an integrated study of the temporal and spatial variation in the supply of propagules, recruitment...

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Journal of Experimental Marine Biology and Ecology

310 (2004) 207–225

An integrated study of the temporal and spatial

variation in the supply of propagules, recruitment

and assemblages of intertidal macroalgae on a

wave-exposed rocky coast, Victoria, Australia

Alecia Bellgrovea,b,c,*, Margaret N. Claytona, G.P. Quinna

aSchool of Biological Sciences, Monash University, Clayton, Victoria 3168, AustraliabQueenscliff Marine Station, Weeroona Parade, Queenscliff, Victoria 3225, Australia

cSchool of Ecology and Environment, Deakin University, Warrnambool, Victoria 3280, Australia

Received 7 November 2003; received in revised form 25 March 2004; accepted 22 April 2004

Abstract

Recruitment is known to influence distributions and abundances of benthic marine organisms. It

is therefore important to document patterns of variability in recruitment and how these relate to

patterns in established assemblages. This study provides an integrated assessment of the temporal

and spatial variation in supply and recruitment of propagules and established populations of several

macroalgae. Propagules in water samples from two stages of the incoming tide, recruitment to

artificial substrata and percentage cover of species established on the shore were recorded every 2

months from December 1994 to October 1995, in two zones of an intertidal, wave-exposed rocky

shore. Variability in recruitment was measured at three spatial scales: 10s cm, 100s cm and 100s m.

Availability and recruitment of most taxa were greatest between April and August, although many

species had available propagules and recruited throughout the year. Temporal variation in the

established assemblages was, however, more species-specific. Differences in established

assemblages between zones were reflected in differences in availability and recruitment of

propagules between zones. Recruitment could not be predicted directly from supply of propagules,

but the two processes were linked. For most species, the greatest variation in recruitment occurred at

the smallest spatial scale of 10s cm, although there was also considerable large-scale (between site)

variation in recruitment of several species. Results indicate that while pre-and post-settlement

mortality are likely to influence macroalgal distribution and abundance, the temporal and spatial

0022-0981/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.jembe.2004.04.011

* Corresponding author. Current address: School of Ecology and Environment, Deakin University, P.O. Box

423, Warrnambool, Victoria 3280, Australia. Tel.: +61-3-5563-3099; fax: +61-3-5563-3462.

E-mail address: [email protected] (A. Bellgrove).

A. Bellgrove et al. / J. Exp. Mar. Biol. Ecol. 310 (2004) 207–225208

variability in supply and recruitment of propagules can explain much of the patchiness in macroalgal

assemblages.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Artificial substrata; Macroalgae; Recruitment; Rocky shores; Supply of propagules; Variation

1. Introduction

Researchers now realise that models of processes structuring communities are

incomplete without some assessment of the variability and importance of recruitment

(Dayton, 1979; Keough, 1983; Underwood and Denley, 1984; Underwood and Fair-

weather, 1989; Watanabe, 1984; Caffey, 1985). Many studies have examined the factors

affecting recruitment and their importance to the distribution and abundance of marine

organisms. However, these studies have largely focused on fish (e.g. Jones, 1984; Sale et

al., 1984; Mapstone and Fowler, 1988; Carr, 1991) and invertebrates (e.g. Keough and

Downes, 1982; Caffey, 1985; Gotelli, 1988; Raimondi, 1988; Petraitis, 1990; Hurlbut,

1991). There remain few studies of macroalgal recruitment, despite macroalgae being a

conspicuous and important component of both intertidal and subtidal communities.

Studies to date show factors such as developmental stage and microhabitat (Harlin and

Lindbergh, 1977; Brawley and Johnson, 1991), predation, sedimentation and season

(Neushul et al., 1976), canopy shading (Reed and Foster, 1984; Schiel, 1988), temporal

and spatial variability in availability of propagules (Hoffmann, 1987; Hoffmann and

Ugarte, 1985), and water motion (Charters et al., 1972; Serrao et al., 1996; Pearson et al.,

1998) may play an important role in macroalgal recruitment (for a thorough review, see

Santelices, 1990).

