Zooplankton induced changes in dissolved free amino acids and in production rates of freshwater bacteria.

This study examined the importance of zooplankton in the flux of dissolved free amino acids (DFAA) in the water and into bacteria. DFAA release rates were followed in laboratory grazing experiments usingDaphnia galeata andEudiaptomus graciloides as grazers, andScenedesmus acutus andSynechococcus elongatus as food sources. Except for minor initial peaks, DFAAs were released continuously during the first 2 hours and made up 6-12% (in one experiment 50%) of the calculated ingestion rates. During three diel studies in lakes, effects of removal and increase of the density of zooplankton (>200μm) on the pools of DFAA as well as on the bacterial production were followed. During two of the diel studies, higher DFAA pools were measured when 3-4 times the natural zooplankton density was present, and in one study a minor increase also occurred in the bacterial production, compared with results from experiments without zooplankton and with a natural zooplankton density. The increase in bacterial growth coincided with a decline in DFAA. During the third study, neither DFAA nor the bacterial production changed significantly when the zooplankton density was increased 3 times. Removal of zooplankton, however, caused a decline in both DFAA and bacterial production. Our data suggest a close relationship between occurrence of zooplankton and release of DFAA, but the factors regulating the amount of DFAA released and its effect on bacterial growth are not yet understood.