Disturbance and reef topography maintain high local diversity in Ecklonia radiata kelp forests

Disturbance of competitive‐dominant plant and algae canopies often lead to increased diversity of the assemblage. Kelp forests, particularly those of temperate Western Australia, are habitats with high alpha diversity. This study investigated the roles of broad‐scale canopy loss and local scale reef topography on structuring the kelp‐dominated macroalgal forests in Western Australia. Eighteen 314 m² circular areas were cleared of their Ecklonia radiata canopy and eighteen controls were established across three locations. The patterns of macroalgal recolonisation in replicate clearances were observed over a 34 month period. Macroalgal species richness initially increased after canopy removal with a turf of filamentous and foliose macroalgae dominating cleared areas for up to seven months. A dense Sargassum canopy dominated cleared areas from 11 to 22 months. By 34 months, partial recovery of the kelp canopy into cleared areas had occurred. Some cleared areas did not follow this trajectory but remained dominated by turfing, foliose and filamentous algae. As kelp canopies developed, the initial high species diversity declined but still remained elevated relative to undisturbed controls, even after 34 months. More complex reef topography was associated with greater variability in the algal assemblage between replicate quadrats suggesting colonising algae had a greater choice of microhabitats available to them on topographically complex reefs. Shading by canopies of either Sargassum spp. and E. radiata are proposed to highly influence the abundance of algae through competitive exclusion that is relaxed by disturbance of the canopy. Disturbance of the canopy in E. radiata kelp forests created a mosaic of different patch types (turf, Sargassum‐dominated, kelp‐dominated). These patch types were both transient and stable over the 34 months of this study, and are a potential contemporary process that maintains high species diversity in temperate kelp‐dominated reefs.     

Spread of introduced Caulerpa species in macroalgal habitats

A short-term field experiment was designed to identify layers of Mediterranean macroalgal assemblage conducive to successful spread of two introduced Caulerpa species (Bryopsidales, Chlorophyta). By manipulation of species presence, three experimental assemblages were obtained: (1) encrusting algae, having removed the turf and erect species; (2) encrusting and turfing algae, having removed erect species; (3) encrusting, turfing and erect algae, that is, unmanipulated assemblages, which served as a control. Fragments of the two introduced species Caulerpa taxifolia (Vahl) C. Agardh and Caulerpa racemosa (Forsskål) J. Agardh were transplanted in each of the three assemblages. Width of the colony, blade density and percentage of the substratum covered by the two species were measured.

The susceptibility of the indigenous community to the spread of Caulerpa species was related to type of assemblage. Blade density and amount of substratum covered by the two Caulerpa species were different between species and generally greater for C. taxifolia than for C. racemosa. Overall, the spread of these species was strongly dependent on the type but not directly on the complexity of the assemblage. Turf was more favourable than encrusting species alone, while the least advantageous habitat was where the macroalgal assemblage is composed of encrusting, turf and erect species. In other words, increased number of species in the assemblage reduces invasion of the Caulerpa species but the type of algae in the assemblage is likely to be more important than number of species. The presence of turf promotes the spread of Caulerpa species.     

Ecosystem engineering by a canopy‐forming kelp facilitates the recruitment of native oysters

Ecosystem engineers are species that influence the abiotic and biotic environment around them and may assist the restoration of associated species, including other habitat‐forming species. We deployed an array of 28 artificial reefs with transplanted Ecklonia radiata, the dominant canopy‐forming kelp species across southern Australia, to investigate how the patch size and density of E. radiata influenced the establishment of the associated communities of plants and animals. Many of the reefs were rapidly colonized by Ostrea angasi, a critically depleted reef‐forming oyster. Over the 24‐month deployment of the reefs, thick oyster mats formed across the entire surface of many of the reefs with estimated biomass densities exceeding 5 kg of live oysters/m²; however, oyster density was dependent on E. radiata patch size and density. Increasing patch size and the presence of kelp resulted in significantly higher densities of oysters 5 months after the reefs were deployed and at the end of the experiment, where oysters were approximately three times more numerous on reefs with kelp compared to those without kelp. E. radiata appeared to facilitate the establishment of O. angasi largely through its capacity to reduce benthic light and thus suppress competition from turfing algae. These results may inform the development of novel approaches to tackle recruitment bottlenecks affecting the restoration of O. angasi reefs.     

