Estuarine and Coastal Marine Science (1976) 49 65-70

Numerical Analysis of Subtidal Communities on Rocky Shores

S. A. Prenticea and Joanna M. Kain (Mrs N. S. Jones) Department of Marine Biology, University of Liverpool, Port Erin, Isle of Man Received 15 Murch 1975

Samples of epibenthic flora and fauna were taken nt various depths from the immediate subtidal zone of a rocky shore using a modified portable airlift in conjunction with a conventional collecting bag and analysed by objective, numrrical methods. Roth non-weighted (i.e. using presence/absence data only) and abundance wcightcd (i.e. quantitative) similarity cocficients were applied in their simplest forms and the results displayed by use of simple dendrograms. Three distinct depth-zones were characterized, thus confirm- ing previous suhjectire observations. The results show that simple mlmerical methods can bc applied to yield meaningful results from hctcrogcncous communities.

Introduction

Until the advent of the aqualung very little work was carried out in the immediate subtidal zone, the most notable being that of Kitching et al. (1934). Methods involving remote sampling (grabs and dredges) inevitably introduce considerable errors, so that by allowing direct sampling and observation the use of the aqualung permits greater accuracy.

Early surveys using the aqualung were qualitative. For example, off the Isle of Man, where the present investigation took place, Kain (1960) listed the species of algae present at each depth at IO rocky sites around the coast. In most sites it was observed that immediately below low water the vegetation was dominated by Laminaria hyperborea, followed in deeper water by a zone dominated by Saccorhixa polyschides and below this an open community often somewhat bare of algae. It was later shown (Jones & Kain, 1967) that in at least one site grazing by fkhinus esculentus was responsible for the relatively bare zone. ‘l’he purpose of the prcscnt study was to apply a more objective method and analyst the spatial distribution of organisms at one of these sites.

Objective, classificatory methods such as those used by tcrrcstrial phytosociologists have shown considerable promise when applied to soft bottom faunal communities (Day et al., 1971; Field, 1971; Hughes & Thomas, 1971) and involve the calculation of similarity cocffi- cients between all possible pairs of samples or individuals.

Since the prcscnt data were primarily hcterogcncous (any one sample contained only a small proportion of the total number of species found) use was made of Jaccard’s and Czckanowski’s cocfficicnts which ignore joint absences in the data. Jaccard’s coefficient is the number of species common to both samples expressed as a percentage of the total number of species in both samples.

JC.-GC x rooyo a-\-b-c

“Present addrex: Department of Industrial Science, CJniversity of Stirling, Stirling, Scotland.

65

66 S. A. Prentice &‘J: M. Kain

where a=number of species in sample A. b=number of species in sample B. c=number of species common to both samples.

Using the same notation, Czekanowski’s coefficient (later attributed to Sorenson) is :

C&Z x mq,& a+b

That is the number of species common to both samples expressed as a percentage of the mean number of species in both samples.

A reliable indication of communities may be gained from an analysis of the abundant species only, especially if an abundance weighted coefficient is used (Day et al., 1971). Czekanowski’s coefficient is easily weighted for abundance, effectively becoming:

cc(w)=(s,$s*+&. . . . . . . . . .fS,) where S,=percentage biomass of species A common to both samples

S,=percentage biomass of species N common to both samples. The resultant matrix of similarity coefficients must then be summarized in a meaningful

and explicit manner. Simple dendrograms constructed by single link sorting methods, as described in Williams et al. (1966), were felt to be most appropriate to hand calculation.

Methods

The collection of the test samples involved the use of a self-contained airlift operated by aqualung divers. The airlift was an adaptation of one used for soft bottom sampling (Brett, 1964; Barnett & Hardy, 1967) and used a modified single hose demand valve, the flow of air being controlled by the valve purge button. The air cylinder was strapped to the airlift tube to provide a completely self-contained unit.

Twenty-three samples from horizontal, upward facing surfaces were taken along a transect (at Lat. 54’05’N, Long. 4”46’W.) to II m below lowest astronomical tide (LAT), two for each metre depth interval with three between 6.0-6.9 m and 7.0-7.9 m and only one between 80-8.9 m. The samples were delineated by a quadrat frame measuring 0.5 m by 0.5 m with one side missing to facilitate positioning. The depth was measured using a fibreglass tape measure attached to a large polystyrene float on the surface (correcting to LAT by reference to tide tables and the local tide pole showing the height of water above LAT at any one time). The macroflora and fauna were removed with a knife and placed in a large collecting bag, the remaining biological material being scraped from the rock surface with a 5 cm wide paint scraper and collected in a small net bag (mesh size I mm) attached to the upper end of the airlift tube. When all visible biota had been removed this bag was transferred to the appropriate collecting bag. Sampling time was approximately 5-10 min depending upon conditions.

Upon return to the laboratory the samples were emptied into large flat dishes and the material identified and weighed after removal of surface water. No attempt was made to quantify the many encrusting Bryozoa, Annelida and Hydrozoa found.

A total of 75 species were identified of which 17 were present in one or more samples with a biomass in excess of 6.0 g/m2. The percentage of each sample occupied by each of these 17

Numerical analysis of subtidal communities 67

species was then calculated (after pooling the replicate samples) and the matrices of similarity coefficients constructed. For the purpose of abundance weighting, biomass below 6.0 g/m2 was awarded the arbitrary value of 0.1 o/O similarity.

Both ‘R’ analyses (comparing the species using their presence or absence in the samples as attributes) and ‘ Q’ analyses (comparing samples using the presence or absence of species as attributes) were carried out on the data.

