RIPARIAN BUFFER ZONES AFFECT the QUALITY of MIDWESTERN STREAMS and RIVERS Despite Similar Land Use Through- (Porter, 2000)

RIPARIAN BUFFER ZONES AFFECT THE QUALITY OF MIDWESTERN STREAMS AND RIVERS Despite similar land use through- (Porter, 2000). Organic enrichment out the Corn Belt region of the resulting from excessive algal produc- Midwest, streams flowing through Minnes ota tion in some midwestern streams may cropland differ considerably in their reduce dissolved oxygen concentra- ecological characteristics, in part tions and be detrimental to other because of differences in riparian requirements of aquatic organisms. buffer zones (see text boxes). This U MIS Shading from tree cover in riparian conclusion is based on an investiga- buffer zones may influence nutrient W is cons in M i tion of 70 streams and rivers within ss concentrations indirectly by reducing is s ip three NAWQA study units in the p the growth of phytoplankton. In i R

i upper Midwest during August 1997 v streams where phytoplankton were

e (fig. A; Sorenson and others, 1999). r abundant (often where buffer zones Specifically, increases in tree cover Iowa were thin or lacking), dissolved nitrate Illinois in buffer zones were associated with E IWA concentrations were significantly aquatic biological communities lower (fig. B; Porter, 2000). The lower indicative of good stream quality, nutrient concentrations may result reduced nuisance algal growths, and L IR B from uptake by the abundant phyto- maintenance of sufficient dissolved plankton. Thus, assessments of oxygen concentrations to support eutrophication would benefit from diverse communities of aquatic consideration of biological communi- organisms. For example, the number ties and the riparian zone, rather than of aquatic insects indicative of good being based solely on nutrient concen- stream quality tended to increase trations in water. Figure A. The affect of riparian buffer zones with increases in percentage of tree ) 15 on the quality of 70 midwestern streams L /

cover, especially in sites where and rivers was evaluated in the Upper g

Mississippi River (UMIS), Eastern Iowa m (

streamflow and dissolved oxygen

(EIWA), and Lower Illinois (LIRB) River 3 conditions were favorable. Fish basins. O 10 N +

communities, which were sampled at 2

24 sites in the Upper Mississippi Streams with less tree cover, and O N

River Basin, also indicated better thus less shading, contained relatively d 5 e v overall conditions in streams with large growths of phytoplankton (algae l o wooded riparian zones than those suspended in the water) at levels con- s s i with more open canopy (Stauffer and sidered indicative of eutrophication D 0 0 50 100 150 200 others, 2000). P hytoplank ton abundance Chlor ophyll-a (mg/L )

Figure B. Dissolved nutrient concentrations decreased in eutrophic streams with exces- sive algal productivity. Rates of nutrient uptake by the algae can exceed rates at which nutrients are transported by streams during low-flow conditions.

Resource agencies, including the U.S. Department of Agriculture, encourage maintenance of strips of trees or grass between cropland and streams as a best management practice. These "riparian buffer zones" are thought to intercept runoff of sediment and chemicals from fields, promote bank stability, and pro- vide shading and habitat for aquatic life (Osborne and Kovacic, 1993). Riparian buffer zones need to be considered along with other important factors that Digital images from USGS topographic maps were used to estimate the percentage of affect chemical and biological indica- trees in a riparian buffer zone (a 100-meter width on each side of the stream) for 2- to 3- tors of stream quality, such as soil drain- mile segments upstream from each sampling site, supplemented by vegetation surveys age properties and stream hydrology at the sampling site (Sorenson and others, 1999). (Porter, 2000).

This figure taken from: Stark, J.R., Hanson, P. E., Goldstein, R.M., Fallon, J.D., Fong, A.L., Lee, K.E., Kroening, S.E., and Andrews, W.J., 2001, Water quality in the Upper Mississippi River Basin, Minnesota and Wisconsin, South Dakota, Iowa, and North Dakota, 1995-98: U.S. Geological Survey Summary Circular 1211, 35 p.