Contrasting effects of giant kelp on dynamics of surfperch populations

Summary The effect of giant kelp, Macrocystis pyrifera, on the population dynamics of two temperate reef fishes, striped surfperch (Embiotoca lateralis) and black surfperch (E. jacksoni), was examined. Based on an understanding of how particular reef resources influence abundances of the surfperch and of the effect of giant kelp on those resources, we anticipated that Macrocystis would adversely affect populations of striped surfperch but would enhance those of black surfperch. The natural establishment of giant kelp at sites at Santa Cruz Island, California, resulted in the predicted dynamical responses of surfperch. Abundances of striped surfperch declined rapidly when and where dense forests of giant kelp appeared, but showed little change where Macrocystis was continuously absent over the 8 y period of study. Abundances of adult black surperch, which increased following the appearance of giant kelp, were lagged by >1 y because the dynamical response involved enhanced local recruitment. No change in abundance of black surfperch populations was evident at areas without giant kelp.The mechanism by which giant kelp altered the dynamics of the surfperch involved modification of the assemblage of understory algae used by surfperch as foraging microhabitat. Foliose algae (including Gelidium robustum) were much reduced and turf was greatly enhanced following the appearance of Macrocystis; these two benthic substrata are the favored foraging microhabitat for striped surfperch and black surfperch respectively. Populations of both surfperch species tracked temporal changes in the local availability of their favored foraging microhabitat. Thus, while neither species used Macrocystis directly, temporal and spatial variation in giant kelp indirectly influenced the dynamics of these fishes by altering their foraging base. These results indicate that the dynamics of striped surfperch and black surfperch were governed to a large degree by density-dependent consumer-resource interactions. The present work underscores the predictive value that arises from a knowledge of the mechanisms by which processes operate.     

The effects of grazing by gastropods and physical factors on the upper limits of distribution of intertidal macroalgae

Summary The cover of foliose algae is sparse to non-existent above a low-level algal zone on many shores in N.S.W., except in rock-pools. Above this algal zone, encrusting algae, mostly Hildenbrandia prototypus, occupy most of the primary substratum on sheltered shores. Experimental manipulations at midtidal levels were used to test hypotheses about the effects of grazing by molluses and of physical factors during low tide on this pattern of algal community structure.Fences and cages were used to exclude grazers: molluscs grazed under roofs and in open areas. Cages and roofs provided shade, and decreased the harshness of the environment during low tide: fences and open areas had the normal environmental regime.In the absence of grazers, rapid colonization of Ulva and slower colonization by other foliose algae occurred in all experimental areas. The rate of colonization by Ulva sporelings was initially retarded on existing encrusting algae, but after a few months, cover of Ulva equalled that on cleared rock.Most species of algae only grew to maturity inside cages, and remained as a turf of sporelings inside fences. No foliose algae grew to a visible size in open, grazed areas. Grazing thus prevents the establishment of foliose algae above their normal upper limit on the shore, but the effects of physical factors during low tide prevent the growth of algae which become established when grazers are removed. Physical factors thus limit the abundance of foliose algae at mid-tidal levels.The recolonization of cleared areas by Hildenbrandia was not affected by the presence of a turf of sporelings, nor by the shade cast by roofs, but was retarded in cages where mature algae formed a canopy. Even under such a canopy, Hildenbrandia eventually covered as much primary substratum as in open, grazed areas. This encrusting alga is able to escape from the effects of grazing by having a tough thallus, and by its vegetative growth which allows individual plants to cover a lot of substratum, and by the tendency for new individuals to start growing from small cracks and pits in the rock, which are apparently inaccessible to the grazers.Mature foliose algae are removed from the substratum by waves, and many individual plants died during periods of hot weather. Sporelings in a turf were eliminated, after experimental fences were removed, by the combined effects of macroalgal grazers, which invaded the areas, and microalgal grarers which ate the turt from the edges inwards.The results obtained here are discussed with respect to other studies on limits to distribution of intertidal macroalgae, and the role of grazing in the diversity and structure of intertidal algal communities. Some problems of these experimental treatments are also discussed.     

