Riverfly 2

David Beeson, January 2022

“The Riverfly Partnership is a network of organisations, representing anglers, conservationists, entomologists, scientists, water course managers and relevant authorities, working together to: – protect the water quality of our rivers; – further the understanding of riverfly populations; – and actively conserve riverfly habitats. The Riverfly Partnership is hosted by the Freshwater Biological Association.”

I am taking a small part in a Citizen Science initiative for the Riverfly Partnership and Environment Agency. I sample a single section of my local chalk river for changes to certain pollution-sensitive invertebrate species. My site is on the River Anton, that feeds into the River Test and that enters the sea at Southampton on the south coast of the UK.

In the first article (Riverfly) there were images of the kick sampling technique and of the collection of stone-clinging invertebrates. The latter invertebrates are limited as the substrate is mostly small gravel, so an unsuitable environment.

Kick sampling is mildly scientific in that it occurs for a specific time, 3-minutes in this case, but the technique is hardly standardised person to person, site to site. But, if carried out by one sampler at a particular location, it will generate a consistent sampling. That data can then be compared month by month. Currently, I am generating a base-line measurement of the target invertebrates of the upper reaches of the River Test.

The project is only looking at eight invertebrate groups and already my data appears similar month by month. Which is a relief, as otherwise I’m incompetent or there has been a big change in the river ecosystem. Phew!

The most common organism encountered has been the freshwater shrimp, Gammarus. Gammarus pulex, or the ‘river shrimp’, is a crustacean related to the crabs and lobsters. It is similar to the ‘sand-hoppers’ commonly seen on our beaches. G. pulex generally lie on their sides under stones, rocks, leaves and wood on river and lake bottoms. They also swim on their sides and can crawl over surfaces and into crevices.

Adult males reach nearly 2cm in length, females are smaller and the young are miniature versions of the adults. They have two pairs of antennae on their head, five pairs of walking legs and two other pairs of leg-like limbs that have hooks on the end. They have been described as looking like a ‘swimming comma’. Males can often be found carrying their mates, and protect them aggressively.

Gammarus are scavengers and feed on the microscopic algae and protozoans normally found in pond water. Their main predators are fish – trout and salmon, bullheads and stone loach all eat them, as do minnows and sticklebacks. In other words, they sit at the base of the animal food chain.

In the River Anton I see trout, sticklebacks and bullheads. All would welcome a snack of a freshwater shrimp or two! Birds such as water rail would also be keen to sample their ‘delights’.

Gammarus pulex

Cased caddis fly larvae are frequently encountered. These are immature flies that spend their young phase in water. To protect themselves they glue materials around their body and crawl forward with their front limbs. The case can be of minute sand particles or vegetation.

Cased caddis fly larva.
Adult caddis fly

The Wildlife Trust website says:

“There are almost 200 species of caddisfly (order Trichoptera, also known as ‘sedge flies’) in the UK, the largest of which is more than 3cm long. Adults are moth-like insects with hairy wings.

Caddisfly larvae live underwater, where they make cases by spinning together stones, sand, leaves and twigs with a silk they secrete from glands around the mouth. Most larvae live in these shelters, which can either be fixed or transportable, though a few species are free-swimming and only construct shelters when they’re ready to pupate.

Adults are often attracted to moth traps, or can be found during the day on vegetation near to the water’s edge, or flying in swarms over the water. Caddisflies are an important food source for all kinds of predators, including Atlantic Salmon and Brown Trout, and birds such as the Dipper.”

The free-swimming caddis are found locally, yet I only encounter them in more rocky areas. So far, they have not appeared in my samples.

Free-swimming caddis that spin an underwater web.

Burrowing mayfly nymphs, Mayfly Ephemeridae, are found in muddy areas. Along my sampling area the substrate is gravel and mud only occurs where weed has established. As I move the sampling transect slightly each time I do not always sample in the weed – so they are not encountered each time.

Ephemeridae adults have ‘up-wings’ while caddis have wings folded along their backs – ‘down-wings’.

Other mayflies are along the river in large numbers. These species enjoy well-oxygenated water and hide during daylight between the gravel, emerging at night to feed. As young nymphs they are herbivores, changing to a more carnivorous diet before emerging. The two types sampled are the blue-winged olive Ephemerellidae and Baetidae. In winter these two types are quite small and distinguishing between them is not easy!

Blue-winged olive Epemererellidae nymph

Finally, I am looking for stoneflies. These are easily distinguished as they have only two tail filaments. So far I have found only a single specimen. I approached the Freshwater Biological Association as to why, and the answer was from Craig Macadam:

“Quite simply, the majority of stoneflies require swift flowing, well-oxygenated water to sustain their populations. They are more common in stony streams in upland areas and in spate-rivers of the north and west. They are relatively tolerant of the slightly acidic conditions found in some of these areas. Whilst chalkstreams are typically of sufficient water quality for stoneflies, the habitat is not conducive to their lifecycle. There are a few exceptions, for example the Yellow Sally (Isoperla grammatica) which is often found in the upper reaches of chalkstreams in Hampshire and Wiltshire.
In contrast, mayflies flourish in the base-rich conditions in the chalkstreams. The relatively constant water temperature and generally consistent flows are perfect conditions for a wide range of mayfly species to develop. The species of stone-clingers (Heptageniidae) are usually absent though as they require faster flows than are present in these watercourses.” Thank you Craig for a 100% answer.

Adult stonefly

The rivers Anton and Test are base-rich chalk streams. And, I have found no Heptageniidae mayflies – the last category for the Riverfly sampling data.

My January data was: Cased caddis – 7; Caseless / swimming caddis – 0; Digging mayfly – 7; Heptageniidae – 0; Blue-winged olive Ephemerellidae – 20+; Olive Baetidae 35+; Gammarus – hundreds, possibly 400 as I gave up counting.

I also encountered three European bullheads (Miller’s thumb), various other mayflies, a few very small leeches, planarians, water beetles and fly larvae.

More on mayflies: They are unique in the insect world in that the adults have two winged forms. The nymph emerges from the water as a dull-coloured sub-adult that immediately seeks a secured covered location, for example in the reeds. After a couple of hours, this sub-adult sheds its outer body covering and emerges as a brightly-coloured full adult.

The HOMEPAGE is http://www.nwhwildlife.org. Here you can scroll down to 130+ ad-free articles on wildlife and ecology.

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