Leaves of a forest-floor plant, Hexastylisarifolia, were subjected to rapid water movement at shallow depths to approximate sheet-flow conditions during flooding. When held <20 mm beneath the water's surface by the basal ends of their petioles, leaves surfaced, apparently through hydrodynamic lift, and aquaplaned with very low drag relative to surface area. When held >20 mm below the water surface, leaves curled into cones that had higher drag. Both measured and extrapolated drag of aquaplaning and submerged leaves were well below petiole breaking strength. Leaves of some other herbaceous plants that might occasionally be exposed to sheet flow behaved similarly; most had blades with upwardly curved basal lobes and fairly long petioles. Tree leaves with similar basal lobes and long petioles did not aquaplane. The biological significance of low drag during aquaplaning remains uncertain because petiole strength and root strength should be enough to hold the plant in place during sheet flow even without aquaplaning. Moreover, flooding with sheet flow is a rare event for most of these plants.

Chironomus salinarius larvae are the dominant benthic organisms in the salt pans of the Odiel marshes, southwest Spain, an area internationally important for migratory shorebirds. Predator exclosures were used to examine the effects of shorebirds on chironomid larvae for periods of 32 to 46 d during spring migration. During the 1^st experimental period, shorebird numbers were relatively low, peaking at 4 feeding birds/ha. During the 2^nd period, shorebird numbers were higher, peaking at 32 feeding birds/ha. The changes in larval density and biomass from the beginning to the end of each period were significantly greater in exclosures than in controls. Predation decreased larval density by 35% and 32% during periods 1 and 2, respectively. Predation decreased larval biomass by 37% and 49% during periods 1 and 2, respectively. Predation also changed prey size distributions, reducing the proportion of larger larvae. These effects did not vary significantly between periods, suggesting that increased predation in period 2 was compensated for by higher larval growth or recruitment in period 2. Variation in predation effects was not related to variation in the initial larval density or biomass in a given plot.

Littoral benthic macroinvertebrate communities in lake reservoirs often are exposed to repeated fluctuations in water level, but little is known of the effects of drawdown on benthic community composition. We compared the taxonomic composition and spatial patterns of benthic macroinvertebrate communities in sediments of littoral areas in a reservoir with >30 y of seasonal drawdowns (Sooke Lake Reservoir [SLR]) and a natural lake with little seasonal change in water levels (Shawnigan Lake [SHL]). Contrary to our predictions, macroinvertebrate density and biomass usually were greater in SLR than in SHL. In SLR, densities and biomasses of macroinvertebrates, especially chironomids, were higher below the drawdown exposure zone than in the upper littoral area. Chironomids with r-selected survival strategies (i.e., smaller size) or desiccant-resistant stages appeared well suited to the fluctuating environment of littoral zones in reservoirs. Orthocladiinae, Chironomini, Tanytarsini, and Tanypodinae dominated at sampling sites immediately below the drawdown exposure zone in SLR, whereas only Orthocladiinae dominated at deeper sites. Warm water from an expanded epilimnion apparently extended the distribution of Diamesinae farther from shore in SLR than in SHL. Chironomini generally were the dominant macroinvertebrate taxon in SHL, and the relative biomass of Diamesinae increased with depth. Distributions of oligochaetes and nematodes extended farther from shore in SLR than in SHL. These contrasting benthic macroinvertebrate communities indicate that variable drawdown regimes could have significant impacts on benthic food webs and the transfer of energy and nutrients to the pelagic area.

