Using Loosely Coupled Models to Assess Climate-Change Impacts on Common Loon Occurrence in Northern Wisconsin

The coupled groundwater/surface-water model GSFLOW was used to simulate the hydrologic response of 27 lakes in the Trout Lake watershed to a variety of climate-change scenarios downscaled by the Wisconsin Initiative on Climate Change Impacts from the IPCC Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Results from the hydrologic model, along with estimates of DOC concentrations were used to develop predictions of dissolved organic carbon (DOC) loading to the study lakes. An empirical DOC model was developed and used to estimate in-lake concentrations of DOC based on the hydrologic inputs. Three scenarios of in-lake total phosphorus change (0, +10% and +25%) were used along with the DOC estimates to simulate changes in secchi depth. The resulting estimates of secchi depth were input to a logistic-regression model to predict the probability of Common Loon occurrence in the study area.

Simulations suggest that all lakes receive a gradual decline in net precipitation (precipitation – evaporation) with time. Drainage lakes in the watershed show relatively minor changes due to the future climate, in part because drainage lakes are relatively low in the watershed, have large contributing areas relative to the lake area, and respond to changing inputs through changes in stream outflow. Seepage lakes, however, show a slight increase in net groundwater and variable net surface-water inputs, resulting in a relatively steady to moderate decline in water levels with time. In almost all cases, the variability of the various hydrologic components increases in the later part of the 21st  century, largely due to increased variability in the downscaled inputs. Because the predicted increase in net groundwater is less than the decrease in net precipitation, the result is a declining water level in the seepage lakes.

The general trend in lakes was toward slightly higher water clarity (secchi depth), as indicated in model results by decreasing DOC loadings from the various hydrologic components. In all but two lakes, DOC concentrations decreased. Decreases in DOC had commensurate increases in clarity for these lakes, even under conditions of elevated in-lake total phosphorus concentrations. The competing effect on clarity, from DOC and chlorophyll, is the result of light extinction caused by DOC, which is three times greater than the light extinction due to chlorophyll. In this setting, decreases in DOC have a much higher influence on clarity compared to increased chlorophyll.

Because water clarity is a primary factor in loon habitat suitability, probabilities of loon occurrence are expected to stay the same or increase slightly between 2010 and 2090 for the study lakes as water clarity gradually increases over time. Changes in nesting habitat characteristics can have large effects on the probability of loon occurrence in lakes. Step changes in nest habitat characteristics generally overwhelm the effect of the estimated changes in water clarity or possible changes in algal productivity. These results point to the critical need to conserve and enhance common loon nesting habitat within the Trout Lake basin, and throughout the current breeding range of loons in Wisconsin.