UC Irvine

Oxygen levels in the global coastal ocean have drastically decreased in past decades with a surge in reported hypoxic events. In most coastal systems, hypoxia is induced through eutrophication, long residence times, and lack of ventilation to the atmosphere. Understanding the complex physical and biogeochemical processes in these systems is crucial for ecosystem health and resilience.

Dongsha Atoll is a coral atoll and Taiwanese National Park located in the northern South China Sea. In summers 2014 and 2015, hypoxia-related fish kills, coral bleaching, and sea grass die offs were observed on the eastern edge of Dongsha Atoll lagoon, motivating further work to understand the key drivers of hypoxia. This body of work focuses on a field campaign in summer 2019. Here, I aim to further our understanding of how physical-biogeochemical interactions drive oxygen availability in tropical reef-lagoon systems.

First, I construct a heat and volume budget for the lagoon to understand the physical processes governing heating, circulation, and ocean-lagoon exchange. Results show that surface heat fluxes drive rapid heating of the lagoon by ~1.5°C over the 22-day study period. Advective transport over the reef flat from tide- and wave-driven flows is an important component of ocean-lagoon exchange that is balanced by a net outflow through the channels on subtidal timescales. Specifically, a near phase-locked interaction between diurnal solar forcing and near-diurnal tidal exchange drives persistent advective cooling over the reef flat.

Next, in order to understand the physical processes governing residence time and renewal of the bottom waters, I construct a bottom water mass budget in Dongsha Atoll lagoon. Results show frequent bottom water renewal events driven by tidally modulated advective processes through the lagoon channels and over the reef flat. Though bottom water renewal events are usually a mechanism for refreshing oxygen levels in stratified systems, the water advected into Dongsha Atoll lagoon from the reef flat can, at times, be hypoxic. The water flowing off of the reef flat into the Dongsha Atoll lagoon is intermittently oxygen deficient, due to seasonal phase-alignments between the primarily diurnal tidal flow and biological respiration on the reef. During the study period, the flood phase of the tide, which drives flow of cooler offshore water across the reef flat from the ocean to the lagoon, occurred at night when strong biological respiration draws down oxygen on the shallow reef flat. These cooler, low oxygen waters sink to the bottom of the stratified lagoon waters, creating a pool of isolated low-oxygen water.

Lastly, I examine the drivers of low oxygen on the east reef flat of Dongsha Atoll. Results show that the diurnal biological cycle on the reef flat is in phase with diurnal flood-at-night tidal forcing, which together leads to strong nocturnal oxygen drawdown. Lagoonward flow at night then transports the low oxygen water to the lagoon boundary. This process results in episodic low-oxygen events at depth inside the lagoon. We show that this pattern is seasonal on Dongsha Atoll due to the annual phasing of the diurnal tides, with the flood phase of the tide consistently occurring at night during summer months. Our findings underscore the potential significance of physical-biological interactions in other reef systems with diurnal tides, emphasizing the need for further work to elucidate oxygen dynamics in coastal tropics.