Physical mechanisms driving biological accumulation in surface lines on coastal Hawaiian waters

Lines of smooth ocean surface water known as "slicks" appear in coastal waters worldwide and can be caused by a number of natural mechanisms. Off the western coast of the island of Hawai'i, USA, internal waves, submarine groundwater discharge, tidal fronts, and headland fronts were all observed to cause the formation of slicks. Neuston net sampling showed slicks to contain enhanced densities of phytoplankton, zooplankton, larval invertebrates, larval fish, and plastic fragments when compared to ambient water. In particular, internal wave slicks were observed to be both the most frequently occurring type of slick and the overall strongest accumulator of neustonic organisms and plastics when compared to the other types of slicks. Satellite imagery illustrated that slicks are visible along most of the West Hawai'i coastline more than 75% of the time. Furthermore, acoustic backscatter observations showed that the accumulation associated with slicks frequently extended from the surface to depths of 5 m. These findings highlight the important roles internal wave slicks can have in coastal ecosystems.