Quantifying importance and scaling effects of atmospheric deposition of inorganic fixed nitrogen for the eutrophic Black Sea

Abstract. Wet atmospheric depositions have been collected in a rural (Katsiveli) and urban (Sevastopol) location at the Crimean coast of the Black Sea from 2003 to 2008. Samples, 217 from Katsiveli and 228 from Sevastopol, have been analysed for inorganic fixed nitrogen (nitrate, nitrite, and ammonium). Data have revealed almost equal contributions of ammonium (44–45 %) and nitrate (52–53 %) and minor contribution of nitrite (2–4 %) for both rural and urban samples. The volume weight mean concentration of inorganic fixed nitrogen (IFN) in urban samples (2.51 mg N L⁻¹) is about 2-fold of that content in rural samples (1.16 mg N L⁻¹). Seasonal variations in volume weight mean monthly concentrations have been revealed for both locations with maximum concentrations in winter and minimum values in summer, but intra-annual variations are statistically significant for only urban samples.

The average annual deposition of IFN with atmospheric precipitations on the surface of the Black Sea is about 0.31 × 10⁶ t N yr⁻¹ (0.75 t N km⁻² yr⁻¹), which is on average 39 % of the riverine input. It does vary in space and time. The relative importance of the atmospheric input increases from coastal to open areas and from winter to summer. Deposition of IFN with wet atmospheric precipitations proportionally increases the concentration of chlorophyll a, as it is traced from satellite data. The traced increase in the concentration of chlorophyll a has reached 1.5-fold for mesoscale processes. In case of individual rain events supporting up to 50–60 mg N m⁻², the influence of IFN deposition is up to 5 % at the north-western shelf of the Black Sea, where most of the river-born IFN is loaded. In the central areas of the sea, where the amount of IFN in summer is low, the contribution of individual rainfall can reach 35 %. The input of IFN to the Black Sea has potential to enhance 2-fold the level of primary production.