Role of Saharan dust on phytoplankton dynamics in the northeastern Mediterranean

Abstract

Effects of atmospheric deposition and other environmental factors on phytoplankton dynamics were evaluated from an open (having offshore characteristics) and a coastal station in the northeastern Mediterranean between December 2000 and December 2001. Data on phytoplankton, chlorophyll a, nutrients, temperature and salinity were obtained at bi-weekly or more intense intervals during 1 yr, whilst transport of Saharan dust towards the sampling region was monitored daily by SeaWiFS (sea-viewing wide field-of-view sensor) images. Diatoms were the group of highest average biomass during the entire investigation period. Although coccolithophores (mainly Emiliania huxleyi) numerically dominated at the open station during the study period, their maximum abundance was as low as 50 x 10 3 cells l -1 . The intensity of dust transport was observed to be highest in spring. Less intense transports were observed in summer and autumn. Several intense episodic dry and wet dust deposition events during the spring season observed by SeaWiFS images caused little or no increase in phytoplankton abundance and biomass in the following days and weeks. Nevertheless, it appears that less intense dust transport events increased phytoplankton abundance and biomass in August, September and October 2001, when water column stratification was at its peak. However, these increases were much weaker than the major winter-early spring bloom (in February and March), which was caused by upwelling, mixing the water column. We suggest that the impact of atmospheric nutrient input on phytoplankton in the Mediterranean is rather low on a yearly basis.