benthic assemblages in sediments associated with intertidal boulder-fields

Benthic assemblages in sediments associated

with intertidal boulder-fields

J.J. Cruz Motta*, A.J. Underwood, M.G. Chapman, F. Rossi

Marine Ecology Laboratories A11, Centre for Research on Ecological Impacts of Coastal Cities,

University of Sydney, Sydney, NSW 2006, Australia

Received 17 May 2002; received in revised form 30 July 2002; accepted 13 September 2002

Abstract

Assemblages on top of and underneath rocks in intertidal boulder-fields have been investigated in

several studies, although macrobenthic assemblages in soft-sediments below these rocks have

generally been ignored. The model where the presence of a boulder changes the sediment below it

and/or the composition of the soft-bottom benthic assemblages living in those sediments, in

comparison to sediments without boulders, was evaluated in this study. Six boulder-fields in New

South Wales (Australia), with different levels of wave-exposure (three exposed and three sheltered),

were sampled at two times (August, 2001 and November, 2001). In each location, at each time,

sediments below boulders and from similar-sized patches without overlying boulders (n = 6) were

sampled using 12 cm diameter cores for analyses of granulometry, organic content and macrobenthic

assemblages. Results showed that total organic content in the sediment was greater below boulders at

all locations and sampling times. Macrobenthic assemblages and distribution of grain-sizes of the

sediments were different below boulders from elsewhere in all sheltered places and in one exposed

location at one sampling time. Where differences were observed, the sediments below boulders had a

greater percentage of gravel than did sediments elsewhere. The taxa associated with most of the

differences between assemblages living below boulders and those living in sediments without

boulders differed across locations. Processes explaining these patterns probably differ among

locations, even among those with a similar exposure to waves.

D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Boulder-fields; Grain-size distribution; Macrobenthos; Sediments

0022-0981/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0022 -0981 (02 )00539 -7

* Corresponding author. Tel.: +61-2-9351-2039; fax: +61-2-9351-6713.

E-mail address: [email protected] (J.J. Cruz Motta).

www.elsevier.com/locate/jembe

Journal of Experimental Marine Biology and Ecology

285–286 (2003) 383–401

1. Introduction

One of the main goals of benthic ecology has been to understand the mechanisms

regulating relationships between sediments and organisms (reviewed by Gray, 1974;

Rhoads, 1974; Snelgrove and Butman, 1994; Aller et al., 2001). All of these have

emphasised the importance of this subject, but views about which factors are important

have changed through time. Very likely, these changes in paradigms have been propelled

by developments of new technology, allowing measurements of a wide range of character-

istics of the physico-chemical environment (e.g. Rhoads et al., 2001).

During the 1970s, a major effort was directed to understand relationships between the

composition and constitution of the sediment and the distribution of organisms (Gray,

1974). These relationships are proving to be complex, with many more physical and

chemical variables (e.g. pore-water chemistry, sediment-transport and flow dynamics of

the boundary-layer) affecting the organisms (Snelgrove and Butman, 1994). It has also

been increasingly demonstrated that the organisms themselves change many characteristics

of sediments (Heip et al., 1995; Herman et al., 1999).

As a consequence, considerable advances have been made in understanding processes

in soft-bottom benthic intertidal and subtidal habitats (e.g. Alongi, 1990, 1998; Brown

and McLachlan, 1990; Warwick, 1997, among many others). There are, however, many

soft-sediment habitats that are still poorly understood, for example, intertidal boulder-

fields. It is not yet understood how the presence of boulders can affect the characteristics

of the sediments underneath them and, ultimately, the assemblages living in those

sediments.

Boulders provide three microhabitats: the upper and the lower surface of boulders and

the substratum underneath the boulders. The former two have been considered in several

studies (e.g. Sousa, 1979; McGuinness and Underwood, 1986; McGuinness, 1987a,b;

Rocha, 1995; Chapman, 2002a,b), but the last has been little studied. Substrata underneath

a boulder can be hard, when the boulder lies either on the rock-platform, or on another

boulder, or it can be soft, if the boulder is on a layer of sediment (Takada, 1999; Chapman,

2002b). A few studies have been done on assemblages living on hard surfaces below

boulders (e.g. Todd and Turner, 1986, 1988), but assemblages in sediments below

boulders have not been investigated, despite this habitat being potentially very different

from sediments where there are no boulders. The effect of a boulder on soft-bottom

macrobenthic assemblages immediately adjacent to them has, however, been studied (e.g.

