Relationships between deforestation, riparian forest buffers and benthic macroinvertebrates in neotropical headwater streams

1. Few studies have evaluated the effectiveness of riparian buffers in the tropics, despite their potential to reduce the impacts of deforestation on stream communities. We examined macroinvertebrate assemblages and stream habitat characteristics in small lowland streams in southeastern Costa Rica to assess the impacts of deforestation on benthic communities and the influence of riparian forest buffers on these effects. Three different stream reach types were compared in the study: (i) forested reference reaches, (ii) stream reaches adjacent to pasture with a riparian forest buffer at least 15 m in width on both banks and (iii) stream reaches adjacent to pasture without a riparian forest buffer. 2. Comparisons between forest and pasture reaches suggest that deforestation, even at a very local scale, can alter the taxonomic composition of benthic macroinvertebrate assemblages, reduce macroinvertebrate diversity and eliminate the most sensitive taxa. The presence of a riparian forest buffer appeared to significantly reduce the effects of deforestation on benthic communities, as macroinvertebrate diversity and assemblage structure in forest buffer reaches were generally very similar to those in forested reference reaches. One forest buffer reach was clearly an exception to this pattern, despite the presence of a wide riparian buffer. 3. The taxonomic structure of macroinvertebrate assemblages differed between pool and riffle habitats, but contrasts among the three reach types in our study were consistent across the two habitats. Differences among reach types also persisted across three sampling periods during our 15‐month study. 4. Among the environmental variables we measured, only stream water temperature varied significantly among reach types, but trends in periphyton abundance and stream sedimentation may have contributed to observed differences in macroinvertebrate assemblage structure. 5. Forest cover was high in all of our study catchments, and more research is needed to determine whether riparian forest buffers will sustain similar functions in more extensively deforested landscapes. Nevertheless, our results provide support for Costa Rican regulations protecting riparian forests and suggest that proper riparian management could significantly reduce the impacts of deforestation on benthic communities in tropical streams.     

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.     

Invertebrate grazing and riparian shade as controllers of nuisance algae in a eutrophic river

1. Algal growth in lotic systems is controlled either from the bottom‐up (e.g. nutrients and light, which determine growth rates) or from the top‐down (e.g. grazing pressure, which reduces accumulated biomass). Nutrient‐enriched streams that support large and diverse grazing macroinvertebrate populations and those with shaded riparian corridors rarely suffer from excessive algal growth. 2. In this study, the density of benthic algivorous macroinvertebrates was experimentally manipulated in shaded and open nutrient‐enriched stream habitats of the Owennagearagh River, south‐west Ireland. The ability of macroinvertebrate grazers and riparian shade to control benthic algal growth [particularly the nuisance alga, Cladophora glomerata (L. Kütz)] was investigated. Three sites with markedly different concentrations of plant nutrients (one site upstream and two sites downstream of the sewage outfall) were selected. The density of grazing invertebrates colonising ceramic tiles was reduced using high‐voltage localised electric pulses. Replicates of treatment (grazer‐excluded) and control (grazed) tiles were deployed in open and shaded (<25 and >80% canopy cover, respectively) patches of stream bed, in each site. 3. After 2‐week Cladophora cover, periphytic chlorophyll a and biofilm ash‐free dry mass (AFDM) were quantified for all experimental tiles. Values for all three parameters were highest on grazer‐excluded tiles from open patches. Grazed tiles from open patches accrued little Cladophora and had significantly lower levels of chlorophyll a and AFDM. Nutrient inputs were found to have an impact on the density of grazing invertebrates, with higher densities of Baetis nymphs at the most nutrient‐enriched site. 4. Our results demonstrate that in eutrophic, high‐light streams, filamentous algae can quickly accumulate to nuisance levels in the absence of invertebrate grazers. In future, greater attention should be paid to the role of grazing invertebrates in controlling nuisance algae in streams, in addition to algal–nutrient relationships.     

