The results of studying the content of nitrogen and mineral phosphorus compounds, as well as the primary production of phytoplankton (PPP) in 2017–2019 are presented. In the marine area of the of the SPNA «Cape Martyan», located on the southern coast of Crimea (Black Sea). It was found that in the summer period in the surface layer of seawater, PP can be limited by both nitrogen and phosphorus. The dependence of the PPF change on the concentration of total nitrogen in water is of little significance, and the dependence on the concentration of phosphorus is high. It is shown that during the entire annual cycle, the concentrations of nitrites, nitrates and ammonium, as well as mineral phosphorus, change, but remain within the limits that do not lead to hypereutrophication of waters. A high ecological significance of atmospheric precipitation was revealed: the associated increase in the concentration of PO4 caused a change in the mode of limiting the PPF from phosphorus to nitrogen. Using theoretical concepts, it has been substantiated that, under oligotrophic conditions, an increase in the concentration of the limiting PPP substrate in water also leads to an increase in the rate of its extraction from the environment in accordance with the negative feedback of natural regulation of ecosystem homeostasis. Under conditions of eutrophication, the influence of production processes on water conditioning by the factor of nutrients decreases.