Transparency, Geomorphology and Mixing Regime Explain Variability in Trends in Lake Temperature and Stratification across Northeastern North America (1975–2014)

Abstract

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.

Browning‐Related Decreases in Water Transparency Lead to Long‐Term Increases in Surface Water Temperature and Thermal Stratification in Two Small Lakes

“…Increases in surface water temperature and strength of thermal stratification are two of the most pervasive changes observed in lakes and oceans worldwide in response to climate change, and these changes are often attributed to warming air temperature (Gao et al, ; O'Reilly et al, ; Schneider & Hook, ; Trenberth & Fasullo, ). The strong increases in surface water temperatures that we observed in Lacawac (0.77 °C/decade) and Giles (1.04 °C/decade) are 2 to 3 times greater than the recent estimates of the global‐mean rate of lake surface water warming of 0.34 °C/decade (O'Reilly et al, ) and greater still than the northeastern North America regional average of 0.52 °C/decade (Richardson et al, ). In addition to surface warming, we found decreasing hypolimnetic temperatures, stronger strength of thermal stratification, and shallower seasonal thermoclines.…”

Effects of Climate Change on Lake Thermal Structure and Biotic Response in Northern Wilderness Lakes

“…Many of these lakes are similar to our study lakes, i.e., they are not undergoing dramatic land-use change or development [26,27]. In similar lakes where land use is changing, the effects of climate drivers are often masked [62,89]. Warming temperatures could not fully explain the increased burial rates; N deposition and cycling were suggested to be important drivers of recent OC burial trends.…”

“…Indirect effects of climate are mediated through the watershed, for example by changes in the transfer of energy to the lake by shading or by inputs of mass from the watershed (nutrients, DOC, and mineral matter, both particulate and dissolved). In many lake-rich regions, human activities in the lake or watershed may mask changes driven by climate [62][63][64]. We focused on changes in wilderness lakes where direct human impact is minimal and watershed disturbances are primarily limited to natural forces.…”

“…Throughout the world, air temperature increases are correlated with surface water temperature increases [62,67,68] and linked to shorter ice cover, decreased cloud cover, increased summer air temperatures. Based on monitoring data for 26 lakes worldwide from 1970 to 2010, lake surface water temperatures, whole lake temperatures, and bottom water temperatures all increased [66].…”

“…In Wisconsin (USA) lakes, observed trends included shallow water warming in all lakes, but deep water warming only in large lakes, potentially due to the enhanced wind sheltering of small lakes [70]. In northeast North America, Richardson et al [62] synthesized broad regional lake records to show that surface water temperatures are rising more rapidly than air temperatures and more rapidly than other regions; rising temperatures are accompanied by increased lake stratification; deep water temperatures are not showing consistent warming patterns among lakes; clear lakes are warming most rapidly; and lakes farther from ocean influence are warming more quickly. Broadly similar warming trends, especially in shallow water temperatures, were evident in our modeled data and are corroborated by worldwide trends.…”