Spatial and temporal dynamics of the phytoplankton biomass in the surface layer of the Вlack sea

Abstract

The aim of the research was analyzing the spatial and temporal variability of phytoplankton biomass in the surface layer of the Black Sea over an 18-year period. To assess the influence of the main currents in the sea on the spatial and temporal dynamics of the biomass of phototrophic phytoplankton. The analysis of spatial seasonal changes in the phototrophic phytoplankton biomass was carried out using regular long-term chlorophyll concentration data obtained from satellite observations using SeaWiFS and MODIS-Aqua/Terra instruments for the period from 1998 to 2015 in the Black Sea. The role of macro and micro-circulations in the spatiotemporal variability of phytoplankton biomass is estimated. An increase in wind activity and a decrease in water temperature from October to March, which lead to an increase in the depth of the mixed upper layer and the intensity of the main synoptic circulations, become a significant factor that promote the winter and spring phytoplankton. «bloom» It was revealed that a decrease in the average water temperature in the cold season in the range from 8 to 7 °C for a duration of more than a month and a half in the deep-water zone leads to the intensive development of biomass in spring. It has been established that the average biomass of phytoplankton for 18 years in the western and eastern cyclonic cycles is 38.0 ± 17.8 mgC m^-3, 37.7 ± 16.8 mgC m^-3, respectively, in the Batumi anticyclone - 38.2 ± 18.0 mgC m^-3. The main Black Seaream current, as a rule, carries phytoplankton formed at the shelf zone along the coastline, and almost do not mix with the deep water. Since 2012, in the summer and early autumn, the phytoplankton biomass decreased on average 1.4 times in the deep sea, and 1.2-1.4 times in the continental slope of the western part of the sea, as well as almost along the entire shelf zone. In the Danube region, the phytoplankton biomass was on average several times higher than in the deep sea and increased by 35% compared to the initial level. In the cyclonic cycles, winter-spring phytoplankton «bloom» is observed on average for a month and a half. Intensive «bloom» in the region of the northwestern rivers flow, observed in May – June, extends to the Bosphorus, while in the cold season it can penetrate into the deep sea in the form of micro whirlwind. In the anticyclonic Batumi and Sevastopol eddies high biomass values ​​were noted in some months. The role of anthropogenic load is the most significant in the coastal zone. However, the influence of coastal waters on the deep sea is possible in late autumn and winter.

Keywords: phytoplankton biomass, synoptic circulation, Black Sea, spatial variability of phytoplankton biomass, water temperature, depth of the mixed layer