Scientists Investigate Eutrophication Mystery and find Oligotrophication Instead

Just like doctors measure body temperature to detect fever, limnologists use a lake's trophic status as a “thermometer” to assess its health. The richer in nutrients a lake is, the more likely it is to develop algal or cyanobacterial blooms and, consequently, the worse the water quality. Limnologists have known for a long time that agricultural runoff can lead to lake eutrophication. What is surprising is that we are now detecting blooms in otherwise pristine lakes, located far from human-impacted areas. A remarkable example of this is the 2014 cyanobacterial bloom reported in Dickson Lake (Ontario) – a bloom that remains so far unexplained. Motivated by this mysterious case, Aleksey Paltsev, a PhD candidate in the University of Western Ontario, became curious about how frequently eutrophication occurs in lakes located in relatively undisturbed regions of the Great Lakes Basin. At the 2018 ASLO Meeting in Victoria (BC), he reported on his investigation, in which he followed changes in the trophic status of 12,600 lakes along a 28-year time course. To classify lake trophic status, Paltsev determined chlorophyll a concentrations based on reflectance values of band 3 (corresponding to green light intensity) from Landsat satellite images. “There is something that makes lakes stable and something that forces them to shift from one trophic status to other,” Paltsev says, “we wanted to know what makes some lakes more resilient than others.” For that, he classified lakes into those that were stable (i.e. permanently oligotrophic or eutrophic) and those that were changing from one stable state to another, either because they were becoming more oligotrophic or more eutrophic. His analysis found that more than 5000 lakes could be classified as stable oligotrophic and about 100 as stable eutrophic. A few lakes were unstable, not showing consistent trends toward eutrophication or oligotrophication. Among lakes experiencing a clear shift in stability, surprisingly, he found that more were undergoing oligotrophication (about 3000) than eutrophication (about 2000). Intrigued by this result, Paltsev wanted to explain why such an unexpected phenomenon was happening in the Great Lakes Basin. He then noticed that the amplitude of variation in both oligotrophying and eutrophying lakes was very similar across the time series, hinting that the same broad-scale environmental factors could be driving these processes. Paltsev noted that increased mean temperatures alone could not explain the observed shifts in lakes stability; instead, a combination of environmental factors and lake physical properties were more important defining a lake's fate across the 28-year period studied. He examined the effect of landscape metrics (e.g. percentage of wetland in the catchment), lake morphometry (e.g., lake fetch, width of the littoral zone, and maximum depth) and weather conditions (e.g., precipitation rates) on lake trophic stability. He observed that lakes undergoing eutrophication were relatively deep, had small fetch, wide littoral zone, were surrounded by many wetlands, and were located in watersheds with a decreasing trend in precipitation rates. Lakes undergoing oligotrophication were also relatively deep, but had a medium-size fetch, narrow littoral zone, were surrounded by only a few wetlands and located in watersheds with an increasing trend in precipitation. He concluded that a lake's morphometry and the catchment area influence nutrients inflow and water residence time, consequently impacting nutrient availability to phytoplankton. Bloom-forming algae and cyanobacteria will have more time to uptake nutrients in eutrophying lakes, which are, in general, more connected to the catchment and exhibit low residence time. This study provides clues to solve the mystery of Dickson Lake, and it goes further by showing an unexpected trend of oligotrophication in pristine lakes. However, it is still too early to predict the consequences of the undergoing changes. “Oligotrophication is less dangerous [to water quality] than eutrophication, but we don't know the consequences of it yet. It is really hard to predict how these lakes will react, lakes also have their own system,” Paltsev explains.

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