The Influence of Phytoplankton Size and Community Composition on Carbon Cycling and Planktonic Food Webs in the Sargasso Sea

The Sargasso Sea is a dynamic physical environment located in the western North Atlantic where strong seasonal variability combines with forcing by mesoscale (~100 km) eddies. These drivers determine nutrient, light, and temperature regimes, and ultimate the size, composition and productivity of the phytoplankton community. My general objective was to determine how the structure and function of planktonic communities affected carbon export from the surface ocean in the Sargasso Sea. On four cruises (2011 and 2012; one eddy per cruise), I investigated links between water column structure, plankton community composition, size, and primary production (PP). I then combined PP data with rates of zooplankton grazing, bacterial production, and carbon export into inverse food web models that reconstructed the major flows of carbon within the Sargasso Sea ecosystem. The major findings of my thesis were: (1) There were substantial effects of mesoscale and sub-mesoscale forcing on phytoplankton community composition: downwelling (in anticyclones) was associated with enhanced cyanobacteria abundances, while upwelling (in cyclones) resulted in enhanced eukaryote carbon biomass. (2) Contributions to phytoplankton biomass were not always proportional to total PP. The picophytoplankton (0.7 – 2 m) contributed 53% or more of total integrated biomass (as chlorophyll-a) and 46% or more of total PP; microphytoplankton (20 – 200 m) contributed only 22% of the biomass, but accounted for 38% of the PP. (3) Microbial pathways dominated carbon flows through our food webs at all times. Due to the relatively low abundance of large phytoplankton prey, the mesozooplankton consumed mostly microzooplankton (47 – 83% of their diet). The majority of carbon being exported from the ecosystem originated with the picophytoplankton via the microbial loop. The robustness of my models relied on our field data that characterized multiple food web interactions involving various plankton size classes taken from the same depths, and geographical locations. Ultimately, my findings emphasize the importance of considering the myriad export pathways of picoplankton carbon as the abundance of this size class is predicted to increase under future climate change scenarios