Habitat structure,resources and diversity: the separate effects of surface roughness and macroalgae on stream invertebrates

Habitat structure has pervasive effects on community composition and diversity, with physically complex habitats often containing more species than physically simple ones. What factors or mechanism drive this pattern is little understood, but a complicating problem is that different sources of habitat structure can be confounded in both surveys and experiments. In this study, we carried out an experiment in which two sources of habitat structure, attached macroalgae and substrate surface texture, were separately manipulated to discern their joint and separate effects upon the diversity and composition of colonizing macroinvertebrates in a stony, upland stream. Because stream algae vary markedly in abundance in both space and time, we also sampled the epilithon of stream stones at two spatial scales on eight dates over 2 years to gain some preliminary data on how stream algae vary between individual substrata over time. Experimental substrata had either a smooth (siltstones, sandstones, crystal-poor felsic volcanics, plain paving bricks) or rough (granodiorites, crystal-rich felsic volcanics, sand-blasted paving bricks) surface. We allowed these substrata to be colonized naturally by macroalgae, mostly the filamentous red alga Audouinella hermannii. Half of each of the rough and smooth substrata were selected at random and the macroalgae gently sheared off. All substrata were defaunated with a household insecticide with little field persistence, set out randomly through the study riffle, and invertebrates allowed to colonize them for 14 days. Some substrata were sampled immediately to check the efficacy of faunal and algal removals, which proved to be successful. Experimental results showed that both surface texture and macroalgae increase species richness independently of each other. Surface texture had no effect on densities, while macroalgae increased colonization densities, but rarefaction showed that both sources of habitat structure increased species richness above values expected simply on the basis of the numbers of colonists. However, reference stones with high macroalgal cover had the same species richness as those with low cover, suggesting that the effects of macroalgae on species richness are transient relative to those associated with surface texture. Epilithon samples taken at different times suggest that the magnitude of spatial variation in plant growth alters with time. If plants generally recolonize rough surfaces more quickly than smooth, then the effects of habitat structure on macroinvertebrates ought to be strongest after major disturbances during growing seasons of plants. Received: 1 September 1999 / Accepted: 10 January 2000     

Microdistribution of benthic invertebrates in a rocky mountain (U.S.A.) stream

A study of the benthic invertebrate community inhabiting a small, foothill trout stream in the Rocky Mountains of Idaho was conducted over a two-year period. Monthly Hess samples and short-term experiments using substratum-filled trays were used to describe the spatial dispersion of the benthos and to examine the response of invertebrate populations to substratum and current. A method was devised for measuring available surface area which involved coating individual stones with latex and measuring the area of the print resulting from inking the impression left on the latex mold.The dispersion of all populations was clumped throughout the year. Alteration of the cross-sectional pattern of current velocity and stream bed composition changed the pattern of distribution but not the extent of clumping. Collections made in areas of depositing and eroding substrata revealed a more diverse fauna in the latter. Most groups of organisms found in the riffle were scarcer in the pools or absent from them. The pool fauna contained no important additions over those found in the riffles.After a year's study of invertebrate populations in an otherwise undisturbed riffle, the substratum was altered and the flow made more uniform; an increase in the abundance of most of the benthic invertebrates followed. No single factor was responsible for the increase, but the change in substratum size and degree of compaction accounted for most of the change. Interpretation of the results was aided by findings from experiments using substratum-filled trays.Two series of stream experiments using the trays were conducted: one to test the relative importance of current and substratum and the other to test the effect of particle size on the distribution of the benthic fauna. In the first series, placement of trays of stones in a pool resulted in an increase in numbers of some but not all of the invertebrates over numbers usually occurring in the pool. Trays filled with stones and placed in a riffle supported fewer animals than found on the adjacent stream bed but more than in the pool. Variations are attributed to differences in current velocity and amounts of imported organic and inorganic debris. Three different relationships of population numbers to current velocity were found for different members of the community (direct, indirect, and parabolic) over the range of 10 to 60 cm/sec. The second series of experiments consisted of two sets of trays filled with stones of medium or large pebbles, respectively. Nine taxa, as well as all of the combined taxa, showed a preference for trays of small stones over the natural stream bed. A few taxa were noticeably more abundant on the small substratum than on the large but most of the fauna showed only slight increases in numbers or remained constant on the two substrata. Only three taxa showed a direct relation of numbers to total surface area presented by the stones.Number and kinds of organisms found in trays filled with a uniform size of substratum did not correspond to those taken in Hess samples from the natural stream bed. This has important implications in terms of currently recommended pollution monitoring techniques. However, it is suggested that if the substratum composition of the trays more nearly matched that of the stream, the correspondence would be much better. The results of the present study also throw considerable doubt on the adequacy of generalizations derived from earlier studies of responses to substratum size and suggest several reasons for reevaluating current ideas regarding the influence of substratum on invertebrate distribution.     

