Release from limitation: Exploring phytoplankton nutrient limitation and community shifts in the Gulf of Mexico

The offshore surface waters of the Northern Gulf of Mexico are predominantly oligotrophic, though nutrients are supplied episodically by the offshore extension of the Mississippi River Plume as well as upwelling events associated with loop-current eddies. The many natural oil/gas seeps in the Northern Gulf can also enhance nutrient availability by promoting vertical mixing and the movement of water and nutrients across the thermocline and into the surface mixed layer. We carried out a series of microcosm experiments to explore nutrient limitation of native phytoplankton communities across a range of offshore habitats in the Northern Gulf of Mexico during a cruise in 2012. In brief, we amended surface seawater with nitrate, phosphate, and silicate at Redfield ratios in a factorial design experiment. We used a combination of flow cytometery, Aquatic Laser Fluorescence measurements, and nutrient measurements to quantify shifts in phytoplankton community abundance and composition, phytoplankton physiological state, and nutrient consumption over the course of a multi-day experiment. Biomass measurements showed co-limitation of phytoplankton by nitrate + phosphate, with clear responses from diatoms in our silicate amendment treatments. Time course analyses of pigment fluorescence reflected differential responses by different phytoplankton taxa, including cyanobacteria, cryptophytes, and diatoms.