Phosphorus uptake and release patterns in overwintering constructed floating wetlands

Understanding nutrient cycling patterns in plants deployed within constructed floating wetlands (CFWs) is critical for improving CFWs’ design and management practices. This study evaluated phosphorus (P) uptake and release patterns during fall/winter plant senescence and spring regrowth. Two mesocosm-scale CFW experiments were conducted characterizing plant growth, plant tissue P levels, and water quality (nutrients and phytoplankton). Experiment 1 quantified P uptake during spring regrowth after overwintering, and experiment 2 quantified P release during fall senescence. Plant treatments (CFWs with Pontederia cordata or Juncus effusus) were compared to an open-water control. In spring, J. effusus removed 0.056 g P m−2 d−1 (19.4% of the load), P. cordata removed 0.034 g P m−2 d−1 (10%), and the open-water control removed 0.03 g P m−2 d−1 (10%). In fall, J. effusus fixed 0.008 g P m−2 d−1 (2.1% of the load), P. cordata released 0.014 g P m−2 d−1 (−2.1%), and controls fixed 0.023 g P m−2 d−1 (6.3%). P was consistently released during the fall experiment and occasionally released in the spring experiment, likely from senescing plant tissues (fall) and from roots sloughing after new root growth (spring). Results demonstrate the potential for multi-season deployment of CFWs using J. effusus for reducing P loads year-round. HIGHLIGHTS Seasonal nutrient cycling should inform constructed floating wetland (CFW) design and management.; Juncus effusus fixed phosphorus (P) both in the fall and spring.; Pontederia cordata released P in the fall and slowly absorbed P in late spring.; Semi-evergreen species may be ideal plants for year-round CFW deployment.;