The ability of coastal wetlands of the Laurentian Great Lakes to reduce pollution from tributaries has not been documented in detail or over multiple seasons. This study developed a surface-water budget for a coastal wetland along Lake Erie and estimated monthly, annual, and storm-related exports of total suspended solids and selected nutrients from the wetland. Water-budget measurements included precipitation, evaporation, surface discharge into the wetland, and net surface discharge into Lake Erie. Water samples collected upstream and downstream and composite dryfall-precipitation samples were analyzed for total suspended solids (TSS), total phosphorus (TP), soluble reactive phosphorus (SRP), nitrate nitrite nitrogen (NO_{2 3}), ammonia nitrogen (NH₃), total Kjeldahl nitrogen (TKN), soluble reactive silica, chloride, and specific conductance. Seasonal and storm-related concentration patterns and a wide variation in monthly, seasonal, and annual loads from the tributary into the wetland were typical of streams draining the western Lake Erie basin. All substances reached higher maximum concentrations upstream than downstream; however, median monthly time-weighted mean concentrations of TP, TSS, NH₃, and TKN were higher downstream. Concentrations without discharge data were inadequate to estimate removal rates. Annual loads of TSS, NH₃, and TKN increased during passage through the wetland, whereas those of TP, SRP, NO_{2 3}, and soluble reactive silica decreased. During storm runoff events, various proportions of TP, SRP, TSS, NO_{2 3}, and soluble reactive silica were removed, despite brief hydraulic residence times, whereas more NH₃ exited than entered. Wetlands occupying the flooded lower reaches of Great Lakes tributaries collectively are probably important in maintaining and enhancing the water and sediment quality of the lakes. Water levels throughout the Laurentian Great Lakes have decreased in recent years; consequently, wetland areas with standing water and hydraulic residence times have decreased, probably reducing the effectiveness of the wetlands in mitigating pollution.