A man pulls in a gill net full of yellow perch into the fishing boat in Lake Ontario. Among the species sampled in a recent study, yellow perch showed the highest liver concentrations of microcystins.JOHNNY C.Y. LAM/The Globe and Mail
From a human-centred perspective, a recent study of sport fish in Lake St. Clair offers some good news: Fillets prepared from the fish that were sampled for the study were deemed safe to eat.
But while the fish may not threaten human health, many had health issues of their own. Their livers were loaded with microcystins – toxic compounds produced by cyanobacteria, which make up the vivid green algal blooms that thrive in the lake’s fertilizer-enriched waters.
The study, published this month in the Science of the Total Environment journal, follows a similar report that found even higher levels of microcystins in fish sampled from Lake Erie, including two commercially harvested species, yellow perch and walleye.
“All of the fish we sampled had microcystins in their livers, and most had them at levels that are known to adversely affect growth and development,” said René Shahmohamadloo, an ecotoxicologist and postdoctoral researcher at the University of Guelph who led both studies.
Since the fish would be expected to swim away if they encountered a high concentration of the toxins in a relatively small area, it is more probable that they are accumulating the microcystins gradually through prolonged exposure, and not just during a single algal bloom, Dr. Shahmohamadloo said.
“I suspect that the fish I collected were exposed in previous years,” he added. “My concern is that they are holding on to these toxins.”
The studies are a potential harbinger of more serious effects on fish populations in the years ahead, as a warming climate sets up conditions for more intense algal blooms.
Algal blooms have been an issue on the lakes for decades. Their impact on public health can be immediate: Such was the case in August, 2014, when more than half a million residents living in and around Toledo, Ohio, were ordered not to drink from the city’s treated water supply, after it was contaminated with high levels of microcystins from an algal bloom. Boiling water does not remove the toxins.
Severity index of algal blooms in Western Lake Erie basin
The severity index is based on the amount of biomass over the peak 30 days
10
8
6
4
Recommended target level
2
0
2002
2006
2010
2014
2018
2022
THE GLOBE AND MAIL, SOURCE: NOAA
Severity index of algal blooms in Western Lake Erie basin
The severity index is based on the amount of biomass over the peak 30 days
10
8
6
4
Recommended target level
2
0
2002
2004
2006
2008
2010
2012
2014
2016
2018
2020
2022
THE GLOBE AND MAIL, SOURCE: NOAA
Severity index of algal blooms in Western Lake Erie basin
The severity index is based on the amount of biomass over the peak 30 days
10
8
6
4
Recommended target level
2
0
2002
2004
2006
2008
2010
2012
2014
2016
2018
2020
2022
THE GLOBE AND MAIL, SOURCE: NOAA
Michael McKay, who directs the Great Lakes Institutes for Environmental Research at the University of Windsor, and who was among those who investigated the incident, said several factors combined to create the water crisis in Toledo, including wind direction near the city’s water intake and the possibility that bacteria were infected by viruses that promoted a large release of microcystins.
Since then, the city has upgraded its water treatment operations to prevent a repeat occurrence. But such measures do not address the root cause of algal blooms, which are mainly the result of phosphorus-rich fertilizer washing into the lakes.
Ohio’s Maumee River and the Thames River of Southwestern Ontario, both of which drain large and intensely farmed regions, are among the main sources of phosphorous that simulate the proliferation of cyanobacteria in the region.
“In both cases, you’ve got a river full of nutrients emptying into a shallow body of water that is warm in the summer and a bit stagnant,” Dr. McKay said.
The two Guelph studies are part of a growing body of new research into the effects of algal blooms on the lakes. For the studies, Dr. Shahmohamadloo used fish that were sampled between 2015 and 2018 and then frozen while researchers developed a high precision method for measuring microcystin concentrations. The effects of those concentrations were studied concurrently by using fish in laboratory studies.
Among the species sampled, yellow perch showed the highest liver concentrations of microcystins, possibly as a consequence of their omnivorous diet which could increase their uptake of the toxic compounds, Dr. Shahmohamadloo said.
Dr. McKay, who was not involved in the studies, said the results were consistent with other recent findings and raise the question of exactly how fish health is affected – for example, by a possible increase in liver cancer rates. He also added that the study only considered one type of toxin, but that algal blooms are known to produce others.
“Microcystins might just be the tip of the iceberg when looking at the influence of cyanobacterial blooms on our Great Lakes,” he said.
Both Canada and the U.S. have adopted a plan to reduce the phosphorous load on Lake Erie by 40 per cent relative to 2008 levels, but the target is seen by many experts as more aspirational than achievable.
Meanwhile, climate change is expected to nudge the lakes in the opposite direction, as warmer waters lengthen the season for blooms and may also benefit cyanobacteria more than competing species of microorganisms.
Another, more subtle climate effect is a shift in rainfall patterns, which is causing more of the region’s annual precipitation to fall earlier in the growing season, sometimes in the form of intense downpours.
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“What we expect is that more of that rainfall is going to come in the spring and there’ll be more of these extreme events pushing things in the direction of more phosphorus flowing into the lakes,” said Anna Michalak, a geophysicist at Stanford University and director of global ecology at the Carnegie Institute for Science, who has studied Lake Erie’s two largest blooms to date in 2011 and 2015.
In those studies, Dr. Michalak and colleagues also showed that repeated loading of phosphorus into the lakes year on year means that it could take a decade before concerted efforts to reduce runoff through more precise application of fertilizers show a pay off.
For Dr. Shahmohamadloo, that lag time is further reason to prioritize efforts that can reduce the stress on fish species in the lakes.
“They’re a voiceless population,” he said. “Just because it’s safe for us to eat them, doesn’t mean we should neglect the conditions that threaten their survival.”
Ivan Semeniuk