Climate‐induced shift in hydrological regime alters basal resource dynamics in a wilderness river ecosystem

1. We integrated a 20‐year ecological data set from a sparsely inhabited, snowmelt‐dominated catchment with hydrologic models to predict the effects of hydrologic shifts on stream biofilm. 2. We used a stepwise multiple regression to assess the relationship between hydrology and biofilm ash‐free dry mass (AFDM) and chlorophyll‐a (chl‐a) under recent climate conditions. Biofilm AFDM was significantly related to the timing of peak streamflow, and chl‐a was significantly related to the timing of median streamflow. We applied these results to output from the variable infiltration capacity hydrologic model, which predicted hydrology under a baseline scenario (+0 °C) and a range of warming scenarios expected with climate change (+1, +2 or +3 °C). 3. When compared to the baseline, the results indicated that earlier peakflows predicted under warming scenarios may lead to earlier initiation of biofilm growth. This may increase biofilm AFDM during the summer by up to 103% (±29) in the +3 °C scenario. Moreover, interannual variability of AFDM was predicted to increase up to 300%. Average chl‐a during the summer increased by up to 90% (±15) in the +3 °C scenario; however, its response was not significantly different from baseline in most years. 4. Because hydrologic change may alter the temporal dynamics of biofilm growth, it may affect the seasonal dynamics of biofilm quality (i.e. chl‐a‐to‐AFDM ratio). The results indicated that hydrologic shifts may increase biofilm quality during the spring, but may decrease it during the summer. Thus, we provide evidence that predicted hydrologic shifts in snowmelt‐dominated streams may alter the quantity and quality of an important basal resource. However, the magnitudes of these predictions are likely to be affected by other environmental changes that are occurring with climate change (e.g. increased wildfire activity and stream warming).     

SPATIAL PATTERNS AND ECOLOGICAL DETERMINANTS OF BENTHIC ALGAL ASSEMBLAGES IN MID-ATLANTIC STREAMS, USA

We attempted to identify spatial patterns and determinants for benthic algal assemblages in Mid-Atlantic streams. Periphyton, water chemistry, stream physical habitat, riparian conditions, and land cover/use in watersheds were characterized at 89 randomly selected stream sites in the Mid-Atlantic region. Cluster analysis (TWINSPAN) partitioned all sites into six groups on the basis of diatom species composition. Stepwise discriminant function analysis indicated that these diatom groups can be best separated by watershed land cover/use (percentage forest cover), water temperature, and riparian conditions (riparian agricultural activities). However, the diatom-based stream classification did not correspond to Omernik's ecoregional classification. Algal biomass measured as chl a can be related to nutrients in habitats where other factors do not constrain accumulation. A regression tree model indicated that chl a concentrations in the Mid-Atlantic streams can be best predicted by conductivity, stream slope, total phosphorus, total nitrogen, and riparian canopy coverage. Our data suggest that broad spatial patterns of benthic diatom assemblages can be predicted both by coarse-scale factors, such as land cover/use in watersheds, and by site-specific factors, such as riparian conditions. However, algal biomass measured as chl a was less predictable using a simple regression approach. The regression tree model was effective for showing that ecological determinants of chl a were hierarchical in the Mid-Atlantic streams.     

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.     

