LANDSCAPE INFLUENCES ON WATER CHEMISTRY IN MIDWESTERN STREAM ECOSYSTEMS

1. Landscape characteristics of sixty-two subcatchments within the Sag inaw Bay Catchment of central Michigan were examined to identify relat ionships with stream water chemistry. Land use, land cover and elevati on were quantified for both entire catchments and the upland-river eco tone (100 m stream buffer strip). Catchment and ecotone data were then empirically compared with stream water chemistry using multivariate a nd regression analyses. Redundancy analysis was used to partition vari ance among land use, geology, and the shared influence of land use and geology. 2. Major catchments dominated by rowcrop agriculture had the highest alkalinity, total dissolved solids and nitrate + nitrite conc entrations. 3. Strong seasonal differences were observed in total nitr ogen and nitrite + nitrate, but not in total phosphorus or suspended s olids. Land use and landscape structure factors such as slope and patc h density (number of land use patches per km(2)) accounted for most of the observed variance in summer. 4. In both autumn and summer, landsc ape factors accounted for much of the observed variation in total diss olved solids and alkalinity. During: autumn, geological factors and th e shared influence of geology/landscape structure plus land use exerte d more influence than did land use alone. 5. Total phosphorus and tota l suspended solids were much better explained by land use within the s tream ecotone in summer than in other seasons. However, total nitrogen , nitrate, orthophosphate and alkalinity were equally well explained b y land use within the ecotone and throughout the whole catchment. Only total dissolved solids in summer and ammonium in autumn were explaine d better by the whole catchment than the ecotone. 6. Our results show that relatively coarse spatial databases can provide useful descriptor s of regional water quality.