Pioneer marine biofilms on artificial surfaces including antifouling coatings immersed in two contrasting French Mediterranean coast sites

Marine biofilm communities that developed on artificial substrata were investigated using molecular and microscopic approaches. Polystyrene, Teflon? and four antifouling (AF) paints were immersed for 2 weeks at two contrasting sites near Toulon on the French Mediterranean coast (Toulon military harbour and the natural protected area of Porquerolles Island). Biofilms comprising bacteria and diatoms were detected on all the coatings. The population structure as well as the densities of the microorganisms differed in terms of both sites and coatings. Lower fouling densities were observed at Porquerolles Island compared to Toulon harbour. All bacterial communities (analysed by PCR-DGGE) showed related structure, controlled both by the sites and the type of substrata. Pioneer microalgal communities were dominated by the same two diatom species, viz. Licmophora gracilis and Cylindrotheca closterium, at both sites, irrespective of the substrata involved. However, the density of diatoms followed the same trend at both sites with a significant effect of all the AF coatings compared to Teflon and polystyrene.     

Antifouling Coatings Influence both Abundance and Community Structure of Colonizing Biofilms: a Case Study in the Northwestern Mediterranean Sea

When immersed in seawater, substrates are rapidly colonized by both micro- and macroorganisms. This process is responsible for important economic and ecological prejudices, particularly when related to ship hulls or aquaculture nets. Commercial antifouling coatings are supposed to reduce biofouling, i.e., micro- and macrofoulers. In this study, biofilms that primarily settled on seven different coatings (polyvinyl chloride [PVC], a fouling release coating [FRC], and five self-polishing copolymer coatings [SPC], including four commercial ones) were quantitatively studied, after 1 month of immersion in summer in the Toulon Bay (Northwestern Mediterranean Sea, France), by using flow cytometry (FCM), microscopy, and denaturing gradient gel electrophoresis. FCM was used after a pretreatment to separate cells from the biofilm matrix, in order to determine densities of heterotrophic bacteria, picocyanobacteria, and pico- and nanoeukaryotes on these coatings. Among diatoms, the only microphytobenthic class identified by microscopy, Licmophora, Navicula, and Nitzschia were determined to be the dominant taxa. Overall, biocide-free coatings showed higher densities than all other coatings, except for one biocidal coating, whatever the group of microorganisms. Heterotrophic bacteria always showed the highest densities, and diatoms showed the lowest, but the relative abundances of these groups varied depending on the coating. In particular, the copper-free SPC failed to prevent diatom settlement, whereas the pyrithione-free SPC exhibited high picocyanobacterial density. These results highlight the interest in FCM for antifouling coating assessment as well as specific selection among microbial communities by antifouling coatings.     

Protozoan communities in chalk streams

This study assessed the individual effects of three mayflies (Paraleptophlebia sp., Ephemerella subvaria McDunnough and Epeorus sp.) and one caddisfly (Psilotreta sp.) on periphyton communities associated with clay tiles and leaves. Algal densities were estimated for leaf discs and tiles from experimental chambers (with individual grazers) and control chambers (i.e., no grazers). Scanning electron micrographs (SEM) of leaf discs and tiles also were taken for all mayfly grazing experiments. Densities of algae on leaf discs were two to five times lower than on tiles. Mouthpart morphology influenced how different insects grazed the periphyton community. Paraleptophlebia had typical collector-gatherer mouthparts and had no effect on diatom densities associated with leaves whereas diatom densities on grazed tiles were higher than densities on tiles from control chambers. Epeorus had brusher mouthparts and had little impact on diatom densities regardless of substratum type. The other two grazers had the blade-like mandibles of a scraper. Psilotreta did not reduce the numerical abundance of diatoms on either substratum, but did alter community structure by significantly reducing densities of stalked Gomphonema olivaceum and large species of Navicula and Nitzschia; densities of smaller diatoms (Achnanthes spp) increased. However, E. subvaria reduced densities of most algal species regardless of size on both substrata and also significantly altered community structure. SEMs of substrata grazed by mayflies showed reductions in fungal hyphae on all grazed leaf discs, decreases in filamentous algal forms on grazed tiles, and greatly shortened stalks of G. olivaceum (Paraleptophlebia only). Thus, periphyton communities are different on leaves versus tiles and grazers with different mouthpart morphologies have varying effects on both algal and heterotrophic microbial community structure.     

