The sublittoral ecology of the menai strait

Estuarine and Coastal Marine Sdence (x979) 9, 663-675

The Sublittoral Ecology of the Menai Strait" I. Temporal and Spatial Variation in the Fauna and Flora along a Transect

R. Hoare a Marine ,Science Laboratories, Menai Bridge, Anglesey, Gwynedd, U.K.

and M. E. Peattie Ministry of Agriculture, Fisheries & Food, Great Westminster House, Horseferry Road, London, O.K. Recdved 5 ffune I978 and in revised form I I September I978

Keywords: sublittoraI organisms; ecology; spatial distribution; temporal variation; tidal currents; current meters; Menai Straits. The Menai Strait is a narrow channel subject to strong tidal currents but little wave action. The major physical factors which interact to control the nature and distribution of the fauna and flora are turbidity, scouring, current strength and substratum. A simple device, based upon a spring balance, was used to measure maximum current strength near the bottom. The distribution of the fauna was studied by a transect survey across the Channel and compared with a similar survey in the same place 20 years earlier. The results suggest a high biological stability although some population fluctuations do take place. It is suggested that such general surveys be supplemented by more detailed investigations of individual species.

Introduction In x956 Knlght-Jones (Knight-Jones & Nelson-Smlth, 1976 ) surveyed a transect across the NIenai Strait. Almost exactly 2o years later this transect has been re-surveyed using much the same methodology. When searching for changes in the biota which may have occurred over the last two decades it was vital to return to the same line. This was possible because the original transect followed the course of an old telegraph cable which is still in position today (Figure I). As shown in Figure x the situation of the transect is unusual in that the middle section of the Menal Strait is very narrow and largely sheltered by the land from all winds. Consequently, wave-action is negligible although there are extremely strong tidal currents of up to 4"5 m s - t at tl{e surface. T h e other major physical factor is the large amount of suspended detritus generally present in the water. According to Buchan et aL (1967 & x973) turbidity is dependent primarily upon temperature and tidal strength. This was substantiated by Kenchington (x97o) who suggested furthermore that the detritus contained sewage effluent as well as other material of terrestrial origin. Certainly, detritus levels are generally higher in the winter as well as on spring tides when currents are greatest. As a result of the high suspended load, light penetration is poor and the algal zone is very narrow. aPresent address: Division of Fisheries and Marine Science, University Pertanian Malaysia, Serdang, Selangor, Malaysia.

663

664

R. Hoare ~ .~. E. Peattie

i

4*W

Anglesey -53 c

Sampling 9 station =--, Main flood 9---* Main ebb

N

// \

--

6..__.m ~.

. , , . . " ' ~ t ~ M ena i

v Suspension Bridge

Figure x. T h e location of the transect in the Menai Strait. T h e position of sampling stations x and 36 are marked at opposite ends of the teleg,~aph cable.

Methods Standard aqualung equipment was used throughout. Prominent species, most of which could be readily identified in situ, together with observations of bottom-type and topography were dictated into an underwater tape-recording system. Profiling was carried out after the manner described by Kain (I962), using a calibrated depth gauge and a 3 ~ m fibre tape to measure distance along the bottom. All depths were later corrected to chart datum. Sampling stations were marked with large wooden labels numbered with paint and coated in vaseline to inhibit fouling. The labels were tied onto the telegraph cable at 5 m intervals all the way across, and each end of the transect was marked with a buoy. The data were computed using a classificatory technique based upon the Index of Affinity described by Bray & Curiis (z957). Qualitative data were employed and pairs of stations were clustered according to the Group Average strategy described by Lance & Williams (i966). An Algol computer programme (U.C.N.W.G.A. Clust) was used to give the clustering levels and the dendrograms were drawn by hand. Species nomenclature is largely according to the Plymouth ~{arine Fauna (1957). Measurements of maximum current speed at different points along the profile were made with a simple device based upon the 'dynamometer' of Jones & Demetropoulos (1968). As shown in Figure 2 a small plastic drogue was attached to a spring balance which was chained to the telegraph cable. The drogue was buoyed-up with a polystyrene pellet float

Subllttoral ecology of the Menai Strait

665

Polystyrene float

disc Nylon / line

,

..9

Current

;

,~-, ~ti

A, I

50 cm t

I"~qiL~ ~ Screw Pointer ~

Slider

Im~/Cham

Figure z. (a) The simple device used to measure maximum current speed consisting of a spring balance with a sliding pointer operated by a small drogue. (b) The current meter in position measures the current flow at heights of less than 50 cm above the bottom.

