Nutrient, particulate matter and phytoplankton variability in the photic layer of the Otranto strait

The distribution of nutrients, suspended matter and phytoplankton in the photic layer of the Otranto Strait, as observed in four seasonal situations (February, May, August and November 1994), is analysed in relation to hydrography. In winter, two water masses were found: the Adriatic Surface Water (ASW) and the Ionian Surface Water (ISW), located at the western and eastern side of the Strait, respectively. In the fresher and cooler ASW, nutrients, suspended matter and phytoplankton contents were higher than in the warmer and saltier ISW (on average: N–NO3=2.3 μM, POC=3.6 μM, chl.a=0.4 μg dm−3 in the ASW, against N–NO3=1.3 μM, POC=2.5 μM, chl.a=0.3 μg dm−3 in the ISW). In the ASW, the mean N:P ratio (50) revealed an excess of nitrogen with respect to phosphorus; the nitrogen supply, as well as the significant presence of diatoms, made us suppose that new production processes were occurring here. In summer, the main features were: (i) high water column stability, (ii) small horizontal differences in hydrological and biological features, (iii) extremely low concentration of nutrients at the surface and (iv) a deep chlorophyll maximum (DCM) at the nutricline level. Phytoplankton summer communities, mainly consisting of nanoflagellates, coccolithophorids and small naked dinoflagellates, were present in low quantities. The low phytoplankton carbon:chlorophyll a ratio (<20), observed at DCM, indicated a high chlorophyll content per single cell, probably as a physiological response to low irradiance. As to the phytoplankton annual cycle, sediment traps and water column observations were highly correlated, evidencing two abundance peaks in spring and autumn, and a summer minimum. Hydro-chemical and biological data suggest that the winter spread of the ASW was the main factor favouring the enhancement of phytoplankton growth and controlling the species composition in the strait, while the DCM formation represents a more typical characteristic of ISW, with an oligotrophic feature similar to that of other Eastern Mediterranean water masses.