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Abstract

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.

5. Conclusions and perspectives

We found the potential in the use of a simple fishermen class hydroacoustic system for the detection of bare bottom and stands dominated by fully developed charophytes or submerged angiosperms in extremely shallow and turbid lagoon. The successful discrimination of echofeatures was based on differences in their height, whereas macrophytes with different morphology could not be distinguished. We could precisely separate the stands of charophytes from submerged angiosperms bellow 1 m depth due to higher difference in their lengths and more monospecific stands than in shallow part of the littoral, where application of the sonar was limited. Since we were able to discriminate stands of charophytes and their maximum colonization depth, which are generally considered as water quality indicators, the use of simple hydroacoustic method could be a very important additionaltool for macrophyte mapping, monitoring and ecological status assessment according to the WFD. Although this study aimed to discriminate stands of submerged vegetation to echofeatures, we clearly see the potential for their quantitative estimation and the development of automatic or semi-automatic assessment methods that can use data from different sonars.