of the challenges of this study was to investigate the impact of the atmospheric nutrient deposition on the phytoplankton dynamics of the basin, i.e. to test the Dust Fertilization Hypothesis (DFH) in a Low Nutrient Low Chlorophyll (LNLC) region. This issue has important scientific and to a non lesser extent technical implications, but the DFH is shown here to play only a minor or even negligible role in the regulation of the phytoplankton dynamics in the MED.

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