Cusson and Bourget, 1997).

There have been few comparisons of physico-chemical environmental variables (such

as temperature, salinity, oxygen and redox) in sediments below boulders compared, to

measures in sediments outside the influence of boulders (Agnew and Taylor, 1985,

1986). No studies have been done on the effects of boulders on variables such as the

distribution of grain-sizes of the sediment. It has been recognised that distributions of

grain-size in sediment, co-varies with more ‘‘meaningful’’ hydrodynamic variables (such

as sediment-transport and bottom boundary-layer flow), which may structure soft-bottom

benthic assemblages (Jumars and Nowell, 1984; Snelgrove and Butman, 1994; Schaffner

et al., 2001). In this study, grain-size was used as an ‘‘indicator’’ of hydrodynamic

variables.

J.J. Cruz Motta et al. / J. Exp. Mar. Biol. Ecol. 285–286 (2003) 383–401384

Intertidal boulder-fields represent important habitats around the world and in temperate

Australia, despite the fact that they can be uncommon and very scattered along the coast

(Chapman, 2002b). The presence of boulders on sediments allows the co-existence of

species living on rocky substrata and in sediments, increasing diversity and richness in

these topographically heterogeneous systems (Cusson and Bourget, 1997). Furthermore,

some of the microhabitats associated with boulders (e.g. the undersurfaces and hard

substrata below boulders) provide habitats for species that may be rare elsewhere (Rocha,

1995; Chapman, 2002a).

The habitats surrounding boulders can be an important source of colonists for

assemblages living on boulders (Chapman, 2002b). Nevertheless, the processes of

colonisation are not well understood, because relationships between the boulders them-

selves, the assemblages living on them, the sediment below/around those boulders and the

assemblages living in those sediments are not known. Before such processes can be

understood, it is important to understand the spatial and temporal patterns of distribution of

these assemblages (e.g. Underwood et al., 2000).

This study describes patterns of spatial variability in the composition of sediments

associated with boulder-fields and the benthic assemblages living in them. It is

generally assumed that physico-chemical conditions in sediments influence benthic

assemblages. We propose that the presence of a boulder will change conditions in

sediments below the boulder and, therefore, the composition of the soft-bottom benthic

assemblages living in those sediments. Based on this model, the present study was

done to test the general hypothesis that sampling below boulders and equivalent areas

of sediment without boulders would reveal consistent differences in sediments and in

assemblages.

2. Material and methods

2.1. Study sites and sampling

Boulder-fields at six different locations in the metropolitan area of Sydney (New South

Wales, Australia) were sampled (Fig. 1). These boulder-fields were very different in terms

of their morphology (Table 1) and were classified into two types: those in estuarine

sheltered areas (Chowder Bay, Bradley’s Head and Hungry Point; Fig. 1) and those in

coastal, wave-exposed areas (Cape Banks, Little Bay and Avalon Beach; Fig. 1). Sheltered

boulder fields were adjacent to reflective beaches in narrow protected bays inside the

Sydney Harbour and Port Hacking estuary. These locations are protected from the oceanic

swell resulting in no wave action during most of the year, except during February and

March, when the winds from the east can cause maximum wave heights of 0.5 m (Short

and Trenaman, 1992; Short, 1993). Exposed boulder fields were located in headlands

directly exposed to the Pacific Ocean. These locations were directly exposed to the

predominant swell and winds from the east and southeast, with an average annual wave

height of 1.59 m and maximum wave height of more than 4 m (Short and Trenaman,

1992). Boulders of the sizes considered in this study in sheltered (pers. obs.) and exposed

(McGuinness, 1984) locations are minimally disturbed by normal wave conditions.