Scales of temporal and spatial variability in the abundance of organisms are also

important in understanding processes influencing structure and dynamics of communi-

ties. Consequently, the need for studies describing these patterns has been advocated for

many ecosystems (Ellison et al., 1993; Brown and Hopkins, 1996; Maugeri et al., 1996;

Hancock and Bunn, 1997). For marine systems, Underwood and Chapman (1996)

emphasised the need to study spatial patterns of organisms at different scales in order

to elucidate the ecological processes that may govern these patterns. Similarly, Morrisey

et al. (1992b) highlighted the importance of a thorough understanding of temporal

patterns of organisms at several scales. While several studies have assessed the temporal

and spatial variation in the abundances of adult (Green and Hobson, 1970; Thrush, 1991;

De Vogelaere, 1992; Morrisey et al., 1992a,b; Underwood and Chapman, 1996) and

larval (Caffey, 1985; Butman, 1987) populations of marine benthic invertebrates, few

studies have examined these patterns for established algal assemblages, particularly in

Australia (Underwood and Kennelly, 1990; Underwood and Chapman, 1998a,b; Chap-

man et al., 1995), and fewer still have examined the temporal and spatial variability in

early life-history stages of macroalgae (e.g. Jernakoff, 1983; Kennelly and Larkum,

1983; Flavier and Zingmark, 1993). However, Fairweather (1991) emphasised the need

to understand variation in early life-history stages, and assess the impact of this variation

on adult life-stages.

A. Bellgrove et al. / J. Exp. Mar. Biol. Ecol. 310 (2004) 207–225 209

Supply-side ecology suggests that populations may be recruitment-limited, or that the

early life-history stages are subject to the greatest selection pressures (Dayton, 1979;

Underwood and Denley, 1984; Underwood and Fairweather, 1989; Keough, 1988). It is

therefore important that studies simultaneously examine a range of demographic stages,

rather than focus on recruitment alone. Santelices et al. (1995) compared temporal

variation in composition and abundance of taxa in ‘the bank of microscopic forms’ found

on boulders in tidal pools, in the water column and in the surrounding macroalgal

vegetation, but did not directly examine recruitment.

Here we test the model that temporal and spatial variability in the supply and/or

recruitment of macroalgal propagules is reflected in the patterns of distribution and

abundance of macroalgae in established assemblages. The aim of the study was to quantify

the temporal (intra-annual) and spatial (10s cm–100s m) variation in the recruitment

processes of intertidal macroalgae on a wave-exposed rocky coast and to determine

whether there is a link between the variability in supply and recruitment of macroalgal

propagules and the variability in established macroalgal assemblages.

2. Materials and methods

2.1. Study sites

The study sites were all intertidal rock platforms of dune limestone/sandstone

conglomerates at Point Nepean in the Mornington Peninsula National Park, southeastern

Victoria, Australia (Fig. 1). This area of coastline is exposed to the weather conditions of

Bass Strait, with prevailing southwesterly winds and associated high swells (for a more

Fig. 1. Location of study sites (arrows) on the Victorian coastline, Australia.


detailed description of this area see Povey and Keough, 1991). The study sites have been

closed to the public since white occupation of Australia, with restricted access only.

Cheviot Beach (38j18VS, 144j37VE) was the main study site for the temporal

investigation, while additional sites were selected for the spatial investigation: Big Rock,

Cheviot Beach West and Fort Pearse (f 300 m west, f 750 m west and f 2 km

northwest of Cheviot Beach, respectively). At Cheviot Beach, two algal habitats were

defined: a ‘Low-shore zone’ in the lower-littoral, composed primarily of a range of turfing

and filamentous species, and a ‘Hormosira zone’ in the mid-littoral, dominated by a

canopy of Hormosira banksii (Turner) Decaisne with an understorey of turfing and

filamentous species. These rock platforms are very gently sloping such that the Low-shore

zone, while closer to the surf break, was at a similar height to the Hormosira zone, and

there was often no time lag in tidal inundations between the zones.

2.2. Temporal investigation

Temporal patterns of supply of macroalgal propagules, recruitment and abundance on

the shore in the Low-shore and Hormosira zones at Cheviot Beach were examined from

December 1994 to October 1995 with sampling every 2 months.

Previous studies (Hoffmann and Ugarte, 1985; Zechman and Mathieson, 1985;

Bellgrove and Aoki, in review; Bellgrove et al., 1997) have suggested that the composition

of propagules in the water column may vary at different tidal states. Thus, to determine

which taxa had propagules potentially available for settlement in both the Low-shore and

Hormosira zones, five replicate 200 ml water samples were collected every 2 months in

each zone at two stages of the incoming tide: from (i) the ‘early wash’—as the water

gently moved over the algal beds with the incoming tide (approx. maximum depth of 30

cm), potentially capturing propagules that may be released at low tide and (ii) the

‘waves’—as the turbulent incoming waves at a later tidal state washed over the algae

(approx. maximum depth of 1 m), potentially capturing propagules that may be released

after re-immersion. Samples were collected by scooping the water washing over the algal

beds on the incoming wash or waves.

