Tropical stream

Freshwater prawns of the genus Macrobrachium are free-living decapod crustaceans that are commonly encountered in tropical streams and lakes. We present a molecular phylogenetic analysis of the diverse Southeast and East Asian fauna based on >3 kb sequence data from three nuclear and two mitochondrial markers for almost 50% of the described fauna. We reconstruct the phylogenetic relationships and track the evolution of key life history traits. Our tree suggests that the last common ancestor of the Asian Macrobrachium laid numerous small eggs and had prolonged larval development (''PLD") in saline coastal waters after which the adults matured in freshwater habitats. We also argue for five independent losses of the marine larval phase to yield five clades of species that develop entirely in freshwater and have fewer and larger eggs than the species with PLD. These species have either semi-abbreviated (two origins) with at least one free-swimming stage or abbreviated larval development (''ALD": three origins) which lack free-swimming larvae. A Shimodaira-Hasegawa test rejects all trees that would imply a single loss of the marine larval phase, but alternative and equally parsimonious optimizations exist that imply a smaller number of losses. However, these scenarios would require the re-acquisition of free-swimming larvae. A concentrated-change test supports Pereira and Garcia's [Pereira, G.A., Garcia, J.V., 1995. Larval development of Macrobrachium reyesi Pereira (Decapoda, Palaemonidae), with a discussion on the origin of abbreviated development in palaemonids. J. Crust. Biol. 15,[117][118][119][120][121][122][123][124][125][126][127][128][129][130][131][132][133] hypothesis of a significant correlation between living in freshwater and the origin of semi-abbreviated and abbreviated larval development. Our phylogenetic tree also reveals that Asian Macrobrachium have independently become cavernicolous at least twice, and invaded the highly acidic waters of freshwater and peat swamps two or three times.

Diatoms are good indicators of water quality in lotic systems. Unlike in the temperate region, the effect of substrate on diatom-based water quality assessment in tropical streams is not fully understood. The purpose of this study was to assess the effect of substrate on diatom-based multivariate water quality assessment. Epilithic, epiphytic, epipsammic and epipelic diatom community and water quality sampling was done four times at 10 sites during the dry season (2008 and 2009). Artificial substrates (brick and glasses) were also placed at the sampling sites during this period and sampled after 1 month. Cluster analysis was performed to show the main differences and similarities in community composition amongst substrates sampled and amongst sampling sites. The IndVal method was used to identify indicator species characterising different substrates. Canonical correspondence analyses (CCAs) were performed to relate the structure of diatom communities from different substrates to predictor variables. A gradient of increasing metal and organic pollution, eutrophication and ionic strength was observed from the agricultural/forested area to the urban area. Diatom community structure closely reflected this gradient, with communities from polluted sites (8, 9 and 10) being different from other communities. Polluted sites were associated with such species as Nitzschia palea, Plantago lanceolata, Achnanthes exigua, Caloneis hyaline, Cyclotella meneghiniana, Gomphonema parvulum, Fallacia monoculata, Luticola goeppertiana, Pinnularia microstauron, Pinnularia subcapitata and Sellaphora pupula. Indicator species analysis showed that common diatom species were not restricted to a single substrate, though preference was generally high for natural (especially macrophytes) compared to artificial substrates. Six CCAs corresponding to six substrates performed to relate diatom community structure to simultaneous effects of predictor variables explained ∼50% of the diatom species variance in all cases and roughly separated highly polluted sites from the rest of the sites. This indicates that the results of diatom-based multivariate water quality assessment based on different substrates may be interchangeable. Only one substrate has to be collected at each site for water quality assessment surveys, thus avoiding unnecessary expensive and time-consuming oversampling. Given the limitations of artificial substrates, sampling of natural substrates is highly recommended.

Landscape-scale patterns of freshwater fish diversity and assemblage structure remain poorly documented in many areas of Central America, while aquatic ecosystems throughout the region have been impacted by habitat degradation and hydrologic alterations. Diadromous fishes may be especially vulnerable to these changes, but there is currently very little information available regarding their distribution and abundance in Central American river systems. We sampled small streams at 20 sites in the Sixaola River basin in southeastern Costa Rica to examine altitudinal variation in the diversity and species composition of stream fish assemblages, with a particular focus on diadromous species. A set of environmental variables was also measured in the study sites to evaluate how changes in fish assemblage structure were related to gradients in stream habitat. Overall, fish diversity and abundance declined steeply with increasing elevation, with very limited species turnover. The contribution of diadromous fishes to local species richness and abundance increased significantly with elevation, and diadromous species dominated assemblages at the highest elevation sites. Ordination of the sampling sites based on fish species composition generally arranged sites by elevation, but also showed some clustering based on geographic proximity. The dominant gradient in fish community structure was strongly correlated with an altitudinal habitat gradient identified through ordination of the environmental variables. The variation we observed in stream fish assemblages over relatively small spatial scales has significant conservation implications and highlights the ecological importance of longitudinal connectivity in Central American river systems.

