PHYTOPLANKTON FLUORESCENCE AND DEEP CHLOROPHYLL MAXIMA IN THE NORTHEASTERN MEDITERRANEAN

Two expeditions in the northeastern Mediterranean by the R/V Bilim (in July 1988 and March 1989) scanned an area of about 3 x 10(5) km2 to d etermine, in situ, the relative fluorescence intensities of the upper layer waters. The in situ fluorescence intensities exhibited a fair co rrelation with the discrete chlorophyll-a concentrations when the conc entrations exceeded 0.1 mug/L. Light intensities indicated that the eu photic zone had an average thickness of 100 m in the open waters. The deep chlorophyll-a maxima (DCM) at the bottom of the euphotic zone usu ally coincided with the maxima observed by in situ fluorometry and wer e a prevalent characteristic of the oligotrophic northeastern Mediterr anean. The formation, maintenance and location of the DCM were control led by the changes in light attenuation and nutrient concentrations oc curring in the anticyclonic and cyclonic gyres. Accordingly, DCM with relatively high chlorophyll concentration formed at shallower depths i n late winter (e.g. 50 m for March, 1989) whilst in summer DCM possess ed lower chlorophyll concentrations and were found as deep as 100 m in the anticyclonic regions. Although the depths of the maximum fluoresc ence intensity varied in space and time, they remained within a relati vely narrow range of isopycnal surfaces, namely, from 28.8 to 29.0 in March 1989, and from 28.6 to 29.0 in July 1988; the appearance of maxi mum fluorescence intensities at larger density values but at shallower depths in late winter is principally the result of lower light intens ity, available nutrients and hydrological changes in the upper layer.