Evaluating effects of weed cutting on water level reductions and ecological status in Danish lowland streams

Sammendrag

At present, scientific evidence documenting effects of weed cutting in streams as a measure to improve flood protection and run-off from agricultural land is scarce, which is surprising considering the huge effect that it has on stream ecology. Instead, weed cutting is performed under the assumption that removal of aquatic plant biomass improves runoff from agricultural land and prevents flood-ing of adjacent areas provided that it is performed regularly.2.In this study, we examined linkages between weed cutting practice and water level reductions in 126 small- and medium-sized Danish streams (catchment size<100 km2) with continuously monitored discharge and water level data (from 1990 to 2012). Specifically, we hypothesised that (1) weed cutting reducesstream water levels more in late summer when the biomass of aquatic plants ishigher than in early summer; (2) the efficiency of cutting declines with increasing cutting frequency as the aquatic plant community changes with increasing abundance of species able to regrow fast following a cutting event; (3) the high frequency cutting in Danish streams lowers the ecological status of the streams asevaluated from aquatic plant assemblages.3.The average effect of weed cutting on the water level was largest in July, August and September with an average reduction of 16 cm and lowest in early spring and late autumn with an average reduction of 11 cm. Regrowth was largest in June, with an increase in water level of 0.41 cm/day, whereas regrowth wasabsent in autumn. Regrowth also varied with the frequency of weed cutting,from an average of 0.04 cm/day in streams subjected to one annual cutting to an average of 0.6 cm/day in streams subjected to >6 annual cuttings. Further-more, we found that the ecological status was either moderate or poor/bad in streams with more than one annual cutting.4.Our findings highlight that it is by no means certain that the current weed cut-ting practice is efficient for flood control since (1) regrowth is stimulated by frequent cuttings and a positive feedback loop may develop, necessitating even more frequent cuttings to maintain the discharge capacity of the streams, and (2)many species stimulated by weed cutting, like for instance Sparganium emersum,form dense canopy beds across the entire stream profile and therefore reduce the discharge capacity of the stream more than species growing in confined patches. We encourage more studies with the aim to identify how stream maintenance should be performed to optimise flood control without compromising the ability to reach good ecological stream quality.