Assessing marine ecosystem model properties from ensemble calculations

Ensemble calculations with a coupled physical–biological model for the coastal Gulf of Alaska are used to characterize the impact of parameter selection on lower trophic level (LTL) ecosystem predictions for different physical and biological regimes. The ocean circulation component is an implementation of the Regional Ocean Modeling System (ROMS) and the LTL ecosystem component is a six-compartment Nutrient-Phytoplankton-Zooplankton-Detritus (NPZD) model with iron limitation. Ensemble statistics (mean, spread) are useful to relate uncertainty in simulated chlorophyll concentrations with expected error in observed chlorophyll concentrations. Information from individual ensemble members can also help identify sets of parameters that minimize model-data error for different sub-regions of the CGOA (e.g., shelf, basin) and over the entire domain. Furthermore, ensemble calculations based on parameter uncertainty are valuable for determining which biological processes fundamentally impact the spatial and temporal variability in LTL ecosystem model solutions, which represents a first step toward being able to specify spatially and temporally dependent distributions for model parameters. Overall, the ensemble approach provides useful insight on how spatial and temporal variability in model solutions relates to uncertainty in the parameterization of key biological processes, a well-known factor limiting the use of LTL ecosystem models to predict observed biological variability in the ocean.