Interactions among members of the microbial loop in an estuary dominated by microzooplankton grazing

Abstract

Modified dilution experiments were employed to characterize the microbial loop in surface waters at sites along an estuarine-offshore gradient in the northern Gulf of Mexico. Estuarine surface waters were more variable than those offshore due to the strong influence of river discharge. Phytoplankton (chlorophyll a) and prokaryotes were both significantly higher in the estuary than offshore, but virus and heterotrophic nanoflagellate abundances did not differ across the gradient. 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, except for locations within strong environmental transition zones. A large, but variable, percentage of phytoplankton (83 ± 149%) and prokaryote (70 ± 64%) production was grazed by microzooplankton across the estuary. While microzooplankton grazing is responsible for the removal of the majority of microbial production, thus supporting productivity of higher trophic levels, the role of viral lysis remains unclear.