The effect of surface colour on the formation of marine micro and macrofouling communities

The effect of substratum colour on the formation of micro- and macro fouling communities was investigated. Acrylic tiles, painted either black or white were covered with transparent sheets in order to ensure similar surface properties. All substrata were exposed to biofouling at 1?m depth for 40?d in the Marina Bandar al Rowdha (Muscat, Sea of Oman). Studies were conducted in 2010 over a time course of 5, 10 and 20?d, and in 2012 samples were collected at 7, 14 and 21?d. The densities of bacteria on the black and white substrata were similar with the exception of day 10, when the black substrata had a higher abundance than white ones. Pyrosequencing via 454 of 16S rRNA genes of bacteria from white and black substrata revealed that Alphaproteobacteria and Firmicutes were the dominant groups. SIMPER analysis demonstrated that bacterial phylotypes (uncultured Gammaproteobacteria, Actibacter, Gaetbulicola, Thalassobius and Silicibacter) and the diatoms (Navicula directa, Navicula sp. and Nitzschia sp.) contributed to the dissimilarities between communities developed on white and black substrata. At day 20, the highest amount of chlorophyll a was recorded in biofilms developed on black substrata. SIMPER analysis showed that Folliculina sp., Ulva sp. and Balanus amphitrite were the major macro fouling species that contributed to the dissimilarities between the communities formed on white and black substrata. Higher densities of these species were observed on black tiles. The results emphasise the effect of substratum colour on the formation of micro and macro fouling communities; substratum colour should to be taken into account in future studies.     

Mechanisms of benthic algal recovery following spates: comparison of simulated and natural events

We conducted a manipulative field experiment to examine individual and interactive effects of scour and short-term nutrient enrichment (4 h exposure) on postspate recovery of benthic algae in a desert stream. We then compared recovery from these simulated-spate conditions to algal recovery patterns following a natural spate that increased water-column nutrient levels for 2 weeks. That event differentially scoured communities on artificial substrata in place for a long-term experiment, significantly reducing biomass in 49-day-old communities but causing no significant reduction of biomass in older, 133-day-old communities. Thus, we were able to examine recovery of scoured and non-scoured benthic algal communities under natural post-spate conditions. Both natural and simulated spates reduced actual and relative abundances of diatoms within communities. In the manipulative experiment, scoured communities accrued biomass more rapidly than those not subjected to scour, but short-term enrichment had not effect. Accrual of diatoms and green algae was stimulated by the scour manipulations, while cyanobacteria maintained equal rates of growth in all treatments. Following the natural spate, diatom and green-algal densities increased in scoured communities, but recovery of algal biomass was slow on both scoured and non-scoured substrata, primarily because cyanobacteria, the dominant algal group on all tiles, did not increase under exposure to highly nitrate-enriched waters. Rates of algal cell accrual were inversely correlated with the amount of algal biomass present at the start of a recovery sequence. Algal immigration rates measured immediately after the natural spate and during an interflood period in the same season did not differ, indicating that the algal drift pool was not augmented by disturbance. Benthic algal recovery following spates is strongly influenced by the degree of scour generated by the event, but recovery patterns are also affected by the length of post-spate enrichment and the taxonomic composition of the affected community.     

The effects of fine-scale substratum roughness on diatom community structure in estuarine biofilms

Benthic diatoms are a major component of biofilms that form on surfaces submerged in marine environments. Roughness of the underlying substratum affects the settlement of both diatoms and subsequent macrofouling colonizers. This study reports the effects of roughness on estuarine diatom communities established in situ in the Indian River Lagoon, FL, USA. Natural communities were established on acrylic panels with a range of surface roughnesses. Smoother substrata exhibited higher cell density, species richness, and diversity. Twenty-three of 58 species were found either exclusively or more abundantly on the smooth surfaces compared to one or both roughened treatments. The results suggest a greater ability of benthic diatoms to recruit and colonize smooth surfaces, which is probably explained by a higher degree of contact between the cells and the surface.     