The samples were returned to the laboratory on ice, where 4 ml of Provasoli ES medium

(Starr and Zeikus, 1987) were added to each sample within a laminar-flow cabinet to prevent

contamination. The samples were then placed under Grolux WS fluorescent lights on a 12-

h light/dark regime with a mean irradiance of 28 Amol m� 2 s� 1. After 1 week, the seawater

was replaced with 100 ml of culture medium in filtered (to 0.22 Am) seawater; this was

changed weekly until algal growth was sufficient for identification to genus level with the

aid of a dissecting microscope (approx. 5 weeks). The base of the culture jar was divided into

32 equal regions, excluding the outer 1 cm margin, and the number of individuals of each

algal genus within eight randomly selected regions was recorded (Bellgrove et al., 1997).

Artificial substrata were used to examine recruitment. A sandstone boulder from Cheviot

Beach with surface structure representative of that found in the Low-shore and Hormosira

zones was moulded and 120, 10� 10 cm clear polyester resin casts made (Bellgrove, in

review(a)). Recruitment to these resin rock panels was comparable to that natural rock

(Bellgrove, in review(a)). At each sampling time, 10 identical resin rock panels were

attached to the rock platform in both the Low-shore and Hormosira zones (i.e. 20 panels in


total). The panels were left for a period of 4 weeks to allow macroalgal recruitment, then

collected in individual containers (without water to prevent disturbance during travelling)

and transported to the laboratory on ice. In the laboratory, 500 ml micropore-filtered (to 1

Am) seawater were added to each container and then 100 kPa of CO2 was bubbled into each

container for 5 min to remove copepods that invariably infested the panels in the field. The

panels were then randomly arranged in four large aquaria with flow-through, micropore-

filtered (1 Am) seawater and a natural light/dark cycle. Panels were left for 4 weeks for algal

growth to reach a stage where recruits could be identified to genus level. CO2 was bubbled

through the aquaria at 100 kPa for 0.5 h per week during this period to kill any copepods

that had developed from resistant stages. Macroalgal recruits were identified and counted in

the laboratory, with the aid of a dissecting microscope with both sub-stage and external light

sources. Counts were made in a random subsample of 1 cm2 plan areas (n = 16) in which the

number of each algal taxon was scored. The outer 1 cm was excluded to allow for any

possible edge effects (Bellgrove, in review a).

As the established assemblages are a source of propagules and an indication of past

recruitment events, the Low-shore and Hormosira zones were surveyed using haphazardly

placed 0.25 m2, 81-point quadrats. Pilot surveys indicated that 30 replicates gave estimates

of macroalgal richness and densities representative of the assemblage in the Low-shore

zone, while 20 were sufficient for the Hormosira zone. Percentage cover of each algal

species present in the quadrats was estimated by recording the number of times the species

occurred within a 1-cm radius below the points in the quadrat (modified method of Levy

and Madden, 1933). Where the algal cover was vertically stratified, both canopy and

understorey species were recorded. Algal samples were collected to confirm identification

in the laboratory, where necessary.

2.3. Spatial investigation

To examine the level at which most of the spatial variation in intertidal macroalgal

recruitment occurs, three spatial scales were used: 10s cm, 100s cm, and 100s m. At the

100s m scale four sites were used as described above. In the Hormosira zone at each site,

four blocks (separated by 2–5 m) of four resin rocks (separated by < 30 cm) were attached

to the rock platform on the 25th March 1996. The panels were left for a period of 4 weeks

to allow macroalgal recruitment, after which they were collected and treated as in the

temporal investigation until recruitment was quantified. During the course of this

experiment three panels from Fort Pearse (1 in one block and 2 in another) were dislodged

from the rock platform in rough weather leaving three and two replicate panels for two of

the four blocks at this site.

3. Results

3.1. Temporal investigation

Over the course of the study 13 taxa were recorded in the water samples (3 Rhodophyta, 3

Phaeophyta and 7 Chlorophyta), 16 taxa recruited (6 Rhodophyta, 5 Phaeophyta and 5


Chlorophyta) and 22 taxa were recorded in the established assemblages (7 Rhodophyta, 4

Phaeophyta, 9 Chlorophyta and 2 Cyanobacteria) in the Low-shore and Hormosira zones.

