Epibiotic Assemblages Research Articles

Effect of a generalist mesopredator on modular and unitary sessile prey associated with a foundation species.

Unitary and modular sessile organisms both dominate in marine benthic communities, commonly preyed upon by the same generalist predators. The differences between unitary and modular defensive strategies may underlie the ways generalist predators control community structure, but this has never been empirically examined. We hypothesize that the individual size of an omnivorous mesopredatory shrimp affects the relative vulnerability of unitary and modular prey and hence translates into community structure. In a short-term laboratory microcosm experiment, we assessed the effect of the shrimp individual size on an epibiotic assemblage of red algae blades initially dominated by three species of modular bryozoans and a unitary serpulid tubeworm. We found that the individual size of a shrimp determines its effect on the prey community composition. Large shrimp stronger than small shrimp increased the proportion of unitary tubeworms among the epibionts surviving predation. While large shrimp reduced the proportions of all the three dominant bryozoan species, small shrimp, in contrast, mostly increased the proportion of a bryozoan species with the smallest modules and largest colonies. This bryozoan, like the tubeworms, demonstrated a higher survival rate with larger individual (colony) size. Yet, against large shrimp this bryozoan was outperformed by the largest tubeworms almost immune to predation. Partial predation by small shrimp modestly improved survival of the largest bryozoan colonies. Thus, relative vulnerability of unitary and modular prey is determined by the predator individual size. Our findings clarify the complex way the size structures of generalist consumers and their prey shape communities by affecting the species-specific relative performance of modular and unitary organisms. The demography of a foundation species and the competitive hierarchy can have additional effects by altering the balance of predation and competition.

Impact of coastal power plant cooling system on planktonic diversity of a polluted creek system

A tropical coastal power plant with a once-through cooling system that pumped sea water along with tiny marine phytoplankton and zooplankton for waste heat discharge recorded reduction in the population density of these organisms by 64% and 93%, respectively, at the discharge site. The depletion of organic carbon is 0.69 tons per annum with loss of 20 to 24 lakhs fish fecundity. The synergistic effect of tropical summer ambiance and waste heat discharge from the power plant considerably reduced the phytoplankton population in the coolant water discharge point during April, June, and July. This resulted in changes in the phytoplankton community structure from Bacillariophyceae > Dyanophyceae > Cyanophyceae to Bacillariophyceae > Cyanophyceae > Dyanophyceae in the Ennore creek system. A unique epibiotic assemblage of the diatoms Licmophora juergensii and Licmophora flabellata was observed on Phormidium sp., a mat-forming Cyanobacterium preharbored along the 4.5-km-long transport channel of the cooling tower blow out of the thermal power plant. These pedunculate fouling diatoms have a symbiotic association with Phormidium sp., which grows few microns high above the substrate, thus creating obstructive flow in cooling water channels of the power plant. Further, loss of fish larvae during zooplankton population reduction creates an impact on the local fishery. However, the emerging scenario of global warming predicts that the migration of fish population toward cooler regions shall further aggravate the fishery reduction near the power plant cooling operation along the tropical coasts. The marine organisms living in tropical coastal waters operated at upper limits of thermal tolerance produce a demand for the regulatory bodies in India to enforce a drop in discharge criteria for coolant water, with the pre-existing power stations permitted to discharge up to 10 °C above the ambient temperature and newer power stations permitted to discharge a maximum of 7 °C. It becomes a requisite for power stations to draw additional seawater along with the plankton. Therefore, an emerging technology of subsurface intake systems called beachwell that resolves the issue of coolant water intake without biota was advocated.

