Limnol. Oceanogr., 44(6), 1999, 1370–1380

The phytoplankton in warm oligotrophic regions of the open oceans is dominated by ,2-mm 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 mm, and their predation indirectly enhanced net growth rates of phytoplankton and 2–5-mm 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 d21 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 mm (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 picoand 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. Warm oligotrophic waters of the subtropical North Pacific support a complex planktonic community with pico-sized (0.2–2 mm) phytoplankton providing most of the primary production and combined autoand heterotrophic bacteria dominating community biomass (Campbell and Vaulot 1993; Letelier et al. 1993; Campbell et al. 1994, 1997). Such organisms are largely unavailable to direct utilization by the crustacean-dominated mesozooplankton because of size constraints on feeding mechanisms (Rassoulzadegan and Etienne 1981; Conover 1982; Berggreen et al. 1988; Hansen et al. 1994). Nonetheless, they are linked in principle to higher order animals by the cascading influences of mesozooplankton grazing on consumers of intermediate size (Sherr et al. 1986; Sherr and Sherr 1988; Wikner and Hagström 1988). The strong coupling of nonadjacent trophic levels via indirect cascade effects is a well-established phenomenon in freshwater systems, where the presence or absence of piscivorous fish can profoundly influence zooplankton community composition, phytoplankton abundance, water clarity, heat budgets, and mixing depth (Carpenter et al. 1985; McQueen et al. 1986; Kerfoot 1987; Mazumder et al. 1990;