Riding the Ferrous Wheel: Mapping Marine Iron Remineralization After a Phytoplankton Bloom

Adrienne Hollister, Kristen Buck

University of South Florida, College of Marine Science

Phytoplankton are a vital part of the ocean food web and global environment, and are responsible for roughly half of our planet’s photosynthetic carbon fixation. Iron and nitrogen are both critical nutrients for phytoplankton and can limit growth in large regions of the surface ocean. Correspondingly, the processes that govern the concentration distributions of these elements are exceedingly important for understanding marine ecosystems. In the water column, phytoplankton deplete nutrients at the surface where they photosynthesize, then release those nutrients as they decay at depth in a process called remineralization. Remineralization is thus a critical aspect of marine nutrient cycling, governing nutrient concentrations at depth and determining how deep the ocean must be mixed to resupply the surface with sufficient nutrients to support new phytoplankton growth. However, compared to the relatively well-known process of nitrogen remineralization (i.e. release of nitrogen from decaying phytoplankton in the water column), the timing and speciation of iron remineralization remains poorly understood. Using natural populations of phytoplankton gathered in oligotrophic Gulf of Mexico waters, I induced a controlled phytoplankton bloom by spiking with iron and macronutrients. I then terminated the bloom by removing all light and monitored the release of macronutrients (ammonium, nitrite, nitrate, phosphate, and silicate), iron, and other trace metals (Zn, Cu, Mn, Co, Ni, and Cd). By monitoring iron remineralization in a controlled environment, my research measures the timing and chemistry of this crucial process in ocean ecosystem dynamics.