Benthic fluxes of nutrients in the northwestern Black Sea

Abstract

The exchange of O2, NH4+, Si(OH)4, ortho-phosphate, Fe2+, Mn2+ and H2S between the sediment and the overlying water (benthic flux) was determined at 6 locations on the northwestern shelf of the Black Sea using an in situ benthic lander. Oxygen consumption by the sediments ranged from 0 to 33 mmol m−2 d−1 depending mostly on the initial oxygen concentration in the bottom water. During the time period of deployment of 22 h, no reduction of sulfate was observed within the lander and no release of methane. H2S was released from the highly eutrophic sediments close to the Dniester river with a flux of 0.3 mmol m−2 d−1. Fluxes of phosphate, ammonia and silica close to the mouths of the Danube and Dniester rivers were high (0.5, 2.6–4.4 and 6.0 mmol m−2 d−1, respectively) and comparable with values measured in other estuaries such as Naragansett and Chesapeake Bay, but higher than fluxes measured in shelf-slope seas like the Skagerrak. The benthic fluxes of phosphorus and silica are in the same order of magnitude as the annual nutrient input by the Danube river, whereas ammonia fluxes from the sediment amounted to ∼10% of the Danube input. Hence, benthic fluxes from the sediments are responsible for a significant flux of nutrients to the waters of the northwestern shelf of the Black Sea.