Spatial variability of epibiotic assemblages on marina pontoons in Singapore

Berthing pontoons, one of the most ubiquitous structures in marinas, are known to provide recruitment substrate for a variety of marine biota but little has been reported on their capacity to support epibiotic organisms in tropical marinas, and even less is known about the factors that shape their distribution in such environments. We surveyed the epibiotic assemblages on the sides of pontoons in three Singapore marinas and examined the environmental conditions that influenced their distribution. A total of 94 taxa were recorded, with each marina hosting 43–65 taxa. Assemblages among marinas were highly distinct, and, key discriminants included components of biotic (alcyonarians, hexacorallians, bivalves, and annelids), as well as abiotic (sediment, bare area and shell fragments) origin. While the assemblage variation among marinas was influenced by local environmental conditions (e.g. water motion and sedimentation rate) and pontoon material, the variation in distribution within each marina was best explained by the distance of the pontoons from the marina’s entrance (epibiotic diversity and taxa richness were lower away from the marina entrance). Knowledge on the distribution of epibiotic assemblages on pontoons is essential to identify the factors that contribute to spatial variation and encourage the design and construction of ecologically-friendly marinas. Our findings suggest that improvements to pontoon design and layout would help to augment marina biodiversity, enhance the ecology of urbanised coasts, and mitigate development impacts.

Macrobenthic assemblages in an artificial reef in central-west Portugal: Preliminary results

Macrobenthic assemblages in an artificial reef in central-west Portugal: Preliminary results

The structure of biogenic habitat and epibiotic assemblages associated with the global invasive kelp Undaria pinnatifida in comparison to native macroalgae

Kelp forests dominate temperate and polar rocky coastlines and represent critical marine habitats because they support elevated rates of primary and secondary production and high biodiversity. A major threat to the stability of these ecosystems is the proliferation of non-native species, such as the Japanese kelp Undariapinnatifida (‘Wakame’), which has recently colonised natural habitats in the UK. We quantified the abundance and biomass of U. pinnatifida on a natural rocky reef habitat over 10 months to make comparisons with three native canopy-forming brown algae (Laminaria ochroleuca, Saccharina latissima, and Saccorhiza polyschides). We also examined the biogenic habitat structure provided by, and epibiotic assemblages associated with, U. pinnatifida in comparison to native macroalgae. Surveys conducted within the Plymouth Sound Special Area of Conservation indicated that U. pinnatifida is now a dominant and conspicuous member of kelp-dominated communities on natural substrata. Crucially, U. pinnatifida supported a structurally dissimilar and less diverse epibiotic assemblage than the native perennial kelp species. However, U. pinnatifida-associated assemblages were similar to those associated with Saccorhiza polyschides, which has a similar life history and growth strategy. Our results suggest that a shift towards U. pinnatifida dominated reefs could result in impoverished epibiotic assemblages and lower local biodiversity, although this could be offset, to some extent, by the climate-driven proliferation of L. ochroleuca at the poleward range edge, which provides complex biogenic habitat and harbours relatively high biodiversity. Clearly, greater understanding of the long-term dynamics and competitive interactions between these habitat-forming species is needed to accurately predict future biodiversity patterns.

Long-Term Field Study Reveals Subtle Effects of the Invasive Alga Sargassum muticum upon the Epibiota of Zostera marina.

Invasive species can alter coastal ecosystems both directly, e.g. through competition for substratum and nutrients, and indirectly. Indirect effects may be mediated by creation of dissimilar or inimical habitats, changes in predator and/or prey assemblages, alterations in associated biota, and perturbations of water movement and thermal regimes. Previous studies have shown that invasive algae can modify native habitat architecture, disrupt intricately linked food webs and alter epibiotic assemblages. In the UK, the seagrass Zostera marina supports a diverse epibiotic assemblage, influencing key factors such as sediment dynamics, depositional regime and trophic linkages. Increasing encroachment of the invasive alga Sargassum muticum into seagrass meadows changes the physical and chemical characteristics of the local environment and creates the potential for changes in the epibionts associated with the seagrass blades, threatening the integrity of the seagrass ecosystem. We investigated the effects of S. muticum invasion upon the epibiota of Z. marina in a drowned river valley in SW England seasonally from spring to autumn over four years in an in-situ manipulative experiment, comparing permanent quadrats with and without artificially introduced S. muticum. Epibiota were weighed, identified to the most detailed operational taxonomic unit (OTU) possible, and unitary organisms were enumerated. Multivariate PERMANOVA+ analysis revealed significant differences in epibiont assemblages between Sargassum treatments. Linear mixed effects models indicated that differences in epibiota assemblage composition were not reflected as significant differences in mean biomass per sample, or number of epibiont OTUs per sample. We conclude that S. muticum invasion into Z. marina meadows may significantly alter the species composition and abundance distribution of epibiotic assemblages found on the blades of the seagrass. Thus S. muticum invasion could have more wide-reaching effects on processes within coastal ecosystems than predicted purely by direct effects.

