SURVEY OF AQUATIC INVERTEBRATES AT WOOLSTON EYES

by Jonathan Guest

• 1. Introduction
• 1.1 The Black-necked Grebe (Podiceps nigricollis) is a rare breeding bird in Britain and protected by Schedule 1 Part 1 of the Wildlife and Countryside Act. During recent years, a small population of breeding grebes has become established in a group of waters in north- western England.
• 1.2 In order to learn something of the freshwater ecology of these breeding waters, a series of site surveys was undertaken during the early autumn of 1998. Surveys took place after the breeding season so as to avoid disturbing the birds but before the invertebrate fauna had begun to die back with the onset of frosts. Survey at Woolston Eyes was carried out on September 21st.

• 2. Method
• 2.1 The method used was based on that developed for invertebrate sampling during the Pondlife Critical Biodiversity Survey of more than a thousand field ponds and other small water bodies in north-western England during the summers of 1995 - 1998. This entailed wading into the water with a standard pond net and searching all accessible habitats until, by the “law of diminishing returns”, no new invertebrate species could be found. Many invertebrates can be identified to species in the field by an experienced pond surveyor, while others have to be removed for identification under the microscope. For each species present, a simple VAFOR code was applied as a measure of abundance: VA = very abundant (thousands of individuals present in a notional three minute sample) A = abundant (hundreds of individuals) F = frequent (tens of individuals) O = occasional (several individuals) R = rare (1 or 2 individuals only) Samples are described as “notional” because it is not practical to continue netting solidly for three minutes. The net is examined repeatedly after short bursts of netting and the contents are summed subjectively to arrive at the results
• 2.2 This method is not as amenable to statistical analyses as are timed samples, but the latter tend to tempt the surveyor to rush around a pond and many of the resultant reports miss out many of the invertebrates present. The present method is felt to give a far better, more complete picture of the macro-invertebrate fauna present in a pond. Where the diet of the grebes is concerned, the abundance ratings of the different invertebrates gives a strong clue to their likely importance.
• 2.3 Four pools at Woolston were visited. The main pool on No.2 Bed proved impossible to sample by this method. Three pools on No.3 Bed were sampled. The habitats within these pools were considered to be representative of the other pools present which could not be visited in the time available.

• 3. Results
• 3.1 No.2 Bed contained only a few centimetres of water over unconsolidated sludge. Sampling was therefore impracticable, but by working the net through the surface of the mud, moderate numbers of chironomid larvae were disturbed. No other invertebrates were found and, given the uniformity of the habitat, it was considered that this situation prevailed across the whole of the pool. Chironomid larvae are available to dabbling ducks, and only Anas species were seen during this brief visit. A few Snipe were feeding at one end of the bed. These may have been taking chironomid larvae, or may have been feeding on worms taken from deeper within the mud.

No.3 Bed

• Three pools were sampled on this bed. The invertebrate fauna appeared rather uniform throughout and was found to be poor in species.
• 3.3 The open water in the pool in front of the southern hide was found to be shallow, with large areas under 30 - 40 cm in depth. Daphnia were very abundant in this shallow water, but are probably too small to be of use to Grebes. The hoglouse (Asellus aquaticus) was frequent on the bed of the shallows, where chironomid larvae were also moderately numerous. Both these taxa would be valuable prey items, as would the small water- boatman (Sigara lateralis) which is also frequent in shallow open water. The bladder snail (Physa acuta) was present in some numbers on the strands of Enteromorpha intestinalis, from which it could be collected with ease by the grebes. In many places the pool is fringed abruptly by tall stands of Greater Reedmace (Typha latifolia). The water within these stands is shaded, cool and relatively unproductive, nor would it be an easy matter for grebes to enter the stands to feed. Where scattered tufts of Typha or Soft Rush (Juncus effusus) stand emergent from the water in sunny settings, these shelter large numbers of the water boatman (Callicorixa praeusta) and the hoglouse (Asellus). Along the southern side of the pool, in contrast, is an area of low-growing Reed Canary-grass (Phalaris arundinacea). The growth of this plant here is unusually low, this being attributed to constant browsing and trampling by waterfowl. Duckweed (Lemna minor) forms a closed mat across the water between the Phalaris and the alga (Enteromorpha) is also abundant here. [In places there are dense mats of a blanket weed, perhaps Spirogyra or Cladophora.] The water boatman (Callicorixa praeusta) was found to be very abundant amongst the grass, with thousands of individuals netted in a few minutes. Asellus and Physa are both abundant in the same area.

