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Ecosystem modeling of coastal acidification and hypoxia and structural uncertainties in the representation of sediment-water exchanges
Citation:
Lehrter, J., L. Lowe, A. Laurent, K. Fennel, J. Pauer, D. Justic, L. Wang, D. Ko, B. Penta, B. Jarvis, R. Devereux, D. Beddick, AND W. Cai. Ecosystem modeling of coastal acidification and hypoxia and structural uncertainties in the representation of sediment-water exchanges. 2016 Ocean Sciences Meeting, New Orleans, LA, February 21 - 26, 2016.
Impact/Purpose:
Test of different sediment model structures in an ecosystem model of coastal acidification and hypoxia.
Description:
Numerical ecosystem models of coastal acidification (CA) and hypoxia have been developed to synthesize current scientific understanding and provide predictions for nutrient management and policy. However, there is not a scientific consensus about the structure of these models and model uncertainties are generally not well characterized. Here, we examine model structural uncertainties and knowledge gaps related to the role of sediment biogeochemistry in development of CA and hypoxia. We evaluate five sediment-water exchange model structures that range in complexity. From most to least complex, these structures include 1) a highly discretized (> 400 layers) method-of-lines sediment diagenesis model that represents the electron-accepting processes and calculates sediment-water exchanges, carbonate chemistry, and sediment burial, 2) a reduced resolution (3 layer) diagenesis model, 3) a sediment flux meta-model derived from a sediment-diagenesis model that relates sediment fluxes to modeled organic matter sedimentation rates and bottom-water concentrations of O2 and nutrients; 4) an instantaneous remineralization model based on modeled sedimentation of organic matter and reaction stoichiometry and 5) an empirical model developed from observed relationships of bottom-water O2 and nutrient concentration with sediment-water exchanges of O2, DIC, and nutrients. We evaluate the five sediment model structures in an application of the Coastal General Ecosystem Model (CGEM) to the northern Gulf of Mexico, which encompasses the major influence of the Mississippi River and where a relation between nutrients, CA, and hypoxia has been observed. The results of this investigation highlight tradeoffs between complex and simple models where the more complex model structures have more realistic lags and feedback responses with the water-column, but at a greater computational cost. Finally, we will present tradeoffs between computational cost and model skill.