Algal and cyanobacterial composition in biofilms is controlled by several factors, including light, nutrients, substratum, hydrodynamics, and grazing. Another potential determinant rarely taken into account is allelopathy, i.e., negative chemical interactions between competitors. We examined the interaction between abiotic variables (light, nutrients, and salt stress) and allelopathy by analyzing the effects of culture conditions on production of allelochemicals by 2 green algal species (Uronema confervicolum and Desmodesmus quadrispina). Production of allelochemicals was greater during the exponential growth phase than during other growth phases and was affected by light, nutrients, and salt concentrations, albeit differently for the 2 species. We then exposed 3 target species (D. quadrispina, Nitzschia palea, and Nostoc PCC 7120) to compounds produced by D. quadrispina, Monoraphidium aff. dybowski, or U. confervicolum. Nutrient-depleted conditions decreased sensitivity, whereas elevated salt concentrations increased sensitivity of D. quadrispina and N. palea to extracts from each of the 3 donor species. Effects of light conditions on sensitivity of target species to donor algal extracts varied among target species. Our results showed that, in benthic biofilms, the magnitude of response to allelochemicals depends on the target species, the donor species, and the abiotic conditions under which they are growing. The role of allelopathy in controlling species composition of biofilms probably is underestimated because the outcome of such complex interactions is difficult to predict.

Benthic grazers can affect the nutrient stoichiometry of periphyton through different pathways: 1) grazers can reduce periphyton biomass, thereby increasing the relative amount of nutrients for the remaining periphyton; 2) grazers can change benthic algal community composition toward grazer-resistant taxa that might differ in their C:N:P stoichiometry from less resistant benthic algae; 3) grazers can differentially recycle or incorporate nutrients, thereby changing periphyton nutrient content; and 4) mechanical removal of high-C detritus by grazers can decrease C:nutrient ratios. Nutrient-mediated grazer effects on periphyton nutrient stoichiometry depend on the relative availability of nutrients in the system in relation to the nutrients made available through grazing. We tested the relative importance of the different pathways under differing nutrient conditions in a 3-wk laboratory experiment in which grazer presence and dissolved N and P concentrations were manipulated. The effects of the gastropod grazer Viviparus viviparus (L.) on periphyton nutrient stoichiometry, algal taxonomy and biomass, and dissolved nutrients were determined with a 2 × 4 factorial design. Factors were grazers ( /–) and nutrient addition (ambient, N, P, and NP). Grazers decreased periphyton C:P and N:P in the N and P treatments. Grazers mainly affected periphyton nutrient stoichiometry by changing benthic algal taxonomic composition because the percentage of mucilage-producing algae Chaetophora spp. was much higher in grazed than in ungrazed treatments. Mucilage has high C and N content, but low P content. Grazer stoichiometry indicated that grazers might have retained more P in the N and P treatment than in the other nutrient treatments. We conclude that nutrient addition interacted with grazer effects on periphyton nutrient stoichiometry and that grazer effects on periphyton nutrient stoichiometry can be explained only by considering the combined effects of nutrient availability, differential nutrient recycling by grazers, and grazer-induced changes in algal taxonomy.

Ontogenetic stage and density of consumers can affect the intensity of their roles within a community. These roles can be further complicated in a heterogeneous environment, where a consumer might have disparate effects in different microhabitat types. We conducted an enclosure experiment in a small temperate stream to test the differential ecological roles of adults and juveniles of the signal crayfish (Pacifastacus leniusculus) in 2 microhabitat types. In addition, we manipulated numbers of each stage to create a gradient of density (and biomass). Enclosures contained 1, 2, or 3 adults, or 4, 8, or 12 juveniles. Control enclosures lacked crayfish. We added leaf packs the 4 corners of the enclosures to provide a primary resource for crayfish and other invertebrates and to provide a detritus-based microhabitat. After 6 wk, we collected the leaf packs and a cobble-bottom (Surber) sample from the center of the enclosures. We used cobble-bottom samples to analyze the composition of the community in the cobble microhabitat. Leaf-pack breakdown rate was positively related to crayfish biomass for adults and juveniles. Leaf-pack invertebrate community composition and abundance were negatively affected by crayfish presence, but these results were independent of crayfish ontogenetic stage and total biomass. The composition of the cobble-bottom invertebrate community was not significantly affected by the presence of crayfish, regardless of density or ontogenetic stage. This result might have been the consequence of the cannibalistic tendencies of P. leniusculus or of possible indirect effects of crayfish (e.g., bioturbation or chemical cues). Our study is among the first to be done on the ecology of P. leniusculus in its native environment. The fact that our results do not agree with those found in other studies for this organism might reflect the fact that most of what we know about this organism comes from research done where it has been introduced.

