Sack Lunch Seminar (SLS)

Modeling Dissolved Organic Matter Cycling in Marine Surface Waters



Dissolved organic matter (DOM) constitutes one of the Earth’s largest reservoirs of bioreactive elements (C,N, P), yet despite decades of study the composition and cycling of DOM is poorly understood. As a result, most marine biogeochemical ecosystem models include highly simplified representations of DOM cycling, if at all. In order to better understand DOM cycling we constructed a model to simulate dissolved organic carbon (DOC) and nitrogen (DON) cycling in marine surface waters. Important features of the model are: (1) carbon and nitrogen are incorporated by means of a set of fixed and varying C:N ratios; (2) DOM is separated into labile, semi-labile, and refractory pools for both C and N; and (3) the production and consumption of DOM is treated in detail. Here we present three case studies that model DOM cycling in the context of: (1) a steady-state comparison of idealized oceanic, coastal, and estuarine ecosystems, (2) the seasonal cycle in eutrophic waters, and (3) a focus on the roles of viruses and microzooplankton. In all cases model runs are able to reproduce DOM and planktonic biomass concentrations and cycling rates that compare directly to or fall within ranges reported for these systems. We show that DOM cycling is intricately tied to the biomass concentration, ratio, and productivity of phytoplankton, zooplankton, viruses, and bacteria. The first case study highlights the importance of certain processes in each ecosystem. The second case study shows how DOM cycling, particularly the sources of DOM, changes seasonally and also suggests the potential importance of viral decay as a source of bioavailable DOM from within the bulk DOM pool. The third case study highlights differences in the top-down and bottom-up roles of viruses and microzooplankton and their subsequent effect on DOM cycling and trophic interactions. These results have important implications for understanding biogeochemical cycling in marine waters and can be used to guide future research endeavors.