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.     

Size‐dependent microhabitat use and intraspecific competition in Cottus gobio

An electrofishing survey of daytime shelter microhabitat use of bullhead Cottus gobio in a southern English chalk stream revealed positive selection for moderate water velocity, vegetation cover and coarse substrata. Water depth, other forms of cover, shade and substratum embeddedness had no significant influence on the distribution of fish. Microhabitat use was size‐dependent, with patches occupied by adult fish containing coarser substrata and less blanket weed ( Cladophora algae) than those occupied by smaller juvenile conspecifics. Differences in substratum use between size‐classes were less pronounced in parts of the stream shaded by the tree canopy. In laboratory tanks stocked at low fish density, both juveniles and adults favoured use of cobbles over pebbles. The response of fish to increased conspecific density was size‐dependent; juveniles reduced use of the coarse substratum whereas adults maintained their predominance in this habitat. An apparently greater shift by juveniles when in the presence of adults was significant at α = 0·10 only, as was an apparent reduction in interactions between size‐classes under low light intensity. The displacement of small juvenile fish from the preferred cobble substratum is consistent with the hypothesis that intraspecific competition contributes to the size‐related microhabitat shift observed in the field. Although there was a tendency for the strength of competition to be reduced at low light levels, the mechanism by which tree canopy cover affects microhabitat use remains uncertain.     

Effects of an introduced piscivorous fish on native benthic fishes in a coastal river

1. We used field surveys to compare the density and mesohabitat-scale distribution of the native coastrange sculpin ( Cottus aleuticus ) and the prickly sculpin ( C. asper ) in coastal rivers in north-western California, U.S.A., with and without an introduced piscivorous fish, the Sacramento pikeminnow, Ptychocheilus grandis . We also measured mortality of tethered prickly sculpin in a field experiment including river, habitat type (pools versus riffles) and cover as factors.
2. Average sculpin density ( C. aleuticus and C. asper combined) in two rivers without pikeminnow was 21 times higher than the average density in two rivers in a drainage with introduced pikeminnow. In riffles, differences in the density of sculpins among rivers could be linked to differences in cover. However, riffles in rivers without pikeminnow had an average sculpin density 77 times higher than rivers with pikeminnow, yet only nine times more cover. In pools, cover availability did not differ among rivers, but the density of sculpins in rivers without pikeminnow was 11 times higher than rivers with pikeminnow.
3. In the field experiment, mortality of tethered sculpin varied substantially among treatments and ANOVA indicated a significant River × Habitat × Cover interaction ( P  < 0.001). Overall, tethered prickly sculpin suffered 40% mortality over 24 h in rivers with pikeminnow and 2% mortality in rivers without pikeminnow, suggesting that predation is the mechanism by which the pikeminnow affects sculpins.
4. The apparent reduction in sculpin abundance by introduced pikeminnow has probably significantly altered food webs and nutrient transport processes, and increased the probability of extinction of coastrange and prickly sculpins in the Eel River drainage.     

The influence of chemistry and habitat features on the microcrustacea of some upland Welsh streams

SUMMARY. 1. Microcrustaceans were sampled during September 1990 from three microhabitats (margins, stony riffles and gravel) at thirteen stream sites in mid-Wales.
2. Patterns in the distribution, abundance and community structure of microcrustacean taxa were related to stream chemistry, physiography, substratum composition and marginal habitat structure. Patterns of distribution between microhabitats were also assessed.
3. Chemical variables were the strongest correlates with microcrustacean parameters. Total numbers of microcrustaceans and harpacticoid copepods were reduced at sites with high aluminium concentrations, and ostracods were scarce or absent at the most acidic sites. In contrast, cyclopoid copepods were more abundant and species rich at low pH.
4. Microcrustacean communities were most species rich at sites with a high percentage cover of macrophytes on the substratum. Cladoceran species richness and abundance, and the abundance of several microcrustacean species, were also positively correlated with percentage macrophyte cover.
5. Three site groupings produced by TWINSPAN classification were primarily related to percentage macrophyte cover and marginal habitat structure, with chemistry and land use also of importance.
6. Total microcrustacean abundance was highest in margins, but of all the species, only the harpacticoid Bryocamptus cuspidatus showed a strong microhabitat preference.     

