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Dryad

Relative phase between tidal and solar cycles influences the heating of a coral atoll lagoon

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Feb 06, 2024 version files 1.19 GB

Abstract

The residence time of water within reef-lagoon systems is of first-order importance to biogeochemical cycles and reef ecosystem functions. Circulation, mixing, and water renewal within these systems is highly dependent upon the geometry of the reef as well as wave, tide, and wind forcing. We are motivated to understand the physical driving a recent warming event as well as observed hypoxia-related fish kills and seagrass dieoff events in Dongsha Atoll lagoon, a 28 km wide coral atoll in the South China Sea. Though extensive work has been done on hypoxia events in temperate coastal systems, there remains a lack of understanding regarding the dynamics of these events in shallow tropical waters. Hydrographic observations were collected in summer 2019 to examine circulation and ocean-lagoon exchange in Dongsha Atoll lagoon. We construct a heat budget for the Dongsha Atoll lagoon to understand the physical processes governing lagoon circulation and ocean-lagoon exchange. We examine the influence of tides, winds, waves, advective exchange, and surface heating on lagoon heating and flushing time. Results show that surface heat fluxes drive rapid heating of the lagoon by approximately 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 oceanlagoon 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. Estimated flushing times for the Dongsha Atoll lagoon range from 2.7 to 3.2 days for spring and neap tide, respectively.