Rapid assessment of epibenthic communities: A comparison between two visual sampling techniques

Quantification and comparisons of the structure of subtidal marine communities are essential for answering a suite of basic and applied ecological questions. However, time constraint associated with sampling underwater by scuba diving is a major obstacle to achieve adequate sampling effort. We tested a technique for rapid-assessment of hard substrate epibenthic communities, based on counts of the frequency of different taxa within sampling units (quadrats). This technique was compared to a broadly used technique, based on visual estimation of percent cover of taxa, to determine whether a significant amount of information is lost when using the rapid assessment technique. We applied both techniques in three case studies from the Ligurian Sea, in the NW Mediterranean. Field measurements showed that the same communities can be surveyed twice as fast with the frequency-count compared to the cover-estimation. Structural variables of communities as species richness and diversity yield almost identical values with the two techniques while dominance showed underestimated by frequency-count when compared to the cover-estimation. Multivariate analyses indicated that the two techniques yield similar results, highlighting similar patterns of variation in all three case studies, particularly when data are transformed (square-root or fourth-root transformation). Our findings suggest that applying the frequency-count technique to the assessment of benthic communities is an effective strategy for increasing sampling effort when working underwater at depth.