Search

Nitrogen stable isotope ratios (δ15N) are routinely used to estimate consumer trophic positions (TPs). This method is very sensitive to the chosen isotopic baseline, which should incorporate all temporal isotopic variability occurring in the base of the food web sustaining the consumers. However, there is still no consensus on the different methods to represent time intervals for isotopic assimilation. It is uncertain if researchers should compute a single baseline obtained by averaging δ15N source values across all time periods, multiple baselines based on δ15N source values collected concomitantly with consumers, or consider a time lag between sources and consumers. Aiming to provide empirical evidence to help select the best option, we designed 4 methods using different temporal lags between consumers and producers based on a 9 yr time series of carbon and nitrogen stable isotopes in an estuary. The 4 tested baseline methods considered the δ15N average of producers sampled in all years (Global); the same season as consumers (Concomitantly); the season before the collection of consumers (Delayed); and the previous season as well as the same season of consumer sampling (Interpolation). Comparison of results with an expected model (based on stomach content data) showed that the Delayed method is the most appropriate. We conclude that time lags in the incorporation of stable isotope values between sources and consumers must be considered in baseline δ15N calculations to improve TP estimates and provide more reliable modeling results. Additionally, we provide other recommendations to improve sampling designs when using stable isotopes in TP estimations.

Food partitioning among coexisting species is often considered advantageous to minimize niche overlap and avoid inter-specific competition. Congeneric fish species such as the mullets Mugil curema and Mugil liza, which co-occur across marine and estuarine habitats, are good models to evaluate resource use and niche overlap or partitioning. We used stomach contents (SCA) and stable isotope analysis (SIA) to assess potential trophic shifts and changes in niche overlap associated with the mullets transitioning from marine to estuarine habitats. SIA included different fractions of organic matter in suspension and in the sediment to estimate the contribution of micro, nano and pico-organisms to the mullets’ diets. We hypothesized higher resource partitioning in the less resource-diverse system (marine surf-zone) than in the more diverse one (estuary). SCA showed diet differences between M. curema and M. liza according to the habitat. They showed distinct diets in the marine area (P < 0.001), but similar diets in the estuary (P = 0.226). A lower niche breadth was observed for both species in the marine area (M. curema = 0.03, M. liza = 0.06) compared with the estuary (M. curema = 0.14, M. liza = 0.16). Isotopic niches of both species were higher in the estuary (64.7%) compared with the marine area (0.7%). These findings corroborated our hypothesis of higher food partitioning in the marine surf-zone. We also demonstrated using SIA the shift from planktonic to benthic feeding following the recruitment of the mullets from the surf-zone into the estuary.

Mercury (Hg) is a contaminant of global concern due to its damaging toxicological effects on organisms. For the vulnerable Dusky Grouper (Epinephelus marginatus) off the coast of Brazil, we investigated: i) spatial patterns in muscle tissue total mercury (THg) contamination; ii) the relationship between muscle THg concentrations and total length iii) the relationship between muscle THg and stable isotopes; and iv) THg concentrations among muscle, liver, and ovary tissues. Out of 134 fish sampled, 21.8 % were higher than 0.5 mg/kg wet weight (above the safe limit for human consumption). THg concentrations increased toward lower latitudes, but an opposite pattern was observed for δ

Biodiversity, biological traits of potential host species, and environmental conditions may mediate the emergence of new diseases. We assessed the relative influence of such factors on patterns of infection by Austrodiplostomum compactum (Digenea, Diplostomidae) in fish assemblages of the Upper Parana River floodplain, Brazil. Multiple infection parameters were modeled at the community and species levels using phylogenetic diversity (PD), abundance (total and for a main reservoir species Plagioscion squamosissimus), local environmental conditions and phylogenetic distance from P. squamosissimus (Dis_Plag). In total, 108 fish species were collected and 28 were infected. At the community level, mean parasite abundance and mean infection intensity were positively associated with PD and the interaction between PD and environmental conditions, whereas host richness was negatively associated with PD. The complementary results indicate a biodiversity sampling effect rather than dilution effect. Environmental conditions often had the strongest coefficients in community-level models and mediated associations between infection parameters and other factors. At the species level, consistent negative associations between infection parameters and Dis_Plag indicate phylogenetic niche conservatism of parasites. Integration of community and species-level analyses demonstrates that phylogenetic diversity can affect host–parasite interactions in multiple ways, but that the associations depend on phylogenetic relationships and environmental conditions.

