Wind-wave driven circulation on the coral reef at Bora Bay, Miyako Island

Abstract

 Current records from three surveys at Bora Bay, Miyako Island, all showed strong unidirectional flows. Ocean water entered the lagoon over the shallower western half of the reef flat and exited the lagoon through a channel on the eastern side. Fourier transform of one of the survey data sets showed that this unidirectional flow is modulated on a cycle with a period half as long as the dominant M2 tidal cycle. The prominent features of the observed time-series current profiles were well reproduced using a numerical simulation that includes a depth dependent formulation of the wind-wave forced cross-reef water flow. The water residence times of the lagoon varied from 1.5 h to 3.7 h when calculated directly from the modeled current field, and from 2.0 h to 9.3 h when calculated as the time required for modeled particles to exit the lagoon. These residence times are surprisingly short and may help to explain how this reef supports high net organic production. Furthermore, the short particle residence times show the importance of analyzing currents on time scales smaller than the dominant tidal cycle to understand the fate of organic material produced in coral reefs.