Lakes are foci of ecological, economic and social processes on many landscapes throughout the world. Small, inland lakes, such as those prominent throughout the Great Lakes region of North America, play a central role in regional hydrologic and biogeochemical cycles, ecological processes, and a wide range of human activities. Over the past two centuries, deforestation, fire suppression, agriculture, industrialization, tourism, and urbanization have transformed landscapes within the region and fundamentally altered the interactions between lakes and their surroundings. For the next century and beyond, the quality of life and the economies of the region will depend upon the quality of the lakes. The North Temperate Lakes Long-Term Ecological Research (NTL LTER) program seeks to understand the long-term ecology of lakes and their interactions with terrestrial, atmospheric, and human processes. Our overarching question is: How do biophysical setting, climate, and changing land use and cover interact to shape lake characteristics and dynamics over time (past, present, future)? Our conceptual framework considers lakes as interactive components of their environment. As collectors of water, energy, and solutes from the landscape and atmosphere, as habitats for aquatic biota, and as attractors of human activities, lakes affect and are affected by natural and human-induced changes in the local and regional landscape and atmosphere. Our perspective extends from analyses of the past (especially the past 150 years of recorded history but also including paleoecological records), through the present, to forecasts of the next 50 years of change in the landscapes and the lakes. We employ a nested set of spatial scales including individual lakes, multiple neighboring lakes, entire lake districts, the Upper Great Lakes region, and lakes of the northern hemisphere. Within the context of this conceptual framework, our program has five inter-related goals. (1) Perceive long-term changes in the physical, chemical, and biological properties of lakes. This goal includes the collection and management of our core datasets. Most of the research we describe depends on these continually evolving, long-term datasets. We are adding three new long-term core datasets: (i) land use and land cover change in the watersheds of our primary study lakes; (ii) human populations and socio-economic characteristics in these core watersheds; and (iii) daily archives of MODIS satellite imagery covering Wisconsin. (2) Understand the drivers of temporal variability in lakes and lake districts. The growth of our long-term database has created several unique opportunities for understanding long-term change in lakes. Five topics have emerged as particularly promising at this stage of the project: (i) dynamics of algal blooms, (ii) patterns in ecosystem production and respiration, (iii) the association of productivity and diversity in lake plankton, (iv) long-term change in the balance of littoral and pelagic processes in lakes, and (v) patterns of fish recruitment. (3) Understand the interaction of spatial processes with long-term change. Lakes are connected dynamically to surrounding landscapes. Long-term change in lakes may depend upon the spatial arrangement of these landscape mosaics. We address three questions in this area. (i) Over what scales of space and time are spatial dependencies important in determining lake characteristics and dynamics? (ii) How do climate, geologic setting, and land use/cover change influence water and solute loading to lakes? (iii) How does spatial positioning of lakes influence their value to humans? (4) Understand causes and predictability of rapid, extensive change in ecosystems. Some ecological changes are rapid and extensive. Such big changes can be difficult to understand or anticipate. Yet understanding the mechanisms underpinning thresholds or regime shifts is a key need for forecasting future change. Our data sets have grown to the point where we can carefully evaluate certain types of big changes. We have selected three for analysis during this grant cycle: (i) eutrophication, (ii) ecosystem consequences of species invasion, and (iii) changes in angler-fish interactions as lakeshores switch between public and private ownership. (5) Build a capacity to forecast the future ecology of lake districts. In coming decades, landscapes and lakes will exhibit complex responses to changes in climate, land use and cover, biota, and human activities such as riparian development and fishing. We will develop methods for forecasting changes in temperate lake districts, as both an emerging research frontier and as a mechanism to generate hypotheses for future long-term research. This research, which is a new theme for NTL LTER, will address two questions. (i) How might climatic, geochemical, ecological, and socioeconomic drivers of the lake districts change in the next 50 years? (ii) How could these changes affect hydrology, biogeochemistry, and ecology of the lakes? In addressing these five goals we use a variety of approaches including long-term observations, small- and large-scale experiments, comparative studies, and process modeling.