Effects of floods versus low flows on invertebrates in a New Zealand gravel-bed river

1. Floods and low flows are hydrological events that influence river ecosystems, but few studies have compared their relative importance in structuring invertebrate communities. Invertebrates were sampled in riffles and runs at eight sites along 40 km of a New Zealand gravel‐bed river every 1–3 months over 2.5 years, during which time a number of large flood and low flow events occurred. Flows were high in winter and spring, and low in summer and autumn. Four flow‐related variables were calculated from hydrological data: flow on the day of sampling (Q_sample), maximum and minimum flow between successive samples (Q_max and Q_min, respectively), and the number of days since the last bed‐moving flood (N_days). 2. The invertebrate community was summarised by relative densities of the 19 most abundant taxa and four biotic metrics [total abundance, taxon richness, the number of Ephemeroptera, Plecoptera and Trichoptera taxa (i.e. EPT richness), and per cent EPT]. Invertebrate density fluctuated greatly, and was high in summer and autumn, and low during winter and spring. Stepwise multiple regression (SMR) analysis was used to investigate relationships between the invertebrate community and season, flow, habitat and water temperature. 3. Seasonal variables were included in almost 50% of the SMR models, while flow‐related variables were included in >75% of models. Densities of many taxa were negatively correlated to Q_min and Q_max, and positively correlated to N_days, suggesting that while high flows reduced invertebrate densities, densities recovered with increasing time following a flood. Although season and flow were confounded in this study, many of the taxa analysed display little seasonal variation in abundance, suggesting that flow‐related variables were more important in structuring communities than seasonal changes in density associated with life‐cycles. 4. Five discrete flood and low flow events were identified and changes to invertebrate communities before and after these events examined. Invertebrate densities decreased more commonly after floods than after low flows, and there was a significant positive relationship between the number of taxa showing reductions in density and flood magnitude. Densities of most invertebrates either remained unchanged, or increased after low flow events, except for four taxa whose densities declined after a very long period (up to 9 months) of low flow. This decline was attributed to autogenic sloughing of thick periphyton communities and subsequent loss of habitat for these taxa. 5. Invertebrate communities changed more after floods and the degree of change was proportional to flood magnitude. Community similarity increased with increasing time since the last disturbance, suggesting that the longer stable flows lasted, the less the community changed. These results suggest that invertebrate communities in the Waipara River were controlled by both floods and low flows, but that the relative effects of floods were greater than even extended periods of extreme low flow. 6. Hydraulic conditions in riffles and runs were measured throughout the study. Riffles had consistently faster velocities, but were shallower and narrower than runs at all measured flows. Invertebrate density in riffles was expressed as a percentage of total density and regressed against the flow‐related variables to see whether invertebrate locations changed according to flow. Significant negative relationships were observed between the per cent density of common taxa in riffles and Q_sample, Q_max and Q_min. This result suggests either that these animals actively drifted into areas of faster velocity during low flows, or that their densities within riffles increased as the width of these habitats declined.     

Periphyton biomass and ecological stoichiometry in streams within an urban to rural land-use gradient

This study examined the effects land use on biomass and ecological stoichiometry of periphyton in 36 streams in southeastern New York State (USA). We quantified in-stream and land-use variables along a N–S land-use gradient at varying distances from New York City (NYC). Streams draining different landscapes had fundamentally different physical, chemical, and biological properties. Human population density significantly decreased (r = −0.739; P < 0.00001), while % agricultural land significantly increased (r = 0.347; P = 0.0379) with northing. Turbidity, temperature, conductivity, and dissolved Mg, Ca, SRP, pH, DOC, and Si significantly increased in more urban locations, but NO₃ ⁻ and NH₄ ⁺ did vary not significantly along the gradient. Periphyton biomass (as AFDM and Chl-a) in rural streams averaged one-third to one-fifth that measured in urban locations. Periphyton biomass in urban streams averaged 18.8 ± 6.0 g/m² AFDM and 75.6 ± 28.5 mg/m² Chl-a. Urban Chl-a levels ranging between 100 and 200 mg/m², are comparable to quantities measured in polluted agricultural streams in other regions, but in our study area was not correlated with % agricultural land. Periphyton nutrient content also varied widely; algal C varied >20-fold (0.06–1.7 μmol/mm²) while N and P content varied >6-fold among sites. Algal C, N, and P correlated negatively with distance from NYC, suggesting that periphyton in urban streams may provide greater nutrition for benthic consumers. C:N ratios averaged 7.6 among streams, with 91% very close to 7.5, a value suggested as the optimum for algal growth. In contrast, periphyton C:P ratios ranged from 122 to >700 (mean = 248, twice Redfield). Algal-P concentrations were significantly greater in urban streams, but data suggest algal growth was P-limited in most streams regardless of degree of urbanization. GIS models indicate that land-use effects did not easily fit into strict categories, but varied continuously from rural to urban conditions. We propose that the gradient approach is the most effective method to characterize the influence of land use and urbanization on periphyton and stream function.     

