The relationship between physical variables on topographically simple and complex reefs and algal assemblage structure beneath an Ecklonia radiata canopy

Estuarine, Coastal and Shelf Science 71 (2007) 232e240 www.elsevier.com/locate/ecss

The relationship between physical variables on topographically simple and complex reefs and algal assemblage structure beneath an Ecklonia radiata canopy Benjamin D. Toohey School of Plant Biology, University of Western Australia, Crawley 6009, WA, Australia Received 2 May 2006; accepted 24 July 2006 Available online 11 September 2006

Abstract Limestone reefs in South-Western Australia range in topographic complexity at the 1e10 m scale from simple, flat, planar reef structures to topographically complex reefs with many rock walls, overhangs and channels. Algal assemblage structure is known to differ between topographically simple and complex reefs. This research assessed if differences in algal assemblage structure could be associated with the physical environment present on each reef type. Sampling of the physical environment and algal assemblage was carried out on simple, planar reefs and complex, rugose reefs. The physical environment described almost 80% of the algal assemblage structure under an Ecklonia radiata canopy. It was found that topographically simple reefs had greater sediment cover and water depth than adjacent reefs with more complex topography which had more downwelling light, stronger and more turbulent water motion and a greater range of substrate slopes. Greater variation in most physical variables on topographically complex reef shows a greater diversity of microenvironments on that reef type. It is argued that these differences in physical environment between reef types have a considerable influence on algal assemblages by altering key biological processes such as recruitment and interspecific competition. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: relief; macroalgae; kelp bed; topography

1. Introduction Algal assemblages in temperate South-Western Australian waters are typically composed of canopy-forming brown algae and an understorey of foliose, corticated and encrusting algae. The canopy is often composed of Ecklonia radiata, Scytothalia doryocarpa, Cystophora or Sargassum species (Wernberg et al., 2003; Goldberg and Kendrick, 2004). Kelp beds form an important part of the marine ecosystem by providing food and habitat to the reef community (Taylor, 1998; Van Alstyne et al., 1999; Wernberg et al., 2005b). Kelp beds in Western Australia have high alpha diversity at small spatial scales of 1e10 m (Kendrick et al., 1999, 2004; Wernberg et al., 2003;

E-mail address: [email protected] 0272-7714/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2006.07.009

Goldberg and Kendrick, 2004). Kelp beds are patchy in their distribution and occur on reefs ranging in topography from simple flat reefs to highly complex rugose reefs. Differences in topography between reefs may explain some of the variations in the distribution of algal species. Limestone reefs in Western Australia are believed to be the lithified remnants of former dunes drowned during the Holocene transgression (Wood, pers. comm.) and form a barrier between the coast and the open ocean (Sanderson et al., 2000). In Hamelin Bay, Western Australia, kelp beds are found on limestone reefs varying in physical structure at the 1e10 m scale. Topographically complex reefs generally rise from the sea floor more than 1 m (but often 4e5 m), and are characterised by an intricate system of channels, crevices, rock walls and overhangs. Topographically simple reefs, in contrast, are generally flat and planar in form with a horizontal profile.

