A water level drawdown index for aquatic macrophytes in Nordic lakes

Many northern lakes are regulated to enhance hydropower production and flood protection. This bears hydromorphological pressures which are important factors causing lowered ecological status. Water level fluctuation triggers erosion on the shoreline and, depending on fluctuation range, also affects species composition or disappearance of sensitive aquatic macrophytes. We developed a water level-drawdown index (WI_c) for Nordic lakes using macrophyte data from 73 lakes with varying water level fluctuation in Finland, Norway and Sweden. The index is based on the ratio between sensitive and tolerant macrophyte species. The sensitive and tolerant species are identified based on a percentile approach, analysing the presence or absence of species along the winter drawdown range. The index correlates well with winter drawdown in Finnish and Norwegian lakes with strongest correlations with winter drawdown in storage lakes (lakes regulated for hydroelectric power and with a considerable winter drawdown). The WI_c-index is applicable in low alkalinity, oligotrophic and ice-covered lakes, and is suggested to be a useful tool to identify and designate heavily modified water bodies in Nordic lakes according to the European Water Framework Directive.