Physical parameters influencing the zonation of surf zone benthos

Estuarine, Coastal and Sherf Science (1989) 28,5 17-530

Physical Zonation

Parameters Influencing of Surf Zone Benthos

P. C. Fleischack

the

and A. J. de Freitas

Oceanographic Research Institute, P.O. Box 10712, Marine Parade 4056, Durban, South Africa Received 26 August 1988 and in revisedform 13 December 1988

Keywords: gradients

surfzone; macrobenthos;

turbulence;

zonation;

sediment; beaches;

Benthic macrofauna were quantitatively sampled seasonally at stations along three transects through a high energy surf zone. Samples were collected on a calm, an intermediate and an exposed, high energy beach. Sediment characteristics were measured and wave parameters were calculated for each station. Two gradients of physical parameters occurred. One decreased from the swash to the nearshore zone and another increased from the sheltered to the exposed beach. Biological parameters showed similar, but opposite, gradients. Multivariate analyses of the 86 benthic species recorded and the physical parameters showed that wave current was the most important factor influencing the fauna1 distribution. Three faunal assemblages were identified which corresponded to the physical swash, breaker and nearshore zones, respectively. Using these, a general zonation scheme for high energy surf zones is proposed.

Introduction Whereas intertidal beaches have been subjected to numerous ecological studies, the subtidal areas, particularly in the breaker zone, have been avoided, probably becauseof their hazardous nature and the physical limitations placed on sampling in this zone. McLachlau (1980a) suggested that inter and subtidal beaches should be studied together as a single system. Subtidal surf zone bentbos has been studied previously (Morgans, 1962; McIntyre & Eleftheriou, 1968; Field 1971; Day et al., 1971; Masse, 1970,1972; Christie, 1976; Dexter, 1978 and McLachlan et al., 1984). Of thesestudies only Field (1971), Christie (1976) and McLachlan et al. (1984) sampled quantitatively in high energy surf zones. The former

authors considered two stations in the breaker zone while the latter workers surveyed only one. Three

aqueous

zones are usually

obvious

to an observer

on the beach looking

out to sea.

These are the swashzones where the waves rush up the beach, the breaker zone and the comparatively calm area seawardof the breakers, or the nearshorezone. These three zones are all represented in the areaconsidered in this study. Zonation classificatory schemesfor subtidal surf zone beacheshave been proposed (Day et al., 1971; Christie, 1976; Field, 0272-7714/89/050517+

14 $03.00/O

@ 1989 Academic

Press Limited

518

P. C. Fleischack & A. -7. de Freitas

1971 and McLachlan, 1983). These authors identified four faunistic zones within the surf zone: the swash, inner turbulent, transition and outer turbulent zones. Wave action and sediment characteristics have been considered as the major physical factors affecting the distribution of benthic fauna on beaches(Field, 1971; Christie, 1976; McLachlan et al., 1984). While the inlluence of sediment on fauna1distribution has been determined by previous workers, the influence of wave characteristics has only been assessed subjectively. This study was undertaken to investigate the fauna1composition of the subtidal benthic community in the surf zone at Durban, and to investigate the influence of wave and sedimentary parameters on fauna1 distribution. By undertaking an intensive sampling programme within the breaker zone it was intended that this study would reveal the true zonation pattern of surf zone benthos in the study area. Study areas The study area lies within the Durban Bight (Figure l), which extends from Durban Bluff (29’52’s; 31”3’E) northwards to Port Dumford (28”55’S; 31”55’E). Three beacheswere selectedfor this investigation, namely Addington, OR1 and Sunkist. These are situated in the southern portion of the Bight, between Durban’s North Pier and the Umgeni River mouth. Oliff (1969) gives ‘ the average height of the highest l/3 of the waves occurring in a specified period ’ (for Durban), from the predominant direction of approach, as 1.31 m with a range of 0.3-2-l m. The annual mean wave period was 10.8 s, with a range of 9.0-12.9 s. Ninety four per cent of the waves to which this area is subjected arise to the south east of the Natal coast (Swart, 1976). The tidal range in the study area averages 1.71 m for spring tides and O-61 for neap tides. The average surf temperature in the study area between January 1982 and December 1984was 21.8 “C, with a range of 17-29 “C. Salinity in the Durban Bight normally ranges between 35.0% and 35*3%0,but may drop to 28% during flood by local rivers (Berry, 1978). The surf zone has recently been described as the area between the swashand the outer limit of surf circulation cells (McLachlan, 1983). Swart (1983) placed this outer limit at a depth of approximately 15 m. This includes both the inner (breaker) and outer turbulent zones. For purposes of this study McLachlan’s (1983) definition of the surf zone was accepted and the nearshore zone was considered to be the area between the breakers and the seaward limit of the surf zone. The study area lay entirely within the surf zone, as it extended from the swashinto the nearshore zone. Methods Sample collection

