Hydrochemical conditions and phytoplankton in the Kievka estuary (Japan Sea) with calculation of primary production by phosphorus utilization

Spatial distribution of chemical parameters, as concentration of dissolved inorganic phosphorus (DIP), inorganic silica, nitrite nitrogen, nitrate nitrogen, ammonium nitrogen, total iron, dissolved oxygen, and biochemical oxygen demand (BOD5) is considered together with species composition, abundance and biomass of phytoplankton on the data of complex survey conducted in the estuary of the Kievka River in May 2020, with special attention to DIP and phytoplankton variations along the salinity gradient in the zone of mixing fresh and saline waters. In the lower Kievka, phytoplankton was studied for the first time. The phytoplankon biomass varied from 0.05 to 0.21 g/m3, proportional to changes in primary production calculated from the DIP balance in the river water moving downstream through the estuary (from 0.02 to 0.23 g/m3km). The highest production and the highest biomass were found in the upper part of the external estuary. The water of Kievka River was initially poor in nutrients, so primary production in its estuary was supported by DIP regenerated in the process of organic matter mineralization in the internal estuary.