Drought leads to a loss of longitudinal and lateral hydrologic connectivity, which causes direct or indirect changes in stream ecosystem properties. Changes in macrohabitat availability from a riffle–pool sequence to isolated pools are among the most conspicuous consequences of connectivity loss. Macroinvertebrate assemblages were compared among 3 distinct stream macrohabitats (riffles [R], pools connected to riffles [Pc], disconnected pools [Pd]) of 19 Mediterranean-climate sites in northern California to examine the influence of loss of habitat resulting from drought disturbance. At the time of sampling, 10 sites were perennial and included R and Pc macrohabitats, whereas 9 sites were intermittent and included only Pd macrohabitats. Taxa richness was more variable in Pd, and taxa richness was significantly lower in Pd than in Pc but not R. These results suggested a decline in richness between Pc and Pd that might be associated with loss of connectivity. Lower Ephemeroptera, Plecoptera, and Trichoptera (EPT) richness relative to Odonata, Coleoptera, and Heteroptera (OCH) richness was observed for Pd than R and Pc macrohabitats. Family composition was more similar between R and Pc than between R or Pc and Pd macrohabitats. This similarity may be associated with greater connectivity between R and Pc macrohabitats. Correspondence analysis indicated that macroinvertebrate composition changed along a gradient from R to Pc and Pd that was related to a perennial–intermittent gradient across sites. High variability among macroinvertebrate assemblages in Pd could have been related to variability in the duration of intermittency. In cluster analysis, macroinvertebrate assemblages were grouped by macrohabitat first and then by site, suggesting that the macrohabitat filter had a greater influence on macroinvertebrate assemblages than did local site characteristics. Few taxa were found exclusively in Pc, and this macrohabitat shared numerous taxa with R and Pd, indicating that Pc may act as a bridge between R and Pd during drought. Drought is regarded as a ramp disturbance, but our results suggest that the response of macroinvertebrate assemblages to the loss of hydrological connectivity among macrohabitats is gradual, at least in Mediterranean-climate streams where drying is gradual. However, the changes may be more dramatic in arid and semiarid streams or in Mediterranean-climate streams if drying is rapid.

Effective catchment management in the face of landuse alteration depends on our ability to quantify ecologically significant changes and to discriminate among varying levels of impact. We compared the efficiency of traditional structural indices of change (species composition) with functional measures based on species traits (including life-history, trophic, and morphological features) in an analysis of grassland streams along a gradient of agricultural development (ungrazed native tussock, grazed tussock, extensively grazed pasture, and intensive dairy and deer farming). Streams were categorized in relation to overall agricultural intensity, and separately in terms of increasing nutrient concentrations and fine sediments on the streambed. Only 5 of 60 individual species demonstrated a significant separation across the landuse gradient, whereas 14 of 53 trait categories did so. Traits associated with population resilience (short generation time, asexual reproduction) became more prevalent with more intense agricultural pressure, reflecting predicted increases in intensity and frequency of stream disturbance. We observed a shift away from a tendency to lay unattached eggs at the water surface of the stream, reflecting the increasing likelihood of smothering by sediment, as well as increases in highly flexible and streamlined body shapes. Principal components and correspondence analyses involving species or trait composition all were able to discriminate landuse practises, but more of the overall between-landuse variance was accounted for by trait composition than species composition. All biological measures (species or trait composition) were correlated with both nutrient concentrations and sedimentation. Nutrient concentrations were better related to species and trait-category densities, whereas sedimentation was related to trait-category relative abundances. Overall, species traits successfully complemented traditional structural measures by helping to differentiate the consequences of landuse intensification in grassland stream communities. Rather than simply recording a loss or reduction of species, our functional approach was able to identify sensitive life-history characteristics linked to the functioning of ecosystems, thereby facilitating the development of targeted management actions.

We sampled 6 rivers of the piedmont region of Venezuela during the dry seasons of 1997 and 1998 to compare physical, chemical, and biological features and evaluate the influence of natural and anthropogenic differences in their watersheds. These 6 watersheds lie in a northeast-to-southwest transect along the interior slope of the Andes. Human disturbance declines and elevation of river origin increases along this transect. All watersheds experience a pronounced dry season from December through March, but rivers to the southwest exhibit less pronounced seasonality in flow and higher water yields. Sediment concentrations were highest in the Río Acarigua, which experiences the greatest human disturbance, but also were high in the least disturbed Río Bumbun, apparently from natural causes. Dissolved nutrient concentrations were lowest in the Río Acarigua, whereas less disturbed rivers contained higher amounts of dissolved nutrients. We collected 46 taxa of invertebrates and 50 fish species. Invertebrates and fish exhibited greatest diversity in rivers intermediate in the gradient. These more diverse systems may experience an intermediate level of disturbance that favors higher diversity, compared to the 2 rivers in the least disturbed landscapes. Cluster analysis done separately using invertebrates and fish separated rivers in the northeast from those in the southwest. However, a full explanation of these trends will have to address the intertwined effects of the natural biogeographical differences and the different human impacts that occur along the gradient.

