By Blaize Denfeld
During the GLEON 18 meeting last summer, the lake metabolism working group started the week by discussing the current state of knowledge of greenhouse gas (GHG) emissions from inland waters – it is evident that inland waters play an important role in the global carbon cycle, emitting both carbon dioxide (CO2) and methane (CH4) into the atmosphere. As new scientific research is published and advancements in technology are made, as scientists, we can fill gaps and improve our scientific understanding. One such gap identified by the lake metabolism working group was the potential sampling bias in greenhouse gas emission estimates from lakes, since most estimates are made during the day. Inspired by the current EuroRun project— a Collaborative European Freshwater Science Project for Young Researchers that focuses on estimating CO2 fluxes from European running waters— and thanks to relatively inexpensive technological advancements in GHG emission measurements, the DC Flux (diurnal CO2 flux variation across latitudinal gradients) project was born.
The main objective of the DC Flux project is to understand how CO2 emissions from lakes vary in time, between day and night, and in space, across a latitudinal gradient. Utilizing the geographically diverse GLEON community, the 24 participating lakes are from Europe (18), North America (5), and South America (1). Each team at the participating lake is responsible for measuring CO2 emission every 4 hours over a 28-hour sampling period during three seasonally different sampling campaigns. For example, since the lake I study is in Northern Sweden and is ice-covered for a portion of the year, we carried out our three sampling campaigns following ice-melt in May, during thermal stratification in summer and during lake mixing in the autumn. While in Sau Reservoir in Spain, Rafa Marcé and Biel Obrador sampled in May when the lake was undersaturated in CO2, and will sample again in the Autumn and during the ‘non-ice covered’ winter early next year.
The DC Flux group is not only geographically diverse but is also made up of scientists at different career stages and disciplines. Dr. Elvira de Eyto and PhD student Brian Doyle, at the Marine Institute in Ireland, graciously volunteered to help chair the DC Flux project. “It’s quite a challenge to organize the group, as it is large (currently 95 members on the email list, based in 25 countries), with people from many different backgrounds and levels of experience,” said de Eyto. “Some have been measuring GHG emissions for many years, using expensive equipment, while some are on a very limited budget and just want to make a start. One of the aims of DC Flux which was vocalized at the inception of the group was the possibility of using relatively cheap sensors to expand the range and geographic spread of lakes for which we have some GHG measurements, and I think this is still one of the main drivers of the working group.”
Since the GLEON 18 meeting, the DC flux working group has had five group meetings using the GLEON Zoom account. At the most recent meeting, 17 GLEON members participated representing 10 different countries.
Although the task of coordinating a large working group can be challenging, de Eyto has had a positive experience, “I have really enjoyed the networking side of the project so far, getting to know lots of new people, and making valuable connections. It’s quite exciting when we successfully pull off a big zoom meeting!” The DC Flux group has been actively carrying out sampling campaigns this summer and looks forward to having an in-person meeting at the GLEON 19 meeting at lake Mohonk, NY, USA this fall.
To get a better idea of what it is like to sample every 4 hours over a 28-hour period in different climate conditions, myself, Blaize Denfeld (Post Doc, Umeå University, Sweden), Ryan McClure (PhD Student, Virginia Tech, USA) and Hannah Chmiel (Post Doc, EPLF, Switzerland) answered a few questions to help share our experience.
Q: How would you describe the lake(s) you are sampling?
Ryan: We are sampling two reservoirs in Virginia during each seasonal sampling campaign and each site is unique. Our most heavily sampled site, Falling Creek Reservoir, has both a hypolimnetic oxygenation system and an epilimnetic mixer. During the 24-hr campaigns, the oxygenation system is running but the epilimnetic mixer is not. The second site, Beaverdam Reservoir, does not have any engineered systems but has experienced a drawdown of almost 4 meters since the beginning of the summer.
Blaize: We are sampling Stortjärn Lake, the northern most lake in the DC Flux project (64˚N, 19˚E), so we have a much shorter open water season compared to many other participating lakes. Stortjärn Lake is dimictic, meaning that it normally turnovers during spring and fall and stratifies during the winter and summer. Typical for the Scandinavian region, Stortjärn Lake is a small ‘brown’ boreal lake (0.04 km2).
Hannah: So far, we have sampled two lakes in Switzerland (Lake Geneva and Lake Cadagno) and one lake in Russia (Lake Onego). Lake Geneva is a 300-m deep lake located in a densely-populated area at the edge of the Alps. Lake Cadagno is a remote, small and quite special lake in the Alps. It is meromictic, meaning the lake never mixes. The reason for this phenomenon is that it has a sulfur oxidizing bacterial layer in the metalimnion, which is so dense, that it prevents the lake from overturn. Lake Onego is the third largest lake in Russia (9,700 km2), and surrounded mostly by boreal forest.
Q: What is it like sampling the lake when it is dark?
Ryan: Sampling in the dark is awesome! Being out at night feels like sampling in a whole new system from what I am used to. I have become accustomed to daily monitoring and oftentimes forget that a large part of a 24-hr cycle is dark and so many cool things happen then. For example, when the winds change, it becomes eerily calm.
Blaize: I agree, sampling at night was a truly awesome experience. Since the lake I study is in northern Sweden, during the Autumn sampling campaign we were fortunate to witness the northern lights! The Autumn sampling was the only sampling campaign with true darkness, during the spring and summer sampling we had almost complete light.
Hannah: Sampling at night is definitely an adventure, and it comes with both good and bad surprises. We had a sampling night out on Lake Geneva, which started with a beautiful sunset and the panorama of the Alps colored in pink. However, when darkness fell, we discovered the true owners of our boat: spiders. Everywhere. In all sizes and quite active in trying to catch the food that our little lights were attracting. They had a good meal that night!
Q: Which sampling campaign/season did you like best and why?
Ryan: Honestly, we have only completed the summer sampling campaign and still have the fall and winter to complete. In previous years, I personally have enjoyed overnight campaigns in late summer because of the cooler nights.
Blaize: I don’t know if I had a favorite, I liked them all for different reasons. Our first sampling in the Autumn was the most challenging and cold but the northern lights made up for it. In spring, it was exciting to be on the lake the day after ice-melt, there were a ton of frogs waking up from the long winter. In summer, we experienced almost 24-hours of complete light!
Hannah: So far, I have only sampled during the summer season, and each lake had its own unique night. Lake Onego in Russia was an amazing experience, since we had only about half an hour of darkness. I am looking forward to the other seasons, though it will probably become more challenging.
Q: Do you have any recommendations for others that want to carry out a 24-sampling campaign?
Ryan: I recommend bringing a lot of snacks. Coffee is also a perk. Also, bring supplies to make a large dinner and breakfast to get through the night and the following morning. Our lab likes to make coals over the fire and then make two big dutch oven meals during the sampling campaign.
Blaize: Make sure to pack layers. It may be warm during the day but temperatures can drop at night. Be sure to have enough light so that you can see what you are doing on the lake. It is also good to leave a light near the shore if you are on a remote lake, so that you know how to get back… I learned that the hard way.
Hannah: I agree that good lights help a lot, especially in remote areas. Also, I recommend that when you do a 24-hour cycle, start during the daytime where there is enough light to figure out your routine. We once started late in the evening, and it was tricky to work in the dark without having figured out a sampling routine before.
Blaize Denfeld is currently a postdoc at Umeå University, Sweden and chair for the GLEON Student Association (GSA).