Lake surface water temperatures are warming worldwide, raising concerns about the future integrity of valuable lake ecosystem services. In contrast to surface water temperatures, we know far less about what is happening to water temperature beneath the surface, where most organisms live. Moreover, we know little about which characteristics make lakes more or less sensitive to climate change and other environmental stressors. We examined changes in lake thermal structure for 231 lakes across northeastern North America (NENA), a region with an exceptionally high density of lakes. We determined how lake thermal structure has changed in recent decades and assessed which lake characteristics are related to changes in lake thermal structure. In general, NENA lakes had increasing near-surface temperatures and thermal stratification strength. On average, changes in deepwater temperatures for the 231 lakes were not significantly different than zero, but individually, half of the lakes experienced warming and half cooling deepwater temperature through time. More transparent lakes (Secchi transparency >5 m) tended to have higher near-surface warming and greater increases in strength of thermal stratification than less transparent lakes. Whole-lake warming was greatest in polymictic lakes, where frequent summer mixing distributed heat throughout the water column. Lakes often function as important sentinels of climate change, but lake characteristics within and across regions modify the magnitude of the signal with important implications for lake biology, ecology and chemistry.

Lakes and reservoirs are recognized as important sentinels of climate change, integrating catchment and atmospheric climate change drivers. Climate change conceptual models generally consider lakes and reservoirs together despite the possibility that these systems respond differently to climate-related drivers. Here, we synthesize differences between lake and reservoir characteristics that are likely important for predicting waterbody response to climate change. To better articulate these differences, we revised the energy mass flux framework, a conceptual model for the effects of climate change on lentic ecosystems, to explicitly consider the differential responses of lake versus reservoir ecosystems. The model predicts that catchment and management characteristics will be more important mediators of climate effects in reservoirs than in natural lakes. Given the increased reliance on reservoirs globally, we highlight current gaps in our understanding of these systems and suggest research directions to further characterize regional and continental differences among lakes and reservoirs. Author Contribution Statement: NMH and BRD co-led the manuscript effort and contributed equally. JRC and BRD conducted the statistical analyses. KES and JRC designed the lake pairing analysis. NMH, NRR, and KES developed the climate change conceptual model. This paper was a highly collaborative effort and all authors contributed equally to the development of the research question and study design as well as the writing of the paper. This is an open access article under the terms of the Creative Commons Attribution NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. Scientific Significance StatementClimate change poses a significant threat to freshwater ecosystems, though the exact nature of these threats can vary by waterbody type. An existing conceptual model describes how altered fluxes of mass and energy will affect standing waterbodies, but it does not differentiate reservoirs from lakes. Here, we synthesize evidence suggesting that lakes and reservoirs differ in fundamental ways that are likely to influence their response to climate change. We then present a revised conceptual model that contrasts climate change effects on reservoirs versus lakes. 47Limnology and Oceanography Letters 2, 2017, 47-62

Globally, lake surface water temperatures have warmed rapidly relative to air temperatures, but changes in deepwater temperatures and vertical thermal structure are still largely unknown. We have compiled the most comprehensive data set to date of long-term (1970–2009) summertime vertical temperature profiles in lakes across the world to examine trends and drivers of whole-lake vertical thermal structure. We found significant increases in surface water temperatures across lakes at an average rate of + 0.37 °C decade−1, comparable to changes reported previously for other lakes, and similarly consistent trends of increasing water column stability (+ 0.08 kg m−3 decade−1). In contrast, however, deepwater temperature trends showed little change on average (+ 0.06 °C decade−1), but had high variability across lakes, with trends in individual lakes ranging from − 0.68 °C decade−1 to + 0.65 °C decade−1. The variability in deepwater temperature trends was not explained by trends in either surface water temperatures or thermal stability within lakes, and only 8.4% was explained by lake thermal region or local lake characteristics in a random forest analysis. These findings suggest that external drivers beyond our tested lake characteristics are important in explaining long-term trends in thermal structure, such as local to regional climate patterns or additional external anthropogenic influences.

Previous reports suggest variable trends in recovery from acidification in northeastern U.S. surface waters in response to the Clean Air Act Amendments. Here we analyze recent trends in emissions, wet deposition, and lake chemistry using long-term data from a variety of lakes in the Adirondack Mountains and New England. Sulfate concentration in wet deposition declined by more than 40% in the 2000s and sulfate concentration in lakes declined at a greater rate from 2002 to 2010 than during the 1980s or 1990s (-3.27 μeq L(-1)year(-1) as compared to -1.26 μeq L(-1)year(-1)). During the 2000s, nitrate concentration in wet deposition declined by more than 50% and nitrate concentration in lakes, which had no linear trend prior to 2000, declined at a rate of -0.05 μeq L(-1)year(-1). Base cation concentrations, which decreased during the 1990s (-1.5 μeq L(-1) year(-1)), have stabilized in New England lakes. Although total aluminum concentrations increased since 1999 (2.57 μg L(-1) year(-1)), there was a shift to nontoxic, organic aluminum. Despite this recent acceleration in recovery in multiple variables, both ANC and pH continue to have variable trends. This may be due in part to variable trajectories in the concentrations of base cations and dissolved organic carbon among our study lakes.

We evaluated trends in dissolved organic carbon (DOC) and associated changes in water transparency and epilimnion thickness to better understand the implications of regional increases in DOC concentration in lakes. Long‐term monitoring of a suite of physical, chemical, and biological data from six to 12 lakes in Acadia National Park in Maine was paired with high‐frequency sensor monitoring of one lake as a model system. Water transparency declined across study sites since 1995 as DOC increased and chlorophyll remained stable, suggesting that this was not a signal of increased eutrophication. As clarity declined, some lakes experienced reduced epilimnion thickness. The degree to which transparency changed across the lakes was dependent on DOC concentration, with a larger decline in transparency occurring in clear water lakes (−0.3 m yr−1) than brown water lakes (−0.1 m yr−1). DOC concentration was an important explanatory variable for reduced epilimnion thickness in short‐term sensor measurements. A regional decline in water transparency across all lakes and reduction in epilimnion thickness in a limited number of systems appeared to be acting as a sentinel for changes in atmospheric deposition and regional weather that modified the delivery of DOC from the watershed.