Diadromous faunas dominate most tropical coastal streams and rivers, but the factors controlling their distribution are not well understood. Our study documents abiotic variables controlling the distribution and abundance of the diadromous snail Neritina virginea (Gastropoda:Neritidae) in the Caribbean island of Puerto Rico. An intensive survey of N. virginea density and shell size, and channel substrate, velocity, and depth was conducted at microhabitat, habitat, and reach scales of a coastal plain reach of the Río Mameyes between August and December 2000. In addition, the inland extent of distribution (stream-network scale) and presence (regional scale) of N. virginea were surveyed in 32 coastal rivers around the island during summer 2001 and 2003. At the microhabitat scale, snail density and microhabitat electivity were greater in patches consisting of a mix of boulders and cobbles than in other types of substrate. At the habitat scale, snail density increased with depth. At the reach scale, snail density increased with fast and turbulent flows (riffle > pools > pond), whereas snail size showed the opposite pattern. At the regional scale, populations were present in 13 of 32 streams. Populations of N. virginea were not found in rivers that were disconnected from the ocean for most of the year because of channel dewatering, formation of sediment bars at their mouths, and low mean monthly discharge (Q = 0.69 m³/s). In contrast, rivers with N. virginea populations had a permanent (Q = 4.04 m³/s) or seasonal (Q = 2.88 m³/s) connection to the ocean over the year. At the regional scale, the inland distribution of populations was not correlated with stream gradient, but was negatively correlated with concentrations of SiO₂, P, and acid neutralizing capacity of the water. Populations colonized montane reaches in only 5 rivers, all of which were forested and protected. Our study highlights the importance of taking a hierarchical approach in managing tropical coastal rivers, and the usefulness of neritid snails as biological indicators of the physical and chemical integrity of rivers.

Extreme events (hurricanes, floods, and droughts) can influence upstream migration of macroinvertebrates and wash out benthic communities, thereby locally altering food webs and species interactions. We sampled palaemonid river shrimp (Macrobrachium spp.), dominant consumers in headwaters of the Luquillo Mountains of northeastern Puerto Rico, to determine their distributions along an elevational gradient (274–456 m asl) during a series of disturbances (Hurricane Hugo in 1989, a drought in 1994, and Hurricane Georges in 1998) that occurred over a 15-y period (1988−2002). We measured shrimp abundance 3 to 6 times/y in Quebrada Prieta in the Espiritu Santo drainage as part of the Luquillo Long-Term Ecological Research Program. In general, Macrobrachium abundance declined with elevation during most years. The lowest mean abundance of Macrobrachium occurred during the 1994 drought, the driest year in 28 y of record in the Espiritu Santo drainage. Macrobrachium increased in abundance for 6 y following the 1994 drought. In contrast, hurricanes and storm flows had relatively little effect on Macrobrachium abundance.

Stream physicochemistry and insect assemblages were studied in lowland tropical streams in Costa Rica to determine physicochemical variables explaining temporal and spatial variations in insect density and biomass. Streams drained a tropical wet forest landscape and had similar geomorphology, but differed in their water chemistry because of differential inputs of solute-rich (i.e., P, Cl^–, and Mg) geothermally modified groundwater. We hypothesized that, within a stream, physical factors, such as discharge, would result in temporal variation in insect assemblages, whereas interstream differences in solute composition would result in differences in insect assemblages among streams. Insect density, biomass, richness, and stream physicochemistry were measured monthly for 1 y in 6 streams. Insect samples were collected with a core sampler in runs where leaf litter overlying fine sediments was the main substrate. Streams with high inputs of geothermally modified groundwater had high conductivity and high concentrations of soluble reactive P (SRP). Annual temporal patterns in stream physicochemistry were related either to rainfall, with subsequent changes in discharge, or to the % geothermally modified groundwater entering streams. Streamwater pH decreased throughout the year, and unbuffered, nongeothermally modified streams changed from near neutral (pH >6) to near acidic (pH <4.5). In all streams, insect density and biomass were highest during the dry season and lowest during the wet season. Insect density and biomass were related positively to pH and to the number of days since the last storm. Insect taxonomic composition was similar among streams. Moreover, interstream variation in insect assemblages was small, and insect density and biomass were not related to any of the measured physicochemical variables, including SRP, which ranged from <5 to 267 μg/L. Overall, the physicochemical characteristics of the study streams were related mainly to geothermally modified groundwater inputs and seasonal patterns in rainfall. Streams with geothermally modified groundwater inputs were better buffered against pH changes than the other streams. Insect assemblages reflected fluctuations in stream physicochemistry during the year and were associated with 2 main factors: floods and pH. Physicochemical characteristics were very different among streams, but insect assemblages were not related to them. We hypothesize that the heterogeneous nature of benthic substrata in the streams resulted in high variation in density and biomass of insects, potentially obscuring differences in insect assemblages among streams.