Succession in a stream invertebrate community: A transition in species dominance through colonization

A field experiment was conducted using ceramic plates as experimental substrates to describe the colonizing pattern of a stream invertebrate community after disturbance, and to ascertain the importance of colonizing ability for succession in a Japanese stream. We employed the simultaneous removal design in which plates were periodically set in place (1, 2, 4, 8, 16 and 32days before sampling) and all plates were sampled on the last day of the experiment, to diminish the influence of seasonal change in stream invertebrates. Total abundance and taxon richness reached a plateau after 4–16days of colonization. In contrast, the relative abundance of six common taxa and community structure changed throughout the 32days of colonization. Differences in the colonizing ability of the stream invertebrates were evident. Taxa with high mobility, such as the mayflies Baetis thermicus, Paraleptophlebia japonica, Cinygmula sp. and Drunella sachalinensis, colonized faster than those with low mobility (e.g. the caddisfly species Brachycentrus americanus and the chironomid midge Diamesinae spp.). The abundance of the most common taxon, Baetis, decreased at late stages of colonization, possibly because of low periphyton biomass. Consequently, we concluded that a difference in colonization ability among taxa is an important factor causing succession in stream invertebrate communities in local habitats.Shigeru Nakano - deceased 27 March 2000.     

The effect of large macroinvertebrate herbivores on sessile epibenthos in a mountain stream

Studies of benthic invertebrates in lakes and streams suggest thatlarge-bodied herbivores are more efficient grazers than smallerones. In order to assess the effect of larger herbivores on smallergrazing invertebrates, the presence of dominant grazer taxa wasmanipulated in streamside troughs in a first order temperaterainforest stream in British Columbia. The presence of mayflies(Ameletus sp.) and tailed frog tadpoles (Ascaphustruei) reduced both algal biomass and the abundance of herbivorouschironomids (Orthocladiinae) on ceramic tiles. This confirms thatlarge mobile grazers in streams have a negative effect on smallersessile invertebrate grazers either through resource competition ordirect consumption (predation).     

COMMUNITY DYNAMICS AND METABOLISM OF BENTHIC ALGAE COLONIZING WOOD AND ROCK SUBSTRATA IN A FOREST STREAM

Colonization dynamics and metabolism of algal communities on wood (small pieces of Douglas fir) and artificial rock (tiles) substrata were investigated in an open reach in Mack Creek, Cascade Mountain Range, Oregon, for 42 days. Chlorophyll a concentration was not significantly different between the two algal communities (ANOVA, P = 0.119). Even though differences in cell densities were not significantly different between the two algal communities (ANOVA, P = 0.063), biovolume and diversity were greater in epixylic than in epilithic communities (ANOVA, P = 0.011 and 0.002, respectively). The same algal taxa occurred on both substrata, but some of them (e.g. Ceratoneis arcus Grun., Cymbella minuta Hilse ex Rabh., Zygnema sp.) were more numerous on wood, whereas a few others preferred tiles (e.g. Achnanthes lanceolata (Bréb.) Grun.). Scanning electron microscopy revealed a much higher surface roughness on wood, which likely led to a patchy distribution of the algae and the development of stalked diatom species. However, adnate species adapted much better to the mainly flat surfaces of tiles. Net community primary productivity (NCPP) measured in 42-day-old substrata was moderate on tiles but negative on wood. Community respiration (CR) was significantly higher on wood than on tiles. Epixylic algal communities exhibited greater nitrogen demand than epilithic communities after ammonium addition. NCPP shifted to positive values on wood but did not increase on tiles. Despite metabolic differences, the variations between the two communities are more closely related to the physical characteristics of the substrata and the attachment abilities of the algal taxa.     

