An Investigation of the Mud Fauna of the Stour Estuary by Suffolk Naturalists' Society

TRANSACTIONS AN INVESTIGATION OF THE MUD FAUNA OF THE STOUR ESTUARY T H E M O R L E Y ESSAY FOR 1 9 5 9 BY FRANCES BULL, H E L E N HONEYMAN AND SUSAN

KNOTT

INTRODUCTION

ONE of the most striking features of any estuary is the replacement of freshwater by marine animals as the river progresses towards the sea. This should be of great interest to the ecologist. In spite of the presence of numerous estuaries in the British Isles, surprisingly few accounts of their ecology have been published (notably SPOONER and M O O R E , 1 9 4 0 ) . It was decided that a study of the River Stour estuary should be made over a period of three weeks during June 1958. The scope of the investigation was limited to burrowing animals and to their distribution in relation to salinity, substrate, and their level on the shore. The estuary of the River Stour is in origin a drowned river valley, and forms part of the boundary between Suffolk and Essex. From Harwich to Manningtree, a distance of 10 miles, the estuary is about a mile wide. Above Manningtree it narrows abruptly but it is tidal for a further two miles up to Fiatford. The vertical tidal rĂ¤nge varies from 13 feet at Harwich to 6 feet at Fiatford. The main Channel is relatively narrow and at low water expanses of shore are exposed. This shore is predominantly muddy but coarser material occurs locally, where there are stronger water currents, or where the shore is more exposed to wave action. During recent years, Spartina townsendii has colonised the Upper part of the shore and in many places vigorous growth has resulted in a dense turf extending downwards almost to mid-tide level. The investigation did not include a study of this specialised habitat.

MUD FAUNA OF THE STOUR ESTUARY

225

METHODS

In an attempt to detect the pattern of distribution of the fauna along the estuary, ten localities were chosen for detailed study (Fig. 1). It can be seen from the figure that these were relatively widely spaced below Manningtree, but that in the upper region where considerable changes in salinity were anticipated the positions were closer together. At each of the ten points a transect line running at right angles to the shore was worked. The lines extended downwards from the edge of the Spartina marsh ; wherever possible they were directed over a shore of soft mud. By limiting the survey to one type of substrate, it was hoped to simplify the interpretation of results. From four to six equally spaced stations were established along each transect line. T h e uppermost Station was usually just below the edge of the Spartina and the lowest was always at the low water mark. An accurate profile of the shore on all transect lines was produced by the use of a dumpy level. Wherever possible the level of the shore was related to Ordnance D a t u m by reference to a bench mark. When no bench mark was available the level of low water on that particular day and its distance below Mid-Tide level (approximately Ordnance Datum on this coast) was obtained from the automatic tide recorder at Harwich. T h e level of every Station relative to Ordnance Datum is thus known. Careful records of the substrate along each transect were made. It was impossible however with the available equipment to analyse the deposit quantitively. Quantitive sampling methods were used for the burrowing animals. A block of mud 8 inches Square on the surface and 12 inches deep was removed from the shore at each Station. This material was sieved through a fine sieve (1 mm. mesh). In practice it was found necessary to place the sample in a bath of water where it was kneaded, and the finer material reduced into suspension. The contents of the bath were then poured through a sieve of 1 cm. mesh to retain coarse materials such as stones and shells, and they were finally passed through the finer sieve. All material left in the fine sieve was sorted in the laboratory where all animals were removed, identified and counted. By using a Standard sample size the numbers of animals for all stations can be compared directly. In the course of this survey, a number of water samples were for salinity records. T h e samples were titrated against silver nitrate using potassium Chromate as an indicator and the salinities were calculated as % 0 C1 (i.e. parts per 1000).

