Estuatine and Coastal Marine Science (1976) 4, 545-554

Variations in the Fauna of Particulate Shores in the Severn Estuary

Colin Little Department of Zoology, University of Bristol, BS8 IUG, England

and C. R. Boyden Portobello Marine Biological Station, Portobello, Otago, New Zealand

Received 4 December 197~ and in revised form 28 January 1976

Variations in invertebrate densities within the Severn Estuary have been examined on four shores over a period of two years. Variability due to seasonal changes was combined with a patchy distribution of some species, while others showed consistent densities at the same tidal level and at different sampling times. Some species showed more consistent densities in winter than in summer. Biological interactions, varying periods of spawn- ing and recruitment and beach instability are thought to be three of the major factors influencing distribution. No marked community changes such as seen in some sublittoral areas have been found.

Introduction

Studies on particulate shores have shown that zonation of the infauna is a widespread phenomenon (e.g. Pirrie et al., 1932; Watkin, 1942; Brady, 1943; Ansell et al., 1972; Boyden & Little, 1973). Distribution patterns on intertidal mud and sand are now well established, but there is little information about the stability of these patterns, especially in estuarine areas. Clearly the type of substratum and its stability are important and these may influence population densities. Sublittoral studies have shown that re-working of the sediments by members of the infauna may reduce the stability of the habitat to such a degree that increased water turbulence destroys the fauna by eroding the sediments (Eagle, 1973, 1975). In such cases very large changes in community structure are seen over periods as short as two months.

In the present study we have examined variations in the densities of invertebrate popula- tions on particulate shores in the Severn Estuary over a period of two years.

Methods and expression of results

Four beaches on the southern shore of the Severn Estuary were examined (see Figure I).

All of these consisted mainly of mud and sand, and descriptions of them are given in Boyden & Little (1973). Profiles of the beaches and approximate vertical heights were estimated by observations of the times at which shore stations were covered by the tide and reference to the tide curves given by the Admiralty Hydrographic Department (1972-4) and (for Sharpness) by Gibson (1933).

Sampling procedures were as described earlier (Boyden & Little, 1973). One transect was taken on each beach, and at each station one quadrat of 0.25 m2 (cu. IO cm deep) was sieved

545

546 C. Little & C. R. Boyden

through a 0.5 mm mesh. Transect positions on each shore were fixed using compass bearings and permanent station poles. Each beach was examined on four occasions : in the summers of 1972 and 1973, and the Winters Of 1972/j and 1973/4.

In order to allow a comparison both between different species and between the same species at different times, we have used a graphical representation of the results which displays densities on a logarithmic scale. While this can give only an impression of order of magnitude, we believe that such accuracy is all that is necessary taking into account the variation that we have found. In Figures 2-4 each quarter segment refers to one of the four sampling dates. Since original densities were measured in areas of 0.25 m2, it must be noted that no density less than 4/ma could be recorded, except as absence. The apex of each quarter segment, i.e. the point at which the two scales cross, represents a density of r/m2.

fed tlouc.rt.r

\ Sharpness

51’30’N

Figure I. Location of sample sites. The inset shows the position of the Sevem Estuary.

Sampling validity

Only if the content of one 0.25 m2 quadrat reflects abundances nearby at the same tidal level (alongshore variation of Eltringham, 1971) will a transect accurately reflect conditions from H.W. to L.W. (downshore variation). On a sub-tropical sandy shore Dauer & Simon (1975) have shown this to be so for dominant polychaetes in widely separated transects, while more variation was found for the rare species. In the present study we investigated the situation at Weston by taking five separate o-25 m2 samples, I m apart, from each of two

Fauna1 variations in the &-vern Estuary 547

tidal levels (stations 0, and A,). The results are shown in Table I. Using standard deviations as a measure, species can be classed as ‘consistent’ (S.D.<o.5 x mean), ‘inconsistent’ (S.D.>o.75 xmean), or ‘fairly consistent’ (between the above extremes). The data have been summarized in Table 2.

