Modeling the nitrogen fluxes in the Black Sea using a 3D coupledhydrodynamical-biogeochemical model: transport versus biogeochemicalprocesses, exchanges across the shelf break and comparison of the shelf anddeep sea ecodynamics

Abstract

Abstract.A 6-compartment biogeochemical model of nitrogen cycling and plankton productivity has been coupled with a 3D general circulation model in an enclosed environment (the Black Sea) so as to quantify and compare, on a seasonal and annual scale, the typical internal biogeochemical functioning of the shelf and of the deep sea as well as to estimate the nitrogen and water exchanges at the shelf break. Model results indicate that the annual nitrogen net export to the deep sea roughly corresponds to the annual load of nitrogen discharged by the rivers on the shelf.The model estimated vertically integrated gross annual primary production is 130 g C m −2 yr −1 for the whole basin, 220 g C m −2 yr −1 for the shelf and 40 g C m −2 yr −1 for the central basin. In agreement with sediment trap observations, model results indicate a rapid and efficient recycling of particulate organic matter in the sub-oxic portion of the water column (60-80 m) of the open sea. More than 95% of the PON produced in the euphotic layer is recycled in the upper 100 m of the water column, 87% in the upper 80 m and 67% in the euphotic layer. The model estimates the annual export of POC towards the anoxic layer to 4 10 10 mol yr −1 . This POC is definitely lost for the system and represents 2% of the annual primary production of the open sea.

Black Sea nitrogen cycling and the preservation of phytoplankton δ15N signals during the Holocene

“…[27] Nitrogen-containing molecules like Chl a may be preferentially scavenged in the N-limiting surface waters, partially accounting for the elevated C:N ratio in the sediments (Figure 8). One biogeochemical model of N cycling in the Black Sea predicts that more than 95% of sinking particulate organic N is recycled in the upper 100 m of the water column [Grégoire and Beckers, 2004]. Konovalov et al [2008] proposed a similar scenario for recycling cyanobacterial biomass in the SOL.…”

The effect of various vertical discretization schemes and horizontal diffusion parameterization on the performance of a 3-D ocean model: the Black Sea case study

“…Currently there are a large number of vertical grids which can be used in ocean circulation models (Ezer et al, 2002;Blumberg and Mellor, 1983;Grégoire and Beckers, 2004). However selection of an optimal grid for a specific application is not straightforward and needs specific experiments and sensitivity tests (Ezer and Mellor, 2004).…”

“…Legg et al, 2008;Berntsen et al, 2010) and exchanges between the shelf and the deep sea (e.g. Huthnance, 1995;Grégoire and Beckers, 2004;Huthnance et al, 2009). An accurate modelling of shelf-sea exchanges is particularly important for the Black Sea, which is the world's largest landlocked basin with an extensive shelf occupying about 20 % of the whole sea area (Shapiro, 2008).…”

Dynamics of the deep chlorophyll maximum in the Black Sea as depicted by BGC-Argo floats

“…These papers highlight a clear differentiation of planktonic community composition in surface and subsurface lay-ers (Mikaelyan et al, 2018(Mikaelyan et al, , 2020 and the importance of environmental factors such as surface winds (Mikaelyan et al, 2017b) and mesoscale vertical dynamics (Mikaelyan et al, 2020) in triggering local surface blooms in autumn. The winter-spring bloom dynamics, and their interannual variations in particular, have been described in detail and used to propose the pulsing bloom hypothesis (Mikaelyan et al, 2017a), an extension of the general critical depth hypothesis and its derivatives (Sverdrup, 1953;Huisman et al, 1999;Chiswell et al, 2015), which applies to highly stratified waters.…”