Hidden Depths: Why do we know so little about what happens to a lake in winter? | Lake District
IIt is a cold January morning and the mist hangs over Windermere like a thick blanket of wool. The lake seems asleep. Researchers who have gathered on its shores to sample the water look into the distance and decide the visibility is too poor to take the boat out today.
Unpredictable weather conditions are one of the reasons why the ecology of lakes during winter has been poorly studied until recently. Dr Stephen Thackeray, lakes ecologist at the UK Center for Ecology and Hydrology (UKCEH) and project manager for the Cumbrian Lakes Monitoring Platform, says there was also a misconception that there was not much going on during the colder months.
“For a long time it was assumed that this was just a really quiet time with muted ecological activity. But studies over the past 10 years have shown that there is actually more going on than we first thought.
Much attention has been paid to how rising temperatures advance the onset of spring – a phenomenon known as “seasonal creep” – and its resulting “phenological asynchrony”, which puts animal and plant species out of step with the others on whom they rely.
However, the duration and temperatures of the winter period are also ecologically important. Growers know that some trees need a certain number of “hours of relaxationso that flowers, fruits and nuts form properly, for example, while freezing temperatures can curb the spread of certain diseases.
Less is known about what happens during winter in freshwater ecosystems, which are fragile and contain many species at risk, and how they are affected by the climate crisis. This is something the UKCEH researchers want to rectify. Every fortnight they go out to monitor four lake basins in the Lake District – North and South Windermere, Esthwaite Water and Blelham Tarn. On Windermere they carry out the work in a boat named after John Lund, who began observing the lake in 1945 because he was interested in the seasonal growth of algae.
“I think it’s fascinating that you start collecting data on these places for a reason, not realizing that sustained monitoring will allow you to answer different questions later,” Thackeray says.
“John was really interested in the dynamics of a particular group of algae called diatoms. But to understand how algae grow and how the lake in general evolves, we also need to look at the physics, chemistry, [and] different components of the food web.
Although they won’t be able to visit their usual sampling sites today, Dr. Glenn Rhodes, a microbial ecologist, and field instrument technician Gareth McShane demonstrate the specialized tools to survey the lake. They drop a probe to measure water temperature at different depths and lower a white disc until it can no longer be seen to measure how deep light can penetrate.
Water samples will be sent to test acidity, oxygen and nutrient concentrations, and volume of phytoplankton and zooplankton. Rhodes pours a sample of seemingly clear water into a plastic bottle, which on closer inspection shows tiny creatures wriggling in it, including crustaceans such as Cyclops and Eudiaptomus which graze on the algae.
“There’s a growing appreciation that important things happen in the winter, that organisms aren’t all completely dormant,” Thackeray says.
“Many species have dormant stages and slow down when it’s cold, but they still have populations that extend into the winter months. Thus, as these conditions warm, these populations may grow more rapidly earlier in the year.
Aided by long-term records from Lund, these observations form a detailed picture of the lake’s evolution. Winter temperatures have increased more on average than those of the rest of the year, and summer starts earlier and ends later. Rising temperatures can affect water temperature, stimulate algae growth and cause fish to spawn earlier.
The way layers of water of different temperatures mix also changes, so that they stabilize or become physically “stable”, earlier in the year, which can have far-reaching effects on the ecology of a lake.
“What’s really important is that all of these species change in different ways, and they all depend on each other and interact with each other. So the changes in the lake at the start of the year have a kind of memory effect that carries over to subsequent seasons. And the ripple effects happen on different time scales for different species,” Thackeray says.
These climate-related impacts are compounded by local environmental stresses that affect most UK freshwater ecosystems, such as sewage pollution and agricultural runoff.
Extreme weather events, such as storms or floods, also have an impact by mixing the layers of water. “The increase in extreme events due to climate change has the potential to erode the stable water structure of the lake. The bottom water is colder, has different chemistry and can be pushed into the lake. C This is an important area of growth in our research field.
In colder countries, research is being done on the decrease in ice cover, which has a significant impact on the functioning of lakes. A paper describes lake ice as “one of the world’s resources most at risk from climate change”.
This type of work presents considerable logistical, methodological and safety challenges, but lake researchers are increasingly using tools such as satellite imagery and environmental DNA to develop their understanding of what is happening during of the year. There will always be a role for getting out on the water and collecting data, says Thackeray, but the possibilities are enormous.
As we stroll along the shores of Windermere, I wonder what the lake will look like in January several years from now. “At this time, we cannot say with complete confidence what future freshwater winters will look like,” Thackeray says. “However, we are already seeing evidence of winter cyanobacteria [blue-green algae] foaming… and further warming could make such events more likely.
“In addition, the gradual loss of cold winter conditions could negatively impact cold-water fish species such as Arctic char. Their eggs are particularly sensitive to water temperature and do not survive well in warmer conditions.
Does it matter? “It’s a very complex picture. You can argue that the lake has value on its own, but ecological processes can also influence things like water quality. It all depends on how we use the lake and what we enjoy there.