Habitat complexity modifies the impact of piscivores on a coral reef fish population

Patterns in juvenile mortality rates can have a profound affect on the distribution and abundance of adult individuals, and may be the result of a number of interacting factors. Field observations at Lizard Island (Great Barrier Reef, Australia) showed that for a coral reef damselfish, Pomacentrus moluccensis, juvenile mortality (over 1 year) varied between 20 and almost 100% among sites. Correlative data showed that juvenile mortality increased as a function of initial densities (recruitment), predator densities and the availability of preferred coral substrata. A multiple regression showed that these three variables together did not explain significantly more variation in mortality than the single factor showing the strongest relationship. This appeared to be because recruitment, predator densities and preferred coral substrata were all highly correlated, suggesting that one, two or all of these factors may be influencing juvenile mortality rates. One hypothesis was that density-dependent mortality in juveniles was the result of an interaction between predators (which appear to aggregate at high-recruitment sites) and the availability of preferred substrata (predator refuges). We tested this hypothesis by using both laboratory and field experiments to see whether fish predation could significantly alter survivorship of this damselfish, and whether this impact was dependent upon the coral substratum. The laboratory experiment was designed to test the effects of three common predators (Pseudochromis fuscus, Cephalopholis boenak and Thalassoma lunare) and three different coral substrata that varied in their complexity (Pocillopora damicornis, Acropora nasuta and A. nobilis) on the survival of juvenile Pomacentrus moluccensis. There was a significant interaction between predator species and microhabitat in determining survival. Pseudochromis fuscus and C. boenak were both significantly better at capturing juvenile damselfish than T. lunare. Juvenile survivorship was significantly better when they were given the more complex corals, Pocillopora damicornis and A. nasuta, compared with those given the open-structured species A. nobilis. This pattern reflects habitat selection in the field. Predators differed in their strike rates and the proportion of strikes that were successful, but all exhibited greater success at prey capture where A. nobilis was provided as shelter. The interaction between the effect of predator species and microhabitat structure on damselfish survival was tested in the field for a cohort of juvenile Pomacentrus moluccensis. We examined juvenile survival in the presence and absence of two predators that co-occur on natural patch reefs (C. boenak and Pseudochromis fuscus). The experimental patch reefs we used for this purpose were constructed from both high complexity (Pocillopora damicornis) and low complexity (A. nobilis) coral substrata. Both juveniles and predators were translocated to reefs at natural densities. The effects of predation were clearly dependent upon the microhabitat. Reefs of the high-complexity coral with predators supported the same high numbers of Pomacentrus moluccensis as the reefs with no resident predators. However, damselfish abundance was significantly lower on low-complexity reefs with resident predators, relative to the other treatments. Background rates of loss were high, even on preferred coral in the absence of the manipulated predator, suggesting that transient predators may be even more important than the residents. We suggest that adult abundances in this species were strongly influenced by the densities of different predators and the availability of preferred refuges. Received: 3 April 1997 / Accepted: 26 August 1997     

Optimal colonization and growth of marine benthic diatoms on artificial substrata: protocol for a routine use in bioindication

Benthic diatoms growing on hard substrata are used for their bioindication ability in freshwater quality monitoring. Artificial substrata are needed in cases where any natural substrate is present or to achieve similar sampling conditions between sites. Prior to use marine benthic diatoms for monitoring, a standardized protocol for sampling on artificial substrata must be set up. Two major types of information are required: (1) the time needed for a diatom community to be well developed and mature (climax stage); (2) the optimal growth conditions, given that the substrataum nature and texture are important parameters for the initial phase of biofilm development and can influence the future diatom assemblage. Three substrataum types were tested: frosted Plexiglass®, frosted glass, and rough enameled tiles. They were submerged for 8 weeks and sampled weekly. The experiment was conducted at five sites of distinct morphology and water chemistry, along the coastal area of Martinique Island, French West Indies. Development of diatom community was studied through biofilm dry weight, valve density, species richness, and species relative abundances. Globally, substratum type had no significant effect on any parameter. Frosted Plexiglass® was found to be the most interesting substratum because of higher valve densities and practical use. The asymptotic phase of biofilm development was encountered between 5 and 8 weeks depending on site and parameter. A compromise between community development and vandalism or loss through time was fixed to 5 weeks. This period is longer than for stream environments and is valid for tropical oligotrophic marine environments.     

