an approach to ecological monitoring in the rocky intertidal: a survey of bantry bay and dunmanus...

Marine Em'ironmental Research 8 (1983) 149-163

An Approach to Ecological Monitoring in the Rocky Intertidal: A Survey of Bantry Bay and Dunmanus Bay,

Ireland

T. F. Cross*& T. Southgate

Department of Zoology, University College, Cork, Ireland

(Received: 8 September, 1982)

ABSTRACT

The methodology of a monitoring study of the fauna and flora of the rocky intertidal zone in Bantry Bay and Dunmanus Bay in southwest Ireland is described and some representative results are presented. Fit'e shores,four in Bantry and one in Dunmanus Bay, were visited monthly from May, 1978 to June, 1980, and more limited monitoring is continuing. On each visit to each shore,four research programmes were undertaken--transect studies, recolonisation of cleared areas, barnacle studies and limpet studies. The transect methods are discussed in relation to other methods in current use. Combining community and key species approaches is shown to be most useful in monitoring programmes. The increased effectiveness of continuous shore monitoring when compared with one-off or before and after impact-type studies is stressed.

INTRODUCTION

Many marine communities are subject to natural fluctuations of varying magnitude and extent. Thirty-one years of continuous plankton record- ing (Colebrook, 1979), as well as a shorter time span for soft bottom fauna (Pearson, 1976) have demonstrated both intra- and inter-year fluctuations and also changes of much greater periodicity. Continuous

* Present address: The Salmon Research Trust of Ireland, Farran Laboratory, Newport, Co. Mayo, Ireland.

149

Marine Environ. Res. 0141-1136/83/0008-0149/$03.00 © Applied Science Publishers Ltd, England, 1983. Printed in Great Britain

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monitoring of the rocky intertidal is a more recent undertaking although limited numbers of algal and animal species have been studied for some years (Jones, 1974; Southward & Crisp, 1956; Southward, 1967; Lewis & Bowman, 1975). Crapp (1971) and Nelson-Smith (1972) have studied the rocky shore biota of Milford Haven, South Wales, during its development as an oil port and subsequent to two major oil spills. The protracted process of recolonisation of rocky shores in Cornwall, denuded by dispersants used to clean up oil from the Torrey Canyon, has also been described (Southward, 1979). The initiation, in 1974, of rocky shore investigations by the Coastal Surveillance Unit at University College, North Wales, marked the first concentrated attempt at regular monitoring of the natural fluctuations of the entire macro algal and animal community in these islands (Jones et al., 1979). The amounts of natural variation demonstrated by the results of the Coastal Surveillance Unit show the necessity of information of a continuous nature in order to distinguish cumulative changes due to human agency from natural events such as severe winter weather or dinoflagellate blooms (Cross & Southgate, 1980, 1981 ;Myers et al., 1980). As argued by Lewis (1976), drastic community changes due to heavy pollution will be immediately apparent, whereas those due to slight chronic pollution will be less easily detected.

From May, 1978 to June, 1980 continuous monthly monitoring was undertaken on five shores in southwest Ireland, four in Bantry Bay and one in Dunmanus Bay, and is continuing on a more limited basis. Several investigations have focused on this area since the setting up of an oil transhipment terminal in Bantry Bay in 1968. These studies have either been one-off general shore surveys (Crapp, 1973), impact studies (Cullinane et al., 1975; Baker et a l., 1981), or studies of individual species (Crapp & Willis, 1975; Thompson, 1979). The methodology used in the present survey is described and some representative results are given.

MATERIAL AND METHODS AND RESULTS

During the first four months of 1978 an extensive preliminary survey was made of the shores of Bantry Bay and Dunmanus Bay (Myers et al., 1978). Particular attention was paid to the sites visited by Crapp (1973) during his survey of the relationship between shore community and exposure to wave action. A large number of these sites have been

Ecological monitoring in the rocky intertidal 151

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Fig. I. Map of Bantry Bay and Dunmanus Bay showing the regular littoral sampling sites. I. Dereenacarrin. 2. Eagle Point. 3. Cooskeen Cove. 4. Pointabulloge. 5. Glengarrif

Castle.

resurveyed by the Oil Pollution Research Unit in 1975, 1978 and 1979 at the request of Gulf Oil Terminals (Irl.) Ltd (Baker et al., 1981 and unpublished results). On the basis of the preliminary survey, four sites in Bantry Bay and one in Dunmanus Bay were chosen for monthly monitoring (Fig. 1). These sites covered a range of exposure to wave action (Table 1). The site in Dunmanus Bay was chosen since it seemed unlikely that oil from a pollution incident in Bantry Bay would reach this shore.