Generally (except for Bathyporeia pilosa), higher densities were recorded at the lower station and here the dominant species were relatively ‘consistent’ in their distribution. Even so, the highly mobile B. pilosa, which occurred at relatively high densities, could only be classed as ‘fairly consistent’ and Hydrobia ulvae was ‘inconsistent’. The spionid species recorded were very small and some of the variability in their abundance was no doubt due to inadequate sampling. Two of the species found to be ‘consistent’ at the lower station (Corophium arenarium and Scoloplos armiger) proved to be ‘inconsistent’ at the upper station, and only one species, Macoma balthica, was ‘consistent’ at both stations.

TABLE I. Invertebrate abundance in five 4 ma samples taken from two stations in Weston Bay, 1975 (May)

Upper station (A,)

(substrate sand) Lower station (0,)

(substrate muddy sand)

Eurydice pulchra 0-l 0.6 0’5 Corophium arenarium S-12 8.0 3.1 Corophium arenarium o-8 2.8 2.7 Bathyporeia pilosa 134-481 268.0 137'7 Bathyporeia pilosa 340-1684 970 592 Crangon vulgaris O-I 0’2 0.4 “Spionids o-6 2.6 2’2 ‘Spionids 4-62 29.6 2x.8 Scoloplos armiger I-IO 3.6 3.6 Scoloplos armiger 3-10 6.2 2.5 Prostomatella sp. o-3 1.8 I'7 Nephtys hombergi 3-7 5’4 I’7 Hydrobia ulvae O-13 7'4 4'5 Nereis diversicolor O-I 0’2 0'4 Adult Macoma balthiea 2-S 2.8 I’3 Prostomatella sp. o-3 1’0 I'3

(>I cm) Hydrobia ulvae 114-898 352’0 316.3 Juvenile M. balthica 6-19 13.0 5'4 Adult Macoma balthica 20-60 42.2 20.3

(<I cm) (>I cm) Total M. balthica 8-22 X5.8 5.6 Juvenile M. balthica 73-181 142.8 47.9

(<I cm) Total M. balthica 93-236 205.0 62.7

“A mixture of Spio jilicornis (0. F. Miiller) and Pygospio elegans Claparede.

It can be concluded that for the species classified as ‘consistent’, the sampling of single 0~~5 m2 quadrats adequately reflected abundance at each station. For the other species, it would have been preferable to increase the number of samples taken at each station, but this was not possible because of practical difficulties. Firm conclusions can therefore in general only be drawn concerning the ‘consistent’ species, and for the others most conclu- sions must be very tentative. In many cases, however, patterns for ‘inconsistent’ species at one station are reinforced by patterns at neighbouring stations (e.g. Hydrobia at Weston, see Figure 3) a factor not accounted for in the consistency index.

Results

The general distribution of species has been described in our earlier paper (Boyden & Little, rg73), where we were concerned solely with populations found in the summer of

548 C. Little & C. R. Boyden

1972. The present sampling was carried out with a view to investigating differences between summer and winter densities over two years. The results are given in Figures 2-4.

Seasonal differences

It is most convenient to consider the beaches in turn and to summarize the general conclu- sions later.

At Sharpness (Figure z) there appeared to be very little difference in the distribution of the species between summer and winter. For three species, Bathyporeia pilosa Lindstrom, Neveis diwersicolor 0. F. Miiller and Macoma balthica L., the winter densities were more consistent than those in summer.

TABLE 2. Classification of the invertebrates at Weston based on their consistency of distribution

S.D. (from Table I)

‘ Consistent’ <0.5 X mean

‘ Fairly consistent’ 0.5-0.75 X mean

‘ Inconsistent ’ >0.75 X mean

Macoma balthica Bathyporeia pilosa Prostomatella sp. Higher station (A,) Spionids Scoloplos armiger

Hydrobia ulvae Corophium arenarium Corophium arenarium Bathyporeia pilosa Prostomatella sp.

