INFLUENCES OF LAND USE ON THE BIOGEOCHEMISTRY OF STREAMS

Land use and land use change are known to influence the biogeochemistry of watersheds. Techniques for assessing this influence and the resulting effects on ecosystems are poorly developed. Our working hypothesis is that different land uses have unique biogeochemical signatures, which are reflected in the health of ecosystems. To examine this hypothesis we assume that in humid climates, groundwater discharge is the primary source of stream water during low-flow conditions. Thus the chemistry of baseflow should provide an integrated signal, or signature, of recent and historical land use and geology along the groundwater flow path from recharge to discharge areas. We are examining biogeochemical signatures of rivers from two major watersheds in Michigan, USA, (Grand Traverse Bay and Muskegon) and relating these signatures to different land use categories (e.g., urban, agriculture, forested) and ecosystem health using algal-invertebrate communities.

Biogeochemical signatures are being developed by synoptic sampling of approximately 80 sites across each watershed during a three to four day time period at low flow. Algal community field assessment for a watershed takes approximately four weeks. Water samples are analyzed for temperature, pH, dissolved oxygen, specific conductance, redox, major, minor and trace ions. Sampling sites were selected based on sourcesheds upstream of each sampling site, stream conditions (e.g., slope) and type of ecological community present. The data are reduced through geochemical modeling and multivariate analysis techniques.

Preliminary results confirm our hypothesis that dominant land uses in sourcesheds leave a biogeochemical fingerprint on surface waters (e.g., Ca and Mg associated with agriculture, Na and Cl with urban land use, and Rb/Sr with domestic and agricultural wastewater). Factors explaining the dominant controls on stream biogeochemistry are related to: agricultural activity; halite dissolution; abandoned farmland distributions; algal activity and wetlands. If the affects of land use on stream and river biogeochemistry are predictable, the results of this research will facilitate watershed management programs designed to mitigate the environmental impacts of land use change and development of protocols for ecological risk assessment.