CHANGES IN THE PLANKTONIC FOOD-WEB OF A NEW EXPERIMENTAL RESERVOIR

Changes in plankton community structure were examined in an experiment al reservoir (Lake 979) before and after impoundment. The role of allo chthonous organic matter in planktonic food webs is unclear, and reser voir creation can be viewed as an extreme manipulation of terrestrial organic matter inputs. After impoundment of Lake 979, concentrations o f phosphorus, nitrogen, and dissolved organic carbon increased as a re sult of decomposition of flooded terrestrial organic matter. In the fi rst year of impoundment, mean bacterial biomass increased 10x, and ind ividual bacterial cell volumes increased 2x over pre-flooding averages . Phytoplankton production and biomass decreased to approximately 25% of pre-flooding levels. Zooplankton biomass and production by Cladocer a increased 10x, and zooplankton community composition changed from do minance by small-sized Bosmina longirostris to dominance by large Daph nia rosea. In the first year of impoundment, production by Cladocera u sually exceeded phytoplankton C-14 productivity, suggesting that the m ain pathway of carbon flow to secondary producers shifted from an auto chthonous to an allochthonous base derived from flooded terrestrial ve getation. In the second year of flooding, bacterial biomass decreased and phytoplankton biomass was higher than in the two previous years of study.