Using diatoms to assess human impacts on streams benefits from multiple-habitat sampling

Benthic diatoms are important indicators of human impacts on streams. Epilithic diatoms are collected most often for bioassessments, but potentially important ecological information from other habitats could be missed. Within our study region, substrata ranged from 100% rock to 100% sandy silt at 61 sites, leading us to question the appropriateness of sampling solely epilithic surfaces in riffles. We compared two protocols for collecting diatoms: (1) epilithic habitat samples (EHS) within riffles and (2) multi-habitat samples (MHS), which proportionately included different habitats (e.g., riffles and pools) and substrata (e.g., sand, silt, and rock). Three streams were not compared because rocks were absent. Nonmetric multidimensional scaling indicated that diatom communities from EHS and MHS responded similarly to alkalinity and agricultural gradients, and Procrustes analysis showed that ordinations were similar (P < 0.01). Percent motile diatoms and metrics indicating high or low P and N from MHS showed stronger relationships with land use variables than those metrics from EHS. Bray-Curtis (BC) similarity between the protocols increased as diversity and richness decreased in MHS, as agricultural impacts increased, and as motile diatoms became more abundant in EHS, which likely indicated greater habitat homogeneity or an overriding effect of water chemistry. The two protocols classified 78% of sites concordantly as minimally impacted or impacted. Multi-habitat sampling is preferred where stream habitats vary greatly because it assessed land use impacts more effectively than EHS, while indicating water chemistry impacts as effectively. Multiple habitats also contained greater species diversity, which is important to documenting species distributions, and were always available for sampling, whereas epilithic habitats could be sparse or absent.     

Surface-immobilised antimicrobial peptoids

Surface modification techniques that create surfaces capable of killing adherent bacteria are promising solutions to infections associated with implantable medical devices. Antimicrobial (AM) peptoid oligomers (ampetoids) that were designed to mimic helical AM peptides were synthesised with a peptoid spacer chain to allow mobility and an adhesive peptide moiety for easy and robust immobilisation onto substrata. TiO₂ substrata were modified with the ampetoids and subsequently backfilled with an antifouling (AF) polypeptoid polymer in order to create polymer surface coatings composed of both AM (active) and AF (passive) peptoid functionalities. Confocal microscopy images showed that the membranes of adherent E. coli cells were damaged after 2-h exposure to the modified substrata, suggesting that ampetoids retain AM properties even when immobilised on substrata.     

Le diatomee bentoniche del porto di Palermo

Abstract

Benthic diatoms in the harbour of Palermo (Sicily).—The first results of a systematic investigation on the benthic diatoms on the fouling settlements, at four selected sites in the harbour of Palermo, are reported. Samples of comparable areas have been examined either on the fouling of vertical walls of the wharfs or on asbestos panels vertically submerged at the same place for at least eighteen months. Seventy-seven species have been found. No marked taxonomic difference is evident between the populations on the stoneworks and those on the panels.     

Interactions among nutrients, algae and invertebrates in a New Zealand mountain stream

SUMMARY. 1. Colonization of nutrient-diffusing substrata by periphyton and invertebrates was investigated at forested and open sites in a small, mountain stream and a spring in the South Island of New Zealand.
2. Substrata had colonization surfaces made from 100μm mesh plankton netting that enabled algal assemblages to be removed intact for scanning electron microscopy. They also allowed small volumes of solvent to be used for the extraction of photosynthetic pigments.
3. At all sites, periphyton assemblages were dominated by species of Achnanthes, Cocconeis and Gomphonema , and except in the forest in winter, periphyton biomass was always greater on enriched (N + P added) than control substrata.
4. Invertebrates colonizing diffusion substrata were principally larval Chironomidae (Orthocladiinac). No larvae were present in winter, but in three spring and summer trials mean larval densities were higher on nutrient-enriched than control substrata at all sites.
5. The inclusion of an insecticide Malathion in diffusion substrata, reduced insect colonization at open and forested sites. After 28 days, no concurrent increases in algal pigment concentration were observed on nutrienl-enriched or control substrata at the forested site. However, pigment concentrations were higher on substrata incorporating Malathion at the open site suggesting that algal standing crop was depressed by the activities of grazers.     