T r a n s e c t s t u d i e s

On each shore a regular slope was chosen and a transect established from the base of the coloured lichen zone to approximately mean low water

TABLE 1 Details of the Littoral Sampling Sites

Site No. Location Degree of exposure to wave action

1 Dereenacarrin Exposed 2 Eagle Point Exposed 3 Cooskeen C o v e Semi-exposed 4 Pointabulloge Semi-exposed 5 Glengarrif Castle Very sheltered

152 T. F. Cross, T. Southgate

springs (MLWS). Stations were positioned at 30cm vertical intervals (approximately one-tenth of the tidal height) using a cross-staff (Moyse & Nelson-Smith, 1963). At each station the position of an 0.25m 2 quadrat was permanently marked using white road paint or masonry nails and nylon twine

The aim of this aspect of the programme was to detect community changes over time. For this reason, it was considered essential to place the quadrat in exactly the same position on each visit to the shore. This was necessary because of the patchiness of distribution of many species and the physical heterogeneity of the substrate. It was reasoned that an increase or decrease observed in a particular quadrat was indicative of similar increases or decreases in that species at that level on the shore in general. Experiments in Anglesey (Jones et al., 1979) and in simulated situations in the laboratory (J. M. Baker and D. H. Dalby, in preparation) have shown this assumption to be well founded.

Quadrat frames (50 x 50cm) were divided into twenty-five 10 x 10cm squares. Thus, each square decimetre represented 4 % of the area of the quadrat. Such an arrangement greatly increased the accuracy of estimates of percentage cover. All macro fauna and flora were recorded (Myers et al., 1979). All plants and certain animals were estimated as percentage cover, while other animal species were either counted in the whole quadrat or, in the case of barnacles and the smaller littorinid species, in certain selected decimetres. Certain animal species were recorded only as present or absent. Black-and-white photographs were taken monthly of all the quadrats and an overview of the shore at one location (Eagle Point) to compare with visual counts.

In the laboratory, the raw data were transcribed onto special sheets from which cards could be punched for computer storage. The methods of data processing used by the Coastal Surveillance Unit (Jones et al., 1979) were adopted and the data were processed in the University College, North Wales, computer laboratory with the assistance of the Coastal Surveillance Unit.

The programmes allow the data to be produced in the form of (i) transect Tables, which list all the species and their concentrations in the quadrats in which they occur, for one visit to the shore and (ii) species Tables, which list the concentration of one species over the transect for all monthly visits. The data, as contained in the species Tables, can be presented graphically in the form of computer-drawn contour diagrams. These are three-dimensional plots in which the position on the shore


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represents one axis, sampling data (the letters denote the month) another and the third dimension, represented by contour lines of different thicknesses, gives the concentration of the species in each quadrat. A typical contour diagram is shown in Fig. 2, which illustrates the changes in numbers of the in situ gastropod predator, Nucella lapillus, and its chief prey species, Balanus balanoides and Mytilus spp., on the exposed shore at Dereenacarrin during the first 15 months of the survey. N. lapillus is rather inactive in winter, forming dense clusters in crevices. It can be seen that, during the spring of 1979, it increased in numbers in the lower quadrats and began to move upshore, first reducing the numbers of Balanus balanoides, the preferred prey species, and, subsequently, the

154 T. F Cross, T~ Southgate

percentage cover of Mytilus spp. The possibility of confusing the results of such natural changes with the effects of pollution has been discussed by Myers et al. (1980).

