Year two of the lake warming experiment is well underway and our team has been staying very busy with sampling, setting up and tending to the warming equipment, and slinging propane. Last summer on the North Slope was very dry and warm, particularly in June. However, this summer has been much cooler and wetter thus far, with the pleasant sunny days few and far between. This difference in weather patterns year-to-year highlights the high degree of inter-annual variation in the Arctic climate, a topic that is of particular interest to our research. The long-term sampling done by the ARC-LTER based out of Toolik is instrumental in tracking how organisms and ecosystems respond to changes in the environment overtime, not only year-to-year, but also over decades. Data obtained by the LTER sampling regime will allow us and other researchers to gain a better understanding of how these arctic ecosystems will respond to changes in climate.
As for the lake warming experiment, our team (Peter MacKinnon, Gary P. Thiede, Tyler Arnold, Ryan West, and myself) began setting up the warming units in early July. As a recap, our warming units consist of eight 100-gallon propane tanks, a propane powered generator which powers a pool pump and a large water heater. Alterations to these units from last year include two more tanks per station, new heaters, and different generators. In addition, this year we have shifted all of our warming efforts to Fog 1, which now has three warming units located on the north and west shores. A spatial layout of the location of our units and large water circulation instrument (SolarBee) is provided in Figure 1. The process carried out by our warming units involves pumping water from a shallow intake hose and then sending this water through the heater and back out into the lake via a long output hose set about 3m below the water surface. Our first few propane slings of the year have gone very smoothly, with many thanks to Pollux aviation and their great coordinators and pilots. Our slings consist of moving 24 full tanks (~650 lbs.) from a turnout off the Dalton Highway to the warming units and 24 empty tanks from the warming units to the road turnout to be re-filled by Crowley Propane. These slings will take place every 4-5 days for the majority of the summer. After a chilly start to the season, we are excited to announce that the warming equipment has been turned on as of 7/14 and warm water is flowing into Fog 1! Currently, the three output hoses are rapidly sending heated water that is about 26°C (more than 10°C above ambient water temperature) into the lake. Temperature profiles obtained from our temperature loggers deployed in each of the Fog lakes are instrumental in tracking thermal dynamics in each of the lakes overtime. Using this data we will be able to track changes in water temperature between our experimental (Fog 1) and reference lakes (Fog 2 & 3). Temperature profiles over time in the form of heat maps are provided in figures 2 & 3. Currently, Fogs 1, 2, and 3, are stratified between 7 and 9m deep. Fog 5, the smaller, shallower lake is currently stratified at about 2m deep, however, this lake has experienced a couple mixing events in which the entire lake became isothermal (equal temperatures throughout the water column). We’ve experienced two substantial wind storms so far this summer where gusts topped out at over 12m/s. Evidence of these wind storms on July 4th and 18th can be seen in the temperature profiles with the deepening of the thermoclines in Fogs 1, 2, and 3 and full mixing of Fog 5. Wind events like we’ve seen here play an important role in lake mixing and can quickly breakdown the density gradients between the warm and cold water, resulting in weakening and/or deepening of the thermocline and even full lake mixing events. Despite the cool and windy weather patterns, the epilimnetic water temperatures of Fog 1 are currently 0.5-1.5 °C warmer on average compared to the reference lakes Fogs 2 & 3. We shall see how these patterns in water temperature pan out for the remainder of the season. Our fieldwork and sampling this season kicked off with setting up our climate stations and de-winterizing our buoys and temperature chains on each of the Fog lakes. Our climate stations are set up on the shore of the lakes and collect data on wind speed and direction, air temperature, humidity, solar radiation, and photosynthetically active radiation (PAR). Our buoys and temperature loggers have overwintered under the think arctic ice and must be pulled up, downloaded, reset with more loggers, and redeployed (i.e., de-winterized). This season our NSF REU (Ryan West) and I had to wait until enough ice melted from the lakes before we could obtain these temperature loggers and re-set our buoys for the summer season. Last summer the Fog lakes were ice-free by mid-June, however this season we had to wait a couple more weeks until the lakes were open. After downloading the data from the temperature loggers, we re-set the buoys with more temperature loggers (every meter) which will provide high-resolution data on the thermal dynamics within each of the Fog lakes. We also deployed three MiniDOT dissolved oxygen loggers set in the epilimnion (surface waters), metalimnion, and hypolimnion (bottom waters) of each lake. The data provided by these MiniDOTs will be used to estimate ecosystem metabolism, which represents the balance of the uptake and production of oxygen and carbon dioxide through photosynthesis and respiration. Ecosystem metabolism and the balance between primary production and respiration is predicted to change as a consequence of warming due to the temperature-dependence of cellular processes. Changes in ecosystem metabolism have the potential to alter ecosystem function, productivity, and food web dynamics. Understanding how warming will affect ecosystem metabolism in arctic lakes is a topic of my dissertation research. Ryan and I also performed early season sampling for benthic macroinvertenbrates and stable isotopes on each of the Fog lakes. This consisted of eckman grabs from shallow (0.5m), intermediate (3m), and deep (7m) depths and performing snail transects via snorkeling. Within these arctic lakes benthic marcroinvertenbrates such as snails, chironomids, and trichoptera are very important food sources for arctic fishes such as arctic char, lake trout, and arctic grayling. The response of arctic fishes to warming will be highly dependent upon the response of their food source. Therefore, investigating changes in macroinvertebrate communities in response to warming will be paramount in understanding how arctic fishes may respond to a changing climate. In addition, we sampled the Fog lakes for stable isotopes. Stable isotopes of carbon and nitrogen can be used to construct and assemble aquatic food webs and provide information on where (carbon) and what (nitrogen) organisms are eating. This sampling consists of collecting organisms from each level of the food web including benthic algae, phytoplankton, benthic macroinvertebrates, zooplankton, small fish such as slimy sculpin, and fin clips from larger arctic char. This stable isotope data will be used to track changes in aquatic food webs overtime, including what and where the organisms are eating. For the remainder of the field season (until late Aug/early Sept.) our team will be staying busy sampling, slinging, and tending to the warming units so be sure to stay tuned for more updates throughout the season! Also, be sure to check out the pictures from this field season (as well as the previous season) in the pictures tab.
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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 |