The ecology of the invertebrate community of Borough Fen Decoy pond

MARGARET PALMER

Introduction topping-up took place on 5 days -28, 29 June, 24 August and 28, 29 September. Borough Fen is the oldest surviving Duck The water in the pond and its eight curved Decoy in Britain. It was constructed 4 miles ‘pipes’ is clear and shallow. Nowhere is it north of Peterborough about 1640 for com­ more than 2 ft deep, in most places only a few mercial duck-catching and continued in that inches. The bottom of fine gravel is overlaid use until the Wildfowl Trust converted it to by a thick layer of black mud. In 1947 it was a ringing station (Cook, 1960). The 2|-acre cleaned out by dragline, and each year a pond and the 14| acres of mixed woodland different area is pumped clear of mud. in which it lies have thus formed an isolated, There are patches of algae on the mud, but managed ecological unit for well over three apart from an island of reeds there are no centuries. With the drainage of the Fens it rooted plants of duckweeds growing in the was one of the few permanent bodies of water central part of the pond. Around the margins remaining, until the relatively recent excava­ Carex acuta, Glyceria maxima and Phrag­ tion of ballast and gravel pits. Woods are mites communis flourish, and the encroaching unusual in the Fen country too, often mark­ vegetation is periodically dug out, as are the ing the sites of disused decoys. edges of the reed island. Aquatic plants grow The pond water is replenished in the sum­ readily in the pipes, which have to be cleared mer, and other times of shortage, through a annually (Figure 1). This vegetation consists drain connecting it with the River Welland. of Glyceria maxima, Lemna minor, Callitriche Indeed the first written record of the Decoy’s stagnalis, Rorippa nasturtium-aquaticum and existence is an application made by the Earl filamentous algae including Spirogyra. of Lincoln to the Bedford Level Commis­ The pond often has a roosting population sioners, in 1670, for permission to pierce of up to 1,500 duck during the autumn, the the embankment of that river. In 1972 such birds flighting out in the evening to feed in

Figure 1. Borough Fen Decoy. The mouth of the North Pipe; a general impression of the vegetation round the pond. 144 Margaret Palmer the surrounding arable land, sometimes as Crustacea far as the coast 20 miles away. In the winter Cladocera the roosting population falls to 400-500. Few Simnocephalus vetulus (Müller) of the duck stay to nest. In 1972 one pair of Daphnia pulex (De Geer) Mute Swans, about ten pairs of Mallard and Daphnia obtusa (Kurs.) several pairs of Moorhen nested around the Daphnia magna (Straus) pond. Scapholeberis aurita (Fischer) Leydigia leydigi (Schodler)

Copepoda Water analysis Cyclops agilis (Koch. Sars) Cyclops vicinus (uljannà) *** A water sample taken in June had approxi­ Isopoda mately 90 parts per million (ppm) of calcium. Asellus meridianus (Racovitza). Two samples of water were taken on a Amphipoda sunny afternoon in July from the central area Crangonyx pseudogracilis (Bousfield) of the pond, where the water was 4 in deep, over 10 in of mud, with a water temperature Insecta of21°C. Oxygen saturation was 135%/122% Hemiptera and the pH 7-98/8-02. The compounds in Gerris lacustris (L.) solution were, in ppm: oxygen 11-7/10-6; Notonecta glauca (L.) ammonia (as nitrogen) 9-2/10-2; nitrate (as Callicorixa praeusta (Fieber) nitrogen) 1 -2/1 -2 ; nitrite not detectable ; phos­ Corixa punctata (Illiger) phate (as P 0 4) 2-1/2-1; total hardness (as Hesperocorixa sahlbergi (Fieber) C aC 03) 250/250. Sigara dorsalis (Leach) Sigara/alleni (Fieber) Sigara lateralis (Leach) Animal survey Trichoptera Limnephilidae Coleoptera This was carried out between March and Haliplus lineatocollis (Marsham) July 1972. Both Pygosteus pungitius (ten- Haliplus ruficollis (Degeer) spined stickleback) and Gasterosteus aculea­ hygrotus inaequalis (Fab.) tus (three-spined stickleback) were present Ostracoda in the pond. Rana temporaria (common frog) Hydroporus granularis (L.) returned in 1972, after some years’ absence Hydroporus palustris (L.) from Borough Fen, but no tadpoles were Hydroporus pubescens (Gyllenhal.) seen. Some fifty species of invertebrates were Diptera found, half of them insects (Table 1). Ptychoptera contaminata (L.) Ptychoptera minuta (Tonnoir) Anopheles maculipennis (Meigen) Table 1. Invertebrates found in Borough Paradixa aestivalis (Mg.) Fen Decoy pond Chironomidae Ceratopogonidae Tricladida Pericoma sp. Dendrocoelum lacteum (Müller) Acarina Polycelis nigra (Müller) Nematoda ., Scarce; *, fairly frequent; **, frequent; ***, very Mermithidae frequent; »***, abundant; **»**, very abundant. Annelida Oligochaeta Tubificadae Nais sp. Notes on the ecology of the invertebrates Hirudinea Erpobdella testacea (Savigny) T ricladida Helobdella stagnalis (L.) Mollusca Gastropoda Dendrocoelum lacteum is a flatworm, found in Lymnaea peregra (Müller) productive waters where the calcium content Lymnaea stagnalis (L.) is at least 10 ppm (Reynoldson, 1967). It feeds Planorbis planorbis (L.) on arthropods, and its distribution is closely Planorbis contortus (L.) linked with that of Asellus, one item of its Lamellibranchiata diet. Polycellis nigra is a widespread species, Anodonta cygnea which feeds on oligochaetes and arthropods. Invertebrate ecology, Borough Fen Pond 145

