Changes in logging and land management practices in the 19th century caused a profound ecological shift in one of the world’s largest freshwater ecosystems according to new research by Trent University postdoctoral fellow Dr. Eric Guiry published in the journal Limnology and Oceanography.
Nitrogen levels in Lake Ontario increased as a result of widespread deforestation across its watershed after at least 800 years of relatively stable levels. The new research shows this balance suddenly shifted when forests were cleared, the land became more vulnerable to erosion, and nitrogen-rich soils washed into the lake.
“Shifting a lake’s nutrient balance, of which nitrogen is a key component, can have significant effects on the health of the plants and animals that live within it,” says Dr. Guiry.
“We know that humans have had an impact on the lake for a long time, and we've known that runoff from forestry and agriculture were an important part of that. Our data allowed us to tightly control the timeframe over which we can see it happening, which helps understand which activity caused the problem.”
Dr. Guiry uncovered this relationship using stable isotope analysis of archaeological fish bones to reconstruct the lake’s historic nitrogen cycle. Trent’s first-ever Banting postdoctoral fellow in Archaeology—a research position funded by all three of Canada’s research councils—Dr. Guiry analyzed the amount of the element nitrogen-15 in the bones of lake trout, Atlantic salmon and whitefish specimens from a number of sites along Lake Ontario’s shores and within its watershed. The oldest among them dated to approximately 980 AD.
Using information about where and when fish specimens were caught, Dr. Guiry was able to identify that after centuries of stability, the level of nitrogen-15 in Lake Ontario began to rise in the 1830s. The findings show that even the largest lakes can be sensitive to human impacts over short timespans.
During the 19th century, the population near the lake was growing. People living in cities like Toronto and Kingston would have been impacting the lake too, but it would have been more localized.
“The increase in nitrogen levels happened on a lake-wide scale, so it had to be broader than just one city,” explains Dr. Guiry. “Widespread industrialization could have caused it, but that had not happened yet, so the cause must have been something occurring across a large area. That was deforestation.”
Indigenous people had long used fire to clear land in the area, and early settlers had cleared forests for their farms. However, the scale of logging in the watershed during that period was unprecedented.
“Originally, when farmers cleared their fields, they often took precautions to avoid nutrient loss in their soils. They would leave stumps behind and wait to start plowing to try to use nutrients without affecting the soil,” says Dr. Guiry.
“As logging increased and more land was cultivated, people started to plow and remove stumps, rocks, and barriers that might have kept more nutrients in the soil. That’s when you start to have large-scale loss of soil nutrients through increased runoff.”