The dependence between bacterial production and environmental conditions in the Gulf of Gdańsk

Bacterial production, primary production and a number of other environmental factors were measured during six cruises in the Gulf of Gdańsk conducted in various seasons from 1995 to 2001. Bacterial production (BP) in the epipelagic layer ranged from 1.5% (April) to 80% (July) of the gross primary production (PP). Significant differences were observed between the BP/PP ratios in estuarine and open-water areas. The highest values were recorded in the coastal area and near the mouth of the river Vistula. It suggests that allochthonous organic matter has a great influence on BP. The correlations between particular parameters and regression analyses indicated that BP in the Gulf of Gdańsk depended on temperature, organic nitrogen concentration, PP, chlorophyll a concentration, organic phosphorus concentration, salinity and biochemical oxygen demand. Of all the independent variables, the temperature had the greatest impact on BP (R2 = 0.62). There was an inverse parabolic relationship between bacterial production and temperature. It appears that above a temperature of 12◦C bacterial production depended on substrates to a higher degree than on temperature. The negative correlation between BP and concentrations of mineral nitrogen and phosphorus in the annual cycle were probably due to an indirect dependence. A multiple regression equation, which included temperature and organic phosphorus concentrations, explained 78% of the variation in BP. Increasing BP resulted in an increasing biomass of bacterivorous nanoflagellates and of bacterivorous ciliates, which is indicative of bottom-up control in this segment of the trophic chain.