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Dßring the summer of 1955, a study was made of the ecology of the sluice at Dedham and the sluice and mill-race at Fiatford Mill. At Dedham the water can be used to power the mill, but some passes over the sluice-gates and seeps through them, thoroughly Wetting the woodwork and iron. T h e adjacent concrete side-walls are also wet, and water about an inch deep flows over the flat concrete floor. The River Stour flows on to Fiatford Mill, where part flows into the mill-pool through a sluice similar to that at Dedham, and the rest goes down the mill-race, now disused. This consists of a concrete sill, a sloping concrete shute (the head-race) with high walls on either side, and a flat concrete bottom (the tail-race) leading also to the mill-pool. At irregulär intervals brackish tidal water fills the mill-pool so that the concrete floors of the sluice and mill-race are submerged to a depth of up to 18 inches. More often the tide does not rise high enough to cover these concrete floors, and only fresh water flows over them. These sites comprise regions of shallow, fast-flowing water and the adjacent areas wetted by splashing and seepage. They are subject to unpredictable irregularities in salinity, rate of flow, and wetness, owing to human activities, and it is these extremes rather than the average or normal conditions that control the environment. HABITATS AT FLATFORD AND DEDHAM

The sites under discussion harbour many different microhabitats close together, which may merge gradually one into another or change abruptly. This made the investigation and presentation more difficult than was expected, and some simplification was necessary in order to " see wood for the trees ".



The habitats have, then, been grouped as follows: 1. Dry Moss . 2. Wet Moss 3.

Walls and gates of the sluices at Dedham Y and Fiatford, and the walls of the millJ race at Flatford.

Head-race at Flatford.


Floor of Dedham Sluice.


Floor of Flatford Sluice.


Tail-race at Flatford.

These habitats are reasonably uniform within themselves and taken in order, illustrate the effects of increasing wetness and water-velocity in mill-races and sluices. The species are listed below and will be discussed later.

1. DRY MOSS This habitat includes the outer limits of colonisation by mosses on the brick walls and gates. Water is obtained only by seepage from below. T h e moss forms a fairly continuous thin feit with yellowish dry, ragged edges. At the lower limit of the habitat, it is just damp to touch. PLANTS f *

ALGAE Protococcus sp.

BRYOPHYTA Mosses Bryum capillare Craioneuron filicinum Eurhynchium confertum E. riparioides Leptodictyum


o. in damper areas. o. in driest areas o. throughout f. in driest areas, d. elsewhere d. in driest areas

ANIMALS Rotifers (single species)

absent in drier areas. Up to 50 per cc moss feit in damper areas. Nematode worms (three species) distribjited like Rotifers. Woodlice Spiders


d. = dominant a. = abundant f. = frequent

0. = occasiรถnal r. = r a r e 1. = local(ly)





This is the part of the moss kept wet by splash. It is green and about f " thick. There is a transition from damp to wet and dripping moss. PLANTS ALGAE

Green Algae Enteromorpha intestinalis Vaucheria sp.

o. o. f. in heavy splash


Liverworts Lunaria cruciata Mosses Bryum capillare Eurhynchium riparioides Fissidens crassipes Leptodictyum riparium

o. r. f. to l.d. l.d. (r. elsewhere) o.

ANIMALS Protozoa : Carchesium sp. Actinophrys sp. Protozoa (Suctoria) (two species) Nematode worms (three species) Rotifers (two species) Oligochaete worms : Nais obtusa Elaeosoma hemprichi

80 to 4,500 per cc. increasing with wetness 100 to 500 per cc increasing with wetness 100 to 2,000 per cc increasing with wetness 1 to 20 per cc increasing with wetness 2 per cc or less increasing with wetness.

Insects : Diptera (larvae): Simulium nolleri Pericoma sp. Chironomus sp. Limnophora sp. Tipula sp.

o. 4 per cc or less 4 per cc or less o. o.

Fish : Elvers


(in June)


30 3.