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Topographically simple and complex reefs are sometimes referred to as low and high relief, respectively (see Harman et al., 2003), as relief and topography have similar geomorphological definitions (Anon., 1997). Topography may alter the physical environment present on a reef, with greater complexity leading to significant changes in key factors such as light, sediments and water motion. Within a single location, the substrate of topographically complex reef can be found over a range of water depths such that some parts of the reef will be shallower than adjacent topographically simple reefs by 3e4 m, affecting light levels and other depthrelated physical variables. In addition to the differences in depth, greater topographic complexity will affect the hydrodynamics (Ferrier et al., 1996), sediment loading (Wolanski and Hamner, 1988) and the distribution of substrate slopes. Kelp beds are influenced by a range of physical variables in their environment. Light levels (Reed and Foster, 1984; Graham, 1996; Connell, 2003; Altamirano et al., 2004; Clark et al., 2004; Toohey et al., 2004), sediment loading (Devinny and Volse, 1978; Kendrick, 1991; Airoldi and Cinelli, 1997; Irving and Connell, 2002a; Eriksson and Johansson, 2003; Gorgula and Connell, 2004; Connell, 2005) and water motion (Harrold et al., 1988; Seymour et al., 1989; Glenn and Doty, 1992; Bell and Denny, 1994; Hurd, 2000; Taylor and Schiel, 2003) are all known to influence the composition of algal assemblages. Other elements of physical environment, such as water depth (Novaczek, 1984; Kirkman, 1985; Dennison and Alberte, 1986; Markager and Sand-Jensen, 1992; Piazzi et al., 2002; Fairhead and Cheshire, 2004; Goldberg and Kendrick, 2004) and substratum slope (Santos, 1993; Baynes, 1999; Goldberg and Foster, 2002; Piazzi et al., 2002) are also reported to influence the physiology and composition of the algal assemblage. Furthermore many of these variables interact to significantly influence the structure of kelp beds (Irving and Connell, 2002b). Kelp canopies can modify the level of some physical variables such that kelp density itself becomes an influence on algal assemblage structure (Kendrick et al., 1999; Wernberg et al., 2005a). Algal assemblage structure differs between topographically simple and complex reefs in Hamelin Bay (Harman et al., 2003; Kendrick et al., 2004). Given the strong influence of the physical environment on algal assemblages, differences in assemblage structure between reefs with simple and complex topography may correlate with differences in the physical environment present on each reef type. The aim of this research was to determine if differences in the physical environment between topographically simple and complex reefs could be related to differences in algal assemblage between these reef types. To assess the physical environment, I measured downwelling light, water motion (both direction and magnitude), sediment cover, water depth, slope of substrate and canopy density on topographically simple and complex reefs. These physical variables were then related to the structure of the algal assemblage beneath an Ecklonia radiata canopy. In this paper algal assemblage structure is defined as the algal species present on a reef and their relative abundance (Fauth et al., 1996).

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2. Material and methods This study was conducted in Hamelin Bay, (34 22 0 S, 115 01 E), a coastal embayment approximately 350 km south of Perth, South-Western Australia. The bay contained many limestone reefs ranging in topographic complexity. Topographically complex reefs can be found both seaward and leeward of topographically simple reefs, forming a haphazard mosaic of topographically simple and complex reefs. 0

2.1. Environmental sampling Six variables were measured on topographically simple and complex reefs. These variables were downwelling PAR light, water motion (both direction and magnitude), sediment cover, water depth, substrate slope and Ecklonia radiata canopy density. The general sampling design consisted of three topographically simple reefs and three topographically complex reefs, with sub-samples taken within each reef. These reefs were 100 m apart and were covered by a monospecific Ecklonia radiata kelp bed. The level of sub-sampling per reef depended on which variable was being measured. Deviations from this sampling design occurred when sampling downwelling light and water motion due to equipment limitations. Unless otherwise stated, each variable was analysed with a 2-way ANOVA with reefs being a random factor nested within the fixed factor of reef topography. To classify reefs into simple and complex topographic categories, the rugosity of each reef used in this study was measured using the ‘‘chain and tape’’ method (Luckhurst and Luckhurst, 1978; McCormick, 1994). Four transects were randomly laid at each reef with a 10 m length of 6 mm chain and then the linear distance between the two ends of the chain was measured. Care was taken to lay the chain along the contours of the substrate such that the chain was in contact with the substrate at points along its length. Rugosity was calculated by dividing the chain length (10 m) by the linear distance between the two ends of the chain after it had been laid across the reef to give a value between 0 and 1. Values closer to 1 indicate that the reef was less rugose. Topographically complex reefs significantly differed from topographically simple reefs in terms of rugosity (F ¼ 199.31, p < 0.001, df ¼ 1, 4) with no difference between replicate reefs within each reef type (F ¼ 0.22, p ¼ 0.923, df ¼ 4, 18). 2.1.1. Downwelling light A single 2p Odyssey PAR light sensor was anchored 1 m above the substrate at two topographically complex and two topographically simple reefs for one complete day (24 h). This sampling design was repeated over three time periods; 16/12/04; 3/2/05 and 1/7/2005e2/7/2005; corresponding to two mid summer readings and one winter reading. Sensors sampled once every minute. Total daily PAR was calculated by summing light reading over the 24 h of deployment. There was a reduction in sampling during the first and second time periods due to the malfunction of some sensors.