The annual flood pulse in tropical lowland rivers creates a continually moving land–water margin that forces aquatic organisms of the littoral zone to relocate to new habitats at intervals ranging from days to weeks. Thus, species assemblages in patchy littoral-zone habitats have large potential to be influenced by stochastic colonization dynamics, whereby organisms may exhibit weak selection for mesohabitat features and arrive on a patch in no particular sequence. We examined species assemblages of fishes and macroinvertebrates occupying habitats of varying structural complexity through an annual flood cycle of the Cinaruco River, a species-rich floodplain river in the southern llanos of Venezuela. We collected 268 standardized samples that contained 54,596 individual fishes representing 156 species (23 families) and 6973 macroinvertebrates representing 8 families. Analysis of species-specific patterns of habitat occupancy based on a randomization procedure indicated greater species selectivity during low water (92% of taxa) relative to rising- and falling-water periods (60% and 63% of taxa, respectively). Community-wide correspondence analysis (CA) revealed similar patterns. Assemblages from similar habitat types clustered together in the CA ordination, and these nonrandom patterns were most apparent during low water. Results from partial canonical correspondence analysis revealed that assemblage structure was strongly influenced by habitat type (patches of different composition located within the shallow littoral zone of either the river main channel or lagoons), which accounted for 56% of the total explainable variation in the species × sample matrix. Mesohabitat parameters (e.g., water velocity, depth), sampling period (month), and location on the landscape explained 30%, 23%, and 17% of variation, respectively. As water level changes and the aquatic–terrestrial interface moves across the landscape, species assemblages in patchy littoral habitats, while highly variable, reconstitute with significant determinism. At the same time, a large proportion of unexplained variation in assemblage structure probably is associated with unmeasured components of landscape variation as well as a stochastic element in colonization dynamics.

Linkages between geology and fish assemblages have been inferred in many regions throughout the world, but no studies have yet investigated whether fish assemblages differ across geologies in Mesoamerica. The goals of our study were to: 1) compare physicochemical conditions and fish-assemblage structure across 2 geologic types in headwaters of the Monkey River Basin, Belize, and 2) describe basin-scale patterns in fish community composition and structure for the benefit of conservation efforts. We censused headwater-pool fishes by direct observation, and assessed habitat size, structure, and water chemistry to compare habitat and fish richness, diversity, evenness, and density between streams in the variably metamorphosed sedimentary geologic type typical of 80% of Belize's Maya Mountains (the Santa Rosa Group), and an anomalous extrusive geologic formation in the same area (the Bladen Volcanic Member). We also collected species-presence data from 20 sites throughout the basin for analyses of compositional patterns from the headwaters to the top of the estuary. Thirty-nine fish species in 21 families were observed. Poeciliids were numerically dominant, making up 39% of individuals captured, followed by characins (25%), and cichlids (20%). Cichlidae was the most species-rich family (7 spp.), followed by Poeciliidae (6 spp.). Habitat size and water chemistry differed strongly between geologic types, but habitat diversity did not. Major fish-assemblage differences also were not obvious between geologies, despite a marked difference in the presence of the aquatic macrophyte, Marathrum oxycarpum (Podostemaceae), which covered 37% of the stream bottom in high-nutrient streams draining the Santa Rosa Group, and did not occur in the low-P streams draining the Bladen Volcanic Member. Correlation analyses suggested that distance from the sea and amount of cover within pools are important to fish-assemblage structure, but that differing abiotic factors may influence assemblage structure within each geologic type. The fauna showed weak compositional zonation into 3 groups (headwaters, coastal plain, and nearshore). Nearly 20% of the fish species collected have migratory life cycles (including Joturuspichardi, Agonostomusmonticola, and Gobiomorusdormitor) that use freshwater and marine habitats. Some of these species probably rely on a natural flow regime and longitudinal connectivity for reproduction and dispersal of young, and natural flow regime and longitudinal connectivity are important factors for maintenance of functional linkages between the uplands and the coast in the ridge-to-reef corridor where the Monkey River is located. Therefore, we suggest that the viability of migratory fish populations may be a good biological indicator of upland-to-estuary connectivity important both to fishes and coastal ecosystem function. We recommend follow-up studies to substantiate the relative strengths of relationships between community structure and abiotic factors in contrasting geologies and to examine potential bottom−up responses of stream biota to the higher nutrient levels that were observed in stream waters draining the Santa Rosa Group geologic type.