Nematoda gracilis is a freshwater shrimp which was introduced from North Américain the 1930s Mermithidae(rainworms) are relatively large and is now fairly widespread. It appears to be roundworms, several centimetres in length, able to stand pollution better than the com­ parasitic in their early stages in insect larvae. mon freshwater shrimp, Gammarus pulex, which was not found at Borough Fen.

Hirudinea Hemiptera Helobdella stagnalis is a very common leech, typical of still waters rich in calcium. Erpob- Gerris lacustris is probably the commonest della testacea, a less common species, is found species of pond skater in the Peterborough in emergent vegetation, eutrophic ponds and district, and Notonecta glauca is by far the organically polluted waters. Both species feed commonest of the back-swimming water on invertebrates such as chironomid larvae boatm en. and oligochaete worms, and Erpobdella test­ Macan (1954, 1963, 1965) has shown that acea also takes water fleas. each species of corixid has precise require­ ments, including the calcium content of the water and the amount of organic matter on Mollusca the bottom. Of the six species of corixid found at Borough Fen, the only numerous one was Lymnaea peregra is the most widespread and Sigara lateralis. This water boatman is often abundant British freshwater snail. L. stag­ abundant in ponds fouled by animals (in this nalis is common in hard-water ponds. case the waterfowl). Callicorixa praeusta is Similarly, Planorbis contortus is a widespread common in rich waters, often where there is ramshorn snail, whereas P. planorbis is re­ some degree of organic pollution. Sigara stricted to hard water. Shells of Anodonta falleni and Corixa punctata are common in cygnea found at Borough Fen measured up calcium-rich waters. Hesperocorixa sahlbergi to 5-j in. in length. Sticklebacks here would is typical of ponds with black mud on the be the hosts for the parasitic larvae of the bottom, where the water is not base-deficient. swan mussel. Sigara dorsalis is typical of waters with less organic matter than is present at Borough Fen. Crustacea

