William J. Mitsch

Restoration of the Mississippi Delta: Lessons from Hurricanes Katrina and Rita

Hurricanes Katrina and Rita showed the vulnerability of coastal communities and how human activities that caused deterioration of the Mississippi Deltaic Plain (MDP) exacerbated this vulnerability. The MDP formed by dynamic interactions between river and coast at various temporal and spatial scales, and human activity has reduced these interactions at all scales. Restoration efforts aim to re-establish this dynamic interaction, with emphasis on reconnecting the river to the deltaic plain. Science must guide MDP restoration, which will provide insights into delta restoration elsewhere and generally into coasts facing climate change in times of resource scarcity.

The value of wetlands: importance of scale and landscape setting

Abstract Wetlands have value because their functions have proved to be useful to humans. The unit value for some wetlands also increases with human development (agriculture and urban) because of increased use and/or increased scarcity. Yet, paradoxically, its functions can easily be overwhelmed in areas of heavy human development, thus lessening those values. Thus wetlands appear to work best in the landscape as spatially distributed systems. Also, the value is partially dependent on where they are found in the landscape, e.g., the degree to which a wetland is open to hydrologic and biological fluxes with other systems, including urban and agricultural landscapes. A paradox of assigning values to wetlands and other ecosystems is that it can argue for the replacement of one system with another if a landscape view is not taken. Estimates of percent of landscape for various functions, e.g. water quality or flood control, are presented. It is suggested that a range of 3–7% of temperate-zone watersheds should be in wetlands to provide adequate flood control and water quality values for the landscape.

Improving the success of wetland creation and restoration with know-how, time, and self-design

The creation and restoration of new wetlands for mitigation of lost wetland habitat is a newly developing science/technology that is still seeking to define and achieve success of these wetlands. Fundamental requirements for achieving success of wetland creation and restoration projects are: understanding wetland function; giving the system time; and allowing for the self-designing capacity of nature. Mitigation projects involving freshwater marshes should require enough time, closer to 15-20 yr than 5 yr, to judge the success or lack thereof. Restoration and creation of forested wetlands, coastal wetlands, or peatlands may require even more time. Ecosystem-level research and ecosystem modelling development may provide guidance on when created and restored wetlands can be expected to comply with criteria that measure their success. Full-scale experimentation is now be- ginning to increase our understanding of wetland function at the larger spatial scales and longer time scales than those of most ecological experiments. Predictive ecological mod- elling may enable ecologists to estimate how long it will take the mitigation wetland to achieve steady state.

The effects of season and hydrologic and chemical loading on nitrate retention in constructed wetlands: a comparison of low- and high-nutrient riverine systems

Abstract We compared the nitrate removal efficiency of two constructed wetlands receiving ambient river water to one constructed municipal wastewater treatment wetland over the same 2-year period in central Ohio, USA. The wastewater wetland represents a high-nutrient system, with an average nitrate plus nitrite load of 12.3 kg N ha -1 day −1 and an average nitrate and nitrite inflow concentration of 12.5 mg N l −1 . The riverine wetland loadings and concentrations were approximately 60% lower (4.6–4.7 kg N ha −1 day −1 and 4.6 mg N l −1 ). Percent nitrate removal by mass ranged from 29% in the wastewater wetland to 37–40% in the riverine wetlands, although differences in retention varied widely by season and were not statistically significant among the wetlands. Retention efficiency was considerably lower in all three wetlands during floods; nitrate outflow was as much as 400% greater than inflow during some flood events. We developed a simple Vollenweider-type model of nitrate retention based on seasonal temperature, hydraulic loading, and nitrate loading. The model is general enough to be useful in describing nitrate retention in both high and low-loaded wetlands and was calibrated and validated with extensive field data. The model was used to predict wetland nitrate removal efficiency as the hydrologic and nutrient conditions change. The ability to make such predictions could be valuable in the design, construction, and management of wetlands for nutrient removal.

Landscape design and the role of created, restored, and natural riparian wetlands in controlling nonpoint source pollution

Abstract General design principles, landscape locations, and case studies of natural and constructed riverine wetlands for the control of nonpoint source (NPS) water pollution are presented. General design principles of wetland construction for NPS pollution control emphasize self-design and minimum maintenance systems, with an emphasis on function over form and biological form over rigid designs. These wetlands can be located as instream wetlands or as floodplain riparian wetlands, can be located as several wetlands in upstream reaches or fewer in downstream reaches of a watershed, and can be designed as terraced wetlands in steep terrain. Case studies of a natural riparian wetland in southern Illinois, an instream wetland in a downstream location in a northern Ohio watershed, and several constructed riparian wetlands in northeastern Illinois demonstrate a wide range of sediment and phosphorus retention, with greater efficiencies generally present in the constructed wetlands (63–96% retention of phosphorus) than in natural wetlands (4–10% retention of phosphorus). By itself, this could be misleading since the natural wetlands have much higher loading rates and actually retain an amount of nutrients comparable to constructed wetlands (1–4 g P m −2 year −1 ).

Creating Wetlands: Primary Succession, Water Quality Changes, and Self-Design over 15 Years

The succession of vegetation, soil development, water quality changes, and carbon and nitrogen dynamics are summarized in this article for a pair of 1-hectare flow-through-created riverine wetlands for their first 15 years. Wetland plant richness increased from 13 originally planted species to 116 species overall after 15 years, with most of the increase occurring in the first 5 years. The planted wetland had a higher plant community diversity index for 15 years, whereas the unplanted wetland was more productive. Wetland soils turned hydric within a few years; soil organic carbon doubled in 10 years and almost tripled in 15 years. Nutrient removal was similar in the two wetlands in most years, with a trend of decreased removal over 15 years for phosphorus. Denitrification accounted for a small percentage of the nitrogen reduction in the wetlands. The wetlands were effective carbon sinks with retention rates of 1800–2700 kilograms of carbon per hectare per year, higher than in comparable reference wetlands and more commonly studied boreal peatlands. Methane emission rates are low enough to create little concern that the wetlands are net sources of climate change radiative forcing. Planting appears to have influenced carbon accumulation, methane emissions, and macrophyte community diversity.

