The intertidal invertebrate fauna of the Orwell estuary

THE INTERTIDAL INVERTEBRATE FAUNA OF THE ORWELL ESTUARY C . H . BEARDALL, S . M . GOOCH a n d R . PILCHER In 1973 the Institute of Terrestrial Ecology ( I T E ) surveyed the intertidal macro-invertebrate fauna of the south east coast of England as part of a wider study to assess the environmental impact of the proposed development of London's third airport at Maplin Sands ( I T E , 1974; Kay & Knights, 1975; B o o r m a n & Ranwell. 1977). The study embraced fourteen sites ranging from the estuarine areas surrounding the Isle of Sheppey in the south, to the Orwell estuary in the north.

The present study was undertaken by the Suffolk Wildlife Trust in September and October of 1986, to assess the distribution and abundance of the intertidal macro-invertebrate fauna of the Orwell estuary and identify any changes in these communities since 1973.

DĂźring the last Century the Orwell estuary has come under increasing pressure from both industrial and recreational activities, many ofwhich have the potential to substantially alter the physical and chemical characteristics of the estuary. The impact such changes have had on the invertebrate fauna is unknown.

The ornithological interest of the Orwell estuary has been well documented over a n u m b e r o f y e a r s (Davidson & E v a n s , 1985; Ravenscroft, 1987; Beardall, Dryden and Holzer, 1988) and the national and international importance of this area recognised by its designation as a Site of Special Scientific Interest in 1985. T h e conservation of the ornithological importance of this site will depend upon careful management and monitoring of all aspects of the ecosystem and especially those communities at the bottom of the food chain upon which the system relies. This study forms part of a wider project researching all aspects of estuarine ecology in Suffolk undertaken by the Suffolk Wildlife Trust Estuaries Project.

Site Description The tidal Channel of the Orwell estuary extends some 18km from Constantine Weir in Ipswich to Felixstowe. The intertidal zone of the upper reaches have been extensively developed to create Ipswich D o c k . The total intertidal mudflat Covers 713ha with the largest expanses found in the mid-reaches on the north shore. Whilst the catchment area of the river Gipping flowing into the estuary is 400km 2 , the freshwater input into the estuary is small, accounting on average for just under 0 . 5 % of the tidal volume (see Beardall et al., 1988). This allows conditions of high salinity to penetrate as far up the estuary as the Orwell bridge (14km upstream). The intertidal flats consist of a wide diversity of substrates but are dominated by fine silt (mean particle diameter + 5 . 5 - + 6 . 5 0 * ) which is replaced in certain regions on the north * A modification of the Wentworth scale is used to define classes of Sediment size. 0 is the - log2 of the particle diameter in millimetres.

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shore by fine sands (mean particle diameter +3.0 - +4.00). At the mouth and head of the estuary silts are mixed with gravel raising the mean particle diameter to 2.40 (Kay & Knights, 1975).

Methods A total of 87 samples were collected during September and October 1986 along transects taken at right angles to the foreshore. Transects were spaced lkm apart along the shoreline (see Figure 1) and covered all the intertidal mudflat present on the Orwell at low tide. Samples were taken at 50m intervals along each transect starting at low water mark and progressing towards the foreshore. The number of sampling points varied depending on the distance of exposed mud from low to high water. At each sampling point a core of 10.3cm diameter and 15cm depth was extracted using a cylindrical core sampler. Each sample was placed in a separate sealed bag and transported to the laboratory for analysis. The samples were sieved through a series of mesh sizes, the smallest of which was approximately 0.5mm. The invertebrates recovered from this process were preserved in 8% formaldehyde for counting and identification at a later date. using a binocular microscope.

To obtain wet and dry weights samples were placed on absorbent paper to remove surface liquids, weighed, dried to constant weight at 60°C and then re-weighed to give a measure of both the total wet and dry weight. Ash free dry weight was obtained by heating each sample in a muffle furnace at 600°C

Figure 1. Distribution of Nereis diversicolor, oligochaetes and Cirriformia on the intertidal mudflats of the Orwell Estuary.

