Method development for mapping of coastal macrophytes

Accurate mapping of macrophyte communities is essential e.g. for evaluating the ecological status of coastal environments according to the Habitats directive. In the Åland archipelago, traditional macrophyte surveys have primarily relied on diver-based methods. This study investigates and assesses an alternative approach employing aquatic sonar in conjunction with a drop-video system. The methodology involves systematic video recording and sonar data acquisition along depth-defined transects, rather than fixed distances from the shoreline, complemented by rake sampling to support species identification. This pilot test demonstrated that the tested method performs reasonably well for estimating both macrophyte cover and maximum depth distribution. Although diver-based transects may yield more precise depth limits of macrophytes, the depth estimates obtained through this approach were sufficiently accurate for use in ecological status assessments. Species detection in the video material was largely limited to macrophytes taller than approximately 10 cm or species forming extensive belts, such as Fucus vesiculosus, whereas smaller taxa e.g., Cladophora spp. or red algae such as Polysiphonia fibrillosa—could not be reliably identified. Sonar-derived percentage cover values tended to shift abruptly between 0% and 100%, suggesting potential overestimation; however, these values generally corresponded well with patterns observed in the video analysis. Species composition also varied across the archipelago gradient, with F. vesiculosus dominating outer archipelago sites while Potamogeton perfoliatus was most prevalent in middle and inner archipelago areas. To summarize, the method has some potential due to its effective time-usage in the field but needs further development for proper data analysis.