T h e head-race is a concave concrete slope designed to accommodate the mill-wheel, which is no longer there. Water cascades down the slope at a speed of about 10 ft. per second, and there is a continuous moss feit, about one inch thick, over its surface. PLANTS ALGAE

Blue-green Alga Oscillatoria (two species) o. Diatoms Gomphonema o. Navicula o. Pleurosigma o. Synedra o. Tabellaria o. Green Algae Cladophora sp. o. Vaucheria sp. o. - f. BRYOPHYTA

Mosses Eurhynchium riparioides Fissidens crassipes Fontinalis antipyretica

d. l.d. o.

on lower part of head-race.

ANIMALS Protozoa (Suctoria) two species) Coelenterata : Cordylophora lacustris (a colonial hydroid) Nematode worms (three species) Rotifers (two species) Oligochaete worms : Nais obtusa Elaeosoma hemprichi Insects : Diptera (larvae) Simulium nolleri Pericoma sp. Chironomus sp. Limnophora sp. Tipula sp. Polyzoa: Paludicella articulata Plumatella repens Fish : Elvers

about 10,000 per cc.

very local about 100 per cc. about 2,000 per cc. about 15 pr cc. fewer than 1 per cc. few to very many, on the moss, not within it. r. o. f. o. very local very local o.




The water flows gently and in most places slowly over the floor of Dedham sluice, which is carpeted with moss except in the faster-flowing parts. PLANTS ALGAE

Blue-green Alga Oscillatoria sp. Diatoms Melosira sp. Other Diatom species present Green Algae Vaucheria sp. Ulothrix sp. Cladophora sp. Enteromorpha intestinalis Red Alga Chantransia sp.

r. a. f. o. o. o. o.

in faster flow

o. in faster water Absent elsewhere


Mosses Eurhynchium riparioides Fissidens crassipes

d. f.

ANIMALS Protozoa : Vorticella sp. o. Protozoa (Suctoria) (two species) about 1,000 per cc. Nematode worms (three species) about 500 per cc. Rotifers (two species) about 300 per cc. Oligochaete worms : Nais obtusa 10 per cc. Elaeosoma hemprichi r. Leeches: Erpobdella testacea o. Diptera (larvae) : Simulium nolleri r. Pericoma sp. r. Chironomus sp. o. Polyzoa : Paludicella articulata f. Plumatella repens o. Hydrochnids (water-mites) o. Fish: Elvers o.

32 5.


More water passes through Flatford sluice than through Dedham sluice, and it flows oVer the floor more rapidly at Flatford. When the gates at Flatford are opened after heavy rain, the floor is subjected to a rushing torrent. There are no mosses. PLANTS ALGAE

Blue-green alga Oscillatoria (two species) o. Diatoms Melosira sp. absent in fast water, increasing to a. in slow water Green algae Cladophora sp. d. Enteromorpha intestinalis o. Red alga Chantransia sp. d. in shaded turbulent parts. ANIMALS

Protozoa (Suctoria) Coelenterata : Cordylophora lacustris

r. to o.

Rotifers (one species) Crustacea : Gammarus pulex Insects : Diptera (larvae) Chironomus sp. different from that above Polyzoa : Paludicella articulata Mollusca : Gastropoda Hydrobia jenkinsi




in dark crannies, in rapid flow. Elsewhere absent.


o. r. o.


The tail-race is some 13 ft. wide and 30 ft long. Part of the Mill building spans the middle 16 ft., is raised about 8 ft. from the water, and cuts out about three-quarters of the light from the tail-race underneath. Water flows much faster down the right hand side than the left hand side ; on the right, it leaves the base of the head-race at 6 ft. per second, and slows to ft. per second, and on the left, it leaves at ft. per secondand slows to ÂŁ ft. per second.




Blue-green alga Oscillatoria (two species) f. Diatoms r. Several species Green alga o. Cladophora sp. Red alga Chantransia sp.

at lower end, absent elsewhere.

a. - f.