Ecology of fouling assemblages associated with mangrove's roots: An artificial substrate for manipulative experiments

Several of the most influential studies in marine ecology are based on results obtained from experiments using units of artificial habitats. However, there are few studies that have measured the effectiveness of those experimental substrates as surrogates of natural ones, in terms of reflecting natural patterns of colonization and development of the natural assemblages. Such considerations are necessary to extrapolate the described patterns and processes in artificial habitats to natural communities. This situation is particularly noticeable in ecological studies dealing with the identification of process that sustain the high levels of species diversity reported for epibiont communities that grow on the roots of Rizophora mangle in the Caribbean Sea. That is, no study has demonstrated the validity of artificial substrate as surrogate of mangrove roots. Consequently, in this study we tested whether wooden sticks can be used as surrogates for natural prop roots. Furthermore, and in order to propose possible mechanisms explaining the observed patterns, we also tested whether differences in epibiotic assemblages on natural mangrove prop roots were due primarily to intrinsic (biological) differences among roots. The experiment was conducted in four randomly chosen localities of La Restinga National Park, Venezuela, to test for the generality of the observed outcomes. We used two types of substrates: broomsticks as artificial mangrove roots (AMR) and natural mangrove roots (NMR) denuded from epibionts. We recorded the presence and estimated the abundance of sessile species growing on the experimental treatments after 30, 130 and 200days. No significant differences in species richness between the two substrates at any locality and at any time were detected. However, the effect of the substrate on the composition and abundance of species varied among localities and increased over time. Multivariate dispersion was not different between substrates, indicating that intrinsic biological characteristics of prop roots are not responsible for the observed differences among neighboring roots. Despite spatial differences at the scale of localities, most of the species recorded in this study colonized both types of substrates, although some species were more abundant over AMR (e.g. cirripeds, bryozoans), while others were more abundant on NMR (e.g. oyster, algae). Typical mangrove sponges and tunicates colonized similarly both types of substrates. Results obtained in this work suggest that broomsticks are appropriate surrogates of mangrove roots to experimentally evaluate processes associated with species diversity, but should not be used to evaluate processes that affect the abundance and structure of epibiont assemblages.

Mechanisms influencing the spread of a native marine alga.

Like invasive macrophytes, some native macrophytes are spreading rapidly with consequences for community structure. There is evidence that the native alga Caulerpa filiformis is spreading along intertidal rocky shores in New South Wales, Australia, seemingly at the expense of native Sargassum spp. We experimentally investigated the role physical disturbance plays in the spread of C. filiformis and its possible consequences for Sargassum spp. Cleared patches within beds of C. filiformis (Caulerpa habitat) or Sargassum spp. (Sargassum habitat) at multiple sites showed that C. filiformis had significantly higher recruitment (via propagules) into its own habitat. The recruitment of Sargassum spp. to Caulerpa habitat was rare, possibly due in part to sediment accretion within Caulerpa habitat. Diversity of newly recruited epibiotic assemblages within Caulerpa habitat was significantly less than in Sargassum habitat. In addition, more C. filiformis than Sargassum spp. recruited to Sargassum habitat at some sites. On common boundaries between these two macroalgae, the vegetative growth of adjacent C. filiformis into cleared patches was significantly higher than for adjacent Sargassum spp. In both experiments, results were largely independent of the size of disturbance (clearing). Lastly, we used PAM fluorometry to show that the photosynthetic condition of Sargassum spp. fronds adjacent to C. filiformis was generally suppressed relative to those distant from C. filiformis. Thus, physical disturbance, combined with invasive traits (e.g. high levels of recruitment and vegetative growth) most likely facilitate the spread of C. filiformis, with the ramifications being lower epibiotic diversity and possibly reduced photosynthetic condition of co-occurring native macrophytes.