J.J. Cruz Motta et al. / J. Exp. Mar. Biol. Ecol. 285–286 (2003) 383–401 385

Four of these locations were sampled twice (August, 2001 and November, 2001),

while Hungry Point and Avalon Beach were sampled only during November, 2001. At

each location and time, six randomly chosen boulders of similar sizes (approximately

1000 cm2 undersurface area) were carefully overturned and the sediment below sampled

using two cores of 12 cm diameter. Boulders fitting tightly through a flexible piece of

wire delimiting 1000 cm2 were selected. Sediments in the cores were retrieved using an

Fig. 1. Map of study sites in the metropolitan area of Sydney, NSW, Australia. Exposed sites: AB=Avalon Beach,

CB=Cape Banks, LB=Little Bay; sheltered sites: Cho =Chowder Bay, Bra =Bradley’s Head, Hun =Hungry

Point.


electrically powered suction sampler. Once sampled, boulders were returned to their

original position. Six randomly chosen areas of sediment of equivalent size (1000 cm2),

at least 0.5 m away from any boulder were sampled in the same way. An extra sample,

using a core of 10 cm diameter, was taken from each area for analyses of granulometry

and total organic content.

Samples were sieved using a 0.5-mm sieve, stored in a bag, fixed with a buffered

7% solution of formalin in seawater and stained with neutral red. Samples were later

sorted and the animals identified to the lowest taxonomic level possible (mainly

families). Within each family, animals were distinguished as morpho-species (Oliver

and Beattie, 1993). Samples for granulometry and organic matter were frozen until

analysed. Grain-sizes were analysed using standard dry-sieving techniques on a dry

sample of 200 g of sediment (Lewis and McConchie, 1994). Total organic matter was

determined in a 4–5 g sub-sample from weight loss after combustion at 450 jC for 12

h (Gross, 1971).

2.2. Data analyses

Bray–Curtis similarity matrices of samples were constructed from the original

biological data matrices. Non-metric Multidimensional Scaling (nMDS) were done on

the similarity matrices to illustrate patterns in the spatial distribution of the samples

in terms of the soft-bottom benthic assemblage structure.

Two-factor nonparametric multivariate analyses of variance (NP-MANOVA; Ander-

son, 2001) at each sampling time tested the hypotheses of: (1) no differences in

assemblages between samples taken below boulders and outside boulders, (2) no

differences between samples taken at different sites and (3) no interaction between

these two factors. Due to significant interactions between these two factors, sites were

then analysed independently using one way analyses of similarities (ANOSIM in

PRIMER; Clarke, 1993).When significant differences were found, taxa making the

greatest contribution to these differences were detected using SIMPER (PRIMER;

Clarke, 1993).

Four-factor analyses of variance (ANOVA) tested for differences between time,

exposure, site and presence/absence of boulder, on the total number of taxa (richness)

and total organic matter content in the sediment.

Table 1

Morphological characteristics of the sites sampled; artificial habitats are boulder fields adjacent to sea walls

Location Cape Banks Little Bay Avalon Beach Chowder Bay Bradley’s Head Hungry Point

Exposure Exposed Exposed Exposed Sheltered Sheltered Sheltered

Length (m) 100 70 300 100 200 30

Width (m) 40 20–30 50 20 20 15

Habitat Natural Natural Natural Artificial Natural Artificial

Composition Sandstone Sandstone Shale Sandstone Sandstone Sandstone

Sediment Calcareous Calcareous Terrigenous Terrigenous Terrigenous Terrigenous

Usage Research

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Table 2

Summary information of characteristics of the sediment and assemblages in the six areas sampled

Location Cape Banks

(exposed)

Little Bay

(exposed)

Avalon

(exposed)

Chowder Bay

(sheltered)

Bradley’s Head

(sheltered)

Hungry Point

(sheltered)

Time Aug Nov Aug Nov Nov Aug Nov Aug Nov Nov

Total no.

of individuals

906 3076 4621 3524 538 2115 811 1143 2070 2533

Total no. of taxa

per sample

148 92 58 72 66 139 75 93 67 111

Range of organic

content (%)

0.82–1.99 1.35–2.93 0.73–2.32 0.75–2.90 1.20–4.25 0.64–3.42 0.40–4.33 0.80–3.80 0.30–5.03 0.70–3.71

