Spatial and temporal variation in the distribution and abundance of Synechococcus spp., picoeukaryotes, nanoeukaryotes and Chlorophyll-a in the Eastern Black Sea

The abundance and distribution of Synechococcus spp., picoeukaryotes, nanoeukaryotes, and chlorophyll- a , were studied through the water column (from 0 – 100 m depth) during four different seasons along the Eastern Black Sea coast. Based on annual average values, Synechococcus spp. were numerically dominant with an annual average of 3.40×10 4 cells mL -1 , ranging between 0.51 and 9.93×10 4 cells mL -1 , followed by picoeukaryotes with an annual average of 0.79×10 3 cells mL -1 , ranging from 0.05 to 3.93×10 3 cells mL -1 . The nanoeukaryotes were the least abundant group in the region, with an annual average of 3.26×10 2 cells mL -1 ranging between 0.12 and 17.98×10 2 cells mL -1 . The minimum and maximum values of Synechococcus spp. and picoeukaryotes were found at stations from Fatsaand Arhavi, while nanoeukaryotes had their minimum-maximum abundance at stations from Yomraand Ordu. Overall, the Synechococcus spp. seasonal cycle exhibited a bimodal distribution, with one peak in summer and the other in autumn. In contrast, picoeukaryote and nanoeukaryote seasonal cycles had unimodal distributions, with peaks during winter. The abundance of Synechococcus spp. was significantly negatively correlated with depth during winter, while picoeukaryotes tended to notably correlate with depth during autumn and winter ( p < 0.05). Contrary, the abundance of nanoeukaryotes was considerably negatively correlated with depth in all seasons. The chlorophyll- a showed a striking negative correlation with depth during spring and autumn. There were particularly positive correlations among Synechococcus spp., picoeukaryotes, nanoeukaryotes and chlorophyll- a , implying their coexistence. However, the picoeukaryotes were significantly negatively-correlated with nanoeukaryotes.