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There was only one functional sensor on topographically simple reef during the first sampling period. Similarly, there was only one functional sensor on topographically complex reef during the second sampling period. 2.1.2. Water motion Two acoustic doppler current profilers (RD instruments, workhorse sentinel: 1200 and 600 kHz) were deployed for five days during late summer (18/3/05e22/3/05) and again for one day during winter (1/7/2005e2/7/2005). One current profiler was placed on topographically complex reef and the other on a flat, planar reef. The sampling interval was set at 5 min. Upon retrieval, the sampling bin (0.25 m in width) closest to the substrate (approx. 2.2 m above substratum) was analysed as these data most accurately reflect the hydrodynamic environment experienced by the algae. The direction of water motion was statistically tested with a circular Chi-squared test to compare its distribution in the first sampling period. In the second sampling period, a lack of data points prevented the use of a Chi-squared test, so an F-test was used to compare mean water direction. The mean magnitude of water motion was compared using a KruskaleWallis test separately for each sampling period. This test was used due to heterogeneous variance structure in the data set (Levene’s test ¼ 224.44, p < 0.001; Levene’s test ¼ 326.59, p < 0.001; March and July, respectively). 2.1.3. Sediment cover Sediment cover was sampled from four 15  15 cm quadrats placed haphazardly on each reef, in areas lacking a seaweed canopy as kelp canopies are known to affect sediment cover (Eckman et al., 1989; Wernberg et al., 2005a). Sediment within the quadrat was collected with a venturi-suction sampler. Sampling effort was held constant at 10 s of suction per quadrat. All sampling was conducted over one day in late summer to reduce the effect of changing hydrodynamic conditions on sediment cover. Once collected, samples were washed with distilled water and any algal material accidentally collected was removed before drying. Total dry weight of samples was analysed. 2.1.4. Water depth At each reef, the water depth was recorded at 12 randomly placed points. Water depth was measured with a Suunto Octopus dive computer at the substrate. All measures were taken over two days to minimise differences due to changes in tidal height (<1 m tides in Hamelin Bay). Data were analysed with a 2-way PERMANOVA based on Euclidean distance using replicate reefs nested within reef topography. Reefs were a random factor and reef topography was a fixed factor. PERMANOVA was used rather than a parametric ANOVA as there were significant differences in variances between treatments groups (Levene’s test ¼ 9.26, p < 0.001). 2.1.5. Slope of substrate At each reef, the slope of the substrate was measured along three randomly placed 10-m transects. The slope was taken at

1-m intervals along the length of each transect. The angle of the substrate was measured using a spirit-level and protractor. The spirit-level was used to determine a vertical plane and the protractor was used to determine how much the slope of the substrate deviated from that vertical plane. Slope was recorded relative to the horizontal plane such that a reading of 0 indicated that the substrate was close to perfectly horizontal and a reading greater than 75þ degrees designated substrate close to vertical. The measuring arm of the protractor integrated over 30 cm sections of the substrate. The angle of the substrate was rounded to the nearest 5 . As each transect constituted a replicate in this test, data from each transect was grouped in seven categories (e.g. 0e15, 16e30, 31e45, 46e60, 61e75, 75þ degrees) which were then used as variables in the resulting multivariate data matrix. This matrix was analysed with a 2-way, nested PERMANOVA with reefs nested within reef topography. BrayeCurtis dissimilarities were used for this test due to the high frequency of zeros in the data set. The data were graphed as the total frequency of each slope category over topographically simple and complex reefs to illustrate the distribution of substrate slope present on each reef type. 2.1.6. Ecklonia radiata density At each reef, the density of Ecklonia radiata was estimated using 12 randomly placed 0.25 m2 quadrats. Only the numbers of adult, canopy-forming kelp thalli (S3 e Kirkman, 1981) with holdfasts within the quadrat were counted. 2.2. Understorey assemblage structure The structure of the algal assemblage beneath an Ecklonia radiata canopy was determined from four 0.25-m2 replicate quadrats that were randomly placed in monospecific E. radiata beds at each of three reefs with simple, flat topography and three reefs with complex topography. All non-encrusting algae within the quadrat were destructively harvested after the density of adult, canopy-forming, E. radiata thalli was noted. Once harvested, each sample was sorted to species level and wet weighted. Species richness was calculated for each sample and analysed using a 2-way nested ANOVA. The algal assemblage data were related to the physical environment on each reef type using DISTLM (Anderson, 2001, 2005; McArdle and Anderson, 2001). DISTLM calculates the similarity between algal biomass samples using BrayeCurtis similarities (Bray and Curtis, 1957) and then performs a principle coordinates analysis (PCO) on the resulting similarity matrix. The principle coordinates from this analysis are then used in a multiple regression against the physical variables. The four assemblage sub-samples for each reef were averaged before the analysis to allow the correct level of replication. The physical variables used for the DISTLM were the average sediment cover, water depth and rugosity of each reef as a full data set was available for these variables. Inclusion of light and water motion data was not possible as they were not sampled to the same level. Kelp density was not included in the