to prevent it from fouling on the bottom at slack water. At each of three sites two such current measuring devices were placed just far enough apart to prevent them from fouling each other and left in position for two tidal cycles. T h e balances were set at zero after placement at slack water and 2 4 h later the readings were taken and the balances moved to the next three stations and reset. A total of I z stations were worked over periods of spring tides (taken as those with a height greater than 9.z m above chart datum) while measurements on neap tides (those less than 7"9 m above c.d.) were confined to three stations. T h e readings on any of the devices which had become fouled with large pieces of weed were ignored. All the balances were checked with a series of known weights to ensure that they weighed accurately for the range of values obtained. Calibration was carried out against a com'entional ' O n o ' current meter towed simultaneously from a boat at different speeds. Although the spring balance current meters only estimated maximum current speeds, they had certain advantages over conventional types. Firstly, they were inexpensive and simple to construct. Secondly, they were small and light, but robust, being easy to read, reset and relocate by a diver. I n addition, as seen from Figure 2, because they had an overall length of less than 5 ~ cm they were able to measure cur?ent to within xo cm of the bottom 9 T h e maximum error on two simultaneous readings was zo% but in most cases readings differed by no more than z %.

666

.R. Hoare ~ M. E. Peattie

Hel/chondr/epen/ce, IO~

0...

Stations I

5

I0

15

20

25

30

:35

40

12.5 m

Spring:

2.9

2.7

2.9

2.9

3"0

3.0

0.7

Neopt

3.0

3.0 3,0

2.B

m/s

1,0

C~ Halichondr/a

: : : : : Sh. gravel

Bedrock

i

9 Boulders

9.* ~ 0 ~-.~ I

2.8

1.6

I

I

I

i

I

i

I

5

7

9

II

13 15 17

i

I

i

I

I

Stones I

i

I

i

i

I

!

!

I

I

!

!

3O 2O

I

3

19 21 2 3 2 5 27 29 31 33 35 37 39

Figure 3. A depth profile of the transect showing the distribution of substrate-type and maximum current speed. The total number of species at each station is shown below.

Results Bottom type and topography The distribution of substratum type along the transect is shown in Figure 3- Stations z and 2 are situated on art outcrop of bedrock, with some small stones and occasional boulders at station I. Stations 3 to 8"consist mostly of small stones together with some shell gravel and occasional rock outcrops. Between stations 9 and I2 boulders and bedrock outcrops are more frequent while stations 14 to z7 are almost entirely mobile coarse shell gravel, which overlies the bedrock to a depth of up to 25 cm. The cable at station 17 is occasionally buried by the gravel but in places boulders remain protruding above it. Bedrock with some pockets of small stones and gravel extends between stations 19 and 22, giving way at station 23 to a predominantly boulder Slope with a dense cover of Ilalichondria panicea. The extent of this is shown separately in Figure 3. From stations 3~ to 35 bedrock and small stones predominate although there are still some boulders. At station 35 the slope flattens out and the Laminaria

Sublittoral ecology of the 2~lenal Strait

667

zone begins. Small stones and boulders occur right up into the intertidal zone. It should be emphaslsed that this bottom type and topography is not typical of the Menai Strait in general. Indeed the whole area is extremely variable, ranging from shelly mud to a complete cover of bedrock.

Current rdglme Also shown in Figure 3 are the values obtained for maximum current speed near the bottom at different points along the profile. It is clear that on spring tides, although the maximum current speed of 3"o m s-x occurs in the centre channel, this value is only slightly higher than those obtained elsewhere on the profile. However, during neap tides not only is the maximum current in the centre channel about half the spring tide value but the current is attenuated much more at the sides of the Strait. On the north side particularly, the current is reduced to a maximum of only 0. 7 m s -1, less than half the value for the centre channel. Thus, during neap tides especially there appears to be a 'current shadow' on the north side of the profile, the reason for which is clear from Figure I. It can be seen that the northern stations (1 to 12) are afforded some degree of shelter from the main current by the rock outcrops which support the bridge stanchions. This is achieved not only by isolating the stations from the main flow in the channel at low water (when the rock dries) but also by providing frictional resistance to current flow especially in the shallower water of neap tides. Consequently, although the maximum current (spring tide values ) varies little across the transect, the duration of maximum flow is reduced considerably on the north side. A current speed of 3.0 m s -x near the bottom corresponds to 4.2m s -1 at the surface. At all points along the profile, currents flow at approximately right-angles to the cable, that is in a north-east/south-west direction as shown by the arrows in Figure I, although at certain states of the tide in the proximity of stations I to 12 eddies are formed which result in a counter flow to that of the main channel.

