Presenter: Pierre Marrec (University of Rhode Island)

Description:
Identifying the factors that drive phytoplankton growth and protist grazing rates is essential for quantifying the flow of carbon in a changing coastal ocean. However, phytoplankton grazing by microzooplankton is often poorly constrained, particularly in dynamic coastal systems. Within the framework of the Northeast US Shelf (NES) Long-Term Ecological Research (LTER) project, we investigated plankton population dynamics by measuring phytoplankton growth rates, primary production and microzooplankton grazing rates. Based on a unique 4-year dataset, robust spatial and seasonal patterns in terms of plankton population dynamics, primary production, and food web structure were identified. Abiotic (temperature and light) and biotic (Chl-a and % of large phytoplankton cells) factors were significantly correlated with phytoplankton growth and primary production on seasonal or annual time scales, while none of these environmental conditions predicted microzooplankton grazing. During our time series sampling, disturbances occurred in the seasonal pattern of planktonic food web dynamics over the NES. The low trophic transfer efficiency typically observed in summer was documented in winter, and was associated with exceptionally high surface water temperatures and salinity, with phytoplankton community structure dominated by small cells. In summer, unusually high trophic transfer efficiency, a winter seasonal pattern, was associated with an intense bloom of large phytoplankton in mid-shelf waters. We are using these seasonal disturbances to investigate whether biotic characteristics, such as phytoplankton size structure or microzooplankton species composition, are significant drivers of carbon flow through the plankton food web. Our results enable us to generally predict seasonal fluctuations in planktonic food web structure and function and hold promise for understanding how disturbances and climate change might affect carbon flow in the future.