Freshwater lakes are warming

Scientists study how Flathead Lake is warming

FLATHEAD LAKE – Over the last few decades, water scientists have noticed a rapid and consistent warming of surface temperatures. Changes beneath the water surface, however, have been subtler, but could still have significant consequences for lake ecosystems.

In a study recently published in Nature’s Scientific Reports, Shawn Devlin, an aquatic ecologist at the University of Montana Flathead Lake Biological Station, and a global team of researchers provide the most comprehensive data set to date of long-term summertime vertical temperature profiles in more than 100 lakes across the world. Led by Rachel Pilla and Craig Williamson of the University of Miami-Ohio, researchers analyzed summer temperature trends, not only in the surface-water layers of the lakes, but also in deep-water layers and vertical thermal structures.

One thing that made this study so remarkable is the scope,” said Devlin, who oversees FLBS’s Flathead Monitoring Program, one of the oldest lake monitoring programs in the world. “We were able to track the long-term surface and deep-water temperature trends of each individual lake and insert that information into a much larger picture to uncover patterns that help us understand what’s happening.”

After examining the data, which spanned nearly four decades beginning in 1970, researchers found consistent, significant increases in surface temperatures – an average of nearly 0.4 degrees Celsius per decade – across the lakes studied. Additionally, researchers were able to use lake models to predict the temperature changes in surface layers on a fairly consistent basis.
But when it came to the deep-water layers, a surprising development emerged. While the temperature trends showed little change on average – an increase of 0.06 degrees Celsius per decade – there was high variability across the lakes. The deep-water temperature of one of the lakes increased by nearly 0.7 degrees Celsius per decade, while the deep-water temperature of another lake actually decreased by the same amount.

This variability couldn’t be explained by trends in surface water temperatures or any other external drivers examined in the study. As a result, lake models were only able to predict deep-water temperature trends in 8.4% of the lakes studied.
“It’s a very interesting phenomenon,” Devlin said. “It’s something we see right here at Flathead Lake, which has a unique ability to mask the impacts of climate change. Even though the surface temperatures have increased quite a bit here in the past couple decades, the average temperature has remained essentially the same.”

According to Devlin, Flathead Lake is able to offset its rise in surface layer temperatures with a large deep-water layer that is bolstered by a steady inflow of snow melt and glacial runoff. As a result, Flathead Lake’s deep-water layer is actually cooling slightly.

While it may at first seem positive that Flathead Lake and others like it aren’t experiencing the dramatic temperature changes in their deep-water layers as they are on the surface, the implications can potentially be quite dire. 
Larger temperature differences between the deep-water and surface-water layers make for stronger stratification. The stronger the stratification, the stronger and more robust the thermocline (the transition layer, or barrier, between waters of different temperatures) becomes, which can prevent the movement of materials and organisms between surface and deeper waters. This can greatly impact the ecology of a lake by creating nutrient imbalances, modifying critical oxygen levels throughout the lake and altering fish communities and production.

“Flathead Lake is now part of our global understanding of one of the biggest threats to our freshwater ecosystems, which is climate change,” Devlin said. “This is an important first step toward a better scientific understanding, and it wouldn’t be possible without the decades of extensive monitoring and modeling efforts of the Flathead Monitoring Program and those who support our work.”
For Devlin, incorporating Flathead Lake’s extensive data into the global research on lakes and working with lake experts from all over the world makes for more powerful science.

“Flathead Lake is a great template of how lakes could work when not greatly influenced by human impacts. We were able to show that our lake is a treasure trove of scientific merit and study, which makes this an important moment scientifically for the Flathead Monitoring Project, the Bio Station and UM.”

Editor's note: This study was made possible through the support of the Global Lake Ecological Observatory Network, in addition to numerous foundations and grants.

How climate change affects fish

MISSOULA –Scientists who study the impact of global warming on the health of aquatic populations have long speculated that rising water temperatures could reduce the ability of fish, particularly larger individuals, to breathe.

However, it has been difficult to measure that impact on a large scale.
Now, researchers at the University of Montana, McGill University in Montreal and Radboud University in the Netherlands have developed a new mathematical model that accurately predicts how the metabolic rates of fish change with temperature, oxygen availability and body size. The team’s research was published in the Nov. 30 issue of the Proceedings of the National Academy of Sciences.

“This represents a significant theoretical addition to an important pre-existing body of theory ­– the metabolic theory of ecology – that focuses primarily on body size and temperature, but does not incorporate oxygen,” said Art Woods, UM professor of biological sciences.  “Including oxygen means that the model does a significantly better job of predicting observed patterns of variation in metabolic rate among fishes worldwide.”
Contrary to warm-blooded animals such as mammals and birds, cold-blooded animals like fish increase their metabolic demand for oxygen with temperature increases. Water temperature already is rising worldwide as a consequence of climate change, and many fish species need to cope with this rapid temperature change either by migrating toward colder regions or by adopting different life strategies, such as growing smaller to avoid respiratory constraints.
“So far, our understanding of the mechanisms linking water temperature, respiratory performance, animal behavior and survival are limited,” said lead author Juan Rubalcaba, a Marie Curie Postdoctoral Fellow at McGill. “And these relationships are complex. For example, if warming increases oxygen uptake in fish, the water enveloping their gills will become depleted of oxygen, which in turn impedes oxygen uptake.

Fish ventilate their gills, but the efficiency of this ventilation depends on water temperature and body size.”

To gain an improved understanding of how these mechanisms work, the research team developed a model based on physicochemical principles describing oxygen diffusion at the gill surface and oxygen consumption by metabolism. Predictions were compared against data from over 200 fish species, measuring oxygen consumption rates at different water temperatures and across individuals of different body sizes.

“Our model predictions matched our observations that aerobic capacity declines with increasing temperature, especially among larger individuals,” Rubalcaba said.

With this model, scientists will be able to better assess the impacts of global warming on fish metabolism and physiological performance. This will give them a more accurate prediction of the future health of the planet’s water bodies and the population of fish that inhabit them.