Density-depended acoustical identification of two common seaweeds (Posidonia oceanica and Cymodocea nodosa) in the Mediterranean Sea

Abstract

Abstract The non-destructive samplings are very important in not damaging seagrasses and seaweed under protection, at the field studies. The grasses are prominent in the assessment of the ecological status of the marine environments. One of the effective non-destructive samplings was the acoustical methods which need a low level of the sea and atmospheric conditions as compared to the other remote sensing system. Like the others, acoustic data alone are inherently ambiguous concerning the identities of the scatterers and need sea-truthing at the field studies. Considering the requirements above and the advantages of the acoustical methods, an acoustical in situ study was conducted to discriminate two dominant seagrasses along the western Turkish Mediterranean coast in August (in the year 2012) when biometrics of both species was at maxima. Four different regions were involved in the study during data collection using a split beam echosounder operated at 206 kHz since each region had different strengths of their biometrics reflecting regional variations of the acoustical measurements. For discrimination, a statistical approach based on the acoustical parameters and their statistics was enriched and validated with multivariate analyses (Silhouette, k-means, PCA and CAP analyses). Posidonia oceanica was correlated with mean, median, SD and maximum value of Sa, whereas Cymodocea nodosa was characterized by hardness and roughness of leaf echo, followed by skewness and kurtosis of Sa. The acoustics of C. nodosa were related to regional differences, but P. oceanica was related to the depthwise difference. Both species had highly different densities (g/cm3), followed by biomass. The present study will interactively help acoustical studies to be more common and comprehensible and the other previous studies published did as well.