VT NEWS | OCTOBER 14, 2020

Scientists have long known that clear water lakes are in danger from land development, pollutants, run-off from storms, and climate change because of increased nutrient pollutants that lead to algae blooms.

However, a recent study from the Virginia Tech Department of Biological Sciences shows that the negative effects from climate change can be mitigated by limiting nutrient pollution from land development in clear-water lakes.

Algae, of course, are a critical part of lake food webs and the zooplankton that eat them. But too much algae in lakes can cause scums on the water, blocking out sunlight for other life in the lake. When the algae die and decompose, they release more nutrients, which can cause even more algae blooms.

“High nutrient pollution can come from many sources: fertilizers and sewage waste are some of the worst sources of nutrient pollution, as far as having the highest concentration of nutrients,” said Nicole Ward, Interfaces of Global Change IGEP fellow and a doctoral student in biological sciences, part of the Virginia Tech College of Science, who led the study.

Ward worked on the study – recently published in the journal Water Resources Research – alongside mentor Cayelan Carey, an associate professor of biological sciences, Kathleen Weathers, a scientist at the Cary Institute of Ecosystem Studies, in addition to collaborators at Dartmouth College in New Hampshire, Bates College in Maine, and the University of Wisconsin-Madison.

The nutrients Ward and Carey refer to are nitrogen and phosphorus – essential building blocks for life, found in DNA, cells, bones, and energy sources. In freshwater systems, the number of organisms living in the water is dependent on the availability of nitrogen and phosphorus. That’s a double-edged sword.

Lake Sunapee in New Hampshire. In the distance is a buoy that has monitored the quality of the water since 2007. Photo by Nicole Ward.

“Erosion and landslides also transport nutrients. Phosphorus is generally bound to sediment particles, so as sediment enters the water it is bringing phosphorus with it,” Ward said. “So, when we add more nutrients, we can really quickly see a lot more life – which seems like a good thing, right? But too much of a good thing is notgood. So, we get huge algae blooms with high nutrient pollution, this can cause scums on the water, potentially harming plants and fish.”

And there’s another issue: “When the algae inevitably die, their decomposition uses up the oxygen in the water, killing organisms in the water that need oxygen,” Ward added.

Further, Ward and Carey found that the negative effects of land use and climate change on a lake depend on if yearly maximum or average phytoplankton concentrations are studied. Average phytoplankton concentrations, during typical summer conditions, show an increase with either warmer air temperatures or higher nutrient pollution. However, annual maximum phytoplankton concentration – or blooms – only increase with higher nutrient pollution.

In the study, Ward and Carey wrote, “Typical summer phytoplankton concentrations will likely increase with warmer air temperatures due to climate change alone and increase even further when combined with higher nutrient pollution. To maintain clear water lakes, nutrient pollution should be reduced even more than previously thought to compensate for increasing phytoplankton in a warmer climate.”

“Oligotrophic lakes – low-nutrient, clear-water lakes with high transparency – are disappearing due to human activities,” said Carey, an affiliated member of Virginia Tech’s Fralin Life Sciences Institute and the Global Change Center. Most of our understanding about how lakes function is from lakes that have already been degraded, so our goal in this study is to understand what factors affect their water quality to best protect them.”

The study focused on Lake Sunapee in New Hampshire, located near Carey’s alma mater, Dartmouth. It was chosen for its pristine water quality and rural location.

The lake also has a goldmine of data thanks to the hard work of the nonprofit, community-operated Lake Sunapee Protective Association. It has 31 years of water quality data, including statistics from a high-frequency buoy monitor that has collected data at 15-minute intervals since 2007. Ward called the group and its data set “critical” to the study.

Ward simulated conditions under five scenarios using the 31-year period data sets. “This study was all data analytics and modeling,” Ward said. “Modeling enables ‘experimentation’ on ecosystems that are not possible in the real world and overcomes logistical limitations of an experiment as large as a watershed. For example, just consider the time to experimentally test each scenario I did in model space: it would have taken 186 years. Our computational abilities, advances in modeling, and advances in sensor technology is completely changing the way we do environmental science.”

Ward added, “Even though this huge global issue of climate change is happening, and we know it is changing water quality across the globe from other research, we can have hope of saving our clear-water lakes from large changes in water quality if we focus on local nutrient pollution. By limiting nutrient availability in the water, the negative climate effects have less ability to wreak havoc.”

The study was funded by a grant from the National Science Foundation’s Coupled Human Natural Systems program. It was part of a larger project to look at several lakes and their water quality under the banner of the project CNH-Lakes, or the Coupled Natural and Human Systems Project, spearheaded by Carey and fellow Virginia Tech faculty Kelly Cobourn, an associate professor in the Department of Forest Resources and Environmental Conversation, part of the College of Natural Resources and Environment, and Kevin Boyle, a professor in the Department of Agriculture and Applied Economics in the College of Agriculture and Life Sciences.

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