Macroinvertebrates of the River Foss


Freshwater macroinvertebrates are a generally abundant and diverse group of animals that spend all or most of their lives on the bottom of still and running water, have no backbone and can be seen with the naked eye. They include worms, molluscs, crustaceans, water beetles and the early life-cycle stages (nymphs and larvae) of several flying insects such as dragon and damselflies, mayflies, stoneflies and caddisflies. They are an extremely important part of the freshwater ecosystem, providing a food source for fish, birds and mammals whilst helping to breakdown organic matter in the environment and consequently helping to maintain water quality. As they have a wide range of tolerances to different environments, levels of pollution and, unlike fish, are relatively immobile, they cannot escape the effects of pollution incidents, so they have been used for many years as indicators of biological water quality. Due to their ease of sampling, using dip or kick nets depending on the nature of the river, monitoring their populations along a river’s course can help to assess the health of the river and identify areas of pollution.

As our landscape has become more developed, the impacts can be seen on river water quality. When it rains, pollutants such as fertilizers, pesticides and loose soil are washed into rivers from agriculture, treated sewage is constantly discharged and raw sewage from storm overflows is an increasing problem. Biological monitoring of the macroinvertebrate communities can directly measure the health of the river ecosystem using water quality, or biotic, indices based on scores given to groups of macroinvertebrates dependent on their known sensitivity to or tolerance of organic pollution in particular. Simple indices only require the animals to be identified to family level, relatively easy for amateur ecologists to carry out. For more accurate indices, identification to species level will be necessary for some groups. The River Invertebrate Prediction and Classification System (RIVPACS), developed by the Freshwater Biological Association, is a free statistical model that enables users to estimate the ecological health of running water sites. Drawing on datasets of the macroinvertebrates that live in ‘pristine’ conditions, the model can predict what macroinvertebrates should be present at a site with a particular habitat type. The difference between the expected population and the observed population then indicates the ecological status of the water.

Those groups of macroinvertebrates that are completely intolerant of pollution, need the cleanest water and high levels of dissolved oxygen to survive, include many of the mayfly and stonefly nymphs and caddisfly larvae. Their presence in a stream or river is indicative of clean water and a good aquatic habitat and lead to high scoring biotic indices.

The next groups are moderately tolerant of pollution and include some mayflies, stoneflies and caddisflies as well as dragon and damselfly nymphs, alderfly larvae, beetles, crustaceans and molluscs

The groups most tolerant of pollution include midge and blackfly larvae, leeches and aquatic worms and these can survive in areas with poor water quality and low dissolved oxygen levels.

Healthy, clean rivers will have macroinvertebrates present from all the above groups.

In 2017 wetland ecologist Martin Hammond led a survey to assess the ecology of the River Foss producing a series of reports, all available on the RFS website, including “The Invertebrates of the River Foss”. This report collates the most recent survey data obtained by the Environment Agency from 1995 along with historical and archaeological data from the Anglo-Scandinavian period (8th-11th centuries) to the 19th century. More than 170 invertebrate taxa (a term for related biological groups) have been found, most identified to species level with some to only family or genus level. Most of the records are from the middle and lower reaches of the river, few from the upper reaches.

High numbers of individuals from a range of mayfly species were found at the middle reach sampling sites, Marton and Lilling, but at the lower sites at Strensall, Huntington and Castle Mills, both individual and species numbers were reduced and those present were from the more pollution tolerant species. Caddisfly larvae showed a gradual decline in individual numbers from the middle reaches to very few at Castle Mills. Few stoneflies were recorded at any site although it is thought they would be found in the upper reaches. Water beetles were present along the river with those found closer to the city being tolerant of nutrient rich, slow-moving rivers.

In 2019, St Nick’s Centre for Nature in York along with Martin Hammond and RFS volunteers, carried out a survey of the upper reaches of the Foss to determine the state of populations of the native crustacean, the white-clawed crayfish.  These are the UK’s only native freshwater crayfish but their numbers have seriously declined in recent years due to the spread of the American signal crayfish. This species was first recorded in south-east England in 1975 and has slowly been spreading across our rivers, outcompeting our native crayfish as they are more aggressive, grow faster, eat a broader range of food therefore reduce food sources for the native crayfish and they also carry a fungal parasite which is particularly deadly to white-clawed crayfish.

Large and healthy populations of white-clawed crayfish were found in the upper reaches of the Foss, which are almost perfect habitat for them, extending the known distribution about 1 mile downstream of previous records.  The upper Foss is a lot more natural in its habitat than the rest of the river and it is hoped to continue surveys further downstream to see if the populations extend further. There have been no reports of signal crayfish in the Foss and, as it is an isolated catchment with no other main rivers flowing into it, there is hope that it will be a refuge for white-clawed crayfish in Yorkshire, but it does need to be protected. Please note that the white-clawed crayfish is protected under schedule 5 of the Wildlife & Countryside Act and it is illegal to search for it without a special survey license.

Thirty-seven species of molluscs (snails and mussels) have been recorded in the Foss over time but many have declined in number or disappeared entirely. As many are sensitive to organic pollution the decline is probably due to deteriorating water quality along with the effects of dredging.

The macroinvertebrate communities of the River Foss show, as with many rivers, a gradual decline in water quality from the upper reaches down to the lower reaches as it flows through built-up areas and into the city. Agricultural run-off, dredging, channel widening and run-off from roads all have a detrimental effect. Working with the Environment Agency and several other wildlife and environmental organisations to improve not just the river’s water quality but also the bankside flora and bankside management will, hopefully, in the long-term lead to improvements for the flora and fauna of the river including those small, generally unnoticed but vitally important macroinvertebrates.