Non-visual feeding behavior of the mottled sculpin,Cottus bairdi,in Lake Michigan

Synopsis Field and laboratory experiments indicate that the mottled sculpin, Cottus bairdi, feed in the dark. Blinded sculpins feed on a variety of motile prey in the laboratory and show stereotyped responses to prey stimuli. The sculpins bite at moving inert objects, even if buried in substratum, indicating that they use their lateral line system to detect prey. Covering portions of the lateral line with an inert paste eliminates response to objects near the covered region of the lateral line. The sculpins can also detect prey (including inert objects) in a stream if the prey is upstream. Collection from two series of presunset, postsunset, presunrise, postsunrise, dives in Lake Michigan indicate nocturnal feeding by the mottled sculpin.     

Coarse Particulate Organic Matter Distribution in the Pools and Riffles of a Second-order Stream

Coarse particulate organic matter distribution was investigated in a 270-m interval of a 2nd-order forest stream, the Yanase River. The dominant tree type was Keyaki (Zelkova serrata). CPOM sampling was conducted, and the sampled CPOM was sorted into leaves and branches, and water depth and current velocity were measured along with stone width, stone height and distance between the stones in the riffles. The collected CPOM was categorized by their accumulation type: LSS, SLP, SLPi, SLPo and DD. LSS was leaf packs at the leading edge of the stones, SLP was the sinking leaf packs in the pools, SLPi was the sinking leaf packs at the inner side of the stream bend in the pools, while SLPo was the sinking leaf packs at the outer side of the stream bend, and finally, DD was the leaf packs in the debris dams. The accumulated CPOM amounts at each leaf pack type were correlated with the measured physical stream variables. From the results, CPOM accumulation in riffles is controlled by stones projecting above the water surface and by their arrangements. In pools, CPOM accumulation occurs at high stream flow where the pools provide low velocities and a thick boundary layer of relatively quiescent flow. At stream meandering points, CPOM accumulation occurs by production of a secondary flow. Finally, CPOM accumulation in debris dams is important where they occur frequently. LSS was the largest in CPOM amount, and the stones in the riffles were the most retentive structures. On the other hand, SLP and SLPo were the least CPOM amounts, and the pools in the mainstream and at the outer side of the stream bend were the least retentive stream morphologies.     

Food,microhabitat, or both? Macroinvertebrate use of leaf accumulations in a montane stream

1. Leaf packs in streams may serve as food and substrate to many macroinvertebrates, but the relative importance of these two functions has not been disentangled. To test the hypothesis that leaf packs are colonized primarily for their food value rather than as microhabitat, the colonization of leaf packs of red alder and of polyester cloth in a natural stream was compared. 2. Species of shredders showed large differences in the colonization of the two types of leaf pack with almost no use of artificial leaf packs. Non-shredders were also more abundant on natural leaf packs, however, they colonized artificial leaf packs six to eighty times proportionally more than shredders. 3. The effect of different leaf types was virtually eliminated for non-shredders when the amount of fine parriculate detritus (food for many non-shredders) trapped in the leaf pack was added to the analysis as a covariate. Therefore, to non-shredders, leaf types differed only in the retention of fine particulate organic matter. 4. Comparisons of the use of both kinds of leaf packs in riffles versus pools revealed that significantly fewer animals colonized pool leaf packs. 5. These results suggest that food value, and not microhabitat, is the primary determinant of leaf pack use for most shredders and non-shredders.     

Submerged undercut banks as macroinvertebrate habitat in a subalpine meadow stream

We compared macroinvertebrate samples from submerged undercut banks with samples from rifles and pools in a low-gradient-meadow reach of a small stream on the Snowy Range, southeastern Wyoming, from July to September 1985. Macroinvertebrates collected from submerged undercut banks were similar to those collected from riffles and pools, but densities varied among the three habitat types. In July Ephemeroptera, Plecoptera, Trichoptera, Coleoptera, and Diptera, as well as Hydracarina, were all substantially more numerous in samples from undercut banks than from riffles or pools. By contrast in September little difference was observed in the abundance of macroinvertebrates in samples from submerged undercut banks, riffles and pools. While forming only 8.5% of the area of aquatic habitat within the study reach submerged undercut banks were estimated to contain 44% of the aquatic insects in July and 30% in August. Further, submerged undercut banks are macroinvertebrate habitat in this subalpine meadow stream, contributing to overall macroinvertebrate abundance and probably an additional food source for trout.     