The temporal patterns of abundance of propagules, recruits and established plants were

not consistent between the Low-shore and Hormosira zones for most species; yet most had

peaks in the density of propagules and recruits between April and August. Propagules and

recruits of several ephemeral algae, such as Ectocarpus, Porphyra and Enteromorpha,

were often found; yet these taxa were never abundant on the shore. Conversely, propagules

of some perennial species, such as Capreolia implexa Guiry and Womersley and

Hormosira banksii, which were abundant on the shore, were rarely recorded. The temporal

variation in the abundance of two ephemeral taxa, Ulva and Enteromorpha, and the

perennial Hormosira are described in detail to illustrate some of these patterns.

The patterns of temporal variation in the density of propagules of Ulva in both the early

wash and waves samples were not the same for the Low-shore and Hormosira zones (two-

factor ANOVA (zone and month), Zone*Month, F(4,20) = 4.32, P= 0.005; F(3,15) = 7.85,

P < 0.001 for early wash and waves, respectively; Fig. 2a and b). For each zone however,

there was significant temporal variation in the density of propagules of Ulva in the early

wash and wave samples (ANOVA simple main effects (SME) comparing months for each

Fig. 2. Ulva rigida in the Low-shore zone (black) and Hormosira zone (white) from December 1994 to October

1995. Density of propagules (meanF SE, n= 5) in the (a) early wash and (b) waves (see text); (c) density of

recruits (meanF SE, n= 10); and (d) percentage cover (meanF SE, n= 30 and 20, respectively) in established

assemblages. ND=no data.


zone, Month, P < 0.001 for all comparisons; for all analyses P indicates the probability of

getting the observed F-value or one larger; Fig. 2a and b). In the Low-shore zone, the

density of propagules peaked in June and again in October in both the early wash and

waves (Fig. 2a and b). However, in the Hormosira zone, densities of propagules of Ulva

peaked at these times only in the early wash, with much lower densities in the wave

samples (Fig. 2a and b). The differences in the density of recruits of Ulva between months

were not consistent between zones (two-factor ANOVA (zone and month), Zone*Month,

F(5,98) = 9.28, P < 0.001), although there was significant temporal variation in recruitment

of Ulva in both the Low-shore and Hormosira zones (SME comparing months for each

zone, F(5,98) = 29.30, P < 0.001 and F(5,98) = 4.61, P= 0.002 for the Low-shore and

Hormosira zones, respectively; Fig. 2c). In the Low-shore zone recruitment of Ulva

peaked in February and then again in June and August, while in the Hormosira zone

recruitment was lower and peaked from June to October (Fig. 2c). On the shore, the

patterns of temporal variation in the percentage cover of Ulva rigida C. Agardh were not

consistent for both the Low-shore and Hormosira zones (two-factor ANOVA (zone and

month), Zone*Month, F(5,278) = 50.74, P < 0.001), although there was significant temporal

Fig. 3. Enteromorpha in the Low-shore zone (black) and Hormosira zone (white) from December 1994 to October

1995. Density of propagules (meanF SE, n= 5) in the (a) early wash and (b) waves (see text); (c) density of

recruits (meanF SE, n= 10); and (d) percentage cover (meanF SE, n= 30 and 20, respectively) in established

assemblages. ND= no data.


variation for both zones (SME comparing months for each zone, F(5,278) = 18.63,

P < 0.001, F(5,278) = 120.44, P < 0.001 for Low-shore and Hormosira zones respectively).

In the Low-shore zone abundance of U. rigida on the shore peaked in December and then

again from August through to October (Fig. 2d). However, in the Hormosira zone,

abundance of U. rigida was relatively low throughout the year, except for a peak in the

cover of this species in the understorey in October (Fig. 2d).

There was significant temporal variation in the density of propagules of Enteromorpha in

theHormosira zone early wash (two-factor ANOVA (zone andmonth), Zone,F(1,40) = 33.46,

P < 0.001) with a peak in June (Fig. 3a). However, there were no significant differences in the

densities of propagules of Enteromorpha in wave samples between zones or months (two-

factor ANOVA (zone and month), Zone, F(1,31) = 0.55, P= 0.466; Month, F(3,31) = 2.26,

P= 0.101; Fig. 3b). The differences in recruitment of Enteromorpha between months were

not consistent between zones (two-factor ANOVA (zone and month), Zone*Month,

F(5,98) = 3.38, P= 0.007). Recruitment of Enteromorpha differed significantly between

months in the Low-shore zone (SME comparing months for Low-shore zone,

Fig. 4. Hormosira banksii in the Low-shore zone (black) and Hormosira zone (white) from December 1994 to

October 1995. (a) Density of propagules (meanF SE, n= 5) in the waves (see text); (b) density of recruits

(meanF SE, n= 10); and (c) percentage cover (meanF SE, n= 30 and 20 respectively) in established

assemblages. ND=no data.


