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

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

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.     

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

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

Periphyton distribution and abundance on substrata of different size along a gradient of stream trophy de Montréal

In stream, substrata of different size present different degree ofstability, current, erosion, and deposition to colonizingorganisms. In this study, we tested the importance of substratumsize ranging from sand to small boulders for periphytondistribution and abundance. Because trophy strongly affect streamorganisms, we sampled at nine sites chosen to represent the rangeof nutrients typical of Eastern Ontario and Western Québec. Alarge part of the variability in algal biomass (as chlorophyll)among sites was explained by trophy (as seston or totalphosphorus). However, there was also an effect of substratum size.Cobbles had the highest biomass, and gravel the lowest; sand andboulders were intermediate. Assemblages on different substrata weredifferent in taxonomy and life forms. Cyanobacterial colonies andmotile diatoms dominated the finer substrata while adnate andfilamentous algae were more developed on the larger ones.Consequently periphyton on fine sediments was more loosely attachedthan on rocks. Average algal size was not related to substratumsize but increased significantly with trophy confirmingobservations in benthic and planktonic assemblages inlakes.     

Adhesion of algal cells to surfaces

This paper reports the cell–substratum interactions of planktonic (Chlorella vulgaris) and benthic (Botryococcus sudeticus) freshwater green algae with hydrophilic (glass) and hydrophobic (indium tin oxide) substrata to determine the critical parameters controlling the adhesion of algal cells to surfaces. The surface properties of the algae and substrata were quantified by measuring contact angle, electrophoretic mobility, and streaming potential. Using these data, the cell–substratum interactions were modeled using thermodynamic, DLVO, and XDLVO approaches. Finally, the rate of attachment and the strength of adhesion of the algal cells were quantified using a parallel-plate flow chamber. The results indicated that (1) acid–base interactions played a critical role in the adhesion of algae, (2) the hydrophobic alga attached at a higher density and with a higher strength of adhesion on both substrata, and (3) the XDLVO model was the most accurate in predicting the density of cells and their strength of adhesion. These results can be used to select substrata to promote/inhibit the adhesion of algal cells to surfaces.     

Three-year study of benthic algal spring bloom development in a small, Danish lowland stream

Seasonal development of benthic algae was studied over a three-year period in a small, nutrient-rich lowland stream to investigate inter-annual variation in the algal spring bloom and differences in algal biomass regulation on two different substrata: fine-grained sediments and stones. The algal spring bloom was initiated when irradiance at the sediment surface exceeded 7 mol photons m^-2 d^-1 and mean water velocity was concomitantly below the threshold for bed load transport in the stream. Large inter-annual and substratum-dependent differences in peak algal biomass were observed, thus suggesting that different parameters regulate algal biomass development on the two substrata. On fine-grained sediments algal biomass development was predominantly coupled to light availability, while on stony substrata algal composition and peak biomass might be affected by invertebrate grazing.     

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.     

Effects of omnivorous shrimp in a montane tropical stream: sediment removal,disturbance of sessile invertebrates and enhancement of understory algal biomass

Freshwater shrimp dominate the faunal biomass of many headwater tropical streams: however, their role in community organization is unclear. Enclosure/exclosure experiments in a montane Puerto Rican stream examined direct and indirect effects of two dominant taxa of atyid (Atyidae) shrimp, Atya lanipes Holthuis and Xiphocaris elongata Guerin-Meneville. Both shrimp taxa caused significant reductions in sediment cover on rock substrata, reducing sedimentation and enhancing algal biovolume on clay tiles in cages. When tiles incubated in shrimp exclosures for 2 wks were placed outside of cages, atyid shrimp removed 100% of the sediment cover within a 30 min observation period. Atyid shrimp appear to play an important role in stream recovery after high discharge events by rapidly removing sediments and detritus deposited on benthic substrata in pools. We evaluated the mechanism by which A. lanipes influences algae and benthic insects by comparing patterns of algal biomass, taxonomic composition, and insect abundance between shrimp-exclusion and shrimp-presence treatments both with and without manual sediment removal. The shrimp exclusion treatment without manual sediment removal bad significantly lower algal biomass and greater sedimentation than all other treatments. The treatment in which shrimp were excluded but sediment was manually removed, however, accrued almost the same algal biovolume as the shrimp enclosure treatment, supporting the hypothesis that sediment removal enhances the biovolume of understory algal taxa. Algal community composition was similar between stream bottom bedrock exposed to natural densities of shrimp and all experimental treatments for both Atya and Xiphocaris: a diatom community strongly dominated (78–95%) by the adnate taxon, Achnanthes lanceolata Breb ex. Kutz. Atyid shrimp are important in determining the distribution and abundance of benthic insects through both direct and indirect effects. Sessile, retreat-building chironomid larvae (Chironomidae: Diptera) are negatively affected by both A. lanipes and X. elongata, through direct removal by foraging activities and/or indirectly through depression of sediment resources available to larvae for the construction of retreats. In constrast, the mobile grazer, Cloeodes maculipes (Baetidae: Ephemeroptera) was not adversely affected and atyid shrimp have the potential to exert positive indirect effects on this taxon by facilitating its exploitation of algal resources and/or through enhancement of understory algal food resources through sediment removal.     

