Attempt to achieve simple measurements of water motion in shallow areas

Abstract

In shallow waters, such as coral reef or seagrass regions, the water motion is one very important factor for the surrounding marine organisms. In these areas it is very difficult to measure the water current with conventional current meters due to the sensor size and topography. To measure currents in shallow waters, a plaster ball technique has been already used. A plaster ball is constituted of a sphere made of plaster (diameter of about 4.0 cm), and a steel bar (length of about 30 cm). Thus, it can be set up not only at many points in the patch reef, but also in coastal regions. This technique to measure time-averaged intensity of water motion is already well known, and could be also valid to measure the water motion in seagrass, seaweed beds and coral reef regions. However, plaster balls are usable only for 2 or 3 consecutive days. The purpose of this study is to improve the material of plaster balls to make them usable for longer periods. The authors tested six different types of plaster balls: viz. plaster for dental use type 1, 2, and 3, polymer type, cement type, and paint type. They estimated the resolution rate using each plaster ball and normal plaster balls in the coral reef region by corroborating the measurements with those observed by electromagnetic current meters. Their experimental results in a patch reef in the Sekisei lagoon (Okinawa prefecture, Japan) indicated that the measurements could be possible for about one-week or ten days when using a sensor with cement in the plaster.