Classification of

grain-size

Medium

sand

Medium

sand

Medium

sand

Medium

sand

Medium

sand

Fine sand Fine sand Fine sand Fine sand Fine sand

Most abundant taxaa Gammaridae sp. B Dorvilleidae sp. B Nematoda Cypridinidae sp. A Exogoninae sp. A Cerithiidae sp. A

(totalling 50% of Nematoda Nemertean sp. A Cypridinidae sp. B Cerithiidae sp. A Gammaridae sp. A Exogoninae sp. B

total abundance) Exogoninae sp. B Gammaridae sp. A Gammaridae sp. A Exogoninae sp. A Oligochaeta sp. A Gammaridae sp. A

Dorvilleidae sp. A Dorvilleidae sp. A Anthuridea sp. A Chaetopteridae sp. A Ischyroceridae sp. A Anthuridea sp. B

Gammaridae sp. A Syllidae sp. A Syllidae sp. A Gammaridae sp. A Oligochaeta sp. A

Syllidae sp. A Sphaeromatidae sp. A Oligochaeta sp. A Cypridinidae sp. B Cypridinidae sp. A Cirratulidae sp. A

Gastropod sp. C Syllidae sp. A Gastropod sp. A

Sipunculid sp. B Ophiuroid

Chrysopetalidae sp. A Trichoptera sp. A

Sphaerosyllis sp. A Trochidae sp. A

Corophiidae sp. A

Oligochaeta sp. A

Gammaridae sp. E

a Letters indicate morpho-species within families and higher groups.

J.J.CruzMotta

etal./J.

Exp.Mar.Biol.Ecol.285–286(2003)383–401

388

3. Results

3.1. Characteristics of sediments

Sediments in all locations and at each time ranged from ‘‘slightly gravelly sands’’ to

‘‘gravelly sands’’, based on the textural classification for gravel-bearing sediments

proposed by Folk (1954, in Lewis and McConchie, 1994). Mean grain-size classified

the sediments as ‘‘fine sands’’ or ‘‘medium sands’’, depending on the locality (Table 2).

The contents of fine fractions ( < 0.063 mm) were always minimal ( < 1%).

Fig. 2. Mean (S.E.) percentage of total organic matter in the sediments of the six locations sampled. = below

boulders, n =without boulders, (a) August, 2001, (b) November, 2001; n= 6.


The percentage of organic matter in sediments below boulders was significantly

greater than in sediments without boulders, during August, 2001 (analysis of variance,

F = 82.4, 1 and 3 df, p < 0.01) and November, 2001 (F = 8.8, 1 and 5 df, p < 0.05).

These differences were consistent across all locations (Fig. 2a,b).

During August, 2001, the characteristics of the sediment below boulders differed

from those without boulders, in sheltered locations only. These sediments were

characterised as having fine skewed and leptokurtotic distributions of grain-size, with

a mode at 0.25 mm. In the two sheltered locations (Chowder Bay and Bradley’s

Head), there was also a significantly greater content of gravel in samples taken from

below boulders than from samples without boulders (F = 9.0, 1 and 10 df, p < 0.05;

F = 5.3, 1 and 10 df, p < 0.05 for Chowder Bay and Bradley’s Head, respectively).

This difference was reflected in the inverse relationship of the finer fractions of the

distribution (Chowder Bay, 2/: F = 5.9, 1 and 10 df, p < 0.05; Bradley’s Head, 3/:F = 6.2, 1 and 10 df, p < 0.05; Fig. 3a,b).

Sediments in exposed locations were characterised as having almost symmetrical

distributions of grain-size, with a coarser mean than in sheltered places. In contrast to

sheltered places, there were no significant differences between sediments below and

those without boulders (Fig. 3c,d). Differences in the percentage of gravel, between

samples from below boulders and those without boulders, were significant (F = 5.4, 1

Fig. 3. Grain-size distribution of the sediment in four locations during August, 2001. / =�Log2 (diameter in mm

of particle size). = below boulders, n =without boulders. (a) Chowder Bay, (b) Bradley’s Head, (c) Cape

Banks, (d) Little Bay. *Significant difference ( p< 0.05), NS = not significant ( p>0.05).


and 10 df, p< 0.05) at Little Bay (Fig. 3d) but this was not the case for finer

components of the sediment.

