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Lake type affects how climate change causes algal blooms in European lakes

September 28, 2018
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Algal bloom on Loch Leven in Scotland. Image: Laurence Carvalho

Blooms of blue-green algae – otherwise known as cyanobacteria – are likely to increase in European lakes as a result of climate change, according to a new study. However, this trend is likely to vary depending on the individual characteristics of different lakes.

Writing in Global Change Biology, Dr. Jessica Richardson from the Centre for Ecology and Hydrology and colleagues used data from lakes across Europe to explore the sensitivities of different types of lakes to multiple environmental stressors associated with climate change and human activities.

The research team – funded by the EU MARS project – wanted to find out how climate change effects, coupled with nutrient pollution, influences cyanobacteria growth. Characteristic ‘blue-green’ cyanobacteria blooms can be a common sight across Europe during the summer months, particularly in shallow lakes in agricultural or urban landscapes.

Whilst cyanobacteria communities are a natural – and valuable – part of almost all aquatic ecosystems, their rapid growth and spread – often following nutrient pollution during hot weather – can have adverse effects for the wider environment. Cyanobacteria blooms can cause hypoxic conditions with low dissolved oxygen levels, block light from entering the water column, and cause toxic health risks to humans and animals.

These effects can cause serious environmental and economic problems. As a result, understanding how and why cyanobacteria blooms occur in response to multiple stressors from human activities is important.

Gathering data from lakes across Europe

The research team used data on 494 natural European lakes to examine how eutrophication, temperature and prolonged periods of drought interact to influence cyanobacteria levels. The lake data was taken from the WISER database, and was based on scientific sampling of the individual lakes between 2000 and 2009.

The lakes were categorised into eight different types based on their alkalinity, the presence of humic substances (and consequently, the colour of the water), and the characteristics of water mixing in the water column. This categorisation was based on the common European lake typology scheme used in the Water Framework Directive. Lake data was paired with corresponding catchment and climate data from the same period, taken from the MARS geodatabase and JRC Agri4Cast Data Portal, respectively.

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Clyndrospermum sp. – microphotograph of a species of cyanobacteria. Image: Matthew Parker | Wikipedia Creative Commons

Modelling the interactions and impacts of multiple stressors

Three key stressors were chosen for analysis in this study: total phosphorus, water temperature and water retention time. Phosphorus is a nutrient pollutant commonly found in aquatic environments – often originating from agricultural fertilisers, run-off from urban areas, or waste-water treatment plants – which at high concentrations can prompt rapid cyanobacteria blooms.

Water temperature increases are widely predicted in lakes globally as a result of ongoing climate change. Previous studies have examined the impacts of temperature increases on lake water quality and biodiversity, particularly in how warming might exacerbate the environmental impacts of nutrient pollution.

Water retention time is a measure of how long water is ‘resident’ in a lake. Retention time is higher during times of drought (when there are low water flows), and lower during periods of heavy rainfall (when there are high water flows). Cyanobacteria blooms can occur when water retention times are high, as a result of increased concentrations of nutrients (which are not ‘flushed out’ of the system) and stable physical conditions. Lakes in regions where lower rainfall and drought is predicted under future climatic changes are likely to exhibit increased water retention time.

The sensitivity of European lakes to climate change is influenced by lake type

Around a quarter of the lakes analysed in the new study had average cyanobacteria levels which exceeded the World Health Organisation ‘low risk’ threshold. The majority of these lakes were located in central Europe. Lakes with lowest cyanobacteria levels were found in the most northerly regions of Europe, following a pattern of decreasing temperature and phosphorus levels with increasing latitude.

Overall, cyanobacteria levels increased with temperature and water retention time in five of the eight lake types. The effects of temperature were greatest in lake types at high latitudes, which suggests that lakes in these regions will become increasingly at risk from climate warming in the future.

However, the sensitivity of cyanobacteria to phosphorus levels, water temperature and retention time varied with lake type. For example, cyanobacteria levels in shallow, humic lakes with medium-high alkalinity were explained by water retention time, and a synergistic relationship between phosphorus levels and temperature. However, in lakes with similar characteristics – but this time with clear (non-humic) water – only water retention time was identified as an explanatory variable for cyanobacteria levels.

In short, the sensitivity and response of European lakes to climate change is shown – in terms of impacts on cyanobacteria levels – to be strongly influenced by lake type. The researchers state that in most lake types, management will become increasingly necessary as the effects of climate change – higher temperatures and retention times –  impact lakes in the future. They suggest that as climate effects cannot be locally controlled, there is a need for effective management of nutrient pollution in order to minimise harmful algal blooms.

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Algal bloom on the Spree River in Berlin. Image: Lars Plougmann | Flickr Creative Commons

European lakes under climate change: a ‘perfect storm’ for cyanobacteria blooms?

Lead author Dr. Jessica Richardson from the Centre for Ecology & Hydrology says, “Lakes are getting warmer and also experiencing longer periods of water retention because of prolonged droughts. In human impacted, nutrient rich lakes we think that these conditions could be the ‘perfect storm’ for promoting blooms of cyanobacteria.”

In our study of almost 500 European lakes, we found that higher temperatures, longer retention times and higher nutrients do increase the amount of cyanobacteria but not to the same degree in all lakes. This is not very surprising as there are many different types of lakes which are sensitive to these stresses in different ways.”

This site-specific variation in the impact of multiple stressors on European lakes has important implications for environmental policy and management, as Richardson explains,

“While climate change and eutrophication are important risk factors in the development of cyanobacterial blooms, other factors which also affect the growth and competition of cyanobacteria with other algae, like the depth of the lake, the colour of the lake, or the location of the lake may be equally as important.”

This means that when it comes to risk management, and predicting future risks of cyanobacterial blooms, a ‘one-size fits all’ approach can’t be applied. Instead, we need to consider that management of different types of lakes may need to be tailored according to their sensitivities.”

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Richardson, J. et al (2018), Effects of multiple stressors on cyanobacteria abundance vary with lake type, Global Change Biology, https://doi.org/10.1111/gcb.14396

MARS Project website

 

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