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Physical processes determine spatial structure in water temperature and residence time on a wide reef flat

Davis, Kristen1; Reid, Emma1; Lentz, Steven2; DeCarlo, Thomas2; Cohen, Anne2

Published Dec 01, 2020 on Dryad. https://doi.org/10.7280/D1S38H

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Dec 01, 2020 version files 728.10 MB

Abstract

On coral reefs, flow determines residence time of water, influencing physical and chemical environments and creating observable microclimates within the reef structure. Understanding the physical mechanisms driving variability on coral reefs, which distinguishes them from the open ocean, can be important for understanding what contributes to thermal resilience of coral communities and predicting their response to future anomalies. In June 2014, a field experiment was conducted at Dongsha Atoll in the northern South China Sea to study the physical forces that drive flow over a broad shallow reef flat. Instrumentation included current and pressure sensors, and a distributed temperature sensing system, which resolved spatially and temporally continuous temperature measurements over a 3-km cross-reef section from the lagoon to the reef crest. Spectral analysis shows that while diurnal variability in temperature was significant across the entire reef flat - a result expected from daily solar heating - water temperature also varied at higher frequencies near the reef crest. These thermal microclimates are shaped by circulation on the wide reef flat, with spatially and temporally variable contributions from tides, wind, and waves. Through particle tracking simulations, we find the residence time of water is short near the reef crest (2.8 hours) and longer near the lagoon (7.7 hours). Tidal variability in flow direction leads to patterns in residence time that are different than what would be predicted from unidirectional flow conditions. Circulation on the reef also governs the source (originating from offshore vs. the lagoon) of the water dominant across the reef flat.