In a country that has more grey days than bright ones, hot weather is usually cause for celebration. But while many in the UK made the most of the recent heatwave, others were more than a little concerned. As temperatures hit a record-breaking high, we saw the uglier side of the sunshine: warped roads, house fires, and sagging power lines.

Humans weren't alone in feeling the heat, and there were reports of distressed wildlife nationwide. We now know that average annual temperatures are rising faster than many species can evolve the traits to match. Those that are less adaptable - or already vulnerable - may even face extinction, especially if damaged habitats make survival a struggle in all four seasons.

Our local chalk streams, for example, provide a steady flow of cool, clear, oxygen-rich water all year round. These unique qualities are of immense value to wildlife, but climate change could negate them entirely. Intense rainfall in the colder months will bring its own hurdles, but today let's look at how rising temperatures will affect these globally rare ecological oddities.

Drying times

The most obvious problem to stem from hot weather is a shortage of water. Chalk streams are fed by an underground aquifer that is 'recharged' by rain, so if the weather has been dry then their levels drop and their flow rates slow down. Natural fluctuations are usually harmless, but long dry spells - compounded by high human water demand - can magnify them to devastating effect.

The adults of many fish species, for instance, need pockets of deep water where they can rest in safety. This is especially important for larger individuals, since they have fewer options for shelter. A drop in water level may force these larger fish to leave the area, thereby skewing the population and limiting breeding. If they are unable to leave, then their health can suffer as a result.

A drop in the surface level will also disconnect the stream from marginal plants, which provide valuable habitat for juvenile fish and access for water voles. Within the channel itself, plants like water crowfoot can be crowded out by those that thrive in slower flows. Duckweed, mare's tail, mosquito fern, and parrot's feather all enjoy a stately pace, and will dominate a stream channel if given the chance.

Toxic cocktail

Less water in the streams is troublesome on its own, but also exacerbates other pressures that these habitats are facing. When it rains, for instance, any water not absorbed by the ground will run downhill. As it passes over land, it picks up pollutants which hitch a lift to the nearest stream. These pollutants become more concentrated when water levels are low, which intensifies their impact.

Fertilisers from farms and gardens - as well as waste from septic tanks - encourage an overgrowth of algae. Spurred on by the warm weather, these algae 'bloom' rapidly within the streams. Their growth uses up oxygen and blocks out sunlight, making the water inhospitable for wildlife. Blue-green algae (actually a bacteria) can also produce toxins that harm people, pets, and livestock.

In contrast to the algal 'boom and bust', the damage from road runoff is insidious. This grim mixture can contain oil, paint, metal, tarmac, tyre dust, gritting salt, and fuel hydrocarbons. Some of these elements poison fish and invertebrates, while others cause problems by using up oxygen. Those animals that are eaten by predators act as gateways for the pollutants to move up the food chain.

Breathe uneasy

Anything that exhausts a stream's oxygen is bad news, particularly if this crucial resource is already in short supply. In normal conditions, our local chalk streams emerge at around 11 °C in all seasons, and so are able to hold high concentrations of dissolved oxygen. If they grow shallow and slow-moving, however, then the warmer water loses more of this oxygen at the surface.

For the resident wildlife, the pace of this process can mean the difference between discomfort and death. As oxygen is lost, the eggs of freshwater invertebrates - as well as fish like salmon, brown trout, and grayling - begin to develop deformities. Beyond a certain point, they simply won't survive. The adults can hold out for longer, but those that can't breathe air are doomed to suffocate.

This situation is only aggravated if the water level continues to drop. Deeper parts of the streams turn from refuges to traps as the shallower sections between them run dry. No longer refreshed by flowing water, these isolated pools are even more prone to overheating and deoxygenation. The wildlife within are unable to leave for better conditions, and make easy pickings for predators.

Downward spiral

It will be clear by now that low water levels set off a domino effect of harmful consequences. Even worse, this is not a linear chain but a vicious cycle of impacts that is difficult to break. To see this in action, we need look no further than the process of sedimentation. It might begin with a stream losing water - and therefore flow - causing suspended sediment to settle on the gravelly bed.

This sediment smothers invertebrates and fish eggs, but also decreases the depth of the stream. This, in turn, raises its temperature, which accelerates deoxygenation. Plants find it harder to grow, leaving the banks vulnerable to erosion. Crumbling banks release more sediment and make the channel wider; the water becomes shallower, slows down, and drops sediment onto the bed below.

This cycle is just part of the larger pattern of decline created by climate change. Anything adverse that happens in the streams during dry times is compounded by the return of rain - especially the erratic and intense bursts that are becoming more common. Concentrated road runoff washes in, banks collapse under floods, and overwhelmed sewers expel waste that encourages algal blooms.

Beat the heat

So, how worried should we be about all this? Heatwaves in the UK last a week or two at most - are they really cause for concern? Well, these extreme weather events aren't the whole picture: the Met Office's 'State of the UK Climate 2021' report shows that last year's summer and autumn were 1.5°C and 1.8°C hotter respectively than those of 1961-1990. All ten of our hottest years on record have happened this century - even without heatwaves, the pressure is on.

Since industrialisation began, average world temperatures have risen by over 1°C. The UN states that keeping this rise below 1.5°C will allow us to avoid the very worst impacts of climate change, although we will still face hurdles beyond what we are already witnessing. But we would need to almost halve our carbon emissions by 2030 in order to achieve this, and current global policies will not do so.

Of course, chalk streams aren't the only wildlife habitat that stands to suffer as a result of climate change. But with only around 200 in existence, they are incredibly precious places that would leave the landscape poorer for their absence. Imagine a Hampshire without the lush greenery of the winding tributaries; without the 'gin-clear' curves of the mighty Test and Itchen - it's unthinkable.

One of the best ways to protect our chalk streams from the effects of climate change is to increase their resilience: the healthier they are, the better they can cope with such pressures. There are many initiatives dedicated to this purpose, including the Watercress and Winterbournes scheme. We need the help of local communities to make our plans a reality, so why not consider getting involved?

Explore the scheme