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DISTLM as there was no significant difference between topographically simple and complex reefs for this variable. To visualise the relationship between the algal assemblage structure and the physical environment, a canonical correlation analysis (to correlate the PCO axes generated from the assemblage matrix with the physical variables) was run to construct a constrained canonical correlation (CAP) ordination. Differences between reefs were tested with the trace statistic (Anderson and Robinson, 2003; Anderson and Willis, 2003).

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3.3. Sediment cover There was an order of magnitude difference in the amount of sediment cover between reef types (Fig. 1C). Simple, flat reefs had significantly more sediment cover in late summer than reefs with high topographic complexity (F ¼ 66.5, p ¼ 0.001, df ¼ 1, 4). Replicate reefs within each reef type did not significantly differ in terms of sediment cover (F ¼ 0.5, p ¼ 0.733, df ¼ 4, 18). Variation in sediment cover was significantly different between topographically simple and complex reefs, with greater variation present on topographically simple reefs (Levene’s test ¼ 22.34, p < 0.001).

3. Results 3.4. Water depth 3.1. Downwelling light Topographically complex reefs had greater downwelling light than topographically simple reefs (Fig. 1A). There were some differences between replicate reefs within a given reef type. Typical diurnal fluctuations in light levels were evident on both reef types with a midday peak around 200e 400 mE m2 s1. One sensor on topographically complex reef gave a readings (29,245 mE m2 day1) lower than those seen on topographically simple reef (72,766 mE m2 day1) during the first (December) time period. This was possibly due to the sensor on the topographically complex reef being shaded by a large adjacent rock wall for most of the sampling period.

3.2. Water motion The direction of water movement significantly differed between reef types during the first (March) sampling period (c2 ¼ 727.908, df ¼ 35, class width ¼ 10 degrees, p < 0.001). Water motion 2.2 m above the substrate in March was bidirectional on the topographically simple reef, whereas water motion was more turbulent on the topographically complex reef, not favouring any particular direction (Fig. 2). During the second (July) sampling period, topographically simple reef significantly different from topographically complex reef in mean direction of water motion (F ¼ 542.47, p < 0.001, df ¼ 1, 1), with water travelling in a mean direction of south-east (115.88  1.14 ) on topographically simple reef and north-east (57.03  2.25 ) on topographically complex reef (mean  std error, n ¼ 347). Similar to the March sampling, directions of water motion were more widely distributed on topographically complex reef than topographically simple reef during the July sampling (Fig. 3). The speed of water flow above the reef was significantly greater on topographically complex reef (Fig. 1B, March: H ¼ 107.4, p < 0.001, df ¼ 1; July: H ¼ 302.1, p < 0.001, df ¼ 1). In the second sampling period during winter (July), the speed of water flow above the reef was almost three times greater on the topographically complex reef than on the topographically simple reef.