The introduction of non-native predators is a matter of great concern, but their impacts on ecosystem functions remain poorly understood. We investigated how changes in fish diversity following the invasion of Cichla kelberi affected ecosystem functions generated by fish populations. Fish assemblages were sampled in macrophyte patches in a Neotropical impoundment over a 5-year period, before and after the introduction of the predator. We assigned seven ecosystem functions (26 trait-states) to each fish species, and examined how these functions behaved after the invasion. We collected 577 fish belonging to 25 species. Species richness, fish biomass and main species declined significantly over periods. The biomass of ecosystem functions changed significantly over time, and most trait-states declined. Few trait-states were lost, but all functions had at least one trait-state reduced by more than 85%. A null model analysis showed that changes in functions were not driven by species identities, while species richness correlated positively with total biomass and with most functions, suggesting that the loss of taxa and biomass drove observed changes in ecosystem functions. Our study provided evidence that community disassembly associated with the invasion of C. kelberi translated to the decline of several ecosystem functions, affecting energy mobilization and transference. Resumo A introdução de predadores não-nativos tem gerado grande preocupação, mas seus impactos sobre a geração de funções ecossistêmicas permanecem pouco investigados. O presente estudo investigou como mudanças na diversidade de peixes, associadas com a invasão de Cichla kelberi, afetaram funções ecossistêmicas geradas por populações de peixes. As assembleias de peixes foram amostradas em bancos de macrófitas em um grande reservatório Neotropical durante um período de cinco anos, antes e depois da introdução do predador. Um conjunto de funções ecossistêmicas (26 trait-states) foi atribuído a cada espécie de peixe, e examinamos como as funções se comportaram depois da invasão. Foram coletados 577 peixes pertencentes a 25 espécies. Registramos declínio significativo da riqueza de espécies, biomassa total e biomassa das principais espécies ao longo dos períodos. A biomassa das funções ecossistêmicas mudou significativamente ao longo do tempo, e a maioria dos trait-states declinou. Poucos trait-states foram perdidos, mas todas as funções tiveram pelo menos um trait-state reduzido em mais de 85%. Uma análise de modelos nulos mostrou que as mudanças nas funções não foram impulsionadas pela identidade das espécies, enquanto que a riqueza de espécies correlacionou positivamente com a biomassa total e com a maioria das funções, sugerindo que a perda de espécies e biomassa impulsionou as mudanças observadas nas funções ecossistêmicas. Nosso estudo apresenta evidências de que a desestruturação da comunidade, associada com a invasão de C. kelberi, se traduziu no declínio de várias funções ecossistêmicas, afetando a mobilização e transferência de energia.

Estuaries are influenced by seasonal and inter-annual variability in marine and freshwater intrusion. This variability affects the abundance and distribution of resources and consumers and may drive estuarine food web dynamics. This study tests for relationships among environmental variables and estuarine consumer δ13C, and assesses whether freshwater-derived sources increase in relative contribution to estuarine consumers during high freshwater inflow events. Data were collected seasonally from 2010 through 2016 for Patos Lagoon, Brazil, and included rainfall and river discharge in the drainage basin, and salinity, chlorophyll-α, and phytoplankton abundance in the estuarine zone of the lagoon. Similarly, stable isotope data (δ13C, δ15N) were collected seasonally for autochthonous sources and consumers (detritivorous and zooplanktivorous fishes) in the estuarine zone, and allochthonous sources of freshwater and marine origin. Mixed-effect models assessed relationships among consumer δ13C and environmental variables, and Bayesian mixing models (MixSIAR) estimated the relative importance of autochthonous and allochthonous sources for estuarine consumers. Hydrology affected estuarine consumer δ13C, corresponding to greater contributions of marine-derived organic material during periods with high freshwater inflow, especially for detritivorous juvenile mullet. This unexpected finding is likely due to high freshwater inflows delaying recruitment of juvenile mullet from coastal areas into the estuarine zone such that data from seasonal sampling reflected marine rather than local feeding and active transport of marine-derived sources into the estuarine zone. In addition to transporting freshwater-derived organic material, high freshwater inflow events may have other important indirect effects on the dynamics of trophic subsidies to estuarine food webs which require further study.

We summarize observed and expected impacts of hydroelectric dams on tropical fishes and fisheries through the lens of fish functional traits and consequences of shifting functional diversity for social-ecological systems. Following impoundment, stocks respond to environmental changes differently according to their functional traits, resulting in fairly predictable shifts in assemblage functional structure. The most vulnerable species are those with traits adaptive for habitats with fast flowing water, structural complexity, flood pulsing or those requiring connectivity across basins to complete their life cycle. Shifts in assemblage functional trait distributions are accompanied by reduced fishery yields and impacts to other ecosystem services. Research employing a functional traits approach should enhance assessment of impacts of impoundments on biodiversity, fisheries and ecosystem services, especially for data-deficient systems.