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lentaculata

T H E INTERTIDAL INVERTEBRATE FAUNA OF THE O R W E L L ESTUARY

35

for 60-90 mins. The remaining material was weighed and subtracted from the total dry weight to give a measure of biomass.

At each sampling point the character of the substrate was noted and the number of lugworm (Arenicola marina) casts per m 2 recorded. This was considered a useful index of the relative abundance of lugworms which otherwise would have been omitted from the data due to the depth of its burrows.

Results A total of 11,490 specimens were counted and 32 species/groups identified (Table 1), three of which had not previously been recorded on the Orwell.

The number of species per sample varied enormously from 1 to 11, with an average of 6 per sample (Table 2). Between 0 to 304 specimens were recorded in each sample (excluding oligochaetes) with an average of 51 specimens per sample (equivalent to 6141 specimens per m 2 ). These figures compare favourably with those reported by Kay and Knights (1975) where an average of 5315 specimens per m 2 were recorded.

The number of species identified in each sample was generally highest in those samples taken from the mid-reaches of the estuary (Table 2). Eleven to 15 species were frequently recorded in samples taken from transects 6 to 14.

In the lower reaches of the estuary, and especially on the south shore, the number of species recorded per sample was lower and in some samples even down to one or two.

The macro-invertebrate fauna of the Orwell consisted mainly of annelid worms, molluscs and arthropods. Annelid worms dominated the benthic fauna, accounting for 67% of the specimens collected (Table 3), and were present in all the samples. Oligocheates (which were not identified further than class) were by far the most abundant class of annelid occurring in 97% of the samples and accounting for 48% of the total specimens collected (Table 3). Of the remaining 15 species of annelid Nereis diversicolor was the most abundant, recorded in 87% of the samples and representing 15% of the specimens collected (Tables 3 & 4). Cirriformia tentaculata was the only other relatively common annelid, occurring in 44% of the samples (Table 4).

The distribution of N. diversicolor and C. tentaculata is shown in Figure 1.

Both species were recorded in high numbers in the mid reaches of the estuary on the north shore. N. diversicolor was however found throughout the sampling area.

Of the ten species of mollusc recorded, four occurred widely; the common cockle, Cerastoderma edule, the Baltic teilin, Macoma balthica, the sand gapper, Mya arenaria, and the laver spire shell, Hydrobia ulvae. H. ulvae was the most numerous and abundant occurring in 59% of the samples but representing only 4% of the specimens collected (Tables 3 & 4). The distribution of the main species of mollusc are illustrated in Figures 2 and 3.

H. ulvae and M. balthica show a similar distribution with peaks in the mid reaches on the north shore and at Fagbury. M. arenaria was restricted to the upper reaches, whilst C. edule was found throughout the sampling area.

Small mussei beds, Mytilus edulis, are found in three regions of the intertidal

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Suffolk Natural History, Vol. 26 Table 1

Invertebrates collected in 1973 (ITE, 1974), 1985 (Bowden & 1985) and 1986 (present study)

Ravenscroft,

1973 PLATYHELMINTHES Turbellarians (Fiatworms) - not identified more specifically N E M A T O D E S (Eelworms) - not identified more specifically A N N E L I D S (Segmented worms) (Phyllodocidae) Polychaetes (Nereidae) (Orbiniidae) (Spionidae)

(Cirratulidae) (Arenicolidae) (Nephydidae) (Capitellidae) (Terebellidae) (Sabellidae) (Opheliidae) (Amphictenidae) Unidentified

Phyllodoce maculata Eulalia viridis Nereis divisicolor N. pelagica Scoloplos armiga unidentified Pygospio elegaes Polydora sp.

Cirrifromia tenticulata Aronicola marina Lugworm Nephtys hombergii Capitella capitata Lanice conchilega unidentified unidentified unidentified Pectinaria koreni

unidentified Oligocheates Sipunculus sp.