Diatoms are frequently used as indicators of eutrophication in temperate systems, but little is known about their application to impacted African tropical systems. Five streams located within Gombe Stream National Park and five streams supporting human settlements draining into Lake Tanganyika, East Africa, were investigated for species composition, richness and diversity of epilithic algae. In addition, a trophic diatom index (TDI) developed for monitoring European rivers was applied to these tropical systems. 54 specific and infraspecific diatom taxa representing 20 genera were identified for all sites with Achnanthes s.l., Gomphonema and Navicula s.l. being the most common genera. Species richness varied between 10 and 21 in disturbed streams and 13 and 19 in undisturbed streams. Nutrients were significantly enriched in streams draining the deforested watersheds but indices of diversity and evenness (Shannon H, J and Simpson-Yule D, E) did not show any significant differences between streams in forested and deforested watersheds. Significant differences were observed between pooled data for the TDI between forested and deforested watersheds. Analysis of percent pollution tolerant diatom taxa indicates that organic pollution of streams in deforested watersheds may be contributing to eutrophication. This study shows that African diatoms, cosmopolitan or resembling well-known North American and European taxa, allows for trophic indices tailored to the autecological preferences of species to be applied to new regions, although intensive studies on these African taxa will lead to more accurate results. Measures of species-richness and diversity, historically used to describe the state of an ecosystem, may not be suitable to evaluate streams which are not grossly polluted.

Managers, researchers and technicians involved in the conservation and management of water resources in the Central Amazonia need a robust tool to assess biological quality in aquatic ecosystems. To provide such a tool, we developed a multimetric index based on stream macroinvertebrate data. We collected samples from eight reference (undisturbed) streams and 12 streams altered by deforestation and domestic sewage during two distinct seasons (dry and rainy) in the municipality of Manaus, Brazil. Metric candidates to compose the index were tested for: Range, temporal variability (stability), sensitivity in separating disturbed from reference streams, correlation with the anthropogenic disturbance gradient and natural stream variability and redundancy between metrics. Our final index included seven metrics: family, Trichoptera and Ephemeroptera–Plecoptera–Trichoptera (EPT) richness as richness measures, EPT percent abundance as a measure of composition, EPT/Chironomidae ratio and sensitive-taxa richness as tolerance measures and percent abundances of gathering-collectors and shredders as trophic measures. All metrics were scored relative to their range quartiles. The final index, derived from the sum of all metric scores (0–70), was divided into five sub-ranges to represent distinct levels of biological quality in streams (bad, poor, regular, good and excellent).

A comparative investigation on aquatic insect diversity was conducted in a tropical stream in Southeast Asia (the Dak Pri stream in southern Vietnam; stream orders II-V, two sites per stream order) with a reference temperate stream in Northeast Asia (the Gapyeong stream in central Korea) in March 2001 and April 2000, respectively. The numbers of aquatic insect taxa in Dak Pri stream (268 species, mostly undescribed, 230 genera, 91 families, and 9 orders; 110.5 ± 17.1 species per site) were about twice those in Gapyeong stream (133 species, 98 genera, 51 families, and 8 orders; 60.3 ± 8.5 species per site). Coleoptera, Trichoptera, Ephemeroptera, and Diptera were the major aquatic insect orders with high taxonomic richness, and Coleoptera, Odonata, and Hemiptera contributed to the higher degree of aquatic insect diversity in Dak Pri stream. The species diversity indices of Dak Pri stream (4.37 ± 0.19) were higher than those of Gapyeong stream (3.73 ± 0.42), whereas the dominance indices of Dak Pri stream (0.195 ± 0.046) were lower than those of Gapyeong stream (0.346 ± 0.113). Collector-gatherers were predominant in both streams; shredders were more abundant in Dak Pri stream while scrapers were more abundant in Gapyeong stream. Factors affecting the higher degree of aquatic insect diversity in Dak Pri stream are discussed.

This study investigated the fish fauna of a stream in southeastern Brazil that runs through sugar cane-growing and urban areas that are subject to sewage discharge. It aimed to determine the importance of spatial and environmental factors on fish distributions and to assess the use of fish as indicators of the environmental conditions. A longitudinal pattern of chemical water quality was observed, with more disturbed conditions at the urban sites. Several opportunistic or tolerant and omnivorous species such as Astyanax altiparanae, Poecilia reticulata, and Astyanax fasciatus were numerically abundant at these urban sites, which resulted in a consistent increase in species density and biomass along the stream. Redundancy analysis extracted three axes, identifying a nutrient–productivity gradient associated with different species composition patterns, as also detected by the indicator species analysis. No species could be considered a potential indicator of the agricultural area in the dry season, whereas four different species were judged potential indicators of three urban sites. In order to explain species distribution, variation in species abundance was partitioned into spatial and environmental components. The latter was more important, explaining 25.4% of the variation. We concluded that the fish assemblage of the Guamium stream is undergoing the initial stages of disturbance, with pronounced effects at the urbanized area, but no detectable changes at the sugar cane sites. Therefore, fish assemblages might be used as ecological indicators to detect urban impacts in similar rivers. However, the potential use of fish to diagnose stressors in areas of sugar cane plantations should be complementary to more comprehensive studies aimed at understanding the influence of this crop on freshwater resources.

