The contrasting oceanography of the Rhodes Gyre and the Central Black Sea

The Rhodes Gyre, a prominent feature of the oceanography of the eastern Mediterranean, is modelled as a vertical, continuous flow, cylindrical reactor illuminated during the day at its upper end. If the Gyre is supposed to be in a steady state whilst the concentrations, C, of a chemical are being measured, the nett rate of formation or consumption of the chemical is given by -w d C/d z + u d C/d r, where w is the upward velocity of the water in the vertical, z , direction and u is the velocity of the water in the radial, r, direction. The behaviour of w and u is analysed to show that the Gyre may be used as a field laboratory in which rates of chemical change may be derived from depth profiles together with values of the surface velocities of the Gyre waters. In contrast, the central Black Sea is modelled as an ideal, strongly stratified sea in which the nett rates of formation or consumption of chemicals under steady state conditions are given by Ds d2C/ds 2, where s is the water density and Ds is an eddy diffusion coefficient. Computations reveal that, given better knowledge of its eddy diffusion coefficients, the Black Sea can also be treated as a field laboratory where rates of reaction mediated by bacteria may be derived from depth profiles

Carbon and nitrogen isotopic ratios of suspended particulate organic matter (SPOM) in the Black Sea water column

Carbon and nitrogen isotopic ratios (delta(13)N and delta(13)C) of suspended particulate organic matter (SPOM) in the water column of the Black Sea were measured at a total of nine stations in September-October (autumn) 1999 and May 2001. For comparison, a station in the Mediterranean Sea and one in the Sea of Marmara were sampled in October 1999. Large-sized particle samples, as well as samples of surface sediment were also collected for N and C isotopic analysis. The results revealed important vertical and regional variations in N and C isotopic composition. Seasonal variations in SpOM delta(15)N and delta(13)C were not apparent. SPOM in the euphotic zone (EZ), oxycline, and suboxic/anoxic interface layers of the water column was characterized by distinct isotopic composition. In the EZ, the N and C isotopic ratios of SPOM were in the range typically observed for plankton-derived SPOM in the surface ocean (EZ means ranged from 2.7%. to 5.9%. for delta(15)N and from -24.0 parts per thousand to -21.5 parts per thousand for delta(13)C). Shelf region SPOM had higher delta(15)N and lower delta(13)C (EZ means of 5.9 parts per thousand and -24.0 parts per thousand. respectively). Large-sized particles (LPOM) collected by zooplankton net tows had similar to 3 parts per thousand higher delta(15)N values compared to SPOM, indicating fractionation during trophic transfer of nitrogen. SPOM in the oxycline increased by 3-6 parts per thousand for delta(15)N, while delta(13)C decreased by -2 parts per thousand to -4 parts per thousand, which may be attributed to greater lipid content. In the suboxic/anoxic interface zone, SPOM isotopic ratios (delta(15)N as low as 0.0 parts per thousand to -8.0 parts per thousand) suggest chemoautotrophic production leading to dominance of new, in situ produced organic matter. The location of the most negative delta(15)N values indicates that chemoautotrophic production is most intense at the shelf-break regions, possibly enhanced by mixing of oxygenated and nitrate-rich Mediterranean inflow waters with suboxic/anoxic Black Sea water

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), photo autotrophic 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 19.9mg m(-2) in the NW shelf region. Integrated photoautotrophic production rates ranged from 112 to 355mg 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, photo autotrophic 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 sigma(theta) = 16.25) in the central gyre and deeper (at sigma(theta) = 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

Nitrogen cycling in the offshore waters of the Black Sea

The purpose of this study was to measure directly the rates of several of the processes responsible for the production and utilization of nitrogenous nutrients, and to use these rates and other data to generate an annual nitrogen budget for the Black Sea. Water column samples and experimentation with (15)N labeled nutrients in the offshore waters of the Black Sea reveal strong seasonal cycles in the utilization of different forms of N, the regeneration of NH(4)(+) and the production of NO(2)(-) in and below the surface mixed layer. There was no opportunity to sample during winter, but historical data and contemporary satellite ocean color data for the study period allow us to make extrapolations to a full annual cycle for the Black Sea N budget. The processes supplying N to, and the microbial processes within, the Cold Intermediate Layer (CIL), which lies below the surface mixed layer, figure prominently in determining the sources of N available for primary production. The uptake of NO(3)(-) by phytoplankton in this system was less sensitive to NH4+ concentration than has been observed in many oceanic waters. The seasonal shift in nutrient uptake kinetics was consistent with seasonality of nutrient availability. Rates of in situ NO(2)(-) production (and inferred nitrification) for the offshore waters was 1.6 x 10(11) mol y(-1), three times the published estimates for NO(3)(-) supplied from the NW Shelf (NWS) region, which originates from riverine discharges. Measured rates of nitrification in the CIL are about 60% of phytoplankton NO(3)(-) + NO(2)(-) uptake (2.8 x 10(11) mol y(-1)). Remineralization is about 25% of the NH(4)(+) phytoplankton utilization rate (3.8 x 10(11) mol y(-1)). Within the CIL NH(4)(+) is utilized in NO(2)(-) production (and implied nitrification) at a rate that is similar to the rate of NH4+ remineralization from organic matter. By preserving the rates that are determined with the most confidence, and making adjustments to the rates least confidently determined, nitrification (+60%, which is within the range of published values) and ammonium remineralization (+13%), the Black Sea N budget can be brought into balance. A balanced annual budget for N cycling in the offshore waters of the Black Sea estimates a particle export rate from the oxygenated surface layer to the deep anoxic waters equivalent to 8% of the total N production. We extrapolate an annual mean f-ratio of 0.38 by the conventional formulation (NO(3)(-) uptake: total N uptake). However, the balanced N budget permits a direct comparison of allochthonous sources of N to total N production in this unusual aquatic ecosystem, resulting in an f-ratio of 0.17, which is reconciled with particulate export when the budgeted losses due to anammox and denitrification are included. The NO(3)(-) content of the CIL is sensitive to year-to-year fluctuations in the source of N from the NWS. These processes plus the intensity of winter mixing, which supplies new N for the fall-winter bloom, are influenced by climate. Oscillations in winter temperature over the past few decades allow inference as to how the Black Sea N budget may be affected by future warmer conditions for this region. (c) 2007 Published by Elsevier Ltd