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

Calbet, Albert, Michael R. Landry

Limnol. Oceanogr., 44(6), 1999, 1370-1380 | DOI: 10.4319/lo.1999.44.6.1370

ABSTRACT: The phytoplankton in warm oligotrophic regions of the open oceans is dominated by <2 µm cells too small for efficient direct consumption by mesozooplankton. However, these primary producers are hypothetically linked to higher trophic levels via the cascading impacts of mesozooplankton grazing on intermediate consumers. To assess the magnitudes of these indirect trophic linkages, grazing experiments, involving different concentration treatments of the mixed mesozooplankton community, were performed during cruises in the subtropical North Pacific at station ALOHA. Mesozooplankton fed on a diverse assemblage of microzooplankton and nanoheterotrophs <5 µm, and their predation indirectly enhanced net growth rates of phytoplankton and 2-5 µm heterotrophs. Increasing the concentration of mesozooplankton also enhanced growth rates of heterotrophic bacteria, but this was more likely the result of organic enrichment than trophic transfer. Scaled to their natural abundance, the indirect grazing impacts of mesozooplankton on lower trophic levels are small, accounting for <0.005 d-1 of the growth rates of each prey category examined. Thus, the larger consumers appear to exert little net influence on the dynamics at the base of the food web. In contrast, size-fraction manipulations of consumers between 2 and 20 µm (i.e., the nanozooplankton) 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 data for developing and validating ecosystem models of the open oceans.

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