Landscape based assessment of human disturbance in Michigan lakes

Anthropogenic disturbances are a major threat to the health of inland lakes in the U.S.  Traditionally, lake condition is assessed using physicochemical and biological indicators measured within a lake.  Because of the natural variation in these indicators among lakes, it is difficult to ascertain whether poor lake condition is natural or a result of human disturbance.  We describe a process for identifying natural condition and assessing disturbance gradients in inland lakes using readily available, geo-referenced databases.  We demonstrate the utility of this process by applying it to all 11,000 inland lakes that are equal or greater than 5 acres in Michigan, and use it to identify which human disturbance factors have the greatest impact on each lake.  Our process for assessing lake health represents a significant advantage over other routinely used methods.  It uses both local and river network catchments, permits the evaluation of lake health statewide, yields an overall disturbance index that is a weighted sum of multiple disturbance factors, and identifies key disturbance factors for individual lake so that management efforts can be prioritized.  Our approach is less robust for identifying lakes that are influenced by local-scale human disturbances that are not captured by our GIS data.