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

Abstract

Upward and regeneration fluxes of inorganic nitrogen and phosphorusin the euphotic zone of the Black Sea deep waters were estimated using data from long-term observations of nitrate and phosphate vertical distribution and modeling of plankton month- and area-averaged nutrients excretion. The upward fluxes were calculated using monthly average nitrate and phosphate concentrations and their gradients beneath the euphotic layer assuming that the vertical transport rates are controlled by the density gradient at the upper layer of the main pycnocline throughout the year. According to the calculations, the nutrient input into the euphotic layer due to physical processes changes gradually throughout the year from minimum values in July-August (0.1-0.3 mg-atN x m(-2) x day(-1) and 0.02-0.04 mg-atP x m(-2) x day(-1)) up to maximum ones in February-March (1.2-1.8 mg-atN x m(-2) x day(-1) and 0.2-0.3 mg-atP x m(-2) x day(-1)), contrary to the seasonal dynamics of the regeneration flux rates. In summer, plankton regeneration rates increase nearly five-fold as compared to the winter months. The contribution of the upward flux to the total nutrient euphotic zone input was calculated and designated as F(N)(F(P))-ratio along the lines with the f-ratio and e-ratio that reflect the ratio of "new" production and particle export to primary production, respectively. The monthly means vary for F(N) from 5 to 50% and for F(P) from 10 to 70% throughout the year in the same manner as the upward flux. Relationships between the monthly mean values of F(N,P) and vertically integrated nitrate and phosphate concentrations, chlorophyll "a", phytoplankton biomass, and primary production in the euphotic zone were established. The obtained relationships correspond to the general relationships between primary production, "new" production and the sinking flux of particles known for the Ocean. Potential values of "new", regenerated, and total production of phytoplankton in the deep-water areas of the Black Sea are identified. Average monthly and annual primary production estimates, calculated on the base of nutrient flows, are consistent with estimates of primary production according to the averaged data of in situ measures.