Microbial food webs in the dark: independence of Lake Plankton from recent algal production

We investigated the development of a heterotrophic plankton food web with or without phytoplankton primary production in a long-term (>1 yr) laboratory experiment. Water from 3 Swedish lakes (humic, oligotrophic clearwater, eutrophic) was exposed to low light or kept in total darkness in triplicate 100 l cylinders. Dissolved organic carbon (DOC) dynamics, bacterial growth and biomass of protozoans, rotifers and microcrustaceans were followed over 18 mo. In the dark treatments, no primary production was detected and DOC concentrations decreased by between 19 and 27% (1.3 to 3.2 mg C l–1). There was bacterial and protozoan growth in the dark during the whole experimental period. However, numbers and production of bacteria, as well as protozoan biomass, were significantly lower in darkness. Dissolved (DOM) and particulate organic matter (POM) initially present in the water (i.e. 18 mo old at the end of the experiment) helped to support substantial metazoan biomasses in dark treatments in the humic and eutrophic waters, but not in the oligotrophic clearwater lake. DOM in the humic water, thus largely of allochthonous origin, gave the highest and most prolonged support to zooplankton living in darkness. Our study indicates that a microbial food web, based on allochthonous organic matter and developing independently from phytoplankton, can act as a link to metazoan zooplankton, especially in oligotrophic humic lakes. These results confirm studies using stable C isotopes, showing a substantial incorporation of terrestrial carbon into zooplankton.