Kelp and sea urchin settlement mediated by biotic interactions with benthic coralline algal species

Species interactions can influence key ecological processes that support community assembly and composition. For example, coralline algae encompass extensive diversity and may play a major role in regime shifts from kelp forests to urchin-dominated barrens through their role in inducing invertebrate larval metamorphosis and influencing kelp spore settlement. In a series of laboratory experiments, we tested the hypothesis that different coralline communities facilitate the maintenance of either ecosystem state by either promoting or inhibiting early recruitment of kelps or urchins. Coralline algae significantly increased red urchin metamorphosis compared with a control, while they had varying effects on kelp settlement. Urchin metamorphosis and density of juvenile canopy kelps did not differ significantly across coralline species abundant in both kelp forests and urchin barrens, suggesting that recruitment of urchin and canopy kelps does not depend on specific corallines. Non-calcified fleshy red algal crusts promoted the highest mean urchin metamorphosis percentage and showed some of the lowest canopy kelp settlement. In contrast, settlement of one subcanopy kelp species was reduced on crustose corallines, but elevated on articulated corallines, suggesting that articulated corallines, typically absent in urchin barrens, may need to recover before this subcanopy kelp could return. Coralline species differed in surface bacterial microbiome composition; however, urchin metamorphosis was not significantly different when microbiomes were removed with antibiotics. Our results clarify the role played by coralline algal species in kelp forest community assembly and could have important implications for kelp forest recovery.     

Impacts and negative feedbacks in community recovery over eight years following removal of habitat-forming macroalgae

The decline or loss of habitat-forming species has affected rocky shore marine communities worldwide. Many short-term studies have documented the initiation of cascading effects due to canopy losses of macroalgae, but relatively few studies have followed recovery dynamics over many years. Here, we show that the experimental removal of a dominant intertidal fucoid in southern New Zealand had numerous community effects up to 8 years later. Even though the dominant fucoid returned to a nearly closed canopy, there remained many differences between disturbed and control communities. The disturbed treatments had lower plant density and biomass of the dominant fucoid, fewer species and more turfing coralline algae than controls. Plots with press disturbances were more affected than those with pulse disturbances. The negative feedback of turfing algae on the recovery of fucoid recruitment resulted in effects on cover and diversity in the wider community being evident after 8 years. We discuss the feedbacks between fucoids, benthic turfing algae and community recovery and argue that if biodiversity impacts on marine rocky reefs are to be understood, the role of non-trophic interactions in structure, function and dynamics must be better delineated.     

Contrasting effects of crustose coralline algae from exposed and subcryptic habitats on coral recruits

Coral recruitment is important in sustaining coral reef ecosystems and contributing to their recovery after disturbances. Despite widespread acceptance that crustose coralline algae (CCA) positively influence coral recruitment success, especially by enhancing coral settlement and early post-settlement stages, there are no experimental data on the effects of CCA species on late post-settlement survival and growth of corals. This study tested the impact of four common, thick-crusted CCA species from two habitats (exposed and subcryptic) on the survival and growth of two recruit size categories of the coral genus Pocillopora. Coral recruits and CCA were transplanted adjacent to each other using epoxy in Petri dishes directly attached to the reef substratum on the forereef of Moorea (French Polynesia) in a 1-year field experiment. In the subcryptic habitat, survival of small-sized recruits adjacent to subcryptic CCA (0–5%) was lower than adjacent to dead CCA (35%), while in the exposed habitat, survival of small-sized recruits adjacent to exposed CCA (20–25%) was higher than adjacent to dead CCA (5%). None of the CCA species affected the survival of large-sized recruits within exposed or subcryptic habitats. However, the growth of large-sized recruits adjacent to subcryptic CCA was lower than adjacent to dead CCA. Recruits adjacent to exposed CCA died less from competition with turf algae relative to dead CCA, while recruits adjacent to subcryptic CCA frequently died from overgrowth by CCA. These results suggest that, in subcryptic habitats, CCA can reduce the survival and/or growth of coral recruits via direct competitive overgrowth, while in exposed habitats, they can enhance coral recruit survival by alleviating competition with turf algae. Importantly, our study demonstrates that not all CCA species are beneficial to the survival and growth of coral recruits and that there is considerable variability in both the outcome and process of competition between CCA and corals.

