Assessment of Spatiotemporal Phytoplankton Composition in Relation to Environmental Conditions of Gorgan Bay, Iran

Phytoplankton seasonal succession and spatial distribution were studied in Gorgan Bay, Iran, which is a semi-enclosed bay located in the southeastern Caspian Sea. Monthly phytoplankton and physicochemical samples were taken from March 2014 to September 2015. A total of 51 genera from six algal divisions were observed with phylum Bacillariophyta being the most represented (16 genera and on average ~ 59% of biomass). While diatoms were prevalent year-round, strong inter-seasonal variations of other phytoplankton groups were observed in response to environmental parameters. The seasonal succession of other taxa started with Chlorophyta (Chlorella sp.) in spring followed by Cyanophyta (Anabaena sp. and Microsystis sp.) and Euglenophyta (Euglena sp.) in summer, then Chrysophyta (Syndera) and Pyrrophyta (Prorocentrum sp., Exuviella sp., and Peridinium sp.) in the autumn and winter. The sites near the mouth of Gorgan Bay were characterized by slightly higher salinity with an annual mean of ~ 12 PSU compared with ~ 11 PSU for inner regions of the bay. Nitrate was higher in the mouth of the bay as well, with an annual mean of ~ 1.4 ppm compared with ~ 0.80 ppm in the inner region. Orthophosphate was also higher in the mouth of the bay with an annual mean of ~ 0.30 ppm compared with ~ 0.23 ppm in inner part of the bay. Euglena sp. and Microsystis sp. performed better in the mouth of the bay compared with their performance in the inner regions. Nutrients and salinity correlated strongest with phytoplankton in Gorgan Bay. During the period when Cyanophyta biomass was maximum, Microsystis sp. was the dominant taxa of the cyanobacteria present. Inorganic pollution from domestic wastewater in the eastern part of Gorgan Bay appears to be the cause of the Euglena maximum. According to the Palmer pollution index, Gorgan Bay experienced pollution from agricultural discharge and domestic wastewater.