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 SEASONAL DISTRIBUTION,ABUNDANCE AND DIVERSITY OF DESMIDS (CHLOROPHYTA) IN A SOFTWATER,NORTH TEMPERATE STREAM1

Suspended and benthic algal communities from a mildly acidic, third-order Rhode Island stream were examined to determine the seasonal distribution, abundance and diversity of the lotic desmids. Within a one-year sampling period, 148 species and 202 subspecific taxa of desmids were identified, representing 23 genera. Species of Cosmarium and Closterium accounted for approximately 70% of the desmids present, and were the most diverse and abundant taxa during all seasons except spring, when Hyalotheca dissiliens was the dominant desmid species. Average abundance and species richness generally were greatest during summer for both suspended and benthic desmids. Most desmids occurred in benthic habitats, and were randomly distributed among substrata. Average seasonal abundance was 7.4 × 10⁴ cells·g^?1 dry wt substratum, among 13 types of substrata. Highest desmid abundance was measured among substrata with intricate morphologies, such as Fontinalis spp., which was associated with 1.2 × 10⁶ desmid cells·g^?1 dry wt substratum, or 1.7 × 10³ cells·cm^?2 substratum. Cell division was observed for 70 desmid taxa, and average seasonal reproduction (based on cell numbers) among all substrata ranged from 4% in winter to 20% during summer. In addition, sexually produced zygospores were found occasionally for H. dissiliens. Desmids were distributed among most substrata examined in this stream, with abundance comparable to reported estimates from softwater lakes and acid bogs. In contrast to established dogma, lotic desmids are not incidental drift organisms, but rather comprise a viable and persistent component of the stream periphyton.     

Impacts of Riparian Forest Removal on Palauan Streams

The 2006 completion of the circum‐island Compact Road on the island of Babeldaob in the Republic of Palau resulted in several deforested stream reaches with modified stream channels. To determine the impacts of deforestation and road construction, various ecosystem parameters were compared between road‐impacted reaches, reforested savanna reaches, and forested reaches. Compared to adjacent forested reaches, road‐impacted reaches received significantly more light (0.4 ± 0.1 vs. 87.8 ± 4.1 % light transmittance, respectively), were significantly warmer (25.7 ± 0.1 vs. 26.1 ± 0.1°C, respectively), and received higher nutrient and sediment loads, all of which were attributed to the removal of riparian vegetation and increased surface runoff from the road. These differences were believed to have shifted the benthic algal community in road‐impacted reaches from diatoms to filamentous algae with significantly greater chl a biomass (10×) and benthic algal ash free dry mass AFDM (3×) compared to adjacent forested reaches. Savanna‐impacted and forested reaches had similar chl a, algal AFDM, and received similar amounts of light. Nutrient and sediment concentrations varied between the two reach types. Results from this study emphasize the need for the maintenance of riparian forests especially with predicted increases in population, development, and deforestation. Future studies are needed to determine effective riparian widths and riparian forest community structure to help resource managers and land owners protect and preserve the many ecosystem services that Palauan streams and watersheds provide.     

Comparing Effects of Nutrients on Algal Biomass in Streams in Two Regions with Different Disturbance Regimes and with Applications for Developing Nutrient Criteria

Responses of stream algal biomass to nutrient enrichment were studied in two regions where differences in hydrologic variability cause great differences in herbivory. Around northwestern Kentucky (KY) hydrologic variability constrains invertebrate biomass and their effects on algae, but hydrologic stability in Michigan (MI) streams permits accrual of high herbivore densities and herbivory of benthic algae. Multiple indicators of algal biomass and nutrient availability were measured in 104 streams with repeated sampling at each site over a 2−month period. Many measures of algal biomass and nutrient availability were positively correlated in both regions, however the amount of variation explained varied with measures of biomass and nutrient concentration and with region. Indicators of diatom biomass were higher in KY than MI, but were not related to nutrient concentrations in either region. Chl a and % area of substratum covered by Cladophora were positively correlated to nutrient concentrations in both regions. Cladophora responded significantly more to nutrients in MI than KY. Total phosphorus (TP) and total nitrogen (TN) explained similar amounts of variation in algal biomass, and not significantly more variation in biomass than dissolved nutrient concentrations. Low N:P ratios in the benthic algae indicated N as well as P may be limiting their accrual. Most observed responses in benthic algal biomass occurred in nutrient concentrations between 10 and 30 μg TP l⁻¹ and between 400 and 1000 μg TN l⁻¹.     