High rates of deforestation, either in the past or the present, affect many of the ecological processes in streams. Integrating deforestation history and the current landscape structure enhances the evaluation of ecological effects of land-use change. This is especially true when contemporary landscape conditions are similar but the temporal path to those conditions differs. One approach that has shown promise for evaluating biodiversity responses over time and space is the β-diversity partitioning, which combines taxonomic and functional trait-based approaches. We tested hypotheses related to stream fish assemblages’ turnover in watersheds with different environmental conditions and deforestation histories. We sampled fish from 75 watersheds in the Machado River basin, Brazil, and environmental factors were quantified at multiple scales. Taxonomic turnover was higher than expected by chance, whereas functional turnover was lower than expected by the observed taxonomic turnover, indicating that deterministic processes are structuring these assemblages. The turnover, and the environmental factors differed among watersheds with different deforestation histories. Besides being scale-dependent, turnover patterns are also likely dependent on land use dynamics and involve time-lags. Resumo Altas taxas de desmatamento, seja no passado ou no presente, afetam muitos processos ecológicos em riachos. Integrar o histórico do desmatamento à estrutura atual da paisagem melhora a avaliação dos efeitos ecológicos da mudança do uso do solo. Isto é especialmente verdadeiro quando as condições da paisagem contemporânea são semelhantes, mas seguiram trajetórias distintas. Uma abordagem promissora para avaliar as respostas da biodiversidade ao longo do tempo e espaço é a partição da diversidade-β, que combina abordagens taxonômica e funcional. Testamos hipóteses relacionadas à substituição das assembleias de peixes de riachos em microbacias com diferentes condições ambientais e histórias de desmatamento. Amostramos 75 microbacias na bacia do rio Machado, fatores ambientais foram quantificados em múltiplas escalas. A substituição taxonômica foi maior do que esperada pelo acaso, enquanto a substituição funcional foi menor do que o esperado pela substituição taxonômica, indicando que as assembleias são estruturadas por processos determinísticos. As substituições e fatores ambientais diferiram entre microbacias com históricos de desmatamento distintos. Além de serem escala-dependentes, os padrões de substituição provavelmente são dependentes da dinâmica do uso do solo, envolvendo defasagem temporal.

The Amazon rainforest has experienced rapid land‐use changes over the last few decades, including extensive deforestation that can affect riparian habitats and streams. The aim of this study was to assess responses of stream fish assemblages to deforestation and land cover change in the eastern Amazon. We expected that percentage of forest in the catchment is correlated with local habitat complexity, which in turn determines fish assemblage composition and structure. We sampled 71 streams in areas with different land uses and tested for relationships between stream fish assemblages and local habitat and landscape variables while controlling for the effect of intersite distance. Fish assemblage composition and structure were correlated with forest coverage, but local habitat variables explained more of the variation in both assemblage composition and structure than landscape variables. Intersite distance contributed to variance explained by local habitat and landscape variables, and the percentage of variance explained by the unique contribution of local habitat was approximately equivalent to the shared variance explained by all three factors in the model. In these streams of the eastern Amazon, fish assemblages were most strongly influenced by features of instream and riparian habitats, yet indirect effects of deforestation on fish assemblage composition and structure were observed even though intact riparian zones were present at most sites. Long‐term monitoring of the hydrographic basin, instream habitat and aquatic fauna is needed to test for potential legacy effects and time lags, as well as assess species responses to continuing deforestation and land‐use changes in the Amazon.

The invasion paradox refers to the existence of biotic resistance and biotic acceptance hypotheses. According to the biotic resistance hypothesis, the higher the richness present in a community, the more resistant it is against invaders. In contrast, the biotic acceptance hypothesis states that if the environment is suitable for a high richness of native species, it will be similarly suitable for establishment of non-native species. Previous studies on terrestrial plant communities considered the scale-dependent nature of native and non-native relationships: the relationship tends to be negative at small scales (biotic resistance) and positive at broader scales (biotic acceptance). We tested the hypothesized role of spatial scale on the relationship between non-native and native species richness using a spatially nested design and a long-term dataset (146 communities sampled during 13 years) of fish species richness from the upper Parana River floodplain, Brazil. Contrary to expectations, non-native fish species richness was positively correlated with native species richness at all spatial scales. Mobility of vertebrates, mode of invasion, and environmental disturbance may affect the role of spatial scale in potentially mediating relative importance of biotic acceptance versus resistance. In this context, the present study provides a unique contribution towards resolving the invasion paradox.

Deforestation is a primary driver of biodiversity change through habitat loss and fragmentation. Stream biodiversity may not respond to deforestation in a simple linear relationship. Rather, threshold responses to extent and timing of deforestation may occur. Identification of critical deforestation thresholds is needed for effective conservation and management. We tested for threshold responses of fish species and functional groups to degree of watershed and riparian zone deforestation and time since impact in 75 streams in the western Brazilian Amazon. We used remote sensing to assess deforestation from 1984 to 2011. Fish assemblages were sampled with seines and dip nets in a standardized manner. Fish species (n = 84) were classified into 20 functional groups based on ecomorphological traits associated with habitat use, feeding, and locomotion. Threshold responses were quantified using threshold indicator taxa analysis. Negative threshold responses to deforestation were common and consistently occurred at very low levels of deforestation ( 70% deforestation and >10 years after impact. Findings were similar at the community level for both taxonomic and functional analyses. Because most negative threshold responses occurred at low levels of deforestation and soon after impact, even minimal change is expected to negatively affect biodiversity. Delayed positive threshold responses to extreme deforestation by a few species do not offset the loss of sensitive taxa and likely contribute to biotic homogenization.

