Distribution of nutrients and particulate organic matter in relation to the prevailing hydrographic features of the Cretan Sea (NE Mediterranean)

The chemical and biological characteristics of the water column in the Cret an Sea (South Aegean Sea, NE Mediterranean) were studied from May 94 to Sep tember 95, in order to understand the underlying structural and functional mechanisms of this pelagic ecosystem. The circulation in the Cretan Sea was dominated at the time by a mesoscale dipole (an anticyclone in the West an d a cyclone in the East), and this created a persistent water flow (jet) in a SE and ESE direction that inhibited offshore transport of material but a ided onslope transport. The intrusion (at intermediate depths of 200-700 m) of the Trans Mediterranean Water (TMW) from the east was detected during e very sampling period and has important implications to the system because i t is a water mass that is older and comparatively richer in nutrients. Mean phosphate, nitrite, nitrate and silicate concentrations within the 1-200 m depth layer (CIW) were 0.08, 0.13, 1.37 and 1.49 mu M, respectively. Accor dingly, at 200-700 m (TMW) the acquired values were 0.17, 0.11, 3.72 and 2. 85 mu M, while at 700-1500 m (CDW) the corresponding values were 0.15, 0.11 , 3.45 and 2.71 mu M. POC and PON concentrations ranged from 8.18-422.58 mu g/l (76.76 mu g/l average) and 0.34-82.58 mu g/l (9.68 mu g/l average), re spectively. The strong seasonal thermocline and the depletion in phosphates (indicated by the high N/P ratio: 20.2-25.9) maintained phytoplankton biom ass at relatively low levels especially during the summer (mean chla, phaeo pigment and CPE concentrations from all sampling periods within the 1-200 m depth layers were 0.202, 0.225 and 0.427 mu g/l, respectively). Thus, alth ough the characteristic Deep Chlorophyll Maximum (DCM), the vertical distri bution of nutrients, and the low levels of chloroplastic pigments, POC and PON in the euphotic zone (0-100 m) are all indicative of a currently oligot rophic ecosystem, the dramatic changes underway in the hydrological structu re of the water column are capable of fertilizing the area thus increasing its productivity and POM distribution. (C) 2000 Elsevier Science Ltd. All r ights reserved.