Water samples were collected from three sites located in the middle reach of the Njoro River, Kenya, and analysed for total phosphorus (TP), orthophosphate, ammonianitrogen, and nitrate-nitrogen to evaluate stressor sources (e.g. factories and wastewater ponds) and the general stream water quality. The stream surface water was also analysed for biochemical oxygen demand (BOD 5 ) to provide an overview of organic matter loading. Mugo, Egerton Bridge and the canning factory sites of the Njoro River had low water quality which is likely to be due to poor farming, partially treated effluents and poor provision of sanitation facilities to the riparian communities. The concentrations of the selected nutrients did not differ significantly among the three sites, presumably due to pollution of the whole stream reach by the catchment nutrient sources. High phosphate concentrations (i.e. $0.76 mgPO 4 l )1 and $0.87 mgTP l )1 ) at Canning Factory were recorded during the low flow dry season. Nitrate-nitrogen concentrations varied significantly with water discharge which explained between 63 and 87% of the nutrient variability in the three sites. BOD 5 differed significantly among the three sites, with historical effects of wastewater and factory effluent discharge being reflected in the results of Egerton Bridge and Canning Factory. The concentrations of ammonia-nitrogen, TP and orthophosphate were higher in the wastewater than in the river water whereas nitratenitrogen was lower. This study indicates that the Njoro River is stressed by nutrients from the activities within its catchment. With the increasing population, the nutrient load to the river will continue to increase and the water quality will continue to deteriorate. Reductions of nutrient loads into the river as well as provision of sanitation facilities to the riparian communities are needed to control further water degradation.

The diet and trophic groups of an assemblage of aquatic insects were studied in a tropical stream. Genera of the orders Ephemeroptera, Odonata, Plecoptera, Lepidoptera, and Hemiptera showed feeding specialization. Others, such as Trichoptera, Coleoptera, and Diptera, showed great diet variation with genera of different trophic groups. Seasonal variation of insect diet, evident only for some genera of the orders Trichoptera, Lepidoptera, Coleoptera, and Diptera, was due to the differences observed in community composition and to generalist habits of these genera. However, the seasonal comparison of trophic groups showed no significant statistical differences. The great importance of organic matter, a non-limited resource, in the diet of Ribeirão do Atalho aquatic insects may be the explanation for the trophic stability in this community organization.

This study analyzes the relationship between physical and chemical factors and the algal communities in tropical streams in micro-watersheds where [70% of their area has different land uses, specifically, cloud mountain forest, coffee plantations, and livestock pastures. Physical, chemical, and biological variables were measured monthly in each stream over a 1-year period. The concentrations of nitrates ? nitrites, total suspended solids (TSS), and silica in the streams were found to differ during the dry and rainy seasons. Coffee-plantation streams showed the highest levels of suspended solids, nitrates ? nitrites, and sulfates. Based on chlorophyll a concentration, the forest and coffee-plantation streams are oligo-mesotrophic, while pasture streams are meso-eutrophic. Forest streams displayed the lowest levels of richness and algal diversity, followed by coffee-plantation streams, whereas pasture streams were the most diverse. Chlorophyll a concentration and species richness depended on land use and season. Forest coverage was positively correlated with acidophilous and oligo-eutraphentic diatom species. Coffee coverage displayed a significant positive correlation with motile species and a significant negative correlation with pollution-sensitive diatom taxa. The results show that diatom assemblages responded to micro-watershed conditions and can be used to monitor the effects of land use on streams in tropical regions.

BouflOn et at. differenc~s in biodiversity are not consisten.t across broad taxonomic groups and have multiple explanations (e.g. Rohde, 1992; RosenzweIg, 1997). When present, these patterns of biodiver_ sity may generate significant differences in food-web structure and productivity that are at odds with predictions from models developed in temperate streams (Covich, 1988). Wide variation in climate, geomorphology, landscape evolution, and geological history across tropical zones potentially masks patterns that might be considered 'unique' to tropical streams (Wantzen et al. 2006). It is also likely that the local variation among streams found within tropical and temper: ate zones is so great that latitudinal trends in regional ecological drivers such as temperature photoperiod, and seasonality seldom are detectable. Here, we address the following questions: is the perception that tropical streams are very different ecologically from temperate ones based on facts or assumptions? Does such a perception result from inappropriate comparisons COn~ founded by differences in geology, flow regime, disturbance history, sampling procedures and taxonomic resolution, biogeography, anthropogenic influence, and uneven data sets? We follow the latitudinal definition of 'tropical' used elsewhere in this book (see Chapter 1) to encompass streams lying between 23°N and 23°S but, where appropriate, extend this to include areas where monsoonal seasonal rains fall. While it is futile to attempt to draw firm boundaries across a climatic continuum, for ease of discussion and from widespread precedence in the scientific literature we use the terms 'tropical' and 'temperate' as adjectives for regional comparisons of their stream ecology. Despite the impossibility of drawing generalizations about an idealized 'tropical' or 'temperate' stream, there is a need to compare ecological processes and diversity in equivalent-sized streams from different parts of tropical and temperate regions. This comparison is necessary because the vast amount of research undertaken on north temperate streams has given rise to ecological models (e.g. the River Continuum Concept: Vannote et al., 1980; the Riverine Productivity Model: Thorp and Delong, 1994) that are now being used to guide research questions and management approaches in rivers worldwide. The flood pulse concept (Junk et aI., 1989; Junk and Wantzen, 2004), developed in tropical rivers, is a notable exception to this generalization. However, if tropical streams are ecologically different from tem