Mosses Fissidens crassipes

Protozoa (Suctoria) (two species) Other Protozoa Carchesium sp. Coelenterata Cordylophora lacustris Rotifers (one species)




on Cordylophora lacustris.


on Cordylophora lacustris.

d. r.

over large areas in fast flow,

Crustacea : Gammarus pulex


Mollusca : Gastropoda Hydrobia jenkinsi


Fish : small flounders, about 1" long




T h e fact that emerges most forcibly from the above lists is the prevalence and dominance of mosses. In all habitats except the floor of Fiatford sluice and the tail-race, they make an almost unbroken cover, which forms the basis on which the rest of the animal and plant life is imposed. They convert bare smooth brick, wood and concrete into a sheltering forest, an ideal home for other plants and many animals.



Three things distinguish the habitats uncolonised by mosses. They are their occasional inundation by brackish water; the normally fast-flowing water over them, and their constant submergence below the water. Appreciable amounts of salt certainly prevent the growth of mosses, but at Fiatford Mill the water in the mill pool is only just brackish, and the mosses in the lower parts of the wet moss zone and the head-race (see Fig. 1) successfully survive submergence in it. It seems therefore that salt is not responsible for the lack of moss. Nor can the rapidity of the flow be entirely responsible, for the water flows even faster, though more brokenly, down the head-race, thick with mosses. The third factor, the constant submergence in unbroken, though very shallow water, may be responsible, though it is difficult to see why a continuous, as opposed to a broken water surface should have this effect. In the lower parts of the wet moss zone, and the head-race just mentioned, salt does appear to have some effect. It allows Fissidens crassipes to displace Eurhynchium riparioides partially or completely, so that isolated patches of pure F. crassipes appear among the E. riparioides. F. crassipes is presumably more tolerant of salt. Another point of interest is that the moss feit seems to be due almost entirely to vegetative spread. All the species except Bryum capillare have a spreading, creeping growth, as opposed to the cushion or tuft habit, and only Leptodictyum riparium growing in the driest parts was fertile. In the wetter parts there would be little or no opportunity for aerial distribution of spores, and no capsules are produced at all. T H E D I S T R I B U T I O N OF CLADOPHORA

Where mosses are dominant, Cladophora is almost completely absent. On the floor of Dedham sluice, it makes an appearance in the fastest flowing water (about \ ft. per second), but is heavily encrusted with diatoms and is not very common. At the lower end of the floor of Fiatford sluice the water flows at \ to 1 ft. per second, and Cladophora is dominant, forming long skeins, again thickly covered with diatoms. Though it is dominant, it forms a much less complete cover and shelter than the mosses, and correspondingly less grows amongst it. Nearer the sluice gates, the water flows faster ; Cladophora remains dominant, but there are few diatoms, and the filaments are a cleaner, brighter green. They are also shorter because in the more active water, the drag is greater, and the lower ends of the filaments break away. Right beneath the sluice gates, the water is too active for Cladophora and very little grows but a close patchy feit of the red alga, Chantransia. In the tail-race at Fiatford, the upper end is too fast-flowing for Cladophora, the middle part too dark, and only at the lower end does Cladophora occur at all, in small amounts.




A comparison of the species lists reveals certain similarities and differences in the animal populations of the six habitats. These will be mentioned and explained briefly. a. The dry moss and wet moss regions are very different. This is because the increased wetness allows of the growth of many aquatic animals that are unable to live in a dry atmosphere. The splashing water also brings food and oxygen, and as the water percolates away, it removes toxic products such as CO 2 . b. The wet moss and head-race regions are very similar because conditions within the moss felts are alike. Although water rushes over the moss on the head-race, it does not penetrate appreciably, but rather compresses the moss and makes it lie flat, so that within, conditions are almost as quiet as in the wet moss regions. c. The floor of Dedham sluice differs in some respects from the wet moss and head-race regions. Similarities are due to the presence of a wet moss feit supplied with food and oxygen and a current to remove toxic products. Differences arise because the feit is not compressed or gummed together by the water, but is floated apart by it. There is enough free or open water to allow such animals as leeches, bryozoans and water-mites to live there. d. The tail-race, and the floor of Fiatford sluice, are similar to each other but different from other habitats. Here the intermittent salinity and water velocity prevent moss growth, and without mosses, little protection from the flow is afforded to other would-be colonizers. F O O D AND FEEDING


