Cobble Substrate Research Articles

Fish Assemblages of Floodplain Lakes in the Ohio River Basin

We sampled fish assemblages in 41 floodplain lakes in the Ohio River Basin in the summer of 2012. We collected 2427 individual fishes in 70 species. Mean abundance of individuals at sites was 66, and mean species richness per site was 8.1. We used two multivariate procedures to predict fish-assemblage variation from habitat and environmental variables: an indirect gradient approach (reciprocal averaging [RA]) and a direct gradient approach (canonical correspondence analysis [CCA]). When we applied a forward selection process in the CCA, the habitat and environmental variables that contributed significantly to explaining variation in fishes were mean elevation, latitude, maximum depth, conductivity, longitude, dissolved oxygen, cobble and sand substrates, and lake-surface area. RA provided different results that suggested the presence of additional environmental gradients we did not quantify. Our results show that floodplain lakes in the Ohio River basin contain high species richness and are important habitats to conserve because they have the potential to act as source pools for river fish populations.

Alternative spawning strategy and temperature for larval emergence of longfin dace (Agosia chrysogaster) in stream mesocosms

Abstract To determine if the strategy of spawning in saucer-like depressions is obligate or facultative for longfin dace (Agosia chrysogaster), we collected adults from four sites in the upper Gila River (southwestern New Mexico), stocked them in separate outdoor stream-mesocosms lined with cobble substrate, and made daily observations for the presence of saucer-nests and hatched larvae. Larvae were observed from three of the four mesocosms and emerged at temperatures ranging from 19.2–24.0°C. The absence of saucer-nests in all mesocosms throughout the study indicates that longfin dace can spawn over cobble substrate and have larvae hatch successfully, suggesting that longfin dace can use an alternate spawning strategy when sand substrate is not available for construction of saucer-nests.

Microhabitat Use and Vertical Habitat Partitioning of Juvenile Atlantic (Gadus morhua) and Greenland (Gadus ogac) Cod in Coastal Newfoundland

Twenty co-occurring juvenile gadids (10 Gadus ogac and 10 Gadus morhua) were surgically implanted with ultrasonic transmitters with depth sensors and continuously monitored for up to 23 days in the summers of 2009 and 2010 to test fine-scale habitat use and vertical distribution overlap in coastal Newfoundland (>18700 positional fixes). A habitat map with 8 substrate and 3 slope classes (low (<5°), moderate (5-10°), and high (>10°)) was generated from acoustic data and coincident video data using seabed mapping software (QTC). Fish locations were integrated with a habitat map to assess and quantify habitat preference. Both species avoided fine gravel/sand substrates with little vegetation and selected for large particle (cobble and boulder) substrates with moderate or dense vegetation, and spent the majority of time in < 10 m of water. Nevertheless, species differences in habitat use were evident. G. ogac typically remained in close proximity to the seafloor whereas G. morhua was often distributed more pelagically and showed greater variation in vertical distribution. Habitat use and vertical distribution patterns were consistent across the diel period. Our results suggest that despite high overlap in habitat use, G. morhua and G. ogac often segregate vertically in the water column, which may reduce competitive interference. We suggest that these patterns are related to differences in diet.

Predation by alewife on lake trout fry emerging from laboratory reefs: Estimation of fry survival and assessment of predation potential

Alewife (Alosa pseudoharengus) predation may be an important mortality source on lake trout fry (Salvelinus namaycush), and could affect the success of lake trout restoration in the Great Lakes. This study tested the prediction that fry showing typical swimming and avoidance behavior over artificial reefs will differ in survival when alewives are present versus when alewives are absent. Six tanks with cobble substrate were each stocked with 153 lake trout fry (density=131m−2), a density comparable to that recorded at Stony Island reef, Lake Ontario during the early 1990s. Four treatment tanks each contained ten alewives (density=8m−2) and two control tanks contained no alewives. After 12days, mean recovery of fry was less in treatment tanks (31.5 fry per tank) than in control tanks (150 fry per tank; P<0.009). Fry mortality in control tanks was about 2% in contrast to 46 to 91% mortality in tanks containing alewives. Alewife predation effects were evident early in the experiment as the mean daily capture of fry by traps set in each tank was always lower after day two in treatment tanks than in control tanks. The rate of consumption of lake trout fry by alewives ranged from 0.57 to 1.16 fryalewife−1day−1 (mean=0.99±0.141; median=1.12). The results of this study support the hypothesis that predation by alewives could cause a high level of lake trout fry mortality, and thus affect natural recruitment of lake trout and the success of population rehabilitation.