A study was conducted to determine the possible influence of recreation on microbiological water quality of a tropical stream. Microbiological water quality was measured at several recreational sites along the stream and a separate experiment was conducted to look at the effect of sediment resuspension on microbiological water quality. Microbiological quality of the water in the stream was generally poor and varied widely with faecal coliform and Escherichia coli levels ranging from 1 to > 16,000 and 14 to 9615 organisms 100 ml(-1) respectively. Levels of faecal coliforms were higher in the wet (median = 700 organisms 100 ml(-1)) than the dry (median = 500 organisms 100 ml(-1)) season while the reverse was true for E. coli (median = 300 and 220 organisms 100 ml(-1) in the wet and dry seasons respectively). Recreational activity resulted in reduced water quality: sites with recreation had poorer water quality than those without; water quality was generally poorer when there were high numbers of recreational users. Wading resulted in a 4-fold increase in mean E. coli densities and a 3-fold increase in total suspended sediments in the overlying water suggesting that the increases were due to suspension of bacteria from the sediments. We conclude that water quality monitoring methodology for assessing recreational water quality should be amended to factor in the effects of wading since environmental strains of bacteria can be pathogenic and thus represent a human health threat.

Telmatogeton torrenticola Terry is a large endemic chironomid (last instar >20 mm) commonly found in high gradient Hawaiian streams on smooth rock surfaces with torrential, shallow flow and in the splash zones of waterfalls. We have quantified benthic water flow in larval habitat in a 50 m segment of Kinihapai Stream, Maui using a thermistor-based microcurrent meter. Under base flow conditions at sites suitable for larval attachment, depth was measured and bottom water velocity measurements were made 2 mm above populations. Larval densities ranged from 386.9-1178 m 2 , habitat bottom water velocities from 13.4-64.2 cm s 1 , and water depths from 1.5-50 cm. Bottom velocities of sites with zero larvae ranged from 20.8-21.8 cm s 1 with depths from 50 to >160 cm. Larval densities were greatest in areas with high bottom water velocities and shallow depths. Stepwise multiple regression analyses showed that density could be confidently predicted best by Froude number (r = 0.81; p = 0.008). In the absence of Froude number as a regression term, the best variable to predict larval density was bottom velocity ratio: relative depth ratio (r = 0.75; p = 0.019). In addition, the torrential habitat of the larvae was always characterized by a periphyton community that appeared to be the primary food resource for the larvae. These data suggest that torrential flows over appropriate substrates are important factors regulating habitat availability for T. torrenticola and that reduced discharge (e.g. affected by water diversions) could significantly reduce the amount of available habitat for this organism and other flow sensitive stream fauna.

Rainbow trout (Oncorhynchus mykiss) and other salmonids have been widely stocked into upland streams throughout the world to provide a basis for sport fisheries, but the effects of such introductions on indigenous and endemic aquatic insect assemblages are poorly documented. In this study, we examine the impact of rainbow trout on the indigenous and endemic entomofauna of upland streams in Kokee State Park, Kauai, Hawaii, with particular emphasis on the potential threat trout pose to populations of endemic damselflies in the genus Megalagrion. Rainbow trout were introduced into the upland streams of Kauai beginning in the 1920s, with over 60 years of subsequent restocking. This study indicates, however, that streams in this area still maintain diverse populations of Megalagrion damselflies and other indigenous and endemic aquatic insects, both in catchments containing naturally reproducing trout populations and in catchments lacking rainbow trout. Our results indicate that the indigenous and endemic aquatic insect communities in the streams under study compare favorably in terms of density and taxonomic richness with other isolated and unimpacted streams elsewhere in Hawaii, and retain high densities and relative percentages of indigenous and endemic aquatic insect taxa. Our results demonstrate that the threats posed by conspicuous introduced species such as trout should not simply be assumed a priori on the basis of postulated negative interactions, because this may divert limited resources from programs aimed at control of other, potentially more destructive introduced taxa such as inconspicuous poeciliid fishes.

1. Few studies have assessed the effects of macroconsumers, such as fishes and shrimps, on detritus and detritivores. 2. We used an underwater electric field to prevent macroconsumers from feeding in and on leaf packs in a lowland stream in Costa Rica and thus to determine their effects on the density of insect detritivores and decay rates of leaves. 3. Exclusion of macroconsumers resulted in significantly higher densities of small invertebrates inhabiting leaf packs. Most of these were collector-gatherers, none were shredders. 4. Despite the increase in invertebrate density, decay rates of leaves were not statistically different. These findings contrast with results from temperate streams showing that increases in the density of invertebrates in leaf packs typically result in an increased rate of decay. 5. Leaf decay rates and invertebrate densities were also compared between leaf packs placed in electric exclusion treatments and those placed in coarse (2 cm) plastic net bags (as used in many previous studies). Our results suggest that using such netting in tropical streams may deter macroconsumers, which can affect insect density and, potentially, decay rates of organic matter.

