Although riparian strips are widely used to protect wetlands, these buffer zones may be too small to do much good. Most wetland buffers are only 30 to 120 m across, and new research shows that wetland water quality can be affected by land use up to 4 km away.
“Our results suggest that current U.S. and Canadian wetland conservation policy and regulations are highly unlikely to sustain wetland water and sediment quality,” say Jeff Houlahan of the University of New Brunswick, St. John, Canada, and Scott Findlay of the University of Ottawa in Ontario, Canada, in the August 2004 issue of Landscape Ecology.
Agriculture, urbanization, and other land uses can degrade streams, rivers, and lakes by adding too many nutrients, which can shift plant communities and kill fish and amphibians. To help protect water quality, many jurisdictions require narrow buffer zones around wetlands. However, little is known about how land uses farther away affect wetland water quality. “Policy makers need to understand the distance at which adjacent land uses affect water quality, if riparian buffer zones are to be an effective management tool,” say Houlahan and Findlay.
To see whether land uses beyond the narrow buffer zones can also affect wetland water quality, the researchers identified both nutrient levels and adjacent land uses for 73 wetlands in southeastern Ontario. Nutrients including nitrogen, phosphorus, and potassium were measured in both water and sediment samples. Adjacent land had forest cover, roads, and a proportion of wetlands and were assessed in areas up to 5 km from the wetland edges.
The results showed that wetland nutrient levels can be affected by land uses far beyond the current narrow buffer zones. Of the land uses assessed, forest cover had the greatest impact on wetland nutrient levels. Notably, potassium and phosphorus were higher in wetlands that had little forest cover 2 and 4 km away. “A loss of about 50 percent forest cover would lead to an estimated increase in phosphorus levels of about 75 percent,” says Houlahan, stressing that these are rough estimates. Less forest often means more agriculture, which typically means more nutrient-rich commercial fertilizers. “It is not surprising that fertilizer runoff from fields 2 and 4 km from a wetland will affect downstream sediment nutrient levels,” say the researchers.
This work suggests that we need a new approach to protecting wetlands. “Small-scale solutions alone (e.g., narrow buffers around individual wetlands) will almost certainly be ineffective,” say Houlahan and Findlay. “Sustaining high wetland water quality will require maintaining . . . relatively large areas of natural forest and wetlands.”
Houlahan, J. and C.S. Findlay. 2004. Estimating the ‘critical’ distance at which adjacent land-use degrades wetland water and sediment quality. Landscape Ecology 19:677-690.
Burreed Photo by Glenn Barrett, Canadian Wildlife Service