UC San Diego

Tropical regions are understudied relative to their temperate counterparts, and there is inadequate data to elucidate the dynamics and function of tropical estuarine systems. Reducing the scientific bias in the understanding of estuarine dynamics is important because coastal processes in the tropics can differ from higher latitudes. Dissolved oxygen (DO) is a critically important ecological variable, and the prevalence of hypoxia is expected to increase due to the combined effects of ocean warming and eutrophication; however, there are still fundamental knowledge gaps in the role that low-DO levels and multiple stressors play in ecosystem health, and dead zones in the tropics are likely severely under-reported.

This dissertation examines the physical processes that are important in regulating hypoxia and temperature inversions in Bahía Almirante, a multi-inlet tropical estuary on Panamá's Caribbean coast, which has experienced documented hypoxia and warming events resulting in coral bleaching and die offs. We use a 10-year record of observations at seven locations in Bahía Almirante to identify seasonal temperature inversions and hypoxia at depth that often co-occur. DO reductions correspond to periods with high freshwater input, resulting in strong salinity stratification that isolates bottom waters, allowing biological oxygen demand to draw down DO.

We conducted a two-year intensive study from September 2019-2021 and find that an alongshore pressure gradient drives net flow through Bahía Almirante, and the depth-mean velocity measured in the channel correlates to an along-coast current on the shelf. Water properties indicate hydrographically distinct sub-basins, with a largely isolated inner bay, even during periods of high net flow conditions. In the inner bay, DO at depth drops to hypoxic levels seasonally. Reoxygenation occurs with inflow at depth and inflow density that exceeds the inner bay bottom layer density, sharing a mechanistic similarity to deep water renewal commonly observed in fjords. In Bahía Almirante, we show that warming of the isolated bottom water mass via radiative heating significantly contributes to density reduction, a novel mechanism for density reduction in deep water renewal dynamics. Hypoxia seasonality is attributed to variability in offshore density and stratification that is associated with annual variations in precipitation.