Suspended material The amount of suspended particulate matter in the ~{enai Strait was not measured in the present survey but general levels are described by Buchan et aL (1967, 1973). However, some of the material drifting close to the bottom was collected in a fine plankton mesh attached to the cable. The major constituents were found to be 'organic debris', small mineral particles and diatoms ~iiss R. Green, personal communication). Other sestonic elements such as crustacean exuvia were present in various quantities and the whole gave a percentage weight loss upon ignition of 9"5% which is a measure of the total organic content. The strong currents in the area of the transect result in most of the particulate matter remaining in suspension except at slack water periods when the larger particles begin to settle out. However,. apart from sheltered microhabitats, such as Lamhtaria holdfasts and amongst stones where a little silt collects, there is no permanent siltation.

Spatial attd temporal variation in the fauna and flora The distribution, based upon presence/absence, of all the prominent species found in the present survey is shown in Tables I and 2 together with the distribution of the same species as recorded by Knight-Jones in 1956. Knight-Jones (Knight-Jones & Nelson-Smith, 1976) had a different number of stations but his station nomenclature has been altered to correspond as closely as possible with the stations in the 1976 survey. The distribution of total numbers of species at each station along the profile is shown in Figure 3 in relation to current speed, topography and bottom-type. The percentage cover of Halichondrla panicea is included in

668

R. tfoare ~ M . E. Peattle

TABLE x. Distribution of prominent flora and sessile fauna along the transect. The :976 data are from all Stations. T h e r956 data are from Stations x, z, 4, 6, 8, ~z, ~6, ~ o , ~4, z8, 3~, 36, 38. STAT~CtqS i

I

I

I

0

1

1

1

~

I

I

I

I

I

I

I

i

I

I

I

I

I

I

I

I

I

I

I

I

i

I

I

i

I

I

I

I

9

0 ~956 X 1976 9 19.56 & 1<)76

HL~;Cs

S~ECXPJ Olva lactuca

X I

9

X

Cladop.~ora r u p e s t r l l

0

9

X

~]-~ra~s~spl~moea I ~ m i z A r i a aacchAri~a

X 0

l ~ 4 ~ r i a dIKitata*

X X X X X X X

DIC t y o t a dtchotonm

X

~alid~s sillquosa

X I

Asc o ~ y l l u ~

X X IXIXX

X

nodoau~

Lithothaanlum ep.