The pond teems with Cladocera. Daphnia T richoptera pulex is typical of polluted ponds, as it feeds on unicellular algae with hard cuticles, which The two limnephilid caddis larvae found abound in these conditions. It can tolerate am ongst Carex stems had constructed their low oxygen concentrations, and its feeding cases from plant material. limbs are not clogged by bacteria. Daphnia obtusa is associated with ‘duck ponds’. D. magna is rather rare, occurring in hard waters in south and east England. Simnocephalus Coleoptera vetulus, which flourishes in hard, weedy waters, was restricted to the thick vegetation All the beetles found at Borough Fen are in the pipe ends. Scapholeberis aurita, a rare very common pond species. species, crawls on the underside of the surface film of the water. Before this survey it had only been recorded from Norfolk and Hamp­ Diptera ton Court (Scourfield & Harding, 1967). Ley- digia leydigi, a widespread species, lives on Anopheles maculipennis is a very common the surface of the mud. mosquito, and Paradixa aestivalis is a very The Copepoda found at Borough Fen are widespread midge. The grub-like larvae of common species. Cyclops vicinus is usually the Ptychopteridae (phantom craneflies) live associated in ponds with Daphnia pulex. in mud beneath shallow water, and breathe Asellus meridianus was not as plentiful in air at the surface by means of a long this pond as might have been expected, as it abdominal respiratory process. Ptychoptera is an abundant water skater in the Peter­ contaminata is a common species, frequenting borough district, and can stand fairly low especially woodland pools. P. minuta is local oxygen concentrations. Crangonyx pseudo- in distribution, and has not previously been 146 Margaret Palmer recorded from this vice-country (Stubbs, with wildly fluctuating oxygen levels, unless 1972). they breathe air at the surface. The Trent River Board has worked out a classification of freshwater, using inverte­ Acarina brate organisms as indicators of the degree of organic pollution. The oxygen require­ Parasitic mite larvae were found attached to ment of the members of the invertebrate the cuticle of Callicorixa praeusta. community is one of the principal criteria upon which this Biotic Index is ultimately based. The Biotic Index ranges from 0 for foully polluted to 10 for thoroughly clean. Discussion The rating for Borough Fen Decoy Pond works out at approximately 6. Many species Apart from Cladocera, Copepoda and in the pond are typical of mildly polluted Ostracoda, only a very few species of animal water, in particular Erpobdella testacea, were at all numerous in the areas of the pond Daphnis pulex, Asellus meridianus, Sigara free of rooted vegetation. These were Den­ lateralis, and Callicorixa praeusta. Tubificid drocoelum lacteum, Helobdella stagnalis, worms and red chironomid larvae are ex­ Crangonyx pseudogracilis, Sigara lateralis, tremely tolerant of pollution, but their pre­ oligochaete worms, and ceratopogonid and sence does not necessarily indicate gross chironomid midge larvae. In addition, the pollution unless all other species requiring small, temporary areas of aquatic vegetation dissolved oxygen are absent. Plecoptera in the pipes accommodated numbers of (stoneflies), Ephemeroptera (mayflies) and Lymnaea per egra, Lymnaea stagnalis, Noto- most species of Trichoptera have aquatic necta glauca, haliplid beetles and mites. stages unable to stand pollution, and, as Three important factors influencing the would be expected, are absent from the pond. composition of the invertebrate fauna of Adult Coleoptera and Hemiptera, and the Borough Fen Decoy Pond are: (a) the high larvae of some Coleoptera and Diptera (in­ concentration of organic material; (b) the cluding Anopheles, Paradixa and Ptychop­ high calcium concentration; (c) the absence tera) obtain gaseous oxygen at the surface, from most of the pond of aquatic plants and so are independent of dissolved oxygen. other than algae. Snails like Lymnaea use dissolved oxygen (a) The concentration of ammonia was unless the concentration is low, when they very high, about double the average for the cometo the surface and breathe by means of effluent from Peterborough Sewage Works. a lung. Some adaptations to low oxygen con­ The source of most of this ammonia is centrations of those aquatic invertebrates undoubtedly the excreta of the very large dependent upon dissolved oxygen are wildfowl population harboured by the pond obvious. One such is the presence of haemo­ from August to March. Leaves drifting in globin in the mud-dwelling chironomid from surrounding trees augment this supply larvae and tubificid worms. The possession of organic material. The high calcium con­ of haemoglobin makes possible the uptake centration of the water encourages rapid of oxygen at very low concentrations. Some decomposition, resulting in the formation of chironomid larvae produce it only in situa­ the thick layer of black mud. tions where oxygen is scarce. Chironomid Water samples collected on a sunny sum­ larvae are able to respire anaerobically if mer day contained abundant oxygen. Most necessary. Both chironomids and tubificids of this must have been derived from the make respiratory movements. Chironomids photosynthetic activity of unicellular and build U-shaped tubes of salivary secretion in filamentous algae. Under these conditions, the mud, and undulate their abdomens to oxygen production was outstripping the create a current through the tubes. Tubificids consumption of dissolved oxygen by the protrude their tails from the mud and un­ plants and animals in the pond, notably the dulate them. The scarcer the oxygen, the bacteria and other micro-organisms break­ greater the length of tail which is protruded. ing down organic compounds. The presence Other adaptations are less obvious. For of nitrate indicates activity by aerobic instance, in dragonflies, the bottom-dwelling bacteria utilizing ammonia. However, at species of nymph appear to have lower night, especially when the temperature re­ metabolic rates, lower growth rates, and to mains high, the water must rapidly become move more slowly than those which live deoxygenated, and anaerobic breakdown among water plants near the surface, where processes must dominate. The aquatic oxygen concentration, temperature and light animals must therefore be adapted to cope intensity are higher (Corbett, Longfield & Invertebrate ecology, Borough Fen Pond 147