Gross primary production and ecosystem respiration together define ecosystem metabolism and help indicate the importance of internal and external carbon sources. Spatial variability of these processes is poorly characterized in rivers. We measured metabolism in the Kansas River: (1) at 10 locations over 100 s of km in tributaries within the watershed and (2) over 20 km with detailed sampling in the main stem. Whole-river metabolism at the larger scale was decoupled from light, algal growth, and nutrient limitation, and was positively related to nutrients. Smaller-scale main stem sampling revealed almost as much variance over a few kilometers as the larger scale sampling. Local processes seemed to dominate dissolved oxygen dynamics, since diurnal dissolved oxygen patterns were better correlated with absolute time than data corrected for travel times. A single-station method compared against two-station metabolism methods indicated that local hotspots of metabolism occur at scales less than 1 km and that single-station estimates average out this variance. The main stem data provide support to the idea that functional processing zones control characteristics used to estimate system metabolism, but the nutrient effect at the whole watershed level indicates that transport from upstream can also be important.

Environmental conditions found along an invasion front can mediate spread dynamics of an invasive species. Conditions that affect survival and individual growth rates of founder propagules and the first young-of-year generation can have a profound effect on establishment dynamics. Responses of younger, more sensitive individuals to these novel conditions, in part, determine establishment success. The southern invasion front of zebra mussels in North America expanded into Texas in 2009. To determine mechanisms that could affect establishment of zebra mussel populations at low latitudes in North America, an in situ study was conducted in Lake Texoma, on the Texas-Oklahoma border. Survival and growth were measured of young-of-year juveniles at multiple sites within the reservoir. Age-specific mortality was significantly positively correlated with temperature at all sites and no mortality occurred when temperatures were

Made available in DSpace on 2018-11-28T06:17:53Z (GMT). No. of bitstreams: 0 Previous issue date: 2017-03-13 Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) International Foundation of Science, IFS ICMBIO US National Science Foundation Resource pulsing is a widespread phenomenon, but its effects on ecosystem dynamics are often difficult to predict. Hydrological pulsing, in particular, is known to influence the structure and dynamics of fluvial and coastal ecosystems, but little information is available about its effects on trophic connectivity between wetlands and estuaries. We investigated the hypothesis that hydrologic pulsing drives 1-way trophic subsidies (e.g. suspended organic matter and freshwater fish) from wetland to estuary. Our study system is a coastal lagoon with an ephemeral mouth that, when closed, stores freshwater as a sustained flood pulse that is subsequently released when a connection with the sea is reestablished. We monitored isotopic composition of consumers and food sources over the course of an entire flood pulse to infer trophic linkages and spatial subsidies. Before the flood peak (April and May), freshwater and estuarine zones were largely dependent on local primary production sources (seston and C-3 plants vs. C-4 plants and microphytobenthos, respectively), essentially functioning as disconnected compartments. A sustained pulse of freshwater inflow (June to August) induced greater habitat connectivity and a net flow of biomass and energy from the freshwater zone into the estuarine zone. The opening of the lagoon outlet channel abruptly terminated the flood pulse and reduced freshwater subsidies to estuarine consumers, and both zones returned to dependence on autochthonous production. Our findings contribute to current concerns that artificial opening of sandbars in coastal lagoons alters natural ecological dynamics with significant effects on biodiversity and ecosystem processes. Rio Grande Fed Univ, Oceanog Inst, BR-96203900 Rio Grande, RS, Brazil Texas A&M Univ, Dept Wildlife & Fisheries Sci, College Stn, TX 77843 USA Texas A&M Univ, Interdisciplinary Program Ecol & Evolutionary Bio, College Stn, TX 77843 USA Univ North Texas, Dept Biol Sci, Denton, TX 76203 USA Univ North Texas, Adv Environm Res Inst, Denton, TX 76203 USA Paulista Julio de Mesquita Filho State Univ, Stable Isotope Ctr Environm & Life Sci, Biosci Inst, BR-18608000 Botucatu, SP, Brazil Paulista Julio de Mesquita Filho State Univ, Stable Isotope Ctr Environm & Life Sci, Biosci Inst, BR-18608000 Botucatu, SP, Brazil CNPq: 482920/2007-6 CNPq: 305888/2012-9 International Foundation of Science, IFS: A/4419-1 ICMBIO: 14523-2 ICMBIO: 14523-4 US National Science Foundation: DEB 1257813 US National Science Foundation: IGERT 0654377

Made available in DSpace on 2020-12-12T02:46:47Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-01-01 Beta diversity quantifies changes in assemblages among sites and can identify how anthropogenic changes affect species distributions patterns. We investigated how spatial and temporal beta diversity of fish assemblages responded to recent environmental changes within an established agroecosystem and investigated whether the observed pattern of spatial and temporal beta diversity was different from null expectations. Fish assemblages in 38 streams were sampled in 2003 and 2013, and local and regional variables were quantified in both periods. Species were classified into functional groups based on 15 ecomorphological traits. We tested for relationships between spatial and temporal patterns of beta diversity and its components (turnover and nestedness) and environmental distances and degree of changes over time, and for differences with null expectations. Spatial beta diversity was mostly due to turnover and remained relatively unchanged between periods. Spatial beta diversity was not different from randomness. Temporal beta diversity was lower than expected by null models and not correlated with environmental changes. Past environmental filters related to a long history of agricultural development homogenized habitats and fish assemblages by selecting for habitat-generalist species. In this context, contemporary fish assemblage dynamics in these streams are driven primarily by stochastic processes, likely stochastic local species loss. Department of Ecology UNESP - São Paulo State University, 1515 24A Avenue Department of Zoology and Botany UNESP - São Paulo State University, 2265 Cristóvão Colombo Street Department of Biological Sciences and the Advanced Environmental Research Institute University of North Texas, 1155 Union Circle #310559 Escuela de Ciencias Biológicas UPTC – Universidad Pedagógica y Tecnológica de Colombia Department of Ecology UNESP - São Paulo State University, 1515 24A Avenue Department of Zoology and Botany UNESP - São Paulo State University, 2265 Cristóvão Colombo Street