i X

Deleseeria s a n ~ u i n e a

9 9 9 Z X Z X

B~odo=,elJ~ conte~voides

X X

0

IX

0

XXXXOX

X

OXI 0

O r i f f i t h ~ i a K].oscu~.os~ X I

PhyllopJ~ora trailli

9 X 9 X X X X 9 9 X

~ocaaiu~ ccrttla~tneu~

~ X

Leucoaolenta eoe~llcata

0

RyCOB r

t~l

0

XX

0

XXXXXXXXXXX

~hAlec

XX

Halichon~a ~cea

s

0

X

0

XXXXIXIXX

0

IXIX

0

OXO

0

IX X

X X

XXXXXXXXXXXX

XXIXXX|XXXIXXXIXXXIXO

IXXOXXXIXXXIX

XXIXXXIXXXIXXXIXXX

XXXXXX

XXX

ga~rf XXX

tinaria a~i etina

0

OXXXIXXXX

XXIXXXIX

X

DipOle ~a p i n a s t e r Eydrallaanla falcata $ertulazta

X

XXXXXXXXXX X

~ ar t u l a r e l l a

XXOX

IXXXIXXXOXXXIXXX

Dyside~ f : ' ~ i l i s

X

XX

|XXIXXXIXIXIXXXIXXXIXXXXXXXXXXXIXXXIXIXX X

~ r g e n t ea

IXXXO X

Set tularia operculata

0 0

XXIXXXXXXXIXXXIXXXIXXXX

XI

0

IX

IXXXIXXXI

Tes~lia f e l ~

XXXIXXXIXXXIXXXIXXXIXXX|XXXIZZXIX

H e t r i d i u ~ senile

XXXO

~as

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ele~

X

0

Alcyoslum dlgi .tatu~

pluma

X

0

IX

Neaertesla a n t e - - ~

XXXXXIXXX|XXXIXXXXXI

X

XXXXXXIXXXXX

PlumA1Aria s e t a c ea s

B

0 X

0

IXXIXXXIXXXIXXXIIXX

Clio]~,a 9 ela+.a

tu~ f u c o r u a

XXIXXXX

0 0

0

Gra~ti& r

X OXX

Cerami~ rubr~

XXXXIX 0

0

XXXXX XXXXXX

0

0

Sabella pevonln~ IX

HydA*oldee norvegica Posatoce~oe trlqueter

IXXIXXXIXXXtXXXIXXXIXXXXXXXIXXX~XXXIXIXX

Bala~uw r

IXIIXXXIXXXXXXXX

~a

XXX

equama

XX

I~ozonla bi~pocrepia Crisia e~trnea

XXO

X

X

X

X

C l a v o l ~ le1~adiro~Is

XX

~o~chelllua r

XXXX

XX

XX

9

X

9

XIXXXXX

9

9 0

9

IXXIXXXIX

AaathAa l e n d l ~ e r a

Di~ei1~ lacttloe~

XX

XIXXXIXXXIXXXO

BuCda f~abelLata

Bot:';11 ~Js e c ~ o ~ e e r l

XXXXXXXXXXIXXXIXXXIXIXX

X

IXXIXXXIXXXXX

Y l u s t r & follar ea Alcyonidiua hirsut~

X

I XXXXXIXX

X

0

XXO

0

IXIZ

XX

0

0

OX 0

XIXOXX X

XXXXXXXX

X

XXXXXXXXXX

XXXXXXXXXXX OXX|XXXO

X

0

0

0

e X n c o r r e c t l y r e c o r d e d by X = l g ~ t - J o n e s and f f e l s o n - ~ e i t h a s L . h y p e r b o r e a (V. ~. J o n e s p e t s . c o m . )

0

Sublittoral ecology of the J~lenai Strait

669

TABLe 2. Distribution of prominent mobile fauna along the transect. See Table x

caption for numbers of Stations sampled in z956. STATXONS e * e e e ~ e e e * e e ~ e e e e e , e ~ e e e e e . e e e e e e , e e e e e e

P~m-I~C3

o

~

X

1976

SP~ZES P ~ o thoe opinl fera

Nere/e p e l a ~ i c a

XX

R

H~p~olTt e v a t i a n s

0

0

0

X

XX 0

0

0

P a l a e ~ n oerratus

XX

9

0

Ho~'us gsmsaxus Galathea s~ua~Ifera porcella~a l o ~ i c o r ~ s

XXXXXXXXXXXX

X 9 9 9 9

Perc~11-~ platycheles

XX

9 X

X X

X

NacropiIras purer

9

| X X 9 | X

Carcinus m a e ~

II

XnXXX|XXXXXXX

Cancer p a ~ a r u s

N 9

0

X

X

9

9

l~Lrte l l ~

Xaachus l ~ i ~

X

Lepidochitona cinerea'

9 9 m X

Oll~'~,,.a ~bilicalls 9

XX

0

XX

9

XX

OX

XOXXXXX 9

0

X

XX

X|XXXX

XXXXXXXXOXXX|

X

|XX XXX

XXI

XX

X

0

XXXXXX X

XX

X

~0

X 9

Gib~ala cineraria

9

XX

X

X X

~acropo~ta Ir~.

X X

XXXXXXXX X 0 X

X

9 X

XXXNX

X X 0 X

0 9

XRX

XX|XXX|XXX|XxX~XXXXXOX

X 9 B XX

X 9 9 | X 9 X |

Pa~A~tS b e r r ~ u s

Pil~u~

XXXX

|XX

0

0

X

X

X

XXO

9

LLt t o r i a a l i t t o r a l i s

XX

YA'Ivla moaacha

X

Nucella l a p i l l u s "

0

0 X X X X

X 9

X X X X

X X 9 II X X X 9 X X 0

l

B 9 9 | X

Coager co~er

0

X

i XX|XXXIXXX|XXX|XX X

M

l II X X

0

0

9

9 X

X|XXXO 9

X

|XOX |

X

XXRXX X

OX|

0

0

0

X

Gadus ~ l l a c ~ u ~

XXXX

X X 9 X X X

Cremllabrus ~ e l o ~

X

X

0

X

Ct eno~a~rus rugestr~s

X

I~bru~ ~ e ~ I t a

X X

X

X

Gob~u~ ~ , g a n e l l u s I~lls ~mnellus Taurulus ~ l s

9

9 X X X 9 9 9

XXX|XXXIXXX|XXX|

fra~ilis

A~hi~holis s~Aamata

0

0

Favorlaua ~ c h l a l l s

X

X

X 9

rac e l i n a a'Ariculata

O]~Kio t h r i x

X

9

Archi~rls p s e u d o ~ ~branchus t r i c o l o r

He~icla ear~uinolenta Aaterias ~ben#

XXX

(~n~e~)