What’s in a site? Variation in lotic macroinvertebrate density and diversity in a spatially replicated experiment

Few studies have tested whether a small‐scale, experimental difference in biotic density and diversity detected at one site has any predictive capacity at other locations, nor whether such experimental differences are significant relative to overall spatial variation. In this study, we tested textural differences between substrata– previously shown to be critical to macroinvertebrate density and diversity at one site on the Steavenson River in southeastern Australia–and replicated the experiment at this site and at two others on the same river, plus at three sites on each of two other rivers (the Acheron and Little Rivers) in the same catchment. Nine rough and nine smooth colonization substrata were set out at each site on 19 December 1995, and at the start of the experiment (26 and 27 February 1996), invertebrates were hand‐picked from substrata while leaving epilithon intact. We re‐collected substrata after 28 days. Substrate texture had a consistent and strong effect upon species richness at all sites, except one, and was also associated with compositional differences at the family level. Textural effects on macroinvertebrate densities were present and varied among sites and rivers somewhat, but only three of nine common species showed significant interactions between textural effects and one of the two spatial scales, and these interactions explained <10% of variance. In most but not all cases, textural effects were the same as that previously observed–higher numbers on rough surfaces compared to smooth. Most variation in species richness and densities was explained by site, which accounted for 30% of variation in species richness, 60% of total numbers of individuals, and 9–75% of variation in abundances in all nine common taxa. Site differences were also associated with large shifts in faunal composition at the family level. Differences between rivers explained 0% of the variance in most cases but it was significant in explaining the abundance of one common species (the beetle Simsonia wilsoni), where it accounted for 59% of the variance. Our results indicate that a small‐scale effect, such as textural differences between substrata, can produce a consistent effect on measures of community structure and have some predictive capacity at other locations. However, the large differences between sites, relative to those seen between whole rivers, suggest that efforts must be directed to discovering what factors cause such localized fluctuations. Sites cannot be reasonably used as ‘representative’ of larger spatial units, such as long sections of entire rivers. Additionally, rivers may not necessarily have signature variation that always overrides localized, site‐based differences.     

Biofilm diatom community structure: Influence of temporal and substratum variability

Abstract

Diatoms, which are early autotrophic colonisers, are an important constituent of the biofouling community in the marine environment. The effects of substratum and temporal variations on the fouling diatom community structure in a monsoon-influenced tropical estuary were studied. Fibreglass and glass coupons were exposed every month for a period of 4 days and the diatom population sampled at 24 h intervals, over a period of 14 months. The planktonic diatom community structure differed from the biofilm community. Pennate diatoms dominated the biofilms whilst centric diatoms were dominant in the water column. Among the biofilm diatoms, species belonging to the genera Navicula, Amphora, Nitzschia, Pleurosigma and Thalassionema were dominant. On certain occasions, the influence of planktonic blooms was also seen on the biofilm community. A comparative study of biofilms formed on the two substrata revealed significant differences in density and diversity. However species composition was almost constant. In addition to substratum variations, the biofilm diatom community structure also showed significant seasonal variations, which were attributed to physico-chemical and biological changes in both the water and substratum. Temporal variations in the tychopelagic diatoms of the water were also observed to exert an influence on the biofilm diatom community. Variations in diatom communities may determine the functional ecosystem of the benthic environment.     

