For the last CAN session of the 2012-13 year, ten of us assembled to hear Bill Bellamy’s talk about inland water quality and how macroinvertebrate surveys are used by the Environment Agency to monitor it. He explained very clearly how factors such as acidity and organic pollution can affect the oxygen content of the water which is crucial for fish and for many sensitive invertebrate groups. He also noted how the nation’s sewage systems have been upgraded in recent decades to reduce the potential for overflow into freshwater systems. Newer threats have come from contraceptive oestrogens changing  the population dynamics in fish and from brominated phenol fire retardants, which are highly toxic and virtually indestructible.

Rostherne Brook

The macroinvertebrates are those exceeding about 1mm in length. Ranging across worms, leeches, molluscs, crustaceans and insects, these have been placed on a scale of 1 (poor) to 10 (good) as an indication of water quality, the classic Biological Monitoring Working Party (BMWP) score. Bill also described the standard kick sampling methodology used in the surveys. Using the CAN microscopes, we spent some time examining preserved samples of many of the important families and the available ID keys before venturing out with nets and trays to practice the method in Rostherne Brook in the Rostherne Mere National Nature Reserve.

Bill shows members whats in the catch

This pleasant winding stream is the main flow into the Mere, and in the past was responsible for an accumulation of phosphate in the sediments until treated sewage was diverted away from the brook in 1991. We divided into three groups so everyone had a chance to do some sampling and get a good look at the catch. We were pleased to find large numbers of freshwater shrimps (Gammarus pulex),  mayfly larvae, several stonefly species and some distinctive caddis fly larvae (Rhyacophilia dorsalis) which rate up to 7 on the BMWP scale, as well as many other more pollution-tolerant  insects, molluscs, leeches and worms.

With not a drop of rain, it made for yet another excellent CAN event, combining not only an attractive location and practical natural history learning, but also an insight into how one of our most basic service industries is controlling the human impact on the environment.