Zooplankton of the Bristol Channel and Severn Estuary

Marine Pollution Bulletin Collins, M. B., Ferentinos, G. & Banner, F. T. (1979). The hydrodynamics and sedimentology of a high (tidal and wave) energy embayment (Swansea Bay, Northern Bristol Channel). Estuar. cstl mar. Sci., 8, 49-74. Davies, A. G. & Sleep, J. A. (1979a). Photosynthesis in some British coastal waters may be inhibited by zinc pollution. Nature, Lond., 277, 292-293. Davies, A. G. & Sleep, J. A. (1979b). Inhibition of carbon fixation as a function of zinc uptake in natural phytoplankton assemblages. J. mar. biol. Ass. U.K., 59, 937-949. Davies, A. G. & Sleep, J. A. (1980). Copper inhibition of carbon fixation in coastal phytoplankton assemblages. J. mar. biol. Ass. U.K., 60, 841-850. Demers, S. & Legendre, L. (1982). Water column stability and photosynthetic capacity of estuarine phytoplankton: Long term relationships. Mar. Ecol. Prog. Ser., 7, 337-340. Joint, I. R. (1980). Phytoplankton production in Swansea Bay. In Industrialized Embayments and their Environmental Problems. A Case Study of Swansea Bay (M. B. Collins, F. T. Banner, P. A. Tyler, S. J. Wakefield & A. E. James, eds), pp. 469--479. Pergamon Press, Oxford. Joint, I. R. & Pomroy, A. J. (1981). Primary production in a turbid estuary. Estuar. cstl Shelf Sei., 13, 303-316. Joint, I. R. & Pomroy, A. J. (1982). Aspects of microbial hetero-

trophic production in a highly turbid estuary. J. exp. mar. Biol. Ecol., 58, 33--46. Mantoura, R. F. C. & Mann, S. V. (1979). Dissolved organic carbon in estuaries. In Tidal Power and Estuary Management, Proc. 30th Symposium of Colston Research Society (R. T. Severn, D. L. Dineley & L. E. Hawker, eds), pp. 279-286. Scientechnica, Bristol. Paulraj, P. J. & Hayward, J. (1980). The phytoplankton of inshore Swansea Bay. In Industrial Embayments and their Environmental Problems--a Case Study of Swansea Bay (M. B. Collins, F. T. Banner, P. A. "13'ler, S. A. Wakefield & A. E. James, eds), pp. 481--486. Pergamon Press, Oxford. Redfield, A. C. (1958). The biological control of chemical factors in the environment. Am. Sci., 46, 205-221. Uncles, R. J. & Joint, I. R. (1983). Vertical mixing and its effect on phytoplankton growth in a turbid estuary. Can. J. Fish aquat. Sci., 40, (Suppl. 1), 221-228. Uncles, R. J. & Radford, P. J. (1980). Seasonal and spring-neap tidal dependence of axial dispersion coefficients in the Severn- a wide, vertical mixed estuary. J. Fluid Mech., 98, 703-726. Ware, G. C. & Anson, A. E. (1979). The bacteriology of the Severn Estuary. In Tidal Power and Estuary Management, Proc. 30th Symposium Colston Research Society (R. T. Severn, D. L. Dineley & L. E. Hawker, eds), pp. 273-278. Scientechnica, Bristol.

0025-326X/84 $3.00 + 0.00 © 1984 Pergamon Press Ltd.

Marine Pollution Bulletin, Vol. 15, No. 2, pp. 66-70, 1984 Printed in Great Britain