For the reefs sampled, topographically simple reefs were between 0.78 and 4.3 m deeper than topographically complex reefs (Fig. 1D). There were significant differences in water depth between reef types and replicate reefs within each reef types (F ¼ 6.12, p ¼ 0.026, df ¼ 1, 4). Variation in water depth was significantly different between reef types and replicate reefs within reef types (Levene’s test ¼ 9.26 p < 0.001) with more variation on topographically complex reefs. 3.5. Slope of substrate There was a significant difference in the distribution of substrate slopes between topographically simple and complex reefs (F ¼ 29.3, p < 0.001, df ¼ 1, 4) with no significant differences between replicate reefs nested with reef types (F ¼ 1.07, p ¼ 0.412, df ¼ 4, 12). Pooled across all three replicate reefs within a reef type, topographically complex reefs had only 33.3% horizontal substrate (<15 ) and 66.6% sloped substrate (>15 ), where as topographically simple reef, had 92.2% horizontal substrate and 7.8% sloped substrate. Of the 66.6% sloped substrate on topographically complex reef, slopes were spread out over a range from 20 to 75 þ, with a slight tendency towards shallower slopes (Fig. 1E). 3.6. Ecklonia radiata density There was no difference in adult Ecklonia radiata density between reefs with simple and complex topography (F ¼ 0.56, p ¼ 0.495, df ¼ 1, 4), nor between replicate reefs within reef types (F ¼ 0.77, p ¼ 0.546, df ¼ 4, 66). Average kelp density across all reefs was 6.58  0.18 per 0.25 m2 (average  std error, n ¼ 72). 3.7. Understorey assemblage structure There was a significant relationship between algal assemblage structure and the average sediment cover, water depth and rugosity (F ¼ 2.31, p < 0.001, df ¼ 3, 2). These three physical variables accounted for 77.6% of the variance in the structure of the algal assemblage. Individual variables were correlated with each other (Table 1), and marginal tests of individual variables were non-significant (not shown),

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Fig. 1. Physical variables on topographically simple and complex reefs in Hamelin Bay. (A) Mean total diurnal PAR during December, February and July. Some error bars missing due to sensor failure. (B) Magnitude of water motion during March and July. (C) Mean and standard error of sediment cover, n ¼ 5. (D) Mean and standard error of water depth in meters, n ¼ 12. (E) Frequency of substrate slope, n ¼ 90. Topographically simple reefs are the white bars and topographically complex reefs are the black bars.

indicating that it was the interaction of these variables that led to the significant multiple regression. Reefs with complex topography had significantly greater species richness than flat reefs (F ¼ 8.08, p ¼ 0.047, df ¼ 1,

4) with the former having 8.17  0.65 species per 0.25 m2 while simple flat reefs had 5.33  0.62 species per 0.25 m2 (average  std error, n ¼ 12). Species common to both reef types included Hennedya crispa, Metagoniolithon radiatum,

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Fig. 2. Direction of water motion on topographically simple and complex reefs during March and July. Note: the difference in ‘‘squared’’ scale for each graph.

Rhodymenia sonderi and Scytothalia doryocarpa. Additional species which were only present on topographically complex reef included Amphiroa anceps, Carpopeltis elara, Corallina sp., Dasya villosa, Delisea pulchra, Dictyota naevosa, Jania pulchella, Laurencia elata, Peyssonnelia capensis, Sarcodia sp. and Sargassum juveniles. Canonical correlation analysis provided good separation of reefs (trace statistic ¼ 1.877, p ¼ 0.006), with reef types separating on the first (horizontal) canonical axis (Fig. 3A). Topographically complex reefs were associated with higher rugosity while topographically simple reefs were associated with greater sediment cover and water depth (Fig. 3A). More species affiliated with topographically complex reef than with topographically simple reef when species abundances were correlated with the canonical axes (Fig. 3B). 4. Discussion The differences in algal assemblages between reefs with simple and complex topography correlate strongly with sediment cover, water depth, and rugosity. This local variation may explain much of the high alpha diversity between replicate quadrats reported by Kendrick et al. (1999, 2004), Wernberg et al. (2003) and Goldberg and Kendrick (2004) from the temperate south and western coasts of Australia. Simple, planar reefs had greater sediment cover and water depth than

