On the production, elemental composition (C, N, P) and distribution of photosynthetic organic matter in the Southern Black Sea

Abstract

Chemical oceanographic understanding of the southern Black Sea has been improved by recent measurements of the optical transparency, phytoplankton biomass (in terms of chlorophyll-a and particulate organic matter) and primary productivity. During the spring-autumn period of 1995-1996, light generally penetrated only into the upper 15-40 m, with an attenuation coefficient varying between 0.125 and 0.350 m(-1). The average chlorophyll-a (Chl-a) concentrations for the euphotic zone ranged from 0.1 to 1.5 mu g 1(-1). Coherent sub-surface Chi-a maxima were formed near the base of the euphotic zone only in summer. Production rate varied between 247 and 1925 in the spring and between 405 and 687 mgC m(-2) d(-1) in the summer-autumn period. The average POM concentrations in the euphotic zone varied regionally and seasonally between 3.8 and 28.6 mu m for POC, 0.5 and 3.1 mu m for PON and 0.02 and 0.1 mu m for PP. Atomic ratios of C/N, C/P and N/P, derived from the regressions of POM data, ranged between 7.5 and 9.6, 109 and 165, and 11.2 and 16.6, respectively. In the suboxic/anoxic interface, the elemental ratios change substantially due to an accumulation of PP cohering to Fe and Mn oxides. The chemocline boundaries and the distinct chemical features of the oxic/anoxic transition layer (the so-called suboxic zone) are all located at specific density surfaces; however, they exhibit remarkable spatial and temporal variations both in their position and in their magnitude, which permit the definition of long-term changes in the biochemical properties of the Black Sea upper layer.