Recolonisation of cleared areas

At quarterly intervals during the first 12 months of the survey squares of rock (0.25 m 2) were cleared of fauna and flora at three tidal levels on each shore (MHWN, MTL and MLWN). The aim of this programme was to study the pattern of localised community development (i)at different levels on the shore, (ii) at different times of year and (iii) on shores of different grades of exposure. It was hoped that this programme would provide information on the factors governing community composition and also have value in predicting the sequence and duration of recovery in the case of human-induced interference denuding an area of shore. The biotic composition of each area was assessed prior to clearance and each area was monitored on each subsequent monthly visit. In addition, black- and-white photographs were taken of all cleared areas on each visit to the shores. An example of the progress of recolonisation at MLWN at Eagle Point is given in Fig. 3. This sh~re was mussel dominated at this level, with a few limpets and isolated fucoid plants. The ephemeral green algae, Enteromorpha spp., quickly colonised areas cleared in the summer of 1978 and the spring of 1979, but did not appear in the areas cleared in the autumn of 1978 and the winter of 1978/79 until late spring. A rapid increase in limpet numbers was evident in the area cleared in the summer of 1978 and there was also a steady increase in mussel cover due to settlement and growth. By August, 1979, the biota of the area cleared in the summer of 1978 was typical of this level on the shore. In this example, it can be seen that the sequence and duration of recolonisation depended on the time of initial clearance relative to settlement and growth periods of both transitional opportunist species such as Enteromorpha spp. and of permanent cover species such as Mytilus spp.

Barnacle studies

At each site a vertical rock face which extended from MLWN to MHWS was chosen to study aspects of the ecology of the four littoral species: Chthamalus montagui, C. stellatus, Balanus balanoides and the in- troduced form, Elminius modestus. The Chthamalus species show an


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exposure gradient of abundance with C. montagui predominating in sheltered to moderately exposed conditions and C. stellatus being dominant in very exposed locations (Southward, 1976). Elminius modestus is currently spreading around Ireland (Crisp & Southward, 1959; Crapp, 1973) but favours sheltered and silty conditions where it tends to replace Balanus balanoides.

The experimental programme was initiated by clearing.10 x 10cm squares at 30 cm vertical intervals from M L W N to MHWS. A marked area immediately adjacent to the first cleared area at each level was chosen as an undisturbed control and was monitored monthly. At each monthly

]56 T. F, Cross, T. Southgate

visit an addit ional area was cleared if sett lement had occurred during the previous mon th or was anticipated dur ing the next month. By May. 1979, there were between four and twelve cleared areas at different levels on the various shores. The p rogramme was modified after 12 months to exclude every second level on each shore. Thus, areas at 60 cm vertical intervals were moni tored for the remaining months . Barnacles were counted in the entire 100 cm 2 if less than a hundred were present or when more abundant by ten r a n d o m counts with a I cm 2 quadrat and the approximate number 100cm -2 calculated. Species were recorded as cyprids, spat (plates undifferentiated), juveniles (up to 1 year old where recognisable) and adults. In addit ion, the numbers of gastropods and other species occurring in the sampling areas were recorded. Particularly relevant were the numbers of limpets and dogwhelks since the grazing activities of the former can displace recently settled barnacles and the latter is the chief in

si tu predator of barnacles (Connell, 1961). The study aimed at investigat- ing settlement, growth and mortal i ty of the four barnacles species relevant to (i) exposure and aspect of shore, (ii) tidal level, (iii) presence of other members of the same species of the same or previous year class, (iv) presence of other barnacle species, (v) presence of other species such as limpets or dogwhelks and (vi) pollut ion or other catastrophic events such as dinoflagellate blooms (Southward, 1979; Cross & Southgate, 1980,

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1981) or extreme weather (Crisp, 1964). Black-and-white photographs were taken of all study areas on each visit to a shore.

When processing the data for the first 16 months, an averaging system was used. In this system the monthly mean figure was taken for each level for the particular species in all squares available during this period of settlement. This system greatly increased the sampling area. It did, however, cause a spurious drop in numbers during the last month of settlement when an area with very few spat was included in the calculation. Thereafter, the same squares were always included. Results of this system for the 1978 C. montagui cohort and the 1979 B. balanoides cohort at Pointabulloge are shown in Fig. 4. Level 1 (MHWS) is not included since only minimal numbers of C. montagui adults were present and no spat were visible. Level 3 was just below MHWN, level 5 at about MTL and level 7 above MLWN.

