Nanoplankton and Microzooplankton grazing in the Northern Gulf of Mexico
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.