     

Spatially variable synergistic effects of disturbance and additional nutrients on kelp recruitment and recovery

Understanding the impact of multiple stressors on ecosystems is of pronounced importance, particularly when one or more of those stressors is anthropogenic. Here we investigated the role of physical disturbance and increased nutrients on reefs dominated by the canopy-forming kelp Ecklonia radiata. We combined experimental kelp canopy removals and additional nutrient at three different locations in a large embayment in temperate southeastern Australia. Over the following winter recruitment season, Ecklonia recruitment was unaffected by increased nutrients alone, but tripled at all sites where the canopy had been removed. At one site, the combination of disturbance and increased nutrients resulted in more than four times the recruitment of the introduced kelp Undaria pinnatifida. Six months after disturbance, the proliferation of the Undaria canopy in the canopy-removal and nutrient-addition treatment negatively influenced the recovery of the native kelp Ecklonia. Given the otherwise competitive dominance of adult Ecklonia, this provides a mechanism whereby Undaria could maintain open space for the following recruitment season. This interplay between disturbance, nutrients and the response of native and invasive species makes a compelling case for how a combination of factors can influence species dynamics.     

Impact of hard-bottom substrata on the small-scale distribution of fish and decapods in shallow subtidal temperate waters

The micro-scale spatial distribution patterns of a demersal fish and decapod crustacean assemblage were assessed in a hard-bottom kelp environment in the southern North Sea. Using quadrats along line transects, we assessed the in situ fish and crustacean abundance in relation to substratum types (rock, cobbles and large pebbles) and the density of algae. Six fish and four crustacean species were abundant, with Ctenolabrus rupestris clearly dominating the fish community and Galathea squamifera dominating the crustacean community. Differences in the substratum types had an even stronger effect on the micro-scale distribution than the density of the dominating algae species. Kelp had a negative effect on the fish abundances, with significantly lower average densities in kelp beds compared with adjacent open areas. Averaged over all of the substrata, the most attractive substratum for the fish was large pebbles. In contrast, crustaceans did not show a specific substratum affinity. The results clearly indicate that, similar to other complex systems, significant micro-scale species–habitat associations occur in northern hard-bottom environments. However, because of the frequently harsh environmental conditions, these habitats are mainly sampled from ships with sampling gear, and the resulting data cannot be used to resolve small-scale species–habitat associations. A detailed substratum classification and community assessment, often only possible using SCUBA diving, is therefore important to reach a better understanding of the functional relationships between species and their environment in northern temperate waters, knowledge that is very important with respect to the increasing environmental pressure caused by global climate change.     

Establishment of the introduced kelp Undaria pinnatifida in Tasmania depends on disturbance to native algal assemblages

Despite recent rapid increases in the occurrence of nonindigenous marine organisms in the marine environment, few studies have critically examined the invasion process for a marine species. Here we use manipulative experiments to examine processes of invasion for the Asian kelp Undaria pinnatifida (Harvey) Suringar at two sites on the east coast of Tasmania. Disturbance to reduce cover of the native algal canopy was found to be critical in the establishment of U. pinnatifida, while the presence of a stable native algal canopy inhibited invasion. In the first sporophyte growth season following disturbance of the canopy, U. pinnatifida recruited in high densities (up to 19 plants m⁻²) while remaining rare or absent in un-manipulated plots. The timing of disturbance was also important. U. pinnatifida recruited in higher densities in plots where the native canopy was removed immediately prior to the sporophyte growth season (winter 2000), compared with plots where the canopy was removed 6 months earlier during the period of spore release (spring 1999). Removal of the native canopy also resulted in a significant increase in cover of sediment on the substratum. In the second year following canopy removal, U. pinnatifida abundance declined significantly, associated with a substantial recovery of native canopy-forming species. A feature of the recovery of the native algal canopy was a significant shift in species composition. Species dominant prior to canopy removal showed little if any signs of recovery. The recovery was instead dominated by canopy-forming species that were either rare or absent in the study areas prior to manipulation of the canopy.     

