Year-to-year variability of the phytoplankton bloom in the southern Adriatic Sea (1998-2000): Sea-viewing Wide field-of-view sensor observations and modeling study

The year-to-year variability in the timing, duration, and spatial extent of the surface phytoplankton bloom over the winter-spring period is examined in the southern Adriatic Sea using Sea-viewing Wide Field-of-view Sensor (Sea WiFS)-derived chlorophyll images for three years (1998, 1999, and 2000). Each year's image time series shows that blooms were intermittent and differed in onset, duration, and intensity with relatively low values observed in 2000. The relation between atmospheric forcing and interannual variability of the bloom timing and intensity is investigated using a coupled physical-biological model. The simulations focus on the effect of cumulative buoyancy loss on convective depths and its implications on surface nutrient availability, chlorophyll concentrations, and other ecosystem components during the study period. We test the hypothesis that the south Adriatic bloom is essentially controlled by the local winter climatic conditions (i.e., maximum convective depth), as suggested by recent findings, rather than the available nutrient pool at the intermediate depths (200-800 m), which also varies from year to year. For all three years the simulations produced convective depths that were in good agreement with in situ observations. However, the fluctuations in Sea WiFS phytoplankton biomass could be reproduced only if the particular year's nutrient pool was also taken into account. Thus the most probable explanation for the low Sea WiFS phytoplankton biomass observed in 2000 is the reduced nutrient pool because of the return from the transient phase to the pretransient regime of the Mediterranean Sea. Our results indicate that the south Adriatic bloom is a complex phenomenon and cannot simply be explained by interannual changes in convective depth.