Zooplankton of the Bristol Channel and Severn Estuary R. W I L L I A M S N a t u r a l E n v i r o n m e n t R e s e a r c h Council, Institute f o r M a r i n e E n v i r o n m e n t a l Research, P r o s p e c t Place, The H o e , P l y m o u t h , U K T h e m o s t c o m p r e h e n s i v e set o f d a t a o n t h e z o o p l a n k t o n o f t h e Bristol C h a n n e l c o m e s f r o m t h e m u l t i - d i s c i p l i n a r y s t u d y m o u n t e d b y I M E R . B e t w e e n J u n e 1971 a n d O c t o b e r 1980, a t o t a l o f 1579 n e t h a u l s was t a k e n o v e r a grid o f 58 s t a t i o n s (see Fig. 1). T h e r e w e r e 52 cruises d u r i n g this

p e r i o d ; n o t all s t a t i o n s w e r e visited o n e v e r y cruise b u t e v e r y c a l e n d a r m o n t h , e x c e p t D e c e m b e r , is r e p r e s e n t e d ; most months during the spring and summer seasons being s a m p l e d in five o r m o r e years. T h e r e w e r e o n l y f o u r s t a t i o n s a b o v e t h e H o l m I s l a n d s ; in c o n s e q u e n c e t h e r e is

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Fig. 1 Chart of Bristol Channel and Severn Estuary showing subregions and sampling sites (1-58).

66

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% Similarity Fig. 2 Dendrograms of percentage similarity in geographical distribution between species for the surveyin April 1974. Asterisksmark numerically dominant species in each of the faunal assemblages which are, from top to bottom: true estuarine ~), estuarine and marine C), euryhaline marine O and stenohaline marine ~). The bottom group, plus juvenile Pleurobrachiapileus, is composed of speciesof low similarities.

little information about the plankton between these Islands and the fresh/brackish water interface. Samples were taken with a Lowestoft High Speed Sampler (Beverton & Tungate, 1967; Harding & Arnold, 1971) fitted with a nylon net of 280 /Jm mesh. In summarizing the results here, most of the illustrations are taken from cruises in April and August of 1974 which typify the beginning of the productive cycle in the spring and its peak in summer.

Species Assemblages The plankton data from the surveys were analysed by two objective techniques; a hierarchical clustering method (Field, 1971; Field et aL, 1982; Collins & Williams, 1982) and multi-dimensional scaling, which is an ordination method (Kruskal, 1977; Field et aL, 1982). Species dendrograms were derived from similarities of geographical distribution of species in the samples from each survey. The dendrogram for April is shown, as an example, in Fig. 2; there are four distinct groups, or associations, of species (plus a list of organisms showing little similarity with each other or any of the groups). Analysis of similarity between stations produced four groups, clearly related to the four main species assemblages. Figure 3 shows, for April and August, the clusters of stations with similarities > 50070. (The station positions in Fig. 3 differ from those shown in Fig. 1 because the large tidal excursions in the Bristol Channel made it necessary to correct the sampling positions for state of tide; a computer program based on Admiralty tidal data was used, therefore, to estimate the high tide position of each station.) The four assemblages showed coherent geographical patterns which shifted according to season and

5o

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Fig. 3 Clusters at high similaritylevelsused to identify the four faunal assemblages from the April and August surveys. Symbols have the same meaningas in Fig. 2. Isohalinesare shown at 27, 30, 32, 33,34and35% oS.

to high and low river run-off into the estuary. This is seen in Fig. 3 where the positions of the groups differ between the two months but the boundaries of their distributions correspond, roughly, to salinities of 29%o, 33%0 and 3 4 ° in April or 3 5 ° in August. Multi-dimensional scaling analysis gave similar results to the classification method and confirmed the relationship between the distributions of the plankton and salinity. The four assemblages or associations of species derived from the dendrograms conformed to the generalized classification of plankton according to their tolerance of salinity ranges in estuaries (Spooner & Moore, 1940; Day, 1951; Jefferies, 1967); that is, (a)true-estuarine, (b)estualine and marine, (c) euryhaline marine and (d) stenohaline marine. The assemblages were characterized in this study by the four copepods Eurytemora affinis (Poppe), Acartia bifilosa var. &ermis (Rose), Centropages hamatus (Lilljeborg) and Calanus helgolandicus, Claus respectively. Their distributions and abundance in August 1974 are shown in Fig. 4.