NUTRIENT LIMITATION OF BENTHIC ALGAE IN LAKE MICHIGAN: THE ROLE OF SILICA1

In the Laurentian Great Lakes, phytoplankton growth and biomass are secondarily limited by silica (Si), as a result of phosphorus (P) enrichment. Even modest levels of P enrichment can induce secondary Silimitation, which, in turn, promotes a shift from the native diatom phytoplankton flora to chlorophyte and cyanobacteria species. However, very little is known about the nutritional status of benthic populations and their response to nutrient enrichment. Two experiments were performed in the littoral zone of Lake Michigan where nutrients were delivered to in situ benthic algal (episammic and epilithic) assemblages using nutrient‐diffusing substrata. In order to test the hypothesis that benthic algae in Lake Michigan are Si limited, a 2 × 3 factorial experiment was used to deliver all combinations of Si, N, and P to resident assemblages growing on artificial substrata composed of natural (Si rich) versus calcium carbonate (Si poor) sand. A second experiment utilized a serial enrichment to evaluate the role of Si in mediating changes in taxonomic composition. These findings indicate that benthic algae in Lake Michigan exhibit signs of secondary Si limitation, and that their response to enrichment is similar to the phytoplankton. Moreover, natural sand substrata may provide a source of Si to resident benthic algae.     

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

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Benthic algal biomass in an unshaded first-order lowland stream: distribution and regulation

The vertical distribution of algal biomass in the bed sediment and the seasonal development of benthic algae on stones and fine-grained sediments were studied in a small unshaded stream. In addition, field experiments were conducted on the role of irradiance and phosphorus in regulating algal biomass. We found that algal biomass was high at a sediment depth of ten centimetres. Comparison of studies on algal biomass where different depths of the sediment are used should therefore be made with caution. Substrata-dependent differences in algal biomass development were substantial. While algal biomass development on stones was controlled by macroinvertebrate grazing, that on the fine-grained sediment followed the dynamics of incident irradiance, but was attenuated by sediment rebedding. Because of the high grazing pressure on algal biomass on stony substrata, no significant response to phosphorus enrichment was attained. In contrast, algal biomass development on fine-grained sediments was phosphorus-limited. Heavy shading of the fine-grained sediments did not significantly affect algal biomass development, thus suggesting that phosphorus limitation prevents algae from fully utilizing the light resource in this stream. This revised version was published online in July 2006 with corrections to the Cover Date.     

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).     

Local disturbance history affects patchiness of benthic river algae

1. Recent research has shown that high‐flow events in streams leave a small‐scale mosaic of bed patches that have experienced scouring, sediment deposition (fill), or remained stable. Few studies have investigated if this ‘local disturbance history’ contributes to the patchy distribution of benthic organisms in streams and rivers. 2. In the present research, we demonstrate that local disturbance history in a mid‐sized river can have both short‐ and long‐term effects on epilithic algae. Chains buried vertically in the substratum of the river bed (236 in a 800‐m reach) indicated that two floods (return periods ≤1 year) caused a mosaic of bed patches with different disturbance histories. Once after the first and twice after the second flood, we sampled epilithic algae (mainly diatoms) in replicate patches that had been scoured, filled, or remained stable during the respective event. Algal biomass and cell density per substratum area were determined. 3. Three months after the first flood, algal biomass, total diatom density, diatom taxon richness, and densities of six of nine most common taxa were highest in fill patches. Six days after the second flood, biomass was highest in stable patches, indicating a refugium function of these patches. The refugium patches consisted of average‐sized stones, in contrast to previous studies of flood refugia for benthic algae in which these refugia were always large and/or immobile substrata. Four weeks after the second flood, diatoms tended to be most abundant in scour patches. With one exception, these differences between patch types could not be attributed to differences in local near‐bed current velocity or water depth. 4. The effects of disturbance history were more complex than a simple refugium function of stable patches because algal patterns changed with time since the last disturbance, possibly depending on the successional state of the algal mats.     