Strikingly large numbers of microscopic animals live in the moss felts of the wet-moss region, the head-race, and the floor of Dedham sluice. An abundant food supply flowing over and into the moss as plankton and flne organic particles, largely accounts for these concentrations. The rotifers " filter-feed " by drawing a current of water past their mouths and straining off the suspended matter. Suctorians capture food with adhesive tentacles. Both animals—the commonest in the moss feit—are sedentary, and wait for the food to come to them. Nematodes are less common, but much larger (though still only about 0.5 mm. long), and their total bulk is comparable with that of the rotifers and suctorians. They too feed on suspended organic matter, but their main diet is probably the bacteria which multiply in the feit. Nematodes do not filter-feed, and so must move in search of food.



The feeding habits of the Oligochaeta worms and most of the dipterous larvae are unknown, but the larvae of Simulium nolleri takes direct advantage of the food supply by filter-feeding. The mature larva is some 6 mm. long and has a pair of collapsible " fans " on its head, which are extended into the current. The food that collects on them is removed by the mouth-parts. These fans are fitted for use in flowing water, and there is insufficient free space in the moss feit for their proper extension, so that S. nolleri lives on the moss, not in it. T h e Polyzoans Paludicella articulata and Plumatella repens and the Coelenterate (Hydroid) Cordylophora lacustris do not live as free individuals, but in colonies, and each unit possesses a crown of tentacles that is spread in the current to catch food. These animals, too, are on too large a scale to filter-feed within the moss matrix, like rotifers. They live like Simulium in the fĂźll force of the current. They cannot live on the moss, and are found where there is none, or where it is sufficiently open for them to grow in between the moss plants. M E T H O D S OF ATTACHMENT

Since Simulium, the Bryozoans and Cordylophora live in the current, they must be attached securely to the substrata. An animal moving from place to place in the current stands a greater chance of being swept away than if it stays put, and only Simulium takes this risk. Its mouth pours out a sticky secretion or web, which sets at once on the moss, and a grapnel just behind the head is immediately fixed in it. Then, by looping or arching of the body, an even larger grapnel at the hind end is embedded in the web, and the head-end lets go. Simulium normally hangs like this, head-downward in the current, but when it is about to move, the mouth spits another web one length ahead of the last, and the looping movement is repeated. Should the larva lose its hold, it is saved by a silken safety-thread, made of the same substance as the web. The silk is also used to bind the larva firmly to the moss when it pupates. Simulium has obviously undergone considerable specialisation in achieving " movement with safety ". Cordylophora and Paludicella form colonies cemented to the bottom by a creeping system of communicating tubes, and from these arise the branched stalks bearing the individual animals. Plumatella is basically the same, but forms cushions rather than spreading forests. S O M E ASPECTS OF


T h e animals in the wet-moss zone, the head-race and the floor of Dedham sluice, are not only very abundant, but very irregularly distributed. For instance, a sample of the moss feit



in the head-race contained about 12,000 rotifers per cc. Another sample close by contained an estimated 20,000 per cc., and in a third, the numbers feil as low as about 500 per cc. Numbers were sometimes very different, even at either end of a particular moss shoot, and no adequate reasons have been found for this. In some instances, crowding was largely confined to the mossbases, and in others, to the apicesof the mossshoots. Sometimes no such preferences were apparent. It is possible that, at the bases, the crowded stems and rhizoids afforded protection from the current and, at the apices, the closely set leaves did the same. Rotifers were nearly always found in the axils of the leaves, and seldom on the bare stem. The leaves of the Willow Moss (Fontinalis) are large, about 4 mm. long and boat-shaped, and where they occurred, they were often packed with rotifers on the concave side in the lee of the flow. Nematodes and Suctorians also occurred in high local concentrations, with comparatively small numbers elsewhere. No corresponding differences in environment were apparent. The Oligochaetes Nais and Elaeosoma, though their smaller numbers made it less dramatic, showed the same patchiness. These irregularities, while very interesting, greatly increased the difficulty of estimating numbers.


