Fish community responses to drying disturbances in an intermittent stream: a landscape perspective

1. Refugia are critical to the persistence of individuals, populations and communities in disturbed environments, yet few studies have considered how the position of refugia within the landscape interacts with the behavioural responses of component species to determine the influence of disturbance events on mobile animals. 2. An 18‐month quantitative electrofishing survey was undertaken on the Selwyn River, a stream that is intermittent in its middle reaches, to determine how the direction and distance to refugia affect the response of fish populations to drying, and how landscape context interacts with flow permanence to produce spatial patterns in communities. 3. Overall, the propensity of fish to take refuge in perennial reaches during drying episodes, and the rate and extent of recolonization from these refugia upon rewetting, depended upon the direction and distance to refugia and the behaviour of component species. 4. In the upper river, Canterbury galaxias (Galaxias vulgaris), upland bullies (Gobiomorphus breviceps) and brown trout (Salmo trutta) migrated upstream to permanent water as the stream dried from the bottom up, but frequent drying and slow recolonization by most species combined to produce a fish community in intermittent reaches that was quantitatively and qualitatively different to that in neighbouring perennial reaches. 5. In the lower river, fish did not appear to migrate downstream to permanent water as the stream dried from the top down, but a lower frequency of drying episodes and faster recolonization by upland bullies and eels (Anguilla spp.) from downstream refugia allowed the fish community in intermittent reaches to converge with that in neighbouring perennial reaches during prolonged wetted periods. 6. Longitudinal patterns of increasing fish density and species richness with flow permanence are interpreted as the product of species‐specific responses to drying events and the spatial position of refugia within the riverscape.     

Hydrological disturbance overrides the effect of substratum roughness on the resistance and resilience of stream benthic algae

1. Habitat heterogeneity in lotic systems is usually associated with the availability of refuges. Heterogeneous habitats (here, rough substrata) should mediate the effect of high‐flow disturbances by protecting benthic algae, thus increasing the resistance and resilience of the system. Additionally, the ability of algae to resist a disturbance and recover after it should be dependent on biological traits that confer resistance and resilience. 2. We designed a field experiment, simulating a high‐flow event with bed movement, to test the effect of substratum roughness on the resistance (assessed as the similarity between samples collected before and immediately after disturbance) and resilience (the similarity between samples collected before and 7 and 15 days after disturbance) of five algal life forms. We evaluated whether algal resistance and resilience were higher on rough than on smooth substrata, and whether the life forms differed in their ability to resist and recover from a disturbance. 3. Rough substrata had higher species richness than smooth substrata at all sampling periods, even immediately after the disturbance. There was no significant effect of substratum roughness on algal resistance and resilience, for both species richness and density of the total assemblage. Neither did roughness affect the resistance and resilience of the total algal assemblage or of the algal life forms separately, when evaluated using multivariate data sets (presence‐absence and quantitative). 4. Algal life forms differed in resistance and resilience; adnate/prostrate and erect/stalked species were more resistant and resilient than the other life forms (filamentous, motile and metaphytic). Additionally, motile species resisted and recovered better than did species that are only loosely associated with the substratum (metaphytic species). 5. Substratum roughness had no pronounced effect on benthic algal resistance and resilience. The results of this and some other studies suggest that the intensity of disturbance determines the importance of habitat heterogeneity and flow refuges for benthic algae in streams.     

Effects of habitat structure on habitat use by Gammarus pulex in artificial streams

1. The influence of coarse substratum and flow, coarse substratum and food, and predation risk and flow on habitat use by Gammarus pulex was studied in three experiments conducted in artificial stream channels. Each stream channel consisted of a riffle and pool habitat.
2. Location of coarse substrata and food was manipulated by placing cobbles (coarse substratum) and leaf packs (food) in different habitats. Predation risk was varied by running experiments in the presence and absence of sculpins ( Cottus gobio ), and flow was varied by pumping water with one or two pumps.
3. In all experiments Gammarus were most abundant in pools but placement of cobbles in riffles increased use of the latter. An even greater percentage of Gammarus used riffles if leaf packs were also placed there. Decreased discharge and the presence of sculpins ( Cottus gobio ) also caused Gammarus to increase use of riffles. These data indicate that Gammarus is able to evaluate differences in habitat quality and respond accordingly.     