Invertebrates present in the southern pool

• 3.4 A pool in the centre of No.3 Bed in front of the metal hide was found to contain similar invertebrate habitats to those in the previous pool. It is surrounded by stands of Typha which end abruptly at the water’s edge. Emergent tussocks of Soft Rush are dominant in places and there are stands of Reed Canary-grass, kept short by loafing waterfowl, around the edge of the pool. Common Duckweed (Lemna minor) is locally abundant amongst the rush tussocks and the Canary grass. Enteromorpha is again abundant. The principal invertebrate species present are again the small corixids C. praeusta and S. lateralis, although the larger Corixa punctata is also frequent, especially amongst the shelter of the rush tussocks. Asellus and the bladder snail Physa are less agile potential prey items, as is the scavenger beetle Enochrus testaceus which crawls amongst floating grasses. Chironomid larvae are frequent, hanging over the bottom sediment.

Invertebrates present in the pool by the metal hide

• 3.5 The third sample was taken from the southern end of the main pool in front of the new screen. Much of the shoreline of this pool is marked by abrupt stands of Typha or locally by Phragmites. Where there are Soft Rush tussocks, these have trapped wind-driven Duckweed in dense mats which made netting difficult. Enteromorpha is again present but in smaller quantities than in the shallower pools.
  
  As in the other two samples, corixids are abundant along with Asellus and the snails Physa and Lymnaea peregra. Chironomids and beetles are other potential prey items for the grebes.

Invertebrates present at the southern end of the main pool

• 4. Discussion
• 4.1 The ecosystem within the pools at Woolston is heavily influenced by the numbers of birds present. The constant input of droppings from gulls and waterfowl keeps the oxygen content of the water low. It will also influence the micro-flora within the pools. [The alga Enteromorpha is not common inland but is known from other waters fouled by waterfowl, e.g. by Canada Geese in Dunham Park, Trafford.] The water boatmen Callicorixa praeusta and Sigara lateralis are both species which flourish in waters fouled by cattle or birds. Both occur widely across the region, for example in field ponds fouled by wading cattle, but large, shallow pools such as those on No.3 Bed are unusual.
• 4.2 S. lateralis frequents shallow water, warmed by the sun, where it may be seen fleeing in swarms when the surface is broken. Large numbers must be taken by waterfowl including Grebes. C. praeusta makes more use of cover, sheltering especially in Juncus tussocks and scattered clumps of Typha. Both species of water boatman swim quickly when disturbed and may be a more suitable prey item for adult grebes than for chicks.
• 4.3 Rush tussocks become more suitable when flattened down by roosting waterfowl or nesting Black-headed Gulls. The blades then lie flat within the water, and the micro-habitat within the tussock is enriched by the droppings of the birds. Many invertebrates shelter beneath the splayed out tussocks. Molluscs and water hoglice crawl along submerged vegetation in search of food.
• 4.4 Reed Canary-grass normally grows into a tall plant which casts moderate to heavy shade. Dense, tall stands have cool water beneath and consequently shelter rather few invertebrates. On No.3 Bed, where waterfowl are constantly present, the growing shoots are browsed by ducks or swans and trampled by loafing birds. The resultant low, matted growth allows sunlight through to the water which will become warm quickly in summer. The water hoglice, bladder snails and scavenger beetles which inhabit these warm micro-habitats will form easy pickings for grebe chicks and for the adults when caring for their young. Unlike the corixids, these more sluggish invertebrates will be less able to flee from a feeding family of grebes.
• 4.5 It is reported that Black-necked Grebes congregate on the large, northern pool on their arrival in spring, but disperse to the smaller pools for breeding. The northern pool has deeper water which will warm up less quickly. It is also likely that invertebrate populations in that pool will be concentrated around the shallower margins. Any fish present are more likely to concentrate in this deeper pool where they can flee into relatively deep water to escape most predators. Shallow, sunlit water is probably of crucial importance to the grebes when feeding young, because of the abundance of easily caught invertebrates.
• 4.6 The nature of the vegetation at the edge of pools is of critical importance. While tall, dense stands of Typha or Phragmites are presumably of importance in screening pools and reducing disturbance, an abrupt interface is far from desirable. This study has shown the importance of micro-habitats such as emergent rush tussocks and browsed, trampled stands of Phalaris in shallow water. It is suggested that, to optimise breeding conditions for Black-necked Grebes, measures should be taken to maintain and perhaps increase the area of short, shallowly inundated vegetation around the edge of pools.