226

MUD FAUNA OF THE STOUR ESTUARY

SUBSTRATES

It was intended to work on substrates of uniformly soft mud, but it was found in practice that considerable Variation occurred even along one transect. The upper three transects had small, steeply shelving beaches and similar types of substrate. At the top stations, soft, black mud was found, but the proportion of gravel gradually increased towards low water. At Buck's Horns, the top two stations were notable for the abundant plant remains ; while there was little soft mud or Spartina marsh at Manningtree Garage, possibly owing to the presence of a sea wall. The mud at Manningtree was black and deoxygenated. The Spartina marsh was extensive, stations I and II being in the surrounding soft mud. Considerable plant remains were found at I. The percentage of gravel increased from Station I I I to Station VI and at low water there were many stones and large Mya shells. In contrast the transect at Mill House was over harder mud except near the Spartina. Soft mud occurred at low water and in the Spartina marsh, which was not sampled. An interesting feature was the presence of Enteromorpha and Zostera at Station III, the former probably indicating fresh water seepage here. At Sutton Ness, there were large quantities of sand and gravel on the upper beach but these were not included in the transect. T h e site was also notable for the large amount of Zostera on the surface. In contrast to previous transects only soft mud was dug but even then variations in sand content occurred, Station II being the sandiest. At both Wrabness and St. Clement's soft mud occurred at higher stations and harder mud nearer low water. The hardness at Wrabness was, however, due to gravel, while at St. Clement's it was due to the presence of clay. Hummocks of Enteromorpha intestinalis at the top of the beach (at St. Clement's) bear out the suspicion that there is a fresh water inflow here. The transect line was chosen to avoid these hummocks. Pure clay was dug at stations I and IV of Erwarton Ness but at the intermediate stations gravelly mud occurred. In contrast the mud was softer at Shotley. At Station I the mud was intermixed with fine gravel which did not occur at Station II. Again at Station I I I pebbles and gravel occurred while stations IV and V at low water were soft mud. These results show that soft mud occurs near the Spartina marsh. However, no generalisation can be made about the rest of the shore.

227

MUD FAUNA OF THE STOUR ESTUARY SALINITIES

Isolated records were obtained, and salinity surveys carried out on two separate days. On both of these occasions water samples were taken at low water. The first survey was made on 20th June when samples from most of the transect sites were collected. At this time the River Stour was running low after a period of fine weather, and thus only a little fresh water was entering the estuary. The highest spring tides of the three weeks' survey period were encountered on this day. Relatively high salinities were therefore anticipated at all transects. Fig. 2 shows the results of this survey in graphical form. It can be seen that there was a tendency for salinity to increase progressively down the estuary. Beyond Stutton Ness there was no significant difference between H.W. and L.W. salinities, but above this point the low water salinity feil successively below the high water figure. Thus at Cattawade Bridge the low water salinity was only 14.5% 0 compared with 26.7 % 0 at high water. The unexpectedly low value for high water at St. Clement's (28.9% 0 ) is thought to be due to a considerable Fresh Water inflow. GMPH

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MUD FAUNA OF THE STOUR ESTUARY

Three days later the sampling procedure was repeated at the upper transects. Heavy rain had fallen on the previous day and the river was now in flood. As would be expected, salinities in the upper estuary were low compared with those of the 20th June, (Fig. 2), but isolated readings (appendix) show that there was little change, owing to rain, in the lower estuary. At Buck's Horns, a low water value of 0.95% 0 was obtained, and even at Manningtree Garage the salinity was still only 2.2% 0 . The relatively small salinity gradient at low water in the upper estuary contrasts strongly with that found at high water, when a difference of 20.15% o was found between Buck's Horns and Manningtree Garage. These results indicate that whereas salinities are relatively constant in the broad lower sections of the estuary below Maningtree, there are wide fluctuations in the upper estuary. These are normally short term fluctuations produced by the rise and fall of tides, but they are greatly modified by more spasmodic changes in the volume of water flowing down the river. It should be remembered that, although it is the rĂ¤nge of salinity that is being measured, this may not be so important as the rate of change. (ALEXANDER et A L . 1 9 3 2 ) . Near high water the animals are subjected to a fluctuation for a shorter time than at low water, where the exposure to high salinities is longer, and the rate of fluctuation slower. ( M I L N E 1 9 3 8 ) . It has been pointed out by ALEXANDER et A L . ( 1 9 3 2 ) and by that the salinity of water overlying the mud is not necessarily that of the water in the mud itself. S M I T H discusses the problems in determining the interstitial salinity of mud and describes a method whereby it could be obtained. This procedure is very lengthy and was not used during the present investigation. We feel, however, that our records show broadly the conditions affecting burrowing animals in the estuary. SMITH (1956)

RESULTS

Although no hard and fast lines can be drawn, the fauna of the estuary roughly falls into three groups, depending on where each species attains a maximum density. It can be inferred that a maximum density implies optimal conditions for the life of the species. There are those species which have their maximum density in the upper reaches of the estuary, although they may occur in limited numbers right down to Shotley. Examples of this group are Corophium volutator, Nereis diversicolor and Streblospio shrubsolii. At the other end of the scale are those species which do not penetrate to the top of the estuary and have a maximum density

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MUD FAUNA OF THE STOUR ESTUARY

