High Resolution Net Ecosystem Metabolism in a Complex Estuary, Evaluated by Biogeochemical Modeling.

The net ecosystem metabolism (NEM) has been widely used to assess the metabolism of a body of water, indicating if it is net autotrophic (positive NEM) or net heterotrophic (negative NEM) by calculating the gross primary production minus community respiration in the system. Even though many techniques have been used to calculate NEM, it is extremely challenging in stratified, dynamic systems, especially at highly resolved temporal and spatial scales. Here we present the use of a biogeochemical model for the calculation of NEM in the Columbia River estuary (CRE), a river-dominated estuary located on the West Coast of the United States characterized by very short residence times. The Bioreactor Model developed for this study is an adaptation of the Spitz et al (2001) model to an estuarine ecosystem, and is coupled to the unstructured- grid finite element SELFE circulation model. It simulates two groups of phytoplankton (marine and freshwater), two groups of zooplankton (meso and micro), dissolved inorganic nutrients, oxygen and an explicit microbial loop. A simple benthos is coupled to the water column model. The results reveal that the water column of the CRE is net autotrophic during most of the year, while the respiration is concentrated in the benthos. When taking into account the water column and the benthos together, the CRE alternates temporally and spatially between net autotrophy and net heterotrophy. The hotspots for net heterotrophy are the estuarine turbidity maxima region, and the marine lateral bays, while net autotrophy is associated with freshwater. The river flow and concentration of chlorophyll in the river control the timing and location of the boundaries between net autotrophic and net heterotrophic regions. Overall, integrating over space and time, the CRE is, like most estuaries, a net heterotrophic system.