Tropical Headwater Streams Research Articles

Diet of invertebrates sampled in leaf-bags incubated in a tropical headwater stream

The diet of macroinvertebrates sampled in leaf-bags incubated in a tropical stream was analyzed on a spatial scale (one forested and one deforested area) and on a temporal scale (dry and rainy seasons). The macroinvertebrates were mostly represented by detritivores specialized in fine detritus (69%), followed by generalist detritivores (10% with a diet based on fine and coarse detritus), carnivores (10%), omnivores (8%), and one detritivore genera specialized on coarse detritus (3%). The detritivores exhibited a broad spatial and temporal distribution and were represented mainly by Ephemeroptera, Trichoptera and Diptera. Phylloicus sp. (Trichoptera) consumed mostly coarse detritus (CPOM) and can be classified as the unique specialist shredder in this stream. Carnivores were represented by Anacroneuria sp. (Plecoptera), Hetaerina sp. and Heteragrion sp. (two Odonata). Omnivory was observed for Anacroneuria sp. and Smicridea sp. (Trichoptera), which evidently varied spatially and temporally in the proportion of the food consumed. The high diversity and the wide distribution of the taxa that used organic matter as food resource demonstrated the great importance of this food item to the macroinvertebrates community in this tropical stream.

Identifying Controls on Water Chemistry of Tropical Cloud Forest Catchments: Combining Descriptive Approaches and Multivariate Analysis

We investigated controls on the water chemistry of a South Ecuadorian cloud forest catchment which is partly pristine, and partly converted to extensive pasture. From April 2007 to May 2008 water samples were taken weekly to biweekly at nine different subcatchments, and were screened for differences in electric conductivity, pH, anion, as well as element composition. A principal component analysis was conducted to reduce dimensionality of the data set and define major factors explaining variation in the data. Three main factors were isolated by a subset of 10 elements (Ca2+, Ce, Gd, K+, Mg2+, Na+, Nd, Rb, Sr, Y), explaining around 90% of the data variation. Land-use was the major factor controlling and changing water chemistry of the subcatchments. A second factor was associated with the concentration of rare earth elements in water, presumably highlighting other anthropogenic influences such as gravel excavation or road construction. Around 12% of the variation was explained by the third component, which was defined by the occurrence of Rb and K and represents the influence of vegetation dynamics on element accumulation and wash-out. Comparison of base- and fast flow concentrations led to the assumption that a significant portion of soil water from around 30 cm depth contributes to storm flow, as revealed by increased rare earth element concentrations in fast flow samples. Our findings demonstrate the utility of multi-tracer principal component analysis to study tropical headwater streams, and emphasize the need for effective land management in cloud forest catchments.

Are autochthonous foods more important than allochthonous resources to benthic consumers in tropical headwater streams?

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.

Riparian forest buffers mitigate the effects of deforestation on fish assemblages in tropical headwater streams

Riparian forest buffers may play a critical role in moderating the impacts of deforestation on tropical stream ecosystems, but very few studies have examined the ecological effects of riparian buffers in the tropics. To test the hypothesis that riparian forest buffers can reduce the impacts of deforestation on tropical stream biota, we sampled fish assemblages in lowland headwater streams in southeastern Costa Rica representing three different treatments: (1) forested reference stream reaches, (2) stream reaches adjacent to pasture with a riparian forest buffer averaging at least 15 m in width on each bank, and (3) stream reaches adjacent to pasture without a riparian forest buffer. Land cover upstream from the study reaches was dominated by forest at all of the sites, allowing us to isolate the reach-scale effects of the three study treatments. Fish density was significantly higher in pasture reaches than in forest and forest buffer reaches, mostly due to an increase in herbivore-detritivores, but fish biomass did not differ among reach types. Fish species richness was also higher in pasture reaches than in forested reference reaches, while forest buffer reaches were intermediate. Overall, the taxonomic and trophic structure of fish assemblages in forest and forest buffer reaches was very similar, while assemblages in pasture reaches were quite distinct. These patterns were persistent across three sampling periods during our 15-month study. Differences in stream ecosystem conditions between pasture reaches and forested sites, including higher stream temperatures, reduced fruit and seed inputs, and a trend toward increased periphyton abundance, appeared to favor fish species normally found in larger streams and facilitate a native invasion process. Forest buffer reaches, in contrast, had stream temperatures and allochthonous inputs more similar to forested streams. Our results illustrate the importance of riparian areas to stream ecosystem integrity in the tropics and provide support for Costa Rican legislation protecting riparian forests.