Sipunculids P R I A P U L I D S (Mud-living worms of uncertain affinities) Priapulus caudatus ARTHROPODS Crustaceans (Cirripedia) unidentified Barnacle larvae (Isopoda) (Woodlice-like forms) Cyathura carcinata Sphaeroma rugicauda Idotea battica Jaera albifrons Anthura gracilia (Amphipoda) ('Sandhoppers') Nototropis falcatus Melita palamata Corophium volutator* C. arenarium Macrodeutopus Sp. Gammarus sp. unidentified

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1985 1986

THE INTERTIDAL INVERTEBRATE F A U N A OF THE O R W E L L ESTUARY

(Decapoda)

MIRIAPODS (Centipedes) INSECTS - unidentified larvae MOLLUSCS Amphineurans (Polyplacophora)

Gastropods

Lamellibranchs

37

(Shrimpsetc.)

Crangon vulgaris Shrimp Carcinus maenus Shore Crab unidentified

(Chitons) Lepidochitona cinerea Hydrobia ulvae (Mesogastropoda) Laver Spire Shell Littorina saxatilis Rough Periwinkle L. litlorea Common Periwinkle Retusa obtusa Alvania cancellata Crepidula fornicata Slipper Limpet Nucella lapitlus (Neogastropoda) Common Dog Whelk (Archaeogastropoda) Gibbula sp.

Top Shell Nucula turgida Mytilis edulis Common Mussei Cerastoderma edule Common Cockle Teilina tenuis Thin Teilin Scrobicularia plana Peppery Furrow Shell Marcoma ballhica Baltic Tellin Mya arenaria Sand Gapper Lutaria lutaria Common Otter Shell Gari dipressa Large Sunset Shell

ECHINODERMS Ophiuroids

(Brittle-stars)

Ophiura sp.

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Suffolk Natural History, Vol. 26 Table 2 Average wet, dry and ash-free weights per transect (glm2)

Transect 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Fl F2 F3 F4

No. species 6 2 13 13 8 1 15 10 14 12 14 11 11 15 9 10 9 5 9 6 6 3 5 4 1 8 5 2

Wet wt.

Dry wt.

30-0 8-4 530-9 1875-0 701-0 433-9 301-2 808-2 123-1 417-3 863-4 1577-4 2886-3 372-6 342-8 457-1 806-3 15-6 470-9 132-0 36-0 6-0 46-8 1-7 375-4 838-4 900-5 2-4

Ash-free dry wt.

21-6 <1 257-7 1187-5 397-1 172-7 102-4 431-7 4M 173-9 241-8 636-5 1491-4 151-9 176-5 253-9 475-9 2-4 285-6 56-9 4-2 <1 4-8 <1 219-8 578-8 686-5 <1

<1 <1 26-1 81-0 40-8 27-4 16-4 52-3 10-3 28-5 69-4 119-4 220-4 28-5 23-4 33-2 54-0 <1 35-0 12.6 3-0 <1 4-2 <1 31-2 39-0 29-3 <1

Table 3 Comparison oftaxonomic groups collected in 1973 ITE study (unpublished data) andpresent study (samples collected in 1986), given as thepercentage ofthe total specimens collected 1973 Annelids

Polychaetes

1986

18

19 28

Oligochaetes Motluscs Arthropods

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10

67 48

67 5

15 17

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Table 4 Occurrence ofselected species (given as the percentage ofthe samples each species was recorded in). Comparison between 1973 (unpublished ITE data), and the present study (1986) 1973

1986

51 74 64

87 44 97

82 89 92

48 34 59

15

29

ANNELIDS Nereis diversicolor Cirriformia tenlaculata

Oligochaetes MOLLUSCS Cerastoderma edule Macoma balthica Hydrobia ulvae

CRUSTACEANS Corophium

volutator

zone, at F a g b u r y , P o n d O o z e and Shotley. H o w e v e r t h e sampling transects did not traverse these regions.

Eight species of a r t h r o p o d were identified. T h e s a n d h o p p e r , Corophium volutator, h o w e v e r was the only crustacean f o u n d in significant n u m b e r s , occuring in 2 9 % of t h e samples. C. volutator had a limited distribution with the highest densities f o u n d on the north shore in the mid-reaches of t h e estuary (Figure 3).