BENTHIC MICROALGAL COLONIZATION IN STREAMS OF DIFFERING RIPARIAN COVER AND LIGHT AVAILABILITY1

Benthic microalgal colonization, light climate, and algal photosynthesis–irradiance relationships were analyzed in two vegetated creeks in southeast Australia, differing in their degree of riparian cover and hence light availability. Diatoms were the main component of the benthic microalgal communities in both creeks. The light received was three times higher in the partly shaded than in the completely shaded creek throughout the 45‐day study period. The algal community in the most shaded stream (Dandenong Creek [DG]) was almost permanently under subsaturating light, with a range of 0.06–0.25 light h a day above saturation irradiance (I_k), whereas in the partly shaded stream (Deep Creek [DC]) a higher period of saturating light, 1.2 light h a day above I_k, was seen. The initial slopes (α) of photosynthesis versus irradiance relationships were similar in the two creeks during the initial stages of colonization, indicating that early algal colonizers could behave as generalists with respect to light. In later stages of colonization, however, α and I_k diverged in the two streams. In DG the α values remained high, up to the end of the colonization, whereas in DC α substantially decreased. In DG there was a higher chl concentration per surface area, and algae had a higher content of chl per cell than in the partly shaded creek. In DC the algal community became adapted to increasing light availability during the course of the colonization, but changes in the photosynthesis–irradiance parameters were accompanied by a shift in the composition of the diatom community. We concluded that the algal community is highly influenced by adaptation of the algal species to governing light conditions throughout colonization.     

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.     

LINKING BENTHIC ALGAL BIOMASS TO STREAM SUBSTRATUM TOPOGRAPHY1

The physical properties of substrata significantly influence benthic algal development. We explored the relationships among substratum surface texture and orientation with epilithic microphytobenthic biomass accumulation at the whole‐substratum and micrometer scales. Unglazed clay tiles set at three orientations (horizontal, vertical, and 45°), and six substrata of varying surface roughness were deployed in a prairie stream for 3 weeks. Substrata were analyzed for loosely attached, adnate, and total benthic algal biomass as chl a, and confocal laser scanning microscopy was used to measure substrata microtopography (i.e., roughness, microscale slope angles, and three‐dimensional surface area). At the whole‐substratum level, vertical substrata collected significantly (P < 0.05) less algal biomass, averaging 34% and 36% less than horizontal and 45° substrata, respectively. Benthic algal biomass was also significantly less on smoother surfaces; glass averaged 29% less biomass than stream rocks. At the microscale level, benthic algal biomass was the greatest at intermediate values, peaking at a mean roughness of approximately 17 μm, a mean microscale slope of 50°, and a projected/areal surface area ratio of 2:1. The proportion of adnate algae increased with surface roughness (26% and 67% for glass and brick, respectively), suggesting that substratum type changes the efficiency of algal removal by brushing. Individual substrata and microsubstrata characteristics can have a strong effect on benthic algae development and potentially affect reach scale algal variability as mediated by geomorphology.     

Saltcedar (Tamarix ramosissima) invasion alters organic matter dynamics in a desert stream

1. We investigated the impacts of saltcedar invasion on organic matter dynamics in a spring‐fed stream (Jackrabbit Spring) in the Mojave Desert of southern Nevada, U.S.A., by experimentally manipulating saltcedar abundance. 2. Saltcedar heavily shaded Jackrabbit Spring and shifted the dominant organic matter inputs from autochthonous production that was available throughout the year to allochthonous saltcedar leaf litter that was strongly pulsed in the autumn. Specifically, reaches dominated by saltcedar had allochthonous litter inputs of 299 g ash free dry mass (AFDM) m^?2 year^?1, macrophyte production of 15 g AFDM m^?2 year^?1 and algal production of 400 g AFDM m^?2 year^?1, while reaches dominated by native riparian vegetation or where saltcedar had been experimentally removed had allochthonous litter inputs of 7–34 g AFDM m^?2 year^?1, macrophyte production of 118–425 g AFDM m^?2 year^?1 and algal production of 640–900 g AFDM m^?2 year^?1. 3. A leaf litter breakdown study indicated that saltcedar also altered decomposition in Jackrabbit Spring, mainly through its influence on litter quality rather than by altering the environment for decomposition. Decomposition rates for saltcedar were lower than for ash (Fraxinus velutina), the dominant native allochthonous litter type, but faster than for bulrush (Scirpus americanus), the dominant macrophyte in this system.     