Our study evaluated spatiotemporal variation of physicochemical parameters and benthic and sestonic algal biomass in littoral areas of the main channel and floodplain lakes of the Cinaruco River, a lowland tributary of the Orinoco River in Venezuela. The Cinaruco is characterized by a highly predictable annual flood pulse, high transparency, and extremely low conductivity and suspended sediment load. During 2002 and 2003, 10 sites in the main channel and floodplain lagoons were sampled on 12 occasions. Correspondence analysis based on water physicochemical variables and algal biomass showed that ˜80% of the variation in the multivariate space among sites and sampling dates was explained by 2 axes. During the high-water periods, river and lagoon sites showed high similarity in water variables and algal biomass. Conductivity, SiO₃, and chlorophyll a concentrations (both benthic and sestonic) were low during the high-water phase. On the other hand, water variables and algal biomass differed between river and lagoon sites during the low-water period. The absence of flow in lagoons and consistently low algal biomass (sestonic and benthic) in river sites were the most important features of the spatial variability between main channel and lagoon sites during low-water phases. Benthic chlorophyll a was highly uniform at small spatial scales and significantly heterogeneous at large spatial scales. The annual flood regime of the Cinaruco, which drives the concentrations of dissolved materials and affects material interchanges between aquatic and terrestrial systems, also appears to be responsible for creating strong patterns of seasonal and spatial variation in benthic algal crops.

We examined primary production, respiration, and nutrient dynamics in littoral areas of the mainstem and lagoons of the Cinaruco, a nutrient-poor river in the Venezuelan llanos. Gross primary productivity (GPP) was relatively high, given the poor nutrient conditions in this river. Seasonal variability in net ecosystem production (NEP) was also high, with highest values occurring in the dry season (March–April) when fish biomass and chlorophyll a (both water-column and benthic) levels were greatest (dry-season NEP = 542 mg C m⁻² d⁻¹, wet-season NEP = 303 mg C m⁻² d⁻¹). NEP and algal biomass (measured as chlorophyll a) were higher in lagoons than at river sites, with more pronounced differences between these 2 habitats during the dry season. Strong N limitation was evident. Dissolved inorganic N (DIN) concentrations always were <2 μM and typically were <0.5 μM. Molar ratios of DIN:SRP (soluble reactive P) varied little seasonally and were always <8. As in other Neotropical aquatic systems, water-column productivity was an important source of organic matter and was >2× benthic productivity, even in shallow regions of the riverine ecosystem. Low nutrient levels combined with high rates of autochthonous productivity in the littoral zone of this river suggest extremely rapid nutrient cycling rates and support the view that the littoral regions may be important in providing the organic matter that maintains secondary production, consistent with the Riverine Productivity Model and other observations in the Orinoco Basin.

The Cinaruco River, an oligotrophic ecosystem in the Venezuelan llanos (savanna), has strong seasonal hydrology and supports large populations of ecologically diverse fishes. The relative contributions of autochthonous and allochthonous production sources that support high stocks of secondary consumers are undetermined in this river. We used excitation–emission fluorescence spectroscopy and absorption spectra of dissolved organic matter to infer degradation of leaf material originating from the surrounding gallery forest. During the low-water period, a large fraction of fluorescent organic matter contained in leaves degraded quickly in river water and was an important allochthonous contribution of C to the system. However, the fluorescence signature of dissolved organic matter in lagoons was different from that of the main river channel during the falling-water period, suggesting that other sources of C were present. Allochthonous organic matter clearly fueled microbial respiration during the falling-water period, but our in-water experiments using light–dark bottle methods indicated that autochthonous production was an important supplementary C source in shallow nearshore waters. During the low-water and falling-water periods, water-column primary production in nearshore waters ranged from 150 to 500 mg C m⁻² d⁻¹ and was 2× greater than community respiration, i.e., the nearshore component of the water column was net autotrophic. Benthic primary production in nearshore areas where light reached the sediments ranged between 350 and 500 mg C m⁻² d⁻¹ and was about equal to community respiration. Primary production was probably limited by the availability of dissolved inorganic N, which sometimes was below detection limits of ˜0.5 μM (mean ˜0.25 μM). Our results support the idea that autotrophic production is an important C source in neotropical rivers.

