The early stage of mucilage formation in the Marmara Sea during spring 2021

Abstract

In the present study, phytoplankton composition in relation to mucilage and accompanied physicochemical conditions were investigated in the sea water samples collected from four stations in İzmit-Kocaeli, Fatih-İstanbul, Biga-Çanakkale (in the Marmara Sea), and Rivaİstanbul (in the Black Sea). Samplings were carried out before (i.e., during September 2020) and at the beginning stage of the mucilage formations (i.e., during April 2021) which was observed in the Marmara Sea during spring and summer months of 2021. Seawater temperature, salinity, pH, and dissolved oxygen levels were measured in situ. Inorganic nitrogen, orthophosphate, and chlorophyll-a concentrations were analyzed spectrophotometrically. In total 83 phytoplankton species (44 Bacillariophyceae-BAC, 36 Dinophyceae-DIN, 2 Dictyochophyceae-DIC, and 1 Chlorophyceae-CHL) were identified during the study. BAC species were generally dominant. Among the 83 phytoplankton taxa identified, Cerataulina pelagica, Cylindrotheca closterium, Pseudo-nitzschia sp., Skeletonema costatum, Thalassiosira rotula, Alexandrium tamarense, Dinophysis acuminata, Dinophysis caudata, Dinophysis fortii, Gonyaulax fragilis, Gonyaulax spinifera, Gymnodinium sp., Gyrodinium sp., and Prorocentrum micans are known as the species associated with mucilage formation. The highest number of species was determined at the Biga-Çanakkale station (MDG25) with 38 taxa in September 2020.Skeletonema costatum (11200 cells/L) was the most abundant species at 10 m depth in the İzmit-Kocaeli station (MDG22) in April 2021. The mucilage formations in MDG22, and MDG25 stations were observed at their beginning stage as a tule curtain form in April 2021 while the seawater temperature and salinity levels were below 15°C and around 30‰, respectively. While the taxa dominance of the BAC in the total phytoplankton remained similar from September to April (i.e., 56%), its number of species decreased dramatically from 59 to 28 in the Marmara Sea. Despite the decreasing biodiversity, there was an increase in total abundance in the sampling stations, particularly in MDG22 and MDG25. In both stations, total abundance increased from 12000 to 42000, and 2000 to 50000 cells/L, respectively, compared to September 2020 and April 2021. Despite the decrease in biodiversity, an increase in the number of cells suggests that eutrophication and, as a result, mucilage formation occurred in the Marmara Sea. TRIX values showed that while the eutrophic conditions increased, the ecological quality in the region generally decreased from September 2020 to April 2021, and it was described as Bad for all stations in the Marmara Sea. The data indicated that the limiting factor for phytoplankton was nitrogen during April 2021 simultaneously with the mucilage formation in the Marmara Sea.