Nutrients, algae and grazers in some British streams of contrasting pH

1. The relationship between algal biomass accumulation, invertebrate colonization, and stream-water pH was investigated in seven streams in three regions of England and Wales. Possible nutrient limitation of algal production at all sites was examined with diffusion substrata. 2. Periphyton assemblages on experimental substrata after 30 days were dominated by diatoms, notably Eunotia spp., at all sites. Algal pigment concentration (chlorophyll a and phaeopigments) was not correlated with stream-water pH, and mean concentrations on control (unenriched) substrata ranged from 0.08 to 1.94 μg cm^?2. 3. The growth response of periphyton to nutrient additions was site specific. Algal production was stimulated by nutrient additions at sites in the English Lake District and Llyn Brianne (south-west Wales), but not in the Ashdown Forest (southern England). 4. Larval Chironomidae were the main invertebrates retrieved from substrata at all sites. Within all three regions, larval abundance was positively related to algal pigment concentration (biomass). Abundance of the stonefly Nemurella pictetii was also positively correlated with algal biomass at the one site where it occurred. 5. Our results indicate that epilithic algal production in small, oligotrophic streams is unlikely to be determined primarily by pH. Neither do they support the view that an absence of grazers from acid streams is necessarily due to an inadequate food supply.     

Distribution patterns of benthic diatoms in a Pampean river exposed to seasonal floods: the Cuarto River (Argentina)

The diatom community growing on cobbles and sand substrata along the Cuarto River (Córdoba, Argentina) was studied during 2000 and 2001. Multivariate analyses of the data (PCA and CCA) showed distinct differences in water chemistry and substrata types between the upstream sites (sites 1–10) and downstream sites (sites 11–19). Sites 1–10 supported an epilithic diatom community associated with low water conductivity and gravel substrata. This consisted of adnate Achnanthes (A. biasolettiana, A. minutissima), as well as stalked (Gomphonema aff. angustum, Reimeria uniseriata, Fragilaria capucina var. rumpens) or prostrate (Nitzschia lacuum) taxa. Downstream sites were associated with high conductivity, fast flowing waters and finer substrata (sand, silt), and were colonised by prostrate diatoms, including several species of Navicula (N. pupula, N. mutica, N. veneta, N. insociabilis) and Nitzschia (N. umbonata, N. palea). Variations in water flow caused significant changes in the diatom communities of the river. During periods of low flow (winter and autumn), chain forming (Diatoma vulgaris, D. moniliformis) or stalked (Synedra ulna) taxa partially replaced the former community of Navicula and Nitzschia in the downstream sites. High flow (in summer) led to diatoms of large size being replaced by smaller size diatoms, such as Achnanthes lanceolata, Navicula mutica, Hantzschia amphioxys and Amphora montana. The severe effect of floods in the lower part of the Cuarto led to these taxa having a much higher proportion in the lower stretch of the river, taking advantage of the subaerial conditions created by the floods. Local episodes of water pollution were associated with a transient shift towards the dominance of Navicula pupula, Synedra ulna, Nitzschia lacuum and Reimeria uniseriata during winter, when inputs were least diluted because of the low flow.     

Comparison of attached algal communities on natural and artificial substrata along a thermal gradient

Colonization rate and community structure of periphyton assemblages was examined on aluminium and glass substrata and compared to populations on four submerged macrophyte species in three temperature zones in Cholla Lake, Arizona, U.S.A. Higher densities were achieved over shorter incubation intervals in the warmer zones (26–35° C). Representatives from the planktonic diatom community were first to colonize artificial substrata during the initial two hour incubation period in all temperature zones. Two periphyton diatom representatives, Amphora coffeiformis and Cocconeis placentula var. lineata were the numerical dominants after one week. Cocconeis placentula var. lineata was most competitive on natural substrata at temperatures <26°C, while Amphora coffeiformis dominated temperature zones >26°C with no significant preferences for artificial or natural substrata. The significance of temperature, specific conductance and availability of living hosts is discussed with respect to regulating populations of these two common periphytic diatom species in alkaline waters in southwestern U.S.A. Similarity indices (SIMI) were used to compare algal assemblages on various natural and artificial substrata pairs. Periphyton assemblages were very similar on all natural substrata within similar temperature zones, with little or no preference for macrophyte species displaying similar leaf morphology. Diatom assemblages were quite similar on aluminium and glass substrata throughout the incubation period in all temperature zones, while blue-green algal populations were significantly different, particularly in the higher temperature zones (>28°C). Natural periphyton communities were best represented after four weeks incubation with aluminium substrata in warmer temperature zones (>28°C) or where filamentous blue-green algae dominated. The selection of adequate incubation time when employing artificial substrata to evaluate natural assemblages for different environmental conditions and algal populations is discussed.     

