Dissolved Organic Matter IN the Ocean (DOMINO)

Quantification and modeling of DOC and DON release in marine systems: a study of increasing trophic complexity

  • Funded by NSF-Biocomplexity. 9/02 to 8/07. $1,699,000.
  • Deborah A. Bronk (PI, VIMS), Walker O. Smith (Co-PI, VIMS), Deborah Steinberg (Co-PI, VIMS), David Malmquist (Co-PI, VIMS), Eric Wommack (Co-PI, University of Delaware), Craig Carlson (Co-PI, University of California at Santa Barbara), Raleigh Hood (Co-PI, University of Maryland, Horn Point Laboratory), and Dan Repeta (Co-PI, Woods Hole Oceanographic Institution)

The enormous size and reactivity of the pool of dissolved organic matter (DOM) in the oceans makes it a critical component of the global C cycle. The primary source of DOM, encompassing both dissolved organic carbon (DOC) and nitrogen (DON), in marine environments is phytoplankton production. Direct release by metabolically active phytoplankton can be substantial depending on the cell's physiological state. Viruses have been shown to release DOM during viral lysis, but the quantitative significance of this DOM source is unknown. DOM release by micro- and mesozooplankton ingestion and excretion is also a major source of DOM. This proposal sought to provide a conceptual and mechanistic understanding of DOC and DON production via the three processes of direct release, viral lysis, and zooplankton grazing.

In the field, we measured in situ rates of DOC and DON production in parallel experiments and then quantified the effect of grazing on these rates using classic grazer release and dilution studies. We used a modification of the multiplicity of infection technique combined with a dilution approach to tease apart release that results directly from phytoplankton with that mediated by viral lysis. We addressed the following questions as part of this research:

  • What processes are quantitatively most important in the release of DOC and DON in in situ direct release, release via viral lysis, and/or release via grazing?
  • Does the relative importance of the three mechanisms change under high nutrient conditions (spring and in recently upwelled water) relative to a low nutrient condition (summer and aged upwelled water)?
  • How is the qualitative composition of in situ DOC and DON modulated by taxonomic composition, growth rates of phytoplankton, viral lysis, and grazing?

In our modeling effort we formulated a mechanistic model to simulate the observed variability and fractionation of DOC and DON between LMW and HMW fractions and refractory and labile forms in the batch culture and field experiments. Focusing on systems with large biomass accumulations is ideal for the modeling because the factors that give rise to phytoplankton blooms and the consumer responses that follow are tractable problems. The culture and field components were designed to provide data on all the relevant processes.

Understanding the sources, composition, and mechanisms that drive DOM formation is critical, because DOM is the largest organic matter pool in the ocean, and small changes in marine DOM can potentially effect a large change in other pools (such as atmospheric CO2. In the past three decades tremendous advances have been made in our understanding of the cycling of organic matter in the surface ocean. We now have much more accurate estimates of primary productivity, vertical particle flux, and the role(s) of bacteria and viruses within the plankton. However, despite our knowledge of the abundance of viral particles, we have no quantitative appreciation of their role in DOM cycling. Similarly, we still lack a simultaneous assessment of the effects of grazers, bacteria, viruses, and phytoplankton on the pools of DOC and DON, as well as of the environmental factors that influence the biological components. DOMINO research provided the first simultaneous determination of three biological constraints on DOM production—direct release, viral lysis, and zooplankton grazing. This information will greatly improve our understanding of the controls of this pathway in the C cycle, an understanding essential to generating predictive models that rigorously include the role of DOM in elemental cycling in the ocean.