Monitoring the subtidal macroalgae percentage cover in an assessment of the human impact on diving sites in Brittany (France): direct estimation versus photography

The fluctuation in macroalgae percentage cover was used as a bioindicator parameter to evaluate the potential impact of SCUBA divers around mooring buoys. The technical aim of the survey was to try out standardised, rapid and easily reproducible methods to assess measures for surveyors potentially unskilled in algal taxonomy, especially in subtidal areas. In this context, we put forward two methods to test macroalgae % cover estimation via, i) direct estimation from a drawing of sampling quadrats with a standardised drawing board and ii) computer treatment of digital pictures. In order to compare the two methods, six algal morphotypes were surveyed by three divers who were novice phycologists. No methodological effect or diver effect was found, indicating that error from direct estimation from drawings was negligible, and that our rapid, cost effective method was highly reproducible.