Biological communities are strongly affected by environmental factors and their variability. Estuarine biota is driven by abiotic factors such as salinity, rainfall, and river flow, varying at distinct spatial and time scales. The El Nino is a coupled ocean–atmosphere phenomenon occurring at irregular time scales that affects biological communities worldwide, including changes in species composition and diversity patterns. One of the most important attributes in community ecology is the food-chain length (FCL) because affects the ecosystem functioning. Based on the ‘intermediate disturbance’ theory and prior evidences showing effects of El Nino on animal diversity, we hypothesized this phenomenon will increase richness (both at species and trophic guilds levels) and FCL in a shallow-water estuarine community. To evaluate our hypothesis, we analyzed a 9-yr dataset of carbon and nitrogen stable isotope ratios of food web components in a shallow embayment of a subtropical estuary. We estimated consumer's trophic positions (TP) using a recent Bayesian approach that incorporates individual variability and sampling error in isotope fractionation, isotopic baselines and consumers. As expected, both species and trophic guilds richness were higher during El Nino events. FCL ranged from 2.45 to 4.60, but against our prediction showed a slight decreased during higher rainfall periods. These finding suggest FCL in estuaries is resilient to environmental variability and natural climatic perturbations.

Our study incorporated a comprehensive suite of parameters (i.e., body size, age, diet and trophic position) to investigate mercury concentration in dusky groupers Epinephelus marginatus. This study was carried out in rocky bottoms in littoral and neritic habitats along the Southern Brazilian coast. We also determined spatial variation in mercury concentrations in individuals inhabiting both zones, which may provide insights into how dietary differences or potential pollution sources affect bioaccumulation. A total of 244 dusky groupers was analyzed to determine total mercury concentrations. Our study revealed that when considering similar body sizes, individuals inhabiting littoral rocky habitats had higher concentrations of mercury probably due to proximity to pollution sources associated with human activities in the estuary and its drainage basin. Furthermore, large individuals (>650mm and >8years old) showed mercury contamination levels that are potentially harmful for this endangered fish species and above the acceptable limits for human consumption.

Isolation-by-distance is recognized as a useful model for describing the spatial distribution of gene frequencies depending on dispersal characteristics of the species under study. However, some species may have populations that occupy the same geographic distribution during the breeding season yet reproduce at different time periods resulting in isolation-by-time (IBT). IBT may complicate investigations of spatial population structure if samples are obtained from multiple discrete time periods or may remain undiscovered if surveys are conducted with limited temporal scope. IBT has been observed in several studies of anadromous fishes (primarily salmon) as well as a few examples in taxa such as frogs, plants, birds and insects, but has not been rigorously tested in freshwater fishes. In this study, we assessed spatial and temporal genetic variation and tested for IBT in Dourado (Salminus brasiliensis), a large and commercially-important potamodromous fish species found in multiple river basins of South America. Using 11 polymorphic microsatellite loci, we estimated genetic differentiation of 317 adult Dourado collected monthly during the breeding season at three locations along the Uruguay River in southern Brazil. Analyses identified three populations that were clustered in time (i.e. early, middle and late), suggesting an IBT pattern of population structure with no significant spatial structure. Our results contribute to the mounting evidence across a wide range of taxa that suggests IBT may be more common that currently considered, even for species with very high dispersal capabilities such as potamodromous fishes.

The Xingu River, located in northern Brazil, is already under the impacts of one of the most harmful projects in the history of the Amazon – the Belo Monte hydroelectric dam. A new megaproject called the ‘Volta Grande Project’, led by Canadian gold mining company Belo Sun will be situated adjacent to the same stretch of the Xingu that is dewatered by the Belo Monte complex. Various aspects of the project are controversial. There are reports that the mining company acquired public lands illegally, the affected area has high biodiversity and endemism of several taxonomic groups, and mining operations would be approximately 9.5 km from indigenous lands. Synergistic impacts of impoundment and mining operations are expected to dramatically alter the Xingu's biodiversity and ecosystem services that support indigenous people and riverine populations. Moreover, the mining activities will generate tons of toxic waste that will be stored in a high-risk tailings dam, which exposes the Xingu River to catastrophic consequences similar to those of the Doce River disaster.

