Using Distributed Temperature Sensing to quantify the influence of internal waves on heat flux and temperature variability on a coral reef

Dongsha Atoll, a coral reef in the northern South China Sea, sits in the middle of an energetic internal wave field. Internal waves shoaling on the shallowing topography of the atoll and surrounding shelf transport cold, nutrient-rich water from the deep ocean onto the shallow fore reef. We will present observations from an experiment in June 2014 designed to quantify the influence of internal waves on water properties on the reef. Historically, measurements of subsurface temperature have been done using point in space or time measurements; however, measurements from fiber-optic temperature sensing or Distributed Temperature Sensing (DTS) makes higher spatiotemporal resolution temperature measurements possible. We deployed 4 km of fiber optic cable across the reef flat and fore reef slope, and using DTS technology we measured temperature every meter along the cable with a sampling interval of 1 minute. Here, we use the DTS data to construct a simple heat budget for the reef flat and quantify internal wave-driven heat flux to the reef. Over the 9-day observation period, the heat budget calculations suggest that subthermocline water delivered to the shallow fore reef by internal waves makes its way onto the reef and contributes to cooling of the reef flat. Additionally, using the high resolution DTS data, we are able to identify very different thermal microclimates across the reef. The highest temperature variance is on the reef slope, and the variance decreases towards the reef flat and lagoon. Furthermore, the frequency of the temperature variability shows a strong diurnal signal on the reef flat and more high frequency variability on the reef slope from the internal waves.