Vancouver Lake is a large, shallow lake located in the urbanized lower Columbia River floodplain in Vancouver, Washington. Seasonal noxious blooms of blue-green algae called cyanobacteria (Aphanizomenon flos-aquae and Anabaena flos-aquae) have been occurring in the lake for the past 20 years, mirroring a global trend. Controlling and/or eliminating these blooms is a high priority for lake managers. With support from the Vancouver Lake Watershed Partnership, we are investigating how cyanobacteria blooms may be influenced by biotic and abiotic factors, to assist in developing appropriate management strategies for this and other temperate, shallow lakes. We examined how microzooplankton (heterotrophic unicellular eukaryotic plankton <200 µm) grazing on cyanobacteria and other algae contributes to bloom formation and decline.

We conducted 16 dilution experiments between April 2008 and January 2009, using treatments with varying ratios of filtered lake water to unfiltered lake water (both with and without added nutrients) to determine the intrinsic growth rate of phytoplankton and the microzooplankton grazing rate.

Results/Conclusions

The bloom occurred in late August (peak chlorophyll was 498.5 µg/l).

Phytoplankton growth rate reached 1.2 d-1 pre-bloom, declined to 0.7 d-1 peak-bloom, then rose to 1.4 d-1 and fell to 0.9 d-1post-bloom. The grazing rate followed a similar trend, ranging from 0.3 d-1 (pre-bloom) to

0.6 d-1 (peak-bloom) to 1.1 d-1 and 0.8 d-1 (post-bloom).  Negative growth and grazing rates occurred mid-June – mid-July with an absence of grazing in January, which may have resulted from a trophic cascade or changes in the community composition. We analyze these results in the context of how the community composition changed.