Role of Nutrient Limitation in Structuring Phytoplankton Communities

Vivian Klotz, Barnard College, Environmental Science, New York, NY, United States, Benjamin Ramcharitar, Lamont-Doherty Earth Observatory, Columbia University, Division of Biology and Paleo Environment, Palisades, NY, United States, Erica Kelley Strope, Georgia Institute of Technology, School of Biological Sciences, Atlanta, GA, United States, Crystal Thomas, NASA Goddard Space Flight Center, Ocean Ecology Laboratory, Greenbelt, MD, United States, Joseph Montoya, School of Biological Sciences, Georgia Institute of Technology, Atlanta, GA, United States and Ajit Subramaniam, Columbia University, Lamont-Doherty Earth Observatory, Palisades, NY, United States
Phytoplankton require essential nutrients such as nitrate, phosphate, and silicate to grow, and thus, to understand the controls on phytoplankton biomass and community structure, we need to know the role of these macronutrients. We tested the hypothesis that the availability of these nutrients affects the health and composition of phytoplankton communities by conducting a series of nutrient amendment experiments in two bodies of water: the Western Tropical North Atlantic, influenced by the Amazon River plume, and the Northern Gulf of Mexico, influenced by the Mississippi River Plume.

Surface waters from various locations in the Western Tropical North Atlantic and the Northern Gulf of Mexico were spiked with 0.8 micromolar Phosphate (P), 8 micromolar Nitrate (N), and 8 micromolar Silicate (Si) in a full factorial experimental design—i.e. Control, +N, +P, +Si, +NP, +NSi, +PSi, +NPSi. The treatments were incubated for a total of 48 hours, with sampling occurring after 0, 12, 24, and 48 hours. At each time point, the treatments were sampled for nutrient concentrations, cell counts using flow cytometry, and chlorophyll and phycoerythrin fluorescence. Samples were also taken for High Performance Liquid Chromatography (HPLC) diagnostic pigment concentrations at the beginning and end of the incubations.

Preliminary results show that the phytoplankton communities in the Western Tropical North Atlantic are co-limited by nitrogen and phosphorus, with some populations benefitting from the addition of, though not being limited by, silica. Further, relative increase in chlorophyll fluorescence between each time point is dependent on initial community structure. The results of all the experiments will be presented in the context of hydrographic and ancillary biogeochemical measurements.