The transect line consisted of a 260 m nylon rope with loops spliced at 20 m intervals along its length. This line was laid using an inflatable craft from the swashthrough the breaker into the nearshore zone. Anchors and buoys attached to the loops in the line marked the positions of the ten sampling stations along the transect. These stations were generally 20 m apart in the breakers and 40 m apart in the nearshore zone. The first station was in the swash zone while the tenth was 260 m offshore, in the nearshore zone. The depth of this outer station varied at each beach, averaging 8.5 m at Sunkist, 6.8 m at ORI and 4.5 m at Addington. When the surf was calm enough to permit

ZnJ%mzceson zonation of surf zone benthos

519

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100

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Figure 1. The study area and the variation in the beach profile of the three study beaches experienced during the period of the investigation. The thick lines on the horizontal axes show the approximate position of the transect.

sampling, the position of the backline was in the vicinity of Station 5 at Sunkist and 4 at OR1 while breakers were absent from Addington. More commonly, however, the breakers extended approximately 180m offshore at Sunkist, 100mat OR1 and 60 m at Addington. A water powered suction sampler (details in Fleischack et al., 1985) wasused to collect ten samplesat each station. This apparatus effectively sampled an area of 315 cm2 to a depth of 30 cm in the sediment. Sampling was undertaken quarterly between October 1983and January 1985. One sediment sample (for particle parameter analysis) was collected at each station on eachoccasion, using a hand corer with a 10 cm diameter. Particle size characteristics of each sediment sample were determined by placing a subsample (approximately 10ml) in a settling column (Flemming, 1977). Horizontal orbital velocity was calculated using tables (Swart, 1981), using depths provided by the

520

P. C. Fleischack 6~ A. J. de Freitas

City Engineer and wave heights for each beach supplied by Swart (pers comm). Longshore sediment transport rate and current velocity were calculated using the computers and software of the Sediment Dynamics Division of the National Research Institute for Oceanology in Stellenbosch. These calculations were developed from the results of hydraulic model tests undertaken as part of the Durban Beach Protection Investigation (Campbell et al., 1985). Fauna1 presence and absence data from samples collected were subjected to three methods of multivariate analysis: (1) (2)

Ordination-Detrended correspondence analysis (Hill, 1979). Classification (cluster analysisflrdination space partitioning after Noy Meir (1973), Hall and Swaine (1976) and Peet (1980). The comparability between clusters produced by this method was verified using a method of hierarchical classification (Field & McFarlane, 1968). (3) Gradient Analysis-The ini%tenceof various environmental variables on the multispeciesrelationships defined by Detrended correspondence analysis was examined using this method recommended by Field et al. (1982) and Gauch (1982). Sample groups were identified by ordination spacepartitioning. Thereafter, each environmental characteristic considered influential was independently superimposed over the ordination. Scaled symbols, each representing a size range of the relevant measured environmental parameter, were used to show the relationship between each variable and the previously identified sample groups. Results Macrofaunal speciespresenceat each station is shown in Table 1. Eighty-six specieswere recorded, many of which were new distribution records for this region. Polychaetes, crustaceans and molluscs dominated the biomass at Addington, OR1 and Sunkist, respectively, while crustaceans, molluscs and polychaetes contributed most biomass in the swash,breaker and nearshore zones, respectively. Table 2 summarizes the sedimentary parameters at each station. Sand was finer at Addington and OR1 than at Sunkist. Sediment size decreasedwith distance offshore at each beach, except at Addington, where larger particles occurred at Stations 6,7 and 8. Mean annual wave parameters at each station are presented in Table 3. Horizontal orbital velocity was similar at Sunkist and ORI, where this value wassubstantially greater than at stations at Addington. This parameter decreasedsteadily with distance offshore. Longshore current velocity and sediment transport rate were lowest at Addington and highest at Sunkist. At each beach thesevalues increasedto a peak at a point approximating the position of the backline, before decreasing with further distance offshore. Space partitioning, when applied to the ordination of data in Table 1 identified three major groups of stations (Figure 2). The dendrogram resulting from verification of these groups by hierarchical classification (Figure 3) supports this grouping of stations according to their fauna1compositions. These zones, asdefined by multivariate analysis, correspond very closely with the observed physical zones at each breach. For this reason group 1 is referred to as the Swash, group 2 asthe Breaker and group 3 asthe nearshore zone. The situation of these three faunistic zones is superimposed on the positions of the sampling stations (Figure 4). Speciesrichnesswas lowest in the swashand highest in the nearshorezone at each beach (Figure 5).

