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

Abstract

Surface water temperatures are warming in many lakes across the globe, and this is widely attributed to warming air temperatures. Yet two lakes in Pennsylvania (USA) have shown long‐term increases in surface water temperatures over the past 27 summers during a period with no significant increase in regional air temperature. We examined the relationship between long‐term trends in seven metrics of whole‐lake thermal structure in two lakes and several potential driver variables. Driver variables included water transparency, lake pH, and meteorological variables. Both lakes exhibited significant surface warming and hypolimnetic cooling, resulting in stronger thermal stratification that further reduced mixing and heat transfer to deep waters. During this time period, there were no long‐term trends in solar radiation or in thawing degree days, but annual precipitation and lake pH increased. Water transparency greatly decreased due to increased dissolved organic matter quantity and color, most likely due to increased precipitation and recovery from anthropogenic acidification. In both lakes, the changes in lake thermal structure and heat distribution were strongly related to the decreases in water transparency and increases in dissolved organic matter. This transparency‐mediated mechanism may augment the effects of air temperature‐driven lake warming in other regions where decreasing transparency is also prevalent, further enhancing increases in surface water temperature and thermal stratification. These results have important ecological and biogeochemical implications, highlighting the need for investigations of multiple drivers to fully understand how lakes will respond to future climate change.

Reconstructing Six Decades of Surface Temperatures at a Shallow Lake

“…The rate of increasing LSWT in the latter phase was also consistent with but higher than the global lake warming trend (0.45 • C/10 year) from the observed and satellite records [10,12,95]. This strong increase of LSWT after 1980 had previously been found in shallow lakes in other regions, such as in northeastern United States [96,97], and northern Europa [61,98].…”

Global lake responses to climate change

“…Moreover, lake specific factors complicate some of these expected thermal responses to climatic variations. Specifically, changes in river discharge regimes 47 and water clarity 48,49 can have a considerable influence on lake surface temperature, amplifying or dampening surface warming rates.…”

The interactive effects of stratospheric ozone depletion, UV radiation, and climate change on aquatic ecosystems

“…This is particularly important in small water bodies. 165 Like DOM, iron-containing compounds selectively absorb UV radiation and have been increasing in many inland water bodies. Increases in concentrations of dissolved iron have been observed in 28% of 340 water bodies examined in 10 different countries across northern Europe and North America.…”

“…185 For coastal or inland waters, the role of weather and land-use changes in altering water clarity and thus the heat budgets can also be important. 165,184 Higher resolution, regional simulations of the Southern Ocean predict latitude-specific changes in marine mixed layer depths, with shallower depths at the northern and southern limits of the Southern Ocean and deeper mixed layer depths in between 58 (Fig. 2).…”