Interrelationships Among Water Quality, Lake Morphometry, Rooted Plants and Related Factors for Selected Shallow Lakes of West-Central Minnesota

The Minnesota Pollution Control Agency (MPCA) is developing eutrophication standards for Minnesota lakes and reservoirs. This effort is in response to a U.S. Environmental Protection Agency (EPA) requirement that states develop nutrient criteria for lakes, rivers, wetlands and estuaries. In the course of work on ecoregion reference lakes, assessment of statewide data sets for 305(b), and the development of guidance for listing of nutrient-impaired lakes on Minnesota's 303(d) list it was apparent that there are some distinct differences in trophic status and potentials of shallow, well-mixed lakes as compared to deeper, stratified lakes. Previous work documented, for instance, the distinct difference in total phosphorus concentrations among dimictic, intermittently mixed, and well-mixed lakes (Fig. 1; Heiskary and Wilson, 1988). From this work it was evident that differences were particularly marked among deep and shallow lakes of the North Central Hardwoods Forests (CHF) and Western Corn Belt Plains (WCP) ecoregions. Also during public comment periods and hearings associated with the establishment of guidance for the listing of nutrient-impaired waters, concerns were expressed that swimming may not be the primary use in many of the states shallow lakes. Among their contentions was shallowness of the lakes, highly organic substrates, and often times over-abundance of rooted submergent and emergent plants. Because of these factors it was recommended that the MPCA consider separate nutrient criteria for shallow lakes that would take these factors into account. This would result in criteria that were more closely attuned to the actual uses of these shallow lakes, which are commonly boating, fishing, aesthetics, wading and waterfowl production, rather than an emphasis on swimming (primary body contact).