As another year of the lake warming experiment is now behind us, let’s look back at some of the results from year two. We had a very fruitful year with our experimental warming, despite the cool and windy “summer” weather. This summer was much cooler than the previous summer (about 2-3°C colder on average) which can certainly make our attempt at warming a 3.5 hectare, 20 meter deep lake all the more difficult. In addition, there were numerous days throughout the summer where wind gusts were well over 10m/s. Given the correlation between air and water temperatures, it’s fairly easy to understand that these low air temperatures caused the lake water temperatures to be cooler relative to last year. Similarly, high winds can also greatly influence lake water temperatures by increasing mixing between warmer surface waters (epilimnion) and colder bottom waters (hypolimnion; typically remain at 4°C all summer long), eventually leading to a reduction or breakdown in stratification within the lake leading to lower water temperatures. In fact, one of these unseasonably “winter-like” storms (low temperatures and high winds) hit our study lakes mid-July, shortly after warming efforts began and partially mixed all of our study lakes.
Nonetheless, our team successfully ran our three warming units non-stop from July 13th until August 31st (44 days). These units were constantly pumping out water 10-15°C warmer than ambient water temperatures (Fig 1). At four meters depth, or about the middle of the epilimnion, water temperatures were, for a period of time, 2-3°C warmer than in our best reference lake (Fig 2). Throughout the entire warming period, average epilimnetic water temperatures were ~1-2°C higher in our experimental lake. This resulted in a larger, warmer epilimnion denoted by the red/orange/yellow regions in Fig 2. This difference in water temperatures may seem relatively subtle, this resulted in over 127,543 m3 (or 1.3 x 108 L) of water that was 1-3°C warmer in our experimental lake than in our best reference lake! Our experimental lake has a small outlet where temperatures were recorded all summer long. Due to lower density than cold water, warmer water sits at the surface of lake and thus, it may be that there is a flux of this warmed surface water exiting our lake through this small outlet. Indeed this is exactly what we observed; the water trickling out of the outlet of our experimental lake was up to 2+°C warmer than the outlet of our best reference lake. Yet another sign of our experimental warming was the higher stability of the thermocline in our experimental lake relative to our best reference lake. Stability is a metric that refers to the resistance to physical mixing due to the potential energy inherent in the stratification of the water column. This stability is a result of the difference in water temperatures between the epilimnion and hypolimnion, the larger the difference, the higher the stability. Thus, warmer lakes will have higher stability compared to lakes with lower epilimnetic temperatures. The stability of the thermocline in our experimentally lake was ~2x higher than our best reference lake throughout the warming period. This increase in stability was due to the warmer epilimnetic temperatures in our experimental lake and resulted in our experimental lake being stratified for 15 days longer than our best reference lake (Fig 2). Eventually, as happens at the end of every growing season in the Arctic, the rapidly dropping air temperatures, lack of sun, and high winds resulted in our experimental lake completely mixing (turning over) on Sept. 1st which marked the end of our warming efforts for the 2018 season. Together, the increased water temperatures, increased outflow temperatures, increased stability, and longer period of stratification observed in our experimental lake served as important indicators of our successful warming efforts during the 2018 field season. We hope to make the lake even warmer in 2019 with more heating improvements, and with any luck, a year in which the weather works in our favor (i.e., an average or warmer than average year)! We also have some exciting results from the modeling portion of our research. Drs. Jiming Jing & Sarah Null (Utah State University) are interested in simulating and projecting lake temperature dynamics using coupled lake-climate models. We have vastly improved the physically based lake scheme embedded in Community Land Model (CLM) version 4.5 by incorporating an oceanic mixing function to predict the summer stratification and thermal regime (including mixing) in all four of our research lakes. In the original CLM lake scheme, the wind is the only factor that drives the lake mixing processes. However, the improved lake scheme with the oceanic mixing function considers multiple factors that affect the water mixing such as surface heat fluxes and density gradient besides the wind. This lake scheme with a 10-layer structure also simulates detailed processes of ice cycling. Driven only by environmental data, the improved CLM lake scheme is mimicking observed mixing processes and thermal conditions in the lake when compared to the original lake scheme (Fig 3). In addition, the discrepancy between observed and predicted in the temperature-controlled lake appears to be due to the manipulated warming, especially later in the season. Importantly, such a discrepancy was minimized if a heat flux of 18 W/m2 was added to the mixing layer (e.g., experimental warming; Fig 3). These improvements to our models now gives us the ability to simulate thermal dynamics of a wide range of arctic lakes and also gives us the ability to project into the future by feeding in projected future climate data and better understand how climate change will affect the thermal dynamics of arctic lakes. Along with our whole-lake manipulation, our team is also initiating complementary smaller scale experiments to better understand how arctic organisms may respond to a warming climate. This includes experiments with microbes (Dr. Byron Crump & PhD student Natasha Christman) as well as experiments with fish. Due to the temperature-dependence of nearly all physiological processes, climatic warming will likely lead to substantial modifications to individual ectotherm performance with consequences culminating at the population, community, and ecosystem levels. In addition, demand for resources will increase with climatic warming due to increases in metabolic demand; as such, the effects associated with climatic warming may be modulated by resource availability. I performed one such set of experiments this past summer at Toolik Field Station. The goal of this study was to investigate the independent and interactive effects of temperature and food availability on slimy sculpin (Cottus cognatus), an abundant mid-level consumer within many arctic lakes, and one that functions as a predator, competitor, and prey source. To accomplish this goal, we used a factorial experiment manipulating temperature (three levels; low (7°C), medium(12°C) and high(17°C)) and food availability (two levels; low and high). We chose the temperatures based on current and projected lake water temperatures as well as the optimal temperatures of slimy sculpin. In summary, our results indicated significant increases in consumption, excretion, and respiration rates with increases in temperature (Fig 4). In addition, when food was not limiting (i.e., high food treatment), growth increased significantly in the medium temperature treatment; however, when food was limiting (i.e., low food treatment), growth decreased with increasing temperatures (Fig 4). With changes in slimy sculpin growth and overall performance, we expect there to be important consequences for top predators such as Lake Trout (Salvelinus namaycush) and Arctic Char (Salvelinus alpinus), which rely on sculpin as a prey source. Furthermore, responses such as decreased growth may be particularly detrimental for individuals in low-grade environments or populations that must overwinter, as is true in the Arctic. Our results indicate the importance of considering the interaction between temperature and resource availability when evaluating the likely effects of climatic warming on individuals, populations, communities, and entire ecosystems. This concludes the summary and reflections from the previous year of the lake warming experiment. I thank all of the hard-working members of our team and our collaborators for another wonderful year! We are all excited to continue and increase our warming and monitoring efforts next year! Be sure to stay tuned for further updates.
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Author:
Nick Barrett- PhD student on Arctic Lake Warming project Check out my personal Twitter page for various tweets about the project: @WaterWorks_NB |