Sessile biota fouling farmed mussels: diversity, spatio-temporal patterns, and implications for the basibiont

The structure of sessile epibiotic assemblages fouling the shells of farmed populations of the common Mediterranean mussel Mytilus galloprovincialis, a species known to be involved in ecosystem engineering processes, was investigated in the north Aegean Sea. Mussel samples were collected from three aquaculture installations and all sessile organisms were examined. Fifteen species were recorded, one macroalga and 14 macro-invertebrates. Four colonial species covered up to 20% of the mussel shell. Among solitary organisms, polychaetes dominated followed by barnacles. The diversity of sessile epibionts associated with farmed mussel populations was comparable to that observed on natural mussel beds in contrast to their abundance/cover, which was higher in the former, possibly due to the higher trophic status in the farming areas. The structure of epibiotic assemblages exhibited limited variability at the spatial scale, in general. On the contrary, strong temporal variability with decreased diversity and abundance/cover values during the cold period of the year was assessed, in accordance with the life cycles of species involved and prevailing environmental conditions. Mussel epibionts appeared to have a negative impact on their basibiont since a reduced condition index was detected; however this impact was rather weak and further data are required to generalize the consequences of epibiosis on farmed mussels.

Epibionts Associated with the Nesting Marine Turtles Lepidochelys olivacea and Chelonia mydas in Jalisco, Mexico: A Review and Field Guide

ABSTRACT The diversity and frequency of epibiota collected over three years (2001, 2002, 2008) from sea turtles (Lepidochelys olivacea and Chelonia mydas) nesting on Teopa Beach in Jalisco State, Mexico, are described. This diversity is compared to epibiotic assemblages procured from these same turtle species nesting on other Mexican beaches, and the role these turtles play in the conservation and dispersal of these epibiota is discussed. Given the increased awareness of epibionts and the desire of many researchers to make positive identifications, specific diagnoses, photographs and a collecting protocol will serve as a basic aid to the collection and accurate identification of epibionts found on turtles living along the Pacific coast of Mexico.

Fouling assemblages associated with estuarine artificial reefs in new South Wales, Australia

Previous studies examining the dynamics of succession on artificial reefs have predominantly focussed on fish communities and largely ignored the role of fouling assemblages in explaining the patterns of community structure associated with artificial reefs. The objective of this study was to record the development of epibiotic assemblages on three "design specific" (Reef Ball®) estuarine artificial reefs systems located in Lake Macquarie, Botany Bay and St Georges Basin in New South Wales, Australia. Recruitment to the artificial reefs was relatively rapid with the majority of taxa identified over the two-year study period observed within the first year post-deployment. The artificial reefs in Lake Macquarie and St Georges Basin were characterised by low diversity with four and nine taxa recorded respectively in contrast to the sixteen taxa observed on the Botany Bay reefs. Results indicated no significant differences in percentage cover of taxa among reefs in either St Georges Basin or Lake Macquarie. In contrast, comparisons between individual Botany Bay reefs identified significant differences in the percentage cover of species between artificial reefs. Analysis of assemblage structure with reef age indicated discrete patterns among estuaries with an overall reduction in the percentage cover of filamentous turfing algae (FTA) identified for all reef systems with an increase in reef age. Variations in environmental and physical conditions (turbidity, water flow, wave action and proximity to naturally occurring reef) may have contributed to the observed differences in fouling assemblages between estuaries and between artificial reefs within Botany Bay.

Fouling assemblages associated with estuarine artificial reefs in new South Wales, Australia

