Nanoplankton and Microzooplankton grazing in the Northern Gulf of Mexico

Gulce Kurtay, University of Louisiana at Lafayette, Biology, Lafayette, LA, United States, Mrunmayee Pathare, Woods Hole Oceanographic Institution, Woods Hole, United States and Beth A Stauffer, University of Louisiana at Lafayette, Department of Biology, Lafayette, LA, United States
Grazing among protistan members of the plankton occurs across several size-based groups, thus complicating our ability to interpret the results of community grazing experiments in natural assemblages. Studies typically measure overall “microzooplankton” community grazing using prey mortality rates based on the dilution method; however, this method does not provide information about which groups or taxa contribute to the observed grazing mortality. To address this knowledge gap, we combined microzooplankton community (micro+nano) and nanoplankton community (nano) grazing experiments, based on the dilution methods. Three replicates each of 100% and 10% dilutions and control bottles (100%, no nutrients) were prepared for communities prescreened with 200 µm (micro+nano) and 20 µm (nano) mesh. T0 and TFinal samples were taken to measure fractionated chl-a biomass. Four in-situ experiments were conducted from May to June 2019 in two onshore; Vermilion Bay (VB) and Terrebonne Bay (TB) and two offshore; Mississippi (MSO) and Texas (TXO) stations to observe onshore-offshore and river-influenced differences in protistan grazing.

Onshore stations showed approximately ten-fold higher chl-a biomass compared to offshore stations; however, relative contributions of small (<5μm), medium (5-20μm), and large (20-200μm) size fractions did not vary considerably. Small size fractions were enhanced in surface samples (25μg/L) near the onshore river-influenced estuarine station in TB while an estuarine station to the west in VB showed much lower small phytoplankton biomass (3μg/L). Preliminary results of grazing experiments showed that the larger phytoplankton (> 20 µm) were more heavily grazed upon at the western estuarine station (VB), while medium phytoplankton (2-20 µm) were more heavily grazed at the other three stations. Finally, net grazing impacts (g/μ) on small- (0.16-1.2) and medium-sized (0.47-2.31) phytoplankton showed similarity between overall micro+nano and nano experiments. These results suggest that nanoplankton grazing on small and medium phytoplankton groups (i.e. < 20 µm) significantly contributes to mortality observed in “microzooplankton” community grazing experiments in estuarine and coastal waters of the northern Gulf of Mexico.