In temperate climates, lakes typically separate into discrete layers because of subtle differences in water temperature and density. Variations in a lake’s thermal structure, such as increased surface water temperature and sharper stratification, can lead to changes in physical processes that may have profound ecological effects like increased oxygen depletion and habitat loss.
Although scientists have determined that increasing air temperature has been a key driver of surface water warming in many lakes around the globe, studies have documented greater variability in bottom water temperature trends even in lakes located within the same region. To better understand which factors regulate the thermal structure of small lakes, Pilla et al. analyzed a 27-year record of temperature profiles collected in two sheltered lakes on Pennsylvania’s Pocono Plateau, a region that has not experienced a significant increase in air temperature during the past few decades.
The team first searched for trends in seven separate metrics of lake thermal structure, including surface and bottom water temperatures, from vertical profiles collected from both lakes each July from 1988 to 2014. The data show that both have experienced increases in surface water temperature at rates that are double or triple the global mean rate while simultaneously observing decreasing bottom water temperatures.
Using a statistical cointegration analysis, the researchers then searched for relationships between these metrics and potential environmental and meteorological drivers, such as water clarity and pH. They found that the observed changes are most likely due to large decreases in water clarity related to higher concentrations of dissolved organic carbon, which increases the absorption of solar radiation near the lake surface and reduces direct heating of the bottom waters. The authors attribute the browning of the water to increases in annual precipitation as well as rising lake pH due to recovery from anthropogenic acid deposition.
This paper offers strong evidence that factors other than increasing air temperature can alter a lake’s thermal structure and heat distribution. Because a decrease in water clarity can enhance the effects of warming air temperatures and accelerate the warming of surface waters, the results have important implications for understanding how lakes will respond to future environmental and climate change. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2017JG004321, 2018)
—Terri Cook, Freelance Writer