Nutrient loading and grazing by the minnow Phoxinus erythrogaster shift periphyton abundance and stoichiometry in mesocosms

1. Anthropogenic activities in prairie streams are increasing nutrient inputs and altering stream communities. Understanding the role of large consumers such as fish in regulating periphyton structure and nutritional content is necessary to predict how changing diversity will interact with nutrient enrichment to regulate stream nutrient processing and retention. 2. We characterised the importance of grazing fish on stream nutrient storage and cycling following a simulated flood under different nutrient regimes by crossing six nutrient concentrations with six densities of a grazing minnow (southern redbelly dace, Phoxinus erythrogaster) in large outdoor mesocosms. We measured the biomass and stoichiometry of overstory and understory periphyton layers, the stoichiometry of fish tissue and excretion, and compared fish diet composition with available algal assemblages in pools and riffles to evaluate whether fish were selectively foraging within or among habitats. 3. Model selection indicated nutrient loading and fish density were important to algal composition and periphyton carbon (C): nitrogen (N). Nutrient loading increased algal biomass, favoured diatom growth over green algae and decreased periphyton C : N. Increasing grazer density did not affect biomass and reduced the C : N of overstory, but not understory periphyton. Algal composition of dace diet was correlated with available algae, but there were proportionately more diatoms present in dace guts. We found no correlation between fish egestion/excretion nutrient ratios and nutrient loading or fish density despite varying N content of periphyton. 4. Large grazers and nutrient availability can have a spatially distinct influence at a microhabitat scale on the nutrient status of primary producers in streams.     

Response of Periphytic Algae to Gradients in Nitrogen and Phosphorus in Streamside Mesocosms

In this study we manipulated both nitrogen and phosphorus concentrations in stream mesocosms to develop quantitative relationships between periphytic algal growth rates and peak biomass with inorganic N and P concentrations. Stream water from Harts Run, a 2nd order stream in a pristine catchment, was constantly added to 36 stream-side stream mesocosms in low volumes and then recirculated to reduce nutrient concentrations. Clay tiles were colonized with periphyton in the mesocosms. Nutrients were added to create P and N concentrations ranging from less than Harts Run concentrations to 128 μg SRP l⁻¹ and 1024 μg NO₃-N l⁻¹. Algae and water were sampled every 3 days during colonization until periphyton communities reached peak biomass and then sloughed. Nutrient depletion was substantial in the mesocosms. Algae accumulated in all streams, even streams in which no nutrients were added. Nutrient limitation of algal growth and peak biomass accrual was observed in both low P and low N conditions. The Monod model best explained relationships between P and N concentrations and algal growth and peak biomass. Algal growth was 90% of maximum rates or higher in nutrient concentrations 16 μg SRP l⁻¹ and 86 μg DIN l⁻¹. These saturating concentrations for growth rates were 3–5 times lower than concentrations needed to produce maximum biomass. Modified Monod models using both DIN and SRP were developed to explain algal growth rates and peak biomass, which respectively explained 44 and 70% of the variance in algal response.     

Effect of periphyton biomass on hydraulic characteristics and nutrient cycling in streams

The effect of periphyton biomass on hydraulic characteristics and nutrient cycling was studied in laboratory streams with and without snail herbivores. Hydraulic characteristics, such as average water velocity, dispersion coefficients, and relative volume of transient storage zones (zones of stationary water), were quantified by performing short-term injections of a conservative tracer and fitting an advection-dispersion model to the conservative tracer concentration profile downstream from the injection site. Nutrient cycling was quantified by measuring two indices: (1) uptake rate of phosphorus from stream water normalized to gross primary production (GPP), a surrogate measure of total P demand, and (2) turnover rate of phosphorus in the periphyton matrix. These measures indicate the importance of internal cycling (within the periphyton matrix) in meeting the P demands of periphyton. Dense growths of filamentous diatoms and blue-green algae accumulated in the streams with no snails (high-biomass streams), whereas the periphyton communities in streams with snails consisted almost entirely of a thin layer of basal cells of Stigeoclonium sp. (low-biomass streams). Dispersion coefficients were significantly greater and transient storage zones were significantly larger in the high-biomass streams compared to the low-biomass streams. Rates of GPP-normalized P uptake from water and rates of P turnover in periphyton were significantly lower in high biomass than in low biomass periphyton communities, suggesting that a greater fraction of the P demand was met by recycling in the high biomass communities. Increases in streamwater P concentration significantly increased GPP-normalized P uptake in high biomass communities, suggesting diffusion limitation of nutrient transfer from stream water to algal cells in these communities. Our results demonstrate that accumulations of periphyton biomass can alter the hydraulic characteristics of streams, particularly by increasing transient storage zones, and can increase internal nutrient cycling. They suggest a close coupling of hydraulic characteristics and nutrient cycling processes in stream ecosystems.     