Catchment urbanisation and increased benthic algal biomass in streams: linking mechanisms to management

1. Urbanisation is an important cause of eutrophication in waters draining urban areas. We determined whether benthic algal biomass in small streams draining urban areas was explained primarily by small‐scale factors (benthic light, substratum type and nutrient concentrations) within a stream, or by catchment‐scale variables that incorporate the interacting multiple impacts of urbanisation (i.e. variables that describe urban density and the intensity of drainage or septic tank systems). 2. Benthic algal biomass was assessed as chlorophyll a density (chl a) in 16 streams spanning a rural–urban gradient, with both a wide range of urban density and of piped stormwater infrastructure intensity on the eastern fringe of metropolitan Melbourne, Australia. The gradient of urban density among streams was broadly correlated with catchment imperviousness, drainage connection (proportion of impervious areas connected to streams by stormwater pipes), altitude, longitude and median phosphorus concentration. Catchment area, septic tank density, median nitrogen concentration, benthic light (photosynthetically active radiation) and substratum type were not strongly correlated with the urban gradient. 3. Variation in benthic light and substratum type within streams explained a relatively small amount of variation in log chl a (3–11 and 1–13%, respectively) compared with between‐site variation (39–54%). 4. Median chl a was positively correlated with catchment urbanisation, with a large proportion of variance explained jointly (as determined by hierarchical partitioning) by those variables correlated with urban density. Independent of this correlation, the contributions of drainage connection and altitude to the explained variance in chl a were significant. 5. The direct connection of impervious surfaces to streams by stormwater pipes is hypothesised as the main determinant of algal biomass in these streams through its effect on the supply of phosphorus, possibly in interaction with stormwater‐related impacts on grazing fauna. Management of benthic algal biomass in streams of urbanised catchments is likely to be most effective through the application of stormwater management approaches that reduce drainage connection.     

Functional redundancy of stream macroconsumers despite differences in catchment land use

1. The conversion of forested landscapes to agriculture and, increasingly, to suburban and urban development significantly affects the structure and function of both terrestrial and aquatic ecosystems. While a growing body of research is examining how biotic communities change in response to human alteration of landscapes, less is known about how these changes in community structure affect biotic interactions. 2. The objective of this study was to examine top‐down control by macroconsumers (fish and crayfish) across a human‐impacted landscape. We predicted that changes in stream macroconsumers and physicochemical characteristics associated with increased catchment development (e.g. decreased abundance of fish that are obligate benthic invertivores, increased sedimentation) would diminish top‐down control of benthic insects. We expected that effects on algal assemblages would be more variable, with increased top‐down control at sites dominated by algivorous fish and diminished control elsewhere. To test these predictions, we experimentally excluded fish and crayfish from areas of the bed of five streams whose catchments ranged from 100% to <50% forested, and examined the effects of exclusion on benthic insects and algae. 3. Despite cross‐site differences in physical, chemical and biological characteristics, the outcome of our experiments was consistent across five sites representing a range of catchment development. Across all sites, macroconsumers reduced total insect biomass, largely due to decreases in Chironomidae and Hydropsychidae larvae. Macroconsumers also affected algal assemblages, reducing chlorophyll‐a and the proportion of upright and filamentous diatoms (e.g. Melosira, Cymbella) but increasing the proportion of adnate diatoms (e.g. Achnanthes) across all sites. 4. We expected that differences in factors such as macroconsumer assemblage composition, nutrient and light availability and sedimentation would result in variable responses to macroconsumer exclusion in the five streams. Contrary to these expectations, only one response variable (ash‐free dry mass) showed a statistically significant interaction (i.e. site × exclusion) effect. Most responses to exclusion were relatively consistent, suggesting functional redundancy in assemblages of macroconsumers among the sites despite differences in catchment land use.     

Assessing the influence of water and substratum quality on benthic macroinvertebrate communities in a metal-polluted stream: an experimental approach

SUMMARY 1. Field and laboratory experiments were conducted to assess the relative influence of water quality and substratum quality on benthic macroinvertebrate communities in the Animas River, a metal-polluted stream in south-western Colorado (U.S.A.).
2. A community-level in situ toxicity test measured direct effects of Animas River water on benthic invertebrates collected from a reference stream (Elk Creek). The effects of metal-contaminated biofilm were examined by comparing macroinvertebrate colonisation of clean and contaminated substrata placed in Elk Creek. A feeding experiment with the mayfly Baetis tricaudatus Dodds (Ephemeroptera: Baetidae) examined metal bioaccumulation and effects of metal-contaminated biofilm on growth and survival.
3. Animas River water was acutely toxic to most taxa, with greatest effects observed on mayflies (Heptageniidae, Ephemerellidae) and stoneflies (Taeniopterygidae and Capniidae).
4. Although Animas River biofilm was characterised by high concentrations of metals and low algal biomass, most taxa colonised substratum from the reference stream and the Animas River equally. The exceptions were Ephemerellidae, Taeniopterygidae and Simuliidae, which were less abundant on Animas River substratum. Mayflies grazing Animas River biofilm accumulated significantly more metals and showed reduced growth compared with organisms feeding on Elk Creek biofilm.
5. Results of our experiments demonstrated that effects of heavy metals on benthic community structure in the Animas River were complex, and that responses to metals in water and contaminated substratum were species-specific. Predicting recovery of benthic communities following remediation requires an understanding of these species-specific responses.