Redox layer, seasonal variability

Here we will describe the main features of the Black Sea biogeochemical structure from the point of view of the changing of redox conditions. We will also analyze the main peculiarities of the seasonal and interannual variability of this structure primarily on the base data received in 1997-2006. The main attention we will placed on describing the structure and temporal variability of the chemical structure of the redox layer, the boundary layer between oxic and anoxic waters. 

Our studies showed that the biogeochemical system of the redox layer is subjected to temporal variability on a seasonal scale (connected with the seasonality of OM production) and interannual changes. Surface ventilation of dissolved oxygen down to the depth of the CIL (ag = 14.5 kg nr3) occurs in the winter from a combination of the NW shelf and the centers of the gyres. The intensity of ventilation is determined by climate forcing which may be determined by large-scale climate patterns like the NAO. This ventilation sets the upper boundary conditions for the downward transport of O2. Therefore, the position of the hydrogen sulfide boundary in the density field is connected with the climate variability, related to the NAO index. 

Proportions of As(lll) and As(V) are particularly variable in stratified lakes with seasonally variable redox gradients (Kuhn and Sigg, 1993). In the stratified, hypersaline, hyperalkaline Mono Lake (California, USA), As(V) predominates in the upper oxic layer and As(lll) in the reducing layer (Maest et al., 1992 Oremland et al., 2000). Oremland et al. (2000) measured in situ rates of dissimilatory As(V) reduction in the lake and found that this could potentially mineralize 8-14% of the annual pelagic primary productivity during meromixis, a significant amount for a trace element, and —1/3 of the amount of sulfate reduction. Such reduction does not occur in the presence of NO3. In fact, NO3 leads to the rapid, microbial re-oxidation of As(III) to As(V) (Hoeft et al, 2002). Iron(III) acts similarly. 

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