Biomass and trophic structure of the plankton community along a trophic gradient in the subtropical and temperate waters of the northwestern Pacific Ocean

Abstract

This study examined biomass structure of autotrophic and heterotrophic plankton along a trophic gradient in the northwestern Pacific Ocean in an attempt to understand planktonic food web structure. Heterotrophic biomass of other microbial constituents exceeded that of autotrophic organisms nearly in all sampling regions, with inverse relationships found between trophic status and the proportion of heterotrophic biomass relative to that of phytoplankton. However, mesozooplankton biomass increased faster than the rate of phytoplankton along the gradient, with the biomass increase being most associated with ciliate biomass, but to a lesser degree with large phytoplankton (>3 &#53446;). Zooplankton in the oligotrophic waters appears to depend more on microbial food web whereas large phytoplankton becomes more important as food in richer waters. Zooplankton grazing pressure on ciliates and heterotrophic dinoflagellates appeared to release both bacteria and picophytoplankton from predation by ciliates and heterotrophic dinoflagellates, but not for heterotrophic nanoflagellates and large phytoplankton. These results indicate meszooplankton can ingest various planktonic groups, but less likely bacteria and picophytoplankton. Smaller size and very low biomass (<5% of total biomass) of zooplankton in the oligotrophic waters and rapid increase of its biomass with the increase of food along trophic gradient suggest that the zooplankton in the oligotrophic region may be severely food-limited. Expansion of the oligotrophic conditions to the north due to continued sea surface warming may have significant ramifications on carbon sequestration capability by mesozooplankton in the region.