The legacy of past land use/cover plays a driving role in current ecosystem state. In addition to correlative relationships, we begin to uncover a mechanistic relationship between lake water characteristics and land use/cover legacies via groundwater pathways. We use a novel combination of principal components analysis and hierarchical regression models to quantify the timescale of legacies for a wide range of limnological variables. Additionally, we created a MODFLOW-2000 groundwater model of a 2600 km2 watershed in Southeastern Michigan to quantify groundwater travel times. We linked these travel times to historical land use/cover information, indicating differences between perceived land use/cover and effective land use/cover.
Results/Conclusions
Our results show that both the presence and timescale of legacy effects depends on the biogeochemical cycles of the variable in question. We found that taking a legacy approach improved the fit of correlative models for total and dissolved nitrogen, reactive ions, and conservative ions. However, the added complexity inherent to correlative legacy models did not substantially improve explanatory power for total and soluble reactive phosphorus. By comparing lake water chemistry variables with vastly different biogeochemical cycles, we can estimate the timescale of legacies. Our results show that highly reactive elements have relatively short legacy timescales, elements of intermediate reactivity have intermediate legacy timescales, and conservative ions have the longest legacy timescales.
Our study explicitly addresses and identifies methods to quantify the temporal component of ecosystem response to land conversion (i.e. land use legacy effect) and provides a mechanistic explanation of why ecosystem responses to management actions may not appear for a number of decades. We also discuss our results within the context of space-for-time substitution.