Depth and water residence time shape dissolved organic matter removal in constructed wetlands

Climate change has intensified the mobility of dissolved organic matter (DOM) from land into aquatic ecosystems leading to increased brownification and hypoxia. Constructed wetlands (CWs) offer a potential mitigation strategy, yet optimal wetland design and water residence time (WRT) with respect to DOM removal remains underexplored. This study evaluates the effects of shallow and deep CWs on the degradation of DOM during two seasons (June and November 2023) in Halmstad, Sweden. Small CWs were used for treatments which received organic matter additions, and the impact of varying depths and WRT on DOM alteration and removal were evaluated. DOM composition and degradation were assessed using fluorescence spectroscopy, bacterial activity (bacterial respiration, production and growth efficiency), and total organic carbon (TOC) analyses. We identified a critical initial period (2 days) during which labile DOM is rapidly degraded by photochemical and microbial processes. Shallow CWs exhibited rapid initial breakdown of DOM but also a release of terrestrial like fractions after 2 days, potentially increasing downstream brownification. In contrast, deep CWs demonstrated sustained DOM degradation and slower internal production, potentially reducing their contribution to downstream brownification. Microbial processes dominated DOM degradation across both seasons, although photodegradation also played a significant role during the summer months. Correlations between DOM composition and bacterial dynamics underscore the role of labile substrates in driving carbon cycling efficiency. These findings inform CW designs, advocating for hybrid approaches integrating shallow and deep systems in series to maximize carbon removal, minimize brownification, and adapt to seasonal variability.