adjacent reefs with more topographic complexity which had greater downwelling light, stronger and more turbulent water motion and more sloped substrate. The physical environment on topographically simple and complex reefs may influence the distribution of algae by altering key biological processes. In coral reefs, reef complexity drives fish assemblage structure by altering predation pressure (Almany, 2004). In this temperate system, reef topography could be altering processes such as recruitment and competition. Both sediment loading and variation in substrate slope affect the recruitment of algae. High sediment loads can reduce recruitment (Devinny and Volse, 1978; Kendrick, 1991; Irving and Connell, 2002a), either through burial or scour, both of which would be greater on topographically simple reef. Vertical rock faces favour the recruitment of invertebrates (Baynes, 1999; Goldberg and Foster, 2002; Piazzi et al., 2002), increasing competition for space and leaving less available space for algae to colonise relative to horizontal rock surfaces. The differences in sediment cover and substrate slope may lead to differences in recruitment patterns across reef types. Due to the greater water motion and downwelling light on topographically complex reefs, competition for light between canopy algae and their understorey may vary between reefs with simple and complex topography. Either reduced interspecific competition for light on topographically complex reef, and/or a difference in recruitment across reef types will lead to the greater

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B.D. Toohey / Estuarine, Coastal and Shelf Science 71 (2007) 232e240 Table 1 The marginal tests of each physical variable against algal assemblage structure (beneath Ecklonia radiata canopy on topographically simple and complex reefs). The second sub-table indicates the correlation between physical variables with the Pearson correlation p-value in brackets Marginal tests Variable

SS (trace)

Pseudo-F

p

Proportion of variance

Sediment cover Water depth Rugosity

1261.483 879.487 1279.069

1.096 0.706 1.116

0.413 0.710 0.403

0.215 0.150 0.218

Correlations among predictor variables

Water depth Rugosity

Fig. 3. (A) Canonical plot of the canonical correlation of PCO axes of algal understorey beneath an Ecklonia radiata canopy to physical variables on topographically simple and complex reefs, using BrayeCurtis similarities. (B) Correlation of species distribution with canonical axes. Origin marked with an X. Species abbreviations are as follows: Amp. anceps, Amphiroa anceps; Cal. rang., Callophyllis rangiferina; Cor. spp., Corallina sp.; Das. villosa, Dasya villosa; Eck. radiata, Ecklonia radiata juveniles; Ery. minuta, Erythrymenia minuta; Glo. brownii, Gloiosaccion brownii; Hen. crispa, Hennedya crispa; Met. radiatum, Metagoniolithon radiatum; Myr. tuberosum, Myriodesma tuberosum; Nit. pulchellum, Nitophyllum pulchellum; Pla. angu., Platythalia angustifolia; Plo. mertensii, Plocamium mertensii; Pte. rect., Pterocladia rectangularis; Rho. sonderi, Rhodymenia sonderi; Sar. juveniles, Sargassum juveniles; Scy. dory., Scytothalia doryocarpa; Scy. dory. juv., Scytothalia doryocarpa juveniles; Zon. sonderi, Zonaria sonderi.

species richness observed under Ecklonia radiata canopy on topographically complex reef. Topographically simple and complex reefs should be considered two distinct habitats as variation in multiple physical variables across reef types shows that reef topography is not a single factor but rather a label given to a collection of covarying factors. Some of the environmental differences between topographically simple and complex reefs are a direct result of the differences in geological structure of the reef. By virtue of the physical structure of the limestone,

Sediment cover

Water depth

0.729 (0.100) 0.921 (0.009)

0.877 (0.022)

topographically complex reefs have greater variation in the slope of the substrate and greater rugosity. Furthermore, as topographically complex reef rise off the seabed; shallower water depths are to be expected between adjacent topographically simple and complex reefs. These primary differences in physical structure of the reef have flow-on effects to other physical variables, such as greater downwelling light in structurally complex reefs. Furthermore, the physical structure of topographically complex reef will brake up water flow to form eddies and other hydrodynamic features (i.e. water motion interacting with boundary conditions), resulting in the more turbulent water motion observed on these reefs. The interrelationship of these physical variables suggests that these reef types should be seen as different habitats. Apart from sediment cover, which was greater but also more variable on topographically simple reef, substrate slope, water depth, water motion and possibly light all had greater variation on topographically complex reefs. This greater variation in physical variables on the topographically complex reefs may be interpreted as a greater diversity in microenvironments present on this reef type. An example of this can be seen in reports of greater water motion at the top of a rock wall than at it’s base (Sebens, 1984), with such spatial differences unlikely to occur on topographically simple reef which lack such walls. In addition, greater diversity in substrate slope will lead to greater variation in light microenvironments on topographically complex reefs (Brakel, 1979). This increase in the number of microenvironments leads to greater variation in assemblage structure on topographically complex reef (Toohey et al., unpub. data). 5. Conclusions This study demonstrates that algal assemblage structure and the distributions of individual algal species are related to the physical environment on topographically simple and complex reefs. Reefs with different levels of topographic complexity have different physical environments in terms of light, water motion, sediment cover, depth and slope of substrate. This local variation may explain much of the high alpha diversity between replicate quadrats reported by Kendrick et al. (1999,

