PLANKTONIC FOOD-CHAINS OF A HIGHLY HUMIC LAKE .1. A MESOCOSM EXPERIMENT DURING THE SPRING PRIMARY PRODUCTION MAXIMUM

The development and metabolism of the plankton of a highly humic lake were followed over the vernal primary production maximum. The study wa s made in a mesocosm in which large filter feeders, typical of this la ke in summer, were absent. During the rising phase of phytoplankton, t he community was predominantly autotrophic. The most important constit uents in the algal biomass were a dinoflagellate, Gymnodinium sp. (40- 50%), and a prasinophycean, Scourfieldia cordiformis (7%). The biomass es 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 t he beginning of the experiment, grazing of algae by phagotrophic proto zoans and phosphate depletion led to a rapid decrease of algal biomass and the community became predominantly heterotrophic. In spite of a l arge variation in algal biomass and primary production, the biomass of bacteria remained of the same order of magnitude as in algae both bef ore and after the algal maximum. Bacteria were mostly responsible for the plankton respiration, which also showed no dependence on primary p roduction. Since exudation by phytoplankton was also low, the nutritio n of bacterioplankton was probably mainly based on allochthonous disso lved organic matter rather than on primary production. Thus the produc tion of bacteria was an additional food source for higher trophic leve ls along with phytoplankton. Because filter feeding zooplankton was ab sent in the experiment, protozoans were the only grazers utilizing alg ae and bacteria. Essentially all growth of bacteria was used by bacter ivores.