Interactions among irradiance,nutrients, and herbivores constrain a stream algal community

Using stream-side, flow-through channels, I tested for the effects of nutrients (NU) (nitrogen plus phosphorus), irradiance (L), and snail grazing (G) on a benthic algal community in a small, forested stream. Grazed communities were-dominated by a chlorophyte (basal cells ofStigeoclonium) and a cyanophyte (Chamaesiphon investiens), whereas ungrazed communities were comprised almost entirely of diatoms, regardless of nutrient and light levels. Snails maintained low algal biomass in all grazed treatments, presumably by consuming increased algal production in treatments to which L and NU were increased. When nutrients were increased, cellular nutrient content increased under ambient conditions (shaded, grazed) and biomass and productivity increased when snails were removed and light was increased. Together, nutrients and light had positive effects and grazing had negative effects on biomass (chlorophylla, AFDM, algal biovolume) and chlorophyll-and areal-specific productivity in ANOVAs. However, in most cases, only means from treatments in which all three factors were manipulated (ungrazed, +NU&L treatments) were significantly different from controls; effects of single factors were generally undetectable. These results indicate that all three factors simultaneously limited algal biomass and productivity in this stream during the summer months. Additionally, the effects of these factors in combination were in some cases different from the effects of single factors. For example, light had slight negative effects on some biomass parameters when added at ambient snail densities and nutrient concentrations, but had strong positive effects in conjunction with nutrient addition and snail removal. This study demonstrates that algal biomass and productivity can be under multiple constraints by irradiance, nutrients, and herbivores and indicates the need to employ multifactor experiments to test for such interactive effects.     

Environmental variability and the ecological effects of spawning Pacific salmon on stream biofilm

1. Variation in resource subsidies can create or reinforce heterogeneity in recipient ecosystems. Related activities of organisms delivering resource subsidies, such as ecosystem engineering by Pacific salmon spawners (Oncorhynchus spp.), also alter heterogeneity. We studied whether heterogeneity in stream environmental conditions and spawner abundances were reflected in the net ecological effects of salmon (i.e. enrichment by resource subsidies and disturbance by ecosystem engineering) on benthic biofilm. 2. We sampled seven Southeast Alaska streams over 3 years, both before and during the salmon run. In each stream and year, stream environmental characteristics and their influence on responses of benthic biofilm [mean and coefficient of variation of chlorophyll a (chl a), ash‐free dry mass (AFDM) and autotrophic index (AFDM:chl a)] to spawners were assessed. 3. Streams and periods before and during the salmon run were distinct based on their environmental characteristics. The responses of most biofilm metrics to spawners were stream‐ and year‐specific, suggesting that the ecological effect of spawners ranged from net enrichment to net disturbance depending on the stream or year studied. The environmental context, especially temperature, large wood, and sediment size, explained >50% of biofilm variability during the run, but <30% over the entire study, suggesting that salmon can alter environmental constraints. 4. Precision of biofilm estimates improved by increasing either the number of streams or the number of years sampled (i.e. spatial or temporal replication). However, combining data from different North Pacific Rim ecoregions inflated the confidence interval as compared with a single ecoregion, indicating the importance of regional environmental contexts for net salmon effects. 5. Our results suggest that biofilm responses to salmon can vary greatly, even within a single ecoregion, and that environmental conditions can modify net salmon effects. Consequently, generalisations about biofilm responses across the native range of salmon may be challenging.     

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.     

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.     

