A conceptual framework for understanding multi-scaled cause-effect relationships between terrestrial and aquatic ecosystems

Background/Question/Methods

In order to make broad-scale inferences about terrestrial-aquatic linkages, ecological studies should span and integrate across multiple spatial and temporal scales. However, because ecological conditions have not been continuously and consistently monitored across broad scales, broad-scale knowledge is often developed by extrapolating from fine-scale studies.  This extrapolation is often misleading because (a) processes at one spatial or temporal scale interact with processes at another scale (i.e., cross-scale interactions; CSIs), leading to non-intuitive relationships, and (b) we do not adequately understand broad-scale (i.e., regional) controls of ecosystem processes. Our objective is to present a framework and approach for studying multi-scaled relationships, including different types of CSIs, that addresses challenges in quantifying multi-scale cause-effect relationships between terrestrial and aquatic ecosystems. We compiled nutrient and organic carbon data from 2,000 lakes in the Midwest and Northeast U.S. We quantified aquatic and terrestrial drivers at two important spatial extents: the local and the regional. We modeled the effects of regional and local predictor variables on lake nutrients and organic carbon to assess the importance of CSIs in modeling these relationships across a broad spatial extent.

Results/Conclusions

We found CSIs were important for understanding broad-scale variation in lake nutrients and organic carbon. For example, the effect of local wetland extent on lake total phosphorus concentrations (TP) differed across regions and was explained by a CSI in which local wetland effects on TP was related to the regional agriculture cover. For lakes in high agriculture regions, the presence of local wetlands was negatively related to TP, whereas the relationship was reversed in low agriculture regions. In contrast, although the effects of local wetlands on total nitrogen concentrations differed across regions, we did not detect any CSIs. Finally, the effect of local wetlands on organic carbon (OC) differed across regions and a CSI existed in which the effect of local wetlands on OC was related to regional groundwater flow. For lakes in regions of high groundwater flow, we found no effect of local wetlands on OC, whereas in regions of low groundwater flow, there were positive effects of local wetlands on OC. Our results show that scientists must consider multiple spatial extents and CSIs that are specific to response variables in order to better understand variation in terrestrial-aquatic linkages.