Nutrient removal in treatment wetlands declines during winter months due to temperature. A 3-mo (wintertime) mesocosm study was conducted to determine the effectiveness of alum in immobilizing P as well as other nutrients during this period of reduced treatment efficiency. Eighteen mesocosms, triplicate alum, and three controls or no alum were established with either Typha spp., Schoenoplectus californicus, or SAV (Najas guadalupensis-dominated). Alum was delivered by timer at a rate of 0.81 g Al m(-2) d(-1) and parameters measured included: pH, soluble reactive phosphorus (SRP), total phosphorus (TP), dissolved organic carbon (DOC), dissolved inorganic nitrogen (DIN), total kjeldahl nitrogen (TKN), and soluble aluminum (Al). Alum-treated mesocosms had significantly lower pH values (8.1) than controls (8.8), but well within the elevated pH range for aluminum toxicity. Alum significantly reduced all measured water column nutrients with the exception of ammonium N, which remained unaffected, and particulate P, which increased. This study demonstrated that seasonal low-dosage alum application to different vegetation communities in a treatment wetland can significantly improve treatment efficiencies for SRP (87 vs. 58%) and TP (62 vs. 44%) but also increase DOC (19 vs. 0%) and TKN (12 vs. -3%) removal capacity to a lesser degree. Alum applications within close proximity of the treatment wetland effluent points should be implemented with caution due to the production of alum floc-bound P which could potentially affect discharge permit compliance for total suspended solids or total P.