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published_or_final_version / Ecology and Biodiversity / Doctoral / Doctor of Philosophy

Choi Li Si. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2003. / Includes bibliographical references (leaves 156-168). / Abstracts in English and Chinese. / Acknowledgements --- p.i / Abstract --- p.ii / Contents --- p.v / List of Tables --- p.ix / List of Figures --- p.xi / Chapter Chapter 1: --- General Introduction / Chapter 1.1 --- Introduction --- p.1 / Chapter 1.2 --- The distribution and growth of coral and algae in Hong Kong --- p.3 / Chapter 1.3 --- Objectives --- p.6 / Chapter 1.4 --- Study Site --- p.7 / Chapter 1.5 --- Coral species chosen for the experiment --- p.10 / Chapter 1.6 --- Thesis outline --- p.11 / Chapter Chapter 2: --- "General Survey on Changes in Percentage Coverage of Coral and Fleshy Macroalgae in AMW and AYW, Tung Ping Chau, over Time" / Chapter 2.1 --- Introduction --- p.18 / Chapter 2.2 --- Methods and Materials --- p.26 / Chapter 2.2.1 --- In-situ survey methods --- p.26 / Chapter 2.2.2 --- Comparison of coral cover in the presence or absence of algae --- p.26 / Chapter 2.2.3 --- Environmental parameters --- p.27 / Chapter 2.2.4 --- "Image, data and statistical analysis" --- p.28 / Chapter 2.3 --- Results --- p.28 / Chapter 2.3.1 --- Coral coverage in AMW and AYW --- p.28 / Chapter 2.3.2 --- Percentage algal cover in AMW and AYW --- p.29 / Chapter 2.3.3 --- Dominating fleshy algal species in AMW and AYW --- p.30 / Chapter 2.3.4 --- Comparison of the coral coverage before and after the algal removal --- p.30 / Chapter 2.3.5 --- Water temperature --- p.31 / Chapter 2.3.6 --- Nutrient levels --- p.32 / Chapter 2.3.7 --- Further observation on the health of the corals during fleshy macroalgal bloom --- p.34 / Chapter 2.4 --- Discussion --- p.35 / Chapter Chapter 3 --- "The Effects of Algal-Coral Interactions on the Photosynthetic Ability of the Coral, Porites lobata in AMW and AYW, Tung Ping Chau" / Chapter 3.1 --- Introduction --- p.59 / Chapter 3.2 --- Methods and Materials --- p.66 / Chapter 3.2.1 --- Settings of the permanent corals --- p.66 / Chapter 3.2.2 --- Measurement of the seasonal changes in the photosynthetic ability of the corals --- p.66 / Chapter 3.2.3 --- Measurement of the diurnal changes in the photochemical efficiency of Porites lobata --- p.67 / Chapter 3.2.4 --- Correlation of quantum yield with the zooxanthellae density and the chlorophyll a concentrations --- p.68 / Chapter 3.2.5 --- Evaluation of zooxanthellae and chlorophyll-a densities --- p.68 / Chapter 3.2.6 --- Statistical analysis --- p.69 / Chapter 3.2.6.1 --- Monthly measurement of the photosynthetic ability of the corals --- p.69 / Chapter 3.2.6.2 --- Diurnal measurements of the photosynthetic ability of the corals in May and July2002 --- p.70 / Chapter 3.2.6.3 --- Relationships between quantum yield and zooxanthellae and chlorophyll a concentrations --- p.70 / Chapter 3.3 --- Results --- p.70 / Chapter 3.3.1 --- The photosynthetic activities of corals --- p.70 / Chapter 3.3.2 --- The photochemical quenching (qP) of the corals --- p.72 / Chapter 3.3.3 --- Diurnal fluctuations in the photosynthetic ability of Porites lobata and the Photo synthetically Active Radiation (PAR) --- p.73 / Chapter 3.3.3.1 --- Photosynthetic quantum yield of Porites lobata --- p.74 / Chapter 3.3.3.2 --- Diurnal changes in the Photo synthetically Active Radiation (PAR) --- p.75 / Chapter 3.3.4 --- The relationship between the photosynthetic ability of the corals and their chlorophyll-a and zooxanthellae densities --- p.76 / Chapter 3.3.5 --- Correlation between photosynthetic activities of corals and eenvironmental parameters --- p.76 / Chapter 3.3.5.1 --- Heights of coral colonies --- p.76 / Chapter 3.3.5.2 --- Photosynthetic ability of the corals and the presence of the drifting algae --- p.77 / Chapter 3.3.5.3 --- Photosynthetic ability of the corals and sea water temperature --- p.77 / Chapter 3.4 --- Discussion --- p.78 / Chapter 3.4.1 --- The photosynthetic activities of the corals --- p.78 / Chapter 3.4.2 --- The photochemical quenching of the corals --- p.80 / Chapter 3.4.3 --- Diurnal changes in the photosynthetic efficiencies of the P. lobata --- p.81 / Chapter 3.4.4 --- Relationship between the fluorescence yield and the chlorophyll-a and zooxanthellae densities --- p.82 / Chapter Chapter 4 --- The effects of drifting fleshy macroalgae