Effects of habitat structure on habitat use by Gammarus pulex in artificial streams

1. The influence of coarse substratum and flow, coarse substratum and food, and predation risk and flow on habitat use by Gammarus pulex was studied in three experiments conducted in artificial stream channels. Each stream channel consisted of a riffle and pool habitat.
2. Location of coarse substrata and food was manipulated by placing cobbles (coarse substratum) and leaf packs (food) in different habitats. Predation risk was varied by running experiments in the presence and absence of sculpins ( Cottus gobio ), and flow was varied by pumping water with one or two pumps.
3. In all experiments Gammarus were most abundant in pools but placement of cobbles in riffles increased use of the latter. An even greater percentage of Gammarus used riffles if leaf packs were also placed there. Decreased discharge and the presence of sculpins ( Cottus gobio ) also caused Gammarus to increase use of riffles. These data indicate that Gammarus is able to evaluate differences in habitat quality and respond accordingly.     

Individual and combined effects of fish predation and bed disturbance on stream benthic communities: a streamside channel experiment

1. The composition and spatiotemporal dynamics of biological communities are influenced by biotic processes, such as predation and competition, but also by physical disturbances, such as floods in running waters. However, the interplay of disturbance with predation is still poorly understood, especially in frequently disturbed streams. Further, different predator species can affect prey communities in different ways depending on their feeding mode and efficiency. 2. We investigated the individual and combined effects of flood‐induced bed disturbance and fish predation on the benthos for 4 weeks in 18 streamside channels fed by a flood‐prone New Zealand river. Bed movements caused by floods were simulated by tumbling the substratum in half the channels. Six channels each were stocked with introduced brown trout (Salmo trutta) or native upland bully (Gobiomorphus breviceps) or had fish excluded. We studied algal biomass and both invertebrate density and daytime activity on surface stones on several dates after the disturbance, invertebrate community composition in the substrata of the entire channels on day 28 and leaf decomposition rates over the 28‐day period. 3. Disturbance affected algal biomass and density, richness and activity of surface stone invertebrates, and overall density and richness of channel invertebrates. Presence or absence of fish, by contrast, did not influence overall invertebrate standing stocks when subsurface substrata were included but did affect invertebrate densities on surface stones in 45% of all analysed cases and invertebrate activity on surface stones in all cases. Leaf decomposition rates were not influenced at all by the experimental manipulations. 4. Native upland bullies featured more often than exotic brown trout in causing invertebrate density changes and equally often in causing changes to grazer behaviour. Overall, our results imply that fish predation can have strong effects on the benthic invertebrate community in frequently disturbed streams, especially via behavioural changes.     

Stream community structure in relation to spatial and temporal variation: a habitat templet study of two contrasting New Zealand streams

• 1 The physical characteristics of two contrasting streams, and habitat types within these streams, are described in terms of a two-dimensional physical habitat templet in which disturbance frequency and the availability of spatial refugia are the temporal and spatial axes.
• 2 It is predicted that habitats experiencing a high disturbance frequency and low refuge availability will be characterized by a low invertebrate species diversity, a low biomass of epilithic algae and particulate organic matter and a community made up of mobile, weedy species. Habitats having a low disturbance frequency and high refuge availability will be characterized by a diverse community containing sedentary and specialist species, with high algal and particulate organic matter levels.
• 3 A lower median substrate particle size and higher shear stress regime in Timber Creek were indicative of a higher disturbance frequency than in the Kyeburn. Substrate diversity was lower in Timber Creek than in the Kyeburn and indicated that the availability of refugia was lower in Timber Creek. In both streams, pools were found to have a higher disturbance frequency and lower availability of refugia than riffles.
• 4 Invertebrate species diversity, the biomass of epilithic algae and particulate organic matter and the representation of sedentary species, filter feeders and shredders were higher in the more temporally stable and spatially heterogeneous Kyeburn. The community of Timber Creek, frequently disturbed and having low refuge availability, had a high proportion of mobile and weedy species, with the highly mobile, generalist-feeding Deleatidium spp. (Ephemeroptera; Leptophlebiidae) being the most dominant organisms.
• 5 The predictions made about stream community structure and species characteristics in relation to disturbance frequency and the availability of spatial refugia are generally supported. Now a larger scale investigation is required to test the generality of the predictions. We conclude that the habitat templet approach offers a sound framework within which to pose questions in stream ecology.