T h e m e a n biomass (ash-free dry weight) for each transect is shown in T a b l e 2 and Figure 4. T h e average biomass for the entire intertidal z o n e was 34.2g/m 2 . This value is lower than 52.9g/m 2 r e p o r t e d by Kay and Knights in 1975. Figure 4 c o m p a r e s t h e distribution of biomass r e p o r t e d in t h e present study with that of 1973 ( I T E , unpublished data) and illustrates a reduction in the productivity of t h e mudflats towards t h e m o u t h of the estuary, particularly o n the south s h o r e .

T a b l e s 3 a n d 4 c o m p a r e t h e present study with that of the I T E (1974). It is a p p a r e n t that t h e d o m i n a n c e of molluscs r e p o r t e d in 1974 (representing 6 7 % of the s p e c i m e n s collected) has changed to a Community now d o m i n a t e d by annelid w o r m s ( r e p r e s e n t i n g 6 7 % of t h e specimens collected in 1976). Figure 5 illustrates t h e c h a n g e s in t h e distribution of N. diversicolor b e t w e e n t h e t w o studies.

Discussion T h e distribution of benthic animals within an estuary is largely dictated by the n a t u r e of the substrate and the prevailing salinity regime. Of all t h e estuaries studied by t h e I T E (1974) t h e Orwell was f o u n d to have the most diverse m a c r o - i n v e r t e b r a t e f a u n a and the highest average biomass per n r .

T h e a u t h o r s suggested that this may be linked to t h e diversity of Sediment characteristics f o u n d within the estuary. T h e present study identified 32 species/groups of m a c r o - i n v e r t e b r a t e whilst in 1973 43 species/groups were

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Suffolk

Natural

History,

Vol.

26 Cerastoderma edule Mya arenaria

m

_

Ii a o O. W IM

12

13

H

16

17

Figure 2. Distribution of Cerastoderma mudflats of the Orwell Estuary.

18

19

20

21

22

23

2«,

edule and Mya arenaria on the intertidal

Corophium volutator Hydrobia ulvae

13

4-> 4J o u w 4J <v <v »—I O

° 8 a, <v w OJ -rH Ol LJ O) VJ a> a o

1!

II

U

15

16

Figure 3. Distribution of Corrophium mudflats of the Orwell Estuary.

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(1990)

17

18

19

20

volutator and Hydrobia

21

22

23

24

ulvae on the intertidal

THE INTERTIDAL INVERTEBRATE FAUNA OF THE ORWELL ESTUARY

41

1973

w.

w m VM

Kdometres alonq north bank

KilMnetresilonq sajth banh

Figure 4. C h a n g e s in the profile o f t h e b i o m a s s of i n v e r t e b r a t e s ( g r a m s d r y w e i g h t p e r S q u a r e m e t r e ) i n h a b i t i n g t h e i n t e r t i d a l m u d f l a t s of t h e O r w e l l , b e t w e e n 1973 ( I T E u n p u b l i s h e d d a t a ) a n d 1986 ( p r e s e n t s t u d y ) .

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reported (Kay & Knights, 1975). Indeed 24 species/groups identified by Kay and Knights (1975) were absent from the fauna recorded in the present studv (Table 1).

Annelids were found to dominate the benthic fauna of the Orwell in 1986.

N. diversicolor has been reported in other studies (Anderson, 1972) to prefer areas of reduced salinity in the mid to upper reaches of an estuary. The distribution of N. diversicolor in the present study would appear to conform to this distribution (Figure 1) with the highest densities found on transect 11 (Black Ooze). The distribution of N. diversicolor has however changed dramatically since 1973 (unpublished ITE data) when the highest densities were reported on the south shore towards the mouth of the estuary (Figure 5). In the present study these areas were found to be relatively poor. The reasons for this shift are unclear but may be related to changes in the physical conditions that now prevail in the lower reaches of the estuary (see Beardall et al., 1988) coupled with a concomitant increase in sewage effluent released into the upper reaches. Limited organic enrichment has been reported to cause an increase in the populations of N. diversicolor on other estuaries (McLusky, 1981).

N. diversicolor was found in 87% of the samples collected in the present study as opposed to 51% in 1973 (unpublished ITE data). A similar increase in the occurrence of oligochaetes was also found (Table 4). This may, to a certain extent, partially reflect the smaller mesh size used in the present study.