Whole-stream rates of nutrient uptake and metabolism vary with season in forested headwater streams, but seasonal variation in rates at the substratum scale, which collectively generate whole-stream patterns, have not previously been quantified. We measured gross primary production (GPP), community respiration (CR), and NO₃⁻ uptake rates (NO₃⁻ U) on 7 substrata using in situ microcosms during spring, summer, and autumn in 3 headwater streams in northern Michigan. We quantified the areal reach coverage of each substratum, scaled NO₃⁻ U and metabolism to the stream reach (100 m), and compared scaled NO₃⁻ U to whole-stream NO₃⁻ U (measured using short-term nutrient additions). Patterns of GPP, CR, and NO₃⁻ U were different among substratum types and seasons. Substratum-specific NO₃⁻ U was positively related to GPP on epilithic biofilms and to CR on epixylic biofilms, bryophytes, and coarse and fine benthic organic matter (FBOM). During summer, large wood and FBOM accounted for 86% of reach-scale NO₃⁻ U, whereas epilithon contributed 28% and 49% of reach-scale NO₃⁻ U in spring and autumn, respectively. This result was unexpected for these forested closed-canopy streams. When substratum-specific rates were scaled up, NO₃⁻ U was not different from whole-stream NO₃⁻ U calculated from short-term enrichment. For large wood, organic matter, and epilithon, high molar ratios of C:N uptake suggested that NO₃⁻ uptake did not satisfy overall N demand. Streambed substratum composition influenced seasonal variability of reach-scale nutrient uptake by its influence on metabolism and changed the relative contribution of several substrata to whole-stream rates through time.

Unionid mussels are among the largest and longest-lived invertebrates in shallow benthic communities, where they often make up most of the biomass. However, they spend their juvenile stages and extended periods of their adult lives buried in the sediments and emerge from the sediments only during part of the year. I tested whether the magnitude and timing of emergence of Elliptio complanata, an abundant native mussel, are related to water temperature, upwelling activity, site exposure, and sediment characteristics. Seasonal changes in the density and average body size of mussels on the sediment surface were measured at 26 shallow (2-m) littoral sites in a multibasin lake to determine which factors are most useful in predicting their emergence. In general, densities started increasing in early spring, peaked in spring or sometime in summer, and declined through autumn, but the magnitude (ratio of minimum-to-maximum surface density) and timing (date of maximum surface density, start of autumn decline) of emergence differed between sites. Maximum mussel surface density was on average (median) 2× higher than minimum surface density and was higher at exposed sites with fine sediment particles than at more protected sites. The timing of mussel emergence and burial were most closely related to site exposure. Mussels reached their maximum surface density earlier, but they also started burying earlier at the most exposed sites (effective fetch >900 m). At more protected sites, maximum densities were reached later in summer, after the main period of reproduction, and densities did not start declining until late summer. The only exceptions were 2 protected sites with deep, fine, highly organic and, presumably, anoxic sediments, where mussels emerged early in the spring and did not bury until late in the season. The timing of mussel emergence varied greatly in different parts of the littoral zone. These results suggest that the functional role of unionid mussels in shallow benthic communities might change seasonally and might change differently at sites exposed to different levels of physical disturbance.