on the corals: A caging manipulation of their effect on the photosynthetic activities of the corals / Chapter 4.1 --- Introduction --- p.114 / Chapter 4.2 --- Methods and Materials --- p.115 / Chapter 4.2.1 --- Setting up of the cages --- p.115 / Chapter 4.2.2 --- Setting up of the corals --- p.116 / Chapter 4.2.3 --- Measurement of the photosynthetic activities of the corals --- p.117 / Chapter 4.2.4 --- Data and statistical analysis --- p.117 / Chapter 4.3 --- Results --- p.117 / Chapter 4.3.1 --- The photosynthetic ability of the corals under different treatments --- p.117 / Chapter 4.3.2 --- The photosynthetic activities of different regions of the corals in each treatment --- p.119 / Chapter 4.4 --- Discussion --- p.120 / Chapter Chapter 5 --- "Interactions between corals, filamentous algal turf and encrusting coralline algae in Tung Ping Chau" / Chapter 5.1 --- Introduction --- p.135 / Chapter 5.2 --- Methods and Materials --- p.138 / Chapter 5.3 --- Results --- p.139 / Chapter 5.3.1 --- Coral-algal turf interactions --- p.139 / Chapter 5.3.2 --- Coral-coralline algae interactions --- p.140 / Chapter 5.3.3 --- General observations on the growth of the algal turf and the CCA on corals --- p.141 / Chapter 5.4 --- Discussion --- p.141 / Chapter Chapter 6 --- Summary and Perspectives --- p.152 / References --- p.156

So Ka Yi Erica. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2005. / Includes bibliographical references (leaves 243-255). / Abstracts in English and Chinese. / Acknowledgements --- p.i / Abstract --- p.iii / Contents --- p.ix / List of Tables --- p.xii / List of Figures --- p.xix / Chapter Chapter 1 --- General Introduction / Chapter 1.1 --- Introduction --- p.1 / Chapter 1.2 --- General Objectives --- p.11 / Chapter 1.3 --- Study Site --- p.12 / Chapter 1.4 --- Organization of the Thesis --- p.13 / Chapter Chapter 2 --- "General Surveys on the Abundance of Algae and Herbivores in A Ma Wan, A Ye Wan and Lung Lok Shui, Tung Ping Chau, Hong Kong" / Chapter 2.1 --- Introduction --- p.16 / Chapter 2.2 --- Materials and Methods --- p.20 / Chapter 2.2.1 --- Study site --- p.20 / Chapter 2.2.2 --- Measurement of water temperature --- p.21 / Chapter 2.2.3 --- Measurement of algal percentage cover --- p.22 / Chapter 2.2.4 --- Measurement of herbivore density --- p.22 / Chapter 2.2.5 --- Investigation on the species richness and diversity of algae and herbivores --- p.23 / Chapter 2.2.6 --- Statistical analysis --- p.24 / Chapter 2.3 --- Results --- p.27 / Chapter 2.3.1 --- Measurement of algal abundance and diversity --- p.27 / Chapter 2.3.1.1 --- Percentage cover and morphology --- p.28 / Chapter 2.3.1.2 --- Species richness --- p.29 / Chapter 2.3.1.3 --- Species diversity --- p.29 / Chapter 2.3.1.4 --- Dominance and composition --- p.30 / Chapter 2.3.2 --- Measurement of herbivore abundance and diversity --- p.32 / Chapter 2.3.2.1 --- Density of herbivores --- p.32 / Chapter 2.3.2.2 --- Species richness --- p.33 / Chapter 2.3.2.3 --- Species diversity --- p.34 / Chapter 2.3.2.4 --- Dominance and composition --- p.34 / Chapter 2.3.3 --- Relationships between algae and herbivores --- p.37 / Chapter 2.3.3.1 --- Pairwise Pearson Correlation between algae and herbivores in different sites --- p.37 / Chapter 2.3.3.2 --- Canonical correlations between algal and herbivorous species --- p.38 / Chapter 2.3.4 --- "Water temperature and its relationships with the abundance, richness and diversity of algae and herbivores" --- p.39 / Chapter 2.4 --- Discussion --- p.40 / Chapter 2.4.1 --- Spatial distribution of algae and herbivores --- p.40 / Chapter 2.4.2 --- Seasonal distributions of algae and herbivores --- p.46 / Chapter 2.4.3 --- Interactions between algae and herbivores --- p.50 / Chapter Chapter 3 --- Growth of Algae in Herbivore-exclusion Manipulative Experiment / Chapter 3.1 --- Introduction --- p.106 / Chapter 3.2 --- Materials and Methods --- p.111 / Chapter 3.2.1 --- Study site --- p.111 / Chapter 3.2.2 --- Manipulative experiment --- p.111 / Chapter 3.2.3 --- Investigation on the manipulative experiment --- p.112 / Chapter 3.2.3.1 --- Species composition of algae and herbivores --- p.113 / Chapter 3.2.3.2 --- Percentage cover of algae and density of herbivores --- p.113 / Chapter 3.2.3.3 --- Sizes of herbivores --- p.113 / Chapter 3.2.4 --- Detecting the cage effect --- p.114 / Chapter 3.2.5 --- Statistical analyses --- p.114 / Chapter 3.3 --- Results --- p.117 / Chapter 