Updated by Brad Plumer
Toxic algae blooms are back. After largely subsiding in the 1980s and 1990s, these algae blooms have beencausing trouble in Lake Erie in the last decade.
This past weekend, the city of Toledo had to warn 500,000 people to avoid all tap water for three days after a toxin created by blue-green algae showed up in the system.
The likely culprit is a massive algae bloom that is trapped on the western edge of Lake Erie by winds and currents. These algae blooms are fed by excess nutrients like phosphorus that are added to the water — fertilizer runoff from farms or effluent from sewage-treatment plants.* Those algae blooms can, in turn, kill off plant and animal life in the lake and poison the freshwater supply.
But why are algae blooms making a comeback? After all, the region around Lake Erie took drastic measures in the 1970s to limit the amount of phosphorus entering the lake in order to curtail large algae blooms. That worked for a few decades. But since the late 1990s, large algae blooms have returned to Lake Erie. Possible culprits include changing farming practices, zebra mussels, and even climate change.
Here’s an overview:
The US curtailed Lake Erie’s algae blooms in the 1970s…
Lake Erie had a massive problem with algae blooms in the 1960s and 1970s, with the lake becoming nearly unusable. Decomposing green algae kept washing up on beaches and had to be removed by bulldozers. Blue-green algae was tainting the water supplies of several cities.
So, starting in 1972, the United States and Canada began taking drastic action. The region spent $8 billionupgrading sewage systems around the region, banned certain types of phosphorus-heavy laundry detergents, and pushed to modernize farming practices so as to prevent so much soil and stormwater runoff.
The ultimate goal was to limit the phosphorus load in Lake Erie to 11,000 metric tons per year — half what it was at the peak between 1967 and 1972. That seemed to work. By the 1980s, algae blooms had mostly subsided, with only an occasional flare-up.
… But since 2002, algae blooms are on the rise again
Recently, however, the blooms have been making a comeback. In 1995, blue-green algae (Microcystis aeruginosa) began flaring up in the western part of Lake Erie (the shallowness of the western side makes it more hospitable territory for algae).
Things only got worse from there: In 2011, that algae covered 1,900 square miles of the lake at its peak — the largest bloom ever recovered.
This year’s bloom is forecast to be the fourth-worst since 2002, according to the National Oceanic and Atmospheric Administration. And, as Toledo discovered, even a small amount of blue algae can do a spectacular amount of damage if it seeps into the water system.
These new blooms are different from the blooms of old: more localized along the shoreline, and with a higher concentration of cyanobacteria (the blue-green algae). Those cyanobacteria can create a variety of toxins — like microcycstin, which cased the panic in Toledo — that can damage the human liver and nervous system and kill other animals. And, while modern water-treatment plants have systems in place to remove the toxins, they can be difficult to deal with.
Farming, mussels, and climate change have all been blamed for the algae resurgence
The question of why algae blooms have returned in recent years has been much discussed — in this 2010 task force report from Ohio, for instance. Various reports have generally pointed to a couple of different factors here:
1) Changing farming practices: There does seem to be an increase in agricultural runoff of phosphorus — particularly dissolved reactive phosphorus into the Maumee River watershed, which feeds into Lake Erie. That could help feed the algae blooms.
The website Circle of Blue has a terrific report on this point — with a telling chart showing the fall and rise of dissolved reactive phosphorus in the river:
So why the extra runoff? There doesn’t seem to be an increase in overall farming acreage in northern Ohio. But more and more of the land in the region is devoted to soybean farming — where farmers are more likely to employ no-till farming techniques.
“No tilling means the farmers do not turn the soil over, reducing erosion but also concentrating phosphorus fertilizers in the top layer of soil where they are more likely to wash off during large storms,” the Circle of Blue report notes.
Meanwhile, the Ohio task force report noted that animal feeding operations in the Lake Erie region have become far bigger and denser over the years — with more animals packed into spaces. Cattle and pig manure is another significant source of phosphorus.
2) Increased rainfall. The number of heavy rain events in the Midwest has increased some 37 percent since the 1950s. That’s significant, since it’s the really heavy storms that wash away the most phosphorus from farm soil and cities into the watershed.
Climate change may play a role here too. One 2013 study in the Proceedings of the National Academies of Sciences predicted that algae blooms in Lake Erie were likely to increase in the years ahead — even if farming practices stay the same. That’s because, as the planet warms, the atmosphere will be able to hold more moisture and heavier rainfall events in the Midwest will become more and more common.
3) Invasive zebra and quaga mussels. Nearly 20 years ago, zebra mussels first invaded the Great Lakes system. Some studies have suggested that these mussels could affect algae blooms in several ways. They may help release phosphorus trapped in bottom sediments in the lake. They may also filter out competitorsto blue-green algae, allowing the organisms to flourish.
This factor is something that’s still being studied and modeled. (And it’s not clear how bad the effects are — after all, the mussels may also filter out phosphorus that’s entering the lake.)
4) Urban storm runoff might be worse than thought. Urban runoff isn’t nearly as big a factor as farming — especially since 82 percent of the land around the Maumee River is devoted to agriculture. But it’s one possible factor.
The Ohio EPA task force report notes, for instance, that certain garden care products can contain high sources of phosphorus. If these are flowing into the river during storms, that could be a minor contributor to the algae blooms.
On the upside, “point sources” like sewage systems and waste-treatment plants aren’t increasing their phosphorus output — the control systems of the 1970s are still working there.
There are lots of proposals for limiting algae blooms
Experts have suggested a couple of different steps over the years for dealing with the resurgence of toxic blooms.
One straightforward way to prevent future algae blooms would be to put less phosphorus into the lake. The Ohio EPA’s task force report notes that conditions in Lake Erie tend to respond pretty quickly — within 10 years — whenever phosphorus loads are reduced.
A separate recent report from the Lake Erie Ecosystem Priority argued for even stricter limits on the phosphorus load. That could be achieved, the report noted, through things like a ban on farmers spreading manure on frozen ground in the winter (which is particularly susceptible to runoff), as well as improved infrastructure to reduce urban storm-water runoff.
The Ohio EPA task force report, for its part, noted that “no single agricultural practice” will result in a massive reduction of phosphorus runoff. But that report, too, made a slew of suggestions for improved farming — and also called for more soil testing and monitoring.
One caveat here: It’s much more difficult, however, to deal with things like the effects of invasive mussels in the Lake. Indeed, if changes to the lake’s internal phosphorus cycle are the biggest reason for the resurgence of algae blooms, then that could prove much more difficult to control. This is something that’s currently being studied more closely.
Other experts have called for improved methods to control the spread of cyanobacterial blooms or better techniques to filter out harmful microcystins. A 2012 paper in thePolish Journal of Environmental Studies, for instance, described a technique for a sand filter that contained bacteria that could degrade microcystin.
That paper noted that toxic algae blooms have become “one of the most serious health risks of the 21st century,” cropping up in places like China, Japan, Brazil, and Australia. Ohio is currently the most visible problem area. But it’s not the only one.
* Note: Here’s a pretty good explanation of why phosphorus stimulates algae blooms in freshwater bodies like the Great Lakes.
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