Limpet studies

At each site, permanently marked quadrats were established at MLWN, MTL and MHWN in which limpet numbers and population structure were monitored. These quadrats measured 50 x 100cm in areas where limpet numbers exceeded 200 m-2, but 1 m 2 where concentrations were lower. At each monthly visit, the shell lengths of limpets within the quadrats were measured to the nearest millimetre with vernier calipers. In addition, a description of the fauna and flora of each quadrat was recorded, since it has been shown that local habitat changes can alter the concentration and population structure of limpets (Lewis & Bowman, 1975). Data on the biota of the permanent quadrats monitored for limpets also complimented the transect data. This programme aimed at monitoring Patella vulgata populations. A complication was the occurrence of Patella aspera at lower levels on the more exposed shores (Thompson, 1979). Individuals of the latter species were provisionally identified in situ on the basis of shell characteristics and excluded from the counts. This method, while effective with younger individuals, was probably less so with adults (Ebling et al., 1962). Additionally, the proportion of the 1979 cohort of P. aspera amongst all the smaller limpets on the lower shores at the more exposed locations was estimated in November, 1979 by destructive sampling and confirmed in situ identification.

The limpet length frequency data were stored on computer tape. They can be reproduced as Tables of length frequencies or as length frequency

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histograms (Fig. 5). Limpets cannot be aged by meristic characteristics, but examination of polymodal length frequency histograms from regular monitoring allows the younger age groups to be identified and their progress followed through time. In Fig. 5 it can be seen that the 1978 cohort averaged 5 mm shell length in May, 1978 and increased in strength as juveniles emerged from rock crevices. By August, modal length was 7 mm and the number of the youngest cohort was greatest, showing a decline thereafter. Growth slowed during the winter, but increased again in the spring, and a modal length of 11 mm was reached by July, 1979. The 1979 cohort was much weaker and, probably due to the cold winter, averaged only 3 mm in May. The population observed in Fig. 5 is typical of limpet populations from exposed shores in that it consists mainly of juveniles (Lewis & Bowman, 1975).


Mussel studies

Mytilus edulis and M. galloprovincialis are abundant from MTL to MLWN on exposed shores in southwest Ireland. In order to pinpoint times of settlement and aspects of the population structure of these species, samples (5 x 5 cm) were taken monthly from MTL and MLWN adjacent to the transect at Eagle Point. Samples were stored at 4 °C in 70 % ethanol and shell length was later measured. The smallest length category included in the measurements was < 1 mm (0.3-0.7 ram) while larger individuals were measured to the nearest millimetre. This smallest category was included because mussel plantigrades are known to settle at 0.3 mm and to grow to almost 1 mm in 1 month (Seed, 1969). Thus, by including this category, settlement could be monitored. From the length frequency histograms it could be seen that the largest settlement at MLWN occurred in January, 1979 and was probably a secondary settlement whereas, as MTL, peaks of settlement were seen in June, 1978 and, to a lesser extent, in January, 1979, probably representing primary and secondary settlement, respectively (Seed, 1969).

In addition to the studies mentioned above, surface water temperature and air temperatures were taken at each site on every visit, to augment data from the nearest meteorological station at Valentia, Co. Kerry (50 km to the north).

DISCUSSION

There has been considerable discussion as to the most effective type of transect for use on rocky shores (Baker & Dalby, in preparation). The method of Moyse & Nelson-Smith (1963) was developed in Milford Haven, South Wales, used by Crapp (1973) in Bantry and is the routine method of the Oil Pollution Research Unit (Baker et al., 1981). In that method, stations are positioned at one-tenth of the tidal height using a cross staff levelling device and then an area extending 1 m above and below the mark and 5 m wide is scrutinised and animals and plants ranked according to abundance scales. This operation is sometimes assisted by the use of random quadrats. In the method employed by the Coastal Surveillance Unit (Jones et al., 1979), a continuous belt of 0.25m 2 quadrats is run over the entire tidal range and percentage cover values are estimated using a 25-point pin frame, with a 100-point frame being used