Effects of early recruits on temperate sessile marine community composition depend on other species recruiting at the time

In many environments recruitment of dispersive propagules (e.g. seeds, spores and larvae) can vary from situations when particular taxa recruit in relative isolation to times when they recruit simultaneously with other, functionally quite different taxa. Differences in the identity and density of recruiting taxa can have important consequences on community structure, but it is still not clear how the effects of individual taxa on communities are modified when they recruit together with other species. Using an experimental approach we compared early development of a temperate marine sessile community after the recruitment of mixtures of botryllid ascidians and barnacles to that when barnacles or botryllid ascidians recruited alone. Communities exposed to recruitment of botryllid ascidians in isolation differed from those that received barnacles, a mixture of botryllids and barnacles or no recruitment in 2-week-old communities. These early differences were driven by higher abundances of the species that were present as initial recruits in experimental treatments. After 2 months communities also differed between barnacle and mixed recruitment treatments but not mixed and botryllid or botryllid and barnacle treatments. These differences were not directly due to differences in the abundances of our manipulated taxa but occurred because of two abundant arborescent bryozoans, Bugula dentata, which occupied more space in communities that initially received mixed recruitment than in those that received barnacle or no recruitment, and Zoobotryon verticillatum, which occupied more space in communities that initially received only barnacle recruitment than those that initially received botryllid or mixed recruitment. These effects did not persist, and communities did not differ after 6 months. These results suggest that, more generally, species may influence community dynamics differently when they recruit alongside other species than when they recruit in relative isolation.     

Biological interactions maintain the boundaries between kelp forests and urchin barrens in the Aleutian Archipelago

This study used benthic surveys and manipulative experiments to examine (1) if boundaries between kelp forests and urchin barrens exist at multiple locations spanning the Aleutian Archipelago, (2) if these boundaries are spatially stable, and (3) how changes in algal density within the kelp forests influence the ability of urchins to invade them. Our results demonstrate that sharply punctuated kelp forest-urchin barren boundaries occur throughout the Archipelago, and they are spatially stable for at least 2 years. Further, when all macroalgae were experimentally removed from the kelp forest side of the boundaries, urchins rapidly moved into these clearings and excluded macroalgae for up to 2 years. However, these movements were not observed where 75% or less of the macroalgae was removed (leaving 25% or more in place), suggesting that even low macroalgal abundances can prevent urchins from invading the kelp forests. Further, urchin densities were negatively related to kelp density, again indicating that kelp can reduce urchin densities. While the ability of urchins to overgraze kelp forests is widely known, our results indicate that kelp can inhibit urchins, that these inhibitory influences are a widely recurrent phenomenon, and that this interaction is important to maintaining kelp forests across the Aleutians.     

Experimental effects of kelp canopies on subtidal coralline algae

Plants are often grouped as canopy species or understorey species because it is thought that that these sets of taxa interact in predictable ways. Mensurative experiments in southern Australia demonstrated that the percentage cover of encrusting coralline algae was greater, and articulated (branching) coralline algae less, on boulders under a canopy of dense kelp (>7 plants per m²), Ecklonia radiata, than on boulders without kelp. Experimental clearances of kelp and reciprocal transplants of boulders between patches of E. radiata and patches without kelp showed that canopies maintained and facilitated the growth of encrusting coralline algae and reduced the cover of articulated coralline algae. Potential artefacts associated with clearing kelp and transplanting boulders were not detected when tested with a series of translocation controls. These results reject the model that the co‐occurrence of E. radiata and encrusting corallines is just an assemblage of plants caused by spatial and temporal coincidence. Instead, they support the model that kelp facilitates the growth and survival of understorey algae.     