Influence of fishes on macroinvertebrate communities and insect emergence production in intermittent stream refuges

1. Drying intermittent stream networks often have permanent water refuges that are important for recolonisation. These habitats may be hotspots for interactions between fishes and invertebrates as they become isolated, but densities and diversity of fishes in these refuges can be highly variable across time and space.
2. Insect emergence from streams provides energy and nutrient subsidies to riparian habitats. The magnitude of such subsidies may be influenced by in-stream predators such as fishes.
3. We examined whether benthic macroinvertebrate communities, emerging adult insects, and algal biomass in permanent grassland stream pools differed among sites with naturally varying densities of fishes. We also manipulated fish densities in a mesocosm experiment to address how fishes might affect colonisation during recovery from hydrologic disturbance.
4. Fish biomass had a negative impact on invertebrate abundance, but not biomass or taxa richness, in natural pools. Total fish biomass was not correlated with total insect emergence in natural pools, but orangethroat darter (Etheostoma spectabile) biomass was inversely correlated with emerging Chironomidae biomass and individual midge body size. The interaction in our models between predatory fish biomass and date suggested that fishes may also delay insect emergence from natural pools, altering the timing of aquatic–terrestrial subsidies.
5. There was an increase over time in algal biomass (chlorophyll-a) in mesocosms, but this did not differ among fish density treatments. Regardless, fish presence in mesocosms reduced the abundance of colonising insects and total invertebrate biomass. Mesocosm invertebrate communities in treatments without fishes were characterised by more Chironomidae, Culicidae, and Corduliidae.
6. Results suggest that fishes influence invertebrates in habitats that represent important refuges during hydrologic disturbance, hot spots for subsidy exports to riparian food webs, and source areas for colonists during recovery from hydrologic disturbance. Fish effects in these systems include decreasing invertebrate abundance, shifting community structure, and altering patterns of invertebrate emergence and colonisation.

     

Plant—herbivore interactions in streams near Mount St Helens

1. In four separate field experiments near Mount St Helens (Washington, U.S.A.) during 1986, the grazing effects of two large benthic herbivores, tadpoles of the tailed frog Ascaphus truei and larvae of the caddisfly Dicosmoecus gilvipes, were investigated using streamside channels and in-stream manipulations. In the experimental channels, abundances of periphyton and small benthic invertebrates declined significantly with increasing density of these larger herbivores. 2. In eleven small, high-gradient streams affected to varying degrees by the May 1980 eruption, in-stream platforms were used to reduce grazing by A, truei tadpoles on tile substrates. Single platforms erected in each tributary and compared to grazed controls revealed only minor grazing effects, and no significant differences among streams varying in disturbance intensity (and, consequently, tadpole density). However, results probably were confounded by high variability among streams in factors other than tadpole abundance. 3. Grazing effects were further examined in two unshaded streams with different tadpole densities, using five platforms per stream. In the stream with five tadpoles m^?2, grazing reduced periphyton biomass by 98% and chlorophyll a by 82%. In the stream lacking tadpoles, no significant grazing effects were revealed. Low algal abundance on both platforms and controls, and high invertebrate density in that stream (c. 30000m^?2) suggests that grazing by small, vagile invertebrates was approximately equivalent to that of tadpoles. 4. The influence of large benthic herbivores on algal and invertebrate communities in streams of Mount St Helens can be important, but reponses vary spatially in relation to stream disturbance history, local environmental factors, and herbivore distributional patterns and abundance.     