There is a purported tendency for non-native species to have larger body sizes in their introduced range, commonly attributed to mechanisms such as enemy release or reduced competition. However, results are equivocal and this tendency may also result from ecosystem differences and/or the selective introduction of larger individuals. For most invasions it is difficult to separate the human from biological influences on body size. In this study, we utilize a natural experiment caused by the elimination of a semi-porous biogeographical barrier to test for body size differences in 12 Neotropical fish species in native and invaded ranges, unbiased by human influence in selecting introduced individuals. Our analyses include an additional 25 fish species native to both basins, enabling tests of consistency of body size patterns across native and non-native species in both ecosystems. Twenty-two species (9 non-native, 13 native), irrespective of life-history or trophic guild, had an interaction of population length-weight relationships which indicated inconsistency in relative body sizes in donor and recipient regions across age classes. Of the 15 species with similar slopes of the length-weight relationships between basins, all non-native species (n = 3) and five species native to both basins exhibited significantly larger body sizes (i.e. body mass at a given length) in the more productive donor ecosystem, and the remaining native species were either larger in the recipient system (n = 3) or were not significantly different between basins (n = 4). Our findings contribute to the growing literature that suggests perceived tendencies of larger body sizes in invaded ranges should not be generalized, especially when environmental conditions differ greatly among regions and when dealing with populations that exhibit significant age structure.

Species or functional guild loss from upper trophic positions, i.e., trophic downgrading, will likely have important consequences for ecosystem functioning due to cascading direct and indirect effects. Using 1000 l mesocosms, we examined how sequential loss of species occupying upper trophic positions influenced ecosystem functioning of experimental floodplain lagoon food webs. Treatments were developed based on fish assemblage data from long-term field surveys of tropical floodplain lagoons, and response variables represented multiple components of ecosystem functioning. Sequential loss of species occupying upper trophic positions significantly influenced multiple ecosystem responses including changes in fish assemblage structure, nutrient concentrations, and zooplankton density. Although loss of species from specific functional roles is expected to facilitate predictive understanding of ecosystem consequences, we observed complex and dynamic responses to trophic downgrading that did not follow expectations of strong predicted top-down effects. The highly connected food web structure in our system and relative balance between top-down and bottom-up processes likely suppressed cascading effects. Consequences of biodiversity loss in highly connected multitrophic ecosystems may be difficult to predict as ecosystem responses will likely deviate from simplified food chain dynamics or from patterns that emerged from single trophic level studies.

Longitudinal and lateral connectivity is a hallmark of natural riverine ecosystems that affects the distribution and diversity of habitats and organisms, flows of energy and matter, and functioning of ecosystems. Dams are ubiquitous in most river basins of the world, and there is a current proliferation of hydropower dams in developing regions of the tropics. Fragmentation interacts with other consequences of dams, such as alteration of flow regimes and habitat change, and ecological impacts depend on dam type, relative permeability, watershed attributes, regional climate, biogeography, and species traits. Dam-induced fragmentation has been documented to affect population sizes, spatial distributions, gene flow and genetic diversity, and community structure for a wide range of aquatic and riparian taxa, as well as aquatic and terrestrial food webs and ecosystem functioning. Technologies to improve connectivity across dams such as fish passages are broadly unsuccessful and may actually do more harm than good in some instances. Costly, ageing, and environmentally unsound dams are now being removed in the U.S. and some other developed countries to restore riverine connectivity. Integrative, strategic planning at the basin scale is needed to minimize impacts of hydropower development on biodiversity in tropical regions and when planning for dam removal.

Groupers are large-bodied fishes with broad global distribution and commercial and ecological importance. Many grouper species are endangered, but information on trophic ecology of groupers outside the Mediterranean Sea is limited. We integrated stomach contents analyses (SCA) and stable isotope analyses (SIA) to test the hypotheses that diets of dusky grouper Epinephelus marginatus differ between littoral and neritic zones in the southwestern Atlantic, but exhibit similar ontogenetic shifts in prey size and consumption of fishes. SCA were dominated by fishes, crabs, and shrimp and differed significantly between habitats, but prey size and consumption of fishes increased with grouper body size at both sites. Grouper stable isotope ratios were similar between sites and among size classes, but integrating SCA and SIA distinguished ingested versus assimilated dietary components, improved dietary resolution, and provided a more comprehensive assessment of grouper in the food webs. Dusky grouper integrate benthic and pelagic trophic pathways, as well as estuarine and marine food webs via consumption of migratory prey. Our findings have applications to other threatened grouper populations, for example, in managing fragmented populations or for reserve designs considering inclusion of artificial substrates as grouper habitat, and more broadly for approaches examining trophic ecology of generalist predators.