Macroinvertebrate organization along a river was examined to relate biological responses to environmental changes observed across 2 discontinuities (a dam and a tributary). Benthic macroinvertebrates and a range of environmental variables were sampled from 4 study segments (above the dam, below the dam, below the tributary confluence, and in the tributary). Substrate was significantly coarser below than above the dam. In contrast, water-quality variables, such as water temperature and dissolved O₂, changed little below the dam. The most striking discontinuity was substrate coarseness at the tributary confluence. Substrate below the confluence was finer than substrate below the dam and similar to the substrate above the dam. Macroinvertebrate organization differed across the 2 discontinuities. Assemblage composition above the dam was more similar to composition below the confluence than to composition below the dam. The longitudinal organization of the macroinvertebrates could be explained largely by changes in substrate characteristics and habitat preferences of the indicator species. The densities of drifting zooplankton and phytoplankton were higher below than above the dam and were higher below the dam than below the confluence. However, the density of drifting plankton did not differ between the reach immediately above the confluence and the reaches below the confluence. This result suggests that the decrease of zooplankton and phytoplankton occurred above the tributary, probably because of biological entrapment or passive deposition rather than the contribution of the tributary inflow. The dam and tributary caused contrasting discontinuities in macroinvertebrate organization. The tributary generally reversed the dam-related changes to the main stem habitat and the macroinvertebrate community. A key management implication of our study is that efforts to restore dam-related environmental impacts would be facilitated by understanding the role of tributaries downstream of the dam.

Annual migrations of anadromous salmon are an important source of nutrients for many coastal streams. Much of the current research on salmon-derived nutrients has focused on nutrient retention via carcass consumption by mammals, birds, and macroinvertebrates, whereas retention and transfer of nutrients by microbiota has received less attention. Our research objective was to investigate nutrient movement from decomposing salmon tissue into periphyton, bryophytes, leaf-pack microbiota, and amphipods in laboratory mesocosm streams. We measured δ¹⁵N of microbiota growing on unglazed tiles (periphyton), microbiota growing on leaf packs, bryophytes on partially submerged stones, and amphipods; C:N and C:P ratios of microbiota and bryophytes; and periphyton biomass (ash-free dry mass and chlorophyll a) in channels with and without decomposing salmon tissue. Periphyton, bryophytes, and leaf-pack microbiota had lower C:N ratios and leaf-pack microbiota had lower C:P ratios in salmon channels than in reference channels. These results indicate increased nutrient quality in salmon channels. Periphyton ash-free dry mass and chlorophyll a were greater in salmon channels than in reference channels. δ¹⁵N values for periphyton, leaf-pack microbiota, and bryophytes were more enriched in salmon channels than in reference channels, a result that demonstrates that salmon-derived nutrients can be retained in streams through multiple mechanisms. Transfer of salmon-derived nutrients through leaf-pack microbiota to a higher trophic level was evidenced by higher δ¹⁵N in amphipods from salmon channels than from reference channels. Last, higher P concentrations (as much as 90% higher) in biota from salmon channels than from reference channels indicate uptake of salmon-derived P in salmon channels. These results suggest that periphyton, leaf-pack microbiota, and bryophytes might play a critical role in capturing salmon-derived nutrients.