Previous studies examining the dynamics of succession on artificial reefs have predominantly focussed on fish communities and largely ignored the role of fouling assemblages in explaining the patterns of community structure associated with artificial reefs. The objective of this study was to record the development of epibiotic assemblages on three "design specific" (Reef Ball®) estuarine artificial reefs systems located in Lake Macquarie, Botany Bay and St Georges Basin in New South Wales, Australia. Recruitment to the artificial reefs was relatively rapid with the majority of taxa identified over the two-year study period observed within the first year post-deployment. The artificial reefs in Lake Macquarie and St Georges Basin were characterised by low diversity with four and nine taxa recorded respectively in contrast to the sixteen taxa observed on the Botany Bay reefs. Results indicated no significant differences in percentage cover of taxa among reefs in either St Georges Basin or Lake Macquarie. In contrast, comparisons between individual Botany Bay reefs identified significant differences in the percentage cover of species between artificial reefs. Analysis of assemblage structure with reef age indicated discrete patterns among estuaries with an overall reduction in the percentage cover of filamentous turfing algae (FTA) identified for all reef systems with an increase in reef age. Variations in environmental and physical conditions (turbidity, water flow, wave action and proximity to naturally occurring reef) may have contributed to the observed differences in fouling assemblages between estuaries and between artificial reefs within Botany Bay.

Patterns of higher taxon colonisation and development in sessile marine benthic assemblages at Casey Station, Antarctica, and their use in environmental monitoring

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 365:77-89 (2008) - DOI: https://doi.org/10.3354/meps07559 Patterns of higher taxon colonisation and development in sessile marine benthic assemblages at Casey Station, Antarctica, and their use in environmental monitoring Jonathan S. Stark* Australian Antarctic Division, Channel Highway, Kingston, Tasmania 7050, Australia *Email: jonny.stark@aad.gov.au ABSTRACT: Colonisation and development of sessile epibiotic assemblages on tiles was studied at Casey Station, East Antarctica, using a mix of higher taxon classifications (family to phylum). Tiles were deployed for 1 and 3 yr at 3 control and 2 impacted locations. Assemblages on upper and lower surfaces of tiles were very different, with little colonisation of upper surfaces (0 to 11% after 3 yr) and extensive colonisation of lower surfaces (60 to 91% after 3 yr), which is greater than previously reported from Antarctica. Hypotheses were tested relating to spatial variation, depth, human impacts (a sewage outfall and a waste disposal site) and period of deployment. Differences between control locations were only apparent after 3 yr, but there were significant differences between control and impacted locations after 1 yr. There were differences between assemblages at 7 to 10 m and 19 to 22 m. Assemblages were initially dominated by spirorbid polychaetes and bryozoans, but by 3 yr there was significant sponge cover at some locations. Both impacted locations had significantly greater cover on upper surfaces than controls. The waste disposal site had the least cover on lower surfaces, with almost no sponge and less bryozoans than controls. The outfall had the greatest cover on the lower surfaces, the greatest cover of spirorbids and sponges but the least cover of bryozoans. Higher taxa assemblage patterns of colonisation on settlement panels are potentially useful as a medium- to long-term monitoring tool for sheltered Antarctic nearshore waters. KEY WORDS: Marine benthos · Colonisation · Settlement panels · Environmental impact · Assemblage succession · Monitoring Full text in pdf format PreviousNextCite this article as: Stark JS (2008) Patterns of higher taxon colonisation and development in sessile marine benthic assemblages at Casey Station, Antarctica, and their use in environmental monitoring. Mar Ecol Prog Ser 365:77-89. https://doi.org/10.3354/meps07559 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 365. Online publication date: August 18, 2008 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2008 Inter-Research.

Evaluation of techniques used in the assessment of subtidal epibiotic assemblage structure

A comparative study was carried out to evaluate the efficiency of a number of techniques commonly used for assessing the structure of subtidal epifaunal communities. Assessments were made of the epifaunal assemblages fouling two substrata: concrete and PVC plastic. Where possible, each technique was undertaken in three ways, namely, in situ underwater, in the laboratory and using image analysis on photographs taken in situ. Comparisons were also made of biomass estimates made on samples taken in situ and in the laboratory. All method and technique combinations assessed detected differences in the epibiotic communities associated with the two fouling substrata. Sampling in situ, in the laboratory and using image analysis gave similar estimates of percent cover. However, there were significant differences in measurements made for most taxa with respect to abundance and frequency counts depending on how the technique was carried out. Laboratory-based sampling of abundance and frequency counts and biomass determinations, rather than in situ or image-analysis based sampling, are recommended for use in future studies of epifaunal fouling.

