Measuring nearshore variability in benthic environments: An acoustic approach

Abstract

Hardbottom and sedimentary habitats in the nearshore are continually affected by natural and anthropogenic disturbances. Currently used monitoring practices rely on visual techniques, aerial and underwater photography, which are conducted on an annual basis. These practices do not allow for rigorous assessment of the effects of human-induced or natural events on the distribution and composition of these habitats. We investigated the use of an acoustic ground discrimination system (RoxAnn) to determine changes in benthic habitats in the nearshore area of northern Indian River County, FL where hardbottom outcrops occur. It was determined that this method was precise and repeatable when deployed in a highly variable surf zone environment. Multiple surveys assessed temporal and spatial changes in bottom types. Spatial distributions of several bottom types were identified: bare rock surfaces, Sabellarid worm reef, sponge covered rock, and rock covered with two dominant algal species. Short-term (weekly) changes in the distribution of sand coverage were detected during a relatively calm hydrodynamic period. At one site (R-15), there was an increase of 11% in sediments, which attributed to approximately 2 750 m2 of coverage; 2.1 km south (R-22) there was a 6% decrease in sediments in six days. The latter equates to a change of almost 1 500 m2 of available habitat. These results suggest that the current practice of annual surveying is limited for assessing the magnitude and impact of episodic disturbances on benthic habitats. Based on these data, acoustic ground discrimination systems provide an unbiased, technology-based data collection method to distinguish benthic habitats in the surf zone. Employing this method will benefit researchers and coastal resource managers.