A detailed ecosystem model of phosphorus dynamics in created riparian wetlands

Abstract A generalized yet detailed wetland ecosystem model was calibrated and validated with 3 years’s data from four similarly constructed wetlands in northeastern Illinois, USA. The model was used to explore the role of different wetland structure and function in relation to phosphorus retention, to integrate collected data and provide a better understanding at the ecosystem level about constructed wetlands, and to predict the sediment and phosphorus retentions under different hydrologic conditions. Four submodels — hydrology, primary productivity, sediments, and phosphorus — were included in the model. Phosphorus cycling was reasonably simulated with one set of parameters for a total of 10 wetland-years. The model showed that autochthonous organic matter production varies from 300 to 1036 g dw m −2 year −1 , with 12 to 103 g dw m −2 year −1 accumulating as bottom detritus. This compares to inflows of sediments from the river (allochthonous) of 192–934 g dw m −2 year −1 . Simulated sediment accumulation ranged from 6 to 29 mm year −1 with high inflow wetlands having higher sediment accumulation rates than low flow wetlands. Model estimates are well below the 50–100 mm year −1 rates predicted by sedimentation trap data in previous studies. Total phosphorus retained with sedimentation is simulated at a rate of 1.08–2.47 g P m −2 year −1 , in the range of values reported for other wetlands. Simulations showed that macrophytes pumped about 0.31–1.66 g P m −2 year −1 out of deep sediments, and increased total phosphorus in the water column mostly during the non-growing season. Simulated phosphorus retention increased by 5.1% when macrophytes were removed from the wetland. Simulated phosphorus retention decreased from 90 to 50% when inflow increases from 8 cm week −1 to 200 cm week −1 . Manipulating the hydrologic regime to increase phosphorus removal efficiency may be a desirable strategy for constructed wetlands. Constructed wetlands are dynamic ecosystems for which we generally have poor predictive capabilities; ecological modelling provides us with a useful tool for understanding wetland function and structure, testing hypothesis, and making predictions.

Modelling nutrient retention of a freshwater coastal wetland: estimating the roles of primary productivity, sedimentation, resuspension and hydrology

A simulation model is developed for a coastal wetland of Lake Erie, one of the North American Laurentian Great Lakes, to determine the fate and retention of phosphorus in the wetland as water flows from an agricultural watershed through the wetland and into Lake Erie. Phosphorus retention in the wetland is a desirable to prevent eutrophication of Lake Erie. The model is developed with sub-models for hydrology, productivity, and phosphorus and a simulated barrier beach that can be opened or closed to Lake Erie. A simulation based on 1988 data is calibrated in step-wise fashion. Resuspension is a necessary inclusion in the model to predict phosphorus concentrations in the wetlands water column. Subsequent simulations are made for various combinations of increased flow from the watershed and changing Lake Erie water levels. Phosphorus retention varies from 17 to 52% with highest retention when high inflows are coupled with high lake levels. A nutrient budget constructed from the model for 1988 conditions showed marked differences with budgets developed from empirical models or field data. The model results suggest a near balance between inorganic sedimentation and resuspension but an active plankton sedimentation that results in a net phosphorus retention rate of 2.9 mg P m−2 day −1.

A new vision for New Orleans and the Mississippi delta: applying ecological economics and ecological engineering

The restoration of New Orleans and the rest of the Mississippi delta after Hurricane Katrina can become another disaster waiting to happen, or it can become a model of sustainable development. Sea level is rising, precipitation patterns are changing, hurricane intensity is increasing, energy costs are predicted to soar, and the city is continuing to sink. Most of New Orleans is currently from 0.6 to 5 m (2–15 feet) below sea level. The conventional approach of simply rebuilding the levees and the city behind them will only delay the inevitable. If New Orleans, and the delta in which it is located, can develop and pursue a new paradigm, it could be a truly unique, sustainable, and desirable city, and an inspiration to people around the world. This paper discusses the underlying causes and implications of the Katrina disaster, basic goals for a sustainable redevelopment initiative, and seven principles necessary for a sustainable vision for the future of New Orleans and the Mississippi delta.

Sediment deposition patterns in restored freshwater wetlands using sediment traps

Abstract Sedimentation rates in constructed wetlands in northeastern Illinois, USA, ranged from 5.9 to 12.8 kg m−2 y−1 in 1989–1990, higher than expected based on concentrations of suspended sediment in influent waters of the wetlands. This predicted an accumulation of 0.5 to 1.0 cm/y. Rates were significantly lower in 1991 growing season, ranging from 1.2 to 4.2 kg m−2 y−1. Factors contributing to the high sedimentation included internal autochthonous production of organic matter and resuspension. Low- and high-flow hydrologic conditions had little effect on sedimentation from 1989–1990 but sedimentation was higher in the high-flow wetlands in the 1991 growing season. Deposition was a function of hydrologic loading near the water inflows; no differences were evident further downstream in the basins. Deposition rates and organic and phoshporus concentrations of the sediments increased through the growing season in conjunction with higher sediment concentrations in the inflow water. The patchy nature of vegetation density within the experimental wetlands created channelized flow, resulting in spatial variability in sediment deposition.