Low oxygen still a concern in L. Manitou

Lake Manitou Area Association keynote speaker Claire Nelligan gives the lowdown on her water quality work on Lake Manitou. photo by Michael Erskine

SANDFIELD—The Old Sandfield Schoolhouse Hall was, as is traditionally the case, filled to capacity by members of the Lake Manitou Area Association (LMAA) and community at 7 pm on August 8 as they gathered eagerly to learn more about the state of their lake. In any event, the news is not great for future development on the Island’s largest lake.

This year, the topic on LMAA agenda was entitled ‘More Secrets from the Depths of Lake Manitou Part II,’ delivered by Queen’s University PhD candidate Clare Nelligan, who brought to the table a clear and concise presentation that outlined the findings to date of her study of sediment cores taken from the bottom of Lake Manitou’s east and west basins. Despite the scientifically complex nature of her work,

Ms. Nelligan was able to effectively share the information she has interpreted to date.

“The sediment acts like a natural archive,” she explained. Information is contained within the layers in the form of the remaining traces of the plant and animal life contained within those layers. “Lots of things live in the lake itself, they die and become trapped in the sediment.”

An innovative coring machine is lowered to the lake bottom and then hammered home with a dropped weight that seals the top of the tube “just like you would by putting your finger over the top of a straw,” she said

Using the science of paleolimnology (the study of the physical, chemical, and biological information preserved in freshwater deposits, primarily from lakes) those cores are divided up into a key chain of slices mere millimetres thick that can help trace the history of the lake’s climate and environment back through the centuries.

To effect there is no arcane magic to this scientific crystal ball gazing, rather the secret lies in understanding the characteristics of the tiny creatures whose glasslike skeletons, teeth and chemical residue are secreted within those sediment slices.

Ms. Nelligan explained that there are miniscule creatures who thrive within very limited and distinct environments. Including, most especially, oxygen levels. But in order for those remains to be useful for investigative analysis, those creatures (or plants) must be very abundant. “So we can find it, there has to be a lot of them in the sediment,” she said. “It also has to be something we can are able to infer information from. Pollen is good, as are certain types of zooplankton.”

Other indicators from the sediment include physical characteristics of the layers, particularly colour, but also the chemical composition of the matter itself. “We normally look at the biological component,” she said. There are species of candidates for study that thrive in a wide spectrum of conditions “but these don’t help much,” she said. Instead, the researchers focus on those whose lives are conducted within a very narrow set of conditions. By knowing those conditions and counting the number of those creatures (perhaps hundreds upon hundreds on a single prepared slide) that are contained within each slice, researchers can gain an accurate photograph of what was taking place in the lake ecology at a set point in the lake’s history.

Three types of creatures/plants are examined: chironomids, diatoms and chlorophyll-a.

Chironomids are those non-biting midges that can be found in clouds around lakes. Their larva, bloodworms, are good indicators of oxygen levels.

Clorophyll-a refers to the pigments to be found in plants. “It is what gives lakes that dark colour,” she said.

Lake trout lakes are relatively rare, with only one percent of Ontario’s 250,000 lakes playing host to the species. Ontario has 21 to 25 percent of the lake trout lakes worldwide. The species are fond of cold lakes of less than 15 degrees Celcius.

Between the influx of sulphur and oxygen depletion, lake trout environment “is being squished.” Increased nutrients equals depleted oxygen. So, between the advent of global warming and the encroachment of human agriculture and urban sources of nutrients, Ontario’s lakes are under severe pressure.

Ms. Nelligan explained the oxygen regeneration cycle, where the spring introduces new oxygen into the water, the summer months create a blanket of warm water that stops circulation and leads to lower levels and the fall, where the water cools and once again oxygen is mixed back into the water.

The west basin has a great deal of agricultural activity, while the east basin is much more heavily forested, she noted.

Interestingly, while preliminary study indicates a lot of variation in the historic levels, there is not a succinct finding of a trend.

Ms. Nelligan then explained the study of water is conducted through the use of a Secci depth disc. Essentially a disc is lowered into the water until it disappears from view, then raised again until it appears. The measurement gives the depth of clarity in the lake.

Climate change is having a remarkable impact on Lake Manitou, with the number of ice free days dropping 21 days in the past 44 years. “That’s half a day per year,” said Ms. Nelligan. In geological terms that is hardly a blink of an eye.

Lake Manitou has benefited from a great deal of citizen science over the years and that has helped a lot with establishing some baselines as well.

At the end of summer, in 2011, the dissolved oxygen levels in the west basin was 2.5 milligrams while in the east basin it measured 6.2 milligrams. “Both fall below the standard,” said Ms. Nelligan. That is what led the province to indicate the lake capacity was reached in 2017.

There are plenty of questions still to be answered, she noted. “Why are oxygen levels so low? Hopefully my research can inform the particular strategies.”

Ms. Nelligan’s work will be posted online at http://post.queensu.ca/~pearl/, but because of the labour intensive nature of the work and its complexity, it may be a matter of at least months if not years before all of it has been analyzed and posted.