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2004), Wernberg et al. (2003) and Goldberg and Kendrick (2004) from the temperate south and western coasts of Australia. The physical environment across topographically simple and complex reefs may influence the distribution of algae by altering key biological processes such as recruitment and competition. Acknowledgements For their help in the field, I thank Patrick Toohey, Emyrs Casali, George Shedrawi, Gary Kendrick, Dave Abdo, Cain Delacy, Zarin Salter, Linda Heap, Brooke Halkyard, Iain Ellis, Cillain Stockdale, Florence Verspecht and Charitha Pattiaratchi. For supplying funding, I recognize the contributions of the Commonwealth Government via an Australian Postgraduate Award, the School of Plant Biology, UWA and an ARC small grant to Gary Kendrick. I am grateful to Gary Kendrick, Thomas Wernberg and three anonymous reviewers for their comments on this manuscript. References Anon., 1997. Topography. In: Jackson, Julia A. (Ed.), Glossary of Geology. American Geological Institute, Virginia. Airoldi, L., Cinelli, F., 1997. Effects of sedimentation on subtidal macroalgal assemblages: an experimental study from a Mediterranean rocky shore. Journal of Experimental Marine Biology and Ecology 215, 268e288. Almany, G.R., 2004. Differential effects of habitat complexity, predators and competitors on abundance of juvenile and adult coral reef fishes. Oecologia 141, 105e113. Altamirano, M., Murakami, A., Kawai, H., 2004. High light stress in the kelp Ecklonia cava. Aquatic Botany 79, 125e135. Anderson, M.J., 2001. A new method for non-parametric multivariate analysis of variance. Austral Ecology 26, 32e45. Anderson, M.J., 2005. DISTLM v.5: A FORTRAN computer program to calculate a distance-based multivariate analysis for a linear model. Department of Statistics, University of Auckland, New Zealand. Anderson, M.J., Robinson, J., 2003. Generalised discriminant analysis based on distances. Australia and New Zealand Journal of Statistics 45, 301e318. Anderson, M.J., Willis, T.J., 2003. Canonical analysis of principal coordinates: a useful method of constrained oridination for ecology. Ecology 84, 511e525. Baynes, T.W., 1999. Factors structuring a subtidal encrusting community in the southern Gulf of California. Bulletin of Marine Science 64, 419e450. Bell, E.C., Denny, M.W., 1994. Quantifying ‘‘wave exposure’’: a simple device for recording maxiumum velocity and results of its use at several field sites. Journal of Experimental Marine Biology and Ecology 181, 9e29. Brakel, W.H., 1979. Small-scale spatial variation in light available to coral reef benthos: quantum irradiance measurements from a Jamaican reef. Bulletin of Marine Science 29, 406e413. Bray, J.R., Curtis, J.T., 1957. An ordination of the upland forest communities of southern Wisconsin. Ecological Monographs 27, 325e349. Clark, R.P., Edwards, M.S., Foster, M.S., 2004. Effects of shade from multiple kelp canopies on an understory algal assemblage. Marine Ecology Progress Series 267, 107e119. Connell, S.D., 2005. The assembly and maintenance of subtidal habitat heterogeneity: synergistic effects of light penetration and sedimentation. Marine Ecology Progress Series 289, 53e61. Connell, S.D., 2003. The monopolization of understorey habitat by subtidal encrusting coralline algae: a test of the combined effects of canopymediated light and sedimentation. Marine Biology 142, 1065e1071.

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