Weed cutting in a large river reduces ecosystem metabolic rates in the case of River Gudenå (Denmark).

Problems related to extensive macrophyte growth are widespread both in modified and man-made canals and streams, and in streams with natural morphology and rich vegetation. The weed cutting is a common management practice in order to reduce flood risk and enhance water conveyance. Although the short- and long-term impacts on the stream physical habitats and biota have been extensively studied, only little information exists on the effects of weed cutting on ecosystem metabolism, especially for larger rivers. This study aims to quantify effects of weed cutting on metabolic rates in a large lowland river in Denmark. We measured Gross Primary Production (GPP), Ecosystem Respiration (ER) and physical parameters (water depth, discharge, water velocity and reaeration rate) one week prior and 2-6 weeks after weed cutting in 2014 and 2020. Physical river conditions changed significantly after the removal of approximately 60% of macrophytic volume, and a significant reduction in water depth and increased water velocity was recorded. We found an immediate 38% and 61% reduction in GPP and 28% and 35% reduction in ER after weed cutting in 2014 and 2020 respectively. We also found that the metabolic rates did not recover to pre-weed cutting levels within 2-6 weeks after weed cutting. The higher decline in GPP compared to that in ER indicates that the heterotrophic contribution to ER was higher compared to the autotrophic contribution. Our results display that even in a large macrophyte-rich river, where only one-third of the channel is managed by weed cutting, GPP and ER can be reduced significantly. The cascade effects of metabolic rates alterations on ecosystem structure and functioning need to be considered in the future management plans, where higher plant biomass and increased flow is anticipated due to the ongoing climate change and thus, the demand for weed cutting might be intensified.