It is a cold January morning and mist hangs over Windermere like a thick wool blanket. The lake looks to be asleep. The researchers that have gathered on its shores to sample the water peer into the distance and decide that visibility is too poor to take the boat out today. Unpredictable weather is one reason lake ecology during the winter has been poorly studied until recently. Dr Stephen Thackeray, a lake ecologist at the UK Centre for Ecology & Hydrology (UKCEH) and project leader of the Cumbrian Lakes Monitoring Platform, says there also used to be a preconception that not much was happening during the colder months. “For a long time, it was assumed that it was just a really quiet time with muted ecological activity. But studies over the past 10 years have shown that actually there’s more going on than we originally thought.” A lot of attention has been paid to how rising temperatures are bringing forward the start of spring – a phenomenon known as “season creep” – and its resulting “phenological asynchrony”, which puts animal and plant species out of sync with others on which they rely. However, the length and temperatures of the winter period are also ecologically important. Growers know that some trees need a certain number of “chill hours” for flowers, fruit and nuts to form properly, for example, while freezing temperatures can curb the spread of particular diseases. Less is known about what happens during winter in freshwater ecosystems, which are fragile and contain many imperilled species, and how they are being affected by the climate crisis. That is something that 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 started observing the lake in 1945 because he was interested in the seasonal growth of algae. “I think it’s fascinating that you start gathering data about these places for a reason, without realising that sustained monitoring will allow you to answer different questions later on,” says Thackeray. “John was really interested in the dynamics of a particular group of algae called diatoms. But to understand how the algae grow and the lake generally is changing, we also have to look at the physics, the chemistry, [and] different components of the food web.” Although they cannot visit their usual sampling sites today, Dr Glenn Rhodes, a microbial ecologist, and field instrument technician Gareth McShane demonstrate the specialised tools for examining 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 gauge how deep light can penetrate. Water samples will be sent off to test the acidity, concentrations of oxygen and nutrients, and the volume of phytoplankton and zooplankton. Rhodes pours a seemingly clear sample of water into a plastic bottle, which on closer inspection is seen to have tiny creatures wriggling around in it, among them crustaceans such as Cyclops and Eudiaptomus that graze on algae. “It’s appreciated more and more that important things happen in the winter, that organisms aren’t all completely inactive,” says Thackeray. “Many species do have dormant stages and slow down when it’s cold but they still have populations that tick over through the winter months. So as those conditions become warmer, those populations can develop more rapidly earlier in the year.” Helped by Lund’s long-term records, these observations are building up a detailed picture of how the lake is changing. Winter temperatures have risen more on average than those during the rest of the year, and the summer starts earlier and finishes later. Rising temperatures can influence water temperature, boost algal growth and cause fish to spawn earlier. How the layers of water of different temperatures mix is also changing, so that they are settling down, or becoming physically “stable”, earlier in the year, which can have far-reaching effects on a lake’s ecology. “The really important thing is that all these species are changing in different ways, and they all depend on and interact with each other. So changes in the lake early in the year have a kind of memory effect that passes through the subsequent seasons. And the knock-on effects are at different timescales for different species,” says Thackeray. These climate-related impacts are compounded by local environmental stresses that affect most of the UK’s freshwater ecosystems, such as sewage pollution and agricultural runoff. Extreme weather events, such as storms or floods, also have an impact by mixing up 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 water at the bottom is colder, it has a different chemistry, and it can get pushed down the lake. It’s a big growth area in our area of research.” In colder countries, research is being done into declining ice cover, which has a significant impact on how lakes function. One paper described lake ice as “one of the world’s resources most threatened by climate change”. This kind 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 happens over the year. There will always be a role for getting out on the water and gathering data, says Thackeray, but the possibilities are huge. As we walk around Windermere’s shores, I wonder what the lake might look like in January several years from now. “At the moment we cannot say with total confidence what future freshwater winters would look like,” says Thackeray. “However, we are already seeing evidence of winter-time cyanobacteria [blue-green algae] scums … and further warming could make such events more likely. “Furthermore, the progressive loss of cold winter conditions could negatively impact coldwater fish species like Arctic char. Their eggs are particularly sensitive to water temperature and do not survive well under warmer conditions.” Does it matter? “It’s a really complex picture. You can argue that the lake has value in itself but ecological processes can also influence things like water quality. It all depends on how we use the lake and what we value in it.” Find more age of extinction coverage here, and follow biodiversity reporters Phoebe Weston and Patrick Greenfield on Twitter for all the latest news and features