Ecosystem effects of water column minnows in experimental streams

We used red shiner (Cyprinella lutrensis) as a model to examine ecosystem effects of water column stream minnows (Cyprinidae) in experimental streams. Benthic primary productivity, benthic invertebrate abundance, water column nutrient concentrations, size distribution of benthic particulate organic matter (BPOM), and sedimentation rates were measured across a range of fish densities (0–26.6 fish m^–2) over a 35-day period. In addition, effects of fish density on algal standing crop and benthic invertebrates in experimental streams were examined over a longer time span (156 and 203 days). After 35 days, benthic primary productivity was positively associated with fish density, with an approximate three-fold increase in productivity between experimental streams stocked with no fish and those with 26.6 fish m^–2. No effects on other ecosystem properties were detected after 35 days. Additionally, there was no effect on algal standing crop after 156 or 203 days and no effect on benthic invertebrates after 203 days. Because red shiners fed primarily on terrestrial insects, this experiment suggests that water column minnows can affect primary productivity in streams by transporting nutrients from terrestrial sources to the benthic compartment of the ecosystem. However, this effect may only be important in streams or during periods when nutrients are limiting.     

Experimental increases and reductions of light to streams: effects on periphyton and macroinvertebrate assemblages in a coniferous forest landscape

Increased light reaching streams as a result of riparian vegetation management is often thought to be responsible for enhanced algal productivity. However, concomitant changes in nutrients and other physical processes confound that interpretation. We manipulated light in two separate experiments to test the role of light as a controlling factor for periphyton productivity and biomass, and to observe invertebrate responses in small streams in central British Columbia, Canada. We did this by adding artificial light to reaches of three forested streams, and in a second experiment we used shadecloth to cover reaches of two streams flowing through clearcuts. Periphyton growth, productivity and composition, and macroinvertebrate benthic densities were contrasted with control reaches within the same streams. Gross primary production (GPP) was increased at least 31% by light addition to forested streams. Periphyton biomass was higher under light additions, but only significantly so in one of the streams. In one stream grazers increased along with the periphyton response, whilst in the other two lit streams invertebrates, including grazers, decreased with increased light. The shading significantly reduced GPP to about 11% of that in clearcut sections, but failed to produce any significant responses in either periphyton standing stock or invertebrates in the clearcut streams. Measures of algal production and biomass responded as predicted; however, invertebrate responses to increased and decreased light were idiosyncratic amongst streams, perhaps indicating lagged responses and limitation by other resources.     

Mechanisms of benthic algal recovery following spates: comparison of simulated and natural events

We conducted a manipulative field experiment to examine individual and interactive effects of scour and short-term nutrient enrichment (4 h exposure) on postspate recovery of benthic algae in a desert stream. We then compared recovery from these simulated-spate conditions to algal recovery patterns following a natural spate that increased water-column nutrient levels for 2 weeks. That event differentially scoured communities on artificial substrata in place for a long-term experiment, significantly reducing biomass in 49-day-old communities but causing no significant reduction of biomass in older, 133-day-old communities. Thus, we were able to examine recovery of scoured and non-scoured benthic algal communities under natural post-spate conditions. Both natural and simulated spates reduced actual and relative abundances of diatoms within communities. In the manipulative experiment, scoured communities accrued biomass more rapidly than those not subjected to scour, but short-term enrichment had not effect. Accrual of diatoms and green algae was stimulated by the scour manipulations, while cyanobacteria maintained equal rates of growth in all treatments. Following the natural spate, diatom and green-algal densities increased in scoured communities, but recovery of algal biomass was slow on both scoured and non-scoured substrata, primarily because cyanobacteria, the dominant algal group on all tiles, did not increase under exposure to highly nitrate-enriched waters. Rates of algal cell accrual were inversely correlated with the amount of algal biomass present at the start of a recovery sequence. Algal immigration rates measured immediately after the natural spate and during an interflood period in the same season did not differ, indicating that the algal drift pool was not augmented by disturbance. Benthic algal recovery following spates is strongly influenced by the degree of scour generated by the event, but recovery patterns are also affected by the length of post-spate enrichment and the taxonomic composition of the affected community.     

