Assessment of a simple hydroacoustic system for the mapping of macrophytes in extremely shallow and turbid lagoon

• We visually derived three acoustic features from the echograms using simple sonar.
• We distinguished 2 macrophytes groups by setting a threshold based on in situ data.
• Short plants (<20 cm) mainly corresponded to charophytes, otherwise to angiosperms.
• The discrimination between these groups was more expressed in deeper areas (>1 m).
• This method can be used for the detection of their maximum depth distribution limit.

In this study, we focused on relatively inexpensive and commonly used fishermen class sonar in order to test its ability to detect stands of charophytes and other groups of submerged angiosperms in the shallow and turbid waters of the Curonian Lagoon. Based on the length of macrophytes, height thresholds (20 and 30 cm) were established in order to distinguish charophytes from submerged angiosperms in obtained echograms. The echograms were visually analysed by two experts. We successfully discriminated 3 echofeatures (bare bottom, stands of charophytes and submerged angiosperms), whereas angiosperms from different morphology groups could not be distinguished. Below 1 m depth, the stands of charophytes and submerged angiosperms were clearly distinguished, thus their maximum colonization depth could be delineated. The accuracy of the discrimination could be reduced by free-drifting mats of filamentous algae, resulting in the overestimation of charophytes. Our approach can be useful for the mapping of monospecific stands of submerged vegetation and could be an important additional tool for macrophyte monitoring and water quality assessment in shallow and turbid waterbodies.