3.3.1 --- Algae --- p.117 / Chapter 3.3.1.1 --- Percentage cover --- p.117 / Chapter 3.3.1.2 --- Species richness --- p.120 / Chapter 3.3.1.3 --- Composition between treatments --- p.121 / Chapter 3.3.1.4 --- Compositions between set-ups --- p.121 / Chapter 3.3.1.5 --- Effects from caging and clearing --- p.122 / Chapter 3.3.2 --- Herbivores --- p.123 / Chapter 3.3.2.1 --- Density --- p.123 / Chapter 3.3.2.2 --- Species richness --- p.124 / Chapter 3.3.2.3 --- Compositions between treatments --- p.124 / Chapter 3.3.2.4 --- Compositions between set-ups --- p.125 / Chapter 3.3.3 --- Relationships between algae and herbivores --- p.125 / Chapter 3.3.3.1 --- Abundance --- p.125 / Chapter 3.3.3.2 --- Composition --- p.126 / Chapter 3.3.4 --- Sizes of herbivores --- p.128 / Chapter 3.3.5 --- Irradiance between treatments --- p.128 / Chapter 3.4 --- Discussion --- p.129 / Chapter 3.4.1 --- Effects of clearing on algal and herbivore dynamics --- p.130 / Chapter 3.4.2 --- Effects of caging on algal and herbivore dynamics --- p.135 / Chapter 3.4.3 --- Effects of seasonality of clearing on algal and herbivore dynamics --- p.139 / Chapter 3.4.4 --- Interactions of algae and herbivores --- p.142 / Chapter Chapter 4 --- Feeding Behavior of Common Herbivores in the Artificial Food Experiment / Chapter 4.1 --- Introduction --- p.216 / Chapter 4.2 --- Materials and Methods --- p.218 / Chapter 4.2.1 --- Sample collections --- p.218 / Chapter 4.2.2 --- Production of artificial foods --- p.219 / Chapter 4.2.3 --- Feeding experiments --- p.219 / Chapter 4.2.4 --- Statistical analysis --- p.220 / Chapter 4.3 --- Results --- p.221 / Chapter 4.4 --- Discussion --- p.222 / Chapter Chapter 5 --- Summary and Conclusion --- p.233 / References --- p.243

Gradients of physical disturbance are central to theories of community organization yet rarely are studies performed in which physical factors are experimentally manipulated. Pothole tidepool algal communities exhibit distinct zonation patterns from top to bottom that result from scouring by rocks and other debris in the pools. Scouring is easily manipulated by removing or adding rocks to tidepools. Thus, the gradient of physical disturbance potentially causing community patterns can be manipulated to test theories of community organization. I documented the distribution pattern of algae inhabiting pothole tidepools and measured a number of physical factors which were hypothesized to be responsible for the observed zonation patterns. Then, I experimentally evaluated the roles of physical disturbance, herbivory, and competition in these tidepool communities. I found that scouring by rocks was primarily responsible for the observed zonation patterns in pothole tidepools. However, not all pools are potholes. Evaluation of the physical properties effecting the cobble-retaining ability of tidepools enabled prediction over a broad geographic range of pools likely to have cobbles and thus show typical pothole algal zonation patterns. Coralline algae (Rhodophyta, Corallinaceae) are a dominant feature of tidepools as well as many low intertidal and subtidal habitats. I evaluated the relative resistance of coralline algae (both articulated and crustose forms) and other common tidepool algae to scouring by rocks. Coralline crusts were highly resistant to scouring while articulated coralline algae are very susceptible to scouring. Erect fleshy algal species showed intermediate resistance to scouring. This corresponds well to observed algal zonation patterns in intertidal potholes. Based on this information, I proposed that wave-induced scouring may have been the selective force for the initial incorporation of calcium carbonate into algal thalli. / Graduation date: 1992

The term "interaction" in evolutionary biology and ecology describes the relationships among variables in two classes of causal models. In the first, "interaction" refers to the influence of a single putatively causal variable on a variable of interest. In the second class of models, the term applies when a third variable mediates the relationship between two variables in the first class of models. The development of multi-factor causal models in evolutionary biology and ecology represents a stage in the construction of theory that usually follows from complexities discovered in single-factor analyses. In this thesis, I present three cases that illustrate how results of simple single-factor models in the