229

toward the seaward end. These, for example Cardium edule and Nephthys hombergii, are generally marine species which have managed to penetrate the estuary. Although they can live in waters of lower salinity than the sea, they appear to be unable to survive the wide fluctuations of the upper reaches. Finally there are those species with a mid-estuarine maximum, for example Heterocirrus sp., Scoloplos armiger and Priapulus caudatus. Thus differing capabilities of dealing with the estuarine conditions are shown ; some species are able to live where the overall salinities are low and tidal fluctuations g r e a t ; others where conditions are essentially marine, and others in the intermediate stretches. A striking feature of the distribution of animals in the estuary (as shown by Fig. 3) is that as soon as salinities become constant between high and low water, and the overall salinity higher, the total number of animal species rises, although the maximum density of any one is usually lowered. T h u s at Buck's Horns a total of 866 Corophium volutator and 720 Nereis diversicolor were collected f r o m four stations, while an odd Scrobicularia plana and Cyathura carinata were the only other species present. T h e other extreme is shown by the transect at Shotley, where eighteen different species were found, but the maximum number of any one collected was 137 Scrobicularia plana from six stations. When comparing the numbers of animals found in agiven volume of mud the difference in size and habits of the different species must be appreciated. For instance, Nephthys hombergii is a large predatory carnivore, and therefore, for a balanced Community, there must be fewer of these and similar animals than there are of herbivores, and small filter-feeders such as Corophium volutator. T h e critical region of change over from large numbers of a few species to smaller numbers of many species appears to be between Manningtree and Stutton Ness. This, as has been mentioned before, is also the Stretch over which the river widens out, so that overall salinity increases and intertidal fluctuations decrease. It can therefore be inferred that the cut-out of many of the species above Mill House (see Fig. 3) is due to the generally decreased salinity and probably even more to the increased fluctuations of salinity, both intertidal and from day to day. It was hoped to find some correlation between the faunal distribution and the tide level, but so many variables wereencountered, particularly the condition of the substrate, that no definite conclusions on this point could be drawn. For instance, considering the population of Corophium volutator at Manningtree, it might be said that there was a falling-off of numbers towards low water, and therefore Corophium prefers the upper part of the shore.

230

MUD FAUNA OF THE STOUR ESTUARY

But there is also a change of substrate Over this region, from shingly mud to gravel and large stones, to which the decrease in numbers is more probably due. Further to emphasise the difficulty in drawing any such conclusions is, taking the same example, the fact that Corophium, under similar substrate and salinity conditions at Manningtree Garage, shows an increase in numbers at the lowwater mark. Similarly, no conclusions can be drawn as to the substrate preferences of any particular animal. Substrate variability is illustrated in the summary, in diagram form, given for each transect at the top of fig. 3. The variability as regards substrate preferences is exemplified by Scoloplos armiger. This polychaete worm is said by many authorities to inhabit clean or muddy sand. It was found in the latter substrate at Stutton Ness. However it also occurred there further down the shore in very soft oxidised mud. Further the maximum density recorded for any Station in the estuary was for the second Station at Mill House, where the substrate was shingly mud with large stones. In addition the animal was found in gravelly mud at Wrabness and hard clay at Erwarton Ness. Thus in the Stour estuary Scoloplos armiger is found in the complete rĂ¤nge of substrates sampled. Equally, no significant correlation can be found between the distribution of animals and that of plant debris in the mud or of Zostera and Enteromorpha on the surface. An interesting Observation was made during the investigation concerning the different distribution of large and small Scrobicularia plana. No large ones (i.e. bigger than one cm.) were found above St. Clement's and most of the really large ones were at Shotley. Very small ones, probably first year specimens (a few species were numerous at isolated stations as far up as Buck's Horns. The average size of these was about two mm. long). It would be interesting to follow up this investigation another year to see whether development of the small ones could continue at the higher levels, or whether the conditions are only suitable for primary settlement. Our work would be complemented by further salinity readings taken at different times of the year ; by investigations of the animals in the Spartina marsh ; and by more transects in the critical area between Manningtree and Stutton Ness. More knowledge of this kind would enable clearer conclusions tobe drawn about the correlations between the distribution of the mud fauna and the tide-levels and substrates colonised. A positive result of our investigations is the detection of a changing pattern of animal distribution to salinity changes in the estuary.

