Marine plankton drive biological processes that exert fundamental controls on the functioning of marine ecosystems and the biosphere. Trophic interactions and feedbacks between plankton primary producers and their main predators influence food-web structure, drive biogeochemical cycles, and regulate climate.
Herbivorous Protists are a diverse group of unicellular plankton organisms, including ciliates, dinoflagellates (represented in this image), and other flagellates . Although most of them are no larger than 100 µm, these organisms have been identified as the main grazers of ocean primary production. As active herbivores, protist grazers exert a significant influence on the species composition, size structure, and abundance of phytoplankton, thereby affecting primary production and the flow of carbon in the ocean. As mediators of nutrient recycling, they are key agents of geochemical cycles and important constituents of the microbial loop. These organisms also are prey to meso- and macro-zooplankton such as copepods and krill, and thus can act as an important trophic link channeling otherwise unavailable primary production up the food web.
Quantifying the feeding rates of herbivorous protists and their impact on ocean primary producers is thus essential to understanding pelagic trophic linkages and biogeochemical cycles.
1- Assessing the role of herbivorous protists on the dynamics of Antarctic phytoplankton biomass: Research I conducted in Antarctic waters aimed to fill gaps regarding the seasonal variability of grazing, and to contribute to building a year-round baseline of protistan grazing rates in fjords lining the Western Antarctic Peninsula (WAP), . These sites of exchange between the cryosphere and the ocean may be particularly sensitive to the WAP region’s fast warming, and expanding the current knowledge of the seasonal dynamics of grazer-induced phytoplankton mortality is essential for observing potential climate-driven changes.
I have measured grazing rates in the WAP both in the late austral Fall and the late austral Spring, during two research cruises associated with the NSF funded S.T.R.E.S. project (https://krillcruise.wordpress.com). A publication of this work is currently under review.
In addition, I have performed perturbation experiments to understand the effect of rising temperature on herbivory rates.
2- Towards increasing resolution of protistan grazing rate measurements: The dilution method is the standard protocol to quantify phytoplankton grazing-mortality rates and has been key in developing an understanding of protistan-grazing impact on ocean primary production. Although the method’s extensive use has facilitated the acquisition of a global dataset, its laborious application hinders the sampling resolution needed to fill remaining knowledge gaps, including the vertical, geographical, and seasonal variability of the magnitude of the rates I measure. Through a thorough assessment of an abbreviated version of the method, I showed that the shorter method yielded reliable rates, and allows to redirect the sampling effort associated with the traditional method towards increasing the number of measurements made and thereby investigate the multiple climate-related factors, such as light and temperature, that drive grazing magnitude. Detailed results can be found in a paper published at L&O Methods.
3- Using an automated image particle analyzer (FlowCAM) to characterize temporal dynamics of the Narragansett Bay phytoplankton community:
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