THE RELATIONSHIP OF LIPIDS WITH LIGHT AND CHLOROPHYLL MEASUREMENTS IN FRESHWATER ALGAE AND PERIPHYTON1

Lipid content and lipid class composition were determined in stream periphyton and the filamentous green algae Cladophora sp. and Spirogyra sp, Sterols and phospholipids were compared to chlorophyll a (chl a) as predictors of biomass for stream periphyton and algae. Chlorophyll a, phospholipids, and sterols were each highly correlated with ash-free dry mass (AFDM) (r² > 0.98). Stream periphyton exposed naturally to high light (HL) and low light (LL) had chl a concentrations (μg chl a-mg^?1AFDM) of 7.9± 0.7 and 12.4 ± 2.9, respectively, while the sterol concentrations of these HL and LL stream periphyton (1.6 ± 0.4) were not significantly different (P > 0.05). Periphyton exposed to an irradiance of 300 μmol photons·m^?2s^?1 in the laboratory for 60 h had 5.6 ± 0.55 μg chl a·mg^?1 AFDM, but the same periphyton exposed to 2% incident light for the same amount of time had 11.0 ± 0.56 μg chl a·mg^?1 AFDM. Sterol concentrations in these periphyton communities remained unchanged (1.5 ± 0.3 μg·mg^?1AFDM), Similar results (i.e. changes in chl a but stability of sterol concentrations in response to irradiance changes) were also found for Cladophora and Spirogyra in laboratory experiments. Sterols can be quantified rapidly from a few milligrams of algae and appear to be a useful predictor of eukaryote biomass, whereas cellular levels of chl a vary substantially with light conditions. Phospholipids (or phospholipid fatty acids) are considered to be a reliable measure of viable microbial biomass. Nevertheless, phospholipid content varied substantially and unpredictably among algae and periphyton under different light regimes. Irradiance also had a significant effect on storage lipids: HL Cladophora and HL periphyton had 2 × and 5 × greater concentrations of triacylglycerols, respectively, compared to their LL forms. HL and LL algae also differed in the concentration of several major fatty acids. These light-induced changes in algal lipids and fatty acids have important implications for grazers.     

Phosphorus and light colimit periphyton growth at subsaturating irradiances

1. This study investigated the combined effects of light and phosphorus on the growth and phosphorus content of periphyton. To investigate the potential for colimitation of algal growth by these two resources, diatom‐dominated periphyton communities in large flow‐through laboratory streams were exposed under controlled conditions to simultaneous gradients of light and phosphorus. 2. Periphyton growth rate was predictably light‐limited by the subsaturating irradiances (12–88 μmol photons m^?2 s^?1) used in this experiment. However, phosphorus concentration also limited growth rate: growth increased hyperbolically with increasing soluble reactive phosphorus (SRP), reaching a threshold of growth saturation between 22 and 82 μg L^?1. 3. Periphyton phosphorus content was strongly and nonlinearly related with SRP, reaching a maximum at 82 μg L^?1 SRP. Contrary to the Light : Nutrient Hypothesis, periphyton phosphorus content did not decrease with increasing light, even at the lowest concentrations of SRP. Periphyton phosphorus was highly correlated with periphyton growth rate (Spearman's ρ = 0.63, P < 0.005). 4. Multiple regression analysis reinforced evidence of simultaneous light and phosphorus limitation. Both light and periphyton phosphorus content were significant variables in multiple regressions with growth parameters as dependent variables. Light alone accounted for 67% of the variance in periphyton biomass, and the addition of periphyton phosphorus as an additional independent variable increased the total amount of variance explained to 81%. 5. Our results did not support the hypothesis that extra phosphorus is required for photoacclimation to low light levels. Rather, the effect of additional phosphorus may have been to accommodate increased requirements for P‐rich ribosomal RNA when growth was stimulated by increased light. The potential colimitation of periphyton growth by phosphorus and light at subsaturating irradiances has important implications in both theoretical and applied aquatic ecology.     