Latitudinal variation in intertidal algal community structure: the influence of grazing and vegetative propagation

Summary The hypothesis that sea urchin grazing and interactions with turf-forming red algae prevent large brown algae from forming an extensive canopy in the low intertidal zone of southern California was tested with field experiments at two study sites. Experimental removal of sea urchins resulted in rapid algal recruitment. Crustose coralline algae which typically dominate the substratum in areas with dense urchin populations were quickly overgrown by several species of short-lived green, brown and red algae. The removal of urchins also significantly increased the recruitment of two long-lived species of large brown algae (Egregia laevigata and Cystoseira osmundacea at one study site and E. laevigata and Halidrys dioica at the other). The experimental plots at both sites were eventually dominated by perennial red algae.A two-factorial experiment demonstrated that sea urchin grazing and preemption of space by red algae in areas where urchins are less abundant are responsible for the rarity of large brown algae in the low intertidal of southern California. The three dominant perennial red algae, Gigartina canaliculata, Laurencia pacifica and Gastroclonium coulteri, recruit seasonally from settled spores but can rapidly fill open space with vigorous vegetative growth throughout the year. These species encroach laterally into space created by the deaths of large brown algae or by other disturbances. Once extensive turfs of these red algae are established further invasion is inhibited. This interaction of algae which proliferate vegetatively with algae which recruit only from settled spores is analogous to those which occur between solitary and colonial marine invertebrates and between solitary and cloning terrestrial plants.It is suggested that a north-south gradient in the abundance of vegetatively propagating species, in grazing intensity and in the frequency of space-clearing disturbances, may account for latitudinal variation in intertidal algal community structure along the Pacific coast of North America.     

Recruitment of the intertidal kelp Lessonia nigrescens Bory in northern Chile: successional constraints and opportunities

The strong 1982/83 El Niño event caused local extinction in populations of the low intertidal kelp Lessonia nigrescens Bory on northern Chilean coasts. The kelp has partially recolonized, but its recovery has apparently been low. This study documents the effect of biological factors that potentially decrease the velocity of kelp recolonization in northern Chile. A removal experiment showed that encrusting coralline algae, which dominate the lower intertidal, significantly reduce the recruitment of L. nigrescens in the presence of herbivores. Epithallium shedding by encrusting corallines is the most probable cause for this inhibition process. On the other hand, grazing on encrusting corallines by the chiton Enoplochiton niger (Barnes), a large-sized herbivore (10–20 cm length), could also affect kelp recruitment. The intestinal content of E. niger, where encrusting corallines are the main item (84.2%), revealed the presence of L. nigrescens. Among the natural substrata on which kelp recruits, measured at five different localities, the coarsely-branched alga Corallina officinalis (Decaisne) Kützing reached the highest frequency, despite its extremely low cover (< 1%) in the field. This suggests that settlement on turfs of C. officinalis allows the kelp to escape from herbivory, thus facilitating its recruitment. The roles of timing of kelp recruitment and seasonal grazing are also discussed.     

Spatial variation in juvenile and adult Ecklonia radiata (Laminariales) sporophytes

Juvenile and adult sporophytes of Ecklonia radiata were counted on in-shore and off-shore reefs at Marmion (Perth, Western Australia), across spatial scales of kilometres, 10s of metres and metres. The position on in-shore reefs vs. off-shore reefs did not influence recruit and adult kelp density. There was considerable site-to-site (kilometres) variation in densities of recruits but not adults. The majority of variation in both recruit and adult densities was found between quadrats separated by a few metres. There was no relationship between abundance of recruits and abundance of adult sporophytes at any of the spatial levels of investigation, suggesting a decoupling of recruitment and canopy processes. The results emphasise the patchy nature of kelp canopy distribution and suggest that relatively small-scale processes, acting on scales of a few metres, are of primary importance in generating heterogeneity in the canopy of these kelps.     

