Plankton Dynamics in a Temperate Estuary:Size Structure and Trophic Implications

Estuarine ecosystems provide goods and services to around 45% of the entire human population. In recent times, these ecosystems have been severely affected by multiple stressors such as pollution, eutrophication, harmful algal blooms, sea level rise, and, most importantly, climate change. Increasing temperature associated with climate change can have pervasive impacts throughout the ecosystem’s functions and processes. Plankton, a conspicuous member of estuarine food web, are among the first to respond to the changes in temperature due to their high sensitivity to changing environmental conditions. In this dissertation, I investigated the plankton dynamics in an estuarine ecosystem through a combination of zooplankton time-series studies and a mesocosm experiment. In the first chapter, I examined the long-term change of copepod community body size using a 40-year (1981-2020) time series, and the potential impact of changing temperature on the copepod community. I found that copepod community body-size has been increasing, especially in the recent decade (2011-2020), but the role of temperature is more complex to disentangle. My results suggest that a change in species composition is driving the overall change in body-size. In my second chapter, I examined the change in body-size in brown shrimp (Penaeus aztecus) post-larvae using a 36-year (1982-2017) time series, which revealed that the size of the brown shrimp post-larvae has not changed over that period. This investigation also showed that there were two distinct size classes of brown shrimp post-larvae that populate the estuary at different times of the year. Finally, in the third chapter, I examined the role of the copepod community in controlling phytoplankton population. By conducting mesocosm experiments during summer, winter and spring, I concluded that the copepod community does not exert top-down control on phytoplankton population even at 4x the natural copepod abundance.