During November, 2001, the four locations sampled previously showed exactly the

same patterns of differences in the grain-sizes. Grain-sizes were not different between

samples below and without boulders at Cape Banks and Little Bay (exposed locations;

Fig. 4b), but were at Chowder Bay and Bradley’s Head (sheltered locations; Fig. 4a).

Of the two additional locations sampled during November, 2001, Hungry Point

(sheltered) gave results as found earlier for sheltered places (Fig. 4c): sediments were

typically leptokurtotic and skewed to the left, with significantly more gravel in

sediments below boulders (F = 45.1, 1 and 4 df, p < 0.01). This difference was

supported by the inverse significant relationship for finer fractions (F = 10.2, 1 and 4

df, p< 0.05; Fig. 4c).

At Avalon Beach (exposed), sediments were more similar to those in sheltered

locations than to sediments in other exposed locations (Fig. 4d). The sediment was

leptokurtotic and strongly skewed to the left, with a big mode in the finer sediments

(0.25 mm). As had been found before for sheltered locations, there was a significantly

greater percentage of gravel below boulders (F = 11.8, 1 and 4 df, p < 0.05) and the

inverse relationship for finer fractions was also significant (F = 12.5, 1 and 4 df,

p < 0.05).

Fig. 4. Grain-size distribution of the sediment in four locations during November, 2001. / =�Log2 (diameter in

mm of particle size). = below boulders, n =without boulders. (a) Chowder Bay, (b) Cape Banks, (c) Hungry

Point, (d) Avalon beach. *Significant difference ( p< 0.05), NS = not significant ( p>0.05).


3.2. Macrobenthic assemblages

In the 240 samples, there were 183 taxa (Table 2). Several (24) taxa found in this

study were singletons (found only once in one sample) and only between 6 and 12

taxa comprised 50% of the total number of animals (Table 2). There was a consistent

higher number of species in sediments below boulders in comparison to sediments

outside boulders (F = 4.13, 1 and 87 df, p < 0.05). Nevertheless, the mean differences

were relatively small (# taxa below = 24.8F 1.5 S.E.; # taxa outside = 22.1F1.5

S.E.).

Fig. 5. nMDS ordinations, of centroids of samples from four locations during August 2001 (a) and in six locations

during November 2001 (b). 4=Chowder Bay, 5 =Bradley’s Head, 5=Hungry Point, o =Cape Banks,

w =Little bay, B =Avalon Beach. Clear symbols = below boulders; shaded symbols =without boulders.


NP-MANOVA showed that during each sampling occasion, differences in assemb-

lages of macrobenthos under and away from boulders varied interactively among

locations (3 and 40 df, p < 0.05; 5 and 60 df, p < 0.05 for August and November,

respectively), as illustrated in Fig. 5a,b. In addition, it can be noted in Fig. 5a,b that

dissimilarities among locations are greater than dissimilarities between below and

outside boulders in each location. Consequently, any influence of boulders was

examined independently in each location at each time.

3.2.1. August 2001

In August, 2001, assemblages in sediments below boulders were different from

assemblages in sediments without boulders only in sheltered locations (Table 3a). The

average dissimilarity values within and between treatments were consistent with

results of ANOSIM tests. Samples from below boulders clearly separated from

samples without boulders in nMDS plots (Fig. 6a,b), although this was not as clear

at Bradley’s Head (Fig. 6b) as in Chowder Bay (Fig. 6a).

SIMPER analyses showed that, in Chowder Bay, 32% of the total dissimilarity

between the two habitats (Table 3a) was associated with taxa almost exclusively found

under boulders: gastropods (Cerithiidae sp. A and Trochidae sp. A) and two unidentified

species of ophiuroids. These taxa were very sparse in sediments without boulders. The

remaining dissimilarity was associated with taxa that were more abundant where there

was no boulder, typically ostracods (Cypridinidae sp. B), polychaetes (Chaetopteridae

sp. A, Exogoninae sp. A and Syllidae sp. A) and amphipods (Dexaminidae sp. C,

Gammaridae sp. A and Corophiidae sp. A). At Bradley’s Head, 93% of the total

dissimilarity between samples from the two habitats was associated with taxa that were

more abundant where there were no boulders. These were polychaetes (Exogoninae sp.

