## 4 Discussion
The ratio of produced organic carbon and utilized nitrogen, phosphorus, and silicon in marine ecosystems (C:N:P:Si) is equal to 106:16:1:15 [33]. The N:P ratio is specifically variable for atmospheric depositions and may influence the Redfield's ratio of carbon (C), nitrogen (N), phosphorus (P), and oxygen (O2) in marine ecosystems. The Redfield's C:N:P:O2 ratio equals 106:16:1:138. [PERSON] et al. (2016) [34] have argued that atmospheric P and N depositions can support new production from 1.9 to 5.7 and from 1.5 to 5.4 mgC\(\cdot\)m\({}^{-2}\cdot\)day\({}^{-1}\), respectively. Increased precipitation of ammonium and nitrate promotes increased dissolved inorganic nitrogen in surface waters and water acidification. If the ratio of nitrogen to phosphorus or silicon is sufficiently shifted towards nitrogen, limitation of primary production for diatoms is expected. These N:P:Si proportions determine limitation for phytoplankton growing and could thus affect the community composition and ecosystem processes [35]. Our data reveal several distinct features in seasonal and site-to-site variations in the N:P:Si ratios (Table 4). Seasonal variations in this ratio are far more pronounced for rural Katsiveli, as compared to industrial Sevastopol. This suggests that natural variations are larger and depend on long-distance atmospheric transport of nitrogen, phosphorus, and silicon from different remote sources and local weather conditions supporting either wet or dry atmospheric depositions. Unlike for rural Katsiveli, powerful local sources of nitrogen, phosphorus, and silicon equal or even override natural sources of these elements, resulting in rather stable intra-annual ratios. The ratio N:P in the atmospheric depositions is significantly shifted towards nitrogen in both locations, as compared to the Redfield ratio. This result is consistent with data published by [PERSON] et al. [36]. The ratio P:Si is significantly shifted towards phosphorus comparing to the Redfield ratio. Thus, atmospheric depositions support the traced shift in the phytoplankton succession from diatoms to coccolithophores and further to dinoflagellates supporting intensive \"red tides\" and oxygen deficit in the coastal area. There is a difference in the contribution of nitrogen in these ratios in Sevastopol and Katsiveli. It is about twofold higher in Katsiveli, as compared Sevastopol (Table 4). However, this is due to lower phosphorus concentrations in the atmospheric depositions in Katsiveli and about twofold higher concentrations in Sevastopol. This is also true for silicon, which is depleted comparing to nitrogen and phosphorus, and far more depleted in Katsiveli comparing to Sevastopol. The situation is dramatically different for rare atmospheric depositions with extremely high concentrations of phosphorus and silicon (Table 5). For these events, the N:P:Si ratio is phosphorus enriched and nitrogen depleted. Yet even these phosphorus and silicon enriched precipitations support silicon depleted ratios in seawater s near both Katsiveli and Sevastopol. Unlike the usual atmospheric depositions (Table 4), these phosphorus enriched depositions support shifts from dinoflagellates to coccolithophores and partially explain extreme bloom of Emiliania huxleyi in the Black Sea in June. Still, atmospheric depositions cannot support a desired shift from non-diatoms to diatoms in the Black Sea. A comparative analysis of the nutrients content in the sea surface layer before and immediately after precipitation for the surface layer of the Sevastopol Bay near the site of sampling atmospheric depositions has revealed that the value of the nutrients concentrations in the surface layer was higher than the background concentration after precipitation [37]. Depending on the trophic status the water regions may react differently to the input of nutrients with precipitation. For example, in the most unproductive areas nutrients can be consumed very quickly and do not lead to an increase in their concentration in the surface layer.