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Lingering live algae cells on Lake Erie floor jumpstart harmful algal blooms

Lingering live algae cells on Lake Erie floor jumpstart harmful algal blooms
December 11, 2018 James Proffitt
Photo courtesy of CSIRO via Wikimedia

Leftover cynobacteria from previous years sitting on lakebed

Photo courtesy of NOAA via Wikimedia

Blue green algae bloom on the shore of Catawaba Island, Ohio in Lake Erie, courtesy of NOAA

Legacy cyanobacteria cells (also called bluegreen algae), which can produce the strain of microcystis responsible for microcystin toxins during late-summer as well as harmful algal blooms (HABS) in autumn on Lake Erie, may be helping jump-start the blooms or otherwise contribute to rapid development of blooms, without respect to current rain events or nutrient run-off from agricultural fields, according to a recently released study.

 
The two-year study, conducted at the Cooperative Institute for Great Lakes Research(CIGLR) at the University of Michigan, was led by three researchers who sampled 16 sites covering about 150 square miles.  The collections included locations from West Sister Island to Toledo and Michigan and pulled material from the very top-most layer of benthos (the top layer of flora, fauna and mud) on the lake floor. 
 
“We were a little bit surprised that where cells had settled in the fall didn’t necessarily match where HABs were most intense the following year,” said Tom Johengen, associate research scientist and associate director of CIGLR, and one of the study’s authors.  He explained the sites were visited for two consecutive years, each autumn and each spring.
 

Photo by GLERL via flickr.com cc 2.0

Tom Johengen at work in the laboratory calibrating instruments for field work, Photo by GLERL via flickr.com cc 2.0

What researchers did find was that there was significant viability found in the lake bottom cells. “We felt like those overwintering cells have a real, significant contribution to the next year’s bloom,” Johengen said. Basically, researchers believe that overwintering cells in the benthos, offer the new season’s bloom a head start.  “There’s a source of seeds that are going to be in addition to the riverine fuel for growth.”

 
While Lake Erie suffered significant algae blooms in the 1970s, they had largely disappeared during the 1980s after phosphorous abatement strategies were utilized to reduce nutrients entering the lake.  While algae blooms re-appeared in the 1990s, they’ve become increasingly more severe in the last decade, despite efforts by scientists, lawmakers and farmers, to combat them.
 
Scientists took the sampled cells and “recruited” them by creating lab conditions of light, temperature and nutrients mirroring the lake’s natural spring and summer states, when HABs begin their growth.

Some overwintering cells found to be more toxic 

“What we’ve found is that early in the season we know that HABs are more toxic,” explained Tim Davis, associate professor of biology at Bowling Green State University, and study author. “What we found in our study is that toxic cells tend to get recruited faster than non-toxic.”
 
Davis said that among bluegreen algae’s microcystis strains, there are both toxic and non-toxic varieties.  In lab tests where Western Basin samples were cultured, the toxic strains woke up and got to business much earlier than the non-toxic.  “I think the number one takeaway is that there are seed populations in the Western Basin and those are driving these blooms.  The blooms are already there – as soon as conditions are favorable, we’re going to see a bloom.”
 

Photo by BGSU via Sarah Bednarski

Dr. George Bullerjahn works with Timothy Davis, and Taylor Tuttle, Photo by BGSU via Sarah Bednarski

Davis said that despite whatever new information comes along, serious nutrient mitigation is likely the key to stop future blooms. 

 
“They need nitrogen and phosphorous.  If we cull that, we can stop these blooms,” Davis said. “We can mitigate the severity.” 
 
But, Davis pointed out, legacy phosphorous, which could be a significant benthos ingredient, could be helping fuel the recent blooms.  “We’re not talking about a one or two-year fix,” he said. “We’re talking about a five or 10-year fix.”
 
And Davis says the effects of climate change on future weather patterns, including more intense and more frequent storms, nutrient loading into the lake and possibly re-suspension of overwintering algae, could bring even more HAB threats to Lake Erie. 

Increasing storms’ effect on HABs still unclear

Studies in other lakes around the world suggest that resuspension events, such as those caused by storms, are a major mechanism for reintroducing those vegetative cells to the water column, where they’re more likely to find the light/nutrients necessary to really start multiplying,” said Christine Kitchens, research technician at CIGLR and the third author.  
But even so, the net impact of increased storm activity on Lake Erie could be undecided.  On one hand, while increased intense storms could exponentially increase re-suspended cells from the benthos deep below, they could also prevent HABs from forming, since HABS thrive in calm water and wave action on the lake tends to break up algal blooms, slowing and preventing their growth, according to Kitchens.
Unfortunately, Kitchens said, there is likely little that can be done about legacy nutrients buried within Lake Erie sediments.  “When you’re working with bodies of water this massive and that share multiple state and international borders, any sort of potential in-lake alteration of sediment chemical make-up is extremely difficult,” she said.  Best option? “I think the best you can do is acknowledge that legacy nutrients do exist and that it might cause a delay in lake response to external nutrient reduction initiatives.”
 

Photo by michigan.gov

Harmful Lake Erie algal bloom, Photo by michigan.gov

Despite the fact that living bluegreen algae cells can be found a foot or more beneath the lake floor, and more than a decade old, the introduction into the lake of the buried cells may be negligible, she said.  Most storms sweep only a few centimeters of sediment off the lake bottom.  Those are the cells which the scientists concentrated on.

 
Whether this study will alter the dynamics used to help manage Lake Erie nutrients is still unclear, according to Johengen.  “If legacy phosphorous proves to be important, that may change the effectiveness of our current nutrients management,” he said. In addition, these findings are the beginning to answering questions that operators at water plants need to know, such as how quickly could HABs form and how toxic can they become?
 
“In just a few days a large change in water quality can occur,” he said. “The study showed that blooms can develop on a larger spatial area and much more rapidly because of these overwintering cells.”
 
And according to Kitchens, one of the next steps could be determining whether the overwintering, vegetative cells participate in what’s called “luxury uptake.”  That’s where they lounge around on the bottom of Lake Erie, nibbling on legacy nutrients in the benthos. “So that they are primed and ready for when they’re reintroduced to the water column in the spring/summer,” she explained.  “Second, we want to better quantify how much microcystis comes out of the sediment over the course of the entire bloom season so that we can start incorporating that information into models.”
 
The study was funded by Great Lakes Restoration Initiative  and published in the journal Plos One on Nov. 21.

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