A and Sabellidae sp. A), amphipods (Gammaridae sp. A, Hyalidae sp. A, Ischyroceridae

Table 3

Summary of Bray–Curtis dissimilarities between and within samples of assemblages taken in sediments below

and without boulders, during tow sampling times and results of one-factor ANOSIM tests

Location Dissimilarities within

groups (%)F S.E.

Dissimilarities between

groups (%)F S.E.

ANOSIM ( p)

Below Without(below vs. without)

(a) August, 2001

Chowder Bay 63F 1.6 71F 2.8 77F 1.3 0.002

Bradley’s Head 66F 1.7 64F 4.2 73F 2.2 0.04

Cape Banks 70F 3.5 71F 2.4 70F 2.1 0.49

Little Bay 74F 9.0 83F 3.0 77F 4.1 0.55

(b) November, 2001

Chowder Bay 72F 3.1 67F 5.1 74F 1.7 0.002

Bradley’s Head 53F 4.6 39F 2.2 67F 2.8 0.002

Hungry Point 60F 3.4 57F 1.8 67F 2.8 0.01

Cape Banks 70F 3.5 71F 2.4 70F 1.5 0.50

Little Bay 25F 2.4 28F 4.3 27F 1.9 0.43

Avalon Beach 60F 2.6 55F 1.9 64F 1.6 0.03


sp. A), ostracods (Cylindroloberididae sp. A, Cypridinidae sp. A) and bivalves (Tell-

inidae sp. A). Contrary to Chowder Bay, no taxa were found mostly or exclusively

below boulders.

nMDS ordinations and ANOSIM tests for exposed locations (Cape Banks and Little

Bay), did not show any differences between the two habitats (Fig. 6c,d; Table 3a).

3.2.2. November 2001

During November, 2001, the four locations sampled previously showed the same

patterns of distributions of assemblages (Table 3b). There were significant differ-

ences between habitats in the two sheltered locations (Bradley’s Head and Chowder

Bay; Fig. 7a), but not in the exposed locations (Little Bay and Cape Banks; Fig.

Fig. 6. nMDS ordinations of samples from (a) Chowder Bay, (b) Bradley’s Head, (c) Cape Banks and (d) Little

Bay; during August, 2001. Clear symbols = below boulder; shaded symbols =without boulder.


7b). The taxa associated with most of the dissimilarity observed between habitats in

sheltered locations were similar to those found in August, 2001.

At Hungry Point (the additional sheltered location), as expected for sheltered

places, there was a similar difference in the assemblage in sediments below boulders

from that where there was no boulder (ANOSIM, p < 0.05; Fig. 7c). SIMPER

analyses showed that, 41% of the total dissimilarity between the two habitats was

associated with taxa that were found almost exclusively below boulders: gastropods

(Cerithiidae sp. A and Trochidae sp. A) and amphipods (Gammaridae sp. A and

Podoceridae sp. A). Apart from amphipods, the other taxa were very sparse where

there were no boulders. The remainder of the dissimilarity was associated with taxa

that were more abundant where there were no boulders, typically nematodes,

Fig. 7. nMDS ordinations, of samples taken in (a) Chowder Bay, (b) Cape Banks, (c) Hungry Point and (d)

Avalon Beach; during August, 2001. Clear symbols = below boulder; shaded symbols =without boulder.


polychaetes (Exogoninae sp. B and Cirratulidae sp. A), amphipods (Ischyroceridae sp.

A), anthurids and oligochaetes.

Contrary to what was expected from other exposed locations, Avalon Beach

showed a difference in the assemblages below boulders from those where there were

no boulders (Table 3b; Fig. 7d). No taxa were conspicuously found below boulders,

but 96% of all the taxa that were reported for this location were more abundant

below a boulder than in sediments without a boulder.

4. Discussion

4.1. Characteristics of sediments

Sediments below boulders had a greater percentage of organic matter than sediments

not overlain by boulders. In addition, the sediment below boulders had a greater

percentage of coarse material (gravel) in all locations, although these differences were

significant only in the sheltered locations and at one exposed location, Avalon Beach.