Effects of algal canopy clearance on plant, fish and macroinvertebrate communities on eastern Tasmanian reefs

Changes in assemblages of plants, macroinvertebrates and fishes on three eastern Tasmanian reefs were monitored over 12 months in replicated control blocks and adjacent 10×12-m blocks cleared of fucoid, laminarian and dictyotalean algae. Removal of canopy-forming plants produced less change to biotic assemblages than reported in studies elsewhere, with the magnitude of change for fish and invertebrate taxa lower than variation between sites and comparable to variation between months.The introduced annual kelp Undaria pinnatifida exhibited the only pronounced response to canopy removal amongst algal taxa, with a fivefold increase in cleared blocks compared to control blocks. Marine reserves are suggested to assist reef communities resist invasion by U. pinnatifida, through an indirect mechanism involving increased predation pressure on sea urchins and reduced formation of urchin barrens that are amenable to U. pinnatifida propagation.Large invertebrates were more associated with turfing algae or the reef substratum than the macroalgal canopy. The herbivorous sea urchin Heliocidaris erythrogramma and abalone Haliotis ruber showed the strongest response to clearing amongst common macroinvertebrate species, with a halving of population numbers. Observed densities of the common monacanthid fish Acanthaluteres vittiger also declined by about 50%. The relatively high level of resistance shown by eastern Tasmanian reef biota to patch disturbance was attributed largely to high diversity and biomass of turfing macroalgae damping effects of canopy clearance.     

Stability of strong species interactions resist the synergistic effects of local and global pollution in kelp forests

Foundation species, such as kelp, exert disproportionately strong community effects and persist, in part, by dominating taxa that inhibit their regeneration. Human activities which benefit their competitors, however, may reduce stability of communities, increasing the probability of phase-shifts. We tested whether a foundation species (kelp) would continue to inhibit a key competitor (turf-forming algae) under moderately increased local (nutrient) and near-future forecasted global pollution (CO(2)). Our results reveal that in the absence of kelp, local and global pollutants combined to cause the greatest cover and mass of turfs, a synergistic response whereby turfs increased more than would be predicted by adding the independent effects of treatments (kelp absence, elevated nutrients, forecasted CO(2)). The positive effects of nutrient and CO(2) enrichment on turfs were, however, inhibited by the presence of kelp, indicating the competitive effect of kelp was stronger than synergistic effects of moderate enrichment of local and global pollutants. Quantification of physicochemical parameters within experimental mesocosms suggests turf inhibition was likely due to an effect of kelp on physical (i.e. shading) rather than chemical conditions. Such results indicate that while forecasted climates may increase the probability of phase-shifts, maintenance of intact populations of foundation species could enable the continued strength of interactions and persistence of communities.     

Dispersion and phagokinesis in the echinoid Evechinus chloroticus (Val.)

Movement in the echinometrid sea urchin Evechinus chloroticus (Val.) was investigated in relation to the availability of a preferred food species, the laminarian alga Ecklonia radiata (C. Ag.). Patterns of microhabitat occupancy, dispersion, and movement were described in two habitats: Coralline Flats, dominated by encrusting coralline algae; and Sediment Flats, occupied by turfing corallines, Ecklonia and the mussel Modiolus areolatus (Gould). In both habitats, sea urchins were positively associated with encrusting coralline algae, despite the fact that encrusting corallines occupied a minor proportion of the substratum in the Sediment Flat habitat. Sea urchins were significantly clumped in both habitat types, and more so in sites within the Sediment Flats habitat. Movement was not directional in either habitat, but the magnitude of movement was almost twice as high in sites from the Coralline Flats habitat, where kelp was absent, as in the Sediment Flats sites, where kelp was present. Behaviour in response to the presence of Ecklonia in both habitats was investigated experimentally. In both habitats, urchins exposed to a “drift” kelp plant at a distance of 1–2 m exhibited net movement approximately twice as great as that shown by control urchins, not exposed to a kelp plant. Despite the relatively small distances the urchins were from the kelp, there was no evidence of directionality in movement. We conclude from the present study that further understanding of the feeding behaviour of urchins requires that a distinction be made between attached and drift algae, and between directionality and magnitude of movement, and that the interaction of these and other factors are best investigated under field conditions.