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More Postcards from Heatwave Europe

August 17, 2018

Low water levels on Thirlmere in the English Lake District in late July 2018. Image: Stephan Brzozowski

We recently published the first in a series of ‘Postcards from Heatwave Europe‘, in which aquatic scientists from across Europe offered their observations on how this summer’s extreme weather was affecting rivers and lakes in their local landscapes.

Today we have two more contributions to the series, from Sweden and the English Lake District. In Britain, at least, the weeks of hot, dry weather experienced this summer have recently been replaced by more unsettled conditions and regular thunderstorms. However, water levels in many lakes and rivers have yet to return to typical summer levels as a result of the prolonged dry spell.

You can read the first set of ‘postcards’ here.


In the Swedish county of Uppland, we have been racing to complete fieldwork for the BiodivERsA-facilitated CROSSLINK project before our streams dried out or went into extreme low flows. Fortunately, there was good snowfall over winter which meant the streams all started with high base-flows in spring. However, it is not difficult to imagine a scenario where a poor snow year coincides with another extremely dry summer, which could place catastrophic strains on systems already under pressure from human activities.

Notably, the spring transition this year was extraordinarily rapid, leading to record warm temperatures in May. This warming seemingly bolstered seasonal emergence of stream insects, leading to a noticeable pulse of riparian insect activity around our study sites early in the season. We hope to quantify the effects of this spatial subsidy on receiving terrestrial consumers using biomarker analyses.


A stream study-site near Uppsala in early May 2018. Image: Francis J. Burdon


Extreme low flows in the same stream in late June 2018 after a long period of hot, dry weather. Image: Francis J. Burdon

Another aspect of the CROSSLINK project is assessing the benefits of woody riparian buffers to stream ecosystems. In this year’s heatwave, our buffered sites in Sweden have been on average 0.5°C cooler than the un-buffered sites upstream. This observation adds further credence to calls for riparian plantings as a climate adaptation tool in catchments facing multiple pressures. However, we have noticed that our forested headwater reference sites all face hydrological challenges during the summer months. Whilst this natural feature of our systems has likely been exacerbated by drainage practices, there may also be a perverse tradeoff between the benefits of shading and the transpiration exerted by trees.

The severity of the drought in Sweden has caused an unprecedented number of wildfires throughout the country. Our study sites have not been affected, and with temperatures starting to cool and some much-needed rain, the situation seems to be stabilising. Similar to reports from Wales, we have experienced torrential thunderstorms in the past two weeks, leading to surface flooding in urban areas of Uppsala. However, like the situation in the UK, these rain events have only marginally improved the low-flow conditions in our streams. It is safe to say we wait with bated breath to see what more the weather will deliver in 2018.

Dr. Francis J. Burdon, Swedish University of Agricultural Sciences


Dry lake bed at Thirlmere in the English Lake District. Image: Stephan Brzozowski


The height of the ‘heatwave’ broke here in mid-July, and a threatened hose-pipe ban has been postponed thanks to some rain, cooler temperatures and a drop in water demand. However the hot, dry conditions through May, June and early July – with average air temperatures 2 – 2.5°C above the 1981 – 2010 average and rainfall being only between a third and just under three-quarters of the average for this time of year – have meant water levels in our lakes and rivers have declined.

By the end of July, river levels in a number of the rivers in the northwest of England were at historically low levels and reservoir stocks were much lower this year, particularly in Cumbria and the Lake District where levels were 10 – 30% lower than on average. So what are the impacts of these hydrological and meteorological conditions on some of the lakes in the Lake District?

temp levels2

Figure 1: Surface temperature data from Windermere, north basin. The blue line represents the 1981–2010 average, and the dotted lines one standard deviation from this average. The yellow line represents the 2018 data. All data are from the CEH Cumbrian lakes long-term monitoring programme.

At CEH we have been monitoring lakes in the Lake District since the 1940s, so we’re in a good position to see how this year fits into the longer-term pattern. We can see by looking at our long-term record of surface water temperatures, that average temperatures during May, June and July 2018 were between 2.4 and 4°C higher than the 1981–2010 average.  (See the example from Lake Windermere’s north basin in Figure 1).

The additional heating of the surface waters also means that the seasonal water column temperature stratification is particularly strong this year, meaning the difference in temperature between the surface and deep water of Windermere is around 16°C, which also has implications for aquatic life in the lakes.

cyanobacteria levels

Figure 2: A measure of total algal biomass, chlorophyll a, for Blelham Tarn and Windermere, north basin. The blue line represents the 1981 – 2010 average and dotted lines are 1 standard deviation from the average. The yellow line represents the data from 2018. All data are part of the CEH Cumbrian lakes long-term monitoring programme.

Interestingly, the warmer water has not – during this early part of summer at least – increased the overall amount of algae in the lakes. One measure of algae levels – total chlorophyll a – is not generally above the long-term average (See Figure 2).

At first this may seem at odds with the incidences of cyanobacteria blooms that have been reported for a number of Lake District lakes. However, these species of algae – or more strictly speaking, photosynthetic bacteria – are particularly well adapted to the warm, stable water column conditions that we have been experiencing, because they have the ability to float to the surface and out-compete other species for light. Their buoyancy also means that they can be readily moved by water currents into shoreline areas where they accumulate and become a problem for humans and other animals.


Examples of some of the cyanobacterial genera present in the Cumbrian lakes Windermere and Esthwaite Water this summer. From top left clockwise: Microcystis, Anabaena (straight form), Anabaena and Woronichinia, Anabaena (coiled form) and Aphanizomenon. Image: CEH

Our algal community data suggest that this group are particularly dominant this year, with cyanobacterial genera Anabaena, Aphanizomenon, Woronichinia and Microcystis being present in the community. The lower amount of algae that we have seen this year compared to the long-term average, is likely to be a response to the nutrients available, which enable the algae to grow.

The depletion in the surface water concentrations of these nutrients has been particularly rapid this year, which may be limiting the overall amount of algae that is being supported. The more recent rainstorms at the middle to end of July may have altered this availability by bringing in nutrients from the lake catchments, which could promote more algal growth over the remainder of the summer.

Dr. Ellie Mackay, UK Centre for Ecology & Hydrology, Lancaster, UK.

Read Part One of ‘Postcards from Heatwave Europe’

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