Physical processes in formation and breakdown of hypoxia in a tropical bay

Researchers: G. Pawlak (MAE/SIO UCSD), K. Davis (UC Irvine), R. Collin (STRI), S. Giddings (SIO UCSD), A. Adelson (SIO UCSD), K.C. Clark (STRI)

Funding: National Science Foundation

Overview

Hypoxia is one of the most widespread deleterious anthropogenic influences on estuarine and coastal waters globally. The incidence of coastal hypoxia (< 30% oxygen saturation or < 2 mg/l) and anoxia (complete lack of oxygen) has risen over the last century as a result of increasing discharges of anthropogenically-derived nutrients and organic matter and climate-driven adjustments to regional physical forcing. Eutrophication and hypoxia have been primarily reported and studied in temperate coastal waters, but relatively little work has been done in shallow tropical waters, despite the important implications for tropical ecosystems which sustain important fisheries habitats and coastal biodiversity. Our ongoing work includes an observational campaign and numerical simulations that aim to address the knowledge gap in shallow tropical systems. We are presently carrying out a suite of physical measurements designed to resolve the physical processes throughout the evolution of a seasonal hypoxic event in Bahia Almirante, a tropical embayment in the Bocas del Toro region of Panama, that has experienced repeated, seasonal hypoxia that has caused widespread bleaching and mortality of corals and other invertebrates.  Further, we are developing a hydrodynamic model of the region with a simplified oxygen model to test our dynamical understanding of these events.

Studies of eutrophication-induced hypoxia in temperate estuaries find that spatial and temporal patterns in the extent and severity of hypoxia cannot be explained by nutrient loading alone and that the physical processes controlling the ventilation of deep waters are of first-order importance.  The rarity of comprehensive observations of hypoxia events in tropical ecosystems points to a critical need for work to understand the physical dynamics which determine their impact on coastal ecosystems.  The physical processes regulating the ventilation of low-oxygen bottom waters in temperature coastal regions include inflow of fresh water and atmospheric fluxes, which affect density stratification, advection, and wind, wave, and tidal-driven turbulent mixing. A better understanding of the physical mechanisms and environmental conditions responsible for oxygen variability in tropical bays will yield rich insight into benthic ecology; turbulent mixing; cross-shelf exchange of nutrients, pollutants, and low DO/pH waters; larval connectivity; coastal hypoxia; and ocean acidification.

Observational array for September 2019-November 2020 in Bahia Almirante, Panama.  Stations for November 2020 to September 2021 are the same apart from removal of Sol, BBP moorings and BB ADCP.