Composition and dynamics of allochthonous organic matter inputs and benthic stock in a Brazilian stream

Riparian vegetation provides the nutrient and energy input that maintains the metabolism and biodiversity in tropical headwater streams. In the present study, it was hypothesised that ~30% of riparian plant species contribute over 70% of coarse particulate organic matter and, because tropical plants are perennial and semi-deciduous, it was expected that leaf fall would occur year round. The aims of the present study were to evaluate the composition and structure of the plant riparian zone and the input and associated benthic stock of organic matter. The riparian vegetation was composed of 99 taxa. The most abundant plant species were Tapirira obtusa, Sclerolobium rugosum, Croton urucurana, Byrsonima sp. and Inga sp. The input and benthic stock showed a seasonal pattern, with higher values recorded at the end of the dry season and at the beginning of tropical storms. The biomass contributed monthly by the vegetation ranged from 28 ± 6 g m–2 to 38 ± 11 g m–2, and the mean monthly benthic standing stock was 138 ± 57 g m–2. The results illustrate the importance of riparian vegetation as an energy source to tropical streams and how individual plant species contribute to organic matter inputs in these ecosystems.

Leaf Breakdown in a Tropical Headwater Stream (Puerto Rico): The role of Freshwater Shrimps and Detritivorous Insects

ABSTRACT Current ecological theory suggests that some organisms have overlapping roles, such that species loss may not disrupt ecosystem processes. In a tropical headwater stream, we hypothesized that the loss of major shredders (two species of shrimps) would decrease leaf breakdown rates. We tested our hypothesis by conducting a leaf breakdown experiment using Cecropia schreberiana leaves under the following treatments: (1) shrimp exclusion, (2) invertebrate exclusion (i.e., shrimp and insect), and (3) control, where all groups had access to leaves. Treatments were replicated in three pools in Quebrada Prieta, Luquillo Experimental Forest, Puerto Rico. Leaf packs were collected from each treatment on days 1, 3, 6, 13, 25, and 35. We found that breakdown rates between the control and shrimp exclusion treatments were not significantly different, whereas the total invertebrate exclusion treatment broke down the slowest. However, ash-free dry mass remaining at day 35 was significantly different among all treatments; control treatments (with shrimps) had the lowest mass remaining and the total invertebrate exclusion had the largest mass remaining. Our study demonstrated that even in a tropical island stream dominated by large populations of decapods, there was a significant degree of leaf breakdown by small, less abundant groups of detritivorous insects. In the long-term, however, shrimps may have a crucial role in the ecosystem process of leaf breakdown by maintaining faster rates of organic matter recycling.

Effects of different types of conditioning on rates of leaf-litter shredding byXiphocariselongata, a Neotropical freshwater shrimp

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 (Xiphocaris elongata) 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 (Rourea surinamensis) 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.

Sediment and wood accumulations in humid tropical headwater streams: Effects of logging and riparian buffers

The amount of sediment, wood, and detritus (leaves and small branches) accumulations were examined in four headwater tributaries subjected to different treatments (unlogged, logged with riparian buffer, and logged with no buffer) 16 and 40 months after timber harvesting in the Bukit Tarek Experimental Watershed, Peninsular Malaysia. Sediment transport from hillslope logging roads and skid trails resulted in large sediment storage in the stream channels. Riparian reserves appear to mitigate logging impacts to streams; however, more sediment was found in a tributary channel with a 20-m buffer than an unlogged channel because some of the nearby roads and skid trails were connected to the stream. Sediment storage in channels declined within 40 months after logging in catchments with and without riparian buffers. Vegetation recovery on skid trails and logging roads reduced both sediment supply and delivery. The volume of large wood was 1.5 to 2-times greater in the stream without riparian reserves than the stream of an unlogged catchment or in a stream with a riparian buffer. No consistent change in detritus accumulations was found among streams in 2001 and 2003, although more detritus was present in 2003 in all streams. The effects of logging and the effectiveness of riparian buffers appear to depend on the hydrologic connections between hillslopes and headwater streams.