F(5,98) = 6.31, P < 0.001), rising slightly in February and April, but was consistent over time

and higher in the Hormosira zone (SME comparing months for Hormosira zone,

F(5,98) = 1.75, P= 0.141; Fig. 3c). In contrast to the abundance of propagules and recruits,

Enteromorpha sp. was never abundant on the shore in either zone (Fig. 3d), with too few

records for analysis.

Hormosira was never abundant in either the early wash or wave water samples (Fig.

4). However recruitment of Hormosira did occur but the patterns of abundance over

time differed between the Low-shore and Hormosira zones (two-factor ANOVA (zone

and month), Zone*Month, F(5,98) = 5.65, P < 0.001). In the Hormosira zone recruitment

of Hormosira differed significantly between months (SME comparing months for

Hormosira zone, F(5,98) = 12.18, P < 0.001) peaking in April, while it was consistently

low throughout the sampling period in the Low-shore zone (SME comparing months for

Low-shore zone, F(5,98) = 0.20, P= 0.961; Fig. 4b). The patterns of temporal variation of

Hormosira banksii on the shore were not consistent between zones (2 factor ANOVA

(zone and month), Zone*Month, F(5,278) = 2.79, P= 0.018). In the Hormosira zone there

was significant temporal variation (SME comparing months for Hormosira zone,

Fig. 5. Mean number of taxa (F SE) in the Low-shore zone (black) and Hormosira zone (white) from December

1994 to October 1995: as propagules in the (a) early wash and (b) waves (see text) (n= 5); (c) as recruits (n= 10);

and (d) in established assemblages (n= 30 and 20, respectively). ND= no data.


F(5,278) = 4.31, P < 0.001), with less cover of H. banksii in December and February than

in April, June and October (Tukey HSD multiple comparisons (hereafter Tukey’s tests),

P < 0.05 for all comparisons; Fig. 4c). In the Low-shore zone the percentage cover of H.

banksii was low ( < 15%) throughout the year and did not vary significantly over time

(SME comparing months for Low-shore zone, F(5,278) = 0.77, P= 0.573; Fig. 4c).

Fig. 6. NMDSOrdination plots from the (a) early wash water samples; (b) wave water samples; (c) the recruitment,

plotted with the October Low-shore zone outlier out of range; and (d) established assemblage. In all plots means are

shown differentiating zones (black = Low-shore zone, white =Hormosira zone) and sampling times (Y December;

n February; . April; x June; E August; 1 October). Stress = 0.02, 0.02, 0.01 and 0.01, respectively.


The mean number of genera in the water samples was consistently low compared with

the established assemblages and recruitment (Fig. 5). The temporal variation in the number

of genera in the early wash was not consistent between zones (two-factor ANOVA (zone

and month), Zone*Month, F(4,40) = 4.37, P= 0.005; Fig. 5a). In the Low-shore zone there

were significantly fewer genera in the early wash in June and October than at other times

of the year (SME comparing months for Low-shore zone, F(5, 40) = 50.400, P < 0.001;

Tukey’s test, P < 0.05 for all comparisons; Fig. 5a). There was no clear temporal pattern in

the Hormosira zone (Fig. 5a). There were consistently more genera in the waves from the

Low-shore zone than from the Hormosira zone (two-factor ANOVA (zone and month),

Zone, F(1,31) = 4.29, P= 0.047), and although the supply of propagules varied between

months, there were no clear patterns (two-factor ANOVA (zone and month), Month,

F(3,31) = 3.31, P= 0.033; Fig. 5b). While the number of genera recruiting to the Low-shore

and Hormosira zones was not consistent over time (two-factor ANOVA (zone and month),

Zone*Month, F(5,98) = 5.16, P < 0.001; Fig. 5c), there was generally a peak in richness in

April and June in the Low-shore zone and through to August in the Hormosira zone (Fig.

5c). The number of species in the established assemblages differed with time, although the

patterns were not the same for the Low-shore and Hormosira zones (2 factor ANOVA

(zone and month), Zone*Month, F(5,278) = 17.83, P < 0.001; SME comparing months for

each zone, F(5,278) = 7.39, P < 0.001 and F(5,278) = 20.33, P < 0.001 for Low-shore and

Hormosira zones, respectively; Fig. 5d). In the Low-shore zone there was a significant

decrease in the number of species on the shore in June compared to December (Tukey’s

test, P= 0.027) and February (Tukey’s test, P < 0.001), when the numbers of species were

highest for the year (Fig. 5d). Conversely, the lowest number of species on the shore in the

Hormosira zone occurred in August (Tukey’s tests, P= 0.002 for comparison with

February, P < 0.001 for comparisons with June and October) and the highest number in

October (Tukey’s tests, P < 0.001 for all comparisons) (Fig. 5d).