Znjluenceson zonation of surf zone benthos

521

The swashzone lies in shallow water (generally lessthan O-7m) between the shorebreak and the water’s edge. The landward boundary of the swashcorresponds to the sublittoral zone (Dahl, 1952), or the zone of saturation (Salvat, 1964). From here this zone extends approximately IO-40 m seawards. Classification techniques resulted in a loosegroup (Group 1) consisting of Station 1 at Addington and Sunkist and Stations 1 and 2 at ORI. Station 2 at Sunkist wasnot sampled during this study, but results of previous dredging indicated the virtual non-existence of fauna at this extremely turbulent station. It is unlikely then, that the swashzone would have included this station at Sunkist. At Addington, Station 2 supported speciescharacteristic of the swashzone, but animals common to the nearshore zone were usually also present, causing this station generally to be ordered in the latter zone. Ten specieswere found in the swashzone (Table 1). Those commonly occurring here were: the bivalves Donax madagascariensis,Tivela natalensisand T. polita at Addington and the gastropod Bullia natalensisat Sunkist. The hippid Emerita austroafricana was probably the most important speciesin this zone at all three beaches,while the polychaetes Glycera natalensiswas common at Addington, and Goniadopsismaskallensisoccurred at Sunkist. No speciesoccurred exclusively in the breaker zone. All specieshere were alsopresent either in the nearshoreor, to a lesserextent, the swashzone. Common speciesin this zone were: D. madagascariensis, D. bipartitus, D. simplex,Zactra trotterina and T. polita; the echinoderms-Echinodiscus bisperforatusand Ophionemmasp.; the crustaceans-Diogenes brevirostris, Phylira globosa, Pontogeloideslatipes, Cunicus profundus and Gastrosaccusbispinosa;the polychaetes-Glycera natalensis, Goniadopsis maskallensisand Sigalion capense;and a nemertean (Table 1). The nearshore zone was characterized by calmer conditions which extended seawards from the swashzone at Addington and from the breaker zone at OR1 and Sunkist. Waves do not usually break in this area. All specieswhich were common in the breaker zone alsooccurred in the nearshorezone. However, some specieswhich were common in the nearshore zone were either absent from, or found only occasionally in the breaker zone. These included the bivalves Sunetta contempta, Siliqua fasciata and T. rejecta; the ophiuroid Paracrocnida persica, the crustaceans Albunea symnista and Ogrydes sp. and the polychaetes Lumbrinereis sp., Magelona papillicornis, Nephtys capensis,Pareulepisgeayi and Scoloplosmadagascariensis (Table 1). The gradient analysesapplied to the ordination of the 28 stations, using the values of each measured and calculated sedimentary and wave parameters, yielded the results expressedbelow. (a) Median and meansandparticle diameter Coarsest sand generally occurred in the swashzone and fine sand at most stations in the nearshore zone. However, coarsesandwas alsofound at stations in the nearshore zone. A range of grain sizes was present in the breaker zone. The range of sizes in both the nearshore and swashzones suggeststhat, although a general trend of decreasing particle size from the swashto the nearshore zone occurred, no strong direct relationship existed between zonation and sandgrain diameter. (b) Sedimentsorting values Each zone contained a range of sorting values. While the widest range occurred in the breaker zone, neither the nearshore nor the swash zones had particular sorting values.