The Effect of Macroinvertebrate Exclusion on Leaf Breakdown Rates in a Tropical Headwater Stream1

ABSTRACTThe role of macroinvertebrates in the process of leaf breakdown is well studied in temperate streams, but less is known about their role in the tropics. We investigated the effect of reducing macroinvertebrate access to leaf material on leaf breakdown rates in a forested headwater stream in the Luquillo Experimental Forest, Puerto Rico. We measured leaf mass loss using fine and coarse mesh bags over 12 weeks for two common riparian species: Cecropia schreberiana (Moraceae) and Dacryodes excelsa (Burseraceae). Coarse mesh allowed freshwater shrimp and other macroinvertebrates to access leaf material, while fine mesh did not. Leaf breakdown rates did not differ between C. schreberiana and D. excelsa in coarse mesh bags (−0.0375/day vs. −0.0395/day, respectively), but C. schreberiana breakdown was significantly slower than D. excelsa in fine mesh bags (−0.0159/day vs. −0.0266/day). Additionally, breakdown in fine mesh bags was significantly slower compared to coarse mesh bags for C. schreberiana, but less so for D. excelsa. Breakdown rates for all treatments were fast relative to those in temperate‐zone streams indicating that both macroinvertebrates and macroinvertebrate‐independent processing can strongly influence leaf decomposition in tropical streams. The difference between C. schreberiana and D. excelsa indicates that the effect of macroinvertebrate exclusion can change with leaf type.

Relative Importance of Bacteria and Fungi in a Tropical Headwater Stream: Leaf Decomposition and Invertebrate Feeding Preference

Bacteria and fungi provide critical links between leaf detritus and higher trophic levels in forested headwater food webs, but these links in tropical streams are not well understood. We compared the roles of bacteria and fungi in the leaf decomposition process and determining feeding preference for two species of freshwater shrimp found in the Luquillo Experimental Forest, Puerto Rico, using experimental microcosms. We first tested the effects of four treatments on decomposition rates for leaves from two common riparian species, Cecropia scheberiana (Moraceae) and Dacryodes excelsa (Burseraceae), in laboratory microcosms. Treatments were designed to alter the microbial community by minimizing the presence of bacteria or fungi. The fastest decay rate was the control treatment for D. excelsa where both bacteria and fungi were present (k = -0.0073 day(-1)) compared to the next fastest rate of k = -0.0063 day(-1) for the bacterial-conditioned D. excelsa leaves. The fastest decay rate for C. scheberiana was also the control treatment (k = -0.0035 day(-1)), while the next fastest rate was for fungal-conditioned leaves (k = -0.0029 day(-1)). The nonadditive effect for leaf decomposition rates observed in the control treatments where both fungi and bacteria were present indicate that bacteria and fungi perform different functions in processing leaf litter. Additionally, leaf types differed in microbial colonization patterns. We next tested feeding preference for leaf type and microbe treatment in microcosms using two species of freshwater shrimp: Xiphocaris elongata, a shredder, and Atya lanipes, a scraper/filterer. To estimate feeding preferences of individual shrimp, we measured change in leaf surface area and the amount of particles generated during 5-day trials in 16 different two-choice combinations. X. elongata preferred D. excelsa over C. scheberiana, and leaves with microbial conditioning over leaves without conditioning. There was no clear preference for fungal-conditioned leaves over bacterial-conditioned leaves. This lack of preference for which microbes were responsible for the conditioning demonstrates the importance of both bacterial and fungal resources in these tropical stream food web studies.

