Spatiotemporal extent of diel and episodic hypoxia in bottom water of a shallow, well-mixed estuary

Globally, low dissolved oxygen levels are among the most pressing ecological issues facing coastal oceans and estuaries. Yet, relatively few data are available to describe the spatiotemporal extent of hypoxia in shallow, well-mixed systems. In the Banana River Lagoon, a shallow, well-mixed, subtropical estuary within the Indian River Lagoon system in Florida, USA, hypoxia impacts fish populations, tourism and property values. It also contributes to feedback loops that sustain eutrophication by enhancing benthic fluxes and internal nutrient loading. A network of 80 dissolved oxygen monitoring stations was deployed to measure dissolved oxygen concentrations, diel ranges and the cumulative duration of hypoxia. Basin wide, monthly average concentrations and percent saturation in bottom water ranged from 4.2 ± 1.5 mg/L and 62 ± 23% in summer to 9.0 ± 1.3 mg/L and 102 ± 14% in winter, following temperature and light related patterns for solubility, photosynthetic production and bacterial metabolism. Concentrations in bottom water were, on average, 0.7 mg/L lower than those at mid-depths. Spatial patterns emerged, with concentrations on average 0.4 mg/L lower, with more frequent instances of daytime hypoxia in areas within 500 m of anaerobic deposits of fine-grained, organic-rich sediments. Diel ranges and the cumulative duration of hypoxia were lowest in areas with sandy substrates compared to areas with higher chlorophyll concentrations from either extensive macrophyte cover or hotspots of algal blooms. Wind stress at speeds less than 5 m/s, mixed oxygenated water downward and mitigated the development of bottom water hypoxia leading to overall higher DO percent saturation. As wind speed increased above 5 m/s, concentrations trended toward atmospheric equilibrium. Quantifying the spatiotemporal extent of hypoxia in a shallow, well-mixed system contextualizes nutrient cycles, providing information to inform sediment and internal nutrient flux calculations and to help resource managers target areas for restoration or remediation projects.