522

P. C. Fleischack

&J A. J. de Freitas

TABLE 1. The occurrence of the 86 species collected their distribution in the study area Addington Species Bivalves Arcopsisgibba Donax bipartirus D. madagascariensi~ D. simplex Iacrra trotteriana Macoma refrorsa Mactra aequisulcata M. glabrata Petricola bicolor Siliqua fasciata Solemya africana Sunetta conrempta Timoclea arakana Thracia sp. Tivela natalensis T. polira T. rejecta Gastropods Bullia naralensii B. similis Hastula longiscata Juliidae sp. Natica gaultericana Oliva caroliniana Polinices didyma P. tumidus Ophiuroids Amphioplus hastatus Ophionema sp. nov. Ophiothela venusta Ophiothrii fragilis Paracrocnidapersica Echinoid Echinodiscus bisperforarus Holothuroid sp. 1 Holothuroid sp. 2 Crustaceans Albunea symnista Dehaanius dentatus Diogenes brevirostris Emerita austroafrikana Macropetasma africanum Matuta lunaris Ogrydes sp. Ovalipes puncratus Phylira globosa Pontophilus megalocheir Porcellana dehaanii Isopods Cyathura estuaria Pontogeloides latipes Amphipods Cunicus profundus Basuto stimpsoni

Code No.

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Influences on zonation of surf zone benthos

523

TABLE 1. (Continued) Addington Code No.

Species Mysids Gasttosaccus bispinosa Ostracoda sp. Pycnogonida sp. Polychaetes

Aonides oxycephala Aricidea curviseta Armandia leptocirrus Capitella capitata Dispio sp. Eunice sp. Eteone ornata Flabelligeridae Glycera natalensis Goniadella gracilis Goniadopsis maskallensis Loimia medusa Lumbrinereis sp. Magelona cincta M. papillicornis Nephtys capensis Onuphis quinquidens Orbinia angrapequensis 0. bioerti 0. cuvieri 0. monori Ophelia roscoffensis Paraonidae Pareulepisgeayi Prionospio sp. Sabellidae Scoloplos johnstonei S. madagascariensis S. uniramus Sigalion capense Spionidae Sthenelais boa Sipunculida Nemertea Zoanthid

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As with grain size, no clear direct relationship between zonation and sorting value was evident. (c) Horizontal

orbital

velocity

Stations in the swashzone were situated in water too shallow to be accommodated by the tables used in the calculation of horizontal orbital velocity. With the exception of one station in each, highest velocities occurred in the breaker zone and lowest in the nearshore zone. The two exceptions (Sunkist Station 8 in the breaker zone and OR1 Station 6 in the nearshore zone) were both situated on the boundary between these two zones. It is, therefore, not totally unexpected that they each had mean annual velocity values slightly

524

P. C. Fleischack 13 A.J. de Freitas

TABLE 2. Average mean and median diameters of sand particles for the three beach sites and for each of the ten stations at each site, where Station 1 is in the swash zone, Station 10 is in the nearshore zone Addington Average diameter (PI

Beach

OR1 Beach

Average diameter (wd

Average diameter (Id

Sunkist

Average diameter (P)

Average diameter (P)

Beach Average diameter (wd

Station

Mean

SD

Mean

SD

Mean

SD

Mean

SD

Mean

SD

Mean

SD

1 2 3 4 5 6 7 8 9 10

330 336 277 263 240 319 351 447 248 -

32 51 32 18 6 85 81 99 22 -

301 303 264 236 207 298 325 410 224 -

66 123 72 30 17 223 177 221 51 -

435 355 337 293 325 279 303 260 239 262

68 17 51 32 72 27 57 14 16 68

396 347 329 272 307 269 295 240 230 247

106 23 99 65 95 59 136 34 36 73

544 448 384 339 289 327 273 242 237

148 33 40 39 33 87 14 13 14

537 375 335 319 238 299 254 230 219

327 39 72 60 42 117 15 20 17

Site

312

37

285

19

310

34

297

14

338

50

311

18

TABLE 3. Mean

wave and current Bed horizontal orbital velocity (m s-l)

parameters

at each site

Longshore current velocity (m s-l)

Longshore sediment transport (m’ day-‘)

Beach

Wave height Cm)

Mean

SD

Mean

SD

Mean

SD

Addington OR1 Sunkist

0.34 0.93 1.24

0.576 0.981 1.113

0.188 0.221 0.271

0.016 Cl27 0.218

0.037 0.163 0.214

2.531 57.692 213.046

6.892 84.772 424.827

different from those common in zones in which they occurred. A direct relationship existed between horizontal orbital velocity and the faunistic zonation in this study. (d) Longshore current velocity (Figure 6) With the exception of one station in each zone, lower current velocities were calculated for stations in the swash and nearshore zones and higher values in the breaker zone. As was the case with horizontal orbital velocity each of these stations lay on the boundary of each zone. In this case the station having longshore current velocities different to others in their zone were Stations 2 at OR1 and Addington and, again, Station 8 at Sunkist. Despite these differences at only one station in each zone, a direct relationship existed between longshore current velocity and zonation.

