Seasonal dynamics of brownification mitigation in constructed wetlands

Boreal freshwaters are becoming darker. This brownification cooccurs with increasing dissolved organic carbon (DOC) concentrations and affects crucial ecosystem services. Our study investigated constructed wetland optimisation in terms of depth and water residence time (WRT) during different seasons to remedy dark water colour and high DOC concentrations, while retaining nitrogen removal. We conducted eleven-day experiments with deep-brown, shallow-brown and shallow-control treatments, in summer (June) and autumn (November) 2023, using 18 small-scale constructed wetlands at an experimental wetland area in southern Sweden. At the beginning of both experiments, the flow through the wetlands was halted and extracted peat was added to the brown treatment wetlands, thus simulating brownification by increasing absorbance and DOC concentrations. Thereafter, changes in absorbance, DOC concentration and total nitrogen (TN) concentration were measured. Our results showed that optimal WRT for increasing water clarity and reducing DOC concentrations varied between one and two days. A WRT of more than two days during summer, resulted in internal carbon production and darker water colour. The optimal WRT for TN removal was not affected by DOC addition. We conclude that constructed wetlands increase water clarity and boost carbon degradation if their WRT, especially during summer, is sufficiently short. If WRT exceeds two -three days in summer, internal carbon production together with low oxygen levels and increased iron (Fe) mobilization, may instead increase downstream brownification. Overall, our study shows that properly designed wetlands are suitable measures to mitigate the effects of brownification.