16(! ] F. Cross, T. Southgate

every five quadrats to increase accuracy. All plants and many animal species, including barnacles, are assessed in terms of percentage cover and then converted to abundance values to maintain congruity with numerical data from other animal species. It is suggested that while the former method gives a more adequate description of the shore biota in general, it is less useful for continuous monitoring or, indeed, for before and after studies, since its accuracy when attempting to detect change in such a large area is much lower than the latter method. The placing of random quadrats (Clancy & Kelly, 1980) is considered to be an even less accurate method of assessment of change. The method used in the present survey was designed to incorporate fixed quadrats but at one-tenth of the tidal height instead of a continuous belt, thus reducing the time taken to complete this phase of the programme. The use of strung quadrat frames to assess percentage cover is similar in accuracy to a 25-point frame iT. F. Cross & T. Southgate, unpublished results) but less accurate than a 100- point frame, which, in turn, is too time-consuming to use over entire shores. There was no visual indication of change of shore biota adjacent to experimental areas due to trampling as had been observed in some areas by the Coastal Surveillance Unit (Jones et al., 1979) and great care was exercised to prevent undue disturbance of experimental areas. The counts made of individual animal species in the present study were not converted to abundance values. It was felt that numerical values gave more accurate representation for individual species. It is. however, recognised that conversion to abundance values may be necessary for community analysis. It is reasoned that changes occurring on the shore at a particular level will be reflected in the quadrat, provided it is accurately positioned on each visit. Exceptions would be for species such as mussels which might sometimes occur in varying mosaic patterns (Seed, 1969). The usefulness of the method described here for detecting change was confirmed by the fact that the five sites showed many similar trends. In addition, species trends similar to those observed on the transects were detected in the limpet and barnacle studies with their attendant faunal and floral investigations (Myers et al., 1979).

The limpet and barnacle investigations included in the present survey were designed to conform with the COST 47 Littoral programme. Considerable information relevant to this programme has already been collected for the limpet, Pate l la L, ulgata, in Scotland, northeast England and southwest England (Bowman & Lewis, 1977) and Anglesey (Jones et al., 1979).


Continuous monitoring, as described in this paper, is felt to be superior to before and after impact-type studies in terms of recognising and understanding natural or human-induced changes. For this reason, even though formal funding ended in November, 1980, certain aspects of the research are being continued. The combination of community and key species approaches (Lewis, 1976) is designed to lend power to the method.

Constant monitoring provides an early warning of environmental deterioration and produces continuous background data against which impact information in relation to developments can be assessed.

ACKNOWLEDGEMENTS

We thank Dr A. A. Myers for advice and discussion. The financial support of the National Board for Science and Technology is gratefully acknowledged.

REFERENCES

Baker, J. M., Hiscock, S., Hiscock, K., Levell, D., Bishop, G., Precious, M., Collinson, R., Kingsbury, R. & O'Sullivan, A. J. (1981). The rocky shore of Bantry Bay: A re-survey. Ir. Fish. Invest., Ser. B, 23, 27pp.

Bowman, R. S. & Lewis, J. R. (1977). Annual fluctuations in the recruitment of Patella vulgata L. J. mar. biol. Ass. U.K., 57, 793-815.

Clancy, J. A. & Kelly, D. (1980). Environmental aspects of Killala Bay, Co. Mayo. Ir. J. Envir. Sci., 1, 57-64.

Colebrook, J. M. (1979). Continuous plankton records monitoring the plankton of the North Atlantic and the North Sea. In: Monitoring the marine environment (Nichols, D. (Ed.)). London, Institute of Biology.

Connell, J. H. (1961). The influence ofinterspecific competition and other factors on the distribution of the barnacle, Chthamalus stellatus. Ecology, 42, 710-23.

Crapp, G. B. (1971). In: The ecological effects of oil pollution on littoral communities (Cowell, E. B. (Ed.)). London, Institute of Petroleum, 102-13.

Crapp, G. B. (1973). The distribution and abundance of animals and plants on the rocky shores of Bantry Bay. Ir. Fish. Invest., Series B, 9, 1-35.

Crapp, G. B. & Willis, M. E. (1975). Age determination in the sea urchin Paracentrotus lividus, with notes on the reproductive cycle. J. exp. Mar. Biol. Ecol., 20, 157-78.

Crisp, D. J. (1964). The effects of the severe winter of 1962-63 on marine life in Britain. J. Anita. Ecol., 33, 165-210.

162 T. F. Cross, T. Southgate

Crisp, D. J. 8~; Southward, A. J. (1959). The further spread ofElmmius modestus in the British Isles to 1959. J. mar. biol. Ass. U.K., 38, 429-37.