Life-history variations in the fluvial sculpin, Cottus nozawae (Cottidae), along the course of a small mountain stream

Life-history variations in male and female fluvial sculpins, Cottus nozawae, were studied in a small mountain stream in Hokkaido, Japan, primarily by using capture-mark-recapture methods. At three study areas established along the stream course, the majority of marked sculpins were recaptured in their original location over one or more years, indicating their long-term occupation of each restricted habitat area. Sculpin densities increased toward the upstream habitats, whereas individual growth rates were more rapid downstream. In both sexes, sculpins distributed downstream matured at a larger body size and later in life than upstream sculpins, clearly demonstrating a clinal variation in these respects. A comparison of life-history variations in C. nozawae with those in amphidromous C. hangiongensis suggests that intrapopulational life-history variations in the former might be environmentally induced, and that one of the most important determinants for the variations in Cottus species might be population density.     

Comparative Microhabitat Use of Ecologically Similar Benthic Fishes

Although benthic insectivorous fishes such as darters and sculpins represent a significant component of riffle communities, few studies have compared the habitat use of these non-related but ecologically similar fishes. The objectives of this study were to examine the habitat use of Etheostoma olmstedi (tessellated darter) compared to Cottus bairdi (mottled sculpin) in Nescopeck Creek and Cottus cognatus (slimy sculpin) in Jack's Creek, Pennsylvania through underwater observation. Etheostoma olmstedi occupied habitats with significantly deeper waters than those available, whereas adult and young of the year Cottus occupied habitats with significantly faster water velocities than those available. Canonical discriminant analysis revealed microhabitat partitioning between E. olmstedi and each Cottus species. Cottus bairdi and C. cognatus occupied significantly shallower habitats with faster water velocities than E. olmstedi. Sculpin species were observed most frequently under substrate whereas E. olmstedi occurred most frequently on the top surface of the substrate. Hurlbert's standardized niche breadth values indicated that C. bairdi and C. cognatus were habitat specialists with regard to water velocity measures, but exhibited generalistic patterns of depth and substrate size use. Etheostoma olmstedi was a habitat specialist with respect to depth, but exhibited generalistic patterns of resource use for substrate size. Differential habitat use by these benthic fishes is consistent with the hypothesis that resource partitioning facilitates species coexistence among stream fishes.     

Effects of temperature on persistence times of native and invasive gammarid species in the stomachs of Cottus gobio

Gastric lavage was used to investigate the effects of temperature on persistence time of two amphipod species, one native Gammarus pulex and one invasive Gammarus roeseli , in the stomachs of bullhead Cottus gobio . Persistence time was strongly influenced by temperature and prey type, such that G. pulex species degraded faster than G. roeseli .     

Microhabitat use and preferences of the endangered Cottus gobio in the River Voer, Belgium

Microhabitat use and preferences of juvenile and adult bullhead Cottus gobio , from the River Voer, Flanders, were studied and compared across different seasons. Water depth, water velocity near the substratum, surface water velocity and substratum type used by C. gobio differed between seasons. These differences, however, were not attributable to differences in microhabitat availability. Adults appeared to prefer higher water velocities and coarser substrata than the average ones available in the basin. Although water depth appeared to have little influence on seasonal variation of microhabitat use in adult C. gobio , juveniles preferred deeper water and coarser substrata in winter, whereas in summer they appeared to use shallower water. There was a difference in microhabitat use between juvenile and adult bullhead only in summer.     

Annelid sperm and fertilization biology

This paper provides data on fine particulate organic matter (FPOM) and macroinvertebrates associated to natural and artificial leaf packs in a small woodland stream (Schlaube, Brandenburg). Macroinvertebrate colonisation and the dynamics of FPOM were studied in oven-dried alder leaf packs, air-dried alder leaf packs and packs with artificial leafshaped substrate exposed in the stream during a 68-day period. The importance of FPOM as a potential food source for macroinvertebrates especially in artificial leaf packs was evaluated. Changes in the quantity as well as in the chemical composition of the accumulating FPOM (>63 and <63 μm) was determined using soluble carbohydrates, proteins and chlorophyll a as parameters of the nutritional quality. Mass loss and the chemical changes of alder leaves during the decompositional process were also described. The loss of soluble carbohydrates due to leaching was more rapid in oven-dried alder leaf packs than in air-dried ones. After 3 days of leaf pack exposure weight loss of oven-dried and air-dried leaf packs was nearly comparable, as the similar decay coefficients, k = 0.0228 (oven-dried leaf packs) and k = 0.0214 (air-dried leaf packs), respectively, show. The amount of FPOM per unit leaf area constantly increased in artificial packs, although it remained below that of alder leaf packs at all sampling dates. The nutritional quality of FPOM <63 μm was constantly greater than that of FPOM >63 μm and decreased in both size-fractions with length of exposure. Referring to leaf area the abundance of macroinvertebrates continually increased in all packs till the end of exposure, whereas the numbers in artificial packs remained below that in alder leaf packs. The taxonomic composition of all treatments was very similar with Gammarus pulex being the most abundant taxon in all packs until day 42, while afterwards the caddis fly genus Hydropsyche gained in importance. The amphipod Gammarus pulex in general did not show a preference for air-dried alder leaf packs compared to oven-dried alder leaf and artificial packs. Corresponding dynamics of macroinvertebrate colonisation and FPOM content in artificial packs support the hypothesis that FPOM functions not only as an important food source for macroinvertebrates including gammarideans but also as a control mechanism of macroinvertebrate abundance in stream habitats. Even if the accumulation of FPOM and drifting macroinvertebrates might be influenced by the same abiotic factor (e.g. by reduction in stream velocity inside the packs) it is quite unlikely that only physical properties caused the invertebrates to stay.     