Connections of a stream to its floodplain are important ecological linkages that affect spatial and temporal dynamics of the basal resources available to primary consumers in streams. Suspended organic material and associated microorganisms (seston) vary in quality seasonally and interannually within streams because of changing inputs from riparian and floodplain sources. Researchers have investigated the quality of different size fractions of material, but these differences have not been assessed with respect to the hydrology and the geomorphic structure of streams. We investigated how quality, represented by the stoichiometric ratio C:N, and stable isotopic signature (δ¹³C and δ¹⁵N) of 3 seston size classes varied in Ichawaynochaway Creek, a 5^th-order tributary of the lower Flint River in the Coastal Plain of southwestern Georgia, USA. Samples were collected throughout the basin during varying flow regimes to estimate the quality and source of materials available over different temporal and spatial scales. Our results indicate significant differences in quality and stable isotopic signature based on particle size, discharge, and geomorphic structure of the stream and floodplain (constrained vs unconstrained reaches). The constrained portions of this stream occur in the lower portions of the basin. During low flow conditions, seston had higher quality with less depleted δ¹³C and more enriched δ¹⁵N signatures in the constrained than in the unconstrained portions of the stream. However, during high flow conditions, higher quality seston entered the stream from the adjacent floodplain in all portions of the basin. Insights gained from our study indicate how terrestrial and aquatic linkages and the natural flow regime affect the dynamics of basal resources and their availability to primary consumers in streams.

Omnivorous central stonerollers (Campostoma anomalum) and crayfish (Orconectes meeki meeki) inhabit streams in the Boston Mountain ecoregion of Arkansas, USA, and can have substantial effects on stream ecosystem structure and function through foraging and ecosystem engineering activities. Seasonal summer drying is extensive in these systems and might alter interactions between these consumers and stream benthic communities. Electrical exclusion of consumers was used to examine changes in the effects of fish and crayfish on algal abundance, sediment deposition, sediment organic matter content (as ash-free dry mass [AFDM]), and invertebrate density on tiles during stream drying. Replicate paired exclusion/exposed treatments were placed in 8 pools for two 30-d periods in June and August 2006 during an extended period of stream drying. Consumer exclusion increased algal chlorophyll a 1.5×, sediment organic matter content 2.3×, sediment dry mass 3.4×, and chironomid density 21× relative to levels on exposed tiles. Exclusion effects were greater in August than in June, but effects were not evident in all pools. In June, the strength of consumer effects on chlorophyll a was positively related to % canopy cover, whereas effects on dry mass were positively related to crayfish density, fish predator presence, and % canopy cover but negatively related to depth. In August, the strength of consumer effects on chlorophyll a was positively related to fish predator presence and effects on AFDM were positively related to stoneroller density. These results demonstrate the importance of environmental context when determining how fish and crayfish influence stream functioning and suggest that mechanisms that generate heterogeneity in streams deserve greater attention, especially because disturbances, such as stream drying, are predicted to intensify with climate change.

Microbes are important in stream ecosystem processes and ubiquitous in stream environments, but limitations of study techniques have left most of these microbial communities poorly described. In stream ecosystems, fungal and bacterial communities play critical roles in leaf decomposition and release energy and nutrients to higher trophic levels of the food web. Our research examined microbial communities in Fossil Creek, Arizona, USA, to elucidate effects of litter quality and abiotic habitat characteristics on early microbial colonizers of leaves. High- and low-quality leaf litter was placed in the creek at 5 study sites with heterogeneous environmental conditions (including differing stream morphology, water flow, water chemistry, and travertine deposition). Microbial assemblages that colonized the decomposing leaves were characterized using terminal-restriction fragment length polymorphism analysis and clone library comparisons. Our study revealed differences in microbial community structure along environmental gradients and, to some extent, between high- and low-quality litter in Fossil Creek. Leaf decomposition rates were strongly influenced by both litter quality and abiotic site characteristics, but microbial communities were more strongly influenced by site than by litter quality. Bacterial and fungal communities differed with incubation times: bacterial diversity increased between 2-d and 8- to 9-d incubations, whereas fungal diversity decreased. Fungal community diversity was negatively correlated with decomposition rates after incubation in the creek for 2 d when the community still included nonaquatic fungi, but this relationship did not exist after longer incubation. Bacterial community diversity was not related to litter quality or decomposition rates.

