Primary productivity in the oligotrophic Cretan Sea (NE Mediterranean): seasonal and interannual variability

Primary productivity and the associated phytoplanktonic parameters were stu died from July 1994 to October 1995 at two stations, over the continental s helf and slope of the Cretan Sea. An offshore increase in oligotrophy was o bserved, from a coastal ecosystem with consistently higher standing stock a nd productivity rates, to a more stressed oligotrophic pelagic environment. All phytoplanktonic variables exhibited a strong seasonal signal and an ev en stronger interannual one, with mean values being consistently higher at the coastal station. Maxima in primary productivity rates and chlorophyll a concentrations occurred in late winter-early spring and minima in late sum mer. The depth integrated rates of primary productivity averaged over seaso n were 220 (+/-64.3) mg C m(-2) d(-1) at the shelf and 162 (+/-61.4) mg C m (-2) d(-1) at the slope station. The annual gross primary productivity was 80 and 59 g C m(-2) y(-1) at the shelf and slope station, respectively. At both stations, throughout the year, dinoflagellates were the dominant group followed by diatoms; the only exceptions were observed during winter (at b oth stations) and late winter-early spring (at the coastal station). Late w inter-early spring maxima in primary production rates and chl a concentrati on at the shelf station were associated with increased diatom abundance (re aching 88% of total cell counts). The picture evolving from the coastal sta tion characteristics supports the contention that favorable hydrological co nditions (physico-chemical features and enhanced nutrient supply through wa ter column mixing) may generate diatom blooms and elevated productivity rat es. (C) 2000 Elsevier Science Ltd. All rights reserved.