Year one of the lake warming experiment provided our team with endless opportunities to learn about what works and what doesn’t work when trying to warm whole lakes off the grid in Arctic AK. At the end of the last blog post, we indicated two of our three warming units were turned on and warmed water was being pumped into our lakes. Unfortunately, due to technical difficulties with the generator, the third unit remained inactive. When the units were initially turned on, the weather had been warm and winds calm. This allowed for ideal conditions to start our lake warming, surface water temperatures were very warm, and the lakes were strongly stratified. However, shortly after turning on the units the weather took a turn; temperatures decreased drammatically and winds picked up. This resulted in unfavorable conditions for warming, including decreased water temperatures and increased mixing. In addition, we encountered numerous mechanical difficulties (i.e., learning experiences) throughout the summer driven by the malfunctioning of generators and heaters. As a result, the delivering of warm water into our lakes was inconsistent over time. That said, many of the issues we confronted have now been addressed including simply learning which types of generators and pool heaters work best in this situation. We also learned a lot about the most efficient strategy for switching out the propane tanks that power our warming units. After 4-5 days of operation, the 100-gallon propane tanks are switched out via helicopter. By the end of the season, we perfected the art of propane switch-outs, completing the entire process of shutdown, switch-out, and power on in less than an hour. We now feel poised and confident to warm the larger lake this upcoming season (discussed further below). In order to track the input of warm water into our lakes, we mounted temperature loggers to the end of our output hoses, which carried the warm water from the heaters and delivered it in to the lake. Figure 1 (downloadable file provided below) displays water temperatures at the outputs hoses and compares these to the water temperatures at one and two meters below the surface. As you can see, we were successful in pumping warm water into our lakes. However, as the figures indicate, there were numerous interruptions in the deliverance of warmed water into the lake. Nonetheless, the warmed water ranged from ~4-10°C warmer than the water in the lake. The thermal regimes of the “warmed” lakes and the reference lakes were very dynamic in 2017. Figure 2 displays water temperature profiles from within each of the Fog lakes throughout the summer. These plots clearly display the thermally stratified layers within our lakes. The epilimnion is indicated by the yellow and green colors (~13°C and above), the hypolimnion in green and light-blue, while the hypolimnion is in darker blue. As you can see, throughout the summer months the temperatures and stratification within our lakes are quite dynamic, particularly in the epilimnion. Furthermore, the epilimnion became deeper over the course of the summer. Eventually, the cold temperatures and wind lead to the thermally stratified layers breaking down and full mixing of the water column (occurred during late Aug). In Figure 2, Fog 1 and Fog 5 represent our experimental lakes while Fog 2 and Fog 3 served as references lakes. Although we can see the signal of the warmed water in the epilimnion soon after the lake warmers were turned on, the extremely cold and atypical weather and air temperatures, in addition to an unanticipated mixing event, minimized the whole lake effects of warming. Despite all of the time spent implementing and working with the lake warming equipment, we did manage to get a large amount of sampling done on the Fog lakes. Throughout the summer the lakes were sampled by the "limnology crew" headed by Dan White. This crew takes weekly measurements of various physical and chemical parameters, including temperature, dissolved oxygen (DO), nutrient concentrations, pH, conductivity. The limnology crew also takes samples of the phytoplankton and zooplankton within the lake. The Fog lakes were also sampled for benthic invertebrates (e.g., caddisflies and snails), stable isotopes (for food web analysis), and fish (However, our most successful fish sampling is performed through the ice in mid-May). In addition, the temperatures within our lakes were closely tracked using a profile of temperature loggers that logged water temperature at a given depth every hour. Eventually, all of this data collected from the lakes will be added to the massive ARC-LTER database and used to better understand the ecology of these systems. Furthermore, This data will be paramount in tracking changes in these important lake characteristics over time. As part of my dissertation research, I am interested in understanding how warming may affect the availability of suitable habitat for fish as well as ecosystem-scale processes such as lake metabolism. Warming may lead to substantial reductions in suitable habitat for arctic fishes through increases in surface water temperatures and reduced levels of DO (i.e., hypoxia) in hypolimnetic waters (leading to a “temperature-oxygen squeeze”). To begin to look at this more closely, we quantified the volume of unsuitable habitat (temperatures >15°C & DO < 5mg/L) for Arctic Char (Salvelinus alpinus) using three-dimensional temperature and DO profiles. Figure 3 and Table 1 show that both increased temperatures and decreased levels of DO can lead to significant reductions in suitable habitat for Arctic Char. A combination of increased temperatures, prolonged stratification, and hypoxia lead to the largest reductions in suitable habitat in Fog 5 (up to 73% during the summer of 2017). Consequently, fish will need to locally adapt to changing conditions to avoid negative consequences such as reduced growth and survival. Future work will include the collection of more data in order to analyze these relationships further and to develop models that can be used to predict warming-induced reductions in habitat for multiple species of fish found on the North Slope. Due to the temperature dependence of physiological processes at the cellular level (photosynthesis and respiration), warming may also have substantial effects on important ecosystem processes such as lake metabolism. Furthermore, by calculating lake metabolism we can gain valuable information on food web structure, carbon dynamics, sources of energy, and overall productivity of these systems. In order to calculate estimates of gross primary production (GPP), ecosystem respiration (R), and lake metabolism (NEP), this summer we deployed sensors to continuously track changes in DO (10-min time steps) in the Fog lakes. Figure 4 shows preliminary estimates of GPP, R, and NEP from each of the Fog lakes. In general, these lakes display low productivity, reflecting their oligotrophic state. However, Fog 5 displayed higher levels of productivity, likely due to it’s morphology, low position in the landscape, and higher levels of organic matter. This may suggest that more productive (i.e., higher rates of GPP & R) systems such as Fog 5 may be particularly responsive to climatic warming. Future work will include more data collection during subsequent field seasons and model development in order to evaluate the affects of warming on lake metabolism As year one came to a close and our lakes began to form ice a thin sheet of ice, it was time to shutdown, clean up, thank our wonderful crew for all of their hard work, and prepare ourselves for year two. Despite some of the roadblocks encountered, we couldn’t be more excited and anxious to start year two. We learned an incredible amount during year one and by using the knowledge gained and applying it to our efforts moving forward, we are now fully prepared to take on year two of the warming experiment. In addition, our team has hired a full time lake warming technician (Tyler Arnold) who’s sole responsibility will be managing our warming units and keeping that warm water pumping. Furthermore, our team has decided to slightly alter our plan of attack. This summer we plan to house all three warming units at Fog 1 and therefore concentrate all of our warming efforts to this lake. Plans are to begin set up of the equipment in Late May/early June so that it will be ready to be turned on when the conditions within the lake are most ideal (i.e., warm surface waters and strong thermal stratification). Thanks for your interest in our project and be sure to stay tuned for updates during year two!
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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 |