Professor Stephen Maberly outlines some of the water quality results from seven Cumbrian Lakes taken in 2016 as part of a significant long-term monitoring exercise...
Lakes are very sensitive to pressures such as climate change, pollution and the introduction of non-native species. For many, the intricacies of how underwater conditions and wildlife respond to such pressures are hidden from view. However, long-running research in the English Lake District is helping us understand the changes and challenges faced by underwater life, and possible knock-on effects upon the benefits that society derives from fresh waters.
The lakes in Cumbria are among the most iconic in England and are some of the best studied in the world, with continuous weekly or fortnightly records extending back over 70 years at some sites. They are extremely diverse in their chemistry and nutrient levels, because of the varied geology and land use in the region, and because of local patterns in human activity. They also vary greatly in their size and depth. As a result, although they experience similar weather, the water quality and organisms that live in them are very different.
The Lake Ecosystems Group of the Centre for Ecology & Hydrology currently undertakes a long-running programme of fortnightly research on seven lake basins in Cumbria. So far this work has produced an impressive 389 lake-years of accumulated data and hundreds of scientific papers.
Using these data, we show here how conditions in the lakes varied over the course of 2016. Figure 2 below gives annual ranges and average values for four key water quality variables. Water temperature varied between a winter minimum of 3.1°C in Grasmere and a summer maximum of 23.2°C in Esthwaite Water, with only small differences among lakes because they all experience similar weather. Oxygen concentrations were more variable: surface waters in the most productive lake, Esthwaite Water, were oversaturated in the summer (nearly 140%) but, at the same time, there was no oxygen present in the deepest waters of this lake.
Oxygen concentrations even fell in the deeper waters of the least productive lake, Derwent Water, although the average water column concentration was higher than in other lakes because the depletion was less severe. Deep-water declines in oxygen concentration can be a major survival challenge for many underwater animals and can also lead to release of nutrients from the sediment back to the water.
Fig. 2: Annual ranges and average values for water quality variables in seven Cumbrian lakes in 2016. Seasonal water column temperature is shown in purple, oxygen concentration in grey (high concentrations are often found in surface waters, and lower concentrations in deeper waters), phytoplankton chlorophyll a concentration in green and the depth of disappearance from view of the Secchi disc in blue. In each case the average values are shown by an orange circle. The lakes are ordered by chlorophyll a concentration.
Deep-water declines in oxygen concentration can be a major survival challenge for many underwater animals and can lead to release of nutrients from the sediment back to the water
We use the lake water concentration of the pigment chlorophyll a, used for photosynthesis, to indicate the total biomass of phytoplankton (“algae”) in each lake. During 2016, these concentrations were variable among lakes as a result of variations in the availability of key nutrients needed for growth - especially phosphorus.
Maximum summer chlorophyll concentrations in Grasmere, Blelham Tarn and Esthwaite Water were two to five-times greater than in the less productive lakes. Phytoplankton are a major cause of reduced water clarity, as measured here by the Secchi depth (the depth at which a standard white metal disc disappears from view, see photo right). Based upon this measurement, Derwent Water has the highest, and Blelham Tarn and Esthwaite Water the lowest, water clarity. Bassenthwaite Lake, however, has a much lower transparency than would be expected from its chlorophyll concentration because, in this shallow and exposed lake, sediment derived from the catchment or re-suspended by wind decreases water clarity.
These and other measurements, some made automatically every few minutes, help us to document how different types of lakes respond to environmental change. The understanding of the causes of change that this generates is used to help manage lakes sustainably for future generations.