Increasing surface water temperatures from climate change is allowing lakes to experience longer periods of time with less oxygen towards the bottom, researchers said. With oxygen depleting in the lakes, there is a resemblance to what the world looked like within the first billion years of existence.
Located in Lake Huron off the shores of Middle Island is a 75-foot sinkhole.
Two Department of Earth and Environmental Sciences graduate students are taking on a research project on the concerns of the microbial communities found at the bottom of the sinkhole to determine how they impact life, how lakes may change and nutrient cycles.
Diana Valezquez and Cecilia Howard are both P.h.D candidates at the University of Michigan.
They are working in collaboration with the Thunder Bay National Marine Sanctuary’s divers to collect samples and further their studies regarding the sinkhole and the microbial mats conditions and how it is changing.
“We always have a great time working with them,” Valezquez said. “They're always very helpful.”
For the last couple of years Stephanie Gandulla Research Coordinator and John Bright Maritime Archaeologist and Research Coordinator have been the main divers.
“They’re phenomenal,” Howard said.
The marine sanctuary covers 4300 square miles of northwestern Lake Huron. Gandulla said this body of water is “an amazing place to do all sorts of science.”
“Of course, we are designated because of the rich history and the shipwrecks here,” she said. “But the ecosystem is as rich. And we're always excited to welcome and facilitate such research that Cecilia and Diana are doing.”
The sinkhole has been apart of the sanctuary’s research since it’s designation in 2000.
“It’s really a fascinating part, something that piques people's interest from all over, of course, they love to hear about shipwrecks,” Gandulla said. “But when you hear about a mysterious sinkhole with a purple bacterial mat, I think people are very fascinated by that.”
Gandulla and Bright work together to help map the lake bottom through sending out vessels that have multibeam sonar attached to be mounted to the sinkhole. The sound waves that come back through the sonar tells the team what the lake bottom looks like.
“What's really exciting is that the Great Lakes have not been mapped to high resolution,” Gandulla said. “And you know, we've got probably another 80% left.”
Howard chimed in and said the location of the sinkhole not only has unique biodiversity, but also has easier access to execute research. Other sinkholes have been found by the sanctuary but are closer to over 100 feet deep which the divers and tools are unable to reach.
The team is also collaborating with the Max Planck Institute in Germany by sending them samples to look at the diatom living in the sediment.
“A lot of our work is collaborative,” Valezquez said.
Besides the sanctuary and the institute in Germany, the team is working with other labs in the University of Michigan as well as a group at Grand Valley State University. So, there are all kinds of moving parts and people involved in this project.
What is happening down below?
Valezquez and Howard are studying different aspects of the sinkhole.
Howard is looking into the sediment coming down from the water column since the glaciers retreated 10,000 years ago, they said.
“In this particular sinkhole, one of the unusual things is that it has anoxic groundwater feeding up into it,” Howard said. “And so, the bottom water chemistry -- there's no oxygen. So, you don't have fish and shellfish and worms living there.”
With these diverse microbial communities, Howard is studying what micro-organisms are living there, what they’re doing and how they are using the different chemistry of the sediments to live.
“And then also looking at kind of how this system is changing over time, because we have pretty extensive sampling back to 2013,” they said. “And so, one of the things I'm looking at is how it's changing over that 10-year span.”
Valezquez is looking at a slightly different issue. She said she is looking at the carbon and nitrogen cycling in the sinkhole to try and understand how the low oxygen environment contributes to the carbon and nitrogen cycling.
“And also looking at how the microbial communities play into nutrient cycling of carbon and nitrogen, because those are very big global cycles and regional cycles in the Great Lakes,” she said.
This environment is very unique, Valezquez said.
“I have been looking at literature for other anoxic lakes as well, there are a few that contain microbial mats,” she said. “But Middle Island sinkhole is unique because it has this low oxygen setting and these benthic microbial mats that really have a diverse set of microbial communities living in them.”
Other sinkholes in the area are feeding off the same water source, Howard said. They made the example that the fountain at the library in Alpena is feeding from the same aquifer. It has low oxygen levels and a lot of sulfur, so that is why the fountain’s water has a bad smell.