     

Transitional ecological requirements for early juveniles of two sympatric stichaeid fishes,Cebidichthys violaceus andXiphister mucosus

Synopsis The distribution patterns, diets, and substratum (refuge) requirements of early juveniles of two sympatric stichaeid fishesCebidichthys violaceus andXiphister mucosus, were investigated in a rocky intertidal habitat at Diablo Canyon, California. Monthly investigations were conducted at low tide for four consecutive months, to assess ontogenetic differences in distribution, diet, and refuge requirements within and between the two species. Distinct differences in vertical zonation were exhibited by both stichaeids throughout the study. Interspecific zonation patterns were similar to those recorded for adults of both species. Diet analyses showed that early juveniles of both stichaeids were zooplanktivorous, differing markedly from the primarily herbivorous diets of adults. Changes in diet were largely due to the selection of larger prey taxa as both fishes, and their mouth size, grew over the study period. Predation by both fishes on water-column planktors (calanoid copepods, zoea and polychaete larvae) was greatest following initial intertidal settlement and habitat establishment by early juvenile fishes. Greater dependence on substrate-oriented and/or benthic prey (harpacticoid copepods, gammarid amphipods and mysid shrimp) was exhibited by both fishes as they grew in size. Affinities for sand, gravel, and pebbles during monthly field surveys were similar for both species throughout the study. As the fishes grew, their substratum preferences changed in relation to the substrata which provided the best refuge. Results from laboratory experiments indicated that young stichaeids select very specific substrata based on fish age (size) and substratum suitability (i.e. adequate refuge). Similarities in diet and substratum preferences, and changes in those preferences over time, appear to be the result of morphological similarities (body size and shape and mouth gape) for both species at a given age.     

Habitat use of redfin bullies (Gobiomorphus huttoni) in a small upland stream in Manawatu, New Zealand

This study uses Passive Integrated Transponder (PIT) technology to describe the habitat use patterns of the small, benthic redfin bully (Gobiomorphus huttoni). A 100-m reach of small upland stream in Manawatu, New Zealand was mapped to scale and inventoried for microhabitat variables. Flow velocity, depth, surface turbulence, substrate size and interstitial refuge space availability were measured in 0.25-m² quadrats throughout the reach. A total of 67 redfin bullies were PIT tagged within the reach and monitored during day and night surveys in 2008 with a portable PIT monitoring system. Of these, 72 % were detected at least once and 64 % were detected 5 times or more within the study reach. Univariate analyses showed that redfin bullies utilized microhabitats that had larger substrate particles and more interstitial refuge spaces compared with all microhabitats available in the reach. During the day, redfin bullies were found in areas with larger substrates than areas where they were found at night. No differences in microhabitat use were found regarding gender, body size or season. Multivariate analysis revealed four macrohabitat types and showed that redfin bullies used deeper, complex macrohabitats during the day, then spread out to occupy all available macrohabitats at night. These findings have implications for river managers trying to cope with increasing anthropogenic impacts such as sedimentation.     

Substratum‐dependent responses of ciliate assemblages to temperature: a natural experiment in Icelandic streams

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The influence of moss on grazers in high‐altitude streams: food,refuge or both?

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Regrowth or dispersal? Recovery of a freshwater red alga following disturbance at the patch scale

Abstract Disturbance is an important factor in species coexistence. Disturbance models require knowledge about whether disturbed patches must be colonized anew from dispersal or whether species left behind can dominate, hence altering recovery trajectories of patches. The red, filamentous alga Audouinella hermannii Roth is a common macroalgal species present at sites in the Steavenson River, a stony, upland stream in south‐eastern Australia. We conducted an experiment in which we contrasted the recovery trajectory of the alga on overturned rocks compared with those that were not overturned, and for rocks that had remnants of the alga left behind compared with others where the alga was scrubbed off completely. Rocks had either a rough or smooth texture. Experimental rocks were set out in riffles and algal recovery monitored in 8 × 8 cm quadrats at approximately 4–6 weekly intervals for 8 months. We found that overturning caused a lasting impact on A. hermannii cover, whereas rocks that were abraded by scrubbing recovered very quickly, suggesting that this alga can re‐grow quickly from fragments (a result confirmed by a second experiment). Both surface texture and resident algae affected recovery on abraded substrata. Quadrats surrounded by resident algae on rough‐textured rocks had lower algal cover compared with all other treatments. We hypothesize this effect is caused by higher densities and grazing intensities of herbivorous macroinvertebrates on those sorts of substrata, analogous to findings for marine habitats. Abrasion does not kill A. hermannii, whereas overturning likely does, necessitating new colonization. Floods often create a mix of abrasion and overturning, producing a mosaic of patches, the complexity of which is not represented well by measures of average disturbance intensity over a whole site. The use of the latter may explain some recent contradictory results among stream disturbance studies. A patch‐level perspective is needed when disturbance creates mosaics over the landscape.     

