Linking nutrient loading and oxygen in the coastal ocean: A new global scale model

• Published in: Global biogeochemical cycles 2016-03, Vol.30 (3), p.447-459
• Main Authors: Reed, Daniel C., Harrison, John A.
• Format: Article
• Language: English
• Subjects: Aerobic respiration; Aquatic plants; Biochemical oxygen demand; Biogeochemistry; Bottom water; bottom water oxygen; coastal ocean; environmental change; eutrophication; Hypoxia; Marine ecosystems; Nutrient concentrations; Nutrient cycles; Nutrient dynamics; Nutrient loading; Organic matter; Oxygen; Oxygen demand; Oxygen depletion; Oxygen uptake; Sulfate reduction
• ISSN: 0886-6236; 1944-9224
• DOI: 10.1002/2015GB005303

Recent decades have witnessed an exponential spread of low‐oxygen regions in the coastal ocean due at least in‐part to enhanced terrestrial nutrient inputs. As oxygen deprivation is a major stressor on marine ecosystems, there is a great need to quantitatively link shifts in nutrient loading with changes in oxygen concentrations. To this end, we have developed and here describe, evaluate, and apply the Coastal Ocean Oxygen Linked to Benthic Exchange And Nutrient Supply (COOLBEANS) model, a first‐of‐its‐kind, spatially explicit (with 152 coastal segments) model, global model of coastal oxygen and nutrient dynamics. In COOLBEANS, benthic oxygen demand (BOD) is calculated using empirical models for aerobic respiration, iron reduction, and sulfate reduction, while oxygen supply is represented by a simple parameterization of exchange between surface and bottom waters. A nutrient cycling component translates shifts in riverine nutrient inputs into changes in organic matter delivery to sediments and, ultimately, oxygen uptake. Modeled BOD reproduces observations reasonably well (Nash‐Sutcliffe efficiency = 0.71), and estimates of exchange between surface and bottom waters correlate with stratification. The model examines sensitivity of bottom water oxygen to changes in nutrient inputs and vertical exchange between surface and bottom waters, highlighting the importance of this vertical exchange in defining the susceptibility of a system to oxygen depletion. These sensitivities along with estimated maximum hypoxic areas that are supported by present day nutrient loads are consistent with existing hypoxic regions. Sensitivities are put into context by applying historic changes in nitrogen loading observed in the Gulf of Mexico to the global coastal ocean, demonstrating that such loads would drive many systems anoxic or even sulfidic. Key Points A new global model quantitatively links nutrient loading to coastal hypoxia Coastal systems vary greatly in their sensitivity to nutrient loading Physical mixing defines susceptibility of systems to hypoxia