Effects of a top invertebrate predator (<Emphasis Type="Italic">Dytiscus alaskanus</Emphasis>; Coleoptera: Dytiscidae) on fishless pond ecosystems

We investigated the predatory effects of Dytiscus alaskanus, a large predaceous diving beetle, on the biomass, species composition and diversity of fishless pond communities. The effects were tested using presence and absence treatments of D. alaskanus in 24 mesocosms distributed among six ponds. We sampled phytoplankton, zooplankton and macroinvertebrates every two weeks for a six week period. Periphyton was sampled from the mesocosm walls on the final day. Total macroinvertebrate biomass decreased in the presence of dytiscids while species richness was not affected. Macroinvertebrate predators, snails and Gammarus lacustris decreased in the dytiscid treatments. Laboratory feeding experiments confirmed feeding preferences consistent with the mesocosm results. Periphyton biomass was six times greater in the dytiscid enclosures, concomitant with the decreased grazing by gastropods and other invertebrate primary consumers indicating a benthic trophic cascade. Top–down effects of dytiscids on other predatory invertebrates led to increased total zooplankton biomass, largely due to increased abundances of large and small cladocerans. Zooplankton species richness increased in the dytiscid enclosures. Inconsistent with trophic cascade theory, phytoplankton did not respond to top–down effects of D. alaskanus within the study period. Overall, the results show D. alaskanus predation caused trophic effects via two distinct food chains, a dytiscid–snail–periphyton trophic cascade, and a dytiscid–predatory macroinvertebrates–zooplankton partial trophic cascade.     

Stream food web response to a salmon carcass analogue addition in two central Idaho, U.S.A. streams

1. Pacific salmon and steelhead once contributed large amounts of marine‐derived carbon, nitrogen and phosphorus to freshwater ecosystems in the Pacific Northwest of the United States of America (California, Oregon, Washington and Idaho). Declines in historically abundant anadromous salmonid populations represent a significant loss of returning nutrients across a large spatial scale. Recently, a manufactured salmon carcass analogue was developed and tested as a safe and effective method of delivering nutrients to freshwater and linked riparian ecosystems where marine‐derived nutrients have been reduced or eliminated. 2. We compared four streams: two reference and two treatment streams using salmon carcass analogue(s) (SCA) as a treatment. Response variables measured included: surface streamwater chemistry; nutrient limitation status; carbon and nitrogen stable isotopes; periphyton chlorophyll a and ash‐free dry mass (AFDM); macroinvertebrate density and biomass; and leaf litter decomposition rates. Within each stream, upstream reference and downstream treatment reaches were sampled 1 year before, during, and 1 year after the addition of SCA. 3. Periphyton chlorophyll a and AFDM and macroinvertebrate biomass were significantly higher in stream reaches treated with SCA. Enriched stable isotope (δ¹⁵N) signatures were observed in periphyton and macroinvertebrate samples collected from treatment reaches in both treatment streams, indicating trophic transfer from SCA to consumers. Densities of Ephemerellidae, Elmidae and Brachycentridae were significantly higher in treatment reaches. Macroinvertebrate community composition and structure, as measured by taxonomic richness and diversity, did not appear to respond significantly to SCA treatment. Leaf breakdown rates were variable among treatment streams: significantly higher in one stream treatment reach but not the other. Salmon carcass analogue treatments had no detectable effect on measured water chemistry variables. 4. Our results suggest that SCA addition successfully increased periphyton and macroinvertebrate biomass with no detectable response in streamwater nutrient concentrations. Correspondingly, no change in nutrient limitation status was detected based on dissolved inorganic nitrogen to soluble reactive phosphorus ratios (DIN/SRP) and nutrient‐diffusing substrata experiments. Salmon carcass analogues appear to increase freshwater productivity. 5. Salmon carcass analogues represent a pathogen‐free nutrient enhancement tool that mimics natural trophic transfer pathways, can be manufactured using recycled fish products, and is easily transported; however, salmon carcass analogues should not be viewed as a replacement for naturally spawning salmon and the important ecological processes they provide.     