Summer habitat use and partitioning by two reintroduced rare madtom species

Habitat use by smoky madtoms (Noturus baileyi) and yellowfin madtoms (Noturus flavipinnis) was quantified in lower Abrams Creek within the Great Smoky Mountains National Park during the summers of 2007–2010. Variables were measured at both macrohabitat and microhabitat spatial scales within locations where each species was present. Reach-level macrohabitat data were analyzed using principal component analysis to identify variables associated with each species’ presence within a reach. Classification trees were developed to describe microhabitat use for each species and to compare differences in microhabitat use between species. Distributions of both species were similar in lower Abrams Creek with each becoming more abundant within downstream reaches. Smoky madtoms predominantly used riffle and run microhabitats with gravel and cobble basal substrates, while yellowfin madtoms used pool microhabitats away from riffles with low velocity and cobble-sized cover rocks. Neither species was ever encountered within the same microhabitat, suggesting summer habitats were partitioned. Models developed in this study can be used to identify potential reintroduction zones or to assist in conservation efforts.

Day and Night Substrate Use in Six Minnow Species

We tested for diel substrate use in artificial stream experiments for six minnows: redfin shiner (Lythrurus umbratilis), silver shiner (Notropis photogenis), spotfin shiner (Cyprinella spiloptera), central stoneroller (Campostoma anomalum), striped shiner (Luxilus chrysocephalus), and bluntnose minnow (Pimephales notatus). Our experimental pools contained two of three possible substrate types (sand, cobble, and gravel) per trial, and we tested if habitat use differed in day and night observations. Over 85% of individuals and all species were observed over cobble substrate during the day, and no species occurred in higher abundances over a single substrate type at night. Fish locations during night observations were over cobble and sand or cobble and gravel. Our results suggest that minnows use coarse substrates during the day and do not have substrate preferences at night. Habitat use studies with only daytime observations of small-bodied fishes likely cannot be interpreted as complete. Further investigations of substrate use variation with 24 h observations and additional body size or life stage variation will provide species-specific habitat requirements and better understanding of stream assemblage structure.

Relationship between lake trout spawning, embryonic survival, and currents: A case of bet hedging in the face of environmental stochasticity?

Lake trout, Salvelinus namaycush, spawning in the Great Lakes occurs primarily on cobble substrate at relatively shallow water depths that can experience strong water currents. Strong currents may limit embryonic survival by damaging or displacing eggs, but may also reduce the accumulation of fine material and limit foraging by potential egg predators. To better understand the importance of currents, we evaluated the role of currents in spawning habitat selection, egg density and survival, and egg predator density at a spawning reef in Lake Champlain (USA). Most spawning occurred one week after the largest storm event associated with the strongest currents and greatest upwelling. Highest spawning activity was associated with a relatively shallow part of the reef that had the highest current velocity and greatest potential for egg displacement. Within the interstices, the survival of naturally deposited eggs was unrelated to the concurrent loss of artificial eggs. We propose that the reproductive strategy of spawning on shallow areas of a reef that have the highest current velocity and high potential for egg loss represents a type of bet hedging to optimize survival of those embryos that remain within interstices. This strategy may have evolved in response to environmental stochasticity that resulted in higher egg survival.

Impact of scallop dredging on benthic epifauna in a mixed-substrate habitat

AbstractExperimental scallop dredging was conducted to assess the vulnerability of emergent epifauna on hard substrates. Three sites were sampled before and after dredging to examine changes in the coverage of faunal turf (hydroid and bryozoan) assemblages and the composition of the wider epifaunal community. Each site had an “impact” box that was dredged, a control box that was in an area that was still open to fishing, and a control box in a special area of conservation (SAC) that had not been fished for two years. Community composition differed significantly after dredging in two of the three sites, with dredged communities becoming less similar to those in the SAC. There was no clear evidence that dredging in the impact boxes reduced the coverage of faunal turfs on hard substrates. However, the coverage of faunal turfs on hard substrates in the SAC was typically greater than in areas that were still being fished commercially, consistent with a dredging effect. The results highlight the role that substrate morphology might play in modifying the severity of dredging effects. This has relevance to marine spatial management, as it suggests that emergent epifauna living on hard substrates that are morphologically suited to dredging, such as pebble and cobble substrates, could be particularly vulnerable to dredging.

