ELEMENTAL COMPOSITION OF SESTON AND NUTRIENT DYNAMICS IN THE SEA OF MARMARA

The Sea of Marmara, an intercontinental basin with shallow and narrow straits, connects the Black and Mediterranean Seas. Data obtained duri ng 1991-1996 have permitted the determination of the elemental composi tion of seston in the euphotic zone and the N:P ratio of the subhalocl ine waters of the Marmara Sea. Since primary production is always limi ted to the less saline upper layer (15-20 m), of the Marmara Sea, the subhalocline waters of Mediteranean origin are always rich in nutrient s (NO3 + NO2 = 8-10 mu m, PO4 = 0.8-1.2 mu m) but depleted in dissolve d oxygen (30-50 mu m) throughout the basin, yielding an -O-2:N:P ratio of 178.9:1. Pollution of the surface waters since the 60s has modifie d the subhalocline nutrient chemistry slightly. In the euphotic zone, the N:P ratio of the seston changes from 5.9 to 9.5 between the less a nd more productive periods. Though the biology of the Marmara has chan ged significantly during the previous two decades, the close relations hip observed between the elemental composition of the surface seston a nd the NO3:PO4 ratio of the subhalocline waters strongly suggests that during the whole year primary production throughout the basin and POM export to the lower layer remain nitrogen-limited. This suggestion ne eds to be confirmed by bio-assays, biological studies and sediment tra p data from the upper subhalocline depths. Nonetheless, the counterflo ws in the Marmara basin possess relatively low N:P ratios in both diss olved and particulate nutrients and extend as far as the adjacent seas .