Stream shredders have been reported as scarce in several tropical areas. This pattern is in contrast to observations in temperate streams, which support an abundant and diverse shredder fauna. Two possible explanations for this pattern are that most shredders are adapted to cool conditions and that temperate riparian vegetation often produces more palatable and more nutritious leaves than do the more diverse, tropical rainforests. In peninsular Malaysia, most streams flow through lowland dipterocarp forests that are characterized by strikingly high tree diversity and by many species with tough leathery leaves that are high in lignin and toxic secondary compounds and low in protein. In contrast, highland streams flow through montane rain forests and are more similar to temperate streams. We hypothesized that shredder fauna would be distributed along an altitudinal gradient, with more abundant and diverse assemblages in highland streams. We sampled leaf litter in 12 sites at altitudes from 55 to 1560 m above sea level. As expected, highland sites supported higher abundance and diversity of shredders (9–15 species per site) than did lowland sites (3–8 species per site). Shredder densities were similar among lowland sites, but species composition was variable. Large snails (Brotia spp.) were the dominant shredders in nonacidic streams in Taman Negara, but they were absent from acidic streams and from streams elsewhere in peninsular Malaysia. Shredder biomass was generally high because of the large body size of most species (e.g., crabs, snails, semiaquatic cockroaches, calamoceratids, and tipulids). Large mouthparts might allow these species to shred tough leaves efficiently, and large body size might enable them to tolerate a wide variety of toxic secondary compounds. Our results suggest that shredder diversity might depend on elevation, water temperature, characteristics of the riparian vegetation along altitudinal gradients, and variation in water chemistry at local scales.

Aquatic fungi are important decomposers of plant litter in temperate streams. However, these microorganisms have been poorly studied in tropical streams. We assessed the dynamics of aquatic hyphomycetes during the decomposition of Ficus sp. (Moraceae) leaves in an intermittent lowland stream of northwestern Venezuela. Our goals were to: 1) determine the composition of aquatic hyphomycetes during leaf decomposition, 2) assess the effect of different mesh sizes (i.e., coarse vs fine mesh) on assemblage composition and activity, and 3) assess the role of habitat (i.e., pools vs riffles) on fungal assemblages. Coarse- and fine-mesh litter bags were incubated in a pool and a riffle for 31 d. For each habitat and treatment, we determined physicochemical variables, sporulation activity, and the structure and composition of fungal assemblages. Leaf litter decomposition rates ranged from 0.058/d in fine-mesh bags in the riffle to 0.031/d in coarse-mesh bags in the pool. Decomposition rates were significantly influenced by habitat type, with the highest decomposition rates found in riffle habitats. Sporulation rates were significantly different between habitats but not between mesh sizes. The highest sporulation rate (399 spores mg⁻¹ leaf dry mass d⁻¹) and fungal diversity (16 spp.) was found in coarse-mesh bags in the riffle. Twenty-one fungal species were identified during the study. Anguillospora longissima and Lunuluspora curvula contributed up to 75% of the conidia produced in the coarse- and fine-mesh bags in the riffle, whereas Filosporella aquatica and Lemonniera terrestris accounted for 65 to 75% of the conidia produced in the coarse- and fine-mesh bags in the pool. In ordination space based on species composition, fungal assemblages formed 2 clear groups that corresponded to habitat type. A temporal change in fungal species composition was observed in riffle samples. Early succession assemblages (2–14 d) were characterized by the presence of A. longissima, L. curvula, and F. aquatica, whereas late assemblages (24–31 d) were dominated by Pyramidospora fluminea, Pyramidospora casuarinae, and unidentified sp 4. Overall, habitat type (pool or riffle) influenced sporulation rates and assemblage composition of aquatic hyphomycetes. Our study suggests that environmental factors associated with hydrology (e.g., stream velocity, width, depth) regulate fungal composition and activity and, hence, leaf litter decomposition in this tropical stream.

