Interactions between members of the microbial loop in an estuary dominated by microzooplankton grazing (2009-2011)

Quantifying the linkages between primary production and higher trophic levels is necessary to understand why particular regions can support high fisheries production. Modified dilution experiments were employed to characterize microbial communities in surface waters at four sites from within a bay to the shelf in the northern Gulf of Mexico (nGOM). Inshore surface waters were more variable than shelf surface waters due to the strong influence of river discharge. Phytoplankton (Chl a) and prokaryote biomass were both significantly higher inshore than on the shelf, with phytoplankton significantly higher than prokaryotes inshore. Virus and heterotrophic nanoflagellate abundances, however, did not differ between inshore and shelf waters. Samples were amended with nutrients (N + P) to examine the impact of nutrient limitation. Prokaryotes were nutrient limited in 14 (28%) of the experiments, while phytoplankton were nutrient limitated in 26 (52%) of the experiments. When phytoplankton were nutrient limited, prokaryote growth rates were significantly altered. A similar impact on phytoplankton growth rates occurred when prokaryotes were nutrient limited, suggesting that the two groups are in competition for resources. Grazing was detected in the majority of experiments, while viral lysis was only detected in 24% of phytoplankton and 12% of prokaryote experiments. Growth and grazing rates for both phytoplankton and prokaryotes were tightly coupled inshore and on the shelf, with significantly more phytoplankton and prokaryotes grazed inshore (average = 106% and 75%, respectively) than on the shelf (average = 55% and 57%). These findings indicate that surface waters across the estuary are highly productive, with microzooplankton grazing transferring the majority of the microbial production to higher trophic levels.