For many lakes and rivers around the world, too much phosphorus is a crisis, super-fertilizing aquatic ecosystems and spurring unusually high algae blooms, weed growth and other problems.

Just look at Lake Erie, where high-phosphorus runoff into the lake in recent summers is blamed for massive algae blooms so bad they endangered municipal water supplies.

Historically large algae blooms also are sprouting on giant but shallow Lake of the Woods on the Minnesota-Canada border, where phosphorus that built up in the sediment over a century is stirred up by wave action and algae is fueled by higher water temperatures.

But a Canadian scientist says Lake Superior has just the opposite issue, too little phosphorus, that may be spurring its own trouble.

Max Bothwell, a researcher for Environment Canada, said the recent appearance of a gooey algae called Didymosphenia geminata in the St. Mary’s River at the outlet of Lake Superior is a sign that phosphorus levels in the big lake may be declining.

Superior has always been less fertile than other Great Lakes and most inland waters. Now that fertility may be dropping even more.

“It’s totally bizarre how different the problem is from one end of the (Great Lakes) to the other. In Erie, you have a huge oversupply of phosphorus causing problems. But with Huron and especially Superior, this is just an intriguing mystery how there could be so little phosphorus,’’ Bothwell told the News Tribune.

Scientists have determined that, unlike most algae that thrive in high phosphorus levels, didymosphenia - didymo for short, often called rock snot - thrives only in very low phosphorus conditions in clear, infertile waters.

But Bothwell thinks something else is happening to Lake Superior, too, something global, combining with low phosphorus and spurring the unusual algae to pop up. It’s happening in new places in multiple continents.

“I’m thinking that it has to have something to do with climate change. But we don’t know what the mechanism is yet,’’ he said.

As low as it can go?

Bothwell said Environment Canada data appears to show phosphorus levels on Lake Superior continuing to decline. But Euan Reavie, a scientist at the University of Minnesota Duluth’s Natural Resources Research Institute, said his assessment is that the lake’s phosphorus already is about as low as it can go.

Based on sediment samples he’s studied, Reavie said the big lake’s phosphorus and fertility rose a bit in the mid-20th century when farming was a little more common in the Minnesota and Wisconsin portions of the lake’s watershed.

Over the past half-century, farming has diminished here, and tilled land now covers just a tiny fraction of the lake’s watershed. That land-use change spurred phosphorus levels, as measured in lake sediment, to decline in recent decades back to historic low levels, Reavie said. (The advent of functioning sewage treatment systems during the last 50 years also helps keep phosphorus out of the lake.)

Infertile lakes are called oligotrophic. Reavie says Superior is so infertile it’s “ultra-oligotrophic.”

“I don’t see how it could go much lower than it is now. It’s essentially as low now as we have the ability to detect it,’’ Reavie said of Superior’s phosphorus level.

Reavie noted that Bothwell is a rock star of rock snot science so “it definitely would pay to keep looking at what he thinks is happening out there.”

If a “higher resolution” analysis of phosphorus were available it might be used to confirm Bothwell’s hypothesis, Reavie notes. Bothwell says didymo seems to respond to just miniscule declines in phosphorus and agreed “our inability to better detect those small changes is frustrating.”

Native or invader?

Didymosphenia - a single-cell diatom - has been found in western Lake Superior before, including on Minnesota’s North Shore where it has never reached problem levels. But it’s unclear why it’s suddenly showing up in the lake’s eastern outlet.

The odd algae has been confounding scientists like Bothwell for years.

As it showed up for the first times in new places, experts assumed it was an invasive species that was being moved around by unsuspecting people, especially stream anglers as they traveled from river to river.

Now, Bothwell says didymo may well be naturally present in many waters and depends on changes in the water - lower phosphorus levels and some other trigger - to set colonies producing en masse. He is moving away from the invasive species label for the algae and says efforts to prevent its spread are missing a broader point.

In some areas where didymo pops up, it has caused serious problems.

The colonies can grow to levels where they cover streambeds. And didymo supports tubifex worms, the only known host of the fish parasite Myxobolus cerebralis. That parasite causes whirling disease, which leads to fatal neurological damage in fish and can cause fish populations to decline.

Experts still debate whether didymo has been moved around for the past 30 years or is suddenly sprouting up in places like the Rocky Mountains, Vancouver Island, New Zealand, Poland and Chile because of some sort of global problem like climate change.

Sentinel species

The fact that didymo colonies also are appearing in far-flung waters where few if any people fish or recreate hints that Bothwell and his camp, including Dartmouth University researcher Brad Taylor, may be right about climate’s impact - about looking at global phenomenon like earlier springs, faster snowmelt and rising water temperatures as potential didymo cohorts with low phosphorus.

For Lake Superior, Bothwell says he’s less worried about didymo than whatever is causing it to flourish.

The rock snot “itself is not a threat to the lake’s ecosystem. But it’s a sentinel species, and I think it’s telling us that something else is going on beyond low phosphorus levels,’’ Bothwell said.

The combined forces of low phosphorus and a changing climate may be enough to change the lake’s ecosystem, even affecting other creatures such as fish.

“But we really don’t know how it’s related,’’ Bothwell said. “We need to keep looking.”

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Looks like wet toilet paper

According to Lake Superior State University, didymo resembles toilet tissue or wadded up paper and forms thick yellow-brown mats along river bottoms. It is easy to miss when just a few cells are present, but it occasionally produces long, branching stalks creating nuisance conditions. Contrary to a slimy appearance, the texture of didymo actually resembles that of wet wool.

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Stay clean and dry

While several scientists now believe didymo is probably a native species in most of the places it has popped up, many agencies such as Minnesota SeaGrant still are asking anglers and others to reduce its spread by cleaning and drying equipment used in infested waters, especially waders, before moving to a different waterway.