Assessment of phytoplankton group composition in the Golden Horn Estuary (Sea of Marmara, Turkey) determined with pigments measured by HPLC-CHEMTAX analyses and microscopy

Phtytoplankton group composition determined by microscopy was compared with high performance liquid chromatography (HPLC) derived from pigment signatures in surface water samples taken bi-weekly and monthly between October 2018 and September 2019 in the Golden Horn Estuary (Sea of Marmara). A total of 80 eukaryotic phytoplankton taxa belonging to eight algal classes were identified in surface water during the study period. Forty-three taxa (54%) were diatoms, 29 taxa (36%) were dinoflagellates and eight taxa (10%) were other phytoflagellates. The average contribution of diatoms to total phytoplankton abundance decreased considerably (41 to 25%), while the average contribution of dinoflagellates and other phytoflagellates increased markedly (59 to 75%) from the lower to the middle estuary. Chlorophyll-a and seven other group-specific pigments, including fucoxanthin, peridinin, chlorophyll-c1 + c2, alloxanthin, 19′-hexanoyloxyfucoxanthin, 19′-butanoyloxyfucoxanthin and divinyl chlorophyll-a were identified in the study area. The relative contribution of the major phytoplankton groups to chlorophyll-a was estimated on three different initial ratio matrices by CHEMTAX. The results obtained were compared with those from microscopic examination. It was concluded that the CHEMTAX method was not accurate enough to characterize the phytoplankton community in the Golden Horn Estuary ecosystem and microscopic analysis was essential to determine the major contributing species to chlorophyll-a.

Changes in phytoplankton composition of the Golden Horn Estuary (Sea of Marmara) following remediation

Changes in phytoplankton composition of the Golden Horn Estuary were investigated following remediation, through seawater transfer from the Strait of Istanbul to the estuary. Average values of Secchi depth, salinity and dissolved oxygen increased during this study when compared with a previous study. The average number of species (S) and species diversity (H′) increased and they correlated positively with Secchi depth, salinity and dissolved oxygen. There was a similar phytoplankton group composition between this and a previous study, however, the species composition differed. A total of 127 taxa consisting of diatoms (66 taxa), dinoflagellates (49 taxa) and others (12 taxa) were identified. Abundance of dinoflagellates and phytoflagellates and their relative contribution to the total phytoplankton abundance increased during this study, however, the abundance of diatoms and their relative contribution decreased notably, as compared with the previous study. There was a significant positive correlation between salinity and dinoflagellates and phytoflagellates (P < 0.01), and also between Secchi depth and dinoflagellates (P < 0.01) in the upper estuary. Additionally, salinity and Secchi depth correlated positively with species diversity (H′) and number of species (S) (P < 0.01). The increase in water transparency probably contributed to the increase in abundance of dinoflagellates and phytoflagellates. The results revealed that water transparency was one of the most important factors affecting phytoplankton composition in the study area. Changes in some environmental conditions following seawater transfer appear to have changed phytoplankton composition. As a result, phytoplankton species were confirmed as a very good indicator of changed environmental conditions.

Variations in abundance and diversity of phytoplankton in the surface waters of the Golden Horn Estuary (Sea of Marmara)

The abundance pattern and species diversity of phytoplankton in the surface waters of the Golden Horn Estuary were investigated between August 2011 and July 2012 in relation to environmental factors. Seventy-eight phytoplankton taxa (38 diatoms, 30 dinoflagellates and 10 phytoflagellates) belonging to eight taxonomic classes were identified in bottle and net samples. Phytoplankton abundance increased in spring and summer (from March to August) and reached its highest (10,429 × 103 cells L−1) during the bloom of Heterosigma akashiwo (Raphidophyceae) in the middle and upper estuary in late May. In general, phytoplankton abundance was higher in the middle estuary indicating more suitable conditions for phytoplankton growth, while species richness and diversity was higher in the lower estuary. The most abundant species were Skeletonema marinoi and Thalassiosira sp. among diatoms; Scrippsiella trochoidea among dinoflagellates; Plagioselmis prolonga and Heterosigma akashiwo among phytoflagellates. Diatoms were more abundant in the lower and middle estuary, while dinoflagellates and phytoflagellates in the upper estuary. The main factors causing the spatio-temporal variation of phytoplankton in the study area were temperature, salinity, water transparency and nutrients. Water transparency in the upper estuary is mostly influenced by organic and inorganic matter carried by two streams. As a result, this area should be considered a potential risk area for future algal blooms.