X

9

X

9

9 X X

X

X

0 X

X 9 9

X 9

X

X

XXXXXXXXXXX|XX

XX

XX

X

X 9

XXXXXXXXXXXXXXX

9 ~ o r m a l l y recorde~ only in the l i t t o r a l zone.

this figure because it covers a large proportion of the rock on the south side of the transect and as such forms an important substratum. It can be seen from Knight-Jones & NelsonSmith (x976) that there was a very similar cover of IIalichondria in x956. Indeed, there are many similarities in the gross distribution of the species common to both surveys. Another constant feature is the higher number of species (see Figure 3) recorded on the north side of the transect than elsewhere, while near the centre of the channel there is a gravel-swept area with few species. The differences that do exist in the prominent fauna found on both surveys can be seen from Table 3 to be largely in the distribution of species rather than in presence or absence. In order to eliminate possible discrepancies in station locations between the x956 and xo76

67 ~

R . ttoare ~ 3 f . E . Peattie

TABLE 3. Apparent differences in the distribution of prominent faunal species bev.veen the 1956 and x976 surveys. Species Leueosolenia r Dysidea fragilis 2ffyxilla rosaeea Arnphilectus fitcorum Hyrneniacidon sanguinea Tubularla larynx Sertularella gayl Sertularella polyzonias Ilalecium halednum* Diphasla pinnaster Sertularia operculata Garveia nutans Sagartia elegans Phoronis hlppocrepia Itydroides norvegica Verruca stroemia Galathea squamlfera Porcellana platycheles Pagurus bernhardus 2~[onia squama Gibbula umbilicalis Nucella lapillus 11uccinum undatum* Calliostoma zizyphinum ~ Trivia monacha Itenricia sangubzolenta Amathia lendigera Alcyonidium gelatbmsum Botrylloldes leachi Sidnyum turbinatum* 3lorchellhtm argus Didemnum maculosum Botryllus schlosseri Dendrodoa grossularla ~

1956 B N B o B S o B S o S B S o o B o S S o o B N N o N o B N B o o B N

1976 S ]3 o B o o S o o N B o~ N B N o B B B B B S o o S B N o o o B N N o

N, north side; S, south side; B, both sides; scarce and/or inconspicuous itinerant species are omitted. *Recorded as scarce in 1956 by ICnight-Jones & Nelson-Smlth (t976). bAbundant in spring 1977, exemplifying seasonal differences.

surveys the transect has been.divided into north and south sides. Also, the algae have been omitted from Table 3 as they were not the subject of an intensive search in 1976 and so comparison with the original survey would be invalid.

Data analysis The major groupings of stations produced by the cluster analysis for the 1976 data are shown in Figure 4(a). Group I contains predominantly northern stations while Group II consists mostly of stations on the south side of the transect. Group IIa stations are in the centre channel and the stations in Group IIb are those largely dominated by Hallchondria. The stations of Group III correspond to the region of mobile shell-gravel while Group IV represents the lower intertidal zone. As might be expected the latter two groups are the least similar to the others. When the data of Knight-Jones & Nelson-Smith (i976 , pp. 385-389) were analysed in the same way similar trends emerged in the station clustering. The resulting dendrogram is

Subllttoral ecology of the 2~IenaiStrait

67 x

shown in Figure 4(b), station numbers having been changed so that they areequivalent(as far as can be known) to those of the I976 transect. Stations z and 38 clustered at a low level but correspond to Groups Ia and Id respectively in Figure (4a). Stations 28 and 32 are equivalent to the Halichondria-dominated Group IIb and are linked to stations 4, 8 and 36, which group corresponds to another part of Group I shown in Figure 4(a). As in the 1976 data the centre channel stations (28 and 32) are slightly dissociated from the adjacent ones while the gravel area (station I6) is the least similar of all. Basically similar groupings arose from the analysis employed by Knight-Jones & Nelson-Smith (z 976) although this was based upon a different index of affinity--Sorensen's Index. Discussion and conclusions

The main physical factors operating in the Menai Strait appear to be turbidity, current and substratum, although there are many others (see, for instance, Lilley et aL, x953) which must interact with them.