Taxonomic and functional homogenisation of macroinvertebrate communities in recently intermittent Alpine watercourses

1. Mountain streams in southwestern European Alps are currently shifting from perennial to intermittent flow due to the combined effects of climate change and local anthropogenic pressures. Given that flow intermittency is a recently documented phenomenon in the Alps, only scattered studies have investigated functional and taxonomical diversity of benthic invertebrate communities in recently intermittent Alpine streams.
2. We used a hierarchical sampling design to investigate patterns in taxonomic and functional diversity of benthic invertebrate communities in 13 recently intermittent Alpine streams in north-west Italy. in April 2017, we sampled benthic communities in two reaches of each stream with different hydrological conditions: a control reach, with permanent flow; and an intermittent reach, which recently experienced non-flow periods in summer.
3. We tested for the response of taxonomic richness at multiple spatial scales by partitioning total diversity into the average richness of local communities and the richness due to variation among local communities both within and among reaches. By partitioning total diversity (γ) into its local (α) and turnover (β) components we showed a decrease in local and regional species richness both within and among reaches, whereas variation among communities was significantly lower in intermittent reaches at the reach scale only.
4. The analysis of multidimensional trait space of macroinvertebrate communities in reaches with different hydrological conditions revealed a significant reduction of functional diversity, dispersion, and evenness in intermittent reaches. There was trait overdispersion in intermittent reaches, as these hosted both typical Alpine taxa and organisms adapted to flow intermittency. In particular, we observed the replacement of taxa with aquatic respiration and those preferring medium- to fast-flowing oligotrophic waters by taxa adapted to lentic habitats, air breathing and with larval dormancy phases.
5. These results indicate that recent flow intermittency has caused drastic changes in benthic invertebrate communities in Alpine streams. Our work highlights the importance of integrating taxonomic and functional diversity to thoroughly assess the impacts of flow intermittency.

     

Periphyton biomass dynamics in gravel bed rivers: the relative effects of flows and nutrients

SUMMARY. 1. Periphyton chlorophyll a and ash free dry weight (AFDW) were monitored in nine rivers to examine the relative importance of flows and nutrients for regulating periphyton biomass in gravel bed rivers. 2. Mean annual flows in the rivers ranged from 0.94 to 169 m³ s^?1, mean dissolved reactive phophorus (DRP) from 1.3 to 68 μ g 1^?1, periphytic chlorophyll a from 4.6 to 73 mg m ^?2. and AFDW from 2.8 to 16 g m^?2. 3. For eight of the nine rivers NH₄-N. DRP, total Kjeldahl nitrogen, total phosphorus and total suspended solids were correlated (P<0.01) with flow, and for seven rivers conductivity was inversely correlated (P<0.05) with flow. 4. There was a hyperbolic relationship between flows and biomass, with chlorophyll a >100 mg m ^?2 and AFDW >20 g m^?2 occurring most frequently in flows of <20 m³ s^?1. 5. Floods prevented the development of medium term (i.e. up to 2 months) maxima in biomass in five of the rivers, but maxima occurred over summer-autumn and winter-spring in the three rivers where floods were absent. 6. Chlorophyll a biomass was more resistant to flooding than AFDW. Only 5993 of the forty-six recorded floods caused chlorophyll a scouring, whereas 74% of the floods caused AFDW scouring. The efficiency of scour was more influenced by the pre-flood biomass than the magnitude of the event. 7. Biomass maxima were significantly correlated (P<0.01) with mean DRP concentration during the accrual period. Overall, up to 53% of the mean annual biomass difference between rivers was explained by the mean annual DRP concentrations. However, the high correlations between nutrient concentrations and flow indicated that the nutrient data were also carrying hydrological information and that simple causal relationships between nutrients and biomass are difficult to establish in rivers. 8. It is concluded that hydrological factors contribute at least equally with nutrients to the differences in periphyton biomass between the gravel-bed study rivers. They combined to explain up to 63.3% of the variance in biomass, compared with 57.6% for nutrients. It is recommended that periphyton data from gravel-bed rivers should always be viewed within the context of the flow history of the site, and not just as a function of nutrient concentrations.     