Temperate headwater streams with closed canopies rely on inputs of terrestrially derived organic matter to provide the major energy basis for their food webs. Microbial colonization, or conditioning, makes leaf litter more nutritional and palatable to stream detritivores, but few studies have investigated the relative importance of litter source to macroshredders in tropical streams. We determined the source (terrestrial, aquatic, or aerial), quantity, and species composition of allochthonous inputs into the Quebrada Prieta, a tropical headwater stream in Puerto Rico, as a first step toward understanding the importance of conditioning history to rates of tropical leaf-litter processing by decapod consumers. Fresh leaves of 4 common species of leaves were treated by exposing them to different conditions for 2 wk. These exposure treatments (conditioning histories) represented routes by which leaves might enter streams and included submersion (aquatic input), incubation on the streambank soil (terrestrial input), and suspension above the ground (aerial input). Conditioned leaves were placed in small experimental microcosms with or without shrimp (Xiphocariselongata) for 20 d. Shrimp significantly increased the rate of decomposition of all leaf species independent of conditioning history. Conditioning history had little effect on breakdown rates independent of the presence of shrimp. One species (Roureasurinamensis) had faster mass loss when the leaves were conditioned as aquatic inputs rather than as terrestrial or aerial inputs. Our results indicate that conditioning history has little effect on the ability of some macroconsumers to alter detrital foodweb dynamics in tropical streams. Tropical stream ecosystems may function differently from temperate ecosystems because of the dominance of large detritivores such as shrimps.

The aim of our study was to determine experimentally the influence of the chemical composition of native leaf species on feeding preferences and growth of the tropical shredder Phylloicus sp. We tested whether Phylloicus was able to discriminate among leaf species, and whether leaf species affected its growth rate. The native leaf species selected for the study were: Anacardiumexcelsum (Anacardiaceae), Ficus sp. (Moraceae), Huracrepitans (Euphorbiaceae), and Tabebuiarosea (Bignoniaceae). Phylloicus larvae and leaves of native tree species were collected from Caño Carichuano, a 2^nd-order intermittent stream located in northwestern Venezuela. Larvae were acclimated to laboratory conditions, and leaves were air dried and stored before beginning the experiments. Additional leaves were used to determine initial chemical composition (P, lignin, N, and polyphenols). Feeding experiments were conducted in which larvae were given a choice of different leaf species. Phylloicus preferred to feed on leaf species with high nutrient content and low lignin and polyphenol concentrations (Ficus sp.) and rejected leaves with low nutrient content and high lignin and polyphenol concentrations (Anacardium). However, Phylloicus selected Anacardium leaves for case-building. Growth of Phylloicus sp. fed leaf discs of Ficus or Tabebuia did not differ.

Leaf litter is a major component in the organic matter budgets of streams worldwide. Shredding invertebrates are widely considered to be of central importance for the breakdown of allochthonous organic material in temperate-zone streams. However, various authors report an absence of this group in tropical streams. Various phenomena, including hydraulic disturbance, chemical leaf quality, and biotic control through macroconsumers, may cause variable shredder performance in streams. Our paper discusses the hypothesis that biogeographic distribution and the life-cycle strategies of the shredders are additional factors affecting the contribution of invertebrates to lotic decomposition processes. As a case study, we analyzed the type of organic matter inputs, the community of benthic invertebrate shredders, and the decomposition of temperate zone leaves (alder, Alnusglutinosa) in the Breitenbach (BRB), a temperate stream in Hesse, Germany, and in the Córrego Tenente Amaral (CTA), a neotropical Cerrado stream in Mato Grosso, Brazil. Quantities of natural leaf-litter inputs were comparable in the 2 systems (CTA: 820 g m⁻² y⁻¹, BRB: 700 g m⁻² y⁻¹), but the diversity of trees varied from 60 species/ha in CTA to 5 in BRB. Several shredding species were found in CTA, but in very low abundances. Larvae of the shredding calamoceratid caddisfly Phylloicus sp. were found only in lateral pools and not in the stream channel at CTA. Earlier decomposition experiments with naturally fallen native leaves did not indicate any importance of shredders at CTA, but green leaves of A. glutinosa were rapidly consumed by mining chironomids (Stenochironomus sp.). Decomposition rates were faster in CTA (–k = 0.035 ± 0.006) than in BRB during wintertime (0.0095 ± 0.0021). We conclude that detritivorous neotropical insects may have 2 different species traits. Either they have mass occurrences during short periods when their food source is available and they must be adapted to a secondary, permanently available resource on which they can survive for the rest of the time (facultative specialists), or they are restricted to the few sites that provide a permanent food source (localized specialists). Apart from these special cases, however, the community in the neotropical stream was largely composed of omnivores.