Stream geomorphology regulates the effects on periphyton of ecosystem engineering and nutrient enrichment by Pacific salmon

1. Pacific salmon (Oncorhynchus spp.) returning to streams deliver substantial quantities of nutrients (nitrogen and phosphorus) that may stimulate primary production. Salmon can also affect the phytobenthos negatively via physical disturbance during nest excavation, a process that may counteract the positive effects of salmon‐derived nutrients on benthic algae. The ability of salmon to disturb benthic habitats may be a function of substratum particle size, and therefore, the geomorphology of streams could determine the net effect of salmon on benthic communities. 2. Based on surveys of 17 streams in southwest Alaska before the salmon run and during peak salmon density, we identified size thresholds for the disturbance of substratum particles by salmon and classified particles as vulnerable (<60 mm B‐axis), invulnerable (>110 mm) or transitional (61–110 mm). At the scale of individual rocks, algal biomass on vulnerable substrata decreased at peak spawning (relative to values before the run) as a power function of salmon density; transitional and invulnerable substrata showed no quantifiable pattern. However, invulnerable substrata in streams with more than 0.11 salmon m^?2 showed net algal accrual, or relatively smaller declines in algal biomass, than vulnerable substrata, indicating that large rocks provide refuge for benthic algae from salmon disturbance. 3. We expected that streams with proportionally larger rocks would respond positively to salmon at the whole‐stream scale, after accounting for the relative abundance of rocks of different sizes within streams. Invulnerable rocks made up only 0–12% of the total substratum particle size distribution in salmon‐bearing streams, however, and algal accrual on invulnerable substrata did not outweigh the strong disturbance effects on the more spatially extensive vulnerable substrata. The change in whole‐stream benthic algal biomass among streams was negatively related to salmon density. 4. Stable isotopes of nitrogen (δ¹⁵N) were used to track nutrients from salmon into benthic biota. Periphyton δ¹⁵N on rocks of all size classes was higher at peak salmon spawning than before the salmon run, indicating the uptake of salmon‐derived nitrogen. Peak δ¹⁵N values were positively related to salmon abundance and followed a two‐isotope mixing relationship. The per cent of N from salmon in periphyton was also related to salmon density and was best explained by a saturating relationship. Spring δ¹⁵N was unrelated to salmon returns in the previous year, suggesting little annual carryover of salmon nutrients.     

Substratum simplification reduces beta diversity of stream algal communities

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Colonization and substratum preference of an introduced burrowing crustacean in a temperate estuary

Habitat use in marine invertebrates is often influenced by multiple abiotic and biotic factors. Substratum composition is one factor known to have a dramatic effect on habitat selection. The Australasian burrowing isopod (Sphaeroma quoianum, H. Milne Edwards 1840) is a common introduced species in many estuaries on the Pacific coast of North America. S. quoianum burrows into a variety of firm substrata including marsh banks (composed of peat, clay, and/or mud), wood, friable rock, and Styrofoam floats. In some areas, isopods achieve high densities and may accelerate the rate of shoreline erosion and damage marine structures; thus, understanding the substratum preference of this species may be important for conservation and management efforts. Field experiments were conducted in Coos Bay, Oregon to examine substratum preference, burrowing rates, and the life stage of colonizers. In three experimental trials (Fall 2005, Spring 2006, Fall 2006), replicates of four intertidal substrata (marsh banks, decayed wood, sandstone, Styrofoam) were deployed near intertidal populations of S. quoianum. The numbers of burrows created in each substratum were enumerated weekly or daily (depending on trial). After the trials were completed, the total numbers of isopods inhabiting each substratum were counted. In weeks, S. quoianum extensively burrowed the substrata but exhibited a distinct preference for decayed wood. Significantly more isopods were present in wood than the other substrata at the end of the experiments and rates of burrowing were greatest in wood, although significance varied across time in one trial. Nearly 90% of colonizing isopods were under 5 mm in length suggesting that juvenile isopods primarily colonize intertidal substrata. Differences between burrow densities measured in the field and the results from these preference trials may indicate other factors, such as relative availability of substrata, recruitment and dispersal limitations, and possible gregarious behavior also influence local isopod densities.     

