The depreciation of an ecosystem: A comparison of natural and created wetland quality

High-quality natural depressional wetlands are nutrient-rich, productive ecosystems, capable of supporting a variety of flora and fauna. Current U.S. wetland policy permits the destruction of these complex systems with mitigation by the construction of new wetlands. The created wetlands, however, are often of inferior quality, primarily due to construction in nutrient-poor (i.e., upland) soils. We compared plant and soil carbon and nitrogen content between ten created and five natural freshwater depressional wetlands located in central Ohio. The created wetlands ranged in age from <1 to 39 years since construction; the natural wetlands ranged in quality from low to high quality. Peak standing plant biomass and soil (10 cm depth) were sampled from 3-5 plots per wetland. Biomass was sorted by species, and both the biomass specimens and soil subsamples analyzed for carbon and nitrogen. Plant and soil C and N stocks were calculated for each wetland and compared by type (created or natural) and by age given type. As expected, the created wetlands contained significantly less C and N (p<0.05). Only plant C increased significantly with age (p=0.06). The discrepancy between created and natural wetlands and the lack of improvement with time was further confirmed by examination of two subsets of wetlands in close proximity: two young created wetlands contained significantly less C and N than a nearby natural wetland; three created wetlands (ages 6, 10 and 32 years) were undifferentiated by age. The lower quality substrate of constructed wetlands is likely an insurmountable obstacle on any feasible timescale.