Seasonal distribution of pikoplankton (Heterotrofic bacteria, Synechococcus and Prochlorococcus) in the surface water and water column of Goksu river discharge area (Northeast Mediterranean)

Although the Northeast Mediterranean (NEM) is known as the most oligotrophic sea in the world, wide shelf basin are fed by nutrient rich rivers which lead to the highly eutrophic conditions. For that reason, understanding the effect of nutrients salts carried by rivers on the primary producer groups is significantly important for the determination of biological dynamics in the region. In the first part of this study, the amount of nutrient salt transported from Göksu River to the Mediterranean was determined and effect of Göksu River on the primary production was investigated by using satellite images. The annual NO2, NO3, NH4 and PO4 load carried by Göksu River to NEM was found as 33 tons/year, 2294 tons/year, 887 tons /year and 342 tons/year, respectively. Satellite images showed that the average chlorophyll-a (chl-a) concentration of was decreased from 0.25 mg/m3 to 0.10 mg/m3 indicating 2.5 folds decrease from river discharge area to offshore waters. When the relationship between nutrient load and the surface chl-a concentration was examined, PO4 load was found as the main factor affecting primary production in the region. On the other hand, there was no direct relationship detected between NO2, NO3, NH4 loads and surface chl-a concentration. In the second part of the thesis study, the distribution of picoplankton, which is the main contributing group to the primary production in the Northeast Mediterranean, has been analyzed seasonally both on the surface water and in the water column. Seasonal monitoring studies were conducted at 50 different stations covering Mersin Bay, Göksu River impact area and oligotrophic offshore waters. Minimum and maximum pico-eukaryotes, Synechococcus, Prochlorococcus, heterotrophic bacteria abundance (cell / mL) in the sea surface were found as 1-1.2x103, 0.6x103-9.7x104, 8x102-7.3x104 ve 3x104-58x104, respectively. Pico-eukaryotes abundance found to be highest at the Göksu River discharge area and their annual abundance was decreased by 1.4 folds from inshore to offshore waters. On the other hand, Synechococcus which is the most dominant group in photosynthetic picoplankton was increased by 1.2 folds from inshore to offshore waters. The average Prochlorococcus abundance was increased by 4 folds from the river impacted area to offshore area. The average abundance of heterotrofic bacteria was decreased by 1.5 folds from coastal areas to open water. Heterotrofic bacteria found as the most dominant group in the picoplankton composition, and the ratio of their abundance to the photosynthetic picoplankton has reached up to 35 times. While the abundance of Synechococcus, Prochlorococcus and bacteria increased in deep chlorophyll maximum (DKM) in October and July when stratification was effective, the pico-eukaryotes abundance was generally decreased with depth. As a result of principal component analysis, the primary factors affecting the seasonal variation of picoplankton abundance and composition were determined as temperature, NO2 + NO3 concentration, Si concentration and depth. Similarly, primary factors explaining the regional variation were determined as salinity, PO4 concentration, NO2 + NO3 concentration, Si concentration and depth.