Die-Back and Regeneration of Reed Wetlands on Lake Balaton in Connection With Local Sediment Dynamics

Abstract

The coastal wetlands of Lake Balaton are the most diverse habitats of the lake and are essential for the regeneration of fish stocks and for shore protection. The area of these wetlands has been decreasing for the last decades, and previous studies have demonstrated that this is in connection with raised and stabilized water levels. However, to date the effect of changes of lake bottom topography on reed stands has not been investigated. Our objective was to compare the lake bottom surveys of 1975 and 2015 within the wetlands, together with quantitative information on the changes of wetland area at the same locations. We found that, on average, the accumulation of sediment dominates, but a wide range of situations occur from sediment removal resulting in changes of elevation of up to a meter, to sediment accumulation up to more than half a meter. Reeds were in decline everywhere where sediment loss was identified, and also in locations with sediment accumulation below 50 cm. Reed regeneration was only observed where sediment accumulation was higher than 50 cm in the studied period. From the perspective of water level regulation, this means that most wetlands cannot sustain the ongoing rise in water level, and that short periods of lower water levels are not sufficient for regeneration of most wetlands. Dedicated studies will be necessary in order to clarify the connection between lake bed topography, water levels, and wetland change, but it is already proven that the lake topography cannot be assumed as constant from the perspective of the wetlands, even at the time scale of a few decades.