Biomass The zooplankton species were allocated into an omnivore group of 135 entities (genera, species and developmental stages) and a carnivore group of 31 entities. Each entity was given a carbon value (~gC) and the biomass for each trophic type calculated from the numbers of organisms in the samples. There was a gradient from high biomass at the seaward end to low values up the Channel, which was more marked in spring for the omnivores and summer for the carnivores. Figure 5 shows the distributions for July when the peak of the omnivore biomass in the six regions occurred; that is one month after the maximum standing crop of phytoplankton, as

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Marine Pollution Bulletin

-51 ° 30'

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Fig. 5 Distribution of omnivore and carnivore plankton biomass in the Bristol Channel and Severn Estuary in July 1974. Contours are drawn at 0. l, 1.0 and 10.0 nag C m - 3

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i I

Fig. 4 Distribution and abundance of (a) Eurytemora affinis, (b) A cartia bifilosa var. inerrnis, (c) Centropages hamatus and (d) Calanus helgolandicus in the Bristol Channel and Severn Estuary for August 1974. Contours are drawn at 1.0, 10,0 and 100.0 animals m - 3

measured by chlorophyll-a (Fig. 6). The biomass values in the Outer Channel in July are comparable with those found in summer in the Celtic Sea (Williams & Conway, 1982). The peak of the carnivore biomass occurred in August/September when the carnivorous chaetognatha (Sagitta) dominated the biomass of the plankton.

Seasonal Cycle of Zooplankton The permanent planktonic animals (holoplankton) in the Bristol Channel and other estuaries in the British Isles are mostly copepods. The temporary plankton (meroplankton) is represented by phyla such as decapods, molluscs, echinoderms, annelids and fish. The samples from the IMER surveys were dominated by calanoid

68

40

copepods, but at certain times of the year mysids, especially Schistomysis spiritus, constituted the major part of the zooplankton biomass from the Inner Channel, reaching 80°7o of the total in summer (for reasons arising from sampling and survey techniques, that is probably a gross under-estimate). The seasonal variations in distribution and abundance of the common zooplankton species are related to the changes in salinity regime (Collins & Williams, 1981, 1982). The reduced freshwater flows in the summer allow the penetration of higher salinity water, with its associated fauna, further up the estuary while in winter the converse is true (Radford et al., 1981; Collins & Williams, 1981). These seasonal patterns of variability in the penetration of seawater have important effects on the patterns of distribution and abundance of the plankton. As well as the changes in distributions produced by the hydrodynamics within the estuary, there are seasonal changes in abundance and succession of species. The major peaks in biomass were found in the omnivorous zooplankton; the carnivores, such as Sagitta spp. (mostly S. elegans), tended to be more important in the second half of the year when they roughly equalled the omnivore biomass in the Bristol Channel, see Fig. 6. The dominance of S. elegans in the plankton would suggest that, for up to six months of the year, the structure of this planktonic ecosystem is probably regulated by this predator.

Fish Thirty-eight species of post-larval fish have been recorded from the IMER surveys (Russell, 1980). The most abundant was the sprat (Sprattus sprattus) which spawns in spring; its eggs were absent from the plankton after June. The distribution and abundance of the eggs and larvae in April, which is the month of their maximum abundance, are shown in Fig. 7. The main concentrations of eggs were in Carmarthen Bay and off Hartland Point but the larvae were found appreciably further up the Channel, the highest numbers being off Barnstaple Bay.

Volume 15/Number 2/February 1984

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Fig. 6 Seasonal variability of chlorophyll-a and biomass (mg carbon m-3) in the six sub-regions of Fig. 1 for all omnivorous and carnivorous zooplankton for the period November 1973 to February 1975. The absence of data for chlorophyll in the Outer Estuary is a consequence of the inadequate sampling in that region.

