Biogeochemical dynamics underlying equilibrium between nitrogen fixation and denitrification and its impact on a coastal marine ecosystem model

Many ecosystem models have chronic issues that result in unrealistic oligotrophic conditions in the shallow coastal regions. This problem is attributed to the poorly resolved bottom boundary condition for ecological tracers; detritus reaching the bottom boundary in shallow water escapes the model domain resulting in continuously decreasing total nitrogen levels. The scaling for the problem is determined by a vertical length scale wd/δ, where wd is detritus sinking speed and δ is remineralization rate. For shallow water depths h≪wd/δ corresponding to shallow marginal coastal ocean regions where loss of nitrogen is significant, ecosystem models predict unrealistic oligotrophic water masses. To alleviate the problem, the classical Nutrient-Phytoplankton-Zooplankton-Detritus (NPZD) model is expanded here to consider denitrification and nitrogen fixation. Internal dynamics of the expanded model are examined through steady-state solutions. The intensity of denitrification, scaled as the ratio between wd/δ and water depth, plays an important role sustaining nitrogen fixers by regulating phosphorus competition with normal phytoplankton. The theoretical equilibriums of the expanded model well represent characteristics of coastal and pelagic ecosystems. The expanded ecosystem model is then fully coupled with a numerical hydrodynamics model and compared with the classical NPZD model. It is shown that unrealistic oligotrophic conditions along the coast predicted by the classical NPZD model are not present when denitrification and nitrogen fixation processes are considered because strong denitrification (loss of nitrogen) in the shallow region enhances nitrogen fixation that compensates the loss.