State of Nature 2016: Disappearing riverflies…

The recently published State of Nature 2016 report records the decline in species as habitat degrades across the UK. Salmon & Trout Conservation UK’s Riverfly Census reinforces the State of Nature’s sombre message in exposing the continuing fall in aquatic invertebrate species as our rivers’ water quality declines.

Aquatic invertebrates matter. They are the foundation of the food chain for fish, birds and mammals which inhabit wetlands. River life is dependent on a thriving aquatic invertebrate population.  Invertebrates are also excellent measurers of water quality: they spend a massive proportion of their lives in water and different species are sensitive to various forms of pollution – organic, nutrient enrichment, sedimentation and low flows.

The Riverfly Census has used species-level data to illuminate the state of our rivers in terms of these various subtle but often lethal threats. Yet one very simple statistic stands out above all else in our data where we can make historic comparisons: the decline in invertebrate species richness continues unabated in many of our river systems.

Declining species richness has long been recognised as a powerful, easily understood indicator of ecological distress:

“Whether it is a rainforest, tundra, coral reef or wetlands like a river, stream or a lake… reduced species richness is the most consistent indicator of ecosystem distress. It is one of those refreshing simplifications that natural systems, despite their diversity, respond to stress in very similar ways.”

Clements, W.H. and Newman, M.C. (2002)

On the chalkstreams, many well-known species such as the Southern Iron Blue or the Blue-winged Olive are no longer present in our samples. They have fallen victim to declining water quality in general and, we believe, in particular to the combined impact of phosphates and sedimentation.

We have compared mayfly (Ephemeroptera) species data from a 1998 study by the Institute of Freshwater Ecology (IFE) with our own and Wessex Chalkstream Rivers Trust data for 2015 and spring 2016. In almost all cases, species numbers have declined. The target of 10 mayfly species[1] which you would expect to find on any clean river, illustrated by the dotted red line on the chart, is also not being met .

The loss is not confined to southern Chalkstreams. We have compared our data to historic species records on the Coquet and the Eden at Temple Sowerby. The Coquet has seen a significant decline at most sites. The Eden site has witnessed a significant decline from 2002 to 2015 though the mayfly species richness is still (just) over 10. The pollution impacts on these rivers in the past probably included cypermethrin from sheep dips. The current downturn may well involve livestock medication again, especially associated with cattle along with other forms of pollution. [JG1]    We plan to investigate this.

We are continuing to gather data on our census rivers to help understand the impact of various forms of pollution on invertebrates and through this work we will challenge the Environment Agency (EA) to act. The chart below plots the relationship between sedimentation and species richness though many other forms of pollution also have an impact on species richness. If the ecological protectorate can persuade the EA to adopt and implement PSI targets of a sufficient magnitude along with targets for phosphates, for example, the rot can be reversed.