The eggs and larvae can tolerate a wide range of salinity and its is unlikely that larvae would be adversely affected by being carried into the Central Channel, where salinities are between 30 and 34%0. Fisheries for sprat are found primarily along the east coast of Britain; most of the fish being caught off the River Thames, Wash and Firth of Forth; however, there is a fishery in the Bristol Channel during the winter months when the fish shoal close inshore. A characteristic of this fishery is that abundance in any one area is very variable from year to year and there is no reliable estimate of stock size (Macer, 1974). Commercial trawling is carried out for demersal fish at the mouth of Carmarthen Bay, the area north of Lundy and to a lesser extent in Barnstaple Bay. The inner part of Carmarthen Bay is considered to be an important nursery ground for demersal fish (Warwick et al., 1978).

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Fig. 7 Geographical distribution and numerical abundance o f the eggs and larvae o f the sprat (Sprattus sprattus) in the Bristol Channel and Severn Estuary in April 1974. Contours are drawn at 0.1, 1.0 and 10.0 eggs or larvae under each m 2.

Discussion The basic distribution and composition of the zooplankton in the Bristol Channel and Severn Estuary is now understood. The communities, or associations of species, described in this report are normal for estuaries in northern latitudes, both in their abundance and species composition. Salinity is the most important environmental variable affecting the distribution of plankton in estuaries (although other factors play their part). Any changes in the pattern of residual currents will not necessarily be reflected precisely in the pattern of salinity distribution; there would be an interaction between the effects on the communities of physical transport and those of salinity. The introduction of a barrage into the Bristol Channel ecosystem would have a complex impact on the zooplankton (Williams & Collins, 1980). The reduction of salinity above the barrage would have a profound effect on the True Estuarine community which would penetrate further down the estuary and establish larger breeding populations in the lower reaches of the Outer Estuary (see Fig. 1) than at present. The increased production of phytoplankton following the construction of a barrage would also enhance the growth of zooplankton populations. The effects on the estuarine marine species are not easily predictable; the reduced salinity above the barrage would compress their distributions and it seems likely that there would be major changes in this group of species. Radford & Young (1980), using the GEMBASE model, predicted that the biomass of zooplankton in the Outer Estuary, above the barrage, would increase by an order of magnitude in the first summer after the barrage comes into operation but this would be a transient effect, the biomass of carnivores settling down quickly to its present level and that of omnivores to about two or three times the level of the unmodified system. However, this prediction requires further examination and too little is known about the biology of the key species in the Estuarine and Estuarine-

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Marine Pollution Bulletin

Marine groups; further fundamental research is needed on Eurytemora affinis and Schistomysis spiritus, particularly in view of the very large part of the biomass which is contributed by mysids.

Beverton, R. J. H. & Tungate, D. S. (1967). A multi-purpose plankton sampler. J. Cons. perm. int. Explor. Mer, 31, 153-157. Collins, N. R. & Williams, R. (1981). Zooplankton of the Bristol Channel and Severn Estuary. The distribution of four copepods in relation to salinity. Mar. Biol., 64, 273-283. Collins, N. R. & Williams, R. (1982). Zooplankton communities in the Bristol Channel and Severn Estuary. Mar. Ecol. Prog. Ser., 9, 1-11. Day, J. H. (1951). The ecology of South African estuaries, Part 1. A review of estuarine conditions in general. Trans. R. Soc. S. Afr., 33, 53-91. Field, J. G. (1971). A numerical analysis of changes in the soft bottom fauna along a transect across False Bay, South Africa. J. exp. mar. Biol. Ecol., 7, 215-153. Field, J. G., Clarke, K. R. & Warwick, R. M. (1982). A practical strategy for analysing multispecies distribution patterns. Mar. Ecol. Prog. Ser., 8, 37-52. Harding, D. & Arnold, G. P. (1971). Flume experiments on the hydrodynamics of the Lowestoft high-speed plankton samplers: I. J. Cons. int. Explor. Mer,, 34, 24-36. Jefferies, H. P. (1967). Saturation of estuarine zooplankton by congeneric associates. In Estuaries (G. H. Lauff, ed.), pp. 500-508. American Association for the Advancement of Science, Washington, D.C. (Publs. Am. Ass. Advmt. Sci. No. 83).