Effects of food‐web structure on periphyton stoichiometry in eutrophic lakes: a mesocosm study

1. Aquatic herbivores typically have much higher concentrations of nutrients (e.g. N and P) in their tissues than there is in the food they eat. These stoichiometric differences can cause herbivores to be limited by the elemental quality of their food, which could affect, in turn, the structure of consumer communities and even alter key ecosystem processes. 2. In streams and in the littoral zone of shallow lakes, periphyton is an important food resource for benthic animals. Studying the elemental composition of periphyton may help us to understand food‐web structure, and any reciprocal effect of this structure on periphyton stoichiometry. 3. To understand how alterations in the food‐web structure affect the elemental composition of periphyton in a eutrophic lake, we carried out a long‐term experiment (14 months) in large‐scale mesocosms (40 m³), in which we manipulated food‐web structure, and which were dominated either by planktivorous fish (Rutilus rutilus) or herbivorous invertebrates (without fish). Periphyton was sampled monthly at three depths (0.5, 1.5 and 2.5 m) to determine its biomass and elemental composition (C/N/P ratio). Food‐web structure, physical and chemical parameters were monitored throughout the experiment. 4. Fish had indirect positive effect on periphyton biomass, leading to twofold higher levels than in herbivore‐dominated mesocosms. This result was probably due to control of benthic consumers by fish, suggesting a strong top–down control on periphyton by their consumers in fishless enclosures. 5. The elemental ratios C/P and C/N were lower in deep water in both treatments, mainly mediated by light availability, in accordance with the light/nutrient ratio hypothesis. These ratios were also lower in fishless treatments, probably due to increases in inorganic nutrient availability and grazing pressure in herbivore‐dominated systems. During winter, periphyton elemental composition was similar in both treatments, and was unrelated to inorganic nutrient availability. 6. These results indicate that any alteration of food‐web structure in lakes, such as in biomanipulation experiments, is likely to modify both the biomass and elemental quality of periphyton. Resultant effects on the consumers of periphyton and macrophytes could play a key role in the success of biomanipulations and should be taken into account in further studies.     

Exotic crayfish in a brown water stream: effects on juvenile trout, invertebrates and algae

SUMMARY 1. The impact of the introduced omnivorous signal crayfish (Pacifastacus leniusculus) on trout fry, macroinvertebrates and algae was evaluated in a brown water stream in southern Sweden using in situ enclosures. We also examined the gut content of all surviving crayfish in the enclosures. Two crayfish densities in addition to a control without crayfish were used in replicate enclosures (1.26 m²) in a 1‐month experiment. Additionally, 20 trout fry (Salmo trutta) were stocked in each enclosure to assess the effects of crayfish on trout survival and growth. 2. Detritus was the most common food item in crayfish guts. Animal fragments were also frequent while algae and macrophytes were scarcer. Crayfish exuviae were found in crayfish guts, but the frequency of cannibalism was low. 3. Trout survival in enclosures was positively related to water velocity but was unaffected by crayfish. 4. Total invertebrate biomass and taxon richness were lower in crayfish treatments. The biomass of all predatory invertebrate taxa was reduced but only three of six non‐predatory taxa were reduced in the crayfish treatments. 5. Epiphytic algal biomass (measured as chlorophyll a, on plastic strips) was not related to crayfish density, whereas the biomass of epilithic algae (measured as chlorophyll a) was enhanced by high water velocity and high crayfish density. The latter was possibly mediated via improved light and nutrient conditions, as active crayfish re‐suspend and/or remove detritus and senescent algal cells during periods of low water velocity. 6. We conclude that the introduced signal crayfish may affect stream communities directly and indirectly. Invaded communities will have reduced macroinvertebrate taxon richness and the signal crayfish will replace vulnerable invertebrate predators such as leeches. In streams that transport large amounts of sediment or organic matter, a high density of crayfish is likely to enhance benthic algal production through physical activity rather than via trophic effects.     