MUD FAUNA OF THE STOUR ESTUARY

231

NOTES ON SOME OF THE ANIMALS FOUND

1. Nereis diversicolor. (Fig. 4). The common name for Nereis diversicolor is " the ragworm ". It lives in temporary burrows in the soft mud of the estuary, and when the shore is covered by the tide the worm roams over the surface of the mud looking for food. To avoid being eaten by birds or dried up, when the tide is out the ragworm makes a new burrow in the mud. Then the only sign of the animals are the small holes left on the surface. 2. Corophium volutator. (Fig. 5). Corophium is a small relative of the shrimps and prawns. Unlike the ragworm, Corophium is covered by a hard jointed shell, and also has jointed legs. When the tide is up the animal swims on its back just over the surface of the mud, sieving tiny food particles from the water. When the tide goes out Corophium lives in a permanent burrow, with walls lined and strengthened by a sticky substance. 3. Scrobicularia plana. (Fig. 6). Locally this animal is known as a " clam ". It does not leave the mud to feed but pushes a tube to the surface to draw in food particles. The other shorter tube shown in the diagram is used to get rid of waste matter. The long, food-collecting tube leaves characteristic star-shaped marks on the mud surface. 4. Priapulus caudatus. (Fig. 7). Although this animal looks something like a lug-worm, it is not closely related. In fact its relations are uncertain. Priapulus caudatus wanders continuously in the mud, eating animals such as the ragworm, and is quite a rare animal. It is therefore interesting that relatively large numbers are found in the Stour estuary. 5. Sabanaea ulvae. (Fig. 8). This snail is about one quarter of an inch long and is common in estuaries and salt marshes. Sabanaea lives on the surface of the mud and in shallow burrows, feeding mostly on surface plants. 6. Heterocirrus sp. These worms are related to the ragworm, but are very much smaller. They live in tubes which they build in the mud by sticking together small grains of sand. These tubes are so well made that they often outlive the animal which built them. They feed by drawing a stream of water through the tube and sieving out the food particles.

Fig.

5 COROPHIUM

VOLUTATOR,

234

MUD FAUNA OF THE STOUR ESTUARY

SUMMARY

1. An investigation of the mud fauna of the river Stour estuary was made in June, 1958. 2. Marked local variations in substrate were found and described qualitatively. 3. Salinity readings were taken at high and low tide-levels at the transect sites, mainly on two days. 4. Salinities were found to fluctuate most between high and low tide-levels in the Upper reaches. The overall salinity decreased with distance up the estuary. 5. No correlation between the distribution of animals and either tide-levels or substrate could be made. 6. A definite zonation of animals was found and appeared to be correlated with the salinity changes in the estuary. ACKNO WLEDGMENTS.

The work was carried out at Fiatford Mill Field Centre with the financial support of the Claude Morley Bursary Fund. It is a pleasure to be able to record our grateful thanks to the Warden, Mr. Bingley, and the Assistant Warden, Tony Hopson, for their guidance and practical assistance in choosing and executing the project. We would also like to thank the Harbour Master at Harwich for his kind co-operation.

Appendix

1. SALINITIES

Date Tide 20th J u n e H W LW 23rd J u n e H W LW 24th J u n e L W 26th J u n e L W 27th J u n e L W 28th J u n e H W LW

— —

— 0.95

0.25 0.35

0.3

THE

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in 30.4 15.2 23.35 2.2 —

—

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Identification

SURFACE

29.3 14.6

27.6

—

— — 0.45

WATER

VII

vi 30.1 31.2

31.2 31.8

28.8

—

— —

— 32.7

—

24.65

19.7 9.65 —

31.7

of numbers :

0 — N e a r B r a h a m Lock. 1—Buck's H o r n s . I i — C a t t a w a d e Bridge. Iii—Manningtree Garage, iv—Manningtree.

v—Mill House. vi—Stutton Ness. vu—Wrabness. v i n — S t . Clements, ix—Erwarton Ness. x—Shotley.

33.2 32.1

33.3 32.7

31.7

235

MUD FAUNA OF THE STOUR ESTUARY

Appendix 2. Number of animals found in each sample, 8" X 8" X 12" deep : volume approximately half a cubic foot (0.445 cu. ft.) ; sieve mesh 1 m.m. Nereis diversicolor :— locality Height above chart datum (in feet) at which sample taken. VI V IV II VII in i 194 134 142 63 91 62 160 53 56 246 153 80 90 50 30 136 108 7 9 none 31 — — — 2 — — — 16 8 — — — 37 21 7 — 2 — — _ _ _ _ _ 5 — — XII