Turbidity In a detailed review ~foore (z977) showed that the ecological effects of turbidity are largely ill-defined at present. The differential distribution of species along the transect is unlikely to be the result of turbidity conditions as turbulent mixing causes a uniform turbidity across the IV[enai Strait. However, the high suspended load probably does influence the overall structure of the community (Moore, x978). In the present situation it is very important to distinguish between suspended and sedimented material as pointed out by Moore (1977). Thus although there is a high particulate content in the water column for most of the year the strong currents prevent much of this material from settling out. Consequently, 'silting-up' is not a problem for the benthos although some animals may be excluded by the inability of their feeding or respiratory mechanisms to cope with the generally high suspended load. One such example may be the sponges, there being a relatively low diversity of sponge species in the Menai Strait compared with current-swept but clearer waters elsewhere on the North Wales coast (see Knight-Jones & Clifford Jones, x955; Moore z977). The degree to which turbidity might affect the zonation of some animals by reducing light penetration is not known. However, Knight-Jones (i956) and Knight-Jones & Nelson-Smith (x976) suggested that the light-reactions of larvae and adults could be influenced by the turbid waters in the Menai Strait. Certainly, those animals commonly associated with algae will be restricted to the shallower depths. Notwithstanding disadvantages there is little doubt that the detrital content (with a high organic fraction) of the water is a valuable source of current-borne food for many of the " bottom-living species. Substratum attd current The importance of substratum is reflected in the distribution of the fauna along the profile. The large cover of Ilalichondria panlcea on the south side probably provides an unsuitable substrate for many species or, perhaps, competitively excludes them due to rapid growth. For instance, Henricia, Metridium, Didemnum, Botryllus, Flustra and Alo'onidium are absent or very rare in the Halichondria zone while they are common on the north side of the channel. It may be the lack of a suitably stable substratum (i.e., bedrock and large boulders) which restricts the growth of Halichondria on the north side. Alternatively, the greater species diversity on the north side might result from the longer period of reduced water flow caused

672

R. Hoare & 3~. E. Peattie

by the local coastal topography. Here, the settling conditions for some larvae must be enhanced because they have more time to establish themselves. Knlght-Jones (i956) stated that the north side of the transect was protected from strong currents but this is contrary to the evidence produced in the present survey. It is the duration and not the intensity of current flow which is important. The other major physical influence on the faunal distribution is the area of gravel just north of the centre channel. Not only is the gravel unsuitable for the settlement of epilithie species but it is highly mobile resulting in a total lack of infauna and small epifauna. Furthermore, the gravel must cause considerable scouring of adjacent rocky areas. In this area, between stations 13 and x7 (Figure 2) occasional boulders harbour the few hardier species which are resistant to gravel abrasion. Tealla fellna, Abletlnarla abfetina, Hydrallmania falcata and Flustrafollacea are the only prominent macrofauna in this gravel region. Indeed, at station 13 Tealia achieves its highest density---equivalent to 9o/m ~. The distribution presented in Table I indicates that Flustra may be less resistant to abrasion than the other species mentioned above as it was not recorded at stations x6 and x7 where most of the gravel occurred in x976. Furthermore, Flustra did not survive experimental 'transplantation' from station xo to station 17 although both stations were within the depth range of the species as well as being subject to the same current flow. The presence of species such as Sycon coronatum (Ellis & Solander) (Arndt, x934) , Grantia and Alcyonidlum in the region influenced by the gravel is explained by their position either on the tops of boulders or growing epizoically upon the hydroids present. This serves to raise them above the most severe conditions of abrasion close to the bottom. Other sessile species such as Balanus crenatus and Pomatoceros trlqueter which transcend the gravel zone are afforded protection by their calcareous external structures. Clearly, although the maximum current-strength near the bottom has been shown not to vary significantly along the transect, there is a major interaction between current and substratum to produce various mlcrohabitats. Apart from scouring, the nature of the bottom determines the formation of boundary layers and patches of 'dead water' where flow is reduced. Consequently, many species will be isolated from the full effects of the current. For instances, Sa3ella pavonlna (albeit rare) is able to survive in such sheltered microhabitats in the Menai Strait although it is normally associated with quieter water. Thus the current-conditlons to which many animals are subjected are inextricably linked to the nature of the substratum and in many cases gross estimates of current speed above the bottom give little more than an indication of the general biotope with respect to water movement.

