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

Abstract

Primary productivity and the associated phytoplanktonic parameters were studied from July 1994 to October 1995 at two stations, over the continental shelf and slope of the Cretan Sea. An offshore increase in oligotrophy was observed, from a coastal ecosystem with consistently higher standing stock and productivity rates, to a more stressed oligotrophic pelagic environment. All phytoplanktonic variables exhibited a strong seasonal signal and an even 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 summer. The depth integrated rates of primary productivity averaged over season 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 both stations) and late winter-early spring (at the coastal station). Late winter-early spring maxima in primary production rates and chl a concentration at the shelf station were associated with increased diatom abundance (reaching 88% of total cell counts). The picture evolving from the coastal station characteristics supports the contention that favorable hydrological conditions (physico-chemical features and enhanced nutrient supply through water column mixing) may generate diatom blooms and elevated productivity rates.