High assemblage persistence in heterogeneous habitats: an experimental test with stream benthic algae

1. The persistence of biological assemblages is positively affected by spatial heterogeneity. This influence may be indirect, through increased species richness. Another possibility is the increased availability of refuges from disturbances, which would prevent local loss. 2. We conducted a field experiment to test the hypothesis that greater roughness (a form of spatial heterogeneity) on the surface of substrata allows higher persistence of assemblages of stream benthic algae and that this relationship does not depend on species richness. Samples were taken on six occasions from smooth and rough artificial substrata used for algal colonisation. We calculated the persistence of assemblages using two analytical approaches: the mean distance to group centroid and the sum of the Euclidean distances between consecutive sampling occasions, both in a multivariate space. We also subsampled the data to take into account differences in species richness between treatments and thus to evaluate the effect of species richness on persistence. 3. Assemblages on rough substrata were more persistent than assemblages on smooth substrata. The effects detected were not due to the greater species richness on rough substrata, since a higher persistence of the assemblages on rough substrata remained after the subsampling procedures. 4. Our results indicate a strong positive relationship between substratum roughness and the persistence of stream benthic algal assemblages. We suggest that this is due to the presence of physical refuges in heterogeneous habitats.     

How protective are refuges? Quantifying algal protection in rock crevices

1. Refuges can be functionally important if they harbour sufficient organisms during disturbance to augment population recovery. I quantified the protection of stream algae in crevice refuges using the applied, severe disturbance of scrubbing. Scrubbing effectively removed visible surface algae, and algae remaining on stones were considered protected. 2. In a field experiment, substrata with different quantities of crevice (glass bottles; greywacke; schist; pumice) were incubated in a channelised stream. The possible influence of growth conditions was investigated concurrently; half of the substrata were suspended in the water column, the rest were placed on the bottom (providing differential access to grazers and exposing them to different flow conditions). 3. Rougher substrata had greater total algal biomass than smoother substrata; this pattern resulted from more algal biomass in crevices of the rougher substrata. Protection from scrubbing ranged from about 5% of total algal biomass on glass and greywacke to 80% on pumice. In contrast, algal biomass removed by scrubbing was similar among the experimental substrata. Suspended substrata had more chironomid grazers than those on the bottom, and also greater algal biomass, possibly because of high algal concentrations in the chironomid retreats. 4. A survey of stones from three rivers supported the experimental results; namely, rough pumice protected more algae from scrubbing than did smoother greywacke. 5. In a separate experimental assay, there was no difference in algal growth on agar plates with and without added powdered rock substrata, suggesting that crevice characteristics and not substratum chemistry produced the differences in algal assemblages between rough and smooth surfaces. 6. Results indicate that rough stream stones may protect sufficient algae to augment their recovery in streams following disturbance.     

Effects of disturbance: disruption and enhancement of coral reef cryptofaunal populations by hurricanes

Samples of solitary cryptofauna were collected from reef habitats before and after Hurricanes David and Frederic struck St. Croix, U.S.V.I. The intertidal beachrock zone suffered severe and sustained damage from waves, sand scouring and moving debris. Most bioeroded cavities in the substrate and several previously abundant cryptic taxa were completely obliterated from this environment for more than 6 months; numbers remained low or the pre-hurricane body size distribution had not been regained by the end of the 2 year study in a number of taxa. In 3 habitats at 2–3 m depths (fringing reef, patch reef, back of the bank barrier reef), some taxa underwent temporary decreases or dislocations, particularly on the fringing reef. Overall, however, this cryptofauna exhibited strong succession, with burrowers and then nestlers increasing above pre-hurricane densities. Many populations of nestlers were still increasing at the end of the 2 year study. Thus, the hurricanes enhanced densities of benthic invertebrates per piece of rubble (probably by providing fresh unburrowed substrate) in these moderately shallow subtidal habitats. On a deeper fore reef (12 m), we detected very few changes in cryptofaunal populations. The enhancement of invertebrate populations in reef habitats with intermediate levels of hurricane diturbance parallels recent findings that some species of corals have evolved colonizing life history tactics which allow them to prosper in moderately disturbed environments. Because the cryptofauna represent an important route of trophic flow (via predatory fishes) through the reef ecosystem, these relatively long term (2 y) changes in abundance of invertebrates have important implications for the structure and function of coral reef communities.     