Influences on zonation of surf zone benthos

525

A: Addington

AXIS

I

0:ORI S:Sunklst

Figure 2. Ordination of the 28 stations by DCA using species presence data from Table Groups 1,2 and 3 correspond to the swash, breaker and nearshore zones respectively.

2 21 .; .= E iii

1.

60-

50-

40-

30-

20-

10 -

0’

Figure 3. Classification of data in Table 1 using the Bray-Curtis and group average method of sorting. Groups and station numbers Figure 3.

similarity index are those given for

526

P. C. Fleischack Q A. 3. de Freitas

Sunktst

&:;-.-;; 111 3456

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i ,

7

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9

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Predominant wave direction

Figure Swash,

4. Situation of faunistic zones derived breaker and nearshore zones correspond

from ordinations in Figures to groups 1,2 and 3.

3 and 4.

80 : z 0 % 6

70

L z ;

40

60 50

30 20

0 Figure 5. Species beaches.

Swash

richness

Breaker zone

Nearshare

of each zone at Addington

( W), OR1 (El) and Sunkist

(H)

The velocity of water over the substratum appears to have an important influence on the distribution of benthic fauna on sandy beaches. With respect to water movement, Station 8 at Sunkist may be regarded as a borderline station between the breaker and nearshore zones.

527

Injluences on zonation of surf zone benthos

Longshore

current

velocity

(m 5-I)

N f d Axis

I

Figure 6. Relationship between the three sample groups and longshore current velocity (0, ~0.10; H, 0.11-0.50). Groups 1, 2 and 3 correspond to the swash, breaker and nearshore zones, respectively.

(e) Longshoresedimenttransport rate All stations in the swashand nearshorezones (with the exception of Station 2 at OR1 in the swashzone) had lowest longshore sediment transport rates, while highest values occurred in the breaker zone. Sediment transport provides an indication of the amount of turbulence experienced in each zone. This parameter was the most directly related of all environmental characteristics to zonation. Low sediment transport values in the swash zone may suggestthat fauna1distribution here should be similar to those in the nearshore. However, the nature of the swashzone and its resident fauna wasunique and the zone was therefore different from others in the study area. Discussion Two gradients, each of physical and biological parameters, were observed on subtidal beaches studied. Wave and turbulence parameters decreased across the beachesfrom Sunkist to Addington and along each beach from the swash(in somecasesthe breaker) to the nearshore zone. Biological parameters such as biomass,density and speciesrichness were inversely related to these physical gradients. These values generally increasedfrom Sunkist to Addington and from the swash to the nearshore zone. Similar trends were reported elsewhere (McIntyre & Eleftheriou, 1968; Day et al., 1971; Masse, 1972; Christie, 1976; Oliver et al., 1980; McLachlan et al., 1984). Sand grain sizehasbeen suggestedasan important factor influencing the distribution of benthic species(Jones, 1950; Gray, 1974, 1981; McLachlan, 1977). However, Day et al. (1971) found no evidence that the faunistic zones on the North Carolina shelf were related to particle size. In False Bay, Field (1971) found a major change in fauna which corresponded to reduction in the wave induced water movement. However, he considered it impossible to determine whether this effect of water movement was direct or through its effects on sediments. Turbulence and sediment texture were considered by Christie

