Development of a Coupled Ecosystem Model for the Coastal Ocean Environment

Many marine ecosystem models assume that nitrogen is the limiting nutrient for primary production and that nutrient components follow Redfield stoichiometry. Research focus is often on resolving physical-ecological dynamics in large-scale pelagic ocean settings where primary production depends on nitrogen transport into the surface euphotic layer. However, many ecosystem models cannot resolve dynamics in the shallow coastal ocean. Observations show that coastal waters are productive and include high concentration of phytoplankton and nutrients; however, many model simulations predict that oligotrophic coastal waters will have low phytoplankton concentration, even if the model contains forcing by river discharge with large nitrogen loading. In this study, we use Nutrient-Phytoplankton-Zooplankton-Detritus model (NPZD; Franks et al., 1986) with the Regional Ocean Modeling System (ROMS; Shchepetkin and McWilliams, 2005), and show that the shallow water problem is caused by bottom boundary conditions for ecological tracers that poorly resolve interactions between seabed sediments and water. An idealized coastal ocean is simulated with the coupled physical-ecological ecosystem model to resolve the shallow water region. Sensitivity experiments are conducted for various bottom boundary conditions. Results show that proper parameterization for sediment-water mass exchange and sediment denitrification process is required to resolve coastal ecosystem dynamics.