The community The availability to individual species of the heterogenous complex of microhabitats which results from the nature of the substratum and its influence over the hydraulic conditions above it, probably explains the blurring of divisions between putative faunal associations. Hence, while the dendrograms show some division of the fauna into fairl~r distinct groups there is generally some overlap. The major groupings appear to be related to distance along the profile rather than directly to depth. In general depth is not considered as a limiting factor in the l~enai Strait as most of the species extend into deeper water under suitable conditions in other areas. The only case where depth seems to be of primary importance is in Group Id (Figure 4), consisting of 3 southern stations, of the shallow, algal-dominated circalittoral which have clustered at a similar level to the equivalent northern stations of Group Ia. The Group IIa stations in the centre channel are subject to maximum current-flow and duration as well as some abrasion from the adjacent gravel area. However, probably because they are

Sublittoral ecology of the 3[enai Strait

673

largely bedrock these stations cluster out as more similar to Group IIb than to Group I stations. Thus, the nature of the bottom substratum seems to be of prime importance in determining the qualitative distribution of the fauna but effects are modified by the degree of water movement (]aag & Ambiihl, x96z). Knight-Jones & Nelson-Smith (z976), also suggested that qualitative data did not produce clear divisions between communities because of the diversity of microhabitats. However, their use of a simple and subjective abundance scale produced little additional information. It seems that a clearer answer would be obtained by the use of quantitative data simultaneously related to a classification of microhabitats rather than to a general cover over a very heterogenous bottom. I a i

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Temporal stability Many of the species recorded in z956 showed a similar distribution in x976. The majority of apparent differences in species distribution between the two surveys are most readily

674

12. Hoare ~ M. 1?,.Peattie

explained by such factors as season and the different approaches to data collection. The I956 data (Knight-Jones & Nelson-Smith, x976) were based largely upon random collections, which inevitably contain many of the smaller less conspicuous species at a few stations, while the x976 data arise from observations of prominent species at more numerous stations. Evidence of a seasonal difference between the surveys is shown by the hydroid, Garvda nutans. This species was recorded on both sides of the transect in x956 (loe. tit.) but not at all in the summer of x976 when the second survey was carried out. However, in the spring of i977 Garveia was abundant in the shallower water on both sides of the Menai Strait. Many other species such as nudibranchs are largely seasonal in their appearance. Nevertheless, the distribution of a few species suggest real differences between x956 and i976. It seems unlikely that Galathea squamlfera, which was very conspicuous throughout x976 , would have been missed in the previous survey, when it was not recorded. Consequently, this species may now be much more abundant than in i956. Other species which come into this category are 3lorchellium argus and Phoronis hippocrepia. Although conspicuous differences in the distribution of scarce itinerant species such as Solaster, Calliostoma, Buccinum and Haledum can be given little weight. More substantial evidence of real change in the Menai Strait fauna is provided in the case of Antedon bifida and .flrchidoris pseudoargus. Crisp & KnightJones (x954), reported an abundance of Antedon near Menai Bridge from I933-x939 but not after x946, and assigned the cause to the severe winter of x946/47. The species is still absent, and Hiscock (i976), suggested that this might be due to a deterioration in water quality. Personal observations (R.H.) show that a similar, although not total, population collapse of Archldoris has taken place since x974, prior to which it was abundant on Halichondrla on both sides of the transect in ~?ay and June. Now it can be found only rarely by turning over boulders. Whether such events are natural or not will only be determined when much more long term data on individual species have been collected. In terms of the Stability]Time hypothesis of Sanders (1968) the region of the Menai Strait where the transect lies may be considered as a harsh but predictable environment. Because short-term environmental variation is tidal (or largely linked to the tides as in the case of turbidity) it follows a predictable pattern and possesses, ipso facto, a type of stability. The absence of wave-surge (which becomes most severe during storms) removes the major irregular factor to which an open coast fauna is subjected. Apart from the examples already mentioned the overall impression is one of a high degree of faunal stability over the last 2o years. The results of the two surveys suggest a stability of dominance, stability of relative abundance patterns (as far as can be estimated subjectively) and to a large extent stability of species composition. In order to assess the relative importance of wave action and strong currents it would be necessary to investigate a similar community on the open coast. Species diversity in areas of differing water movements has been discussed briefly by Lewis (x968). The surveys described above are largely descriptive in their approach and, inevitably there is much speculation in interpreting the data. The need now is for a more functional approach which would, for instance, assist in separating the lithophilic and rheophilie elements of the community. Attempts to study whole sublittoral communities simultaneously are constrained for practical reasons and it seems more useful to concentrate upon individual species (vide Lewis, x972). Certainly, detailed and long-term information on the life-cycles and behaviour of all sublittoral species is required. Equally there is room for improvement of survey methodology and enumeration techniques. In particular, a classification of microhabitats and a standardization of abundance estimates. The use of sophisticated mathematical analyses cannot replace a better quality and type of data.