The nomenclature follows that of the Marine Fauna of the Isle of Man (Bruce et al., 1963) and the latest algal check-list (Parke & Dixon, 1968).

Results

The zonation with depth is most readily apparent from the abundance weighted analysis [Figure r(a)], using Czekanowski’s coefficient weighted for abundance. A similar but less distinct grouping is obtained using Jaccard’s coefficient [Figure I(b)]. Three are distinguish- able, an upper zone from o-4.9 m below LAT, a central zone from 5.0 m and 7.9 m below LAT and a bottom zone from 8.0-1 I m below LAT.

The R analyses were less consistent, giving rather different results with different coeffi- cients. Using Czekanowski’s coefficient weighted for abundance [Figure I(C)] only one group, containing Laminaria hyperborea, Membranoptera alata, Debxaria sanguinea, Odonthalia dentata and Pilumnus hirtellus, is decisively indicated. The use of Jaccard’s coefficient [Figure r(d)] separates two groups, one containing Pilumnus hirtellus, Plocamium cartilagi- neum, Phycodrys rubens and Membranoptera alata which appear to be restricted to the upper zone described above, the others containing Laminaria hyperborea, Callophyllis laciniata, Cruoria pellita, Odonthalia dentata and Saccorhiza polyschsdes which appear to be restricted to the upper two zones.

Discussion

In such an investigation as this it is very difficult to draw any firm conclusions without a clear idea of the inherent limitations and accuracy of the methods employed. The sampling process used may be divided into four sections: sample selection, macrosampling, micro- sampling and analysis. Sample selection introduces errors due to the extreme difficulty of random sampling underwater. Restrictin g samples to horizontal, upward-facing surfaces means that some species such as Alcyonium digitatum and Echinus esculentus which seem to favour more vertical surfaces may be partially excluded. Macrosampling introduces few quantitative errors and these tend to be insignificant in view of the large biomass of the sample. Fragmentation of Laminaria hyperborea holdfasts, particularly in older individuals may cause some loss of material as may the gelatinous secretions from the cut surfaces but again these are usually small. Microsampling (collection of all other biological material using the airlift) however, involves errors that are both large and variable. These arise from the difficulty of removing all material, much of which may not be visible underwater. The efficiency of the airlift (and thus of collection) depends upon the relative pressure differential between the ends of the airlift tube and it is thus less effective in deeper water, while any swell or current increases handling difficulties, thus reducing efficiency. Damage due to abrasion and turbulence in the collecting bag can make identification of some algal fragments im- possible without considerable knowledge of the cellular morphology of the algae in question.

Thus the errors introduced in microsampling are considerable and the method can only be considered quantitative for species of reasonably high biomass. For a semi-quantitative or

68 S. A. Prentice M J. M. Kain

qualitative study however, this method is probably more successful than those used pre- viously. For example, the use of a hand scraper with collecting bag attached (Kain, 1960) would not collect all the fauna. The collection of small boulders with the flora still attached

Ib) I 1

r

(a)

=-I

9 10 11 7 6 6 5 3 1 2 4 1 2 6 11 9

r GFDHPOAIMCBQLNEKJ M KO N HLJBCEDGOPFIA

Figure I. Dendrograms of percentage similarity between samples and species. (See text for further details.) Depths [(a) and (b)] are lower limit of depth interval in metres below LAT. Species [(c) and (d)] as follows: A Calliostoma zizyphinum; I3, Callophyllis laciniata; C, C ruoria pellita; D, Cutleria multi’da; E, Delesseria sanguinea; F, Desmarestia aculeata; G, Echinus esculentus; H, Gibbula cineraria; I, Gibbula umbilicalis; J, I,aminaria hyperborea; K, Membranoptera al&a; L, Odonthalia dentata; 31, Phycodrys rubens; N, Pilumnus hirtellus; 0, Plocamium cartilagineum; P, Saccorhiza polyschides; Q, Rhodymenia palmata.

Numerical analysis of subtidal comminuties 69

(Edelstein et al., 1969) would also result in the loss of some epifauna and the method was in any case impracticable in the present study in view of the large size of the boulders observed. It is thus felt that the method used here represents a considerable increase in collection efficiency of both flora and fauna over any method used previously. The high suction pressure generated by the airlift allows little loss of material, even small fish and free-swimming crustacea being successfully collected.

Differences in the groupings using the two different coefficients are inevitable due to their different parameter emphasis. It would seem that while the use of an abundance weighted coefficient is advantageous for a Q analysis the use of a non-weighted coefficient is more suited for the corresponding R analysis in terms of separation efficiency. However, by combining the results of both analyses it is possible to confirm quantitatively the presence of the three zones which were described subjectively by Kain (1960). These can now be charac- terized as follows :

Zone I (o-4.9 m below LAT)

The Laminaria zone, distinguished by the presence of Laminaria hyperborea, Odonthalia dentata and Delessaria sanguinea in quantity and the presence alone of Phycodrys rubens, Plocamium cartilagineum, Membranoptera alata and Pilumnus hirtellus.

Zone 2 (5-7.9 m below LAT)

The Saccorhiza zone, distinguished by the presence in quantity of Saccorhixapolyschides and Desmarestia aculeata.

Zone 3 (&II m below LAT)

The Echinus zone, characterized by a general lack of species except Echinus esculentus which may be present in considerable numbers.

Acknowledgements

We should like to express our thanks to Mr M. Bates for his invaluable assistance under- water during sampling and the crew of the M.V. Silver Spray for their aid on the surface.

References

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70 S. A. Prentice &:‘J. M. Kain

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