NMDS and ANOSIM showed that the assemblage of algal propagules in the early wash

differed significantly both between the Low-shore and Hormosira zones (R = 0.170,

P= 0.020), and between months (R = 0.439, P < 0.001), although not all pairs of months

were significantly different (Fig 6a). Similarly, the assemblage of algal propagules in the

Table 1

Variance components calculated after nested ANOVAs comparing the density of recruits of designated taxa to

panels within blocks (scale of 10s cm; n= 4a) and blocks within sites (scale of 100s cm; n= 4) at four sites in the

Point Nepean National Park: Cheviot Beach, Big Rock, Cheviot Beach West and Fort Pearse (scale of 100s m;

n= 4)

Source Bryopsis Capreolia Ceramium Encrusting

Corallines

Hormosira Porphyra Ralfsia Ulva

Site (100s m) 0.14 0.04 0.08 0.20 0.56 0.01 1.65 0.24

Block within Site

(100s cm)

0.04 0.10 0.00 0.03 0.14 0.06 0.13 0.00

Panel (10s cm) 0.31 0.19 0.35 0.22 0.21 0.13 0.32 0.16

a At Fort Pearse, one panel from one block and two panels from another were not recovered leaving n=3

and 2 for two of four blocks at this site. Consequently, REML was used for estimating variance components

(see Quinn and Keough, 2002). Data were fourth-root transformed.


waves differed significantly both between the Low-shore and Hormosira zones (R = 0.596,

P < 0.001), and between all months (R = 0.684, P < 0.001) (Fig 6b). NMDS of the

assemblages of macroalgal recruits identified the October, Low-shore zone sample as an

extreme outlier. However, the results of an ANOSIM were not changed when this outlier

was removed, and showed significant differences between both the Low-shore and

Hormosira zones (R = 0.305, P < 0.001) and between months (R = 0.477, P < 0.001), with

all months differing significantly except December and February (Fig. 6c). The NMDS

plot of the established assemblage not surprisingly showed a marked separation between

the Low-shore and Hormosira zones, which differed significantly (R = 1.000, P= 0.020),

but no difference over time (R =� 0.422, P= 0.938) (Fig. 6d).

Fig. 7. Ordination plots from the Spatial Investigation NMDS showing (a) whole data set differentiating sites

(symbols); (b) whole data set differentiating blocks within sites (letters); and (c) means differentiating sites

(symbols) and blocks within sites (shading). Stress=(a and b) 0.20; (c) 0.05.

3.2. Spatial investigation

Partitioning the variances revealed that most of the variation in recruitment occurred at

the site level (100s m) for three out of eight genera: Hormosira, Ralfsia and Ulva (Table 1).

No taxa were found to have the most variation in recruitment at the block level (100s cm),

although Capreolia and Porphyra had more variation at this level than at the site level

(Table 1). Most of the variation in recruitment occurred at the panel level (10s cm) for five

out of eight taxa: Bryopsis, Capreolia, Ceramium, encrusting corallines and Porphyra

(Table 1).

The NMDS showed a separation between sites (100s m) (ANOSIM, R = 0.817,

P < 0.001), but the pairwise differences between sites were non-significant after adjusting

for multiple comparisons. In contrast, there was no separation between blocks within sites

(100s cm) (Fig. 7).


4. Discussion

The established assemblages were strongly differentiated, with respect to species

composition and abundance, between the Low-shore and Hormosira zones not only due

to the abundance of Hormosira banksii, but also differences in the composition and

abundance of understorey species. However, the assemblages in the Low-shore and

Hormosira zones did not differ temporally. Whilst the composition and abundance of

propagules in the water column and recruits also differed between zones, these showed

more variation with time. This indicates that the patterns of macroalgal distribution and

abundance on the shore are not solely a reflection of the supply of propagules and

recruitment.

