Bacterial response to seasonal changes in primary production and phytoplankton biomass in Lake Constance

Abstract

Parameters characterizing bacterial biomass and metabolic activity are compared with phytoplankton biomass and daily primary production rates throughout the year. Between late March (before the onset of the phytoplankton spring bloom) and mid-July (diatom maximum), bacterial degradation of organic matter was more closely related to phytoplankton productivity than during the rest of the year. Bacterial production (as estimated by amino acid net uptake) was significantly correlated with concentrations of chlorophyll α and pheopigments. However, bacterial production was correlated less closely with primary production and only weakly with bacterial biomass. Bacterial biomass was also only weakly correlated with primary production but significantly with pheopigments. Numbers of active bacteria as estimated by autoradiography covaried closely with bacterial production and cell numbers. Whenever bacterial production was low, enhanced proportions of amino acids were respired. Oxygen consumption measurements showed that the size fraction <3 μm contributed 25–75% to total respiration. On average, bacterial biomass comprised 11 % of paniculate organic matter and roughly equalled phytoplankton biomass. During the growing season, bacterial production in the uppermost 20 m comprised about 20% of phytoplankton production with large seasonal fluctuations. A tentative carbon budget of the euphotic zone including primary production, zooplankton grazing, bacterial production and sedimentation is presented.