The influence of resident adults on larval settlement: experiments with four species of ascidians

Residents within any community can affect the larval settlement of both their own and other species. In marine sessile communities resident adults can affect larval settlement by preying on settling larvae, removing or adding space for the larvae to colonize, or stimulating or prohibiting larval settlement on available substratum nearby. To examine those processes by which residents affect settlement, we exposed experimental substrata with three densities of adults of a single species at a site in eastern Long Island Sound, USA for a 24-h period. Four species of common ascidians, Botryllus schlosseri (Pallas), Botrylloides diegensis Ritter and Forsyth, Diplosoma macdonaldi Herdman, and Molgula manhattensis (De Kay), were used in 11 separate experiments. Few individuals of any species settling attached to the surfaces of these species and this resulted in the main effect of these residents being the usurpation of space and the restricting of settlement to unoccupied areas. A model is also presented to explain the apparent aggregated settlement of several species in open areas adjacent to the resident ascidians. From this model we suggest that the aggregated settlement can result from limited larval mobility such that some larvae that contact and reject the resident species as settlement sites may subsequently contact open surfaces of the same substratum and increase settlement densities there over those observed on control substrata. Finally, settlement data for several species indicate that Molgula may influence settlement by preying on larvae.     

Development of the initial diatom microfouling layer on antifouling and fouling-release surfaces in temperate and tropical Australia

Diatoms are a major component of the slime layers that form on artificial surfaces in marine environments. In this article, the role played by diatoms during the pioneering stages of colonization of three marine antifouling (AF) coatings, viz Intersmooth 360®, Super Yacht 800® and a fouling-release (FR) coating Intersleek 700®, was investigated. The study was conducted over three distinct seasons in two very different marine environments in Australia, ie temperate Williamstown, Victoria and tropical Cairns, Queensland. Diatom fouling occurred more rapidly on the FR coating Intersleek 700, compared to both biocidal AF paints. However, colonization by diatoms on all three coatings was generally slow during the 16-day study. Benthic diatoms do not subsist by floating around in the water column, rather they only gain the opportunity to colonize new surfaces when they either voluntarily release or are displaced from their benthic habitat, thereafter entering the water column where the opportunity to adhere to a new surface presents itself. However, once settled, fouling diatoms grow exponentially from the site of attachment, spreading out until they populate large areas of the surface. This mode of surface colonization correlates more with an ‘infection’ type, epidemiology model, a mechanism that accounts for the colonization of significant regions of the coating surface from a single fouling diatom cell, forming ‘clonal patches’. This is in comparison to the bacterial colonization of the surface, which exhibits far more rapid recruitment and growth of cells on the substratum surface. Therefore, it is hypothesized that fouling diatoms may be characterized more by their ability to adhere and grow on surfaces already modified by bacterial biofilms, rather than on their strength of adhesion. Cell morphology and the ability to avoid shear may also be an important factor.     

Effects of clay discharges on streams

The impact of clay discharges on benthic invertebrates was investigated by comparison of communities upstream and downstream of alluvial gold mining on 6 streams on the West Coast of the South Island, New Zealand. Mean turbidity was increased by 7–154 NTU above background (mean 1.3–8.2 NTU) by the mine discharges during the 2 months before sampling. Patterns of increase in suspended solids (strongly correlated with turbidity, r=0.95) were similar. Invertebrates densities were significantly lower at all downstream sites, ranging from 9 to 45% (median 26%) of densities at matched upstream sites. Downstream densities as a proportion of those upstream were negatively correlated with the logarithm of the turbidity loading (r=–0.82, P<0.05). The densities of the common taxa were also generally lower downstream of mining. Taxonomic richness was significantly lower at downstream sites in the four streams receiving higher turbidity loads (mean turbidity increase = 23–154 NTU). Lower epilithon biomass and productivity, and degraded food quality at the downstream sites probably explain the lowered invertebrate densities. At some sites, reduced bed permeability and interstitial dissolved oxygen, and avoidance reactions of invertebrates (i.e., increased drift), may have also contributed to lower invertebrate densities.Formerly DSIR Marine and Freshwater.