Decomposition of allochthonous material is a central process in organic matter budgets of low-order streams. Shredding invertebrates contribute substantially to the decomposition of leaves in northern-temperate-zone streams, but their role in tropical streams is less clear. Hydrology has an overarching control on ecological processes in streams, especially on floodplains, where flooding of the mainstem river may cause backflooding in the tributaries. Decomposition experiments with leaves of Cecropialatiloba, Tessariaintegrifolia, and Symmeriapaniculata were used to assess the importance of hydrology and benthic invertebrates to decomposition in a neotropical floodplain stream. Leaf breakdown rates and leaf-associated aquatic invertebrate assemblages were compared among leaf types in a typical 1^st-order stream of the Amazonian whitewater floodplain forest near Leticia, Colombia. The stream hydrograph was influenced by regular rainfall throughout the year and by seasonal backflooding from the Amazon River. Stream discharge was more variable when the Amazon was low (flashy-discharge phase [FDP], September–November) than during the high-water period (backflooded phase [BFP], March–April). Cecropialatiloba and T. integrifolia decomposed much faster than S. paniculata; however, breakdown rates (mean k ± SD = 0.031 ± 0.003, 0.029 ± 0.006, and 0.010 ± 0.003, respectively) were significantly higher during FDP than during BFP (k = 0.009 ± 0.002, 0.009 ± 0.002, and 0.001 ± 0.003, respectively). Thirty-nine taxa of benthic invertebrates (mainly Chironomidae, Trichoptera, and Ephemeroptera) were identified from the litterbags. The tough leaves of C. latiloba were most densely colonized. Most invertebrate taxa (60%) were collectors, whereas shredders were very scarce (0–5%). The proportions of functional feeding groups did not differ significantly between leaf species and hydrologic period. Our results indicated that irregular hydrological pulses in these Neotropical streams had a significantly stronger impact on leaf breakdown than leaf-associated invertebrates.

The Cinaruco River, a lowland floodplain river in the Venezuelan llanos, has a strongly seasonal hydrology, low nutrient concentrations, and high fish diversity and abundance. Fish exclosure/enclosure experiments were conducted in the littoral zone of the river channel and connected lagoons to examine seasonal variation in the magnitude of fish effects on benthic organic matter and algal biomass. During the dry season, large-fish exclosures in the channel accrued significantly more sediment, organic material, and chlorophyll than control cages after 20 d. Grazing scars suggested the bocachico, Semaprochiloduskneri, was a major consumer of organic-rich sediments. Further experiments were conducted to test the hypothesis that the relative strength of top−down (grazer) control of organic matter in sediments varies according to species, hydrologic period, and habitat. At flooding onset (May), S. kneri migrate to the Orinoco River to reproduce and feed. Thus, their densities are extremely low in the Cinaruco during the interval when nutrient inputs from newly flooded plains should be greatest, whereas densities are highest during the low-water season. Experiments conducted during the low-water period in the river channel and floodplain lagoons revealed significant treatment (large-fish exclosure, total fish exclosure, S. kneri enclosure, control) effects for accumulation of sediment mass, organic material mass, and chlorophyll a on tiles after 8 d. Chlorophyll a concentrations were significantly greater in lagoons than river-channel sites. Mean mass of sediments and organic material matched our prediction of grazer control during the low-water season. Experiments during the early rising-water period, when S. kneri emigrate from the Cinaruco, yielded no significant habitat or treatment effects after 3 d. Overall, our results support a model predicting continuous, gradual change in the magnitude of top−down effects of benthivorous grazing fishes on organic material on sediments as a function of seasonal changes in water level.