Increasing evidence suggests that autochthonous foods are the principal basis of consumer production in tropical forest streams, despite the predominance of terrestrial detritus inputs. The relative importance of autochthonous and allochthonous energy for the dominant benthic consumers was investigated in 3 tropical headwater streams with different shading conditions in Hong Kong with a combination of assimilation-based analyses: stoichiometry, C and N stable isotopes, and fatty acid (FA) profiling. The snail Brotia hainanensis (Pachychilidae), shrimps Caridina cantonensis (Atyidae) and Macrobrachium hainanense (Palaemonidae), and their potential basal food sources (leaf litter, fine particulate organic matter [FPOM], periphyton, cyanobacteria, and filamentous algae) were collected in Tai Po Kau Forest Stream (shaded 1), Shing Mun Stream (shaded 2), and Pak Ngau Shek Stream (open) during the 2004 dry season (January and February). All samples were analyzed for C:N ratios, δ¹³C, and δ¹⁵N values. Total FAs were extracted from each sample, and concentrations of 35 important FAs were analyzed by gas chromatography–mass spectrometry (GC–MS). C:N ratios of algal foods were markedly lower than those of terrestrial detritus and similar to those of the test animals at all 3 sites, a result that suggested that autochthonous sources were relatively more nutritious than were allochthonous sources. Autochthonous foods were more ¹³C and ¹⁵N enriched than were allochthonous foods at all sites. The algal sources contributed to 29 to 98% of consumer biomass, generally more than was attributed to the terrestrial sources (2–71%). Consumers also showed distinctive FA profiles indicating consumption of autochthonous foods, especially periphytic diatoms and cyanobacteria, as revealed by the elevated concentrations of FA biomarkers such as palmitoleic (16:1[cis-9]) and eicosapentaenoic acids (20:5[all cis-5,8,11,14,17]) in the consumers, periphyton, and cyanobacteria. Our results suggest that autochthonous resources are possibly more important than allochthonous foods to secondary production in tropical headwater streams.

Comparisons of the effects of leaf litter chemistry on leaf breakdown rates in tropical vs temperate streams are hindered by incompatibility among studies and across sites of analytical methods used to measure leaf chemistry. We used standardized analytical techniques to measure chemistry and breakdown rate of leaves from common riparian tree species at 2 sites, 1 tropical and 1 temperate, where a relatively large amount of information is available on litter chemistry and breakdown rates in streams (La Selva Biological Station, Costa Rica, and Coweeta Hydrologic Laboratory, North Carolina, USA). We selected 8 common riparian tree species from La Selva and 7 from Coweeta that spanned the range of chemistries of leaf litter naturally entering streams at each site. We predicted that concentrations of secondary compounds would be higher in the tropical species than in the temperate species and that high concentrations of condensed tannins would decrease breakdown rates in both sites. Contrary to our predictions, mean concentration of condensed tannins was significantly greater (2.6×, p < 0.001) for species at Coweeta than for species at La Selva. Concentration of condensed tannins was negatively correlated with breakdown rate among Coweeta species (r = −0.80), not among La Selva species, and negatively correlated when the 2 sites were combined (r = −0.53). Concentrations of structural compounds were strongly correlated with breakdown rate at both sites (Coweeta species, lignin r = −0.94, cellulose r = −0.77; La Selva species, cellulose r = −0.78, C r = −0.73). The chemistries of 8 riparian species from La Selva and 7 riparian species from Coweeta were not as different as expected. Our results underline the importance of standardized analytical techniques when making cross-site comparisons of leaf chemistry.