Cordylophora lacustris

Cordylophora deserves a little more attention because of its intrinsic interest and because, according to the textbooks, it is in entirely the wrong habitat. In June, all the Cordylophora was greyish, but in August, the colonies or mats in the faster water were a bright salmon-pink, those in slower water remaining grey-white. The pink colonies were fertile at least during the period August lOth to 29th, when they were studied. They produced both male and female gonophores on different upright stems, the gonophores replacing tentacled heads at the end of the short lateral branches. The male gonophore is an oblong whitish sac about 1.5 mm. long and contains tadpole-shaped sperms. The female gonophore is a sac at first opaque, then transparent, containing 5 to 15 spherical eggs. When fertilised, an egg elongates to resemble a small lozenge, develops cilia, and escapes as a planula larva, which soon settles down to produce a new colony. It was soon evident that Cordylophora was growing more vigorously in the faster water ; the upright branches were longer, and the tentacled heads bigger. But Pennak in " Fresh-water Biology of the United States " says that it grows in static or slowly flowing water. Allman in his monograph on Cordylophora



D i a g r a m o f the sluice and millrace at F i a t f o r d


FIG. 2.

Perspective v i e w o f the m i l l race at F i a t f o r d

M i l l w i t h one w a l l

cut away.

Some microscopic animals and plants, all approximately X 100. A. and E. suctorian protozoa ; B. the protozoan Stentor ; C. the colonial protozoan Carchesium ; D . the diatom Gomphonema ; F. the red alga Chantransia.

CORDYLOPHORA LACUSTRIS. 5 gonophore containing mature ova. Natural size of gonophore : 1 mm. long.



T u f t s are about 3 m m . long. Note minute sphereical spores.


Mucus webs of SIMULIUM NOLLER1 larva. A web is about 0.5 m m . across.


A n unidentified suctorian protozoan abundant in the Wet Moss and Headrace zones. Actual length 0.25 m m .



Cordylophora lacustris. A C r e e p i n g n e t w o r k of tubes ; B. u p r i g h t s t e m b e a r i n g f e e d i n g heads ; C . with tentacles and t e r m i n a l m o u t h s ; D . female g o n o p h o r e c o n t a i n i n g eggs ; E. m a l e g o n o p h o r e containing s p e r m s .



says of it "in aquis dulcibus quietus", and Hincks, in "British Hydroid Zoophytes", quotes habitats such as canals, and a cistern in Kensington. These authorities make no reference to actively flowing water. The Cordylophora at Fiatford Mill may well be described as "locos obscuros amans"— as it was by Allman—for it thrives best under the building on the mill-race, and in obscure crannies and fissures in the sluice ; but it does not grow " in aquis dulcibus quietus ". The answer to the riddle appears to be that Allman studied one local strain, and this is another. He was not aware that this species is extraordinarily plastic, and that several of these local strains occur, each with its own characteristics. If this Cordylophora also grows in slow-flowing water, the mill pool would be the ideal place to look for it, for many planula larvae must be washed down from the colonies above. A superficial search revealed none, but a more exhaustive investigation might be interesting. CONCLUSION

This paper is based on only a few weeks' work during June and August, and some of the conclusions arrived at must be tentative. This type of work does, however, illustrate how communites of very small animals and plants are built up, how they function and interact, and how they become adapted to or take advantage of specialised conditions. Above all, it should be remembered that life and all its forms defy definition, and that any demarcation of a Community into habitats is a convenience, an approximation of the infinite gradation that is Nature. USEFUL


Allman : " Monograph of the Freshwater Polyzoa ". Dixon : " Student's Handbook of British Mosses ". Fritsch : " Structure and Reproduction of the Algae ". Garnett: " Fresh Water Microscopy ". Hincks : " British Hydriod Zoophytes ". Mellanby : " Animal Life in Fresh Water ". Watson : " British Mosses and Liverworts ". M r . Milne received research work.—Ed.

the assistance of the Morley Bursary for this

A Study of Mill Races and Sluices at Flatford Mill and Dedham  
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