Effects of point-source nutrient addition and mussel removal on epibiotic assemblages in Perumytilus purpuratus beds

The objective of this study was to evaluate the effects of point-source nutrient addition and mussel removal on species diversity of the epibenthic assemblage of the purple mussel Perumytilus purpuratus. It was hypothesised that mussels cushion the effects of disturbance on the associated organisms and thereby favour species diversity in patches with added nutrients. In order to test this hypothesis, a two-factorial field experiment was conducted at an exposed rocky shore of northern-central Chile in which nutrient addition and mussel removal were manipulated with two levels. Experimental units consisted of 100-cm 2 mussel plots that were sampled after two months of experimental manipulations. Local nutrient addition was done with coated slow-release fertilisers. Mussel removal treatments consisted in three destructive events applied throughout the experimental phase, and each event consisted in the removal of 20% of the mussel biomass from each plot. While point-source nutrient addition had only minor effects on diversity, mussel removal had significant and negative effects on faunal species richness and the abundance of suspension-feeders and sessile organisms. The abundance of predators/scavengers significantly decreased with mussel removal in the presence of nutrient addition. Grazers and mobile organisms were very abundant in the mussel assemblage but their abundance remained unaffected by treatments. Finally, species richness and total abundance of algae were also not affected by the treatments. Although the duration of our experiment was relatively short, we suggest that the effects of local nutrient addition are of minor importance at the study site in comparison to the effects of mussel removal. Based on our results and previous studies conducted in soft-bottom systems, we suggest that physical and biological stress acting on exposed hard-bottom communities overshadows the potential effects of local nutrient addition.

Low-crested coastal defence structures as artificial habitats for marine life: Using ecological criteria in design

Coastal defence structures to protect sedimentary coastlines from erosion and flooding are increasingly common throughout Europe. They will become more widespread over the next 10–30 years in response to rising and stormier seas and accelerating economic development of the coastal zone. Building coastal defences results in the loss and fragmentation of sedimentary habitats and their replacement by artificial rocky habitats that become colonised by algae and marine animals. The engineering design and construction of these structures have received considerable attention. However, the ecological consequences of coastal defences have been less extensively investigated. Furthermore, due to their rapid proliferation, there is a growing need to understand the role of these man-made habitats in the coastal ecosystems in order to implement impact minimisation and/or mitigation measures. As part of the DELOS project, targeted studies were carried out throughout Europe to assess the ecological similarity of low-crested coastal defence structures (LCS) to natural rocky shores and to investigate the influence of LCS design features on the colonising marine epibiota. LCSs can be considered as a relatively poor surrogate of natural rocky shores. Epibiotic communities were qualitatively similar to those on natural rocky shores as both habitats are regulated by the same physical and biological factors. However, there were quantitative differences in the diversity and abundance of epibiota on artificial structures. Typically, epibiotic assemblages were less diverse than rocky shore communities. Also, LCSs offered less structurally complex habitats for colonisation and in some locations experienced higher disturbance than natural shores. We propose several criteria that can be integrated into the design and construction of LCSs to minimise ecological impacts and allow targeted management of diversity and natural living resources.

A survey of the epibiota of Eretmochelys imbricata (Testudines: Cheloniidae) of Mona Island, Puerto Rico.

Epibiotic organisms inhabiting non-nesting hawksbill sea turtles, Eretmochelys imbricata (Linnaeus, 1766), are described from Mona and Monito Islands, Puerto Rico. Epibiont samples from 105 turtles of shallow (< 40 m) water foraging habitats were collected and identified to the lowest possible taxon. This epibiotic assemblage consisting of at least 4 algal functional groups and 12 animal phyla represents the greatest phylogenetic diversity for marine turtle epibiota. Six groups are considered new reports for marine turtles. Most epibiont colonization was found on posterior marginal scutes and under overlapping scutes. Ecological attributes of epibiota and their symbiosis with E. imbricata provide a tool to understand basi and epibiont populations.

