Evaluating change in seagrass meadows: A time-framed comparison of Side Scan Sonar maps

Abstract

Side Scan Sonar (SSS) is a common method for mapping seagrass meadows that allows to define meadow extent and typology. Should sonogram interpretation be efficient, repeated mapping would allow evaluation of spatial meadow dynamics through time. Here, maps of Posidonia oceanica (L.) Delile meadows from western Liguria (NW Mediterranean Sea) produced in 1990, 1991 and 2006 through SSS surveys, were processed with GIS (Geographical Information System) and compared by means of diachronic analysis to establish concordances and discordances. While any difference between maps of 1990 and 2006 might provide information about change in seagrass cover with time, the availability of maps taken at only one year of distance (1991 vs 1990) offered the unique opportunity to evaluate the efficiency of SSS mapping, as the slow dynamics of P. oceanica makes significant change over such a short time scale unrealistic in absence of any large-scale environmental catastrophe. Results showed obvious differences in P. oceanica meadow distribution among the three periods in most of the areas. The largest differences were found between the maps of 1990 and 1991. This discrepancy was probably due to erroneous interpretation of small, fragmented shallow-water meadows on sonograms. Errors in sonogram interpretation combine with inefficient positioning in old cartographies to blur the evaluation of change over large time scales (i.e. between 1990 and 2006). Extensive sea-truthing based on a rigorous design is mandatory for efficient acoustic mapping of seagrass meadows.