These findings contradict the notion that total organic content is positively correlated with

content of fine material in soft-bottom intertidal ecosystems (e.g. Gray, 1981; Alongi,

1998). One possibility is that boulders function as traps for large pieces of detritus. It has

been suggested that introduced structures (e.g. artificial reefs, breakwaters and pier pilings)

can entrap drifting organic material, such as algae and seagrass debris, which could

potentially increase the levels of organic matter in the adjacent sediments (Davis et al.,

1982; Agnew and Taylor, 1986). It is also very likely that organic material in sediments

below boulders is enhanced by the death of organisms living on the undersurfaces of the

boulders.

Once this organic material is trapped, it is possible that fragmentation of this material is

slowed, because important factors involved in fragmentation, such as tidal or wave-energy

(Alongi, 1998), cannot operate below boulders. In addition, the typical reduction in

numbers or absence of autotrophs below boulders means that respiration by the fauna

decreases the PO2, especially during low tide, creating anoxic conditions in these

environments (Agnew and Taylor, 1986). It is known that anoxic conditions can affect

rates of maceration, fragmentation and incorporation of organic matter (e.g. Kerner, 1993).

Cusson and Bourget (1997) found that the percentage of fine fractions (and conse-

quently the total organic content) was greater near to boulders ( < 0.75 m) than away

(>0.75 m). The authors attributed these findings to faster rates of deposition of fine

material near boulders, due to slower flow in these regions. In contrast, authors studying

sediments in areas close to ( < 1 m) and away from (10–20 m) subtidal rocky reefs have

found that the percentage of coarse material increased with proximity to the rocky

structure, potentially due to faster water-movement near the reef (Ambrose and Anderson,

1990; Barros et al., 2001).

Our results suggest that there may be faster flow below boulders, causing poor retention

of fine materials (Jumars and Nowell, 1984). This contrasts with the intuitive idea that

there should be slower flow below boulders than elsewhere. The effects of individual and

groups of boulder-like objects on the flow of water and, consequently, the immediate


surrounding sediments are well known (e.g. Jumars et al., 1981; Nowell and Jumars, 1984;

Paola et al., 1986), although no attempt has been made to measure flow below boulders.

This may be due to the assumption that no component of flow will extend underneath a

boulder, or to logistical constraints in measuring this variable. Nevertheless, in future this

variable must be measured, so that the different models can be evaluated.

4.2. Macrobenthic assemblages

Soft-bottom benthic assemblages living in sediments associated with boulder-fields

were very different from one location to another, in agreement with previous findings on

assemblages living on top of boulders (e.g. McGuinness and Underwood, 1986). These

different assemblages had different patterns of spatial distribution in relation to the

presence of boulders.

Cerithiidae sp. A were always abundant below boulders compared to sediments

elsewhere in Chowder Bay and Hungry Point, which has been reported before for other

species of Cerithiidae (e.g. Rao and Sarma, 1979; Ayal and Safriel, 1982). This suggests

that crevices and habitats below boulders could act as refuges for these snails. Similarly,

ophiuroids were more abundant in sediments below boulders at Chowder Bay, which has

also been extensively reported in other studies (e.g. Sides and Woodley, 1985; Soliman,

1991; Chapman, 2002a). These results would suggest that, at least for these locations,

boulders are functioning as refuges (sensu Woodin, 1978) for some taxa. Boulders have

been reported to offer refuge not only for these taxa, but also for crabs (e.g. Snyder-Conn,

1981) and chitons (e.g. Smith and Otway, 1997). Alternatively and given that we sampled

only during the day during low tide, greater abundances below boulders of these taxa

could be due to activity rhythms related to tide/light. Previous studies have reported such

diurnal patterns for cerithiids on rocky platforms with boulders (Ayal and Safriel, 1982),

cerithiids in mangroves (Cockcroft and Forbes, 1981), ophiuroids below coral rubble

(Sides and Woodley, 1985) and crabs below boulders (Snyder-Conn, 1981). All of these

studies reported greater densities of organisms below boulders or other refuges during the

day or during low tide.

