Wetlands are often constructed to counteract eutrophication or brownification of downstream waters, although our understanding of their differences in efficiency and effects is still scarce. Therefore, we here used the upper and lower 95% confidence limits of the average retention capacity of 11 wetlands to represent efficient and inefficient wetlands, respectively. We then modelled future scenarios in water colour (brownification) and phosphorus (eutrophication) in a downstream lake. We show that efficient wetlands may delay eutrophication and brownification rates of downstream aquatic systems by up to 15 and 4 years, respectively, thereby providing a temporal gain in time that could be used to implement complementing measures. Inefficient wetlands, on the other hand, may instead deteriorate the water quality by increasing levels of phosphorus and water colour and thereby reducing the euphotic zone of downstream lakes. Our study underscores the difficulty of having realistic expectations of the efficiency of wetlands as tools in water management. We conclude that despite high variation in efficiency among wetlands, they may still hold a strong potential when properly designed and placed for mitigating both eutrophication and brownification. However, further evaluation or systematic monitoring after restoration or construction is needed to investigate optimal placement and design in relation to management objectives. © 2026 The Author(s). Ecohydrology published by John Wiley & Sons Ltd.