The importance of terrestrial carbon in plankton food webs

Abstract: Allochthonous substances, i.e. produced in terrestrial ecosystems, are known to fuel bacterial production in humic lakes. I observed that allochthonous carbon subsidizes bacteria even at high levels of phytoplankton biomass. In line with that, my results also show that dissolved organic carbon is largely constituted of terrestrial substances irrespective of lake characteristics. Since bacteria constitute a quantitatively important food source for lake zooplankton, zooplankton growth can be subsidized by allochthonous carbon. Moreover, in the absence of phytoplankton derived substances I observed bacteria to grow and sustain the microbial food web. This was demonstrated by increasing populations of copepods in the absence of any input of newly produced algal carbon after a period of 1.5 years. Photochemical breakdown of organic substances, affects utilization of detrital carbon by bacteria and consequently the contribution of terrestrial carbon to food webs in humic lakes. Increase of bacterial production following abiotic cleavage of dissolved organic matter stimulated biomass production of lake zooplankton. Stimulation of zooplankton may depend on a direct trophic coupling to bacteria and flagellates, which is probably stronger in rotifers and cladocerans than in copepods. Mixotrophic flagellates can feed large zooplankton as much as heterotrophic flagellates. The flux of allochthonous substances and photosynthetic production to zooplankton is strongly coupled when bacterivorous algae dominates the flagellated community. I observed that ingestion rates of bacteria by the mixotrophic chrysophyte Poterioochromonas malhamensis is as high as for the heterotrophic Spumella elongata, whereas photosynthesis plays an important role on P. malhamensis nutrition. I conclude that allochthonous energy sources have great ecological relevance in terms of zooplankton nutrition, not only in special circumstances found in humic lakes.