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Title: Influence of hydrologic loading rate on phosphorus retention and ecosystem productivity in created wetlands
Authors: Mitsch, William J.
Cronk, J. K.
Keywords: Freshwater marsh productivity
River restoration
Wetland creation
Phosphorus retention
Restoration ecology
Constructed wetlands
Lake County (Ill.)
Publisher: U.S. Army Engineer Waterways Experiment Station
Series/Report no.: Technical Report (Wetlands Research Program (U.S.)) ; no. Technical Report WRP-RE-6
Abstract: Four 2- to 3-ha constructed freshwater riparian wetlands in Lake County, lllinois, were subjected to two hydrologic regimes of pumped river water to simulate nonpoint source pollution. The experimental wetlands at the Des Plaines River Wetland Demonstration Project were designed to develop and test wetland design principles, construction methods, and management programs needed to create and maintain wetlands for the purposes of water quality management, flood control, and fish and wildlife habitat. High-flow wetlands (HFW) with short retention times received 34 to 38 cm of river water per week, and low-flow wetlands (LFW) with high retention times received 10 to 15 cm per week. This report summarizes research results for phosphorus dynamics and retention, macrophyte development, periphyton productivity, and overall water column metabolism through 1992. All of these functions were hypothesized to be related to hydrologic conditions. Measurements of soluble reactive phosphorus and total phosphorus were made weekly at the inflow and outflow of each constructed wetland from 1990-1992 and at permanent reference quadrats throughout the wetlands in 1991. Intensive sampling with six samples per day was also carried out in 1992 on two of the wetlands. The wetlands retained over 60 percent of incoming phosphorus with general improvement from the first to second year. In the third year, inflow to one of the wetlands more than doubled, and phosphorus retention decreased from 85 to 53 percent. Spatial patterns of phosphorus concentrations indicate that in the LFW, over 60 percent of the incoming phosphorus was removed from the water column within 15 to 20 m of the inflow. In the HFW, phosphorus decreased gradually from inflow to outflow with 60-percent removal within 70 to 100 m of the inflow. Intensive sampling in 1992 with over 800 analyses of phosphorus from the outflows of two wetlands showed an 81-percent decrease in phosphorus concentration from inflow to outflow in the LFW and a 74-percent decrease in the HFW. Intensive sampling also showed an 87-percent decrease in phosphorus mass from inflow to outflow in a LFW and 83-percent decrease in phosphorus mass in a HFW. Analysis of other processes in the wetlands illustrated that most of the phosphorus was probably retained through sedimentation of inorganic sediments, with macrophyte uptake accounting for a substantial sink of phosphorus. Periphyton and planktonic uptake was less significant. Macrophyte biomass production and successional changes in wetland species composition were monitored over 5 years, from 1988 through 1992. From dry conditions in 1988 to flooded conditions beginning in 1989, the species composition of each wetland's plant community changed to include almost 100-percent wetland species soon after flooding. The number of species the four wetlands had in common decreased from 1988 to 1989 as the plant community adjusted to flooded conditions and then increased as the wetlands developed and the plant communities converged. Biomass increased from 1989 to 1990 in all of the wetlands as the plant communities changed from upland to wetland species. After 1990, biomass increased progressively each year in LFW. HFW showed more variable patterns from year to year. Peak biomass of emergent macrophytes ranged from 250 g dry weight/m² in one HFW to 1,470 g dry weight/m² in another HFW after 4 years of flooding. Emergent macrophyte productivity was determined to be more a function of antecedent conditions and water levels than flow-through conditions over this relatively short time. Periphyton samplers were placed near the inflow and outflow of all four wetlands, and dry weight, organic dry weight, and chlorophyll a was measured every 2 weeks from May through August 1991. Periphyton growth on macrophytes was estimated three times in all four wetlands. Periphyton chlorophyll a on artificial surfaces was higher in the HFW than in the LFW. Samplers in one HFW (HFW 5) had higher dry weight and organic dry weight than the other wetlands. Inflow samplers had higher chlorophyll a, dry weight, and organic dry weight than outflow samplers (p < 0.10). These data suggest that periphyton responded to the hydrologic treatment as well as to position within the wetland. Of the three macrophyte species examined for epiphytes (Polygonum spp., Phalaris arundinacea, and Typha spp.), epiphyte dry weight, organic dry weight, and chlorophyll a were highest on Polygonum spp. and lowest on Typha spp. Net epiphyte biomass was estimated to be highest in a HFW (HFW 5). Diurnal changes in dissolved oxygen concentrations were used during three growing seasons to estimate water column primary productivity. Productivity was generally higher in the HFW (84 kJ·m⁻²·day⁻¹ in 1990; 69 kJ·m⁻²·day⁻¹ in 1991) than in the LFW (73 kJ·m⁻²·day⁻¹ in 1990; 46 kJ·m⁻²·day⁻¹). In 1992, LFW 4 had the highest water column primary productivity (129 kJ·m⁻²·day⁻¹). Chlorophyll a concentration did not correlate to productivity. Water column primary producers contributed an estimated 17 to 67 percent of the net aboveground carbon production of each wetland, but no consistent pattern of increased or decreased importance of water column productivity over 3 years was observed.
Description: Technical Report
Gov't Doc #: Technical Report WRP-RE-6
Rights: Approved for public release; distribution is unlimited.
Size: 98 pages
Types of Materials: PDF
Appears in Collections:Technical Report

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