population genetics and community ecology of seaweeds may be affected by incorporation of a second causal factor. In Chapter II, we consider how the effect of natural selection on genetic variability in seaweeds and other plants may be mediated by life history variation. Many seaweeds have haplodiplontic life histories in which haploid and diploid stages alternate. Our theoretical analysis and review of the electrophoretic literature show that the level of genetic polymorphism in haplodiplonts is not necessarily reduced relative to that in diploids. In Chapter III, I take an experimental approach to understanding how herbivory may mediate the effect of desiccation on the upper intertidal limit of a red alga, Iridaea cornucopiae. Iridaea appears to be grazer-limited in dry, but grazer-dependent in moist environments, suggesting that a third factor may mediate the interaction of desiccation and herbivory. Finally, in Chapter IV, we consider research strategies for studying how the outcome of competitive interactions is affected by seaweed traits. Some of the problems that arise in applying simple models of competition to plants suggest the need for theory that explicitly incorporates plant traits in two- (or more) factor models of interspecific competition. In particular, we note that unique traits of seaweeds require development of new approaches to understanding competition. Single-factor causal models represent an indispensable stage in the development of evolutionary and ecological theory. Properly conceived theoretical and empirical studies focus attention on the assumptions under which such models will hold and suggest lines of inquiry that ultimately lead to the integration of additional causal factors in conceptual models of natural processes. Identifying the circumstances under which simple models will suffice remains one of the most important challenges of evolutionary and ecological scholarship. / Graduation date: 1993

Four experiments were performed to test the feasibility of using taxonomic composition and abundance of attached algae to identify treatments of zinc (Zn) alone and in combination with treatments of phosphate, snail grazing, and pH. In the experiment presented in chapter 2, three treatments of zinc (0.05, 0.5, 1.0 mg Zn•l⁻¹) and a control could be identified by different algal communities in outdoor, flow-through, stream mesocosms. Established communities were continuously exposed to Zn, and samples were collected on days 0, 2, 5, 10, 20, and 30 after treatment began. Experiments were conducted in spring, summer, and fall 1984. Control stream mesocosms could be identified by diatoms in all seasons. The 0.05 mg Zn•l⁻¹ treatment could be identified by certain diatom taxa being more abundant than in the control in all seasons and by a filamentous green·alga in summer and fall. The 0.5 mg Zn•l⁻¹ treatment could be identified a filamentous green alga in fall. The 1.0 mg Zn•l⁻¹ treatment could be identified by unicellular green-algae in all seasons and by a filamentous blue-green alga in summer. A similarity index (SIMI) indicated that Zn stressed samples generally became less similar to control samples as Zn concentration increased from 0.05 to 1.0 mg Zn•l⁻¹. Total biovolume·density of all taxa responded more slowly than did individual taxa in spring and failed to distinguish between Zn treatments in summer and fall. Zn bound to periphyton (microbial community on solid substrates) was more reliable than total Zn in water for identifying Zn treatments. Zn treatments as low as 0.05 mg Zn•l⁻¹ changed algal species composition. This conflicts with the criterion (0.047 mg Zn•l⁻¹) of the U. S. Environmental Protection Agency for the 24-hour average of total recoverable Zn. ln the experiment presented in chapter 3, individual and combined effects of phosphate (P) and zinc (Zn) on the abundance of dominant algae and protozoa in a community were observed. Nutrient·diffusing artificial substrates were colonized in Douglas Lake, Michigan, and then placed in laboratory microcosms containing one of five Zn treatments (control, 0.1, 1.0, 3.0, and 10.0 mg Zn•l⁻¹). After one week of exposure in the laboratory the substrates were scraped and algal and ciliated protozoan abundances determined. Ten of thirteen algae and five of eight ciliated protozoa responded to experimental treatments. Some algae (diatoms and green algae) and ciliated protozoa were stimulated by high P, some