Habitat Suitability Index Relationships for the Northern Clearwater Crayfish,Orconectes Propinquus (Decapoda: Cambaridae)

We evaluated habitat models that determined relative abundance relationships among microhabitat, reach and watershed-scale factors important for predicting habitat selection. Thirty stream reaches in central Indiana were sampled to determine relationships between habitat associations and relative abundance, size, and age associations of the Northern Clearwater Crayfish, Orconectes propinquus. Females are significantly more abundant than males and the frequency of crayfish in gravel substrate was significantly higher than that of cobble substrate. The sizes of crayfish in cobble substrate were significantly larger than individuals found in gravel substrates, while females were significantly larger than males in gravel substrates. Watershed variables were not significantly related to crayfish abundance. The only reach scale variable that proved to be significant was boulder substrate. Microhabitat variables showed a significant increase between CPUE and cobble and gravel substrates. Habitat models provide valuable information on the conservation status and habitat parameters responsible for determining species preferences, life history strategies, and relative abundance.

Biodiversity of hillstream fishes in Bangladesh.

Bangladesh is a country of 1,47,570 km2 of mostly flat topography, but about 12 percent is hilly. The hilly areas are confined to the northeast and the southeastern parts of the country bordering India and Myanmar. Hill streams are highly variable and very important for the study and understanding of the aquatic biodiversity of Bangladesh. Hillstream ecosystems include a variety of habitats including those with sand, clay, cobble, gravel, mud, and rock substrates. In a recent field survey, 82 species of fishes have been identified from those habitats. The ichthyofauna belongs to the following families (numbers of species in parentheses) Notopteridae (1), Engraulidae (1), Cyprinidae (32), Psilorhynchidae (3), Nemacheilidae (2), Cobitidae (6), Bagridae (6), Schilbeidae (5), Amblycipitidae (1), Akysidae (1), Sisoridae (4), Erethistidae (1), Clariidae (1), Olyridae (1), Aplocheilidae (1), Ambassidae (2), Badidae (1), Mugilidae (1), Gobiidae (2), Osphronemidae (2), Channidae (3), Mastacembelidae (3), Belonidae (1) and Tetraodontidae (1). This paper provides a checklist of the hillstream fish species with their habitat preferences and associated fauna.

Habitat utilization, movement and use of groundwater seepages by larval and juvenile Black Redhorse, Moxostoma duquesnei

Black Redhorse (Moxostoma duquesnei) larval and juvenile habitat was characterized in the Grand River, Ontario from June to September 2007–2012. Similar to adult Black Redhorse and their congeners, larval Black Redhorse were most likely to be located in clean, clear, stable runs with low to moderate flow, over pebble, gravel and cobble substrate, mixed with sand. Areas (n = 22) where 0+ Black Redhorse were observed and collected were 1.4 ± 0.2 m from shore, with a mean water temperature of 22.0 ± 0.5 °C, mean depth of 0.20 ± 0.02 m and mean water velocity of 0.12 ± 0.05 m/s. Larval and juvenile Black Redhorse occupied riffles, runs, pools and backwater areas; however, there was a strong preference for runs. Juvenile Black Redhorse moved upstream in the early evening and at night to overwintering areas in the Grand River in November when water temperature approached 5 °C. The persistence of Black Redhorse populations in the Grand River may be related to the presence of groundwater, which provides refuge from extreme temperature and poor water quality during the summer.

Habitat preferences of the burbot (Lota lota) from the River Elbe: an experimental approach

An experimental approach was used to analyze preferences of the burbot (Lota lota) with regard to water depth, substrate type and flow velocity. In total, 30 burbots used in the experiments were caught in the middle reaches of the River Elbe by electro-fishing. Immediately after capture, they were placed in a 180 L transportation box with aerated freshwater and taken to the research aquarium of the Zoological Museum Hamburg where they were divided into two groups according to total length: group I, 10.0–16.5 cm; group II, 20.0–30.0 cm. Prior to the experiments the fish were adapted to laboratory conditions for four weeks at 20°C in four tanks, each containing 720 L water. For each experiment, three L. lota were transferred to an additional 1350 L experimental tank. After a one-day adaptation phase each experiment ran for three continuous day and night phases. Burbot preferences were tested for (i) water depth, (ii) substrate size, (iii) water depth and substrate size combined, and (iv) water depth, substrate and flow velocity in combination. Diurnal and nocturnal activities were recorded and analyzed throughout all experiments. Burbots of both length groups showed a significant preference for deeper water depths and large cobble substrate. Substrate type was found to be the dominant factor determining habitat preferences of L. lota when investigated in combination with water depth. In experiments combined with flow velocity, substrate and water depth were the most important factors determining burbot habitat preferences, and flow velocity of minor importance. In general, burbots were most active at night, while daytime activity was much lower. However, in all experimental approaches in the smaller length group the burbots displayed much higher day- and night-time activity than did the individuals in the larger length group.

