Technical Report Number 144

Cumulative Impacts of Development on Lakes in the North Central Hardwood Forest Ecoregion of Minnesota: An Exploratory Study

P. L. Brezonik, R. A. Osgood, L. Olmanson, E. Day, L. Hatch, J. Doyle, J. A. Perry, M. Bauer, E. MacBeth, and T. Anderle

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Abstract

This report presents the results of an exploratory study funded by the Minnesota Department of Natural Resources on cumulative impacts of development on lakes, and is modified from the project completion report submitted to the MN DNR in January 2000. In the context of this study, “development” refers to urban, suburban, and near-shore residential development in the vicinity of lakes that may have deleterious effects on lake quality. The study focused on the North Central Hardwood Forest ecoregion of Minnesota. This region includes the already urbanized but still expanding Twin Cities metropolitan area (TCMA), as well as two large, lake-rich regions further north and west—the Brainerd lakes area and the Alexandria lakes area (ALA)—that are experiencing suburban, rural, and shoreline development pressures. Work described in this report focused on lakes in the TCMA and ALA. Related objectives of the study included (1) a review and analysis of lake classification approaches relevant for evaluating cumulative impacts of development on lakes and (2) a review of lake monitoring activities in the region and state. The review of lake monitoring activities is included in this report; results regarding lake classification approaches can be found in WRC Technical Report 143 entitled, Methods for Classifying Lakes Based on Measures of Development Impacts.

Because of the short time frame available for the study, our analysis was exploratory and relied primarily on existing databases and statistical approaches. Because of the lack of comprehensive information from a short time period for water quality conditions in lakes across the TCMA and ALA, we used satellite imagery to infer lake clarity synoptically for late summer of 1991.The relationships we found between various landscape conditions and lake trophic response factors (primarily water clarity) generally were weak, which is not surprising given the enormously complicated behavior of land-lake interactions. Nonetheless, some trends were found to support the supposition that watershed development leads to poorer water quality in lakes. The results also are important in providing guidelines for developing more definitive conclusions and in demonstrating deficiencies with regard to the (a) temporal consistency, (b) extent of coverage (number of lakes), and (c) types of data being collected on Minnesota lakes.

Specific conclusions include the following:

Satellite imagery is a useful, cost-effective tool to assess lake conditions in regional studies.
Satellite imagery has great potential for quantifying changes in landscape conditions, specifically to determine percent-impervious surface area for urban areas.
Lake monitoring programs in Minnesota, while extensive in number, are fragmented and inadequate with regard to the variety of information and temporal consistency of the collected data.
No single variable related to landscape development is a strong predictor of lake water clarity/trophic state.
More (in the sense of greater sample size) is not always better in exploratory statistical analyses; classifying lakes into smaller and thus more homogeneous groups (based on size, depth, conductivity) led to better correlations between landscape (stressor) and lake (response) variables.
Inclusion of variables that express a lake’s sensitivity to impacts of development led to better fits in stressor-response multiple regressions.
Large scatter in the relationships precluded the finding of any evidence for threshold effects of landscape stressors on water clarity; moreover, the current results do not lend themselves to development of criteria to defining the extent of watershed development that will protect lake quality.
Water quality degradation and eutrophication are only a subset of the impacts of urban/suburban/ shoreline development on lake values and uses, and further studies should be initiated to quantify these impacts and to develop guidelines that will protect these values and uses.