Contrary to findings in Chowder Bay and Hungry Point, no taxa were exclusively

found below boulders at Bradley’s Head. In addition, almost all taxa in this location were

more abundant in sediments without a boulder than in those below a boulder. Boulders in

this location may represent a disturbance, instead of a provider of refuge as the movement

of boulders has been previously reported to be an important factor in disturbing macro-

benthic assemblages on hard-substrata (e.g. Shanks and Wright, 1986). Nevertheless,

results obtained at the exposed location Avalon Beach, where greater abundances of

organisms were found in sediments below boulders than elsewhere, would suggest

differently.

Alternatively, it is possible that food was a limiting factor in sediments below boulders

at Bardley’s Head as there was less total organic contents in the sediments below boulders

here than in any other location. It is also possible that physico-chemical conditions of the

substratum below boulders at Bradley’s Head are not different from those in other

sheltered locations, but larvae of the taxa likely to recruit below boulders are not present

at Bradley’s Head. This has been extensively reported for many taxa (see review by


Olafsson et al., 1994) and more specifically, for other cerithiidae (e.g. Ayal and Safriel,

1983).

4.3. Assemblage–sediment relationships

Based on our main results, it appears that the presence of a boulder only affects soft-

bottom macrobenthic assemblages in locations of low energy regimes. Based on the

assumption that distribution of grain-sizes co-varies with ‘‘more meaningful’’ factors (such

as water-flow) that explain the distribution of soft-bottom macrobenthic assemblages

(Jumars and Nowell, 1984; Snelgrove and Butman, 1994), it could be argued that

hydrodynamics conditions are different below boulders from places without boulders

only in sheltered locations.

Alternatively, changes in the sediment can also be created by the fauna inhabiting

them, by changing the shear-stress, roughness of the bed or granulometry (e.g. Hall,

1994; Heip et al., 1995; Herman et al., 1999). It is also known, that these biogenic

influences are maximised in areas where influences of physical factors (e.g. tidal currents,

waves) are not very strong (Probert, 1984). Consequently, it could be argued that

differences in the grain-sizes of the sediment below boulders could be due to an indirect

effect of the different assemblages living in those sediments and not to a direct effect of

physical factors on those sediments. It is very probable that the effects of physical factors,

such as tidal currents, are minimised in sediments below boulders, especially in sheltered

conditions.

Nevertheless, the above two models exclude the fact that, at one exposed location

(Avalon Beach), there were differences in distribution of grain-sizes and macrobenthic

assemblages in sediments below boulders compared to those without boulders. Although

these results were found at only one location at one time, it is interesting to note that this

location, despite being wave-exposed (Short, 1993), had grain-sizes more similar to those

in sediments in sheltered places. Therefore, the sediment and patterns in the fauna

resembled sheltered shores.

The literature reporting direct relationships between grain-sizes and macrobenthic

assemblages and populations is extensive, although it has been criticised for being

generally correlative, with few experimental studies of factors influencing these assemb-

lages (Snelgrove and Butman, 1994). Nevertheless, some studies have experimentally

examined the effects of grain-sizes and other factors on benthic assemblages/populations

(e.g. Skilleter and Underwood, 1993). More specifically, relationships between grain-size

and number of chitons have been reported for sediments below boulders (Smith and

Otway, 1997). Some taxa (e.g. Nereis virens) are positively correlated with conditions

similar to those reported in this study below boulders (i.e. much gravel and organic matter;

Miron and Desrosiers, 1990).

In summary, this study showed that soft-bottom benthic assemblages are affected by the

presence of boulders only in some circumstances. These circumstances appear to be

related to the energy of the system and/or to the grain-size. To determine whether the

grain-size itself and/or other hydrological variables are affecting the benthic assemblages

living in sediments associated with boulder-fields, further work, including the relevant

manipulative experiments, is needed.


Acknowledgements

We thank S. Cruz, A. Kazandjian, S. Monteiro and W. Widmer for their assistance in

the field, S. Cummins, A. Grigaliunas and L. Lee for their assistance in the field and

laboratory and two anonymous referees for valuable improvements to the paper. Financial

support was given by the Australian Research Council through the Centre for Research on

Ecological Impacts of Coastal Cities and an International Postgraduate Scholarship

(awarded to JJCM) at the University of Sydney. [RW]

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