Studies in freshwater ecosystems are seeking to improve understanding of carbon flow in food webs and stable isotopes have been influential in this work. However, variation in isotopic values of basal production sources could either be an asset or a hindrance depending on study objectives. We assessed the potential for basin geology and local limnological conditions to predict stable carbon and nitrogen isotope values of six carbon sources at multiple locations in four Neotropical floodplain ecosystems (Parana, Pantanal, Araguaia, and Amazon). Limnological conditions exhibited greater variation within than among systems. δ15N differed among basins for most carbon sources, but δ13C did not (though high within-basin variability for periphyton, phytoplankton and particulate organic carbon was observed). Although δ13C and δ15N values exhibited significant correlations with some limnological factors within and among basins, those relationships differed among carbon sources. Regression trees for both carbon and nitrogen isotopes for all sources depicted complex and in some cases nested relationships, and only very limited similarity was observed among trees for different carbon sources. Although limnological conditions predicted variation in isotope values of carbon sources, we suggest the resulting models were too complex to enable mathematical corrections of source isotope values among sites based on these parameters. The importance of local conditions in determining variation in source isotope values suggest that isotopes may be useful for examining habitat use, dispersal and patch dynamics within heterogeneous floodplain ecosystems, but spatial variability in isotope values needs to be explicitly considered when testing ecosystem models of carbon flow in these systems.

Ecosystem processes and biological community structure are expected to change in a relatively predictable manner along fluvial gradients within river basins. Such predictions are heavily based on temperate rivers, and food web variation along fluvial gradients in Mesoamerican rivers has received limited attention. In this study, we analyzed carbon and nitrogen stable isotope ratios of basal carbon sources and dominant consumer species to examine aquatic food web structure along the fluvial gradient of the Monkey River Basin, Belize. Similar to previous studies in other regions, consumer species richness and functional diversity increased along the downstream fluvial gradient, due in part to the addition of estuarine species in lower reaches and increasing diversity of piscivorous species along the gradient. Aquatic food webs in upstream reaches were primarily supported by allochthonous production sources, and in-stream sources increased in importance along the downstream gradient. Our study system traversing the Maya Mountain Marine Area Transect also provided a unique opportunity to test the utility of primary consumer d 15 N as an indicator of watershed impacts within a tropical basin with a diverse biota and a different type of agricultural impact than typically studied (i.e. banana plantations vs. tilled row cropping). As expected, primary consumer d 15 N at sites draining impacted watersheds was enriched compared to values from forested reference sites. Assessment of primary consumer d 15 N may be a feasible option for monitoring watershed impacts on aquatic food webs in service of the ridge-to-reef conservation strategy adopted for this watershed as well as in other tropical river basins. Copyright # 2010 John Wiley & Sons, Ltd.

Mangrove and seagrass habitats are important components of tropical coastal zones worldwide, and are conspicuous habitats of Centla Wetland Biosphere Reserve (CWBR) in Tabasco, Mexico. In this study, we examine food webs in mangrove- and seagrass-dominated habitats of CWBR using stable isotope ratios of carbon and nitrogen. Our objective was to identify the importance of carbon derived from mangroves and seagrasses to secondary production of aquatic consumers in this poorly studied conservation area. Carbon and nitrogen isotope ratios of basal sources and aquatic consumers indicated that the species-rich food webs of both habitats are dependent on riparian production sources. The abundant Red mangrove Rhizophora mangle appears to be a primary source of carbon for the mangrove creek food web. Even though dense seagrass beds were ubiquitous, most consumers in the lagoon food web appeared to rely on carbon derived from riparian vegetation (e.g. Phragmites australis). The introduced Amazon sailfin catfish Pterygoplichthys pardalis had isotope signatures overlapping with native species (including high-value fisheries species), suggesting potential competition for resources. Future research should examine the role played by terrestrial insects in linking riparian and aquatic food webs, and impacts of the expanding P. pardalis population on ecosystem function and fisheries in CWBR. Our findings can be used as a baseline to reinforce the conservation and management of this important reserve in the face of diverse external and internal human impacts. Manguezais e pradarias de gramíneas são importantes componentes das zonas costeiras tropicais em todo o mundo, sendo habitats comuns nos ''Pântanos de Centla'', uma Reserva da Biosfera localizada em Tabasco, México. Nesse trabalho, são investigadas as teias alimentares de habitats dominados por manguezais e pradarias de gramíneas, através de isótopos estáveis de carbono e nitrogênio, tendo como objetivo identificar a importância do carbono derivado desses produtores para a produção aquática secundária nessa unidade de conservação tão pouco estudada. As razões isotópicas de carbono e nitrogênio das fontes basais e dos consumidores aquáticos indicam que as teias alimentares, ricas em espécies, de ambos os habitats, são dependentes da produção ripária. O abundante mangue-vermelho Rhizophora mangle parece ser a fonte primária de carbono no habitat dominado por manguezais. Em contraste, muito embora as pradarias de gramíneas sejam conspícuas no ambiente lagunar estudado, muitos dos consumidores da teia alimentar nessa região parecem depender do carbono oriundo da vegetação ripária (e.g. Phragmites australis).A espécie de bagre exótica Pterygoplichthys pardalis possui assinaturas isotópicas que se sobrepõem a das espécies nativas (incluindo algumas de elevado valor comercial na pesca), sugerindo competição potencial por recursos. Novas investigações deveriam avaliar o papel dos insetos terrestres como elo de conexão entre as teias alimentares da mata ripária e do ambiente aquático, bem como os impactos da expansão da população de P. pardalis sobre o funcionamento do ecossistema e das pescarias no ''Pântano de Centla''. Considerando os múltiplos impactos antrópicos na região, os resultados obtidos no presente trabalho podem ser utilizados como uma base de referência em programas de gerenciamento e conservação da diversidade nessa importante Reserva da Biosfera.

