High abundances and negligible grazing during winter by the mixotrophic chrysophyte Dinobryon

Grazing on bacteria by planktonic organisms is a channel for the biological transfer of organic matter through the aquatic food web. Much attention has been given to the importance of heterotrophic nanoflagellates and ciliates as primary grazers of bacteria. However, the prevalence of phagotrophy by phytoplankton across many environments emphasizes a need to include mixotrophy in studies of the protistan food web. Few studies have addressed the seasonal dynamics nor grazing activity of mixotrophs in freshwater environments, especially those that extend below surface waters. The goal of this work was to examine temporal patterns in mixotroph abundance and taxon–specific bacterivory, with a focus on Dinobryon. Results shown here support our general predictions of increased bacterivory by Dinobryon where a low light and nutrient environment may promote grazing. Dinobryon spp. were numerically dominant members of the community under–ice in winter, possibly as a result of their ability to supplement photosynthesis with phagotrophy of bacteria in conditions of reduced irradiance and day length. However, daily grazing rate and associated impact on the bacterial community during winter were not substantial. These results highlight the importance of including winter sampling when an active community of phagotrophic phytoflagellates may play a major role in ecosystem functioning. Results underscore the importance of including measurements of grazing activity with mixotroph occurrences, as high abundance of Dinobryon did not align with high rates of bacterivory.