Effects of dissolved organic matter and ultraviolet radiation on the accrual, stoichiometry and algal taxonomy of stream periphyton

1. We investigated the effects of dissolved organic matter (DOM) and ultraviolet‐B (UVB) radiation on periphyton during a 30‐day experiment in grazer‐free, outdoor artificial streams. We established high [10–12 mg carbon (C) L⁻¹] and low (3–5 mg C L⁻¹) concentrations of DOM in artificial streams exposed to or shielded from ambient UVB radiation. Periphyton was sampled weekly for ash‐free dry mass (AFDM), chlorophyll (chl) a , algal biovolume, elemental composition [C, nitrogen (N) and phosphorus (P)], and algal taxonomic composition. 2. Regardless of the UVB environment, increased DOM concentration caused greater periphyton AFDM, chl a and total C content during the experiment. Increased DOM also significantly increased periphyton C : P and N : P (but not C : N) ratios throughout the experiment. Algal taxonomic composition was strongly affected by elevated stream DOM concentrations; some algal taxa increased and some decreased in biomass and prevalence in artificial streams receiving DOM additions. UVB removal, on the other hand, did not strongly affect periphyton biomass, elemental composition or algal taxonomic composition for most of the experiment. 3. Our results show strong effects of DOM concentration but few, if any, effects of UVB radiation on periphyton biomass, elemental composition and algal taxonomic composition. The effects of DOM may have resulted from its absorption of UVA radiation, or more likely, its provision of organic C and nutrients to microbial communities. The strong effects of DOM on periphyton biomass and elemental composition indicate that they potentially play a key role in food web dynamics and ecosystem processes in forested streams.     

Biotic disturbance and benthic community dynamics in salmon-bearing streams

1. Organisms can impact ecosystems via multiple pathways, often with positive and negative impacts on inhabitants. Understanding the context dependency of these types of impacts remains challenging. For example, organisms may perform different functions at different densities. 2. Anadromous salmon accumulate > 99% of their lifetime growth in marine ecosystems, and then return to spawn, often at high densities, in relatively confined freshwaters. While previous research has focused on how salmon nutrients can fertilize benthic communities, we examined how an ecosystem engineer, sockeye salmon Oncorhynchus nerka, influences seasonal dynamics of stream benthic communities through their nest-digging activities in south-western Alaska, USA. Benthic invertebrate and algal abundance were quantified every 7-14 days during the open water seasons of 10 streams in riffle and run habitats across multiple years, leading to 25 different stream-year combinations that spanned a large gradient of salmon density. 3. In streams with few or no salmon, benthic algal and insect biomass were fairly constant throughout the season. However, in streams with more than 0.1 salmon m(-2), algal and insect biomass decreased by an average of 75-85% during salmon spawning. Algal biomass recovered quickly following salmon disturbance, occasionally reaching pre-salmon biomass. In contrast, in streams with more than 0.1 salmon m(-2), aquatic insect populations did not recover to pre-salmon levels within the same season. We observed no positive impacts of salmon on algae or insects via fertilization from carcass nutrients. 4. Salmon, when their populations exceed thresholds in spawning density, are an important component of stream disturbance regimes and influence seasonal dynamics of benthic communities. Human activities that drive salmon densities below threshold densities, as has likely happened in many streams, will lead to altered seasonal dynamics of stream communities. Human activities that alter animal populations that are sources of biogenic disturbance can result in shifts in community dynamics.     

The introduced bivalve <Emphasis Type="Italic">Limnoperna fortunei</Emphasis> boosts <Emphasis Type="Italic">Microcystis</Emphasis> growth in Salto Grande reservoir (Argentina): evidence from mesocosm experiments

In order to assess the effects of the introduced bivalve Limnoperna fortunei on water-column properties of Salto Grande reservoir, experiments were conducted using six 400 L mesocosms: 2 with 100 mussels, 2 with 300 mussels, and 2 controls (without mussels). At 0, 1, 2, 3, 7, 14, 21, 28, and 35 days we measured nutrient and chlorophyll a concentrations, counted and identified the phytoplankton, and estimated the density, size, and number of cells of the colonies of Microcystis spp. Cumulative periphyton growth and total accumulated sediments were assessed in all enclosures at the end of the experiment. Throughout the experiment, in the controls ammonia and phosphates dropped to near zero, whereas in the mesocosms with L. fortunei they increased two- to tenfold. Nitrates decreased in all mesocosms. In the presence of the mussel, chlorophyll a and algal cells dropped until day 3 increasing thereafter, whereas in the controls they increased from day 0. Periphyton growth and sediment accumulation were significantly higher in the mesocosms with mussels that in the controls. Cell density, proportion of colonial cells and colony size of Microcystis spp. increased in all enclosures, but these increases were dramatically (and very significantly) higher in enclosures with 100 and, especially, with 300 mussels, than in the controls. Our results indicate that L. fortunei modifies nutrient concentrations and proportions, and promotes aggregation of solitary Microcystis spp. cells into colonies; both these effects can favor blooms of this often noxious cyanobacteria.     

