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

The distribution of nutrients, suspended matter and phytoplankton in the ph otic layer of the Otranto Strait, as observed in four seasonal situations ( February, May, August and November 1994), is analysed in relation to hydrog raphy. 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 warme r and saltier ISW ton average: N-NO3 = 2.3 mu M, POC = 3.6 mu M, chl.a = 0. 4 mu g dm(-3) in the ASW, against N-NO3 = 1.3 mu M, POC = 2.5 mu M, chl.a = 0.3 mu g 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 w ell as the significant presence of diatoms, made us suppose that new produc tion processes were occurring here. In summer, the main features were: (i) high water column stability, (ii) small horizontal differences in hydrologi cal 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 nanoflagella tes, coccolithophorids and small naked dinoflagellates, were present in low quantities. The low phytoplankton carbon:chlorophyll a ratio (< 20), obser ved at DCM, indicated a high chlorophyll content per single cell, probably as a physiological response to low irradiance. As to the phytoplankton annu al cycle, sediment traps and water column observations were highly correlat ed, evidencing two abundance peaks in spring and autumn, and a summer minim um. Hydro-chemical and biological data suggest that the winter spread of th e ASW was the main factor favouring the enhancement of phytoplankton growth and controlling the species composition in the strait, while the DCM forma tion represents a more typical characteristic of ISW, with an oligotrophic feature similar to that of other Eastern Mediterranean water masses. (C) 19 99 Elsevier Science B.V. All rights reserved.