| Summary: | Phagotrophic protists have been established as the major consumers of ocean primary production and as such occupy a pivotal position in pelagic food webs, yet knowledge gaps remain regarding the seasonal and spatial variability of protistan grazing, and of its drivers, both environmental and biotic. The aim of the present research was to address such gaps. To do so, I gathered field measurements and observations and evaluated modifications and alternatives to current methods used to estimate grazing rates and characterize plankton communities. In a study conducted in the Western Antarctic Peninsula aimed at quantifying the seasonal variability of protistan herbivory (Chapter 2), the magnitude of grazing rates measured during austral fall 2013 and austral spring 2014 did not vary with prey biomass. Despite contrasting levels of phytoplankton biomass assessed by chlorophyll a measurements (< 0.4 µg L-1 in the fall, up to 18.5 µg L-1 in the spring), grazing rates measured during austral fall (0-0.26 d-1 ) were as high or higher than rates measured during austral spring (0-0.1 d-1), and approximately half of the experiments in both seasons yielded no measurable grazing. Overall low grazing rates could not be explained by a lack of predators. Small cells dominated the austral fall phytoplankton community, and during austral spring, grazing was detected when the prey size-structure resembled fall conditions most, suggesting an association between detectable grazing and the dominance of small cells. These results indicate a lack of predators’ functional response in the WAP, which is contrary to the assumption made when describing zooplankton grazing in models. Instead, plankton population dynamics and ultimately phytoplankton biomass accumulation rates in the WAP region may be best predicted as a function of plankton community composition, emphasizing the importance of characterizing these communities concurrently while measuring rates of protistan herbivory. Results also underline the need to extend measurements for the global ocean to less productive seasons in order to verify whether the assumed enhancing effect of prey abundance on grazing rates is always observed in the field. Quantifying the variability of protistan grazing requires increasing the sampling resolution of grazing rate measurements, which is currently precluded by the sampling logistics associated with the standard multiple-dilution technique used to quantify grazing rates. In Chapter 3, I assessed an abbreviated version of the method that uses only two dilutions. I found that rate-estimates for either phytoplankton growth or grazer-induced mortality obtained using only two dilution levels did not substantially deviate from those obtained when using multiple dilutions, and that their accuracy was satisfactory and similar in magnitude to the inherent error associated with the dilution-series estimates (± ~0.1 d-1), supporting the usefulness of the abbreviated method. Routine characterization of phytoplankton communities in terms of their size structure and overall taxonomic composition is needed to decipher patterns of association between these characteristics and the level of grazing. In Chapter 4, I showed that a qualitative characterization of plankton populations could rapidly be achieved using the FlowCAM, an automated plankton imaging system. Expanding the spatial and temporal resolution of protistan grazing rate measurements and further investigating the factors influencing grazing magnitude, including plankton species composition, is essential to provide reliable parameters for plankton models, and to underpin the importance of phagotrophic protists in pelagic food webs.
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