Deforestation alters the resource base and biomass of endemic stream insects in eastern Madagascar

1. Rainforest streams in eastern Madagascar have species‐rich and diverse endemic insect communities, while streams in deforested areas have relatively depauperate assemblages dominated by collector‐gatherer taxa. We sampled a suite of benthic insects and their food resources in three primary rainforest streams within Ranomafana National Park in eastern Madagascar and three agriculture streams in the park's deforested peripheral zone. We analysed gut contents and combined biomass and stable isotope data to examine stream community responses to deforestation in the region, which is a threatened and globally important hotspot for freshwater biodiversity. 2. Gut analyses showed that most taxa depended largely on amorphous detritus, obtained either from biofilms (collector‐gatherers) or from seston (microfilterers). Despite different resource availability in forest versus agriculture streams, diets of each taxon did not differ between stream types, suggesting inflexible feeding modes. Carbon sources for forest stream insects were difficult to discern using δ¹³C. However, in agriculture streams dependence on terrestrial carbon sources was low relative to algal sources. Most insect taxa with δ¹³C similar to terrestrial carbon sources (e.g. the stonefly Madenemura, the caddisfly Chimarra sp. and Simulium blackflies) were absent or present at lower biomass in agriculture streams relative to forest streams. Conversely, collector‐gatherers (Afroptilum mayflies) relied on algal carbon sources and had much higher biomass in agriculture streams. 3. Our analyses indicate that a few collector‐gatherer species (mostly Ephemeroptera) can take advantage of increased primary production in biofilms and consequently dominate biomass in streams affected by deforestation. In contrast, many forest stream insects (especially those in the orders Plecoptera, Trichoptera and Diptera) depend on terrestrial carbon sources (i.e. seston and leaf litter), are unable to track resource availability and consequently decline in streams draining deforested landscapes. These forest‐specialists are often micro‐endemic and particularly vulnerable to deforestation. 4. The use of consumer biomass data in stable isotope research can help detect population‐level responses to shifts in basal resources caused by anthropogenic change. We also suggest that restoration of vegetated riparian zones in eastern Madagascar and elsewhere could mitigate the deleterious effects of deforestation on sensitive, endemic stream taxa that are dependent on terrestrial carbon sources.     

Catchment- and site-scale influences of forest cover and longitudinal forest position on the distribution of a diadromous fish

1. The hydrologic connectivity between landscape elements and streams means that fragmentation of terrestrial habitats could affect the distribution of stream faunas at multiple spatial scales. We investigated how catchment‐ and site‐scale influences, including proportion and position of forest cover within a catchment, and presence of riparian forest cover affected the distribution of a diadromous fish. 2. The occurrence of koaro (Galaxias brevipinnis) in 50‐m stream reaches with either forested or non‐forested riparian margins at 172 sites in 24 catchments on Banks Peninsula, South Island, New Zealand was analysed. Proportions of catchments forested and the dominant position (upland or lowland) of forest within catchments were determined using geographical information system spatial analysis tools. 3. Multivariate analysis of variance indicated forest position and proportion forested at the catchment accounted for the majority of the variation in the overall proportion of sites in a catchment with koaro. 4. Where forest was predominantly in the lower part of the catchments, the presence of riparian cover was important in explaining the proportion of sites with koaro. However, where forest was predominantly in the upper part of the catchment, the effect of riparian forest was not as strong. In the absence of riparian forest cover, no patterns of koaro distribution with respect to catchment forest cover or forest position were detected. 5. These results indicate that landscape elements, such as the proportion and position of catchment forest, operating at catchment‐scales, influence the distribution of diadromous fish but their influence depends on the presence of riparian vegetation, a site‐scale factor.     

Inter-annual variation in responses of water chemistry and epilithon to Pacific salmon spawners in an Alaskan stream

