Plankton vary in size from less than a micron to over a meter (>6 orders of magnitude variability) in size and come from all major domains and many phyla. With this incredible taxonomic diversity comes a similar diversity in trophic modes and hence ecosystem structures. Biogeochemists tend to focus solely on the total primary productivity of an ecosystem or the proportion of primary productivity stemming from large phytoplankton. However, the productivity and efficiency of an ecosystem is often determined by a complex interplay between phytoplankton, grazing pressure, bacterial remineralization, and various biotic and abiotic particle packaging and disaggregating mechanisms. For instance, an ephemeral bloom of a single salp species can shift an oligotrophic system from an almost completely recycling regime to a high export regime. Similarly, top-down grazing pressure from krill swarms can terminate diatom bloom formation, leading to transitory increases in export, but an overall decrease in total export due to the decreased bloom formation. Meanwhile, particle aggregation and remineralization by particle-attached protists and bacteria may control fluxes into the deep ocean.

 

Unraveling these relationships is a complex task. My research involves the use of simple trophic models to synthesize in situ rate measurements. Recently, I have focused on the effects of alternate grazing pathways on ecosystem efficiency. In the California Current Ecosystem, Gulf of Mexico, and Costa Rica upwelling dome, my collaborators and I have simultaneously measured taxon-specific phytoplankton growth rates, losses to protozoans and mesozooplankton, and POC export. We can then estimate fecal pellet production - a potentially dominant component of sinking particles - if we assume a fixed efficiency for protozoans and that protozoan production is consumed by mesozooplankton. Such simple relationships can be a powerful hypothesis-testing tool when combined with in situ measurements, but are critically dependent on the efficiency of the protozoan community (which in turn depends on both gross growth efficiency of individual organisms and the number of trophic steps within the protozoa).

 

Plankton trophic dynamics are further complicated by the incredible overlap of different 'trophic levels'. For instance larval, crustaceans may be preyed upon by the ciliates that serve as food for their adult stages. Mixotrophic 'phytoplankton' can supplement their energy or nutrient requirements by preying upon smaller phytoplankton and bacteria - and in the process cull their competitors. Kleptoplastidic ciliates ingest phytoplankton in order to maintain their chloroplasts and become de facto phytoplankton themselves, while diatom-diazotroph assemblages harbor endosymbiotic prokaryotes capable of fixing dinitrogen for their hosts. These and many other examples highlight the complexity of the marine microbial community, which simultaneously makes determining biogeochemical functions difficult and serves as a fascinating laboratory for studying ecological interactions.