Predicting invertebrate diversity from disturbance regimes in forest streams

The link between substrate disturbance and stream invertebrate species richness is often complicated by the fact that substrate disturbance removes both invertebrates and periphyton (a potential food source). It is never clear whether disturbance acts directly on species diversity by removing animals or indirectly by reducing one of their food sources. To examine this relationship invertebrate diversity patterns were examined in 25 forest streams in Urewera National Park, New Zealand, where light attenuation from the forest canopy was postulated to limit periphyton biomass and remove the confounding influence of periphyton on the link between substrate disturbance and invertebrate diversity. Invertebrate species richness declined linearly with increasing substrate disturbance. Although periphyton biomass was comparatively low, species richness was more strongly related to periphyton biomass than to any disturbance measure. The highly mobile nature and terrestrial reproductive stage of many lotic invertebrates suggest that colonisation dynamics may have a more important influence on diversity patterns than monopolisation of resources for population growth. Although both the intermediate disturbance hypothesis and the dynamic equilibrium model encompass colonisation as a critical determinant of diversity both models also require a trade-off between the colonising and competitive ability of individual species; a phenomenon which does not appear to occur widely in lotic communities. Rather, it is postulated that resource levels will set an upper limit to the species richness of a benthic community that can be achieved through colonisation of taxa in the absence of disturbance, while disturbance removes taxa and resets the colonisation process.     

Local adaptation of fish consumers alters primary production through changes in algal community composition and diversity

Ecological research has focused on understanding how changes in consumer abundance affects community structure and ecosystem processes. However, there is increasing evidence that evolutionary changes in consumers can also alter community structure and ecosystem processes. Typically, the effects of consumer phenotype on communities and ecosystem processes are measured as net effects that integrate numerous ecological pathways. Here, we analyze new data from experimental manipulations of Trinidadian guppy Poecilia reticulata presence, density and phenotype to examine how effects on the algal community cause changes in gross‐primary production (GPP). We combine analytical tools borrowed from path analysis with experimental exclosures in mesocosms to separate the ecological and evolutionary effects of guppies into direct and indirect components. We show that the evolutionary effects of guppy phenotype act through different ecological pathways than the effects of guppy presence and density on GPP. As reported in previous studies that used a different measure of algal biomass, adding guppies and doubling their densities decreased algal biovolume through direct effects. In contrast to these previously reported results, exchanging guppy phenotypes that live without predators for phenotypes that live with predators did not affect algal biovolume. Instead, guppies from populations that live with predators increased the diversity of algal species and increased GPP compared to guppies that live without predators. These changes in the algal community were driven primarily by guppy phenotypes that live with predators—algal communities in mesocosms without fish were similar to those with guppies from predator‐free locations, but both were different from mesocosms with guppies from populations that live with predators. Changes in the algal community were driven directly by differences in foraging behavior between the two consumer phenotypes. We reconcile these results with our previous findings, thereby enhancing our understanding of the relationship between ecological and evolutionary processes.     

Influence of macroconsumers, stream position, and nutrient gradients on invertebrate assemblage development following flooding in intermittent prairie streams

Climate change in the US Great Plains is expected to result in less frequent but more severe floods. This will affect hydrologic cycles, stream organisms, and ultimately ecosystem structure and function. We examined factors influencing invertebrate assemblages following flooding in 3 reaches (20 pools) of Kings Creek, an intermittent prairie stream on the Konza Prairie Biological Station, using replicated macroconsumer enclosures (fishless, dace, shiners, ambient). Invertebrate densities and biomass increased rapidly following scouring, including rapid colonizing taxa and relatively long-lived taxa, but macroconsumers had no significant effects. Rather, distance, which was negatively correlated with the concentration of dissolved inorganic nitrogen, from the downstream confluence with a larger stream significantly influenced assemblage structure, with higher richness and greater nutrient concentrations closer to the confluence. Results support previous findings that recovery patterns following flooding in this grassland stream are strongly influenced by proximity to refuges. Furthermore, physical rather than biological factors appear more influential in structuring invertebrate assemblages in these frequently disturbed systems. Predicted increases in the intensity and duration of hydrologic disturbances will increase direct impacts on stream communities, relative to indirect effects through potential changes in macroconsumer communities. Human activities that alter refuges may further impede recovery following hydrologic disturbances.     

