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Mesocosm research: Testing the waters

An innovative facility consisting of 32 tanks is helping researchers in Lancaster understand how large bodies of water such as reservoirs behave in response to the elements

By Dr Heidrun Feuchtmayr, Centre for Ecology & Hydrology (CEH)

Acquiring detailed process understanding in a ‘real-world’ reservoir is very challenging due to a lack of environmental controls and the lack of replication. In order to bridge the gap between the real-world environment of a natural reservoir and experiments in a small beaker inside a laboratory, mesocosm experiments are conducted at the Centre for Ecology & Hydrology (CEH) in Lancaster. The modern mesocosm facility consists of a large number of 3,000-litre tanks; in essence each of the 32 tanks simulates a small lake where all variables can be controlled.

Each mesocosm is open to the elements like a real lake and, by the addition of lake sediment, water and lake organisms, we can simulate a range of highly realistic lake environments under controlled conditions. The advantage this facility has over natural lakes or reservoirs is that it provides us with replication and continuous monitoring. Whatever treatment or manipulation we choose, we can replicate it in various mesocosms as we have so many of them. This gives us statistically robust evidence of how freshwaters react to whichever factors and manipulations we want to investigate.

The facility at CEH’s site in Lancaster is especially well equipped, including sensors within each mesocosm that allow us to monitor physical and chemical conditions 24/7. For example, we perform climate warming experiments by having all mesocosms fitted with heating elements mounted above the sediment. Computer-control allows us to warm the water temperature of half the mesocosms by up to 5 degrees Celsius higher than the non-heated mesocosm water temperature. Temperatures in the warmed mesocosms are adjusted every minute, ensuring a constant temperature difference between the heated and non-heated mesocosms. Data feeds from sensors to our analytical software allows us to view a near-real-time picture of the processes taking place in each mesocosm.

How has the facility been used so far?
So far, the mesocosm facility has been used extensively to run climate change experiments. Specifically, we have investigated the combined effects of climate warming, organic matter increase, nutrient addition and extreme rainfall events.

Brownification - the increase in organic matter concentrations in lakes and reservoirs across the temperature region - is of growing concern and the impact upon freshwater communities is largely unknown. By manipulating water temperature and organic matter input in nutrient-rich systems, we aimed to test if algal growth would be stimulated by warming and the additional nutrients within this organic matter, or whether organic matter actually decreases algal growth due to reductions in underwater light.

We found high dissolved organic matter increase, as predicted to occur over the next 21 years, reduced algal growth via shading. However, low organic matter additions, predicted to take place over the next four years, stimulated algal growth, especially cyanobacteria. It is likely that cyanobacterial blooms will be favoured by climate warming; they represent an increasing threat to water quality, with implications for treatment costs, human health, taste and odour.

An understanding of the combined effect of climate warming, nutrient input and high rainfall events on cyanobacteria was the focus of another recent mesocosm experiment. We simulated rainfall events every 12 weeks by adding 1,500 litres of water from a nearby reservoir to each designated mesocosm. Temperature increase and nutrient addition combined did not stimulate cyanobacterial growth as much as anticipated. In fact, at very high nutrient concentrations, other limiting factors seemed to alter interactions. High rainfall events only had a short-term effect on algal abundance, meaning that water quality recovered quite quickly from the perturbation.

Given the growing popularity of floatovoltaics - floating solar panels - as a potential renewable energy source, we set up a mesocosm experiment to investigate the effects of such installations upon lake ecosystems. We focused upon changes in the physical properties of the water, temperature and solar radiation, but whole ecosystem implications are yet unknown.

The smooth and successful running of the mesocosm experiments has proven that the site is a very reliable facility, scalable for both large and small-scale studies. We can set up controlled, replicated, treatments in large ecosystems open to the atmosphere, and the site is easily accessible. CEH staff in Lancaster have years of knowledge and expertise on running and managing mesocosm experiments, and interpreting the evidence that is generated by them.

Potential future water industry applications
The mesocosm facility is a highly versatile site and can be used for an array of different applications: from short-term experiments of a few weeks, to long-term experiments over years; from a few mesocosms to the entire facility of 32 mesocosms. If research is aimed at a particular reservoir, water from the site (if accessible) can be transported to the mesocosms to ensure identical communities. There is the potential to run a suite of different experiments, but two current water industry priorities are ideally-suited to mesocosm research: floatovoltaics and taste and odour.

Currently, floatovoltaics are installed at various reservoirs within the country. However, the positive and negative environmental impacts of such installations are not understood. Possible water quality effects could be investigated in the mesocosms. Another potential application for mesocosm research could be taste and odour issues, and the key question: what could the industry do to manage cyanobacterial blooms, the main producer of geosmin? Geosmin causes taste and odour problems as it can be detected by end users in very low concentrations.

Whatever the chosen focus of a mesocosm experiment might be, results could inform decision making and potentially reduce water treatment costs.

For more information visit www.ceh.ac.uk.

-This article appears in the April 2019 issue of WWT magazine.


Topic: Innovation , Water Quality
Tags: Ecology , Water Quality , research & development

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