Kruskal, J. B. (1977). Multidimensional scaling and other methods for discovering structure. In Statistical Methods f o r Digital Computers (K. Enslein, A. Ralston & H. S. Wilf, eds.), pp. 296-339. John Wiley, New York. Macer, C. T. (1974). Industrial Fisheries. In Sea Fisheries Research (F. R. Harden Jones, ed.), pp. 193-221. Elek Science, London. Mills, E. L. (1969). A community concept in marine zoology, with comments on continua and instability in some marine communities: A review. J. Fish. Res. BdCan., 26, 1415-1428. Radford, P. J. & Young, K. M. E. (1980). Predicted effects of proposed tidal power schemes on the Severn Estuary ecosystem. Volume 2: Ecosystem effects, Department o f Energy Report, Contract E/5A/CON/SB/1598/51/036, pp. 45 (mimeo). Radford, P. J., Uncles, R. J. & Morris, A. W. (1981). Simulating the impact of technological change on dissolved cadmium distribution in the Severn Estuary. WaterRes., 15, 1045-1052. Russell, F. S. (1980). On the distribution of postlarval fish in the Bristol Channel. Bull. mar. Ecol., 8, 283-290. Spooner, G. M. & Moore, H. B. (1940). The ecology of the Tamar estuary. VI. An account of the microfauna of the intertidal muds. J. mar. biol. Ass. U.K., 24, 283-330. Warwick, R. M., George, C, L. & Davies, J. R. (1978). Annual macrofauna production in a Venus community. Estuar. cstl mar. Sci., 7, 215-241. Williams, R. & Collins, N. R. (1980). The implications to the benthic and planktonic faunas of presumed alterations to salinity regimes in relation to the proposed Severn Barrage. Part 2. Department o f Energy Report, Contract No. E/5A/CON/4012/51/072, pp. 14 (mimeo). Williams, R. & Conway, D. V. P. (1982). Population growth and vertical distribution of Calanus helgolandicus in the Celtic Sea. Neth. J. Sea Res., 16, 185-194.

0025-326X/84$3.00+0.00 © 1984PergamonPressLtd.

MarinePollutionBulletin,Vol. 15, No. 2, pp. 70-76, 1984 Printedin GreatBritain

The Benthic Ecology of the Bristol Channel R. M. WARWICK

Natural Environment Research Council, Institute for Marine Environmental Research, Prospect Place, The Hoe, Plymouth, UK In view of the importance of the Bristol Channel as an area of present industrial development and future potential, and of its proximity to several institutions actively engaged in marine research, it was surprising to find as recently as 1972 that very little research had been undertaken on the subtidal benthos, and certainly none of a comprehensive nature (Natural Environment Research Council, 1972). At that time the Institute for Marine Environmental Research began to undertake such a study. The objectives were not immediately concerned with the pollution status of the region or with predicting the consequences of future events such as the construction of a tidal barrage, but rather with gaining a more fundamental understanding of the factors underlying the distribution, structure and functioning of benthic communities, and the interactions between the benthos and other components of the ecosystem.

The Distribution of Benthic Communities Samples of the bottom fauna have been taken on a grid of 155 stations in the Bristol Channel from Lundy Island

70

to just above the Holme Islands, using a Day grab and Naturalist's dredge (Warwick & Davies, 1977). Stations were initially ordered into Petersen (1913) communities subjectively, on the basis of published descriptions of the species composition of these communities (particularly those of Thorson, 1958). This was followed by an objective computer classification which facilitated the recognition of sub-groupings and gradations in species composition across communities which had previously escaped subjective identification. The nature of the bottom substrates and the bed topography were determined from a combination of grab sampling, Admiralty charts and the extensive coverage of the Bristol Channel with side-scan sonars (Belderson & Stride, 1966; Kenyon, 1970; Belderson etal., 1971; Lloyd etal., 1973). The inner part of the Channel is a zone of erosion with a rock bed, and such zones are also apparent off Caldy Island and as a continuous band between Hartland Point and Lundy Island (Fig. 1). A Modiolus community in its pure form is typical of these rock floors (Fig. 2), but the strong tidal scour in the up-channel region results in a very