Planktonic food web structure at a coastal time-series site: I. Partitioning of microbial abundances and carbon biomass

Biogeochemistry in marine plankton communities is strongly influenced by the activities of microbial species. Understanding the composition and dynamics of these assemblages is essential for modeling emergent community-level processes, yet few studies have examined all of the biological assemblages present in the plankton, and benchmark data of this sort from time-series studies are rare. Abundance and biomass of the entire microbial assemblage and mesozooplankton ( > 200 mu m) were determined vertically, monthly and seasonally over a 3-year period at a coastal time-series station in the San Pedro Basin off the southwestern coast of the USA. All compartments of the planktonic community were enumerated (viruses in the femtoplankton size range [0.02-0.2 mu m], bacteria + archaea and cyanobacteria in the picoplankton size range [0.2-2.0 mu m], phototrophic and heterotrophic protists in the nanoplanktonic [2-20 mu m] and microplanktonic [20-200 mu m] size ranges, and mesozooplankton [ > 200 mu m]. Carbon biomass of each category was estimated using standard conversion factors. Plankton abundances varied over seven orders of magnitude across all categories, and total carbon biomass averaged approximately 60 mu g C l(-1) in surface waters of the 890 m water column over the study period. Bacteria + archaea comprised the single largest component of biomass ( > 1/3 of the total), with the sum of phototrophic protistan biomass making up a similar proportion. Temporal variability at this subtropical station was not dramatic. Monthly depth-specific and depth-integrated biomass varied 2-fold at the station, while seasonal variances were generally < 50%. This study provides benchmark information for investigating long-term environmental forcing on the composition and dynamics of the microbes that dominate food web structure and function at this coastal observatory.