Changing patterns of bacterial substrate decomposition in a eutrophication gradient

Abstract

Bacterial variables are expected to respond differently to eutrophication. This was investigated along the eutrophication gradient in the narrow Schlei fjord (northern Germany). Bacterial extracellular enzyme activities (peptidase, aand P-glucosidase, chitinase) were measured together with a large number of autotrophic and heterotrophic biological variables. Increases of values measured along the eutrophication gradient were generally higher for bacterial substrate uptake and growth than for bacterial counts and enzyme activities. Annual patterns of activities (per volume of water) obtained from stations with different degrees of eutrophication were clearly different from each other. In contrast, annual patterns of activities per bacterial cell at the stations with different degrees of eutrophication were not clearly different from each other, Indicating that they did not depend on eutrophication. Size fractionation of enzyme activities revealed that most of the peptidase activity was generally associated with free-living bacteria ( 3 pm particle size class (43 and 5 2 % , respectively). Free enzyme activities ( < O 2 pm, % of total) were generally higher in less eutrophic areas than in eutrophic areas. Enzyme activities associated with the bacterial size class were generally higher in eutrophic areas than in less eutrophic areas. It is concl.uded that particle hydrolysis In eutrophic waters is mainly affected by attached bacterial glucosidic and chitinolytic potentials, while peptides are the preferred substrates of free-living bacteria. To cover bacterial C demands for growth, exudation by phytoplankton had to be supplemented by bacterial substrate hydrolysis in eutrophic waters, which was, however, not as high as expected.