T h e shift f r o m a b e n t h i c Community d o m i n a t e d by m o l l u s c s to o n e

dominated by annelids has also been reported in the Stour estuary (ITE, 1974; Anglian Water, 1985) where annelids accounted for 9% of the total specimens collected in 1973 compared with 70% in 1985.

In contrast to annalids, molluscs on the Otwell have decreased since 1973, accounting for 67% of the specimens collected in 1973 (ITE, 1973) and only

Figure 5. Changes in the distribution of Nereis diversicolor along the intertidal mudflats of the Orwell between 1973 (ITE unpublished data) and 1986 (present study).

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15% in 1986. A n even greater decline from 90% to 10% was reported on the Stour during a similar period (Anglian Water, 1985). On the Stour the occurrence of H. ulvae and C. edule decreased by almost exactly the same as on the Orwell during the same time period. The reasons for this decline are unclear although C. edule is known to suffer large population fluctuations due to cold weather mortality and predation (Anderson, 1972). It is interesting to note that M. balthica, which is reported to be more able to tolerate cold weather conditions (Anderson, 1972) did not suffer a similar decline on the Stour, but did so on the Orwell.

H. ulvae and C. volutator have been reported to have a similar distribution (Anderson, 1972) with a preference for high shore levels in the mid-reaches of an estuary where salinity ranges f r o m 10-35 parts per thousand and the silt content of the substrate is high (Meadows, 1964). In the present study C. volutator had a restricted ränge (occurring in only 29% of the samples) with the highest densities found at mid to high shore level on transects 7 to 10. H. ulvae was also found in high densities at mid to high shore levels, but displayed a wider distribution throughout the estuary (Figure 3) occurring in 59% of the samples. Whilst H. ulvae appears to have decreased since 1973, C. volutator increased from occurring in 15% of the samples in 1973 to 29% in the present study.

In conclusion, the invertebrate Community of the Orwell estuary appears to have changed from one dominated by molluscs in 1973 to the dominance of annelid worms in 1986. The reasons for such changes are unclear, but it is evident that many other changes have occurred during the same period.

Increased effluent discharges from Cliff Quay sewage plant may have resulted in organic enrichment creating more favourable conditions for annelid worms (particularly oligochaetes), whilst adversely affecting mollusc communities, a p h e n o m e n o n that has been observed in organically enriched estuaries elsewhere (McLusky, 1981). It is interesting to note that 20% of all oligochaete species and 19% of N. diversicolor were recorded from transect 11 adjacent to the Cliff Quay sewage out-fall. The introduction of tri-butyl tin anti-fouling paints during the same period may have caused some changes, especially to the mollusc communities known to be sensitive to this poison (Waldock, Thain & Waite, 1987).

In addition to changing chemical factors the distribution and abundance of benthic macro-invertebrates are also influenced by changes in the physical conditions of an estuary (i.e. wave action, river and tidal currents) resulting in modifications in the patterns of Sedimentation and changes in the nature of the substrate (see McLusky, 1981). T h e paucity of species and numbers of specimens collected from the lower reaches of the estuary, and particularly on the south shore (transects 22-24), may well be related to changes in tidal currents and substrate characteristics that have occurred since 1973. The physical characteristics of the mouth of the Orwell have altered dramatically with the construction of the Trinity terminal of Felixstowe Port, increased boat movements and size of vessels operating from the Ports of Felixstowe and Ipswich, and increased Channel dredging to accommodate such vessels (the tidal volume of the Orwell estuary has been estimated to have increased by 56% as a result of dredging activities, see Beardall et al., 1988). Since no

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Suffolk Natural History, Vol. 26

data have been collected to monitor the environmental impact of these activities it is impossible to conclude with any degree of certainty whether such factors have played a part in causing the observed changes in the benthic communities or whether these changes are merely natural population fluctuations.

Acknowledgements The authors wish to thank Dr. R. C. Dryden and Judy Powell for their assistance in processing the specimens for biomass data and Mr. N. Ravenscroft for his identification skills.

We also wish to thank the Institute of Terrestrial Ecology for the use of unpublished data.