Cross, T. F. & Southgate, T. (1980). Mortalities of fauna of rocky substrates in south-west Ireland associated with the occurrence of Gyrodinium aureolum blooms during Autumn, 1979. J. mar. biol. Ass. U.K., 60, 1077-9.

Cross, T. F. & Southgate, T. (1981). Observation on plankton blooms in Bantry Bay and Dunmanus Bay during August, 1978. In: Red Tide Seminar (Parker, M. (Ed.)). Dublin, Fisheries Seminar, 9-11

Cullinane, J. P., McCarthy, P. & Fletcher, A. (1975). The effect ofoil pollution in Bantry Bay. Mar. Poll. Bull., 6, 173--6.

Ebling, F. J., Sloane, J. F., Kitching, J. A. & Davies. H. M. (1962). The ecology of Lough Ine. XII. The distribution and characteristics of Patella species. J. anita. Ecol., 31,457-70.

Jones, W. E. (1974). Changes in the seaweed flora of the British Isles. In: The changing flora and fauna of Britain (Hawksworth, D.L. (Ed.)). London, Academic Press, 97-113.

Jones, W. E., Bennell, S. J., Beveridge, C. M., Cooke, F. P., Fletcher, A., McConnell, B. J., Mack-Smith, S. & Mitchell, J. S. (1979). The Fifth Report of the Coastal Surreillance Unit. Bangor, University College of NOrth Wales, 184 pp.

Lewis, J. R. (1976). Long-term ecological surveillance: Practical realities in the rocky littoral. Oceanogr. Mar. Biol. Ann. Rer., 14, 371-90.

Lewis, J. R. & Bowman, R. S. (1975). Local habitat-induced variations in the population dynamics of Patella rulgata L. J. exp. Mar. Biol. Ecol.. 17, 165- 203.

Moyse, J. & Nelson-Smith, A. (1963). Zonation of animals and plants on rocky shores around Dale, Pembrokeshire. Field Studies, 1, 1-31.

Myers, A. A., Cross, T. F. & Southgate, T. (1978). Bantry Bay Survey, Zoology Dept., University College, Cork. First Annual Report, 25 pp.

Myers, A. A., Cross, T. F. & Southgate, T. (1979). Bantry Bay Survey, Zoology Dept. University College, Cork. Second Annual Report, 124 pp.

Myers, A. A., Southgate, T. & Cross, T. F. (1980). Distinguishing the effects of oil pollution from natural cyclical phenomena on the biota of Bantry Bay, Ireland. Mar. Poll. Bull., 11,204-7.

Nelson-Smith, A. (1972). Oilpollution and marine ecology. London, Paul Elek. Pearson, T. H. (1976). The benthic ecology of Loch Linhe and Loch Eil, a sea-

loch system on the west coast of Scotland. IV. Changes in the benthic fauna attributable to organic enrichment. J. exp. mar. Biol. Ecol., 20, 1- 41.

Seed, R. (1969). The ecology of M. edulis on exposed rocky shores. II. Oecologia, 3, 317-50.

Southward, A. J. (1967). Recent changes in abundance of intertidal barnacles in south-west England: A possible effect of climatic deterioration. J. mar. biol. Ass., U.K., 47, 81-95.

Southward, A. J. (1976). On the taxonomic status and distribution of Chthamalus stellatus (Cirripedia) in the North Atlantic region: With a key to the common intertidal barnacles. J. mar. biol. Ass., U.K., 56, 1007-28.


Southward, A. J. (1979). Cyclical fluctuations in population density during eleven years recolonization of rocky shores in West Cornwall following the To rrey Canyon oil spill in 1967. In: Cyclical phenomena in marine plants and animals (Naylor, E. & Hartnoll, R. G. (Eds)). Pergamon Press, Oxford and New York, 93-100.

Southward, A. J. & Crisp, D. J. (1956). Fluctuations in the distribution and abundance of intertidal barnacles. J. mar. biol. Ass., U.K., 35, 211-29.

Thompson, G. B. (1979). Distribution and population dynamics of the limpet Patella aspera (Lamarck) in Bantry Bay. J. exp. mar. Biol. Ecol., 40, 115-35.

an approach to ecological monitoring in the rocky intertidal: a survey of bantry bay and dunmanus...

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