Post-recruitment mortality pressures or other modes of regeneration are also likely to be

important to the structure and dynamics of macroalgal assemblages. Many studies have

implicated competition (Dayton, 1971; Lubchenco, 1978, 1982; Paine, 1984; Reed, 1990),

predation/herbivory (Petraitis, 1990; McCook and Chapman, 1993); and disturbance

(Dayton, 1971; Peckol and Searles, 1983; Gunnill, 1985; Farrell, 1989) as important

post-recruitment mortality pressures in marine systems. Johnson and Brawley (1998)

found differences in the settlement and recruitment of the fucoid Silvetia compressa

(previously Pelvetia compressa) and suggested that this was indicative of post-settlement

mortality. Additionally, vegetative regeneration has been found to be very important to the

persistence of some algal species (McCook and Chapman, 1992; Kim and DeWreede,

1996). Regeneration from a ‘bank of microscopic forms’ has also been suggested as a

potentially important factor contributing to macroalgal assemblage structure (Hoffmann

and Santelices, 1991; Santelices et al., 1995; Blanchette, 1996).

Despite the broad assemblage patterns, most species showed temporal variation in

cover on the shore, densities of propagules in the water, and recruits, in both zones. For

most taxa, the supply of propagules and recruitment was greatest between April and

August, continuing into October for Ulva and Enteromorpha only, whereas temporal

patterns of abundance on the shore were more species-specific. These recruitment patterns

contrast with those described by Jernakoff (1985), who found the fastest recruitment of


macroalgae into a barnacle zone in New South Wales, Australia, in summer and the

slowest recruitment in autumn. However, the present study had no replication within

seasons or between years and, as such, does not aim to describe seasonal patterns, but

rather temporal variability within a year.

The peaks in supply of propagules did not directly correspond to peaks in recruitment

of associated species, although in general, propagules of most species were present in the

water samples when recruitment was occurring. Similar results were found by Hruby and

Norton (1979) for the Firth of Clyde. Also, in the only similar study to this, Santelices et

al. (1995) found that most taxa found in the water column were also represented in the

assemblage of microforms (which would include new recruits as well as potentially

suppressed individuals). The water sample data in this study, however, must be viewed

with caution as merely a snapshot indication of the propagules potentially available for

settlement. There is likely to be considerable variation in supply between and within days;

however the only measure of small-scale temporal variation (see Morrisey et al., 1992b)

was between the early wash and wave samples. The differences between these samples

give some indication of tidal effects on supply of propagules. Likewise, Santelices et al.

(1995) had no measure of small-scale temporal variation in composition or abundance of

species. The possibility of spatial and temporal variation in the distribution of propagules

in the water column (Hruby and Norton, 1979; Amsler and Searles, 1980; Hoffmann,

1987; Hoffmann and Ugarte, 1985) and diurnal and tidal differences in propagule release

(Ngan and Price, 1983) and planktonic abundance (Zechman and Mathieson, 1985;

Bellgrove and Aoki, in review) must be taken into account when determining the influence

of the availability of propagules on recruitment patterns.

Generally, densities of most taxa found in the water samples were equivalent or slightly

lower in the waves compared to the early wash. This may in fact indicate that there are

actually more propagules in the waves than the early wash, given a dilution factor of at

least 2:1 at the later tidal state. This is consistent with the results of Zechman and

Mathieson (1985) who found the greatest abundances and number of intertidal macroalgal

taxa in high tide samples (i.e. after a period of desiccation at low tide, and then rehydration

and discharge of propagules with the incoming tide: reviewed by Santelices, 1990).

The number of taxa recruiting to both the Low-shore and Hormosira zones was

generally consistent with the richness on the shore. However, there were consistently

fewer taxa recorded in the water samples. Santelices et al. (1995) found similarly lower

taxon richness in the water samples than in either the assemblage of microforms on tidal

pool boulders or in the surrounding established assemblages. This is possibly a conse-

quence of the potentially great spatial and temporal variation of planktonic propagules,

beyond the scales measured in the two studies (Amsler and Searles, 1980; Zechman and

Mathieson, 1985; Morrisey et al., 1992b). It is also likely that propagules of some species,

such as Hormosira banksii that has relatively large eggs (64 Am; Clayton, 1990), rapidly

sink and settle (Santelices, 1990; Clayton, 1992) therefore successfully recruiting, but

never being a significant component of the plankton.

In the spatial investigation, most taxa showed the greatest variation in recruitment

densities at the smallest spatial scale of the panels within blocks (i.e. 10s cm). Similarly

high variation was found between sites (i.e. 100s m) for a few species, and the overall

assemblages of recruits also differed significantly between sites. However, the spatial


investigation in this study was only conducted once (March–April 1996) and as such, the

generality of these patterns of variability is unknown (see Underwood and Chapman,

1998a,b; Coleman, 2002).