Conservation Aquaculture of Northern Leatherside Chub and Effects of Temperature on Egg Survival

AbstractWe present 4 years of data that refine aquaculture protocols for the northern leatherside chub Lepidomeda copei, a species of conservation concern in the Intermountain West. Experiments examined life history traits (age at first spawning and thermal limits to egg hatching success) and aquaculture techniques (brood density, spawning substrate type and surface area, and feeding methods for fry). Tests showed that leatherside chub can reproduce as early as age 2. Multiple spawns per female during a year were also documented. Survival of eggs was compared at incubation temperatures of 18.4, 23.0, 24.6, and 26.8°C. Eggs at 18.4°C had the highest survival to hatching (54.0%); eggs at 26.8°C had significantly lower survival (1.5%). Egg survival at 23.0°C and 24.6°C (32–33%) was significantly lower than survival at 18.4°C. Aquaculture experiments showed that the mean total number of eggs produced did not significantly differ between brood densities of 8.4 (1,246 ± 1,236 eggs [mean ± SD]) or 16.8 (2,224 ± 1,600 eggs) fish/m3. Studies showed that leatherside chub preferred spawning over natural cobble substrate to spawning over marble substrate. More eggs were recovered from a three‐substrate tray treatment (1,350 cm2) than from a single tray treatment (450 cm2). Fry given brine shrimp Artemia spp. with probiotic bacteria or fed with an automated, more continuous drip feeder did not show any advantages in growth over time. Juveniles at rearing densities of 800, 1,700, and 3,400 fish/m3 did not differ significantly in growth rates, deformities, or mortalities. This research provides general guidelines for rearing northern leatherside chub and some additional information on the species’ life history.

Impacts of the Eurasian round goby (Neogobius melanostomus) on benthic communities in the upper St. Lawrence River

An invasive benthivorous fish, the Eurasian round goby ( Neogobius melanostomus ) is abundant throughout the lower Great Lakes – St. Lawrence River system. We examined the round goby’s potential to alter benthic communities on cobble substrates in the upper St. Lawrence River. During the summers of 2008 and 2009, macroinvertebrates and benthic algae were sampled across sites with varying goby densities. Archived data from various sites in 2004–2006 (prior to invasion) were available for comparison. Macroinvertebrate community composition varied significantly among samples grouped into categories based on goby density and time since invasion. Macroinvertebrate diversity and dominance by large-bodied taxa declined with increasing goby density. Surprisingly, dreissenid biomass did not vary consistently with goby density, in contrast to studies in the Great Lakes. The biomass of all non-dreissenid taxa was negatively correlated with increasing goby density across sites and over time at three of four sites. Negative effects were most pronounced on the biomass of gastropods. Benthic algal biomass increased with goby density across sites, suggesting a trophic cascade driven by the impacts of gobies on gastropods and other algivores. Our study highlights the potential ecosystem impacts of an expanding goby population in a large river.

Microhabitat association of Hemimysis anomala on fish spawning reefs in Grand Traverse Bay, Lake Michigan

In 2006 the bloody-red mysid (Hemimysis anomala), a new invasive species to the Great Lakes, was discovered in the Muskegon channel which flows into Lake Michigan. As predicted at the time of introduction, it quickly expanded its range in Lake Michigan and has recently been documented in Grand Traverse Bay near Elk Rapids, Michigan. Its effects on Great Lakes ecosystems, however, remain unclear owing to a lack of information on its abundance. Using a gear intended to sample lake trout and lake whitefish eggs from cobble substrate during spawning, we found variable densities of Hemimysis at three near shore reefs at Elk Rapids, Lake Michigan over a two year period. The highest densities of Hemimysis were found at the reef with the highest quality fish (i.e., lake trout and lake whitefish) spawning habitat based on the proportion of rounded cobble and rubble substrates, and the amount of interstitial space. Abundance of Hemimysis on all reefs was highly seasonally dependent, ranging from 0 to 31.4 per m². The highest numbers were seen during the fall when water temperatures were between 6 and 12°C. Based on the association of Hemimysis with cobble substrates and the abundance of this type of habitat in northeastern Lake Michigan, we predict substantial expansion of Hemimysis in this area of the lake. Additional evaluation of Hemimysis on fish spawning habitat is needed to determine potential interactions with other Great Lakes biota, particularly larval fish with which they may compete.

