Interactions between environmental factors and vertical extension of helophyte zones in lakes in Finland

Abstract

Sheltered shores in boreal lakes are typically dominated by emergent helophytes in which the perennating organ lies in soil below the water level, but the aerial shoots protrude above the water. Changes in natural water-level regimes due to hydropower development may significantly affect the longitudinal extent of the helophyte zone. We explored the vertical distribution of helophyte zones under water-level regulation to develop tools for estimating hydropower impacts. We recorded the vertical extension of the three most common helophyte species (Carex L. spp., Equisetum fluviatile L., and Phragmites australis (Cav.) Trin. ex Steud.) along 238 macrophyte transects in four regulated lakes and one nonregulated lake in Finland. We used random forest (RF) models to quantify the degree to which water-level fluctuation and site-specific environmental variables uniquely explained the variation in helophyte zonation. The vertical extension of Carex spp. was most strongly associated with the water-level fluctuation of the open-water period (OWP), followed by Phragmites and Equisetum. Overall, the RF models explained 4–41% of the variation observed in the helophyte zones. The models indicated that OWP fluctuation, slope, openness, and cover of other macrophyte groups were key factors explaining the extent of the helophyte zones. OWP fluctuation was the most important variable for zonation of Carex spp. and the upper limit of Phragmites. Slope was the most significant variable for the lower limit of Equisetum and Phragmites. Our study demonstrates the importance of taking into account natural habitat-level variability to increase confidence in quantifying the environmental impacts of hydropower production. This approach could be used in future impact assessments of new water-level regulation practices.