Inland lakes readily respond to changes in external forcing and therefore serve as sentinels of climate change. Many parts of the world continue to experience declining groundwater levels due to anthropogenic activities such as high-capacity pumping for agriculture or decreases in natural recharge rates of aquifers while lake surface temperatures continue to rise and show a clear warming trend. The responses of individual lakes to these stressors could vary depending upon the positions of the lakes within the landscape and the nature of lake-groundwater interactions. Since temperature is a key driver that affects the structure and function of ecosystems including biological productivity, nutrient cycling and hypoxia, groundwater-fed lakes could be altered drastically due to declining groundwater contribution. Thus, it is crucial to understand the role of groundwater in biophysical processes and to determine what regime shifts may occur in the absence of lake-groundwater interactions. To address this question, extensive field datasets were collected in the Gull Lake, a deep, dimictic, groundwater-fed, inland lake in Michigan, with bottom cooling and strong stratification during summer. The lake supports diverse warm and cold water fisheries. Detailed three-dimensional hydrodynamic and temperature models of Gull Lake coupled to nutrient and algal dynamics were developed to study the effect of groundwater on physical, chemical, and biological processes in the lake. Coupled biophysical processes in the water column are closely linked to meteorological forcing. Therefore, meteorological forcing fields were carefully reconstructed from a network of weather station data, and were assessed using outputs from a mesoscale numerical weather forecasting model (WRF). A novel manifold method of reconstructing dynamically evolving spatial fields is presented for assimilating data from sensor networks in lake and watershed models. The manifold method has been developed based on the assumption that geophysical and meteorological data can be mapped onto an underlying differential manifold. A comparative evaluation of turbulence models was also conducted to improve descriptions of vertical mixing and thermal structure of the lake. The performance of the biophysical model was first evaluated against high-resolution in situ observations, including currents, lake levels, temperature, nutrients, dissolved oxygen, and chlorophyll data. After successfully applying the model to describe current conditions, the developed models were used to understand the responses of the lake ecosystem when the groundwater contribution is absent. Results suggest that groundwater-fed lakes have the ability to buffer seasonal water temperature variations in the hypolimnion, which helps them to withstand disturbances from surface-induced changes. However, groundwater depletion was accompanied by changes in the structure and function of lake ecosystems including lake level changes, rising water temperatures, increased growth rates of algae, oxygen depletion, early anoxia, reduction of light availability, and eutrophication. These results highlight the significant role played by groundwater in inland lakes and indicate that groundwater-dependent ecosystems tend to show greater resilience. In addition to providing insights into key biophysical processes in inland lakes, this study is expected to help strengthen management efforts to improve or maintain the resilience of lake ecosystems.
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