Upward and regeneration fluxes of inorganic nitrogen and phosphorus in deep-water areas of the Black Sea

Abstract

Upward and regeneration fluxes of inorganic nitrogen and phosphorus in the euphotic zone of deep waters in the Black Sea have been estimated using data from long-term observations of nitrate and phosphate vertical distributions and modelling of plankton monthand area-averaged nutrients excretion. The upward fluxes were calculated using monthly average nitrate and phosphate concentrations and their gradients below the euphotic layer, with the assumption that the vertical transport rates were controlled by the density gradient at the upper boundary of the main pycnocline throughout the year. According to the calculations, the nutrient input into the euphotic layer changed gradually through the year because of physical processes from the minimum, which was observed in July–August (0.1–0.3 mg-atN m–2 day–1 and 0.02–0.04 mg-atP m–2 day–1), up to the maximum, which was registered in February–March (1.2–1.8 mg-atN m–2 day–1 and 0.2–0.3 mg-atP m–2 day–1), contrary to the seasonal dynamics of the regeneration flux rates. In summer, the plankton regeneration was nearly five times higher than in the winter months. The contribution of the upward flux to the total nutrient input into the euphotic zone was calculated and assigned as the FN(FP)-ratio. The monthly averages varied for FN as 5–50% and for FP as 10–70% throughout the year following the curve of the upward flux. Relationships were found for the monthly averages of FN, P and vertically integrated nitrate and phosphate concentrations, chlorophyll a, phytoplankton biomass, and primary production in the euphotic zone. These relationships corresponded to the general relationships between the primary production, “new” production, and the flux of the sinking particles, which are well known in the world’s oceans. The theoretically potential values of a “new,” regeneration, and total production of phytoplankton in the deepwater areas of the Black Sea were calculated. Monthly and annually averaged primary production rates, which were calculated under nutrient fluxes, referred to the range of the primary production calculated under in situ measurements.