Mapping of submerged vegetation and fish breeding grounds in the Mönsfladan, Åland

During the summer of 2013 mapping of the Mönsfladan, Åland, was performed under assignment of the Åland Government. There is a local wish to plan to digging of a canal between the Mönsfladan and the Engrundsfjärden in order to increase water exchange and create a new boating route. The goal of this study is to determine the ecological state of the Mönsfladan, predict the possible consequences of the proposed canal and to establish an ecological baseline to serve as reference for future monitoring. The ecological state of the Mönsfladan was established by mapping the submerged vegetation, fishing for juvenile fish and taking hydrographical samples together with temperature reading from three locations in Mönsfladan and one in Engrundsfjärden. The vegetation mapping showed that the flad was dominated by Najas marina, Chara tomentosa, Chara aspera and Potamogeton pectinatus. Other species that were found are Chara baltica, Chara canescens and the red listed Chara horrida. The abundance and cover of Charophytes (26.2 % cover of the total Mönsfladan) indicates a healthy ecological situation. The status is further supported by the low turbidity of the water. Charophytes also form important spawning and nursery habitats for fish. This nursery function is also indicated by the catch of fish in Mönsfladan, which caught in the proximity of charophyte beds, was dominated in number by juvenile perch and roach (36 % and 29 %, respectively). The temperature readings together with the hydrographical results indicate that the inside of Mönsfladan and Engrundsfjärden are sheltered and have little water exchange with the sea compared with the outer Mönsfladan. Possible consequences of increased water exchange due to the construction of a canal and increased boat traffic are eutrophication and increased turbidity caused by resuspension of sediment, which may result in decline or disappearance of charophytes and their ecological functions. Another effect is that the overall temperature of the flad may drop, rendering it too cold for the survival of small fry and larvae of fish