Modeling dissolved oxygen dynamics and coastal hypoxia: a review

Abstract

Hypoxia occurs in marine ecosystems throughout the world, influences biogeochemi-cal cycles of elements and may have severe impacts on marine life. Hypoxia results from complex interactions between physical and biogeochemical processes, which can not be addressed by observations alone. In this paper, we review oxygen dynamical 5 models that have been applied in studies of factors controlling coastal hypoxia and in predictions of future states. We also identify scientific issues that need further development and point out some of the major challenges. Over recent decades, substantial progress has been made in the development of oxygen dynamical models. Considerable progress has been made towards the parameterization of biogeochemi-10 cal processes in the water column and sediments, such as the dynamic representation of nitrification-denitrification. Recent advances in three-dimensional coupled physical-ecological-biogeochemical models allow better representation of physical-biological interactions. Several types of modelling approaches, from simple to complex, have significantly contributed to improve our understanding of hypoxia. We discuss the applica-15 tions of these models to the study of the effects of oxygen depletion on biogeochemical cycles, links between nutrient enrichment and hypoxia development, impacts of hy-poxia on marine ecosystems and predictions of climate change responses. However, for some processes models are still crude. For example, current representations of organic matter transformations and remineralization are incomplete, as they are mostly 20 based on empirical parameterizations at few locations. For most of these processes, the availability of validation data has been a limiting factor in model development. Another gap is that, in virtually all nutrient load models, efforts have focused on nutrient utilization and organic matter degradation, whereas three-dimensional mixing and advection have been less well represented. Explicit inclusion of physical and biogeo-25 chemical processes in models will help us answer several important questions, such as those about the causes of the observed worldwide increase in hypoxic conditions, and future changes in the intensity and spread of coastal hypoxia. At the same time, recent 9196 quantitative model intercomparison studies suggest that the predictive ability of our models may be adversely affected by their increasing complexity, unless the models are properly constrained by observations.