M oore, 1960). A m ongst the leeches, Erpob- all confined to the temporarily uncleared della octoculatais widespread, but is replaced ends of the pipes. The notable absence of by the less common E. testacea in anaerobic dragonflies is due at least in part to this situations such as emergent vegetation or factor, as many species lay their eggs in waters otherwise rich in organic material. aquatic plants, and most nymphs are weed- Mann (1965) has carried out experiments on dwellers. The thick, soft mud does not pro­ the oxygen consumption of these and other vide a suitable substrate for some plants, but leeches, and has shown that E. testacea is conditions are favourable for duckweed. This physiologically better adapted to life in low is eaten by the ducks as quickly as it grows. oxygen concentrations than is E. octoculata. If left undisturbed, the pond would support The oxygen consumption of E. octoculata is a greater variety of invertebrates, but dependent upon the concentration of dis­ obviously plants would quickly engulf the solved oxygen, whereas, at low levels, the pond if vegetation was left uncleared. oxygen consumption of E. testacea is inde­ Lack of food probably contributes to the pendent of oxygen concentration. Mann high mortality rate amongst the young of the (1961) also points out that whereas E. octo­ resident wildfowl population of Borough culata breed in summer and lives for 2 years, Fen. In 1972 only three Mallard ducklings E. testacea dies after breeding early in the survived, and all the cygnets died. Olney year, so that in the warmest months, when (1967) has found in the stomachs of adult oxygen is likely to be scarcest, it is small or Mallard, seeds, aquatic plants, and many of in the egg stage. It is significant that E. the invertebrate species present in the Decoy testacea is present at Borough Fen, but pond. An examination was made of the E. octoculata was not found. stomacn contents of three Mallard killed at (b) Amongst freshwater invertebrates there Borough Fen in October 1972 as a contribu­ are some groups in which the presence of tion to another programme of research. calcium appears to favour all the species, and These birds may not have been feeding with decreasing amounts of calcium below a exclusively at Borough Fen, but the food certain level the number of species decreases. items found were all present in the pond. For instance Boycott (1936) has shown that Much of the food consisted of ‘seeds’, pre­ som e species of M ollusca (e.g. Lymnaea dominantly grain scattered on the pond to peregra and Planorbis contortus) can live in attract the ducks. However, one stomach hard and soft waters, whereas other species contained only about sixty ‘seeds’ and a large (e.g. Lymnaea stagnalis and Planorbis plan­ number of insects. These were one land orbis) are only present where there is at least beetle, one larva of Pericoma, four larvae of 20 ppm of calcium. One reason for this may Ptychoptera, ten chironomid pupae, and be that, as calcium encourages decomposi­ about 300 chironomid larvae. tion of organic material, hard waters are more productive than calcium-deficient ones. In other groups, for instance the Corixidae, Acknowledgments some species are characteristic of hard waters, while others are confined to soft I am grateful to the Wildfowl Trust for allowing waters (Macan, 1954,1963,1965). By inverte­ me to carry out this investigation, and I would like brate standards the water at Borough Fen to thank the Decoyman, M r W. A. Cook, for his is extremely rich in calcium. Dendrocoelum help and encouragement. lacteum, Helobdella stagnalis, Lynmaea stag­ I am obliged to Dr J. P. Harding for confirming nalis, Planorbis planorbis, Simnocephalus my identification of Scapholeberis aurita. vetulus, Corixa punctata, Callicorixa pra­ eusta. Sigara f alleni, S. lateralis and Hes- perocorixa sahlbergi are all species confined Summary to or favoured by hard water, and present at Borough Fen. The invertebrate fauna of the pond, which has (c) The scarcity of vegetation in the pond existed for over 300 years, is listed. The ecological limits the number of animals, especially snails requirements of the diiferent species are described, and insects. Gastropods, Coleoptera and and the ways in which they are adapted to the som e H em iptera (e.g. Notonecta) were nearly particular conditions of Borough Fen discussed.

References

Boycott, A. E. 1936. The habitats of freshwater Mallusca in Britain. J. Anim. Ecol. 5: 116-186. Cook, W. A. 1960. The numbers of ducks caught in Borough Fen Decoy 1776-1959. Wildfowl Trust Ann. Rep. 11:118-122. 148 Margaret Palmer

Corbett, P. S., Longfield, C. & Moore, N. W. 1960. Dragonflies. Collins, London. Macan, T. T. 1954. A contribution to the study of the ecology of Corixidae. J. Anim. Ecol. 23: 115-141. Macan, T. T. 1963. Freshwater Ecology. Longman, London. Macan, T. T. 1965. A Revised Key to the British Water Bugs (Hemiptera-Fieteroptera) Freshwater Biological Association Scientific Publication 16. Mann, K. H. 1956. A study of the oxygen consumption of five species of leech. J. exp. Biol. 33:615-626. Mann, K. H. 1961. The life history of the leech Erpobdella testacea (Sav.) and its adaptive significance. Oikos, 12:164-169. Olney, P. J. S. 1967. The feeding ecology of local M allard and other wildfowl. Wildfowl Trust Ann. Rep. 18:47-55. Reynoldson, T. B. 1967. A Key to the British species of Freshwater Triclads. Freshwater Biological Association Scientific Publication 23. Scourfield, D. J. & Harding, J. P. A Key to the species of British Cladocera. 1967. Freshwater Biological Association Scientific Publication 5. Stubbs, A. E. 1972. A review of the information on the distribution of the British species of Ptychoptera (Diptera-Ptychopteridae). Entomologist, 105:23-28. In addition, the standard guides and keys for the invertebrates were consulted.

Margaret Palmer, ‘Garthside’, Main Street, Barnack, Stamford, Lincs.