Contrasting effects of an invasive crayfish (Procambarus clarkii) on two temperate stream communities

Summary1. The effects of omnivorous exotic species on native communities are often difficult to predict because of the broad diets and behavioural flexibility of the omnivore, and the diverse abiotic and biotic characteristics of invaded systems. We investigated experimentally the effects of a gradient of density of the introduced, omnivorous red swamp crayfish Procambarus clarkii (Decapoda: Cambaridae) on two stream communities in southern California, U.S.A.2. The Ventura River is a clear, flowing stream with a cobble substratum, with abundant algae but low densities of large invertebrates, small herbivores and snails. The Santa Ynez River at the time of the study consisted of a series of drying pools underlain by sand, with abundant charophytes, large predatory invertebrates and herbivores, including snails.3. In the Ventura River, periphyton biomass and inorganic sediment decreased with increasing crayfish abundance, but in the Santa Ynez River, periphyton and sediment were unrelated to crayfish densities.4. In the Ventura River, the biomass and density of all benthic invertebrates combined, chironomids, micropredators, the meiofauna (chydorid cladocerans, copepods and ostracods), and specific predatory and herbivorous taxa, as well as taxon richness, were negatively related to crayfish density. In the Santa Ynez River, the biomass and average body size of benthic invertebrates, predatory invertebrates, herbivores and chironomids, but not total invertebrate density or taxon richness, were negatively related to crayfish density.5. Fewer large predatory invertebrates and snails (Physella gyrina) in both streams, and baetid mayflies in the Ventura River, were visible at night in channels where crayfish were abundant. Snails responded to crayfish by moving above the water line in the Santa Ynez River, but not in the Ventura River.6. We suggest that the same omnivore had different effects on these neighbouring streams because of crayfish predation on large invertebrates in the Santa Ynez River and the scarcity of such prey in the Ventura River, leading to increased crayfish grazing on periphyton, and reductions in periphyton‐associated invertebrates, in the Ventura River.

The influence of low-head dams on fish assemblages in streams across Alabama

We quantified fish assemblages in 20 streams containing mill dams in various physical conditions (dams intact, partially breached, or relict with normal flows) in Alabama, USA, during the period from 2006 to 2008. We used a backpack electroshocker to sample three 150-m reaches per stream: 500 to 1000 m downstream of the dam, 0 to 100 m downstream of the dam, and 100 m upstream of the impoundment. Species- and trait-based analyses revealed slightly different, but often complementary, information about fish assemblages. Fish species richness and benthic conditions differed longitudinally among reaches in streams with dams. In streams with breached dams, species richness, but not trait richness, was lower in upstream reaches than in downstream reaches. Overall, species and trait richness were correlated with benthic-habitat variables in streams with relict dams and were significantly correlated with water physicochemical variables in streams with intact and breached dams. Nonmetric multidimensional scaling ordination failed to resolve any discernable site groupings based on species abundance data, and indicator species analysis revealed 1 indicator species, Esox americanus, upstream of relict dams. Fourth-corner trait analysis revealed more trait associations in reaches in streams with breached dams than in those with intact or relict dams. Generalist spawners (nest-guarding polyphils) increased and taxa with a preference for cobble substrates decreased upstream of breached dams. Few longitudinal differences were observed in streams with relict and intact dams. Taken together, dams, particularly those that are breached, appear to exert a strong upstream influence on fish species richness and functional composition and could alter the trophic structure of the entire stream through habitat modifications or limitation of fish movements.

