Within the content of this thesis, it was aimed to understand the changes in the biomass and abundance of heterotrophic bacteria and marine cyanobacteria Synechococcus in time with respect to multitude of ambient physical, chemical and biological factors. For this, monthly samples from discrete depths in the offshore (0-20-40-60-80-100-125-150-175-200 m) and in the near shore (surface and 10 m) stations over a period of one year in the Cilician Basin (eastern Mediterranean) were collected via rosette sampler. Epifluorescent microscope and the image analysis system were used to estimate abundance and biomass of both groups. Coastal station was more abundant and had much higher bacterial (heterotrophic bacteria) and cyanobacterial (Synechococcus) biomass than the offshore station as it receives substantial amount of freshwater from the nearby Lamas River throughout the year. The surface annual averages for bacterial and cyanobacterial abundance and biomass were 9600000 cells/ml - 56.5 microgram C/l and 400000 cells/ml - 24.1 microgram C/l, respectively, at the coastal station. The surface annual averages for bacterial and cyanobacterial abundance and biomass were 8100000 cells/ml &ndash / 49.1 microgram C/l and 210000 cells/ml &ndash / 10.6 microgram C/l, respectively, at the offshore station. Bacterial population always found to exceed Synechococcus abundance within the water column. In general, bacterial and cyanobacterial abundance and biomass tend to decrease with depth. On a seasonal basis, bacterial population was found excessively dominant at the surface or near-surface waters during the second half of the year. Synechococcus were also found more abundant during late summer and autumn. Temperature and nitrate concentration seemed to affect efficiently the abundance of both populations in the area. Based on Spearman Rank Correlation analysis, highly significant correlations between bacterial abundance as well as biomass and ambient temperature were observed at both stations. However, a significant correlation was found between Synechococcus and temperature only at the offshore station. Significant negative correlations are found between nitrate and bacterial abundance and biomass at both stations and between Synechococcus abundance and biomass only at the offshore station. At the offshore station, salinity was also found to be positively correlated with the bacterial and cyanobacterial abundance and biomass.