Seepage Lakes as Indicators of Climate Change

Problem and Research Objectives:

Climate changes predicted for North America over the next century include warming temperatures, long periods of drought, and increased levels of precipitation, all of which pose serious risks to freshwater ecosystems (Poff et al., 2002). Global Climate Models have been used to predict the type and magnitude of physical climate changes. Several scenarios have been suggested and more than one may be accurate, causing great uncertainty in the future of lakes and streams (New England Regional Assessment, 2001).

As climate change is likely to be a complex set of shifts in rainfall and temperature that may have annual, seasonal, and cumulative components, we propose that lakes may provide a sensitive integrator of hydrologic effects of climate change. Prolonged climate shifts such as drought alter the transport of water and solutes to a lake, affecting its water budget and generating strong chemical responses. For example, drier and warmer periods disconnect lakes from their catchments and from local groundwater flowpaths, altering transport of substances such as dissolved organic carbon from adjacent wetlands (Magnuson et al. 1997; Schindler 1997). In addition, during drought lakes often have higher concentrations of more conservative solutes, reflecting increases in evaporation over precipitation and lower lake water levels (Webster et al. 1996; 2000). This response can be complicated in extreme cases where drought is sufficient to reverse groundwater inputs and cause decreased in-lake ion concentrations (Webster et al. 1990).

Landscape position (the position of a lake along a local hydrologic flowpath) can influence the nature of chemical responses to climatic forcing. Lakes located high in the landscape, near groundwater and surface water divides, are subject to more transience in local flowpaths of groundwater and greater variability in lake water levels as climate shifts (Anderson and Cheng 1993; Cheng and Anderson 1994; Winter 1999). Because surface water inlets and outlets are lacking, the hydrologic budgets of high-order, precipitation-dominated, seepage lakes are dominated by precipitation, some groundwater input, evaporation, and inflow from adjacent wetlands. Thus, during climate change, the magnitude of shifts in ionic strength for these water bodies becomes a function of the relative importance of wetland connections, evaporative losses, and groundwater inputs. This close integration between climate and hydrology, make seepage lakes sensitive indicators of climate change (Winter and Rosenberry 1998; Fritz 1996).

In this study we are evaluating chemical indicators that potentially reflect climate-induced shifts in hydrologic connections between lakes and (1) wetlands and (2) groundwater inputs. We expect to find that these precipitation-dominated seepage lakes located higher in the landscape, with weaker connections to groundwater flow systems are responsive to climate and thus provide more sensitive indicators of climate change. In addition, we have access to long-term data on the chemistry of seepage lakes in Wisconsin for comparison with lakes in Maine.

We have three general objectives for this research project:

1. a) Determine if changes in the chemistry of sensitive seepage lakes in Maine reflect climatic shifts over the past two decades.
   b) Determine if there is a strong signal of chemical response to climate-induced shifts in a lake’s hydrologic connections to wetlands and local groundwater systems.
   c) Compare chemical responses of Maine lakes with drought-induced changes in water chemistry observed for similar seepage lakes in Wisconsin.
   d) Interpret responses to climate in the context of decreased acid deposition over the past 20 years

2. Evaluate the potential for using seepage lakes in Maine as sensitive sentinels of climate change.

3. Recommend a research program to monitor lakes for climate change in the future and to identify impacts of concern to the health of lakes and availability of water resources in the future.

In addition to the main objective in this proposal to evaluate the use of Maine lakes as sentinels of climate change, we cannot ignore the value of basic data for increasing our understanding of the ecological responses of lakes to climate.  Climate change has the potential to alter physical and chemical features of lakes in ways that could dramatically change community structure of aquatic organisms and ecosystem processes. In addition, we need better information on how climate influences lake ecosystems in order to understand multiple effects of regional disturbances such as acid rain, UV radiation, and land use alteration. Placing results of both short and long-term studies in a context of climate variability greatly improves our ability to make informed decisions on policy and management actions that affect lake ecosystems.

Report:
Annual Report to USGS July 2003