The influence of sediment mobility and channel geomorphology on periphyton abundance

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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.     

The relationship between invertebrate assemblages and available food at forest and pasture sites in three southeastern Australian streams

1. Levels of ash-free dry matter (AFDM) and chlorophyll a in epilithon, benthic participate organic matter (BPOM), invertebrate assemblage composition, and biomass of functional feeding groups were compared in winter and summer at forest and pasture sites in three Victorian streams. 2. Chlorophyll a concentrations of epilithon were significantly higher at pasture than forest sites in winter but not in summer while BPOM was not significantly greater at forest sites in either season. Epilithic biomass as AFDM did not show consistent differences between land uses or seasons. 3. Total biomass of invertebrates did not differ between forest and pasture sites but the biomass of shredders was significantly higher, and that of grazers significantly lower, at forest than pasture sites. A site shaded with an artificial canopy behaved as a forest site for grazers but as a pasture site for shredders. 4. Cluster analyses of invertebrate assemblages grouped pasture sites with forest sites on the same stream at the same season, indicating that assemblage composition was less influenced by land-use differences than by between-stream and seasonal differences. 5. Biomass of functional feeding groups appeared to be a more sensitive indicator of invertebrate assemblage response to land-use alteration than either species diversity/ richness measures, or multivariate assemblage composition measures.     

Environmental correlates of plant and invertebrate species richness in ponds

Ponds (lentic water bodies <2 ha) constitute a considerable biodiversity resource. Understanding the environmental factors that underlie this diversity is important in protecting and managing the habitat. We surveyed 425 ponds for biological and physical characteristics with 78 of those also surveyed for chemical characteristics. A total of 277 invertebrate species and 265 plant species were found. Species richness varied between 2 and 99 (mean 27.2 ± 0.6 SE) for invertebrates and 1 and 58 (mean 20.8 ± 0.4 SE) for plants. Generalised additive models were used to investigate variables that correlate with the species richness of plants and invertebrates, with additional models to investigate insect, Coleoptera, Odonata, Hemiptera, Trichoptera and Mollusca species richness. Models performed reasonably well for invertebrates in general (R ² = 30.3%) but varied between lower-order invertebrate taxa (12.7–34.7%). Ponds with lower levels of shading and no history of drying contained higher numbers of species of plants and all invertebrate groups. Aquatic plant coverage positively correlated with species richness in all invertebrate groups apart from Trichoptera and the presence of fish was associated with high invertebrate species richness in all groups apart from Coleoptera. The addition of chemistry variables suggested non-linear relationships between oxygen demand and phosphate concentration and higher-order richness. We demonstrate that the composition of biological communities varies along with their species richness and that less diverse ponds are more variable compared to more diverse ponds. Variables positively correlated with richness of one taxon may be negatively correlated with that of another, making comprehensive management recommendations difficult. Promoting a high landscape-level pond biodiversity will involve the management of a high diversity of pond types within that landscape.     

Invertebrate drift along a longitudinal gradient in a Neotropical stream in Serra do Cipó National Park,Brazil

The diversity and composition of drift invertebrate assemblages were evaluated along a longitudinal gradient of an altitudinal stream in southeastern Brazil. The main goal of this study was to evaluate the influence of seasonality, stream order, and some abiotic factors on invertebrate drift and the use of drifting invertebrate assemblages to assess aquatic invertebrate diversity. Drift samples were collected over a 24 h period using nets (open area of 0.08 m²; mesh 0.250 mm), partially submerged (60%) in the water column. Taxonomic richness, Pielou evenness (J), Shannon–Wiener diversity (H), and total density of drift invertebrate assemblages were used in unpaired t-tests, Kruskal–Wallis and stepwise multiple regression analysis. The results showed a high taxonomic richness of aquatic invertebrates, with 91 taxa found. Chironomidae and Ephemeroptera represented together c. 80% of the total density of drift organisms. The drift approach allowed the collection of new and rare taxa, besides the knowledge of pupae stage of several chironomid genera. Significant differences in the taxonomic richness and diversity of drift invertebrate assemblages were found between the rainy and dry periods, indicating a significant influence of seasonality. An increase in water flow and electrical conductivity were associated with the increase in the taxonomic richness and diversity in the rainy period. No significant differences were found among the other abiotic variables among the stream orders.     

