Surface and mid-water sources of organic carbon by photoautotrophic and chemoautotrophic production in the Black Sea

The multilayered surface waters of the Black Sea contain aerobic, suboxic and anoxic layers that support both photoautotrophic (PP) and chemoautotrophic (ChP) biological production. During the R/V Knorr cruise in May–June 2001, phytoplankton biomass (represented as chlorophyll-a), photoautotrophic and chemoautotrophic production (ChP) rates were determined in the western Black Sea. Integrated chlorophyll-a concentrations in the euphotic zone were as low as 2.2 mg m−2 in the central gyre, while they were as high as 19.9 mg m−2 in the NW shelf region. Integrated photoautotrophic production rates ranged from 112 to 355 mg C m−2 d−1. The lowest values were determined in the central gyre and the highest values were found at the shelf-break station near the Bosphorus, the NW shelf/shelf-break area and in the Sevastopol eddy. Primary production and chlorophyll-a data revealed that post-bloom conditions existed during this sampling period. Bioassay experiments showed that under optimum light conditions, photoautotrophic production was nitrogen-limited. ChP increased in the redox transition zone and coincided with the lower boundary of the fine particle layer. The maximum values were shallower (at σ θ = 16.25 ) in the central gyre and deeper (at σ θ = 16.5 ) in the shelf-break region near Sakarya Canyon. Integrated ChP rates were 63 and 1930 mg C m−2 d−1, which were equivalent to 30% and 89% of the overall water-column production for the central gyre and Sakarya Canyon regions, respectively.