Landscape-scale variation in streamwater phosphorus (P) concentration can affect aquatic food webs. Such variation occurs naturally in streams at La Selva Biological Station in Costa Rica due to spatially variable inputs of geothermally modified groundwater. We examined effects of this gradient on detrital food web components at 16 stream sites. The Michaelis-Menten model provided a good fit of the relationship between soluble reactive phosphorus (SRP) and leaf decay rate, fungal biomass, and invertebrate biomass, indicating that these variables were controlled by P concentration and that half-saturation constants were relatively low (7-13 g L Ϫ1 SRP). In a subsequent short-term (3 week) whole-stream P enrichment study, we found no effect of P addition on leaf decay rate or on biomass or density of invertebrates. However, laboratory tests of P, N, and Ca concentrations on mass loss of leaves showed detectable stimulation by both N and P after 3 weeks. A fourth study assessed the relative contribution of invertebrate consumption versus P concentration in determining decay rates among streams. The majority of variation was due to P concentration (71%), compared to effects of invertebrates (3%) or invertebrate ϫ P interactions (14%). Overall, we found that a landscape-scale natural gradient in P concentration influenced decay rates of organic matter and biomass of consumers, providing evidence that benthic detrital food webs can be limited from the bottom up by nutrients. Microbial processes appeared to be most important in driving differences in organic matter decay among sites, but invertebrates also contributed to elevated decay rates at high-P sites.

Temperature and daily emergence of seven genera of Ephemeroptera (Insecta) in a cloud forest stream of tropical Andes. Daily emergence of mayflies in Neotropical rivers and their causes have been poorly studied. In temperate zones, this process is better known and attributed to several factors. In this work, we studied the daily emergence of subimagines of several Ephemeroptera genera in La Picón River of a Venezuelan Andean cloud forest and its relation with changes of environmental temperature. Four emergence traps were placed along a reach of 50 m of the stream, each one was examined each two hours in a 24 hr cycle to capture the newly emerged subimagos. This procedure was repeated for eight dates between November-2007 and February-2008 for a total of 32 observations in each sampling hour. The subimagos were reared to adults and identified to genus. The relative density of emergence per trap was calculated for each genus and sampling hour. Water and air temperature were measured each hour during the daily cycle of observation, and the averages of temperature and hour-degrees of air and water were calculated for each hour from the eight dates studied. Seven genera were identified: Leptohyphes Eaton, 1882 and Haplohyphes Allen 1966 (Leptohyphidae); Prebaetodes Lugo-Ortiz and McCafferty, 1996, Andesiops Lugo-Ortiz and McCafferty, 1999, Baetodes Needham and Murphy, 1924 and Americabaetis Kluge, 1992 (Baetidae); and Thraulodes Ulmer, 1920 (Leptophlebiidae); being the more abundant Leptohyphes (38.4 %) and Thraulodes (20.5 %). The emergence occurred between 11:00 am and 23:00 pm showing the following: a) an emergence initiated during daylight hours by organisms of Leptohyphes, Prebaetodes and Haplohyphes; b) a nocturnal emergence, in Thraulodes, Andesiops, Baetodes and Americabaetis; and c) two peaks: one diurnal produced by Leptohyphes and other nocturnal with predominance of Thraulodes. These results are the first records on the diurnal daily emergence in Andesiops, Prebaetodes, Americabaetis, Haplohyphes, and Leptohyphes, as well as the nocturnal emergence in Thraulodes. It was evidenced that Leptohyphes, with small nymphs (average head width = 1.05 mm) needed to accumulate less hour-degrees to initiate the emergence than those required by Thraulodes whose nymphs are larger (average head width = 2.01 mm). This disparity in the emergence energy requirements must be consequence of differences between the sizes of mature nymphs of both genera; facts which rely on the constancy of sizes shown by these taxa along an altitudinal-thermal gradient and the little daily and seasonal variability of water temperature in La Picón River. In the daily lapse when the emergence occurred, the air and water average temperatures were higher than those registered in the no-emergence lapse; therefore; it is suggested that during the daily lapse, when this process occurs, the environment is thermally favorable for the emergence of subimagos and their survival out of water. Rev. Biol. Trop. 64 (1): 117-130. Epub 2016 March 01.

We studied the effects of nutrient availability and grazers on periphyton in the littoral zone of Lake Tanganyika. Using a combination of dissolved nutrient ratios, nutrient diffusing substrates, and benthic productivity responses to nutrient supplementation, we evaluated whether nitrogen (N) or phosphorus (P) limited periphyton productivity near Kigoma, Tanzania, during the dry seasons of 2001 and 2002. The relative effects of grazers and nutrients on periphyton were quantified by manipulating grazer access to nutrient diffusing substrates. We found very low ambient concentrations of inorganic N and soluble reactive P, but generally higher concentrations of dissolved organic nutrients. Nutrient diffusing substrates indicated a shift from P limitation in 2001 to co-limitation by N and P in 2002, probably as a consequence of unusual coastal upwelling in 2002. Productivity responses of natural epilithic algae to nutrient supplementation also indicated N–P co-limitation in 2002. However, fish and other large grazers had much stronger effects on periphyton than nutrients. Grazers strongly suppressed periphyton biomass, but had weaker negative effects on area-specific gross primary productivity as a result of large increases in biomass-specific gross primary productivity. We conclude that littoral nutrient availability influenced periphyton productivity, but that top-down control predominated.

