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
Hard-bitten explorer (and Scot) though he was, S.J. Dawson thrilled to vistas “of the most lovely description” when he paddled the warm waters 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.
What cottage groups can do
Like the fancy forensic work done by TV sleuths, this detective work doesn’t come cheap. The type of intensive analysis undertaken by the DFO costs roughly $10,000 per core, and few cottage groups have that kind of cash. Fortunately, the Lake of the Woods association can draw on 4,000 members, each paying $27 in annual dues. Members also make voluntary contributions to a fund that pays for environmental work on the lake, and help bankroll their own water-quality research program. This spending power allowed the association to build a research partnership with the DFO in 2002, formalized by a legal contract. By rolling the association’s project into other research underway, the DFO’s Winnipeg-based Freshwater Institute analyzed six cores from various spots in Lake of the Woods for the bargain rate of $33,000. The association paid $20,000 for two of the cores, and used a $13,000 grant (from the DFO) to support the rest of the work.
Since the feds began their work, the MOE has also responded to the algal concerns by launching its own study, using 17 cores to look at the difference between the south end of Lake of the Woods – where phosphorus has historically been higher – and the more pristine north. The MOE project takes samples only from the bottom and the top of the cores, a less expensive form of analysis. “You get a snapshot approach from comparing the top and bottom of the cores,” Paterson says, adding the survey will illustrate how much – and where – phosphorus levels have changed in the last 150 years. Paterson’s wider-ranging efforts should provide more context for the DFO’s work.
In her Winnipeg lab, Hedy Kling, a private algal specialist working with the cores, plots the changing makeup of Lake of the Woods through the lens of her microscope. Each glimpse into the sediment is a venture into an alien world of exquisitely beautiful creatures: sputnik-shaped grains of pollen, lace-like scales of chrysophytes, and the mandibles of zooplankton. In one core, as she sorts through the bodies of long-dead algae, plants, and animals, Kling says, “I can see a gradual increase in algae between the ’50s and ’70s, then a very rapid one starting in the early ’90s” – and the spike includes a burgeoning population of those toxic blue-greens. Moreover, the algal increase in some cores seems to follow a rise in Lake of the Woods’ cottage development.
That cottaging plays a role in making lakes murkier is nothing new, of course: Faulty septic systems and shoreline clearances typically cause the kind of nutrient-laden runoff that algae love. But with its 4,350 square kilometres and 14,600 islands, Lake of the Woods is no tiny inland lake, where cottagers are the most obvious impact. While its north end is hard-rock Canadian Shield country, the southern shore is fertile Minnesota prairie with a patchwork of farms and towns. Unfairly or not, the south end has long been blamed as the source of algae-feeding nutrients such as fertilizer and manure. The prairie soil is also naturally higher in phosphorus, so any erosion contributes to algal blooms.
How to make positive changes
The paleo survey is the first step in proving the lake is changing, and untangling the roles that cottages, farms, industries, and municipal sewage treatment systems play in that change. It’s also the first step in tackling the pollutants that turn beauty into beast. Gerry Wilson hopes the results will be a lever for political and environmental change. “There are always going to be naysayers. That’s why you need this kind of science. If you’re going to approach key decision makers, you have to have evidence. You’ve got to have facts and figures in order to change legislation.”
Backed with the knowledge gleaned from the Lake of the Woods sediment, David Hewlett envisions a cleanup effort that will turn back the clock – perhaps not restoring the lake to the days of Dawson, but “to the conditions of 1945, or 1960, or even 1980. The lake is a major source of relaxation and release from the stress of life, and we don’t want to see this incredible resource diminished,” he says.
For his part, Dawson might be gratified to know the “minute vegetable growth” he puzzled over so long ago is reappearing beneath a microscope and illuminating the workings of the aquatic world. It’s an example of how something as seemingly worthless as sediment can yield a treasure house of information – how the muck oozing through your toes can hold the key to a cleaner lake. L
Ray Ford, a writer and farmer in Powassan, Ont., still likes playing in the mud.
How other sleuths got their sludge
Lake of the Woods cottagers may be in on the ground floor of sediment research, but other lake associations have done similar work – and found innovative ways to pay for it. A few years ago, in Saskatchewan, the Big Shell Lake Watershed Stewardship Association used core samples to investigate phosphorus build-up linked to farm practices and shoreline development, and was able to fund it with a mix of federal and provincial in-kind grants.
During the summer of 2000, cottager Heather Stuart took cores from Ontario’s Salerno Lake to study the lake’s phosphorus cycle as part of her honours B.Sc. Now environmental director of the Salerno Devil’s Lake Cottagers Association, Stuart found the bark, needles, and debris left in the sediment from 19th-century log runs were contributing to the lake’s phosphorus levels. Because that phosphorus will be trapped in the lake for a long time, Stuart says cottagers have to be doubly careful not to add to it. “It means we have to be vigilant when it comes to inspecting septic systems and holding tanks.”
This article was originally published on October 3, 2005