Grazing impacts of rotifer zooplankton over a cyanobacteria bloom cycle in Vancouver Lake, WA

Kathryn E Sweeney and Gretchen Rollwagen-Bollens, School of the Environment, Washington State University, WA, Stephanie Hampton, Center for Environmental Research, Education and Outreach, Washington State University, WA

Background/Question/Methods
Vancouver Lake in western Washington (USA) is one of many lakes characterized by annual harmful cyanobacteria blooms. Cyanobacteria are microscopic primary producers found naturally in freshwater and marine systems; however, when cyanobacteria grow rapidly to produce “blooms” they may also produce toxins which can be harmful to humans or other animals. Zooplankton grazers, including copepods and cladocerans (mesozooplankton), as well as rotifers, ciliates, and dinoflagellates (microzooplankton), consume cyanobacteria and other phytoplankton thereby influencing and possibly controlling, harmful bloom dynamics. Previous studies have shown copepod grazing to contribute to bloom formation in many freshwater lakes, including Vancouver Lake, but bloom decline to be largely driven by microzooplankton community grazing. While the trophic role of ciliates and other heterotrophic protists have been investigated in recent years, little is known about the influence of rotifer grazing on cyanobacteria, particularly in Vancouver Lake. Therefore, monthly from June to October 2019, two series of monthly feeding experiments were conducted: 1) incubation experiments with field-collected rotifers feeding upon the natural assemblage of phytoplankton from Vancouver Lake; and 2) concurrent dilution experiments to measure the grazing rates of the full microzooplankton community. These experiments captured the conditions prior to, during, and following a large cyanobacteria bloom.
Results/Conclusions
Results from incubations show that rotifers had a small but significant feeding effect on chlorophyll-a, carbon biomass, and abundance of phytoplankton during August (immediately following peak bloom) and October (end of bloom). In October, rotifers showed significantly higher clearance rate for some cells, particularly dinoflagellates, over other prey categories (p-value = 0.0169). However, the rotifers significantly consumed cyanobacteria cells at the highest rates (p-value < 0.0001), likely due to the over-abundance of cyanobacteria cells overall. Additionally, dilution experiments demonstrated that the entire microzooplankton community (including rotifers, ciliates and dinoflagellates) had a large grazing impact on chlorophyll-a biomass both before and after the bloom peak. These findings suggest that both rotifers and non-rotifer microzooplankton are responsible for suppressing cyanobacteria blooms during multiple bloom stages. The results of this study fill an important gap in our understanding of how zooplankton of all sizes may limit bloom magnitude and timing, and provide a new, clearer picture of the role of micrograzers. With the growing body of work examining the role of zooplankton grazing in this system, Vancouver Lake can be used as a model for the study of cyanobacteria bloom dynamics in shallow, temperate lakes.