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 Cretan Sea (South Aegean Sea, NE Mediterranean) were studied from May 94 to September 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 and 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 aided onslope transport. The intrusion (at intermediate depths of 200–700 m) of the Trans Mediterranean Water (TMW) from the east was detected during every sampling period and has important implications to the system because it 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 μM, respectively. Accordingly, at 200–700 m (TMW) the acquired values were 0.17, 0.11, 3.72 and 2.85 μM, while at 700–1500 m (CDW) the corresponding values were 0.15, 0.11, 3.45 and 2.71 μM. POC and PON concentrations ranged from 8.18–422.58 μg/l (76.76 μg/l average) and 0.34–82.58 μg/l (9.68 μg/l average), respectively. The strong seasonal thermocline and the depletion in phosphates (indicated by the high N/P ratio: 20.2–25.9) maintained phytoplankton biomass at relatively low levels especially during the summer (mean chla, phaeopigment and CPE concentrations from all sampling periods within the 1–200 m depth layers were 0.202, 0.225 and 0.427 μg/l, respectively). Thus, although the characteristic Deep Chlorophyll Maximum (DCM), the vertical distribution 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 oligotrophic ecosystem, the dramatic changes underway in the hydrological structure of the water column are capable of fertilizing the area thus increasing its productivity and POM distribution.