Slightly hidden but only a short walk into Yarner Woods, lies the reservoir. It’s a popular place to sit and watch waterfowl species, such as the mandarin ducks, and on the sunny autumn days we have been having recently, it’s easy to overlook that underneath the surface of the water, it’s an ecosystem that’s not performing at its best.
Results of a recent fish survey suggested its waters are deprived of nutrients, a condition known as ‘oligotrophic’. Fish were found to have stunted growth, suggesting that they are not gaining either enough or the right type of food from the creatures that reside within it.
Small freshwater bodies, like the reservoir, typically have a small catchment area. As a result, they’re influenced chiefly by local environmental conditions. This creates unique habitats that have potential to support a diverse array of differing species. Biodiversity, in its most simplistic form, is the variety and diversity of life. Freshwater biodiversity is threatened and is decreasing more rapidly than in terrestrial or marine environments. Yet it receives very little protection.
As part of its management, it’s important not to think of the reservoir as an isolated ecosystem – i.e. one in which we only focus on the animals that are contained within its waters. An ecosystem is not an individual and discrete unit and there are an endless number of interactions between an aquatic system and its terrestrial interface. For example, emerging adult insects that were once larvae within the water are preyed upon by bats, birds and spiders. You may have seen the kingfisher darting along the surface of the water feeding and then resting on the tree branches around the water’s edge – it’s all part of a highly connected ecosystem that doesn’t work in isolation.
American signal crayfish – monitoring for an invasive species
Before any reservoir management can begin, the team here at Yarner have been closely monitoring for the presence of a damaging invasive species, known as the American signal crayfish. For a species to be considered invasive, it must be found in its non-indigenous environment and have the potential to cause that environment harm.
In the last 50 years, at least eight non-native species of crayfish have been introduced into the Great Britain, and most of these have now established viable breeding populations. Reasons for introductions vary but most commonly include being imported for the food and pet trade and through unintentional introductions, such as being carried unknowingly by boats along waterways.
The American signal crayfish is a carrier of the crayfish plague, an infectious disease caused by the water mould Aphanamocyes astaci. All American species carry the infection (but it is only lethal to individuals that are already stressed and easily prone to illness). To European crayfish species, such as the endangered native white-clawed, the infection is rapidly fatal.
They are voracious predators, predating not only native crayfish, but also many other aquatic invertebrates and can wipe out native fish stocks through consuming there eggs. They now occupy 80 per cent of rivers in England and Wales, but an effective control technique is yet to be established.
It’s essential to prove that this invasive crayfish is absent from the reservoir, before proceeding with management. If we were to improve the ecological health of the reservoir, and later find that the crayfish was present, it being there could remove any beneficial effects that the management has had on other species, for example through predation.
Last year’s monitoring was successful, with no evidence of signal crayfish presence (see blog – Initial Investigation for Invasives Shows all Clear). This year seems to be on a similar trend, so let’s hope it continues and that we can proceed with the exiting upcoming plans!
The equipment we use here at Yarner to monitor for the American Signal Crayfish
Interestingly, not all non-native species are considered negative. You may have noticed waterfowl with an elaborate orange plumage floating on the reservoir. These are male mandarin ducks (females are duller by comparison, being mottled brown with a white stripe behind the eye) and they are experiencing a severe decline in their natural habitat in Asia. Europe seems a stronghold habitat for this species now and because they have a minimal impact on our native wildlife, it’s important to allow them to live here at Yarner.
What’s next for the reservoir? Future management plans
As an artificial feature, the reservoir edge lacks features often found in natural bodies of water. Its edges are uniformly steeply angled and made of concrete. There are no crevices in the substrate floor in which animals can hide and no throughput of water making the reservoir isolated from streams or nearby water sources. Changes need to be made to improve this.
In future months, ‘artificial floating islands’, in the form of floating mats containing beneficial vegetation, will be placed within its waters.
An example of artificial floating islands (Image courtesy of contractor, Frog Environmental; Rafts made by BioHaven)
Floating islands offer alternative nesting sites to waterfowl species and reduce predation risks by providing safe, inaccessible nesting refuges. Often, dragonflies and damselflies are seen flying above the reservoir waters, and the mats will help to provide areas in which they, and other similar insects can rest. A key advantage is that, because they float, they can adapt to water level fluctuations.
One of their most important features is to help re-oxygenate the surrounding waters because oxygen is released as the floating plants photosynthesise. Aquatic invertebrates are highly sensitive to deoxygenation, so aeration is key benefit to these mats. They also help to clean the water as nutrients, via nutrient recycling, are removed thorough direct plant root uptake.
At the moment, we’re in the design phase, but over this winter, trees will be felled and the rafts will be installed. As with any form of natural management, these changes won’t happen immediately, but we should start to see them over the coming months following their installation as animals start to colonise. Hopefully work will be completed by April, and future water testing should show these gradual improvements.
We’re all looking forward to seeing the future changes, so watch out for upcoming blogs on how work to the reservoir is coming along!
Written by Katherine Hewitt, Conservation Assistant with Natural England
The works at Yarner Wood reservoir are part of the Moor than meets the eye scheme, with support from the Heritage Lottery Fund.