The daily cycle of heating and cooling of the near-surface ocean may be quite different in the open ocean compared with a shallow lagoon with a seafloor that is a few meters deep and can be heated directly by the sun. This solar radiation can affect the local benthic communities. To study the physical processes associated with the daily cycle of the South Pacific lagoon surrounding Bora Bora, a vertical string of five high-resolution temperature sensors is moored at a 2-m-deep site for 3 weeks. Besides standard ocean warming (mostly during the day) and cooling (mostly at night), the sensors show relatively highest temperature near the lagoon floor during the warming phase and weakly stable stratification toward the end of the cooling phase. During the warming phase, highly variable stratification is observed extending into the water column, but only under conditions of calm weather and turbid waters. Under trade winds and clear waters, the lowest sensor or sensors show consistently higher temperature variability than higher sensors with spectral slopes indicative of shear and/or convective turbulence. During the cooling phase, the lower sensor shows consistently very low variance (nonturbulent), while other sensors show a spectral slope around the buoyancy frequency, evidencing weakly stratified waters supporting internal waves. These observations contrast with open-ocean near-surface observations of stable stratification during the warming phase and turbulent free convection during the cooling phase. Thus, lagoons seem to resemble the atmosphere more than the ocean in daytime thermodynamics and possibly act as a natural solar pond with bottom conductive heating (when salinity compensates for unstable temperature variations).
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