1. Pacific salmon (Oncorhynchus spp.) deliver salmon‐derived nutrients (SDN) to the streams in which they spawn. However, many stream parameters, such as discharge and spawner abundance, can vary from year to year, which could alter the quantity and flux of SDN. 2. Over six consecutive years, we studied responses in streamwater chemistry and epilithon (i.e. the microbial community on submerged rocks) to salmon spawners in Fish Creek, southeastern Alaska, U.S.A. The lower reach of Fish Creek receives spawners of several salmon species, while the upper reach does not receive spawners because of an intervening waterfall. 3. We estimated salmon spawner biomass, analysed water chemistry [ammonium, nitrate, soluble reactive phosphorus (SRP) and dissolved organic carbon (DOC)], and measured epilithon abundance [as chlorophyll a (chl a) and ash‐free dry mass (AFDM)] in Fish Creek. Measurements were made in both the upper and lower reaches, before, during and after the major salmon runs. 4. Absolute values and relative differences indicated that the presence of salmon spawners consistently increased dissolved ammonium (by 58 μg L⁻¹ on average, 41× over background), SRP (by 6 μg L⁻¹, 14×), epilithon chl a (by 35 mg m⁻², 16×), and epilithon AFDM (by 3 g m⁻², 8×). Salmon spawners did not increase nitrate or DOC in either absolute or relative amounts. The persistence and magnitude of spawner effects varied among years and appeared to reflect weather‐driven hydrology as well as spawner biomass. 5. Salmon‐derived nutrients can stimulate the growth of primary producers by increasing streamwater nutrient concentrations, but this positive influence may be modulated by other factors, such as water temperature and discharge. To better assess the ecological influence of SDN on stream biota, future studies should explicitly consider the role of key environmental factors and their temporal and spatial dynamics in stream ecosystems.     

Effects of inorganic sedimentation and riparian clearing on benthic community metabolism in an agriculturally-disturbed stream

An in situ chamber technique was used to obtain seasonal estimates of benthic community metabolism at three stations in an agriculturally disturbed stream. Two stations with open canopies were examined. Sand was the dominant substrate at one site, cobble at the other. The third station was shaded by riparian vegetation and had a sand substrate.Seasonal estimates of net community productivity (NCP) and community respiration (CR) at the cobble section were significantly higher than those calculated for the sand sections (p>0.05). Ratios of gross community productivity (GCP) to 24 h respiration indicated autotrophic conditions in the cobble and extreme heterotrophy in the sand. NCP was significantly higher (p<0.05) in the open canopy sand than in the riparian shaded sand only when turbidity and discharge were low. Measurements of periphyton ash-free dry mass (AFDM) and chlorophyll a support metabolism estimates. Measurements of loose detrital AFDM were very low and variable compared to others reported in the literature. Therefore, allochthonously derived detritus may not be an important energy source for the benthic community.     

Impacts of land use at the catchment scale constrain the habitat benefits of stream riparian buffers

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Impacts of an Invasive N2‐Fixing Tree on Hawaiian Stream Water Quality

N₂‐fixing trees can affect stream water quality. This has been documented in temperate streams, but not in tropical ones, even though N₂‐fixing trees are prevalent in the tropics. We investigated the effects of the introduced, invasive tree, Falcataria moluccana (albizia) on water quality of Hawaiian streams. Nutrient concentrations were measured in reaches above and below F. moluccana‐dominated riparian zones of four streams over 14 mo, and benthic algal nutrient limitation was examined in one stream. NO₃^?+NO₂^? concentrations were up to 600 percent higher in reaches below F. moluccana stands than in ones above them. In contrast, dissolved organic nitrogen concentrations were 24 percent lower in most reaches below F. moluccana stands, and NH₄⁺ and particulate nitrogen concentrations were similar above and below the stands. Dissolved organic carbon concentrations were up to 30 percent lower below F. moluccana stands, but particulate carbon concentrations were similar between reaches. Total dissolved phosphorus concentrations were similar above and below F. moluccana stands, whereas H₄SiO₄ concentrations were higher below the stands. In the stream where benthic chlorophyll a was measured, concentrations were three times higher below the F. moluccana stand than above it. Benthic algae were co‐limited by nitrogen and phosphorus above the F. moluccana stand, and limited by phosphorus below it. These results suggest that F. moluccana's presence relieved nitrogen‐limitation and caused the benthic algae to become solely phosphorus‐limited. Overall, our results demonstrate that F. moluccana can strongly affect the chemistry and primary producers of these tropical streams.