An invasive lineage of sculpins, Cottus sp. (Pisces, Teleostei) in the Rhine with new habitat adaptations has originated from hybridization between old phylogeographic groups

Fish abundance surveys in the Rhine system have shown in the past two decades that there is a rapid upriver invasion of a freshwater sculpin of the genus Cottus. These fish are found in habitats that are atypical for the known species Cottus gobio, which is confined to small cold streams within the Rhine drainage. Phylogeographic analysis based on mitochondrial haplotypes and diagnostic single nucleotide polymorphisms indicates that the invasive sculpins are hybrids between two old lineages from the River Scheldt drainage and the River Rhine drainage, although it is morphologically more similar to the Scheldt sculpins. Most importantly, however, the invasive population possesses a unique ecological potential that does not occur in either of the source populations from the Rhine or the Scheldt, which allows the colonization of new habitats that have previously been free of sculpins. Microsatellite analysis shows that the new lineage is genetically intermediate between the old lineages and that it forms a distinct genetic group across its whole expansion range. We conclude that hybridization between long separated groups has lead to the fast emergence of a new, adaptationally distinct sculpin lineage.     

Common sculpin Cottus gobio as a natural paratenic host of Proteocephalus longicollis (Cestoda: Proteocephalidae), a parasite of salmonids, in Europe.

Common sculpins Cottus gobio L. (Pisces: Cottidae), from the Mlynsky Brook near Ceské Zleby in the Sumava National Park, southwestern Bohemia, Czech Republic, were found to harbour in their intestines juvenile cestodes Proteocephalus longicollis (Zeder, 1800), a common parasite of holarctic salmonids, with a prevalence of 60% and intensity of 1 to 11 (mean 5) parasites per fish; undoubtedly, these prey fish serve as paratenic hosts. In this locality, the definitive host of P. longicollis is the brown trout Salmo trutta m. fario L., large specimens of which apparently acquire infection of this parasite by feeding on infected sculpins. C. gobio is the first known natural paratenic host of P. longicollis in Europe.     

Leaf breakdown and associated macroinvertebrates in alpine glacial streams

1. The relationship between macroinvertebrate assemblages and the breakdown of alder [Alnus viridis (Chaix), Dc.] leaves was examined by exposing leaf packs in four streams in an alpine glacial floodplain over 8 months. Although glacially fed, the four sites (pro-glacial, glacial lake outlet, main channel, and a side-channel with a mix of water sources) differed physically and contained different benthic communities.
2. Leaf breakdown and associated fungal properties differed widely among sites. Leaf decay rate varied by an order of magnitude ( k ranged from 0.0029 to 0.0305 day^–1), and was fastest at the lake outlet (< 20% leaf mass remaining by day 45) and slowest at the pro-glacial site (> 75% remaining on day 45). Rapid processing at the lake outlet was because of the presence of Acrophylax zerberus Brauer, a shredding caddisfly.
3. There were few macroinvertebrate taxa at the pro-glacial site (two to four taxa present in packs) and leaf breakdown was attributed primarily to micro-organisms. Leuctra abundance in leaf packs was strongly correlated with fungal biomass but not with the sporulation activity of any specific aquatic hyphomycete. Other taxa, such as Baetis and chironomids, showed no relationship with any leaf characteristic, suggesting that leaf packs were used mainly as a habitat and not as a food resource.
4. The predatory stonefly Isoperla was significantly associated with the abundance of macroinvertebrate prey ( Baetis , Chironomidae and Leuctra ) in leaf packs at the main and side-channel sites. The results indicate that leaf breakdown can vary widely in alpine lotic environments, reflecting site-specific differences in habitat characteristics, and in macroinvertebrate and fungal composition.     

