Assessing ecological status of regulated lakes with littoral macrophyte, macroinvertebrate and fish assemblages

Anthropogenic alterations of hydrologic regime are a major stressor for aquatic ecosystems. In boreal lakes water levels are often drawn down several meters for hydropower production in winter, which may have negative impacts particularly on littoral biota. We examined macrophyte, macroinvertebrate, and fish assemblages in littoral zone of 30 regulated and non-regulated reference lakes in Finland. We applied community metrics which correspond to demands of EU Water Framework Directive (WFD) to (i) assess and classify ecological status of the lakes, and to (ii) estimate critical water level drawdown in winter that would allow for acceptable status (“Good” ecological quality class in WFD). All three organism groups indicated strong response to winter drawdown, but with rather high mutual variation. Macroinvertebrates showed the strongest response (r2 = 0.73 between Ecological Quality Ratio (EQR) and winter drawdown), followed by macrophytes (r2 = 0.43) and fish (r2 = 0.40). Mean-EQR over the three organism groups indicated that Good ecological status class was usually not attained if winter drawdown exceeded 3.46 m, whereas minimum-EQR (lowest of the three groups, i.e. one-out, all-out principle) indicated a critical threshold of 1.76 m. Generally, our results suggest that monitoring and ecological status assessment of regulated lakes should focus on littoral biota which is most vulnerable to water level fluctuations.