Of spates and species: responses by interstitial water mites to simulated spates in a subtropical Australian river

The 'hyporheic refuge hypothesis' predicts that the hyporheic zone, the saturated sediments below and alongside rivers and streams, is a refuge from the scouring effects of spates for many aquatic invertebrates including water mites. We tested this hypothesis in two lateral gravel bars and two riffles in a subtropical Australian river by collecting water mites from the hyporheic zone at two depths (10 and 50 cm) at two 'pre-flood' sampling times before experimentally diverting water through the sites for 14 h to simulate a spate. Taxon richness of mites washigh (46 taxa) and dominated by the Prostigmata, with nearly half the species being new to science. Oribatids were also common at the four sites. Samples were collected twice during each 'spate', and again soon after flow was returned to normal. The experimental spate induced changes in the strength and even direction of subsurface-surface water exchange; however, these changes seldom persisted after the experiment, nor after a subsequent natural spate. The hyporheic refuge hypothesis was not supported by our water mite data. Neither during nor shortly after the experimental spates did we find more epigean (surface-dwelling) water mites in downwelling zones where surface streamwater enters the hyporheic zone, demonstrating that these mites were not using the hyporheic zone as a refuge at these locations. There was also no evidence for a 'wash out' effect, because hyporheic mitedensities did not significantly decline late in the spate. Our data indicate that floods of the low magnitude simulated in this study apparently do not pose a lasting disturbance for hypogean water mites. The fact that the same response was found at four sites indicates that the hyporheic refuge hypothesis may not always be an appropriate explanation for rapid post-flood recolonisation. Possibly, the use of the hyporheic zone as a refuge from floods may be dictated by the strength of the disturbance and substrate composition and stability.     

Seasonal reversals of upland-riparian diversity gradients in the Sonoran Desert

Flood disturbance and water resource availability vary sharply over time and space along arid‐region rivers and can interact in complex fashion to shape diversity patterns. Plant diversity showed spatial patterning along a topogradient from the floodplain of the San Pedro River (Arizona, USA) to the arid upland, but the patterns shifted temporally as the suite of limiting factors changed. During two of three sampling times, spatial diversity patterns were shaped primarily by gradients of water availability, the regional limiting factor. In the summer dry season, microscale diversity (species richness per 1 m²) and mesoscale diversity (cumulative species and functional types in 20, 1‐m² plots) of herbaceous plants decreased along the topogradient from floodplain to upland, reflecting the greater water availability on the low surfaces. During a summer wet season with moderate rains and flooding, diversity increased in all hydrogeomorphic zones (floodplain, terrace, upland), but the spatial pattern along the topogradient persisted. Following a very wet winter, patterns along the topogradient reversed: scour from large floods limited diversity on the floodplain and competitive exclusion limited the diversity on undisturbed river terrace, while abundant rains allowed for high microscale diversity in the upland. Disturbance and resource availability thus interacted to influence plant species diversity in a fashion consistent with the dynamic‐equilibrium model of species diversity. In contrast to the microscale patterns, mesoscale diversity of species and functional types remained high in the floodplain during all sampling times, with 58% more plant species and 90% more functional types sampled in low floodplain than arid upland for the year as a whole. Species with a wide range of moisture and temperature affinities were present in the floodplain, and seasonal turnover of species was high in this zone. The floodplain zone of a perennial to intermittent‐flow river thus had greater plant diversity than arid Sonoran Desert upland, as measured at temporal scales that capture seasonal variance in resource and disturbance pulses and at spatial scales that capture the environmental heterogeneity of floodplains. Although periodically limited by intense flood disturbance, diversity remains high in the floodplain because of the combination of moderate resource levels (groundwater, seasonal flood water) and persistent effects of flood disturbance (high spatial heterogeneity, absence of competitive exclusion), in concert with the same climatic factors that produce seasonally high diversity in the region (temporally variable pulses of rainfall).     