The Suffolk Wildlife Trust Estuaries Project is sponsored through the Worldwide Fund for Nature by the Central Electricity Generating Board.

References Anderson, S. S. (1972). The Ecology of Morecambe Bay. II. Intertidal invertebrates and the factors affecting their distribution. J. appl. Ecol.. 9.

161.

Beardall, C. H., Dryden, R. C. & Holzer, T. J. (1988). The Suffolk Estuaries - A report by the Suffolk Wildlife Trust on the wildlife and conservation of the Suffolk estuaries. Suffolk Wildlife Trust, Saxmundham Boorman, L. A. & Ranwell, D. S. (1977). Ecology ofthe Maplin Sands and the coastal zones of Suffolk, Essex and North Kent. Institute of Terrestrial Ecology ( N E R C ) Publication, Cambridge.

Bowden C. G. R. & Ravenscroft, N. O. M. (1986). An assessment of the macro-invertebrate resources of the Orwell estuary in relation to proposed developments at Fagbury. Suffolk Wildlife Trust. Internal report.

Davidson, N. C. & Evans, P. R. (1985). Implications for nature conservation of the proposed Felixstowe dock expansion. Nature Conservancy Council Final Report on Contract No. 2/85 (ref. E F 02-02-01).

Institute of Terrestrial Ecology (1974). Aspects ofthe ecology ofthe coastal area in the outer Thames estuary and the impact of the proposed Maplin Airport. A report submitted to the D o E by the Natural Environment Research Council.

Kay, D. G. & Knights, R. D. (1975). The macro-invertebrate fauna of the inter-tidal soft sediments of south east England. Journal of the Marine Biological Association ofthe U.K., 55, 811.

McLusky, D. S. (1981). The estuarine ecosystem. Blackie, Glasgow.

Meadows, P. S. (1964). Substrate selection by Corophium species: the particle size of substrates. J. Anim. Ecol., 33, 387.

Ravenscroft, N. O. M. (1987). Waders and Wildfowl o f t h e Orwell estuary 1983-1986; changes in status and feeding patterns. Suffolk Wildlife Trust.

Internal report.

Trans. Suffolk Nat. Soc. 26 (1990)

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Waldock, M. J., Thain, J. E. & Waite, M. E. (1987). T h e distribution and potential toxic effects of T B T in UK estuaries during 1986. Applied Organometallic Chemistry, 1, 287.

Dr. C. H . Beardall, S. M. Gooch & R. Pilcher, Suffolk Wildlife Trust, Park Cottage, Saxmundham, Suffolk, IP17 1DQ

Perching on Overhead Wires by Barn Owls Perching on overhead wires by Tawny Owls (Strix aluco) has been recorded (Martin, 1988) but it was considered unusual and did not appear to have been recorded outside Suffolk. Recently it has come to our notice that some Barn Owls ( T y t o alba) have been seen perching in the same manner. As this also appears to be unusual and not recorded outside Suffolk we list the recorded occurrences below: Date 4. 5.1983 13.11.1984 12.7.1987 2. 7.1989

Place Martlesham Hinderclay Rendham Mildenhall

Observer J. R. Martin S. Bishop A. Parker E. Seymour

We had particularly good views of a Barn Owl perching on an overhead power cable just Over the County boundary in Essex on the nights of 2 2 - 2 3 October, 1987, whilst driving along a lane at Great Horkesley.

On the first night we stopped the car and watched the owl by torchlight for several rriinutes. The next night it was perched at the same location and we stopped again. On this occasion it flew off and alighted quite easily on the wire some 50m away. We followed, and it again flew off and perched on a wire. This was repeated several times. There were other perching places available, such as trees and the tops of posts, but the owl made no attempt to use them.

Reference Martin, J. R. (1988). Unusual perching behaviour by Tawny Owls. Trans.

Suffolk Nat. Soc., 24, 5.

Jeff and Tina Martin

Trans. Suffolk Nat. Soc. 26 (1990)

Plate 7: lnicrtidal mud Oats of the Orwell Estuary and (inset) a sample of the rich invertebrate fauna they support (p . 33) .

(Photos: Charles H. Bearda ll )