We investigated the breakdown of 2 leaf species, Croton gossypifolius (Euphorbiaceae) and Clidemia sp. (Melastomataceae), in a 4th-order neotropical stream (Andean Mountains, southwestern Colombia) using leaf bags over a 6-wk period. We determined the initial leaf chemical composition and followed the change in content of organic matter, C, N, and ergosterol, the sporulation activity of aquatic hyphomycetes, and the structure and composition of leaf-associated aquatic hyphomycetes and macroinvertebrates. Both leaf species decomposed rapidly (k ϭ 0.0651 and 0.0235/d, respectively); Croton lost 95% of its initial mass within 4 wk compared to 54% for Clidemia. These high rates were probably related to the stable and moderately high water temperature (19ЊC), favoring strong biological activity. Up to 2300 and 1500 invertebrates per leaf bag were found on Croton and Clidemia leaves after 10 and 16 d, respectively. Shredders accounted for Ͻ5% of the total numbers and biomass. Fungal biomass peaked at 8.4 and 9.6% of the detrital mass of the 2 leaf species, suggesting that fungi contributed considerably to leaf mass loss. The difference in breakdown rates between leaf species was consistent with the earlier peaks in ergosterol and sporulation rate in Croton (10 d vs 16 d in Clidemia) and the faster colonization of Croton by macroinvertebrates. The softer texture, lower tannin content, and higher N content were partly responsible for the faster breakdown of Croton leaves. The rapid breakdown of leaf litter, combined with a low influence by shredders, is in accordance with previous findings. The high fungal activity associated with rapid leaf breakdown appears to be characteristic of leaf processing in tropical streams.

Coarse particulate organic matter is often broken down by specialist shredder invertebrates in temperate streams. In some tropical streams, larger, non-specialist, omnivorous fauna, (macroconsumers), particularly decapod shrimps and crabs, have been found to process coarse particulate matter. Larger shrimps and fish may also prey on or inhibit smaller invertebrates. Depending on the relative importance of larger and smaller fauna in leaf processing and in predatory interactions, we could expect that exclusion of larger fauna could either result in a decrease in leaf processing (if they were important in shredding or bioturbation) or increase in leaf processing if they Handling editor: B. Oertli did not increase leaf processing. We conclude that macroconsumers and not smaller fauna had a positive effect on leaf processing, and this confirms a pattern observed in some other coastal Neotropical streams.

Stream shredders play an important role in the breakdown of allochthonous leaf litter-a well-known, key process in temperate headwater streams. In contrast, it has been suggested that litter breakdown in tropical streams is driven by microorganisms, shredders being scarce or absent. We propose that shredders have been overlooked in some tropical streams for two reasons: (1) assuming that tropical shredders belong to the same taxa as temperate ones, without determining the diet of tropical litter fauna; and (2) the small spatial scale of most tropical stream studies, which do not account for intra-and inter-site comparisons. We explored shredder abundance and species richness in six streams in each of two tropical regions, the Australian wet tropics (AWT) and Panama (PAN), finding 734 individuals of 12 shredder species in AWT and 391 individuals of 16 species in PAN. Shredder species richness was positively related to altitude in AWT, but not in PAN. Shredder contribution to total leaf breakdown in the field was 24 AE 3 SE percent in AWT and negligible in PAN, but this was probably due to the unsuccessful colonization of experimental cages by PAN shredders. In the laboratory, shredder contribution to total leaf breakdown was higher than in the field (35% AE 2 SE in AWT and 64% AE 3 SE in PAN) and varied with leaf decomposability. Our results support earlier indications that shredders are not scarce or functionally unimportant in the tropics, and suggest that their contribution to litter processing should be determined along altitudinal gradients.

In a tropical stream (at the Soberanía National Park, Panama), different environmental factors were quantified in riffle habitats (water characteristics: velocity, depth, turbulence, and direction; stone characteristics: surface area, sphericity, and degree of burial; and others: substrate type, and canopy cover). Characteristics of macroinvertebrate assemblages (mean density of individuals, mean taxon richness, and cumulative taxon richness in three stones at each riffle) were related to both mean values and variability of these environmental factors at riffle scale. Macroinvertebrate density was higher in shallow, fast flowing, stony riffles, with low variabihty in dominant substrate type. Taxon richness was also higher in shallow rilfles with loose, not buried stones, and water direction more or less parallel to the bank. Environmental variability resulted as important as mean values of environmental factors to explain variation in macroinvertebrate assemblages. This is the first study, to our knowledge, that quantifies substratum variability and demonstrates its influence on macroinvertebrate assemblages in a tropical stream.

