In highly agricultural areas, a clear lake does not always mean clean water, according to new research. Although a scum of green algae typically signals unhealthy nutrient levels, scientists have now found that nitrogen concentrations from agricultural nutrient runoff above a certain threshold kill off algae growth. Thus, highly tainted but limpid lakes give a false impression of their water quality.
“We’re seeing clear lakes under the most extreme nutrient concentrations,” said Christopher Filstrup, a research associate at the Large Lakes Observatory and Minnesota Sea Grant at the University of Minnesota in Duluth. The lakes’ clarity “can give a false sense of good quality lakes when they’re actually the most degraded lakes that we see.”
Many agencies that regulate the use of nutrients and fertilizers in farming use reduced water clarity as a proxy for nutrient enrichment—particularly phosphorus and nitrogen introduced into the water—because increased nutrient runoff typically accelerates algae growth. However, water quality experts could be underestimating the amount of polluting nutrients that end up in regional watersheds, as scummy water may clear up even as pollution reaches new extremes, according to Filstrup and his colleagues.
The researchers also suggest that existing water management strategies may not work as expected. Returning water clarity after a period of intense nutrient enrichment might signal that “you’re seeing even more nutrients and you’ve finally gotten across that nitrogen threshold” rather than the success of mitigation strategies, Filstrup said.
Clarity and Quality, Not Always Equal
In the natural biogeochemical cycle of inland lakes, nitrogen and phosphorus each generally drive algal growth. Conventional wisdom holds that the highest concentrations of both nitrogen and phosphorus yield the highest concentrations of chlorophyll a, a marker for blue-green algae known as cyanobacteria, which use the molecule in photosynthesis.
However, land use practices associated with agriculture and raising livestock can alter the natural biogeochemical cycles in watersheds by introducing additional nutrients into local water supplies from fertilizers, pesticides, and manure. As a result, scientists’ understanding of how algae growth depends on nutrient levels, developed by studying lakes with natural nutrient levels, might not apply to lakes whose elevated nitrogen or phosphorus concentrations result from human activity.
“We wanted to test relationships that have been demonstrated—that more nutrients equal more algae,” Filstrup said, “but we wanted to know whether or not that still occurred under the highest nutrient conditions or whether some other factor became limiting for algal growth.”
To do this, the researchers analyzed 13 years of water quality data from Iowa’s Lake Monitoring Program to explore the possibility of a new relationship between nitrogen, phosphorus, and chlorophyll a concentrations.
Their data set included chemical measurements and algae concentrations from 139 man-made and natural lakes within Iowa, a state where nearly 90% of the land is farmed for crops or livestock. All of the lakes are classified as moderately to extremely enhanced in nutrients.
When phosphorus concentrations were moderately high, the researchers found that increased nitrogen correlated with increased chlorophyll a, as previously thought, but only up to a point. As nitrogen concentrations rose above around 3 milligrams per liter, or 3 parts per million, chlorophyll a concentrations began to drop off precipitously, a previously unrecognized phenomenon. The researchers published these results in Inland Waters on 9 October.
“These new findings, based on intensive sampling of some of the most enriched lake water on the planet, suggest that algae growth is inhibited by oxidation processes stemming from nitrogen fertilizer lost from crop production,” explained coauthor John Downing, director of the Minnesota Sea Grant and a scientist at the Large Lakes Observatory.
Filstrup and Downing have raised “a provocative idea,” said Stephen R. Carpenter of the University of Wisconsin–Madison Center for Limnology, who was not involved with their research. He encourages “further testing using time series from gradually enriched lakes or experiments.”
Implications for Land Management Policies
The Minnesota researchers speculate that this phenomenon went unrecognized for so long because recent expansions in agriculture have led to levels of nutrient enrichment in local watersheds never seen before.
“When we first saw this pattern, with low concentration of chlorophyll in high nutrient conditions,” Filstrup said, “we went to the whiteboard and started listing all possible hypotheses that could lead to reduced chlorophyll under the highest nutrient levels.”
It was no surprise that there was a threshold level of nitrogen, Filstrup said. But he and Downing were taken aback to discover that it’s only 3 parts per million because “that’s not an unheard of concentration in lakes in other very high nutrient systems,” he said.
Now they’re pondering the consequences for watershed management strategies.
“The old adage that you want to reduce nutrient export from these agricultural ecosystems is still correct,” said Filstrup, but the U.S. Department of Agriculture’s guidance to use water clarity as the only proxy for water quality is not. Instead, Filstrup suggested that the U.S. Environmental Protection Agency’s dual-nutrient criteria would better mitigate algal growth due to nutrient runoff, as they consider the effects of both nitrogen and phosphorus on algae growth.
According to Downing, “greater control of agricultural nutrient loss to surface water is a national priority.” However, conveying this latest understanding about water clarity to the public could make or break support for some water management projects because nutrient reductions “result in more algal growth in these lakes and murkier water conditions in the lake until you get back below that nitrogen threshold value,” Filstrup cautioned.
—Kimberly M. S. Cartier (@AstroKimCartier), News Writing and Production Intern