stimulated by intermediate P, and some inhibited by high P. One alga and four protozoa responded positively to Zn. Two algae and three protozoa responded to a significant interaction between P and Zn so that abundances were from 3 to 19 times higher than the added effects of individual P and Zn treatments. Total algal abundance was increased by high P and total protozoan abundance was increased by intermediate P but at control levels for high P. The number of protozoan species was increased by P. Total algal abundance was increased by · combinations of Zn P and the number of protozoan species was decreased by Zn P. Altered abundance by combinations of Zn and P had not been demonstrated for a community of algae and protozoa previously. Although concentrations of Zn were initially above the level considered safe by the U.S. Environmental Protection Agency, many factors may prevent Zn stress. In the experiment presented in chapter 4, effects of 0.5 mg Zn•l⁻¹ and snail grazing (400 snails m⁻²) on density of dominant algal taxa were examined using established (12-day colonization) periphyton communities in flow-through stream mesocosms with four treatments (Zn, snails, Zn and snails, control) for 30 days. Grazing and Zn similarly reduced the abundance of 5 of 10 dominant taxa during the first 10 days of treatment. Temperature may play a very important role in determining the effect of snail grazing on attached algal communities. Cold temperatures (< 15 C) may have inhibited snail grazing to the extent that abundance of four taxa increased to levels found in non·snail treatments. However, one diatom was more than twice as abundant in snail treatment over non-snail treatment -- apparently stimulated by the presence of snails during cold conditions; and two diaoms remained at low abundance in snail treatment despite rapid growth in non-snail treatment -- apparently inhibited or selected as a food source by snails during cold con- ditions. No algal taxa replaced the diatoms inhibited by 0.5 mg Zn•l⁻¹ in this October-November, 1984, experiment by day 10. This is in contrast to an experiment performed one month earlier, in September-October, in which a community characteristic of this treatment developed by day 5. Testing individual and combined variables that affect attached algal communities will enhance understanding of population dynamics in algal ecology and pollutant assessment. In the experiment presented in chapter 5, attached-algal communities were employed to test the US Environmental Protection Agency’s (USEPA) guidelines for zinc (Zn) and pH. The experiment was designed to determine whether algal community composition and abundance would be altered by (a) ph 6 or 9, (b) 0.05 mg Zn•l⁻¹, or (c) the combination of ph 6 or 9 and 0.05 mg Zn•l⁻¹. Stream mesocosms were continuously supplied with natural water from the New River, VA, USA. Established (12-day colonization) communities on artificial substrates were sampled on days 0, 5, 10, 20, and 30 after treatment began on 9 July 1985. Zinc and pH treatments changed algal community composition from diatoms and a filamentous blue-green alga to different diatom taxa, green algae, or a coccoid blue-green alga. Total algal abundance was moderately increased by pH 6 treatment. Treatments of pH 6 and 0.05 mg Zn•l⁻¹ significantly altered attached-algal community composition even though these levels are considered "safe' by the USEPA. The pH 9 treatment did not significantly alter community composition, most likely because ambient pH was near this level. / Ph. D. / incomplete_metadata

The toxicity of methanol, atrazine, and copper to Pseudokirchneriella subcapitata (Korshikov) Hindák historically referred to as Selenastrum capricornutum Printz were determined following 96 hrs growth in a modified Goram's growth media. Methanol and atrazine inhibited fluorescence readings in the cultures by 50% (IC50) at concentrations of 2% and 82 µg/l respectively. These toxicity values compared favorably to other published reports. The IC50 for copper was 160 µg/l which is substantially higher than reported values. This is understandable because of the high chelating capacity of Goram's media. The use of stereologically derived relative volume in the chloroplasts, mitochondria, lipid bodies, phosphate bodies, and nucleus was investigated to determine if it could be used as a sensitive endpoint in toxicity tests. The volume fractions for the chloroplasts and