[

II Iii IV

VI VII VIII IX X

i ii

iii IV v VI

«I

—

250

—

VIII

—

Corophium volutator :— — — 81 — 475 222 314 244 190 188 — 3 51 255 2 1 — — 25

—

— •

249 — — —

—

9 80 178 ÖO Ö 282 O 4 —

—

/iii

Sabanaea — — none — — — — —

ulvae :— — — — — — — — — — — 56 —

Cyathura carinata :— 2

—

none —

— — 6 7

— — 3 —

3 1

—

— 35

1 — — —

— 1 —

9 — — 5 — 25 1

—

180 — 16 —

Scrobicularia plana and Macoma balthica :— (mostly young specimens, where species indistinguishable) — 1 — — 194 II /IV — 2 6 36 1 3 — n 3 2 — 5 VII 3 t'III 4 217 X 21 68 18 14 i :ii :v i'i m

— •

—

— —

_ _ — — — — — —

10 2

—

— —

236

MUD FAUNA OF THE STOUR ESTUARY

Heterocirrus sp. :— Locality Height above chart datum (in feet) at which sample taken. XII XI _ x _ IX _ VIII _ VII _ vi _ v _ IV_ III II5 vi _ _ viii ix

— _

—

Pnyllodoce sp. :— vi IX

vi VIII IX x

— _

—

_ _

— 2

2 —

— 11

Cardium edule :— _ _ _ _ _ _ _ _ _ _ 1 15 _ _ _ _ i _ _ _ _ _ _ _ Nephthys liombergii :—

—

— —

1 — — 1

—

2 —

—

— —

2 _ _

i i o

—

_ ? _

—

2 —

— 2

— — — —

Arenicola marina :— vli IX

in iv

vi v

VII ix

—

Maldanidae :— — — — — —- — — — 110

12— — —

—

8 —

—

_ _ _ _ _ _ _ _ _ Priapulus caudatus :— _ _ _ i 3 1 — — — _ _ _ _ _ _ _ _ 2 — — — — — 1 — — — _

—

— — 3 — — —

Scoloplos armiger :— v vi ix

— _

—

— 18

4 28

— 15

— —

— 7

—

— 16

— —

Streblospio shrubsolii :— In iv

— _

—

MUD FAUNA OF THE STOUR ESTUARY

237

The following species occurred at the localities shown :— Oligochaeta II. Mya arenaria IV, V, VI. Polydora sp., IV. Linens gesserensis V, IX. Amphiporus lacteus V, VI. Gattyana sp., VI. Ampharete grubei VI, V I I IX. Audouinea tentaculata VI, V I I , IX, X. Sphaeroma sp.. VI, IX. Amphitrite johnstom VI. Sagartia elegans VI. Melita palmata ~VIII. Scolecolepis fuliginosa X. Key to LocaVties :— i—Buck's Horns. II—Cattawade Bridge. III—Manningtree Garage. IV—Manningtree. v—Mill House.

vi—Stutton Ness. VII—Wrabness. VIII—St. Clements. ix—Erwarton Ness. x—Shotley.

REFERENCES.

1. Alexander, W. B., Southgate, B.A and Bassindale, R. T h e Salinity of the water retained in the muddy foreshore of an estuary. Journ. Mar. Biol. Assoc. 18, p. 297, 1932. 2. Milne, A. T h e ecology of the T a m a r estuary 3. Salinity and temperature conditions in the lower estuary. Journ. Mar. Biol. Assoc. 22, p. 529, 1938. 3. Smith, R. I. T h e ecology of the T a m a r estuary 7. Observations on the interstitial salinity of intertidal muds in the estuarine habitat of Nereis diversicolor. Journ. Mar. Biol. Assoc. 35, p. 81, 1958. 4. Spootier, G. M. and Moore, H. B. T h e ecology of the T a m a r estuary 4. An account of the macrofauna of intertidal muds. Journ. Mar. Biol. Assoc. 24, p. 283, 1940.

THE PREHISTORIC MAMMALIA OF SUFFOLK by

HAROLD E .

SPENCER,

F.G.S.

THE recent discovery of a portion of a humerus of the so called " Cave Lion " has led to the realisation of the great rarity of the fossil remains of carnivores in this part of the country. For more than half a Century extensive additions have been made to the large collection of Pleistocene mammalian remains in the Ipswich Museum, and during the latter half of this period a number of sites have been kept under Observation by members of the staff with negligible results as far as fossils of beasts of prey are concerned. Detailed records of fossils found in the Crag are seldom preserved as the workmen were, understandably, mainly interested in getting a good price for them, while collectors were chiefly concerned with adding rare specimens to their cabinets. It is fortunate mammalian fossils from the Crags are easy to recognise, although we may never know from what level in the Crag they were obtained.