The roles of predation, substrate supply and temperature in controlling bacterial abundance: interaction between spatial and seasonal scale

Abstract

     Bacterial and heterotrophic nanoflagellates (HNF) abundance, production, growth and grazing rates were studied monthly from January 1995 to December 1997 along the trophic gradient in the middle Adriatic Sea. The role of substrate supply (expressed as a slope of the linear regression of bacterial abundance on bacterial production), bacterivory and temperature in controlling bacterial abundance were studied with respect to the spatial (trophic) and seasonal scales. The most significant relationship between bacterial abundance and production was established in the oligotrophic area, and during the winter-spring period suggesting that substrate supply could be the most important factor in bacterial control. On the other hand, inconsistent relationship between bacterial abundance and productivity in the eutrophic area and during the summer-autumn period suggests conditions in wich substrate supply is above saturating level, or in which mortality factors such as bacterivory and viral lysis are very strong. Variability on the trophic and seasonal scales is shown by two ratios, bacterial production/bacterial biomass (P/B) and bacterial abundance/HNF abundance (B/HNF), which appeared to be good indicators of the control of bacterial dynamics. Variations of P/B ratio were more influenced by the trophic scale which explained 32% of the total variability in comparison to seasonal scale which explained 21%, whilst B/HNF ratio were more influenced by seasonal scale (71% of variability). Furthermore, both the scales interacted, and their interactions explained the significant part (31% and 15% of P/B and B/HNF ratios variability, respectively) of the observed variability in the spatial-temporal continuum. The degree of dependency of bacterial abundance on temperature varied over different trophic and temperature ranges. The control of bacterial abundance by temperature decreased from the eutrophic to the oligotrphic area, thus, control by temperature was negatively correlated with control by substrate supply. This suggests that in eutrophic area substrate set an upper limit for bacterial abundance, whilst other factors like temperature, predation pressure and/or viral lysis determine the level of realized abundance, particularly during summer. Furthermore, temperature was an important factor in controlling bacterial abundance during the nonsummer period (temperature < 20°C) when temperature and substrate supply acted synergistically. In contrast to trophic scale, on the seasonal scale the control of bacterial abundance by substrate supply was positively correlated with their control by temperature.