Lower station (OJ

i

Scoloplos armiger Spionids Nephtys hombergi Hydrobia ulvae Macoma balthica

The overall paucity of species at Portishead is a reflection of the arduous conditions produced by a combination of low and fluctuating salinity, high turbidity and exceptionally strong currents which result in beach instability. The three polychaetes all showed different trends: Nereis showed no seasonal differences, Nephtys hombergi Lamarck was common only in summer and ScolopZos armiger (0. F. Mtiller) occurred only in winter. The molluscs Hydrobia ulvae (Pennant) and Macoma both showed a build-up in density during the study period.

At Weston (Figure 3) a greater number of species was found, including the nemertine Prostomatella Friedrich which exhibited rather erratic densities within the small zone to which it is restricted. N. hombergi here showed little difference between summer and winter densities, whilst Scoloplos and the cirratulid Z’haryx murioni (St. Joseph) were more common in summer. Corophium arenarium Crawford showed no summer-winter differences, while Corophium volutator (Pallas) was much more common in summer, in contrast to its occur- rence at Sharpness. B. pilosa showed a very variable abundance with no obvious seasonality, but since it has breeding peaks in both spring and autumn (Fish & Preece, 1970), summer and winter samples might be expected to be similar. M. balthica and Retusa obtusa (Montagu) both extended their range slightly up the shore in winter while H. ulvae extended its range both up and down shore. This might be related to breeding periodicities (Fish & Fish, 1974), but is more likely to be related to distribution by the tides: these were greater in amplitude at the time of our winter sampling so that greater passive distribution of the molluscs might be expected.

At Minehead additional species were found and there were few in common with the other transect sites. Scoloplos was more common in summer than winter, as at Weston, but was here more widely distributed over the beach. Eteone spp. were more common in winter than summer. Amphipods dominated the beach and were common in summer and

Fauna1 variations in the Severn Estuary 549

Sharpness

Stotions

13

12

Portishead

6

Distance

fr%~W.

0

20

45

90

135

I80

230

275

320

365

410

460

500

Vertical height

above 0.0. rt;; (m)

t4.3 - M.H.W.N.

+2,3 - M.T.L.

+2,7

+4.5 - M.H.W.N.

+5,3

t5.1

t4.9

+4,6 - M.H.W.N.

+3,e

t2.3 - M.T.L.

to.7

0

-0.9 - M.L.W.S. -M.L.W.N.

0 t3.5 - M.H.W.N.

90

I80

275

370

430

t2.7

+0,6--.-w M.T.L.

-2.2 - M.L.W.N.

-5.0 - M.L.W.S.

-6.0

[ Sand

m Mud

Ju;;‘p;Ig Feb/ Mar

June /July * ;:r73

1973 1974

Figure 2. Distribution of invertebrates along transects at Sharpness and Portishead over a z-year period. Densities are plotted on a logarithmic scale; solid quadrants show densities in summer, open quadrants those in winter. The apex of each quadrant represents a density of r/m *. Vertical heights are referred to O.D. (Newlyn). The irregular shore profile at Sharpness results in the appearance of the same tidal height at more than one point on the transect. In this region of the estuary it is to be noted that M.L.W.N. lies below M.L.W.S. Substratum type is indicated diagrammatically.

550 C. Little & C. R. Boyden

Weston

Stations

Vertical Distance heiaht B x Substratum fry#.W. “7;: 0.0. Tidal

level - M.H.W.S.

e, +++++++

Bl +++++-i-+

02 ~-j-j--+++-+-+a A +++++++

Al -++I-++++

O -++++-t-++

-M.H.W.N.

Oo -+4--+++++

01 +++-I-+++

’ +-!-+-l--t-++

2 +++++-i-+ - M.T.L.

3 ++43++++

4 .++-~++-f-+

s ++w-++t

= ++e++++-+-

7 +-t-e+++++

B ++d+H-4++ - M.L.W.N.

s ++4++44

‘O +++4-+-++ -M.L.W.S

a Records not available

Figure 3. Distribution of invertebrates at Weston-super-Mare. Details as in the legend to Figure 2.

