Heterocapsa rotundata is a dinoflagellate species that is known to form winter blooms in coastal and estuarine systems. Despite evidence that winter H. rotundata blooms are a common occurrence, there is a lack of laboratory and field-based research on the ecology of this species. My goal was to understand the impact these blooms had on the plankton food web and whether the winter blooms influenced the spring ecosystem. A majority of my research was done with water collected from the Choptank River, MD over the course of five winters. I conducted dilution and prey removal experiments to address the importance of top-down control to the formation of winter blooms. These experiments showed that appropriate environmental conditions are necessary for high H. rotundata growth, and that a bloom will not form unless zooplankton grazing pressure is reduced. I also used a combination of laboratory and field experiments to address whether mixotrophy helps H. rotundata bloom in winter, and my work showed that H. rotundata typically dominates winter blooms because they are a mixotrophic species that uses phagotrophy to overcome the light limitation of winter. I used cultures of Eurytemora carolleeae and H. rotundata to test if high H. rotundata abundances benefit E. carolleeae populations. I found that H. rotundata has no effect on E. carolleeae egg production rate or hatching success rate, but that they can increase the survival E. carolleeae nauplii. Ultimately, through the use of historical data and a temperature based E. carolleeae developmental model, I discovered that lower winter temperatures improve the potential for high annual recruitment of anadromous fish larvae hatched in spring in Chesapeake Bay. Winters with below average temperatures reduce the development rate of E. carolleeae nauplii hatched in winter, and H. rotundata blooms are likely to form and that increase the survival of E. carolleeae nauplii. The delayed development and increased survival causes distinct peaks in E. carolleeae populations later in spring, increasing the chance of high E. carolleeae abundance when fish larvae start feeding. Overall, my research has shown the winter temperature and plankton community can influence the spring ecosystem, specifically the survival and recruitment of anadromous fish larvae.