“So, some of the bacteria that are constantly growing all over the fountain are the same types of bacteria that we're seeing in the sinkhole,” Howard said.
One interesting thing about there being no oxygen in the bottom of the sinkhole, they said, is that organic matter loses a lot of the different ways it decays.
When there is less decay of organic matter, more is revealed about the history of carbon and nitrogen cycling, and what is happening in the water column and in the sediments of the sinkhole.
On another note, Valezquez said climate change is impacting our lakes.
“There's increasing surface water temperatures in lakes,” Valezquez said. “So that means there's not of a lot of mixing going on, which regularly happens in lakes. And so, the lack of mixing allows for lakes to experience, longer periods of time in which there's this low oxygen bottom layer.”
To clarify, Valezquez said the sinkhole is not anoxic because of climate change, it is because it feeds off the groundwater that is already low oxygen levels. However, with climate change heating up the surfaces of water, as mentioned previously, the mixing that usually happens within lakes is not happening.
Therefore, there are prolonged periods of time where the top layer that is oxygenated and the anoxic bottom layer are not mixing to create a lake that is overall oxygenated.
With this research the University of Michigan team is conducting, Valezquez said they can learn about what life might have looked like in the past.
Earth’s history
Up until around 500 million years ago, we had very little oxygen, Howard said. There were two instances in Earth’s history of increases in oxygen in the atmosphere.
The first one, the Great Oxygenation event, was two and a half billion years ago. Before this, there was no oxygen in the atmosphere.
“Other than maybe occasional whiffs [of oxygen] is our current understanding of that first billion and a half years of Earth's history,” Howard said. “And so that earliest life was developing in an environment that had no oxygen.
“So, it was using other things like hydrogen and sulfur, and metals to get energy. Which feels a little bit weird in our current environment of [because] a lot of the things that you see around you, on a visible scale are relying so much on oxygen.”
They used the example of photosynthesis. In that sense, the way of getting energy through photosynthesis does not require oxygen, it just uses carbon dioxide. Instead, the process creates oxygen, Howard said.
“Which is kind of one of the hypotheses for how we initially got oxygen in the atmosphere is from a big burst of photosynthetic organisms,” they said.
The next increase in oxygen was around 500 million years ago. These two bursts of oxygen were probably catastrophic for some types of life that were accustomed to being in an anaerobic environment, Howard said.
However, from the perspective of organisms that use and require oxygen this was ideal. Howard said oxygen is very energetically favorable way of receiving energy. With the second increase of oxygen on Earth, life such as plants and animals started to develop more broadly and quicker.
Now it is the opposite, Howard said. As the lake becomes more anoxic, there are some microbes that thrive in those conditions such as the ones in the sinkhole, but it’s not a great situation to be in for those that rely on an oxygenated lake.
Luckily, Valezquez said there is not a long of wildlife in the sinkhole due to the lack of oxygen. So they are mostly focused on the microorganisms that live there and how they are transforming.
The impact of the lack of oxygen is taking more of a toll on the bottom of the lake, the benthic zone, rather than the surface. Nevertheless, Howard said for the microbial communities that live in that region of the water, it could potentially be a problem.
“So, seeing this type of system where there's low oxygen can really help us understand what might have been happening in these ancient systems where just the surface water has oxygen,” they said. “Because there's not enough oxygen in the atmosphere to penetrate down into those deeper waters.”
Why does this matter and what now?
Besides getting a better understanding of the influence these microorganisms have on the lake and the lack of oxygen, Valezquez said it is important to understand the balance of carbon sources and sinkholes.
“Because that also impacts lake productivity,” she said. “So, increasing carbon might also increase nutrient availability and increase the amount of bacteria or, for instance, algae that is in the lakes.”
Valezquez used the example of Lake Erie and the large number of harmful algal blooms. This has to do with carbon and nitrogen cycling. Therefore, to better understand this will offer insight as to how this will impact the quality of life and drinking water.
Valezquez and Howard visited throughout the summer allowing the sediment trap instrument to collect samples. Upon arrival and after retrieving the sediment samples, the team freezes them as well as extract the DNA to better analyze the life within the deposit.
They are expected to return in the fall for a few days to conduct further research.