The effect of two scales of habitat architecture on benthic grazing in a river

1. This experiment studied the effects of differing levels of the complexity of substratum architecture at two spatial scales on the distribution and abundance of benthic algae and invertebrates, and the strength of the trophic interaction between invertebrate grazers and algae. Some estimates of the effects on invertebrate colonization rates were also made. 2. Four levels of microhabitat architectural complexity were created using artificial substrata (clay tiles) and placed in Mountain River, Tasmania, in two riffle types (bedrock and boulder-cobble) of differing large-scale substratum complexity. After a colonization period, invertebrate grazers were removed from half the tiles to measure the effects of grazing. Invertebrates on the tiles were also counted and identified. At the end of the experiment, algae were removed from the tiles and analysed for chlorophyll a. 3. Invertebrate grazers did not reduce algal biomass during the experiment, and microhabitat-scale architecture influenced algal biomass more strongly than riffle-scale architecture. Highly complex microhabitat architecture increased algal biomass by providing more surface area, but once standardized for surface area, algal biomass decreased as the complexity of microhabitat architecture increased. 4. Microhabitat-scale architecture was also predominant in determining invertebrate density and the identity of the dominant grazer species. In contrast to algal biomass, invertebrate densities and species density increased with the complexity of microhabitat architecture, suggesting that refuges from flow (and possibly predation) were as important to river invertebrates as the distribution of their food source. 5. Riffle-scale architecture had some effect on the colonization of two slow-moving grazer taxa, but, overall, the colonization processes of slow-moving grazers were determined mostly by the complexity of microhabitat-scale architecture.     

The importance of predation,substrate and spatial refugia in determining lotic insect distributions

An experiment was conducted to evaluate the interaction between predation, substrate, and spatial refugia in the organization of a stream insect community (Reeds Creek, Pendleton Co., West Virginia). Patterns of insect colonization were compared between fish exclusion cages and open controls that allowed access to vertebrate predators. Each cage contained 4 different substrates that varied in the relative amount of spatial refugia. Fish had little influence on the diversity or abundance of any insect taxa, even when spatial refugia were limited. The only significant effect due to predation, was an increased diversity of large (>8 mm) invertebrates in the absence of predators. However, because these taxa were relatively rare, the overall role of fish predation on insect community structure was minimal.In contrast, substrate had a marked effect on insect colonization. Insects were always more abundant (number/basket) on loose substrates containing large numbers of interstitial spaces, compared to cement-embedded substrates with few refuges available. In addition, invertebrates were more abundant on loose gravel compared to loose cobbles. Howver, when substrate preferencesrd were examined according to insect density (number/m²), loose cobbles were generally the preferred substrate. The present experiment rejects the hypothesis that patterns of substrate colonization can be explained as differential insect mortality by predators, due to varying amounts of refugia. Alternative mechanisms such as differing amounts of trapped detritus and substrate surface area may account for substrate preference.     

Trunk invertebrate faunas of Western Australian forests and woodlands: Influence of tree species and season

Abstract Trunk–associated invertebrates were sampled on two rough‐barked tree species (jarrah, Eucalyptus marginata and marri, E. calophylla) at Karragullen, in the hills near Perth, Western Australia, and on these two species plus two smooth‐barked species (wandoo, E. wandoo, and powderbark wandoo, E. accedens) at Dryandra, a drier site situated 150 km to the south‐east. Invertebrates were sampled by intercept traps, which collect animals that attempt to land on the trunks, and photo‐eclector bark traps, which collect invertebrates that move, or live, on the trunk. The range and abundance of invertebrates sampled was generally greater in the intercept than the bark traps. Invertebrate abundance and activity (but not biomass) on bark was strongly seasonal, with greater numbers being found during the moister periods. The two smooth‐barked species supported, and were visited by, more invertebrates than the two rough‐barked species. There was some evidence that jarrah supported more invertebrates than marri at both Karragullen and Dryandra, although the results were equivocal. Within the two smooth‐barked species, wandoo tended to support more invertebrates than powderbark wandoo. These trends are discussed in terms of the characteristics of the bark of these trees and the environments in which they occur.     

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.     

