Simple measurements of small scale water motion in coral reefs

In shallow waters, water motion is one of the most important environmental factors for marine organisms. This is particularly true in coral reefs, where the effect of flow is important for growth processes and spawning settlements of coral inhabiting the area. However, it is very difficult to measure the water motion in these areas with conventional current meters (propeller type and electromagnetic current meters etc.) due to the meter size and bottom topography. For sessile organisms such as coral and oysters, the maximum instantaneous current velocity rather than the intensity of the averaged flow is important. Therefore, to measure water motion in coral reefs, a plaster ball technique has been developed and used. However, plaster balls are usable for only 2 or 3 days. In the case of measuring with plaster balls, such as for spawning settlement and diffusion of coral larvae, it is necessary to examine a large number of points over a long time period. Previously, we presented a way to improve the material of the plaster balls to make them usable for longer periods. In this study, we examined measurements of water motion around coral reefs using a large number of normal plaster balls and improved plaster balls, including plaster for dentistry, cement, and other types. The results of our experiment in a patch reef in the Sekisei Lagoon (Okinawa Prefecture, Japan) clearly showed that the intensity of the flow varied with depth and position, and that a small vortex had been generated around the patch reef. We consider the effect of such differential water motion and small vortices on the growth processes and spawning settlement of coral