Pacific island stream communities are species-poor because of the effects of extreme geographic isolation on colonization rates of taxa common to continental regions. The effects of such low species richness on stream ecosystem function are not well understood. Here, we provide data on community structure and leaf litter breakdown rate in a virtually pristine stream on a remote island in the eastern Caroline Islands of Micronesia. The Yela River catchment on Kosrae, Federated States of Micronesia, is uninhabited, completely forested, and not traversed by any road. At each of 5 sampling stations along the Yela River we measured physical and chemical variables, characterized the benthic invertebrate community, and estimated the relative abundance of macroconsumers (fishes, shrimps, and snails) using snorkeling surveys and trapping. Benthic invertebrate biomass decreased with stream size and was dominated by macroconsumers (i.e., decapods and gastropods). Benthic insect species richness and density were extremely low, characterized by the complete absence of Ephemeroptera, Trichoptera, and Plecoptera, with nonshredding larval Chironomidae making up ∼85% of the exceptionally low insect biomass (∼0.6 mg dry mass/m²). Six species each of fishes and shrimps and 4 snail species were found. We examined the functional role of these noninsect macroconsumers in organic matter processing by comparing litter breakdown rates inside and outside electrified macroconsumer-excluding quadrats at a single site in the lower Yela River. Breakdown rates were slow relative to those reported in most tropical litter decay studies (k = 0.004–0.018 among quadrats). Despite differences in macroconsumer density (p = 0.01), the leaf breakdown rate was not significantly different between reference and electrified treatments (p = 0.77). Thus, macroconsumers at this site had neither a direct role in shredding nor an indirect effect on leaf breakdown through their interactions with any nonexcluded taxa. In the absence of a significant role played by insect or macroconsumer shredders, leaf litter breakdown in streams of remote Pacific oceanic islands might be driven purely by physical and microbial processes. Our results suggest that biogeographic processes have placed some Pacific island streams at the extreme low end of a continuum of shredder biodiversity and biomass, restricting the ecosystem function of their species-poor communities.

Approximately 0.5 million km of tropical stream channel are modified by catchment deforestation annually, but the consequences of this process for community structure are poorly understood because of a dearth of data from tropical regions. We compared the algal communities of epilithic biofilms from 3 tropical rainforest streams draining Ranomafana National Park (RNP) in eastern Madagascar and 3 open-canopy streams draining RNP's deforested peripheral zone. Forest and open-canopy streams differed in canopy cover and mean water temperature but did not differ in substrate composition or major nutrient chemistry. We recorded 137 algal taxa, of which ∼45% can be considered endemic or potentially endemic. Deforestation had significant effects on algal community structure. Complete separation between forest and open-canopy streams was observed in nonmetric multidimensional scaling ordinations based on species cell densities, species presence–absence, and cell densities of algal growth forms. Forest streams were characterized by higher species richness and cell densities of motile and solitary growth forms (e.g., Navicula spp.) than were open canopy streams. Open-canopy streams had more variable community structure than forest streams and were characterized by prostrate and solitary (e.g., Planothidium spp.) and chain-forming/stalked growth forms (e.g., Gomphonema spp.). Community shifts and reductions in species richness observed in open-canopy streams show that diatom biodiversity might be affected adversely by vegetation removal in the catchments we studied. Given that Madagascar has lost most of its rainforest in recent centuries, it is reasonable to assume that historical deforestation has led to shifts in algal assemblages at broader regional scales. Our results also suggest that global algal diversity could be affected by tropical deforestation if similar patterns of endemism and alteration of algal assemblages occur in the 0.5 million km of stream channel affected by tropical deforestation annually.

Landuse changes might alter N cycling in tropical aquatic ecosystems, but understanding of N cycling in tropical streams is limited. We measured actual and potential denitrification rates during the dry season in Río Las Marías, a 4^th-order Andean piedmont stream in Venezuela. Our objectives were to describe spatial and temporal variation in denitrification, quantify the effects of nutrient availability and substratum particle size on denitrification, and explore potential effects of anthropogenic sedimentation on denitrification. In 4 experiments, actual and potential denitrification rates ranged from 0 to 160 and from 0 to 740 μg N₂O-N m⁻² h⁻¹, respectively. Rates were distributed approximately log-normally because of spatial variation. During a 1-mo period, actual denitrification rates decreased exponentially from 37 ± 39 to 5 ± 7 μg N₂O-N m⁻² h⁻¹ (mean ± SD), probably because of a decline in water-column NO₃-N concentration from 41 ± 14 to 12 ± 3 μg NO₃-N/L. The texture (particle size) of stream substrata markedly affected denitrification rates. Actual rates were low in cobble, gravel, and fine sediments (<5 mm), but in fine sediments, rates increased in response to addition of excess NO₃-N and organic C. In a 3-km stream reach, actual (but not potential) denitrification rates increased with the proportion of fine sediments (<2 mm) in mixed substrata. This increase was nonlinear, and the threshold value occurred at 37% fine particles, above which actual denitrification rates were almost always high. An experiment simulating the effects of anthropogenic sedimentation showed that topsoil inputs resulted in denitrification rates ∼8× higher than rates in trials where excess NO₃-N and organic C were supplied. Denitrification is a small but potentially significant sink for available N in this N-limited system. Anthropogenic sedimentation associated with landuse change might significantly increase denitrification rates in streams.

