Comparison of Offshore and River-Influenced Coastal Waters in the Eastern Mediterranean

Abstract

Depending on the environmental condition and antropogenic nutrient supply, organic matter production and chemical composition show spatial and temporal variabilities in marine environment. The Eastern Mediterranean is known to be one of the oligotrophic seas, where the surface inorganic phosphate and nitrate concentrations vary in the range of 10 to 20 nM and 0.10 to 0.30 µM, respectively. Primary production is, therefore, mainly controlled by phosphate. The exceptionally low phosphate concentrations have limited reliable estimates of the N/P (nitrate/phosphate) ratio by traditional measurement methods. In this study, we have used Magnesium induced coprecipitation (MAGIC) technique to measure inorganic phosphate with a sensitivity of 1 nM. Changes in nutrient and hydrographic conditions are monitored at three near-shore stations on Erdemli (Mersin) coastal zone and four off-shore stations in December 2003. Nutrient concentrations in the surface water displayed a strong offshore gradient, decreasing from 0.018 to 0.009 µM for phosphate; 0.40 to 0.06 µM for nitrate; 1.76 to 1.69 µM for reactive silicate towards open sea. On the other hand, deep water concentrations exhibited an opposite trend; phosphate rises from 0.024 to 0.214 µM, nitrate from 0.35 to 5.52 µM, reactive silicate from 1.77 to 13.78 µM in the region.