Sublittoral ecology of the Menal Strait

675

Acknowledgements The authors wish to thank all those who assisted in the diving and boat work, in particular Mr G. Ward and ~llr C. Lumb. Many of the algae were identified or confirmed by Dr W. E. Jones. Thanks are due to Dr D. A. Jones for reading and criticising the manuscript. The work was carried out while M. Peattie was in receipt of a N.E.R.C. Advance Course Studentship. References Arndt, W. *934 Porifera. Tierw. Nord-u. Ostsee, 3 a x, x-4o. Bray, J. R. & Curtis, J. T. I957 An ordination of the upland forest communities of Southern Wisconsin. Ecological 2~lonographs 27j 325-349. Buchan S., Floodgate, G. D. & Crisp, D. J. x967 Studies on the seasonal variation of the suspended matter in the Menai Straits, I. The inorganic fraction. Limnology and Oceanography I2 (3), 4x9-43 x. Buchan S., Floodgate, G. D. & Crisp, D. J. x973 Studies of the seasonal variation of the suspended matter of the Menai Straits. II. Mid stream data. Sonderdruck aus tier Deutschen IIydrographlschen Zeitschrlft 28 (2), 74-83. Iliscock, K. x976 The influence of water movement on the ecology of sublittoral rocky areas. PhD Thesis, University of Wales. z35 f. + 5 pt. Jaag, O. & AmbiJhl, H. x964 The effect of the current on the composition of biocoenoses in flowing water streams. In Advances in lVater Pollution Research Vol. I. (ed. Southgate, B. A.), Pergamon Press, London, pp. 3x-44. Jones, W. E. & Demetropoulos, A. x968 Exposure to wave-action: measurements of an important ecological parameter on rocky shores on Anglesey. Journal of Experimental AIarbze Biology and Ecology 2, 46-63. Kain, J. M. x962 Aspects of the biology of Lamlnaria hyperborea. I. Vertical Distribution. Journal of the 3Iarlne Biological Association, U.K. 42, 377-385. Kenehington, R. A. x97o An investigation of the detritus in Menai Straits plankton samples. Journal of the J~farb~e Biological Association, U.K. 5o, 489-498. Knight-Jones, E. W. & Clifford Jones, W. x955 The fauna of rocks at various depths off Bardsey. I. Sponges, Coelenterates and Bryozoans. Bardsey Observatory Report 1955, x-8 pp. Knight-Jones, E. W. x956 l~arine Biology in Wales. University College of Swansea, 26 pp. Knight-Jones, E. W. & Nelson-Smith, A. I976 Sublittoral transects in the Menai Straits and Milford Haven. In Biology of Benthic organisms, zxth European Symposium on ~arine Biology. (Keegan B. F., Ceidigh, P. O. & Boaden, P. J. S., eds.). Pergamon Press, pp. 373-389. Lance, G. N. & Williams, W . T . x966 A generalized sorting strategy for computer classifications.

Nature 212, p. 218. Lewis, J. R. x968 "~Vater movements and their role in rocky shore ecology. Sarsla 34, x3-36. Lewis, J. R. x972 Problems and approaches to baseline studies in coastal communities. In 2~larine Pollution amt Sea Life, pp. 4ox-4o4. (Ruivo, M., ed.). Fishing News Books Limited, London. Lilly, S. J., Sloane, J. F. Bassindale, It., Ebling, F. J. & Kitching, J. A. x953 The ecology of the Lough Ine rapids with special reference to water currents. IV'. The sedentary fauna of sublittoral boulders. Journal of Animal Ecology 22 (x), 87-x22. l~Iarine Biological Association U.K. x957 Plymouth 21Iarine Fauna (3rd Edn.), 457 PPl~,Ioore, P. G. x977 Inorganic particulate suspensions in the sea and their effects on marine animals. Oceanography and 2~farine Biology Annual Review x5, 225-363. Moore P. G. x978 Turbidity and kelp holdfast Amphipoda I. Wales and SAV. England. Journal of Experimental 2~Iarlne Biology amt Ecology 32, 53-'96. Sanders, H. L. x968 Marine h~nthic diversity--a comparative study. American Naturalist xo2~ 243-282.