Effects of macrograzers and micrograzers on enclosed, in situ phytoplankton assemblages in a Newfoundland lake

Thatcher, Sarah Janice (1987) Effects of macrograzers and micrograzers on enclosed, in situ phytoplankton assemblages in a Newfoundland lake. Masters thesis, Memorial University of Newfoundland.

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Abstract

The effects of macrograzing and micrograzing on enclosed, in situ phytoplankton assemblages were examined in a Newfoundland lake. Chambers with inner and outer compartments, were deployed in Hogan's Pond during three experiments each lasting 13 days. Samples were collected every other day from both chamber compartments and the lake water. In two experiments, the macrograzer Diaptomus minutus (Copepoda) was initially added to the outer compartment of the grazed chamber, and in a third experiment Bosmina longispina (Cladocera) was added. Macrograzer metabolites were able to pass through fine gauze between the two compartments. Thus the chemical effects of the macrograzers were observed in the inner compartment while the physical effects were the dominant treatment in the outer compartment. Micrograzer effects were investigated in chambers without added macrograzers in all three experiments. -- Micrograzing effects, the physical effects of macrograzers, and the chemical effects of macrograzer metabolites on phytoplankton assemblages were evaluated. Grazing effects were examined in each experiment by a priori comparisons of phytoplankton densities between the chamber compartments and the lake water. -- Densities of some of the desmid species were depressed by the physical effects of macrograzers. Some individual taxa, such as Arthrodesmus triangularis var. rotundatus and Mesotaenium sp., were augmented under the chemical effects of the macrograzer metabolites. Physical effects of macrograzers were more marked than the micrograzer effects on the Chlorophyceae and the microflagellates. Micrograzers may affect the species composition and abundance of the chlorophycean taxa within phytoplankton assemblages. -- With the exceptions of Synedra sp. and Tabellaria fenestrata var. lacustris the diatoms were unaffected by the grazers. The Chrysophyceae were little affected by micrograzing and physical effects of macrograzers, in contrast to chemical effects of macrograzer metabolites that were evident for individual species. Several of the Cyanophyceae were augmented in the presence of micrograzers; some individual species, e.g., Microcystis aeruginosa, were augmented by both physical and chemical effects of macrograzers. -- Key words: Macrograzers; micrograzers; phytoplankton; assemblages; Newfoundland; lake.

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