Sedimentation and light penetration interact to maintain heterogeneity of subtidal habitats: algal versus invertebrate dominated assemblages

One of the most striking and widespread patterns observed on subtidal rocky reefs is that up-facing surfaces are monopolized by algae, whereas down-facing surfaces are dominated by sessile invertebrates. This study experimentally assessed the model that light and sedimentation interact with surface orientation to maintain this pattern of habitat heterogeneity. We tested the hypothesis that if epibiotic assemblages on down-facing surfaces were rotated to face upward, then the least change in assemblage structure would occur on shaded surfaces with reduced rates of sed- imentation. In general, the alternate states of algal vs invertebrate dominated assemblages appeared to be primarily maintained by light intensity, which facilitated the cover of algae on up-facing sur- faces (full light) and invertebrates on down-facing surfaces (reduced light). Although sedimentation was only partially responsible for differences between habitat types, it acted as a negative distur- bance on the abundance of algae and survivorship of invertebrates. When combined with differences in light intensity, high rates of sediment accumulation had slight negative effects under natural light, but under shaded conditions these negative effects were substantially increased, causing changes to the structure of the whole assemblage. This result warns that attempts to identify the effects of sedi- mentation in isolation from light intensity, which depends on factors such as turbidity, may not reveal the true effects of sedimentation on epibiotic assemblages. The ability of invertebrates to withstand high rates of sediment accumulation was related to their morphology, whereby erect forms growing above accumulated sediments had greater rates of survivorship than prostrate growth forms, which tended to be smothered by sediments. To properly understand the physical processes of facilita- tion (e.g. light intensity) and disturbance (e.g. sedimentation) we need to assess them in meaningful combinations so that explanations of assemblage structure do not create the false impression that such processes, however complex, produce only small effects relative to other processes.

Why do floating structures create novel habitats for subtidal epibiota?

Urban structures are a conspicuous, yet poorly understood component of the marine environment along urban coastlines. Previous work has shown that different types of structures support different diversities and relative abundances of sessile marine organisms. Studies on the effects of substratum composition, age, orientation and the effects of predation have failed to explain the observed differences in assemblages that develop on different types of structures. We assessed the model that differences in epibiotic assemblages between pontoons and pilings were due to the floating nature of pontoons versus the fixed (relative to the seafloor) nature of pilings, as opposed to other structural differences (colour, shape, surface type, etc.). Two additional (non- mutually exclusive) models were also tested. These were that the presence of a 'swash zone' con- stantly exposed to wave action and/or attachment to the benthos could cause differences between pontoons and pilings. We hypothesized that purpose-built experimental structures that floated would develop different assemblages from structures that were held fixed relative to the seafloor, regardless of whether they were pontoons or pilings. If swash were important, then structures float- ing just below the surface would differ from structures floating at the surface. If attachment to the benthos were important, then pilings attached to the benthos would differ from all the other struc- tures. Multivariate analyses supported the hypothesis that both floating and the presence of swash were important in creating a typical pontoon assemblage, while other factors (type of structure, attachment to the benthos) were not. Several taxa contributed to these differences, including the mussel Mytilus edulis, the polychaete tubeworm Hydroides sp. and several algal taxa. Differences between fixed and floating structures have implications for the interpretation of previous studies done on floating docks. More studies of this kind are needed in order to inform the managers of urban waterways about the implications of adding different types of structures to the coastal environment.

Urban structures as marine habitats: an experimental comparison of the composition and abundance of subtidal epibiota among pilings, pontoons and rocky reefs

There remains little understanding of the relationship between the ecologies of urban habitats (pilings and pontoons) and natural habitats (rocky reef) for sessile plants and animals (epibiota) living on urbanised coasts. This study describes the structure of subtidal assemblages of epibiota among pilings, pontoons and adjacent rocky reef in Sydney Harbour, Australia. I tested the prediction that the experimental provision of substrata of the same age and composition in all three habitats would produce assemblages that: (1) differed among all three habitats; and (2) differed most on floating pontoons relative to the two fixed habitats (pilings and reef). As predicted, the results suggested that both pilings and pontoons, particularly the latter, create novel habitats for epibiotic assemblages independent of age and composition of substratum. It is not fully understood why these urban structures act as such different habitats from natural rocky reefs. The important point is that they are different and we are yet to understand the implications of this for the ecology of coastal areas subject to urbanisation.