The importance of crayfish in the breakdown of rhododendron leaf litter

1. Rhododendron (Rhododendron maximum) is a common evergreen shrub in riparian areas of the southern Appalachians, where its leaves can comprise a large proportion of leaf litter in streams. However, they are relatively refractory and generally considered a low quality food resource for detritivores. 2. Our objective was to assess whether macroconsumers [primarily crayfish (Cambarus bartonii)] influence rhododendron leaf breakdown in a forested southern Appalachian stream in both summer (when leaves other than rhododendron are relatively scarce) and autumn (when other leaves are relatively abundant). We conducted two leaf decay experiments, one in summer and one in autumn, using pre‐conditioned leaves. Macroconsumers were excluded from the benthos of a fourth‐order stream using electric ‘fences’; we predicted that excluding macroconsumers would reduce the decay rate of rhododendron leaves in both summer and autumn. 3. In both experiments, breakdown rate was lower in exclusion treatments. Macroconsumers accounted for approximately 33 and 54% of rhododendron decay in summer and autumn, respectively. We attribute this effect to direct shredding of rhododendron by crayfish. Biomass of insect shredders, insect predators and fungi did not differ between control and exclusion treatments, indicating that insectivorous sculpins (Cottus bairdi) had no effect on rhododendron decay and that omnivorous crayfish did not exert an indirect effect via alteration of insect or fungal biomass. 4. The influence of shredding insects varied between summer and autumn. In summer, when other, more palatable leaf types were not available, rhododendron leaf packs appeared to provide ‘resource islands’ for insect shredders. There was a significant inverse relationship between insect shredders and leaf pack mass in the summer exclusion treatment: insects were the only organisms eating leaves in this treatment and, as shredder biomass increased, remaining leaf pack mass decreased. In the control treatment, however, we did not see this relationship; here, the effect of insect shredders was presumably swamped by the impact of crayfish. In autumn, when other leaves were abundant, insect shredder biomass in rhododendron leaf packs was less than one‐third of summer values. 5. Even at low density (approximately 2 m^–2) crayfish were able to influence an ecosystem process such as leaf decay in both summer and autumn. Given the threatened status of many crayfish species in the United States, this finding is especially relevant. Even small alterations in crayfish assemblages, whether via loss of native species and/or introduction of exotic species, may have significant repercussions for ecosystem function.     

The influence of potential predators on the habitat preferenda of emerging brown trout

Much research has focused on the developmental behaviour of fish and it has been shown that their sensory and physical capabilities evolve very quickly during their early life. Thus, ontogenesis could influence fishes preferences for particular environmental factors. Little is known about the habitat preferences of trout during the post-emergence phase and it is not known if they correspond to the preference curves established by the PHABSIM method for the' alevin phase'. Here, the downstream movement and habitat preferences of young emerging trout were studied in a flume. In the absence of predators, alevins preferred a water depth of 20 to 30 cm and pebble rather than gravel substratum. When emergence occurred in an area with 1⁺ trout and sculpin, Cottus gobio , almost all the emergent trout remained cryptic. When visible, most of them were in the shallowest area (10cm depth) where their preference for pebble substratum was less marked. The presence of 1⁺ trout and sculpin increased the movement downstream of young trout by 20% without changing the general and diel patterns of catches. Their presence also reduced the initial growth of 0⁺ trout.     

Organic matter availability structures microbial biomass and activity in a Mediterranean stream

1. We compared microbial biomass (bacteria, fungi, algae) and the activity of extracellular enzymes used in the decomposition of organic matter (OM) among different benthic substrata (leaves, coarse and fine substrata) over one hydrological year in a Mediterranean stream.
2. Microbial heterotrophic biomass (bacteria plus fungi) was generally higher than autotrophic biomass (algae), except during short periods of high light availability in the spring and winter. During these periods, sources of OM shifted towards autochthonous sources derived mainly from algae, which was demonstrated by high algal biomass and peptidase activity in benthic communities.
3. Heterotrophic activity peaked in the autumn. Bacterial and fungal biomass increased with the decomposition of cellulose and hemicellulose compounds from leaf material. Later, lignin decomposition was stimulated in fine (sand, gravel) and coarse (rocks, boulders and cobbles) substrata by the accumulation of fine detritus.
4. The Mediterranean summer drought provoked an earlier leaf fall. The resumption of the water flow caused the weathering of riparian soils and subsequently a large increase in dissolved organic carbon and nitrate, which led to growth of bacteria and fungi.