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‘Modest’ fine sediment and phosphate pollution in English rivers causes mortality of up to 80% of mayfly eggs

January 12, 2018
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Blue-winged Olive. Image: Francisco Welter-Schultes | Wikipedia Creative Commons

Increased levels of fine suspended sediment and phosphate in aquatic ecosystems can have significant negative impacts on the survival of mayfly eggs, according to a new study. Relatively modest levels of pollution can kill up to 80% of eggs, with potentially devastating effects on mayfly populations and wider aquatic food webs.

Writing in the journal Environmental Pollution, a team of researchers led by Nick Everall of the Aquascience Consultancy carried out experiments on the blue-winged olive, a species of mayfly found across Europe, whose populations have fallen in recent decades.

Fine sediments and phosphate pollution from agricultural run-off and untreated sewage have been identified as key causes of this decline. However, until now, research has focused largely on the response of adult and larval mayfly to such multiple stressors.

Supported by the Salmon and Trout Conservation UK, the research team collected eggs of the blue-winged olive to be incubated in laboratory aquariums under different concentrations of fine suspended sediments and phosphate.

In the wild, blue-winged olive eggs are laid on the beds of fast-flowing streams and rivers, and have to survive over winter for up to eight months before hatching into nymphs. As a result, it is important to understand how stressors affect this crucial early stage in the insect’s life cycle. The researchers found that fine sediments cover mayfly eggs, starving them of oxygen and encouraging fungus growth, whilst phosphate can inhibit egg development.

When low levels of fine sediment were added to experiments with raised phosphate levels, the mortality of mayfly eggs increased significantly. However, when phosphate was added to experiments with increased amounts of fine sediment, mortality was not significantly increased.

This finding suggests that fine sediment has a greater impact on mayfly egg mortality than phosphate. As a multiple stressor relationship, it suggests that the run-off of fine sediments into aquatic ecosystems already stressed by phosphate pollution could have significant negative consequences for mayfly populations.

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Mayfly egg mortality increases as suspended sediment concentrations rise at a constant level of phosphate concentration. Image: Journal Authors.

Significantly, relatively modest levels of each stressor had damaging effects on mayfly egg survival. The concentrations of fine sediment and phosphate used in this experiment were largely below the Water Framework Directive defined thresholds for river management in England. At levels close to the upper limits for management – 25mg per litre of fine sediment and 0.07 mg per litre of phosphate – the mortality rate of mayfly eggs in the experiment was 80%.

Whilst the experimental conditions don’t fully represent the fluctuating nature of pollutant concentrations found in most rivers, the research team argue that their findings show that the annual mean suspended sediment guideline standard of 25 mg per litre for the UK is not sufficient to conserve mayfly populations.

More broadly, they suggest that increased attention needs to be paid to managing fine sediment into rivers across Europe, particularly as many rivers across the continent have raised phosphate levels. The implementation of effective mitigation strategies for reducing erosion and run-off of fine sediments from agricultural land surrounding rivers is clearly needed.

Everall NC et al (2017) Sensitivity of the early life stages of a mayfly to fine sediment and orthophosphate levels, Environmental Pollution, Online: In Press.

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