Planktonic food chains of a highly humic lake

The development and metabolism of the plankton of a highly humic lake were followed over the vernal primary production maximum. The study was made in a mesocosm in which large filter feeders, typical of this lake in summer, were absent. During the rising phase of phytoplankton, the community was predominantly autotrophic. The most important constituents in the algal biomass were a dinoflagellate, Gymnodinium sp. (40–50%), and a prasinophycean, Scourfieldia cordiformis (7%). The biomasses of Chlamydomonas spp. and Chrysococcus spp. reached their maxima a few days later and Cryptomonas sp. became most abundant at the end of the experiment. After the phytoplankton maximum, about one week from the beginning ofthe experiment, grazing of algae by phagotrophic protozoans and phosphate depletion led to a rapid decrease of algal biomass and the community became predominantly heterotrophic. In spite of a large variation in algal biomass and primary production, the biomass of bacteria remained of the same order of magnitude as in algae both before and after the algal maximum. Bacteria were mostly responsible for the plankton respiration, which also showed no dependence on primary production. Since exudation by phytoplankton was also low, the nutrition of bacterioplankton was probably mainly based on allochthonous dissolved organic matter rather than or primary production. Thus the production of bacteria was an additional food source for higher trophic levels along with phytoplankton. Because filter feeding zooplankton was absent in the experiment, protozoans were the only grazers utilizing algae and bacteria. Essentially all growth of bacteria was used by bacterivores.