Spatial variability of coastal flow along a coral reef-lined Pacific Island; implications for larval, nutrient, and pollutant dispersal and retention

Vessel mounted ADCP surveys of currents were conducted along a 15-km section of the northern coast of Tutuila, American Samoa, during February 2015. Over the course of four days, 9 primary transects were successively run to characterize the currents over varying stages of the tide. In addition, wind speed and direction were measured from the vessel to characterize variability in the wind field over the survey area. Large-scale currents along the coast appear to be controlled by regional flows, and not the tidal cycle or even regional winds, resulting in a large eddy on the windward side of a prominent headland that was in the lee of the regional flow. Strong cross-shore shear was noted along most of the transects, sometimes resulting in reversing flow between the nearshore and offshore. The combination of a regional eddy and strong shear in the nearshore that is characteristic of a coastal boundary layer result in a flow field that favors retention of suspended matter such as larvae and/or nutrients and contaminants that can influence the coastline’s coral reef ecosystems.