In many countries of the Global South, aquatic ecosystems such as streams, rivers, lakes, and wetlands are severely impacted by several simultaneous environmental stressors, associated with accelerated urban development, and extreme climate. However, this problem receives little attention. Applying a DPSIR approach (Drivers, Pressures, State, Impacts, Responses), we analyzed the environmental impacts and their effects on urban hydrosystems (including stagnant waters), and suggest possible solutions from a series of case studies worldwide. We find that rivers in the Global South, with their distinctive geographical and socio-political setting, display significant differences from the Urban Stream Syndrome described so far in temperate zones. We introduce the term of ‘Southern Urban Hydrosystem Syndrome’ for the biophysical problems as well as the social interactions, including the perception of water bodies by the urbanites, the interactions of actors (e.g., top-down, bottom-up), and...

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.

The hypothesis of convergence takes the deterministic view that community (or assemblage) structure can be predicted from the environment, and that the environment is expected to drive evolution in a predictable direction. Here we present results of a comparative study of freshwater fish assemblages from headwater streams in four continents (Europe, North America, Africa and South America), with the general objective of testing whether these assemblages display convergent structures under comparable environmental conditions (i.e. assemblage position in the stream longitudinal continuum). We tested this hypothesis by comparing species richness and trophic guilds of those stream fish assemblages represented in available data from multiple sites on each continent. Independent of phylogenetic and historical constraints, fish assemblage richness and trophic structure in the four continents converged along the stream continua to a substantial degree. For the four continents, assemblage richness increased, the proportion of invertivorous species decreased, and the proportion of omnivorous species increased from upstream to downstream, supporting theoretical predictions of the river continuum concept. However, the herbivore/detritivore and piscivore guilds were virtually absent from our small European and North American stream sites, unlike our African and South American stream sites. This divergence can be linked to differences in energy availability between temperate and tropical systems.

Two collections of submerged wood were made from the Mushroom Research Centre in northern Thailand. One collection comprising 100 samples was made from an artificial lake, which had been made by damming a stream running through a secondary forest. The other collection comprising 90 samples was made from an adjacent non dammed shallow stream running through a similar forest. A total of 68 fungal taxa were recorded during the study and the Shannon-Weiner index (H 0 ) was applied to evaluate the diversities of freshwater fungi. Sørensen's index (S 0 ) was calculated to evaluate the similarity of different fungal communities. The variation of freshwater fungi on submerged wood between lentic and lotic habitat is discussed and compared with previous studies. A dramatic decrease in species richness and diversity, with significantly changed species composition were observed in the artificial lake as compared to the non dammed stream.

This study analyzes the relationship between physical and chemical factors and the algal communities in tropical streams in micro-watersheds where [70% of their area has different land uses, specifically, cloud mountain forest, coffee plantations, and livestock pastures. Physical, chemical, and biological variables were measured monthly in each stream over a 1-year period. The concentrations of nitrates ? nitrites, total suspended solids (TSS), and silica in the streams were found to differ during the dry and rainy seasons. Coffee-plantation streams showed the highest levels of suspended solids, nitrates ? nitrites, and sulfates. Based on chlorophyll a concentration, the forest and coffee-plantation streams are oligo-mesotrophic, while pasture streams are meso-eutrophic. Forest streams displayed the lowest levels of richness and algal diversity, followed by coffee-plantation streams, whereas pasture streams were the most diverse. Chlorophyll a concentration and species richness depended on land use and season. Forest coverage was positively correlated with acidophilous and oligo-eutraphentic diatom species. Coffee coverage displayed a significant positive correlation with motile species and a significant negative correlation with pollution-sensitive diatom taxa. The results show that diatom assemblages responded to micro-watershed conditions and can be used to monitor the effects of land use on streams in tropical regions.

The downstream transport of fish larvae is well known from temperate running waters, but there exists remarkably little information for tropical streams. We sampled drift in two Andean Piedmont streams during the dry seasons of 1986-1988. Bryconamericus deuterodonoides (Characidae) larvae were found to be extremely abundant in some drift collections, with peak drift densities exceeding those of temperate streams by as much as an order of magnitude. Drift of fish larvae displayed pronounced die1 periodicity, with more than one-quarter of a million larvae collected during the night compared to two individuals during the day. Fish larvae were more abundant during the latter compared to early part of the dry season, even in years when streams were reduced to small isolated pools by the end of the season. These observations indicate that the dry season represents an important period of reproduction for the characid, Bryconamericus deuterodonoides, and possibly other neotropical fish species.

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.

