Mesozooplankton influences on the microbial food web: Direct and indirect trophic interactions in the oligotrophic open ocean

The phytoplankton in warm oligotrophic regions of the open oceans is domina ted by <2-mu m cells too small for efficient direct consumption by mesozoop lankton. However, these primary producers are hypothetically linked to high er trophic levels via the cascading impacts of mesozooplankton grazing on i ntermediate consumers. To assess the magnitudes of these indirect trophic l inkages, grazing experiments, involving different concentration treatments of the mixed mesozooplankton community, were performed during cruises in th e subtropical North Pacific at station ALOHA. Mesozooplankton fed on a dive rse assemblage of microzooplankton and nanoheterotrophs >5 mu m, and their predation indirectly enhanced net growth rates of phytoplankton and 2-5-mu m heterotrophs. Increasing the concentration of mesozooplankton also enhanc ed growth rates of heterotrophic bacteria, but this was more likely the res ult of organic enrichment than trophic transfer. Scaled to their natural ab undance, the indirect grazing impacts of mesozooplankton on lower trophic l evels are small, accounting for <0.005 d(-1) of the growth rates of each pr ey category examined. Thus, the larger consumers appear to exert little net influence on the dynamics at the base of the food web. Ln contrast, size-f raction manipulations of consumers between 2 and 20 mu m (i.e., the nanozoo plankton) elicited strong responses among bacterial populations indicative of tightly coupled predatory chain of at least two steps. Given the present results, detailed studies of the interactions among pico- and nanoplankton appear to be the most profitable avenue for improving our understanding of community structure and function in this region and for acquiring useful d ata for developing and validating ecosystem models of the open oceans.