Mayfly research emphasises serious need for river regulation re-think

For the first time a new research study, co-authored and financed by S&TC and published in the science journal Environmental Pollution, shows how even modest levels of sediment and phosphate are threatening the very life-blood in our rivers and streams.

This research matters as our Riverfly Census shows rivers are suffering from phosphate and sediment. Phosphate is the single largest cause of water bodies not achieving ‘good ecological status’ in the UK under the EU Water Framework Directive (WFD) and 83% of our rivers are failing to meet this standard.  According to the Environment Agency, phosphorus concentrations have increased in many regions, often linked to human and animal waste. 

The study carried out by Dr Nick Everall from Aquascience Consultancy Ltd in collaboration with us, shows that elevated levels of fine sediment and phosphate are having a devastating impact on the survival of the eggs of the Blue-Winged Olive (Serratella ignita) one of the most common mayfly species in the British Isles and present across most of Europe. 

Once a mainstay species for fish and anglers alike and synonymous with summer evenings on rivers from the Eden to the Test, the Blue-Winged Olive (BWO), is now in rapid decline in many waters across the UK.

The Blue Winged Olive (Serratella ignita) with an egg ball
The Blue Winged Olive (Serratella ignita) with an egg ball

Dr Nick Everall, who conducted this study with S&TC said:

“Mayflies such as the Blue-Winged Olive are a crucial component in the aquatic food chain but numbers have declined substantially in many UK rivers over the past 30 years, particularly in chalkstreams. Their continuing loss can affect the survival of other important species such as wild fish, bird life and mammals.  This research showed that even modest levels of sediment and phosphate below current national thresholds have a significant impact on egg survival to hatching.”

Although knowledge on environmental stresses affecting adult and larval life stages of mayfly such as BWOs has increased, until now there has been very limited research on the survival of their eggs. Their life cycle typically includes a long overwintering period of up to eight months in the vulnerable egg stage before nymphs emerge in the spring with adults hatching from June to September.   

Paul Knight, Chief Executive of S&TC said:

“The results of this ground-breaking study, which add further weight to the findings of our Riverfly Census are irrefutable.  For the first time, we have identified the serious impact that fine sediment and phosphate are having on the vulnerable early egg stages of mayflies.  We believe this is just the tip of the iceberg.  Lose your invertebrates and other species will follow.” 

Nick Everall explains:

“Our study showed that even low levels of both suspended sediment and orthophosphate (phosphate in solution) negatively affect invertebrate egg development.  It would seem that important information is being missed by focusing on the larval and adult stage of invertebrate development rather than their eggs, in what is potentially their most vulnerable development stage.”

Previous research on the larval and adult stages of invertebrates indicates that elevated levels of suspended sediment and orthophosphate are pervasive issues in river management, due to agricultural intensification and population increases coupled with direct discharge of untreated human waste.

Significantly, the sediment and orthophosphate levels used in laboratory conditions in this study, represented relatively modest concentrations for many English rivers and typically below current Water Framework specified thresholds.  These current findings support the growing concern that the annual mean suspended sediment guideline standard of 25mg 1-¹ in the UK is not sufficient.

This new study is in conjunction with our 3-year Riverfly Census, which has been monitoring the health of rivers across England and Wales by measuring water insect abundance and diversity.  The Census, which ends in 2018 has so far shown that many of our rivers (including those with Special Area of Conservation status) are suffering from pollution because of human pressure to the detriment of all water life including insects, wild fish, birds and mammals. 

Using the unique results of the Riverfly Census S&TC has been working with local Environment Agency teams to devise appropriate targets for river invertebrates such as mayfly species but, as Paul Knight, explains:

“These targets need to be agreed on a national basis before we will be able to see any positive progress.” 

Paul Knight adds:

“Current regulations under the Water Framework Directive are simply not rigorous enough to detect the extent of the problem. This latest study supports growing concern about current guidelines relating to suspended sediment and associated organic contaminants and the need for more stringent regulation, more defined targeting and better monitoring to protect all aquatic wildlife is paramount.”

The study published in Environmental Pollution (2017) by Everall N.C. et al is titled: Sensitivity of the early life stages of a mayfly to fine sediment and orthophosphate levels. 

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