Dynamics of picoplankton in microbial loop and their importance at pelajic food web in the South-Eastern Black Sea

In order to reveal structure of picoplankton and heterotrophic nanoflagellates, which are main contributors to microbial loop in the south-eastern Black Sea during October 2010-December 2011, samplings were carried out monthly intervals in three selected stations throughout the 200 m depths water column. Vertical distribution of picoplankton and heterotrophic nanoflagellates associated with environmental parameters and trophic interactions were studied in the field and laboratory with microcosm experiments.In terms of biomass and abundance, heterotrophic bacteria were dominant group at the surface waters. While the highest cell number was obtained as 5.7x109 cell l-1in June, the highest carbon biomass was detected as 120 µg C l-1at 30 m water depths.The mean heterotrophic cell volume exhibited an increase between sub-oxic and anoxic layers. The highest abundance and biomass values of Synechococcus spp. occurred during summer period within the photic depth and increased from coastal to offshore. The highest cell number and biomass were obtained as 33.7x107 cell l-1and 61.67 µg C l-1, respectively. Two blooms of picoplankton were observed at the study area and followed by enhanced heterotrophic nanoflagellates bloom and reached up to 5.16x107 cell l-1.According to grazing experiments, the growth and grazing rate of Synechococcus spp. was varied from 0.03 to 1.24 d-1 and 0.3 to 1.54 d-1, respectively. On the other hand, the growth and grazing rate of Heterotrophic bacteriawere varied from 0.13 to 1.11 d-1 and 0.3 to 1.41 d-1, respectively.Throughout the sampling period, microbial loop supported to food chain. While the food chain is controlled through top-down during December, January and March, it is controlled by substrat supply during the rest of the year.