Microzooplankton-induced mortality of protists in the Gulf of Mexico: across regions, along productivity gradients, and among functional groups

The Gulf of Mexico (GOM) is comprised of several diverse oceanographic regions, spanning the eutrophic waters off the Mississippi River delta to the oligotrophic seas in the western Gulf. Since 2017, we have conducted 25 microzooplankton grazing experiments (based on the dilution method) in coastal to deep waters throughout the GOM. Of these experiments, 13 conducted in the LA-TX shelf, Bay of Campeche, and Yucatan Channel yielded non-zero community microzooplankton grazing rates using apparent growth based on chlorophyll concentrations (8 onshore, 5 offshore). Many of these experiments also showed nonlinear relationships between dilution and apparent growth rate, suggesting grazer saturation and/or other food web interactions. There were no discernible differences in bulk grazing rates (g, d^-1) with salinity, temperature, starting phytoplankton biomass, or between onshore (< 100 m total depth) and offshore (> 100 m bottom depth) locations. However, the ratio of the microzooplankton grazing rate to the phytoplankton growth rate (g:µ) was significantly higher in the offshore experiments (mean: 0.818) than onshore (mean: 0.456), suggesting relatively stronger control of these prey communities in waters off the continental shelf. Notable exceptions occurred off the Texas coast in October 2017, following Hurricane Harvey when g:µ was high across all of the stations (mean: 0.547). Flow cytometric counts of protistan functional groups including picocyanobacteria (PC), picoeukaryotes (PE), autotrophic nanoplankton (AN), and heterotrophic nanoplankton (HN) were also used to calculate apparent growth and estimate group-specific mortality rates in the experiments in the northern GOM (TX-LA shelf). Grazing-induced mortality rates of HN and PC tended to be highest among the four groups in onshore and offshore waters, respectively. The drivers of this variable grazing-induced mortality across onshore-offshore gradients and within size-structured protistan and plankton food webs in the GOM will be discussed.