Fauna1 variations in the Severn Estuary

+ -I- -t-

552 C. Little & C. R. Boyden

winter. Unlike Fish & Preece (1970), we found no evidence for an offshore migration of Bathyporeiu pelagica in summer.

If only the species which were found to be ‘consistent’ at Weston are now considered, these results can be summarized as follows. N. hombergi was commoner in summer than in winter at Portishead but at Weston showed no seasonal differences. S. armiger was commoner in winter at Portishead but in summer at Weston and Minehead. M. balthica showed little seasonality at any site, but extended its range up the beach in winter and C. arenarium showed none at the two sites where it occurs. Taking into account similar patterns in neigh- bouring stations for the inconsistent species, it can also be said that the molluscs in general showed the same trend as Macoma, and that B. pilosa and N. diversicolor showed more consistent densities in winter than in summer,

DiSferences between I97213 and 1973/g

At several places, species were rare in the first summer, including for example, H. ulvae (and less so M. balthica) at Portishead, and H. ulvae together with C. arenarium and R. obtusa at Weston. C. arenarium also appeared to increase in abundance at Sharpness. At Minehead, Prostomatella became more abundant in the second year and the two Eteone spp. were not found at all until the second summer. In general, however, the differences between the two years were not at all marked.

Discussion

Spatial variations in density of species

The patchiness shown by some species, i.e. those recorded as ‘inconsistent’ in Table 2 is not easily explained. This phenomenon has been much discussed in the context of plankton distribution, where the balance between phytoplankton and zooplankton populations has been considered in terms of feeding, migration and possibly exclusion (e.g. Bainbridge, 1953). The idea of exclusion might well be applied to the infauna and such an inter-relationship has already been proposed for Macoma and the amphipod Pontoporeia by Segerstrale (1965). There may also be interactions between individuals of the same species so that a clumped distribution is observed. Such a distribution is suggested by our data for the highly mobile form Bathyporeiu pilosa.

Summer-winter variations

Several species showed population densities which were consistent from winter to summer while others were commoner at one season than another. The periods of spawning and recruitment such as described for the cirratulid Cirriformia (Audouinia) tentaculata by George (1964) could explain the high numbers of cirratulids at Weston in summer. Seasonal migrations might also be important but evidence for these in the infauna is scarce. Brady (1943) showed a small downshore movement of several polychaetes and Tharyx marioni at Weston may behave similarly; but it may be that this species is an annual so that the adults only appear in summer. In contrast, the extension of Hydrobiu over the beach in winter is likely to be due to passive movement by waves and currents, since Hydrobia has not been observed to float on the tide in the Severn (Little & Nix, 1976).

Details of changes in beach profile and in the distribution of sediment types, such as those known for a tropical shore (Ansell et al., 1972) are not available for the Severn Estuary.

Fauna1 variations in the Severn Estuary 553

Since our winter samples were taken during periods of larger tidal amplitude than the summer samples, it may be that tidal scour affected the composition of the samples more in the winter. The small numbers of N. hombergi found at Portishead in the winter may be an example. It may also be that the presence of Scoloplos at Portishead only in winter is related to its ability to colonize mobile substrata. Nevertheless, many species show more consistent densities in winter than in summer.

Di@rences between successive years

The differences between the two sampling years were relatively slight, in contrast to results from such sublittoral areas as Liverpool Bay (Eagle, 1973, 1975). Since it is known that

apart from the effects of beach instability caused by the infauna, many species often exhibit large fluctuations in numbers depending upon the success of each spatfall (e.g. Stephen, 1932; Thorson, 1950), this is surprising. Nevertheless, the only radical change in community structure that we have seen is the appearance of two Eteone species at Minehead in the second year. This general consistency suggests that the data obtained may provide a fairly reliable background against which future figures may be compared.

Acknowledgements

It is a pleasure to acknowledge the help given in sampling by Mr R. A. Milne and Mr P. White. We are also grateful to Dr A. E. Dorey for reading and commenting upon the manuscript.

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