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

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An experimental study of the effects of predatory fish on macroinvertebrates in a Hong Kong stream

SUMMARY. 1. Predation upon macroinvertebrates by the loach Oreonectes platycephalus Günther (Cobitidae) was studied using predator inclusion/exclusion cages in a series of pools along a Hong Kong stream. Treatments employed were predator exclusion, medium (approximately natural) predator densities (1 fish cage⁻¹) and high predator densities (2 fish cage⁻¹). Macroinvertebrate abundance in cages was monitored after 2 and 4-weeks exposure to predators.
2. The presence of fish was associated with significant declines in the total numbers of macroinvertebrates colonizing cages. However, taxa were influenced differently, with mayflies decreasing by a factor of two while the more mobile shrimps (Atyidae) were unaffected. Chironomid abundance (largely Chironominae) was unaffected by predator density and increased in week 4. Detritus acted as a confounding variable at this time because chironomid abundance was significantly correlated with the weight of accumulated detritus in cages.
3. While invertebrates were more abundant in cages lacking fish, there were no fewer invertebrates in cages with 2 fish than with 1 fish. This may indicate the presence of secure refuges among substrates in the cages, preventing the additional fish from depleting prey further, or a lack of precision of methods due to natural variations in prey densities and spatial patchiness.
4. No significant effects of predators on relative prey abundance or species richness were detected.
5. The impact of predation on prey abundance weakened on week 4, perhaps due to extra refuges among the accumulated detritus. However, drying of the stream increased fish densities in pools so that cages may have become zones of relative safety that were colonized readily by macroinvertebrates. This result highlights the need for year-round investigations to quantify predation effects in Hong Kong's seasonal tropical climate.     

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.     

Colonization of natural substrata of different roughness and colour by Ephemeroptera nymphs using retrieval and direct observation techniques

We investigated the colonization of stream substrata by Ephemeroptera nymphs in a Canadian Rocky Mountain stream. Stream substrata used in our study were different textures (smooth and rough) and colours (dark and light); the taxonomic groups studied were Drunella coloradensis Dodds (Ephemerellidae), Heptageniidae (Cinygmula and Epeorus) and a Baetis Ameletus group (Baetidae and Siphlonuridae). We tested the hypothesis that as D. coloradensis nymphs and other abundant Ephemeroptera nymphs increase in size they choose a dark substratum to remain cryptic and a substratum of greater texture to reduce the risk of being swept from the substratum by the water flow. Two approaches were used to test this hypothesis, a substratum basket experiment and direct observations.For the substratum basket experiment, there were no statistical differences in abundance of nymphs on the substratum types. But there was a general tendency that the D. coloradensis nymphs were least abundant on the rough dark-coloured substratum and developmental stage three nymphs were most abundant on the smooth white-coloured substratum.Colonization was investigated for other Ephemeroptera nymphs by making direct observations on the substratum baskets using a glass-bottomed box. For the taxonomic groups and two colonization periods, nymphs were statistically at greatest density on the smooth white-coloured substratum and lowest on the smooth black-coloured substratum. The Baetis-Ameletus groups were also found at greatest densities on the rough dark-coloured substratum.     

Disturbance history influences the distribution of stream invertebrates by altering microhabitat parameters: a field experiment

1. We investigated the effects of local disturbance history and several biotic and abiotic habitat parameters on the microdistribution of benthic invertebrates after an experimental disturbance in a flood‐prone German stream. 2. Bed movement patterns during a moderate flood were simulated by scouring and filling stream bed patches (area 0.49 m²) to a depth of 15–20 cm. Invertebrates were investigated using ceramic tiles as standardized substrata. After 1, 8, 22, 29, 36 and 50 days, we sampled one tile from each of 16 replicates of three bed stability treatments (scour, fill and stable controls). For each tile, we also determined water depth, near‐bed current velocity, the grain size of the substratum beneath the tile, epilithic algal biomass and standing stock of particulate organic matter (POM). 3. Shortly after disturbance, total invertebrate density, taxon richness and density of the common taxa Baetis spp. and Chironomidae were highest in stable patches. Several weeks after disturbance, by contrast, Baetis spp. and Hydropsychidae were most common in fill and Leuctra spp. in scour patches. The black fly Simulium spp. was most abundant in fill patches from the first day onwards. Community evenness was highest in scour patches during the entire study. 4. Local disturbance history also influenced algal biomass and POM standing stock at the beginning of the experiment, and water depth, current velocity and substratum grain size throughout the experiment. Scouring mainly exposed finer substrata and caused local depressions in the stream bed characterized by slower near‐bed current velocity. Algal biomass was higher in stable and scour patches and POM was highest in scour patches. In turn, all five common invertebrate taxa were frequently correlated with one or two of these habitat parameters. 5. Our results suggest that several ‘direct’ initial effects of local disturbance history on the invertebrates were subsequently replaced by ‘indirect’ effects of disturbance history (via disturbance‐induced changes in habitat parameters such as current velocity or food).     

