Research Areas > Wetlands > Habitat Value of Natural and Constucted Wetlands Used to Treat Urban Runoff: A Literature Review

Project: Habitat Value of Natural and Constructed Wetlands Used to Treat Urban Runoff: A Literature Review

Background and Objectives

With increased urbanization in the coastal watersheds of southern California, wetlands and riparian areas have rapidly been disappearing from the landscape, while those that remain are often highly degraded. Concurrent with the loss of wetland habitat, increased pollution discharges and runoff from urbanized watersheds has created a demand for effective, low-cost solutions to improve surface water quality and attenuate storm flows. As a result, there is increasing pressure to restore, enhance, and create wetlands with multiple objectives (i.e., habitat support, treatment of nonpoint source pollution, flood attenuation, and recreation). Unfortunately, many studies have asserted that use of wetlands for these treatment purposes may potentially endanger wildlife.

In order to better understand the risks, considerations, and trade-offs associated with using wetlands to treat urban runoff, SCCWRP conducted a literature review on the topic. The need for this work was identified by the California Coastal Conservancy and the Southern California Wetland Recovery Project (WRP). These groups have received several requests to fund urban wetland restoration projects whose stated objectives include improving the water quality of urban runoff, as well as restoring riparian and wetland habitat. The intended use of this review is to provide a baseline of information from which the WRP partner agencies may begin to address key research, management, and policy questions pertaining to the ability of wetlands to fulfill the dual objectives of treating urban runoff and providing wildlife habitat.

Status

This project was completed in 2003.

Methods

To better understand the complex issues associated with habitat quality and hazards associated with natural or constructed wetlands subject to urban runoff, literature was reviewed to address the following questions:

1) Do natural or constructed wetlands used for treatment of urban runoff become environmental hazards for wildlife?

2) How does directing urban runoff or stormwater through a natural or constructed wetland affect the habitat quality of the wetland?

a) What is the potential that changes to a wetland’s physical or chemical structure will affect the ecological health of the wetland, in terms of its ability to support a diverse and healthy assemblage of typical wetland flora and fauna?

b) How do potential changes in wetland physical and chemical structure impact the biological communities dependent on these wetlands, and are these impacts severe enough to be of concern?

c) To what extent does the intentional manipulation of hydrologic regime, site morphology, and wetland biota (to maximize water treatment capacity) detract from the habitat value of the wetland?

3) How do considerations of the habitat effects of treatment wetlands differ by wetland class (e.g., how are the issues different for riverine vs. palustrine or lacustrine wetlands)?

Findings

SCCWRP’s review of over 200 reports, articles, books, and other references yielded the following general conclusions:

• Adequate research does not exist to address the three central questions above.

• The existing literature is generally tangential to the central topics of interest. Studies typically address either the efficacy of treatment wetlands in removing contaminants or the effects of urbanization on wetland attributes.

• Documented examples exist showing potential incompatibility between water quality and habitat goals in wetlands. Overall, the literature reviewed suggests that concerns of risk to wildlife are valid, but because these studies were not specifically designed to address this question, the degree of risk is unknown.

• Research into the effects of urbanization on wetlands has found that urban runoff can affect biotic communities by altering the hydrology, biogeochemistry, trophic level dynamics, community structure, and diversity of species and habitat types. In southern California, many remaining freshwater wetlands are sustained by urban runoff. It is not clear, given southern California’s extreme loss of aquatic habitat, that the risks posed by urban pollutants would outweigh the benefits of habitat provision.

• There is a general lack of literature on the efficacy and effects of using wetlands to treat urban runoff in semi-arid climates such as southern California. Most of the literature that addresses efficacy of treatment or effects of urbanization on wetlands has been conducted in humid, temperate climates. This literature is helpful, but may be less applicable to southern California’s semi-arid climate.