Freshwater prawns of the genus Macrobrachium are free-living decapod crustaceans that are commonly encountered in tropical streams and lakes. We present a molecular phylogenetic analysis of the diverse Southeast and East Asian fauna based on >3 kb sequence data from three nuclear and two mitochondrial markers for almost 50% of the described fauna. We reconstruct the phylogenetic relationships and track the evolution of key life history traits. Our tree suggests that the last common ancestor of the Asian Macrobrachium laid numerous small eggs and had prolonged larval development (''PLD") in saline coastal waters after which the adults matured in freshwater habitats. We also argue for five independent losses of the marine larval phase to yield five clades of species that develop entirely in freshwater and have fewer and larger eggs than the species with PLD. These species have either semi-abbreviated (two origins) with at least one free-swimming stage or abbreviated larval development (''ALD": three origins) which lack free-swimming larvae. A Shimodaira-Hasegawa test rejects all trees that would imply a single loss of the marine larval phase, but alternative and equally parsimonious optimizations exist that imply a smaller number of losses. However, these scenarios would require the re-acquisition of free-swimming larvae. A concentrated-change test supports Pereira and Garcia's [Pereira, G.A., Garcia, J.V., 1995. Larval development of Macrobrachium reyesi Pereira (Decapoda, Palaemonidae), with a discussion on the origin of abbreviated development in palaemonids. J. Crust. Biol. 15, 117-133] hypothesis of a significant correlation between living in freshwater and the origin of semi-abbreviated and abbreviated larval development. Our phylogenetic tree also reveals that Asian Macrobrachium have independently become cavernicolous at least twice, and invaded the highly acidic waters of freshwater and peat swamps two or three times.

The distribution of the benthic community may be explained as an adjustment between hydrological conditions and life history of the populations (Statzner et al. 1997). The distribution of organisms changes in relation to hydrological and geomorphological conditions at different spatial scales: drainage, reach, and habitat (Hildrew 1996, Maddock 1999). Information about benthic assemblages associated with particular habitats is essential in order to assess environmental changes and to understand ecological segregation within multispecies communities (Bisson et al. 1982). Quantification of habitat use provides a basis for

Freshwater snails usually possess thin unadorned shells lacking structural components such as spines. Exceptions can be found on the high, well-watered islands of the South Pacific. Streams on these islands support a rich freshwater molluscan fauna with several nerite taxa (Neritimorpha: Neritidae) exhibiting extremely long dorsal spines. We sought to assess the defensive capacity of these structures for several co-occurring nerite genera on the Island of Ovalau, Fiji. Our overarching hypothesis was that spines confer a defensive advantage. We tested four predictions for eight common taxa: (i) predator "rich" habitats (the creek entrance) would be dominated by spine-bearing nerites, (ii) spine-bearing species should be smaller in size, (iii) nerites with spines would exhibit lower levels of shell damage and (iv) nerites with spines should invest less in their shells (i.e., their shells should be thinner). Most of these predictions received support. Spine-bearing species dominated the entrance to the creek and were smaller in size. Levels of shell damage were low overall, with 2 of the 3 spinose taxa exhibiting no shell damage, as did many of the nonspinose taxa. Finally, shells of spinose species were 25% thicker, demonstrating increased rather than decreased investment. Taken together, these findings suggest that the elaborate spines of Clithon spp. play a defensive role.

Surveys of wood along 30 forested headwater stream reaches in La Selva Biological Station in north-eastern Costa Rica represent the first systematic data reported on wood loads in neotropical streams. For streams with drainage areas of 0·1-8·5 km 2 and gradients of 0·2-8%, wood load ranged from 3 to 34·7 m 3 wood/100 m channel and 41-612 m 3 wood/ha channel. These values are within the range reported for temperate streams. The variables wood diameter/flow depth, stream power, the presence of backflooding, and channel width/depth are consistently selected as significant predictors by statistical models for wood load. These variables explain half to two-thirds of the variability in wood load. These results, along with the spatial distribution of wood with respect to the thalweg, suggest that transport processes exert a greater influence on wood loads than recruitment processes. Wood appears to be more geomorphically effective in altering bed elevations in gravel-bed reaches than in reaches with coarser or finer substrate. Figure 2. Views of (A) site 11, El Surá, with slope of 5·91%, and drainage area of 3·26 km 2 ; (B) site 17, Quebrada Esquina, with slope of 3·17%, and drainage area of 1·64 km 2 ; (C) site 3, El Surá, with slope of 0·24%, and drainage area of 4·76 km 2 ; (D) site 1, Taconazo, with a slope of 0·32%, and drainage area of 0·28 km 2 .

1.Principles and practices of conservation from temperate, developed-world regions can generally be applied to tropical, developing regions, but the specific solutions are likely to be determined by regional ecological and socio-economic factors.2.This can be seen in the contributions to the Special Issue ‘Conservation of Tropical Streams’: principles of catchment protection, hydrological connectivity and biodiversity conservation are broadly applicable, but regional planning and management need to be based on specific studies.3.Although specific studies and monitoring are necessary, they should not delay the conservation process; in many cases the specific studies need to parallel the management actions and refine the conservation planning.Copyright © 2006 John Wiley & Sons, Ltd.Principles and practices of conservation from temperate, developed-world regions can generally be applied to tropical, developing regions, but the specific solutions are likely to be determined by regional ecological and socio-economic factors.This can be seen in the contributions to the Special Issue ‘Conservation of Tropical Streams’: principles of catchment protection, hydrological connectivity and biodiversity conservation are broadly applicable, but regional planning and management need to be based on specific studies.Although specific studies and monitoring are necessary, they should not delay the conservation process; in many cases the specific studies need to parallel the management actions and refine the conservation planning.

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