The Land Use Legacy Effect: including temporal context in understanding lake eutrophication

The effects of changing land cover on aquatic ecosystem functioning and ecosystem services poses many scientific questions, and is of critical importance to the conservation and effective restoration of these resources. A fundamental challenge facing scientists is how to incorporate the impacts of previous land use (i.e., land use legacies) into our understanding of the effect of current land cover and land use. Prior research (e.g., Harding et. al. 1998) has shown that the current status of stream ecosystems is more related to past land use than recent land use conversions. In this research, I take a similar approach to study lake eutrophication. Changes in land use/cover (urban, agriculture, range, forest, and wetland) over six time steps (1938, 1955, 1968, 1978, 1996, and 2006) were quantified within lake buffer areas and compared to lake nutrient concentrations (nitrogen and phosphorus) from 37 Michigan lakes sampled in 2008. The relationship between time specific land use/cover and lake nutrient concentration was analyzed using multiple regression and classification and regression trees. Regression analyses show that current land use/cover had a strong relationship with current nutrient concentrations, however, land use/cover from other time periods was also related to current nutrient status. Additionally, CART analyses initially split using current land use/cover, indicating that current land use/cover has a stronger relationship with current lake nutrient status than past land use/cover. However, detailed analysis of CART splitting decisions revealed that past land use/cover acts as a surrogate in the initial split. Moreover, the second split has many competitor splits spanning the full range of time steps. Therefore, these results provide evidence for a land use legacy effect in lakes and should be considered when designing effective management strategies for lake eutrophication.