Size-dependent carbon flow in the epipelagic food web of the Western Equatorial Pacific

The present study examines the complexity of plankton food-web structure related to size diversity in lower trophic levels of the Equatorial Pacific and its possible effects on community stability and food availability of mesozooplankton. Food-web interactions and carbon flows in the plankton community were estimated from size-dependent predation, carbon transfer between trophic levels and size–biomass measurements determined in 5 independent field investigations. The community was separated into 10 different functional groups, which consisted mainly of pico-, nano- and micro-phytoplankton, bacteria, heterotrophic nanoflagellates, heterotrophic dinoflagellates, ciliates, nauplii, copepods and chaetognaths. The major carbon flow on all 5 occasions was consistent with a general conceptual food-web model, even though the study region was subjected to different oceanographic conditions—warm pool in 1990 to 1993, equatorial divergence in 1994. The 5 food webs showed distinct differences in structure, dependent on the size composition within each functional group, suggesting potential inherent non-steady-state dynamics at the intra-functional group level. The apparent stability of food-web dynamics in the equatorial plankton community may be a consequence of complex food-web interaction and resulting fluctuations, the effects of which neutralize each other over time.