528

P. C. Fleischack Q A. -7. de Freitas

(1976) as the most important factors which regulate the distribution of species in the turbulent zone. More recently, McLachlan et al. (1984) concluded that wave energy controls the physical environment and the distribution of organisms on high energy coasts. Although previous workers have recognized the importance of water movement on the distribution of fauna, none have measured any wave or current parameters. Conclusions have been drawn from measurementsof sediment parameters, the recognised relationship between sediment size and water movement and subjective appraisal of wave activity (in most casestermed wave energy). The relationship between physical factors and speciesdistribution, as elucidated by classification and ordination, indicated that turbulence, represented primarily by longshore sediment transport rate, but also by horizontal orbital velocity and longshore current velocity, was the most important environmental parameter affecting the benthic fauna1 zonation on the subtidal beachesat Durban. This supported the assumption of McLachlan et al. (1984) that turbulence was the over-riding gradient on beachesstudied in Port Elizabeth. Values for horizontal velocity, longshore current velocity and longshore sediment transport rate greater than 1 m s-i, 0.05 m s-l and 50 m3 day-‘, respectively were found (with very few exceptions) in the breaker zone, while values smaller than these occurred at stations in the nearshore zone. Therefore, it may be possible to predict the species composition of a benthic fauna1assemblageof a particular zone on the basisof measurements of these parameters. Animals characteristic of the nearshore zone in a particular zoogeographic area may be common where measurementsare lower than these values, whilst breaker zone assemblagespredominate where these measurementsare greater. Fauna on subtidal beaches,as in the caseof intertidal organisms, exhibit zonation. In the present study three distinct faunistic zones were identified. The boundaries of these zones closely approximated those of the physical zones except at Addington Beach, where calm conditions generally precluded a faunistic breaker zone. Bally (1981) has stated that zones, being areas dominated by a few speciesoptimally adapted to the environmental conditions found therein, are only evident in fairly narrow bands along intertidal beaches.The fauna1zonation consists of a gradual replacement of one set of speciesby another down a gradient of physical conditions. This was true of subtidal beachesin this study, where speciesand zoneswere not readily identified without sophisticated mathematical analyses.The positions of these zones change asfrequently as the tides and, at least in the swash zone, speciesmigrate with these zones in order to maintain their positions in an optimum environment (Cubit, 1969; Branch & Branch, 1981). Resident fauna (Dahl, 1952) and the degree of moisture in the sediment (Salvat, 1964) have been used to define zones on intertidal sandy beaches. Zones based on physical parameters are more universally applicable than those defined by fauna becausephysical characteristics are not subject to zoogeographic variation. However, zonation defined by Dahl generally can be superimposedon the schemedescribed by Salvat (Bally, 1981). On intertidal beachesbenthic macrofauna selectzones according to the degree of desiccation which they can best tolerate. The air breathing crustaceans (ocypodids) occur on the landward limit, while those specieswhich rely on saturated sediment inhabit the swash or seaward limit of this area. Distribution of fauna on subtidal beaches is affected by turbulence. Generally, those specieswhich have hard bodies and can either burrow or swim rapidly, are most successfulin zones which are subjected to high turbulence, while

Influences

on zonation of

surf zone

benthos

529

soft-bodied, sedentary or slow moving species are prevented from successfully inhabiting these zones. The close relationship between the intertidal and subtidal areas of the beach necessitates a zonation scheme which includes them both. The intertidal faunistic zones described by Dahl (1952) may be coupled with those found in the present study and a modification of those described by McLachlan et al. (1984). The following scheme is proposed: the supralittoral zone characterized by air-breathing crustaceans; the midlittoral zone by cirolanid isopods; the swash or sublittoral zone by Emerita or Donax; the breaker or inner turbulent zone by Gastrosaccus, Donax and other motile bivalves, e.g. Iactra and crustaceans; the nearshore or transition zone with these and other species, e.g. Tivela rejecta, Sunetta contempta, echinoderms and less motile polychaetes; and the outer turbulent zone where species in the nearshore/transition zone are said to become more abundant and where burrowers such as Callianassa and Echinocardium are common. An identical scheme, based on a combination of physical parameters (degree of moisture in the sediment in the intertidal and turbulence in the subtidal) is evident. In this case Salvat’s scheme is combined with the findings of the present study and those of McLachlan et al. (1984). The uppermost level of dry sand reached only by spray or high spring tides; the zone of retention reached by all tides; the swash zone (consisting of the zones of resurgence and saturation of Salvat) where sand, at least within 20 cm of the surface, is permanently saturated with water. Turbulence values here are low. The breaker zone characterized by high turbulence values (e.g. horizontal orbital velocity 1 m s-l or longsore sediment transport rate 50 m3 day-‘; the nearshore or transition zone where turbulence values are lower than these, and the Outer Turbulent zone which is characterised by even lower turbulence values. This last zone was not sampled in the present study and the level of turbulence, which would differentiate between this and the nearshore zone, was not determined. Acknowledgements It is always difficult to enumerate all those individuals who have assisted and advised the authors during the course of the study. Some that need to be mentioned are, however, the South African National Committee for Oceanographic Research, the South African Association for Marine Biological Research, Roy Jackson, Louise Martin, Rudy van der Elst, Stephen Piper, Mike Roberts, Lorna Cameron, Alet Metzger, Drs ‘Tickie’ Forbes, Allan Connel, Alan Ramm, George Begg, Harry Swart, Rod Bally and Professor Alan Bowmaker-all of whom have helped in some way. References Bally,

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