Ecological and toxicological interactions between zooplankton and phytoplankton in eutrophic lakes.

The present thesis is a multiscale study of the interactions between zooplankton and phytoplankton in eutrophic cyanobacteria-dominated lakes. The main objective of this study was to evaluate the effects of increasing trophy, following watershed disturbance (i.e, forest harvesting), on zooplankton community dynamics. An experimental approach, at several temporal and spatial scales, was used to evaluate the response of zooplankton communities to the increase in biomass of colonial and filamentous cyanobacteria species in lakes. Cyanobacterial blooms were experimentally induced in large enclosures by adding different amount of nutrients (N, P). The addition of nutrients produced different responses of the phytoplankton as a function of light climate, whereby cyanobacteria biomass increased in shallow enclosures but not in deep enclosures. The increase in the biomass of inedible algae produced a reduction in the size structure and the biomass of large zooplankters (i.e., daphnids). The same patterns were found in lakes which were subject to a major ecosystemic harvesting experiment (TROLS), where cladoceran size structure and dominance were reduced in favor of small zooplankters (i.e., copepods) after the increase in cyanobacteria biomass. An examination of the cyanobacteria-zooplankton interaction at fine scale, revealed that the decline in cladoceran size and biomass may be related to feeding inhibition caused by the increase in large colonial and filamentous cyanobacteria and other inedible algae. Although the response to the addition of purified microcystin-LR to the food of daphnids revealed the same inhibitory effects, our results point more toward feeding inhibition by colonies and filaments as a cause of the changes in the communities. On the other hand, the examination of the filtering appendages of daphnids exposed to large biomass of cyanobacteria biomass revealed an increase in the size and changes in the ultrastructure of those filtering appendages. These results suggest that daphnids have the ability to respond phenotypically to the changes in their resources as an adaptative strategy. However, the implications of these phenotypic changes on the whole community, on the long term, are yet to be elucidated. The examination of cyanobacteria-zooplankton interactions revealed that the response of zooplankton communities to changes in their resources, from edible to inedible algal particles, can be displayed at the individual, population or community levels as a function the duration of the perturbation. Understanding the nature of the response of biological communities (i.e., zooplankton) to perturbations such as eutrophication improve our knowledge of the ecosystems and help the implementation of better management strategies.
Anas Ghadouani, 2002
anas.ghadouani@umontreal.ca