Research Areas > Wetlands > Study of the Habitat Value of Urban Wetlands
Project: Habitat Value of Urban Wetlands
Background and Objectives
With increasing urbanization in Southern California coastal watersheds, requirements for municipal control of runoff quantity and quality have created a demand for wetlands that serve as effective, low-cost best management practices (BMPs) to improve surface water quality and attenuate storm flows. There is equal pressure to restore, enhance, and create wetlands with multiple objectives (i.e., habitat support, treatment of nonpoint source pollution, flood attenuation, and recreation). The potential risk to wildlife associated with using wetlands for treatment of nonpoint source runoff, and the trade-offs between habitat and water quality objectives are not well quantified.
To address this information gap, a study was conducted to evaluate habitat value associated with urban wetlands having multiple objectives in Southern California. The goal of this project was to provide information on how these urban wetlands can be better managed to improve overall ecological condition and compatibility with wildlife protection in Southern California.
Histogram of CRAM index scores and site photos represent of the range of conditions found among the 40 urban and 3 reference wetlands examined in this study.
This project was completed in 2008.
A phased approach was used to allow data collected in the first phase to drive decisions about objectives and study design in subsequent phases. The major phases of this project included:
1) Develop an inventory of 40 existing urban and treatment wetland projects and 3 reference wetlands.
2) Conduct a biological survey on a representative sample of urban wetlands to evaluate wildlife beneficial use indicators including wetland and riparian habitat mapping, California Rapid Assessment Method, plant community composition, benthic macroinvertebrates, bird use, and catchment land use.
3) Evaluate the exposure and toxicity of sediment-borne contaminants to wildlife.
1) Determining appropriate reference conditions for urban wetlands - The 40 freshwater urban wetlands were highly modified from historical reference conditions, with a large percentage converted to types atypical of the southern California landscape. The reference sites used in this study reflected a state of “best achievable,” but did not necessarily provide an adequate benchmark for the characteristics of a wetland in pristine ecological condition.
2) Presence and magnitude of risk from contaminants - Most wetlands posed a risk of elevated sediment contaminants and/or toxicity. Eighteen of the 21 urban wetlands were either toxic to the amphipod H. azteca, exceeded a sediment quality guideline, or both. An index of the degree of sediment contamination was found to negatively correlate with benthic macroinvertebrate diversity in these wetlands. At all 10 sites that were toxic to H. azteca, pyrethroid concentrations were elevated, suggesting that this class of compounds was responsible for much of the toxicity observed in the study. Confirmation studies would be needed in order to determine the definite source of the toxicity, which could be expected to vary at each wetland site.
3) Urban infrastructure constrains condition - Results from the biological survey showed that the surrounding urban infrastructure creates basic constraints on “best achievable” wetland condition. Levels of Cu, Pb, Zn, PAHs, and cypermethrin were significantly correlated with percent imperviousness of the catchment area, a proxy for percent urbanization. However, sediment toxicity and sediment pyrethroid concentrations were not significantly correlated with the degree of urbanization. Site specific management factors, such as wetland project design criteria and wetland management and maintenance activities (e.g., contaminant source control and pretreatment), can mitigate to some degree the constraints of the urban landscape.
4) Differences by project objective and design criteria - Sediment chemistry concentrations and toxicity were not significantly different among habitat, water quality, and multifunctional types of wetlands. This relationship held true for several other indicators of habitat quality as well, including benthic macroinvertebrate and bird diversity. Thus, habitat wetlands did not necessarily exhibit superior conditions relative to other urban wetland types.
5) Intensity of maintenance activities - This study showed that urban wetlands must be maintained frequently to manage the variety of stressors, but not at an intensity or in a manner incompatible with the seasonal cycles of nesting and reproduction.
6) Analysis of effectiveness of treatment wetlands is hampered by lack of flow data - Analyses conducted on existing monitoring data were inconclusive because of lack of flow data required to calculate loads. Modeling of wetland BMPs is needed to provide a time-integrated picture of water and contaminant budgets that can lead to better calculations of treatment efficiencies. Standardized monitoring of treatment wetland projects can provide the data needed to develop these models.
7) Treatment wetlands reduce contaminant concentrations - Existing monitoring data show that southern California treatment wetlands reduce the concentrations of all constituents of interest (i.e., total and dissolved metals, nutrients, TSS, and bacteria) relative to inflow concentrations. For dissolved Cu, Pb, and Zn, southern California treatment wetlands were effective at reducing wet season inflow concentrations to below water quality criteria. They showed 1-2 order of magnitude reductions in E. coli, Enterococcus, fecal coliform, total coliform and nutrients. Great variability was found in the effectiveness of removal.
Watershed Planning Recommendations:
1) Consider that a watershed-wide plan to reduce urban runoff can be greatly aided by watershed-scale conservation and restoration of natural wetlands and riparian areas.
2) Incorporate low impact development, source control and BMP implementation upstream of wetlands to reduce potential for onsite exposure and toxicity to contaminants.
3) When possible, conduct pretreatment of the wetland water source. A variety of treatment strategies, including detention, pre-treatment, treatment, and infiltration, should be incorporated in series in order to maximize removal efficiencies and minimize wildlife exposure.
4) Incorporate BMPs throughout the watershed.
Recommendations for Design Elements:
1) If habitat is an objective of a project to create wetlands, clearly state what the management endpoints are and how the project design is linked to those endpoints.
2) Locate the site in an area that can support wetland hydrology.
3) Assess the potential risk from contaminants by characterizing major water sources and wetland sediments.
4) Design the site to have good physical structure.
5) Maximize the diversity of habitats within the wetland and transitional upland areas.
6) Design and maintain the wetland and transitional upland buffer to have appropriate width and native vegetation.
Management and Maintenance Recommendations:
1) Wetland stewardship is essential in urban areas.
2) Manage hydrology to mimic the natural hydroperiod.
3) Manage urban stressors (e.g. contaminants, human visitation, invasive plants, trash, excessive sedimentation, etc).
4) Maintain the wetland at a frequency necessary to manage stressors, but not in a manner that is incompatible with the seasonal cycles of nesting and reproduction.
5) Conduct monitoring with intensity scaled to the project size. Core standardized monitoring elements recommended in the report should remain intact regardless of project size.
This project was conducted in collaboration with the Southern California Wetland Recovery Project.
J Brown. 2009. Sediment Quality in Urban Wetlands. Presented at 2nd Annual SCCWRP Symposium.
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