Comparative simulations of dissolved organic matter cycling in idealized oceanic, coastal, and estuarine surface waters

In this paper we used a steady-state ecosystem model that simulates both dissolved organic carbon (DOC) and nitrogen (DON) cycling to study how the planktonic community structure, nutrient availability, and dissolved organic matter (DOM) loading affect these cycles in idealized oceanic, coastal, and estuarine surface waters. The model was able to reproduce DOM and planktonic biomass distributions, uptake rates, and production rates (including DOM) that fell within ranges reported for oceanic, coastal, and estuarine systems. Using a sensitivity analysis we show that DOM cycling was intricately tied to the biomass concentration, distribution, and productivity of plankton. The efficiency of nutrient remineralization and the availability of inflowing nutrients and DON also played a large role in DOM cycling. In these simulations the largest autochthonous source of DOC was always phytoplankton exudation while important sources of DON varied considerably. In the oceanic simulations heterotrophic bacteria were particularly important for mediating DOM cycling because they were the primary agents that controlled nutrient recycling and supply (i.e., strong bottom-up control). In contrast, in the estuarine simulations mortality (mainly from grazing and viral lysis) had the most influence on DOM production. However, DOM cycling was generally less dependent on interactions between plankton in the estuarine case because of high nutrient and DOM loading. The coastal simulations were somewhere in between. In all simulations competition between different size classes of phytoplankton also played an important role in DOM cycling.