How to study lake sediment

If your lake's looking murky, what's on the bottom can clue you in to what's happening on top

By Cottage LifeCottage Life


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Hard-bitten explorer (and Scot) though he was, S.J. Dawson thrilled to vistas “of the most lovely description” when he paddled the warm Sedimentwaters of Lake of the Woods in August 1857. Then the lake did its Jekyll-and-Hyde act – the one cottagers know so well.

“About four miles from land the water became tinged with green, deriving from a minute vegetable growth, which increased as we progressed,” Dawson scribbled in his report. Soon the water was filled with “an infinite quantity of …minute tubular needle-shaped organisms” that formed thickening mats “until they gave the appearance to the lake of a vast expanse of dirty green mud.”

From the sublime to the malodorous, Lake of the Woods still suffers from algal blooms a century and a half after the explorer paddled through on his way to the Red River. Almost every year, a population explosion of the tiny plant-like critters turns boat wakes luminescent green, and leaves a verdant layer of scum on the nether regions of cottagers when they peel off their Speedos.

Dawson toured the lake well before the Speedo era, but to judge from his report, algae was always a problem. Or was it?

Not according to David Hewlett. He argues the increasing frequency of algal scums is turning the lake from beautiful to beastly. For years, the chair of the environmental committee for the Lake of the Woods District Property Owners Association (LOWDPOA) watched the green water sweep by his Sand Lake cottage on its way downstream from Lake of the Woods, and wondered if the algae was getting worse. Now, thanks to a science called paleolimnology and a contract with Fisheries and Oceans Canada (DFO), “we’ve got the evidence to say algae has always been present, but not in the great concentrations we see now.” Equally worrisome, the algae may be more than an eyesore. Some of it is potentially toxic, and could sicken cottage dogs (or even their owners) who drink the water.

The proof of this change lies in the pudding-like guck at the bottom of the lake, which offers a glimpse of the past and points the way to a cleaner future.

Paleolimnology – the study of lake sediment – is a complex name for a simple idea. Since water runs downhill, traces of nearly everything in a lake’s watershed “eventually end up at the bottom of the lake, including things growing in the lake itself. Every second of every day they accumulate in the sediment,” says Andrew Paterson, an Ontario Ministry of the Environment (MOE) scientist who has probed the bottom of Lake of the Woods, as well as others in cottage country. The remains of fish, insects, plankton, and algae – not to mention charcoal from forest fires, pollen from plants, and soot from smokestacks – pile up year after year in layers like the rings of a tree. Researchers sample the layers by dropping or driving a long tube into the sediment. “It works like sticking a straw in the water and putting your thumb on top,” Paterson adds. “As you pull it out, the sediment stays in the core.”

The cores are cut into sections that are lead- or carbon-dated, submitted to chemical analyses, then put under a microscope. By looking at the mix of creatures and chemicals within each layer, scientists reconstruct conditions within the lake, and track changes through time. As science goes, this is messy fun. With his talk of “the power of paleo,” Paterson displays the enthusiasm of a kid ready to make mud pies. “I find the idea that your lake has a record of its past really fascinating.”

The technique is especially valuable when other clues to lake history are absent or unreliable, as in the case of Lake of the Woods. “The ideal situation would be to have 50 years of detailed data from water-quality monitoring,” says Mike Stainton, an analytical chemist and leader of the DFO’s coring research on the lake. “But in the absence of that, the best we can do is reconstruct the past.”

Sometimes the record can be seen with the naked eye. “Do you see that colour change?” Paterson says, pointing to a metre-long clear plastic tube filled with muck from Muskoka’s Blue Chalk Lake. About 30 centimetres from the bottom of the core the sediment shades from a dark-brown colour, like chocolate pudding, to a sandy charcoal-grey. “Just about every lake you core in central Ontario has that horizon, and it dates to European settlement. This is when they cleared the forest,” he adds, tapping the spot on the tube where long-dead settlers cut trees, increased erosion, and left an enduring mark on the lake bottom.

Forest-clearing isn’t the only impact.

The sediment records events happening thousands of kilometres away, including the birth of the atomic bomb. Cesium, a by-product of atmospheric nuclear explosions, rained down on lakes throughout the northern hemisphere, first appearing in sediments laid in the early ’50s and spiking in 1963. For scientists, it’s a reliable way to date sediment from the baby-boom era; for the rest of us, it’s a sobering reminder that not even the cottage lake lives outside the nuclear shadow.

On Lake of the Woods, scientists wanted to delve at least 150 years into the lake’s past, seeing it as it was before waves of settlers carved out farms, built homes and industries and, finally, cottages. They wanted to know whether the algae Dawson described is similar to what cottagers are seeing today, because if the lake is naturally prone to severe algal blooms, fighting them is as pointless as priming a jet pump when the foot valve is broken. But if it wasn’t always this murky, maybe its former nature can be restored.

A valuable guide for lake managers, paleolimnology can also be a defence against the all-too-human tendency to become inured to gradual change. Probing the sediment in Lake of the Woods has helped to move the discussion about the algal blooms beyond the annual “is it worse or isn’t it” dockside chit-chat. “There was so much rumour and hearsay about whether there’s more algae or there’s less, we wanted a more definitive answer,” says Gerry Wilson, LOWDPOA’s executive director. “That’s why we wanted to know about the history of the lake.”

The association had begun its own investigations in 1998, testing algal samples in surface water after blue-green algae was noticed in the lake. Unlike the more-familiar green algae, the blue-green species can release nerve and liver toxins, and has been implicated in the deaths of dogs and cattle in Manitoba. Its appearance in Lake of the Woods “set off some alarm bells, and encouraged us to learn more,” Wilson says. By collaborating with the DFO’s sediment research a few years later, the cottagers saw the opportunity to get to the bottom of their lake.

This article was originally published on October 3, 2005

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