Exotic plants have invaded the riparian zones of many streams worldwide, but their consequences for stream fauna are seldom fully appreciated, especially when effects are sublethal. In northern New South Wales, Australia, the exotic tree camphor laurel (Cinnamomum camphora) has aggressively invaded riparian zones of many subtropical streams, often forming monocultures and outcompeting native vegetation. In forested streams, leaf litter provides a major source of food for some stream fauna and is broken down by shredding invertebrates. We hypothesized that shredder densities would be reduced in streams where the riparian zone was dominated by camphor laurel because of the reputed chemical toxicity of the leaf litter. We also investigated the sublethal effects of camphor laurel litter by comparing growth rates of larvae of the common shredding caddisfly Anisocentropus (Calamoceratidae:Trichoptera) reared on a diet of camphor laurel vs native leaf litter. Shredder densities were significantly lower in streams where camphor laurel constituted >38% of the benthic leaf litter than in streams with a lower percentage of camphor laurel litter (0.43 ± 0.13 vs 1.43 ± 0.28 individuals/g dry mass of litter; mean ± SE). Shredder densities were greater on artificial packs of native leaf litter than on packs of camphor laurel litter in camphor laurel–infested streams (3.37 ± 1.01 vs 1.40 ± 0.46 ind./g dry mass of litter). However, shredder densities did not differ between leaf-litter types in streams lined with native vegetation. Anisocentropus growth was retarded when larvae were fed only camphor laurel compared to those reared on native leaf litter. This native caddisfly used camphor laurel leaves for case construction, but impaired larval growth in infested streams implies a sublethal impact that has repercussions for energy transfer to higher trophic levels (e.g., predatory fish). Our findings suggest effects similar to those often reported for shredder assemblages in temperate streams subject to invasion by exotic riparian plants, indicating that the tropical–temperate differences in this aspect of leaf breakdown might not be as marked as previously suspected. Carefully managed river restoration to remove camphor laurel to allow recovery by native vegetation is recommended in these streams. Population dynamics of Anisocentropus could provide a useful indicator of the ecological success of such restoration.

Opinion about latitudinal diversity gradients in freshwater fauna has varied over past decades. Global data have been compiled for diversity of Ephemeroptera, Plecoptera, and Trichoptera at the site scale, but no assessment of global variability in regional diversity has been done for any taxon except fish. Global variation cannot be inferred from site-scale data because the relationship between site and regional diversity is not necessarily linear. We have assembled global data on regional diversity for 7 freshwater taxa. Here, we plot diversity against regional area, develop diversity–area regressions, and then plot the residuals against latitude to investigate latitudinal diversity gradients. The existence and directions of gradients vary substantially among taxa. For Ephemeroptera and Plecoptera, significant latitudinal gradients exist, and diversity is greater at higher latitudes. For Trichoptera and Caudata, latitudinal gradients are not apparent, although taxa within the Caudata show distinct patterns. For Odonata, Osteichthyes, and Anura, latitudinal gradients are highly significant, and diversity is greatest at low latitudes. A clear distinction between the life cycles of tropically diverse taxa and of other taxa (excluding fish) is a long terrestrial phase in the tropically diverse taxa (e.g., Odonata). We discuss reasons for the gradients and this contrast in ecological and evolutionary contexts (e.g., habitat complexity and cross-habitat adaptation).