mitochondria were normally distributed in control cells while the nuclei, phosphate bodies, and lipid bodies were not. The chloroplasts were the most dominate organelle occupying a mean relative volume of 46% and mitochondria occupied a mean relative volume of 3%. The nucleus and phosphate bodies occupied a median relative volume of 7% and 2% respectively. The lipid bodies were rare in section profile and no meaningful median relative volume could be calculated. Up to the 82nd percentile of sectioned profiles contained no recognizable lipid bodies. The use of relative volume was not a sensitive endpoint for use in toxicity tests. No significant differences in relative volume could be detected in the nucleus or phosphate bodies following any treatment. Limited differences were detected in the mitochondria, chloroplasts, and lipid bodies. The only significant differences that appear to be biologically significant occurred in methanol treated cells where an increase in the lipid bodies' relative volume was apparently concentration dependent. Significant differences in the relative volume of mitochondria and chloroplasts do not appear to be biologically significant.

Generalist herbivores profoundly influence the biomass and species composition of macroalgae assemblages. In subtidal ecosystems of temperate latitudes, large invertebrates are usually the most influential herbivores. I tested the prediction that exclusion of invertebrate herbivores would lead to changes in the biomass and species composition of the macroalgae assemblages that are a prominent feature of the reefs in south-western Australia. The most abundant invertebrate herbivores were sea urchins (Heliocidaris erythrogramma, Phyllacanthus irregularis and Centrostephanus tenuispinus), and these occupied different trophic positions. Heliocidaris was present at virtually all reefs surveyed, and was particularly abundant in the Fremantle region. Analyses of stable isotopes and direct observations of gut contents revealed that it was almost exclusively herbivorous, and that it mainly ate foliose brown algae. In contrast, Phyllacanthus and Centrostephanus were omnivorous; while they consumed large proportions of algae, a substantial proportion of the diet of both species was animal tissue. Because Heliocidaris is a generalist herbivore that occurs at high densities, it could exert a large influence on the macroalgae assemblage. This prediction was tested by a series of press experiments. Contrary to the prediction, Heliocidaris exerted a very minor influence on the biomass, and no detectable influence on the species composition, of attached macroalgae. However, it exerted a major influence on the retention of drift macroalgae and seagrass by trapping and feeding on drift. It exerted a particularly strong influence on retention of the kelp Ecklonia radiata. This kelp was not abundant in the attached algae assemblage (when all plots were pooled it ranked 35th in biomass), but was abundant as drift (ranking 1st). Most of the drift Ecklonia was retained by sea urchins, rather than freely drifting.Herbivorous fish may also influence macroalgae assemblages. To compare the effects of sea urchins versus fish on recruiting and adult macroalgae a 13-month exclusion experiment was conducted. There were no detectable effects of sea urchins (mainly Heliocidaris) on either recruiting or adult macroalgae. There were some patterns in the biomass of recruiting algae consistent with an influence by herbivorous fish; however, these patterns were also consistent with the presence of artefacts (shading and reduced water flow) by fish exclusion devices. I began with the prediction that large invertebrate herbivores were a major influence on the macroalgae assemblages of subtidal reefs in south-western Australia. Overall, there was little evidence to support this prediction: within spatial extents of tens of square metres and over periods of 1-2 years, only minor effects were detected. However, it remains plausible that herbivores exert an influence over long time periods across large spatial extents in south-western Australia. I propose that trophic subsidies support the comparatively high densities of Heliocidaris that exist at some reefs. I further propose that these subsidies mediate the effects of sea urchins on the attached macroalgae assemblage, and that they might play an important role in energy and nutrient cycling in these nearshore ecosystems.