In this study, the role of major factors (i.e. hydrology, nutrient concentration in seawater, atmospheric and coastal input, rain) on phytoplankton dynamics in the northeastern Mediterranean were investigated at one deep and one shallow station in during March-June 1999 and December 2000-April 2002. Nutrient effects on three marine phytoplankton species (the coccolithophore Emiliania huxleyi, the diatom Skeletonema costatum and the dinoflagellate Prorocentrum micans) were also investigated under laboratory conditions.

In order to observe any possible effects of dust on phytoplankton and to find out if reflectance events are due to coccolithophore blooms, Saharan dust transport and reflectance events from the satellite images were regularly observed together with frequent (generally biweekly and sometimes as often as every three days) field sampling of phytoplankton and conventional physical and chemical parameters of seawater.

Precipitation positively correlated with total abundance and biomass of phytoplankton (mainly composed of diatoms). Precipitation also increased phytoplankton abundance in the laboratory cultures.

The 1999 period was more productive than 2001. Highest nitrate, phosphate, chlorophyll-a and phytoplankton abundance and biomass values were found in 1999 (probably due to the much stronger river influence as well as water dynamics). High values were also observed in 2002 (probably due to heavier precipitation as well as strong vertical mixing) compared to 2001 (which was a relatively calm period).

At the deep station, the major phytoplankton bloom along the water column in January-February 2001 was found to be mainly due to winter mixing (i.e. upwelling). Notable increases in phytoplankton abundance and biomass at the surface waters in March-April and August-October 2001 were probably due to precipitation plus river inflow and to dry atmospheric deposition, respectively. It should be noted that the summer biomass during June-October 2001 accounts for 16% of the total integrated phytoplankton biomass (down to 100 m) during one year (December 2000-November 2001).

Highest abundances (max. 50,000 cells l^-1) of Emiliania huxleyi also occurred during October – November 2001 which could be related with the sharpness and thickness of the mixed layer combined with the dry and wet deposition of nutrients from the atmosphere in this period.

Phytoplankton abundance, chlorophyll-a concentration and in-vivo fluorescence values (except Skeletonea costatum abundances in 1-litre flasks) all decreased with a decrease in nutrients in the laboratory cultures. When nitrate, phosphate and ammonium concentrations were separately reduced to 10% of the F/2 medium, Chl-a peaks did not coincide with abundance or in-vivo fluorescence peaks unlike the F/2 Medium. This may indicate that nutrient limitation impedes chlorophyll-a synthesis much more than growth rate (in abundance) and consequently, low chlorophyll a values in the field may not always reflect actual phytoplankton biomass. While all three species consumed similar amounts of nitrate, Skeletonema costatum consumed a higher amount of phosphate compared to Prorocentrum micans and Emiliania huxleyi.