STREAM MORPHOLOGY CONTROLS AMMONIUM RETENTION IN TROPICAL HEADWATERS

Ammonium retention was studied in 12 tropical headwater streams differing in channel morphology and hydraulic characteristics. Results from NaCl and NH4Cl injections combining a one-dimensional transport model with the nutrient spiraling concept were evaluated. Transient solute storage was large in swamp reaches, with ratios of storage zone and main channel cross-sectional area (AS/A) of up to 18, and fractions of residence time due to transient storage (F 200med) of up to 94%. Moreover, transient storage was at an intermediate level in step-pool and meandering reaches and low in run reaches. Maximum ammonium uptake potentials (UMAX) varied by a factor of 15 due to differing stream morphologies, and ammonium retention was high in swamp and step-pool reaches, with UMAX values as high as 1.8 mg N·m−2·min−1. Between 52% and 85% of ecosystem retention occurred in storage zones. Additionally, differences in ammonium uptake efficiency in the storage zones of contrasting stream morphotypes were negligible; hence, ecosystem ammonium uptake potential was determined by storage zone size. Interestingly, surface transient storage, which is usually neglected in stream tracer studies, was both an important transient storage mechanism and a strong determinant of ammonium uptake potentials. Here, we demonstrated that channel morphology controlled transient storage and ammonium retention potentials in these tropical ecosystems.

Particulate organic matter dynamics in tropical headwater streams: a comparison of biotic and abiotic factors

Particulate organic matter dynamics in tropical headwater streams: a comparison of biotic and abiotic factors

Freshwater Shrimp Effects on Detrital Processing and Nutrients in a Tropical Headwater Stream

In this paper, we report on a whole-pool manipulation of leaf litter decomposition in a tropical stream following a hurricane. The study was designed to distinguish how decapod species comprising two functional feeding guilds alter rates and magnitudes of leaf litter processing and nutrient release linking the detrital food web with the overall producer–consumer food web. Streams of the Luquillo Experimental Forest, Puerto Rico, are dominated numerically by two freshwater shrimp species (Atya lanipes and Xiphocaris elongata). To determine how these shrimp affected detrital processing following large leaf inputs associated with a hurricane, we manipulated the presence or absence of two species of shrimp in six fenced pools of a headwater stream with hurricane levels of Cecropia leaf litter over a 23-d period. The experiment was designed to determine how the two different shrimp affected: (1) the rate and amount of size fractionation of leaf material; (2) the localized nutrient concentrations in the pools; and (3) the rate of particulate export from the pools. Both shrimp species influenced detrital processing, but in fundamentally different ways. Xiphocaris shred intact, large leaves and converted them into fine, medium, and coarse particulates. Through this conversion process Xiphocaris increased the concentration and rate of downstream transport of suspended particulate organic matter. Xiphocaris also significantly increased the concentration of both total dissolved nitrogen and dissolved organic carbon, likely by changing the surface area to volume ratio of organic particles. Atya, a scraper/filter feeder, caused a slight increase in rates of leaf breakdown as compared to controls at the end of the experiment but filtered out fine organic particulates, resulting in less downstream export. Both decapod species affected detrital processing, but in fundamentally different ways, illustrating the potential importance guild diversity may have in altering both particulate and nutrient availability to the rest of the food web. In addition, these results suggest that the presence of both feeding guilds can significantly influence ecosystem responses to severe, large-scale disturbance events.

Estimating the potential role of freshwater shrimp on an aquatic insect assemblage in a tropical headwater stream: a bioenergetics approach

Estimating the potential role of freshwater shrimp on an aquatic insect assemblage in a tropical headwater stream: a bioenergetics approach

Organic Matter Budgets for Streams: A Synthesis

Organic Matter Budgets for Streams: A Synthesis

Large-Scale Trends for Stream Benthic Respiration

Large-Scale Trends for Stream Benthic Respiration