Flying up the River

This Monday saw the River Wandle in Sutton receive one of it’s regular health check-ups, using a quick and simple survey of the river’s inhabitants.

Fresh water shrimp (Gammerus)

Fresh water shrimp (Gammerus)

The Riverfly survey is designed to check that the river is clean and healthy for the animals and plants that inhabit it.  A quick way of doing this is to identify and count key insect / invertebrate species that you would expect to see in healthy waters, such as fresh water shrimp (Gammerus), caddis fly larvae (Trichoptera) and mayfly nymphs (Ephemera).

Fresh water shrimp are less than an inch in size and live at the bottom of the river bed. Although called ‘shrimp’ they are actually creatures called amphipods (meaning ‘different leg’), which feed mainly on detritus.

Caddis Fly larvae (Trichoptera)

Caddis Fly larvae (Trichoptera)

Caddis fly larvae can be found at the bottom of rocks; they spin a silken ‘web’ to catch and encase themselves in sand and bits of vegetation and appear as small nodules. They will eventually mature into pupae, and then adults with a full set of wings.

Mayfly Nymph (Ephemera)

Mayfly Nymph (Ephemera)

Mayfly nymphs are small insects with a distinctive three-pronged tail. Like the caddis fly, they will also mature into adults with wings, famously living for only a few days, or even hours, just to mate and reproduce. As nymphs rise to the surface of the river to become adults, they are often taken in huge numbers by fish and fishermen use a huge variety of mimics of these emerging adults as lures when they are trying to catch fish!

As these key species are affected by pollution in the water, even at low levels, there will be very few insects detected in a polluted river. Therefore, the insects that are counted should give us a good idea about how high the quality of the water in the river is.

The river survey showed some interesting results, for example, the site at Beddington Park was particularly rich in all three key species. On the other hand, the other two sites surveyed had noticeably less species, which suggests that these sites are in poorer health.

Male stickleback building his ‘nest’

Male stickleback building his ‘nest’

We had a pleasant surprise at the final site, when several sticklebacks turned up in the net. Sticklebacks are small fish that are common in fresh-water environments. The male stickleback is known for building ‘nests’ at the bottom of the pond /river and uses his bright red sides to attract females to lay their eggs in his nest. He’ll then protect the eggs and young until they are old enough to survive on their own!

The information collected from the river survey today will be sent off and added to the long-term information already gathered; long-term datasets like this are vital in detecting ‘real’ (i.e. statistically significant) trends to see if there is an improvement in water quality, by organisations trying to reduce pollution entering the river and a variety of changes in the management of the river.

I found today a really interesting facet of the variety of work the London Borough of Sutton’s Biodiversity Team and the SNCV participate in. The whole River Wandle is surveyed from its source in Croydon to where it empties in the Thames. All the surveys up and down the river are undertaken by volunteers keen to ensure the river is a great place for both wildlife and people. If you’d like to learn more about the Wandle, please see the pages of the Wandle Trust or to volunteer to help with the Riverfly survey, contact Will Tall at the Wandle Piscators.

Eleanor Kirby-Green
SNCV Biodiversity Assistant


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