Importance of sedimentation for survival of canopy-forming fucoid algae in urchin barrens

Sedimentation can provide indirect benefits to the survival of macroalgae in areas with potentially high grazing pressure. Field studies were performed in an embayment with extensive urchin barrens but also with locally persistent fucoid beds, on the coast of Kagoshima, south-western Japan, to elucidate the physical and biological processes responsible for the maintenance of the beds. Rocky subtidal reefs devoid of fine sediment were almost completely barren and dominated by sea urchins (primarily Echinometra sp. A), while fucoid algae (predominantly Sargassum duplicatum) densely populated cobble sites overlaid with a thin layer of fine sediment (medium grain size: 0.15–0.25 mm). Quadrat samplings in areas intermixed with urchin barrens and sand flats as well as experimental addition of sediment suggested that Echinometra sp. was readily excluded from hard substratum overlaid with even only a thin layer of fine sediment. Quadrat surveys and a transplant experiment conducted at the border area between a cobble bed with a thin cover of fine sediment and a sediment-free boulder one indicated that sea urchins (mainly composed of Echinometra sp. and Diadema spp.) rarely invaded the sediment-covered bed to graze. Wave measurements at the entrance (8 m deep) of the embayment over a 3.5-year period showed that the study area had long-term extremely calm conditions (84% of significant wave heights < 0.1 m) and seasonal or transient moderate disturbances due to relatively high waves (significant wave height: 0.8–1.3 m). However, the 2-year time series of root-mean-square wave orbital velocity estimates at different sites consistently suggested that the wave-action intensities at urchin barrens were still too high for deposition of fine sediments which occurred in more wave-sheltered persistent fucoid area. Nearly 2-year investigations on sediment level change and on cobble substrates together with overlying sediment in the fucoid area suggested that the absence of fatal sediment inundation and maintenance of the thin overlying sediment layer (mean thickness < ca 2 mm) throughout the year allowed settlement and growth of sand-tolerant fucoids by preventing urchin grazing.

Changes in relative abundance of adult walleye and egg density following the addition of walleye spawning habitat in a midwest irrigation reservoir

Cobble substrate for walleye (Sander vitreus) spawning was added at Sherman Reservoir, Nebraska, in January 2008. We evaluated changes in relative adult walleye abundance and egg density in response to the cobble substrate and estimated site fidelity of adult walleye during the spawning seasons of 2007, 2008, and 2009. A 15% increase in electrofishing catch-per-unit-effort (CPUE) of adult walleye was observed on the cobble substrate the first season it was available (2008), but a 5% decrease in electrofishing CPUE was observed the second season (2009) compared to the 2007 season. While gill nets were not run during the first season the cobble was available, a 113% increase in gill net CPUE was observed the second season. During the first season the cobble was available, a 215% increase in egg density was observed, but no increase in egg density was found in the second season. Site fidelity of adult walleye within a spawning season was 94% and was 81% between seasons. These results indicate that the addition of the cobble substrate increased the relative abundance of adult walleye and density of eggs on the cobble substrate.

Influence of substratum on the variability of benthic biofilm stable isotope signatures: implications for energy flow to a primary consumer

Benthic biofilms have been identified using stable isotope analysis (SIA) as an important resource supporting many freshwater food webs. However, biofilm δ13C signatures are highly variable in freshwaters, which may hamper our understanding of energy flow through food webs in these systems. There has been little consideration of the influence that substratum may have on biofilm δ13C signature variability and energy flows to primary consumers. We investigated the effect of organic and inorganic substrata on biofilm dynamics by examining: (1) temporal variability of biofilm stable isotope (δ13C, δ15N) signatures on allochthonous leaf-litter (Eucalyptus camaldulensis) and cobble substrata over 12 months in a lowland river in south-eastern Australia; and (2) the effect of substrata on biofilm energy flows to a grazer snail, Physa acuta (Gastropoda: Physidae), using SIA and ecological stoichiometry in a laboratory experiment. The temporal study indicated that cobble biofilm varied significantly in δ13C signature during the 12 months (up to 11‰), whereas the δ13C signature of leaf biofilm was less variable (less than 2‰). In contrast, biofilm δ15N signatures varied temporally on both cobble (2.6‰) and leaf (1‰) substrata. This suggests that leaf biofilm was more reliant on leaf tissue for carbon and therefore less limited by carbon supply than cobble biofilm whereas for nitrogen biofilm on both substrata was reliant on external sources. In the laboratory experiment, snails fed leaf biofilm reflected more of an allochthonous δ13C signature than cobble biofilm fed snails, suggesting assimilation of leaf carbon via the heterotrophic microbial community within the biofilm. Snails grew largest on cobble biofilm, which had lower C:N ratios than leaf biofilm. Our results demonstrate that the type of substratum can influence the temporal variability of biofilm δ13C signatures and energy flow to primary consumers.