Simulating sediment–water exchange of nutrients and oxygen: A comparative assessment of models against mesocosm observations

How to represent nutrient fluxes resulting from organic matter remineralization in sediments should be an important consideration when formulating a biogeochemical ocean model. Here representations ranging from simple parameterizations to vertically resolved diagenetic models are compared against a comprehensive, multi-year data set from a mesocosm eutrophication study. Observations of sediment–water fluxes of nutrients and oxygen and measurements of the state of the overlying water column were made over 2.5 years in nine mesocosms, six of which received geometrically increasing loads of inorganic nutrients. These observations are used here to force and optimize two simple parameterizations of sediment oxygen uptake, one representative two-layer diagenetic model and one representative multi-layer diagenetic model. In cross-validation experiments the predictive ability of these different representations is compared. The main results are that the optimized multi-layer model fits the observations best and also proved to be the most parsimonious, while the two-layer model failed the cross-validation indicating that it is prone to over-fitting and was less parsimonious even than one of the simpler functional oxygen flux models. We recommend that sediment models that are candidates for inclusion in a biogeochemical model be assessed through a process of optimization and cross-validation as we have done here.