Quantitative Links Between Pacific Salmon and Stream Periphyton

Species’ impacts on primary production can have strong ecological consequences. In freshwater ecosystems, Pacific salmon (Oncorhynchus spp.) may influence stream periphyton through substrate disturbance during spawning and nutrient subsidies from senescent adults. The shape of relationships between the abundance of spawning salmon and stream periphyton, as well as interactions with environmental variables, are incompletely understood and may differ across the geographic range of salmon. We examined these relationships across 24 sockeye salmon (Oncorhynchus nerka) spawning streams in north-central British Columbia, Canada. The influence of salmon abundance and environmental variables (temperature, light, dissolved nutrients, water velocity, watershed size, and invertebrate grazer abundance) on post-spawning periphyton abundance and nitrogen stable isotope signatures, which can indicate the uptake of salmon nitrogen, was evaluated using linear regression models and Akaike Information Criterion. Periphyton nitrogen stable isotope signatures were best described by a positive log-linear relationship with an upstream salmon abundance metric that includes salmon from earlier years. This suggests the presence of a nutrient legacy. In contrast, periphyton abundance was negatively related to the spawning-year salmon density, which likely results from substrate disturbance during spawning, and positively related to dissolved soluble reactive phosphorus prior to spawning, which may indicate phosphorus limitation in the streams. These results suggest that enrichment from salmon nutrients does not always translate into elevated periphyton abundance. This underscores the need to directly assess the outcome of salmon impacts on streams rather than extrapolating from stable isotope evidence for the incorporation of salmon nutrients into food webs.     

SPECIES COMPOSITION,BIOMASS, AND NUTRIENT CONTENT OF PERIPHYTON IN THE FLORIDA EVERGLADES1

Periphyton biomass, nutrient dynamics in the biomass, and species composition were studied in two Florida Everglades sloughs from August 1991 to August 1992. Periphyton biomass on macrophytes was strongly season-dependent. Maximum biomasses, 1180, 161, and 59 g dry mass.m^?2 on Eleocharis vivipara, E. cellulosa, and Nymphaea odorata, respectively, occurred in summer and early autumn; winter and spring periphyton biomass was very low (practically not measurable). Periphyton was dominated by blue-green algae (cyanobacteria) during the summer and autumn; diatoms dominated during the winter and spring. Green algae occurred mostly during the summer and autumn, but their growth was sparse and did not contribute significantly to periphyton biomass. Nitrogen-to-phosphorus ratios in the periphyton were very high (59–121:1), suggesting phosphorus limitation of periphyton growth. The periphyton contained large concentrations of calcium (up to 22.3% on dry mass basis) especially in late summer and autumn.     

Salinity cascading and predator effects on consumer and producer community structure: a mesocosm experiment

1. An observed community structure is often shaped by a combination of bottom-top and top-bottom processes, affected by biotic and abiotic factors. These factors affect community structure either directly (each species separately) or indirectly, via other species. Few studies have observed the combined effects of biotic and abiotic factors on primary producers and consumer communities composed of numerous species. 2. This study investigated the combined effects of a predatory hemipteran and water salinity on abundance and species richness of the periphyton and arthropod communities in an artificial pool experiment. 3. The abundance and species richness of both periphyton and arthropod communities were affected by the combination of salinity and predator. 4. It is suggested that the observed consumer community is composed of arthropod species that vary in salinity tolerance and response to the predator. In addition, the abundance of euryhaline species or species that are not consumed by this particular predator increases indirectly in specific treatment combinations that eventually result in reduced interspecific competition. Periphyton variables were strongly correlated to densities of larval Ochlerotatus caspius (Diptera: Culicidae), suggesting that this species may be largely responsible for structuring the producer community. 5. It is suggested that O. caspius distribution is the result of female oviposition habitat selection that is manipulated by chemical signals. Therefore, chemical signals that inform about habitat suitability also play an important role in shaping both consumer and primary producer communities.     