In a study of the spatial scales at which significant differences in the abundance of

several common, largely mobile, intertidal animals occurred, Underwood and Chapman

(1996) found most spatial variability to be at the smallest scale of cm to m. They attributed

this variability to the spatial patchiness of microhabitats across the shore and behavioural

responses to the small-scale environmental cues, rather than to recruitment and/or

mortality. They also suggested that sessile species, such as algae, may be affected at the

small-scale by mortality caused by environmental variability or localised behaviour of

mobile species, and at the large-scale (i.e.100s m) by variable recruitment, possibly

combined with differential mortality. In support of the latter, large-scale spatial (between

shores separated by 11–555 km) and temporal (between years) variations in local

population size and age structure of the barnacle Tesseporea rosea, were found to be

directly related to variations in settlement and early post-settlement survival of this

barnacle (Caffey, 1985).

Whilst at the small-scale, mortality (such as herbivory: Underwood and Jernakoff,

1984; Bellgrove, in review(b)) is likely to be an important controlling factor of sessile

populations, this study shows that variable recruitment at this scale may also potentially

affect macroalgal abundance. Indeed, Coleman (2002) found that variation in the

composition, abundance and distributions of turfing algal assemblages was consistently

greatest at the scale of 10s cm, and suggested that variable supply of propagules or

recruitment at these small scales may at least partially account for the observed patterns.

Underwood and Chapman (1998a) similarly found variation in established algal assemb-

lages along the NSW coastline at various spatial scales, with the greatest variation at the

smallest spatial scale measured (between quadrats metres apart).

The large-scale (between site) differences in macroalgal recruitment in this study are

possibly due to differences in abundances of various species between sites, which may

potentially be manifest in variable supply of propagules at the same scale. Many algal

species have limited dispersal (Santelices, 1990), which may mean that such populations

are effectively ‘closed’ and therefore highly dependent on the local population for the

supply of propagules (Williams and Di Fiori, 1996). Temporal variation in the onset of

reproduction between the different populations may also contribute to a variable supply of

propagules between sites. Cheviot Beach, Cheviot Beach West and Fort Pearse are all

isolated by headlands. The associated near-shore current systems of each of these sites

may serve not only to isolate propagule populations, but possibly also planktivores or

propagule-grazer populations. This may result in differential planktonic mortality between

sites, potentially accounting for some of the variability in recruitment at this scale.

Instead of treating natural variability, both spatial and temporal, as noise obscuring the

patterns of processes regulating communities, it needs to be identified and incorporated

into ecological theory (Thrush, 1991). The regulatory processes may act at different scales,

and only by understanding the level of variability at the scale appropriate to the

mechanism, can we ascertain the importance of this variation to the population/community

dynamics (Thrush, 1991; Underwood and Chapman, 1996). Not only may recruitment per

se be important to the structure and dynamics of a community (Connell, 1985; Underwood


and Fairweather, 1989), but the action of various post-settlement processes structuring

communities (such as competition (Dayton, 1971; Lubchenco, 1978, 1982; Paine, 1984;

Reed, 1990); predation/herbivory (Petraitis, 1990; McCook and Chapman, 1993); and

disturbance (Dayton, 1971; Peckol and Searles, 1983; Gunnill, 1985; Farrell, 1989) may

hinge on the variable settlement and early survival of recruits of component species

(Caffey, 1985). This study has shown that variation in the supply of propagules and

recruitment of intertidal macroalgae contributes to the variation in established assemblages

and cannot be ignored when considering processes structuring communities. However,

other pre-and post-recruitment regulatory processes are also important to the distribution

and abundance of intertidal macroalgae. Further examination of this variability and its

impact on established assemblages is required for other geographic regions, at a number of

temporal and spatial scales, to assess the generality of conclusions presented here.

Acknowledgements

Access to study sites within the Mornington Peninsula National Park was gratefully

appreciated. Many people, in particular Drs. Bron Burton and Lissa Wheatley, assisted

with fieldwork, without which this study would not have been possible. We gratefully

acknowledge Ken Miles, Rod Watson, Peter Domelow and Ian Stewart for technical

assistance and advice throughout. Drs. Lissa Wheatley, Masakazu Aoki and Belinda

Robson commented on drafts of this manuscript, which was greatly appreciated.

Additionally, comments from Prof. Bernabe Santelices improved the discussion, and

comments from Prof. Tony Underwood and two anonymous referees improved the

manuscript overall. This research was supported by an Australian Postgraduate Award to

A. Bellgrove. [AU]

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an integrated study of the temporal and spatial variation in the supply of propagules, recruitment...

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