Comparisons of benthic invertebrates between riffles and pools

Benthic species assemblages in upstream and downstream ends of riffles and in pools were investigated seasonally in the first five orders of an alluvial gravel stream with distinct pool and riffle channel form. Riffles comprised < 10% of stream area and were separated by pools with extensive bedrock substrate (ca. 15–85% of total surface area) which was scoured during periodic high flow. Virtually all taxa were more abundant in riffles than in pools, except chironomids which were more equally distributed. Inconsistent results were obtained for upstream-downstream comparisons within riffles. Intermittent headwater reaches (orders 1 & 2) which supported half as many taxa retained this pattern during periods of flow, although riffles at these sites were dry from mid-June to mid-November. Pools which contained more gravel, indicating less disturbance during high flow, had a richer assemblage of benthic species than other pools. Many invertebrates in pools may have been there as a result of drift from their preferred riffle habitats, but the presence of gravel in the pools indicates less intense flow disturbance during floods, provides protection from the mild scouring that does occur during floods, and provides refugia from predators.     

Avoiding the flow: refuges expand the swimming potential of coral reef fishes

While many coral reef fishes utilise substratum refuges, the direct influence of water flow and swimming ability on such refuging patterns is yet to be established. This study examined the swimming ability and refuging behaviour of a labrid (Halichoeres margaritaceus) and a pomacentrid (Pomacentrus chrysurus) that inhabit high flow, wave-swept coral reef flats. Field observations of refuging patterns were combined with experimental evaluations in a flow tank using a replica of a substratum hole frequently used by these species. Under a range of flow speeds commonly found on the reef flat (0–60 cm s⁻¹), flow within the substratum refuge was reduced to speeds of 0–12 cm s⁻¹, representing a 75–100% flow reduction. Swimming ability of each species was then tested at 60 cm s⁻¹ with and without access to this flow refuge. Both species were able to maintain activity within the 60 cm s⁻¹ flow for considerably longer when provided with a refuge, with increases from approximately 39 min to 36 h for H. margaritaceus and 8 min to 88 h for P. chrysurus. Despite H. margaritaceus having the strongest swimming ability without access to a refuge, P. chrysurus was able to maintain swimming activity more than twice as long as H. margaritaceus when provided with a refuge. These increases in activity are probably due to energetic savings, with this type of refuge providing an estimated 95% energy saving over swimming directly into a unidirectional flow of 60 cm s⁻¹. These results highlight the major advantages provided by refuging behaviour and emphasise the importance of habitat refuges in shaping patterns of habitat use in reef fishes.     

Studies on Lake Erie's littoral algae; Host specificity and temporal periodicity of epiphytic diatoms

Substratum specificity and temporal periodicity of the attached diatom flora upon three aquatic vascular plants and an artificial substratum were examined in three Lake Erie marshes. No qualitative or quantitative specificity for substrata was observed. Variability of diatom assemblage structure within replicate samples of a particular substratum type was as great as, or greater than, variability between substrata. Diatom assemblages upon dowel rod displayed a mid to late summer density maximum. Variability of density maxima upon natural substrata was attributed to different growth rates of the host macrophytes. Diatom assemblages within each sampling site possessed a distinct temporal periodicity indicating that factors affecting diatom growth are heterogeneous in distribution throughout Lake Erie's littoral zone.     

Charge transfer during staphylococcal adhesion to TiNOX coatings with different specific resistivity

Adhesion of the bacterial strain Staphylococcus epidermidis 3399 to titanium-oxy-nitride (TiNOX) substrata with different specific resistivities was studied in a parallel plate flow chamber, while simultaneously measuring the electric potential of the substrata. During adhesion, bacteria either donated or accepted electrons to the substrata depending on the specific resistivity of the substratum and bacteria that had donated electrons to the substratum adhered more strongly than bacteria that had accepted electrons from the substratum. These results demonstrate that electron transfer plays a role in bacterial adhesion to conducting surfaces, which has hitherto been neglected.