Effects of forestry on the thermal habitat of Dolly Varden (<Emphasis Type="Italic">Salvelinus malma</Emphasis>)

Forestry activities in riparian areas are known to affect stream communities considerably. Not only do riparian deforestation resulting from agriculture or urbanization developments affect stream communities but extensive commercial plantation and forestry practices can alter stream environments adjacent to remaining, intact or secondary forests. Because forestry often includes the construction of logging roads through the riparian zone, this can directly degrade stream environments. Twelve streams in the Shiretoko Peninsula, Hokkaido were investigated so as to determine the effects of forestry practices on stream temperature, periphyton biomass, grazer (benthic invertebrates) biomass and Dolly Varden (Salvelinus malma Walbaum) biomass. The greater the proportion of planted area in the catchment, the higher the stream temperature. Stream temperature directly affects periphyton biomass and Dolly Varden biomass negatively. Neither stream temperature nor periphyton biomass predicted grazer biomass, whereas a positive correlation was found between grazer biomass and Dolly Varden biomass that forage on invertebrates. The overall results indicated that Dolly Varden in the Shiretoko streams were negatively affected by forestry practices and the resultant stream temperature increases. Without effective future riparian forest management, the complex effects of both riparian disturbance and ongoing global warming could further reduce Dolly Varden populations in the region.     

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

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

Nutrients, algae and grazers in some British streams of contrasting pH

1. The relationship between algal biomass accumulation, invertebrate colonization, and stream-water pH was investigated in seven streams in three regions of England and Wales. Possible nutrient limitation of algal production at all sites was examined with diffusion substrata. 2. Periphyton assemblages on experimental substrata after 30 days were dominated by diatoms, notably Eunotia spp., at all sites. Algal pigment concentration (chlorophyll a and phaeopigments) was not correlated with stream-water pH, and mean concentrations on control (unenriched) substrata ranged from 0.08 to 1.94 μg cm^?2. 3. The growth response of periphyton to nutrient additions was site specific. Algal production was stimulated by nutrient additions at sites in the English Lake District and Llyn Brianne (south-west Wales), but not in the Ashdown Forest (southern England). 4. Larval Chironomidae were the main invertebrates retrieved from substrata at all sites. Within all three regions, larval abundance was positively related to algal pigment concentration (biomass). Abundance of the stonefly Nemurella pictetii was also positively correlated with algal biomass at the one site where it occurred. 5. Our results indicate that epilithic algal production in small, oligotrophic streams is unlikely to be determined primarily by pH. Neither do they support the view that an absence of grazers from acid streams is necessarily due to an inadequate food supply.     

Interaction between disturbance and primary productivity in determining stream invertebrate diversity

Invertebrate diversity patterns were examined in 10 streams that differed in substrate disturbance rates, in Taranaki, New Zealand, between April 1999 and January 2000. Two sites on each stream were sampled, one under native forest canopy where light was postulated to limit periphyton growth and a similar site 225–3800 m downstream in open grassland. Periphyton biomass was considerably higher at open stable sites than at closed or unstable sites. Associated with the higher algal biomass, species number and total abundance of animals were higher at open canopy sites. Species number exhibited a negative linear relationship with disturbance but only at open sites. In contrast, rarefied species richness exhibited a negative linear relationship with disturbance at both open and closed sites. This was a result of communities at the more disturbed sites being numerically dominated by only a few taxa compared to the more evenly distributed communities at stable sites. The observed patterns provide little support for contemporary diversity disturbance models but suggest diversity of invertebrates in streams is a function of time since the last disturbance, mediated through recovery of the food base in autotrophic streams.