A comparison of the trophic ecology of the crayfishes (Orconectes nais (Faxon) and Orconectes neglectus (Faxon)) and the central stoneroller minnow ( Campostoma anomalum (Rafinesque)): omnivory in a tallgrass prairie stream

Omnivorous fish, such as the central stoneroller minnow (Campostoma anomalum(Rafinesque)), and crayfish often play important roles in the trophic dynamics of streams. The trophic role of these two omnivores has not been compared within a system even though they both consume algae, detritus and invertebrates and often co-occur in streams in the Midwestern United States. Natural abundance of ¹⁵N and ¹³C isotopes and a whole stream ¹⁵N-labeled ammonium chloride release were used to compare the trophic ecology of the central stoneroller minnow (Campostoma anomalum (Rafinesque)) and two species of crayfish (Orconectes neglectus (Faxon) and Orconectes nais (Faxon)) in a tallgrass prairie stream. The ¹⁵N and ¹³C values of Orconectes spp. were more similar to coarse benthic organic matter (CBOM) and filamentous green algae than to invertebrates, fine benthic organic matter (FBOM), and periphyton. Values for ¹⁵N and ¹³C in C. anomalum were more similar to grazer and collector invertebrates and filamentous green algae than to FBOM and periphyton. Results from a ¹⁵N tracer release also indicated a portion of algae and/or invertebrates were a component of nitrogen assimilated in Orconectes spp. and C. anomalum diets. Gut contents of C. anomalum were also analyzed. In contrast to stable isotope data, amorphous detritus was a significant component of C. anomalum guts, followed by diatoms and filamentous green algae. A significant percentage of invertebrate material was found in C. anomalum guts sampled in the spring. Experiments were conducted in artificial streams to determine if Orconectes spp. and C. anomalum could reduce epilithic algal biomass in small streams. Algal biomass on clay tile substrata was decreased relative to controls in artificial stream channels containing O. neglectus (3.4 fold, p=0.0002), C. anomalum (2.1 fold, p=0.0012), and both species combined (3.0 fold, p=0.0003). Results indicate that Orconectes spp. are functioning more as algal and detrital processors than as predators in Kings Creek. Isotope and gut content data show that C. anomalum includes invertebrates as well as algae and detritus in its diet. Both species have the potential to affect algal biomass and are important omnivores in the stream food web.     

Can artificial substrates enriched with crustose coralline algae enhance larval settlement and recruitment in the fluted giant clam (<Emphasis Type="Italic">Tridacna squamosa</Emphasis>)?

Habitat recognition and selection can greatly increase the early-life survival of sessile reef organisms. This study describes the settlement and recruitment responses of the fluted giant clam, Tridacna squamosa, to concrete tablets and tiles containing different concentrations of crustose coralline algae covered coral rubble (CCACR). Crustose coralline algae is known to induce settlement in a variety benthic animals, but it has not been used previously as an aggregate in concrete—potentially a way of encouraging colonization of man-made structures erected on or near coral reefs. After being given the choice of small tablets made with 0%, 30% or 60% CCACR for 4 days, 11 days old larvae preferred the substrate containing the most CCACR. Recruitment responses of juvenile clams to larger tiles made with the same three CCACR concentrations were also tested. These tiles were further divided into rough and smooth surface textures. After 6 weeks, more juvenile clams had recruited to the rough surfaced tiles than the smooth ones, but no significant differences among the CCACR treatments were found. Thus, even though concrete made with CCACR is initially attractive to larvae, it has no effect on recruitment of juvenile T. squamosa. Handling editor: I. Nagalkerken     

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.