Hydraulic parameters and benthic invertebrate distributions in two gravel-bed New Zealand rivers

1. Benthic invertebrates were sampled over a matrix of about eighty combinations of mean velocity (10–150cms^?1) and depth (10–150cm) in two rivers that differed in substrate size variability. Vertical velocity profiles were measured at each sample site and substratum roughness was measured and estimated from percentage cover by stone size classes. The influence of depth on periphyton biomass was also measured. 2. The hydraulic and substrate data were used to investigate the correlations between conventional (mean velocity, depth, substrate size) and complex hydraulic variables (Froude number, shear velocity, and water column and boundary Reynolds number) that were either calculated from direct measurements or inferred from mean velocity, depth, kinematic viscosity and substrate roughness. The ecological relevance of these hydraulic variables was investigated by comparing their degree of correlation with invertebrate densities and community metrics. 3. The invertebrate variables had similar correlations with mean velocity and the complex near-bed hydraulic variables in the river with uniform cobble substrates. In the river with diverse substrates, however, average correlations with Froude number, and inferred shear velocity and boundary Reynolds number were 25–45% higher than with velocity. Of all the individual hydraulic parameters, the boundary Reynolds number, calculated from simple measures, was most strongly correlated with benthic invertebrate distributions and taxa richness. However, invertebrate distributions were more strongly correlated with predictions of multiple regression models, incorporating substrate size, depth and mean velocity, than with any single hydraulic variable. 4. Hydraulic influences on food availability and oxygen concentration in the benthos are likely mechanisms affecting the hydraulic preferences of several taxa. Lower periphyton biomass with depth, partly attributable to light attenuation, appeared to have a non-hydraulic influence on a collector-browser species.     

Macroinvertebrate food web structure in a floodplain lake of the Bolivian Amazon

Two stable isotopes δ¹³C and δ¹⁵N were used to identify the energy sources and trophic relationships of the main freshwater macroinvertebrates in a floodplain lake of the Beni River (Bolivian Amazonia). Four energy sources (seston, bottom sediment, periphyton, and aquatic macrophytes) and macroinvertebrate communities were collected during three periods of the river hydrological cycle. Macroinvertebrates showed greater temporal variation in isotope values than their food sources. Six trophic chains were identified: four were based on seston, periphyton, C₃ macrophytes, and bottom sediments, and the last two chains on a combination of two carbon sources. One mixed seston and periphyton sources during the wet season while the other mixed periphyton and macrophytes sources during the wet and dry seasons. Periphyton was the most important energy source supporting the highest number of trophic levels and consumers. The macrophytic contribution was only significant during the dry season. Bottom sediments constituted a marginal energy source. As each season is associated with different physical and chemical conditions, processes organizing macroinvertebrate food web structure in the Beni floodplain seem strongly linked to hydrological seasonality.     

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.     

Habitat scale and biodiversity: influence of catchment, stream reach and bedform scales on local invertebrate diversity

Although many studies have investigated the influence of environmental patterns on local stream invertebrate diversity, there has been little consistency in reported relationships between diversity and particular environmental variables. Here we test the hypothesis that local stream invertebrate diversity is determined by a combination of factors occurring at multiple spatial scales. We developed predictive models relating invertebrate diversity (species richness and equitability) to environmental variables collected at various spatial scales (bedform, reach and catchment, respectively) using data from 97 sampling sites dispersed throughout the Taieri River drainage in New Zealand. Models based on an individual scale of perception (bedform, reach or catchment) were not able to match predictions to observations (r < 0.26, P > 0.01, between observed and predicted equitability and species richness). In contrast, models incorporating all three scales simultaneously were highly significant (P < 0.01; r = 0.55 and 0.64, between observed and predicted equitability and species richness, respectively). The most influential variables for both richness and equitability were median particle size at the bedform scale, adjacent land use at the reach scale, and relief ratio at the catchment scale. Our findings suggest that patterns observed in local assemblages are not determined solely by local mechanisms acting within assemblages, but also result from processes operating at larger spatial scales. The integration of different spatial scales may be the key to increasing model predictability and our understanding of the factors that determine local biodiversity.