Elemental composition of seston and nutrient dynamics in the Sea of Marmara

Abstract

The Sea of Marmara, an intercontinental basin with shallow and narrow straits, connects the Black and Mediterranean Seas. Data obtained during 1991-1996 have permitted the determination of the elemental composition of seston in the euphotic zone and the N:P ratio of the subhalocline waters of the Marmara Sea. Since primary production is always limited to the less saline upper layer (15-20 m), of the Marmara Sea, the subhalocline waters of Mediteranean origin are always rich in nutrients (NO3 + NO2 = 8-10 mu m, PO4 = 0.8-1.2 mu m) but depleted in dissolved 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 modified 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 and more productive periods. Though the biology of the Marmara has changed significantly during the previous two decades, the close relationship observed between the elemental composition of the surface seston and 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 needs to be confirmed by bio-assays, biological studies and sediment trap data from the upper subhalocline depths. Nonetheless, the counterflows in the Marmara basin possess relatively low N:P ratios in both dissolved and particulate nutrients and extend as far as the adjacent seas.