Nitrogen budget of the northwestern Black Sea shelf inferred from modeling studies and in situ benthic measurements

Abstract

A 3D eddy-resolving coupled biogeochemical-hydrodynamical model and in situ obser- vations are used to investigate benthic processes on the Black Sea's NW shelf. Measurements of ben- thic fluxes (oxygen, nutrients, redox compounds) with in situ flux chambers are analyzed in regard to sediment dynamics on the shelf. The seasonal/interannual and spatial variability of benthic processes is explained in terms of 3D ecohydrodynamics. The space/time distribution of benthic fluxes depended on the position of the river plume and the associated primary production, intensity of ver- tical mixing and water depth. Model results and in situ observations reveal the presence of a region of intense benthic recycling and high benthic nutrient fluxes nearshore and in the northern part of the shelf. The model estimates that this region covers about 15% of the shelf area and is connected to the high productivity and high sedimentation caused by river input of organic matter. On the off- shore shelf, covering about 85% of the shelf area, benthic nutrient regeneration is low due to low pro- ductivity. Benthic mineralization pathways (aerobic respiration, denitrification, sulfate reduction, methanogenesis) are quantified. In the high flux region, more than 55% and in the offshore low flux region more than 80% of the organic matter is decomposed by aerobic respiration. In the high flux region, sulfate reduction is the main anaerobic pathway, whereas denitrification is more important on the low flux offshore shelf. At the shelf edge, under the influence of anoxic waters, more than 60% of organic matter is remineralized by sulfate reduction. Little organic matter is decomposed by methanogenesis. A mass balance of particulate organic carbon (POC) and particulate organic nitro- gen (PON), integrated over the whole shelf and year, shows that 3.7 to 4.2 × 10 6 t of POC reach the sediments, while model results give a value of 1.4 × 10 6 t C. The annual ammonium benthic outflux is estimated at 85 × 10 3 and 174 × 10 3 t N by in situ data and the model, respectively. The amount of nitrogen lost by burial and denitrification estimated from in situ observations is 57 × 10 3 and 324 × 10 3 t N, respectively. Therefore, NW shelf Black Sea sediments are an efficient sink for riverine nitrogen, trapping about 50% of the annual river discharge in total inorganic N.