We reviewed concepts of patch dynamics and environmental heterogeneity and their applications to the study of fluvial ecosystems, with emphasis on research published in J-NABS. We discuss several important papers synthesizing theories and findings on this topic and reports of descriptive and experimental research. A large body of research, much of it published in J-NABS, has demonstrated how spatial and temporal variation influences population, community, and ecosystem patterns and processes in fluvial ecosystems. Conceptual models of patch dynamics can be traced to 2 basic approaches: 1) the landscape ecology perspective and 2) the metacommunity perspective. The former focuses on how spatial patterns are created and affect ecological processes over variable scales of space and time, whereas the latter emphasizes the important influence of periodic disturbances, refugia, and dispersal in maintaining nonequilibrium communities within patch mosaics. The origin of the metacommunity Patch Dynamics Con...

Over the last few decades, the use of stable isotope ratios in ecological studies has increased greatly, most notably in the area of trophic ecology and food webs (Peterson and Fry 1987, Fry 2006). However, applications of stable isotopes in food web studies have been mostly qualitative to date, prompting the recent development of quantitative approaches to investigate different aspects of trophic ecology and food web structure using stable isotope data (e.g., Bearhop et al. 2004, Newsome et al. 2007, Schmidt et al. 2007). One such contribution (Layman et al. 2007a) proposed a series of six metrics that provide community-wide measures of trophic structure using a dual-isotope framework. However, the metrics as proposed by Layman et al. (2007a) will not accurately represent aspects of trophic structure for most ecological systems. Herein we summarize the framework and metrics proposed by Layman et al. (2007a), identify key limitations that prevent their widespread application, and discuss other directions that may facilitate the development of quantitative food web approaches using stable isotope data.

Food-chain length is a central characteristic of ecological communities that affects community structure and ecosystem function. What determines the length of food chains is not well resolved for most ecosystems. Herein, we examine environmental correlates of food-chain length based on the productivity hypothesis, compare food-chain lengths among aquatic ecosystem types and identify bi-directional effects of river impoundment on food-chain length in the Parana ´ River Basin of South America. Both temperature regime, a surrogate of productivity, and ecosystem type significantly affected food-chain length in independent analyses. However, when analyzed together, only ecosystem type explained significant variation in food-chain length. Food chains were longest in reservoirs, and shortest in high-gradient rivers. The proximate mechanism driving this pattern appears to be body-size ratios of primary consumers to apex predators, which differ among trophic pathways. Food chains based on phytoplankton production may have an additional size-structured link not present in food chains based on other basal sources such as detritus and algae. Hydrogeomorphology is the ultimate mechanism influencing food-chain length because it affects the relative importance of basal carbon sources supporting higher trophic levels, which through differences in the number of trophic links along the different sizestructured pathways, appears to drive the observed patterns in food-chain length. We discuss a hypothesis of food-chain length that integrates energy flow and size-structure, facilitates inclusion of temporal dynamics and which is readily testable in both ‘closed’ and ‘open’ ecosystems.

The world's most biodiverse river basins—the Amazon, Congo, and Mekong—are experiencing an unprecedented boom in construction of hydropower dams. These projects address important energy needs, but advocates often overestimate economic benefits and underestimate far-reaching effects on biodiversity and critically important fisheries. Powerful new analytical tools and high-resolution environmental data can clarify trade-offs between engineering and environmental goals and can enable governments and funding institutions to compare alternative sites for dam building. Current site-specific assessment protocols largely ignore cumulative impacts on hydrology and ecosystem services as ever more dams are constructed within a watershed ( 1 ). To achieve true sustainability, assessments of new projects must go beyond local impacts by accounting for synergies with existing dams, as well as land cover changes and likely climatic shifts ( 2 , 3 ). We call for more sophisticated and holistic hydropower planning, including validation of technologies intended to mitigate environmental impacts. Should anything less be required when tampering with the world's great river ecosystems?

Efforts to conserve, restore, or otherwise manage large rivers and the services they provide are hindered by limited understanding of the functional dynamics of these systems. This shortcoming is especially evident with regard to trophic structure and energy flow. We used natural abundances of carbon and nitrogen isotopes to examine patterns of material flow in ten large-river food webs characterized by different landscape-scale hydrologic characteristics (low-gradient river, high-gradient river, river stretches downstream of reservoirs, and reservoirs), and tested predictions from three ecosystem concepts commonly applied to large-rivers: The River Continuum Concept, The Flood Pulse Concept and the Riverine Productivity Model. Carbon derived from aquatic C3 plants and phytoplankton were the dominant energy sources supporting secondary consumers across the ten large-river food webs examined, but relative contributions differed significantly among landscape types. For low-gradient river food webs, aquatic C3 plants were the principal carbon source, contributing as much as 80% of carbon assimilated by top consumers, with phytoplankton secondarily important. The estimated relative importance of phytoplankton was greatest for food webs of reservoirs and river stretches downriver from impoundments, although aquatic C3 plants contributed similar amounts in both landscape types. Highest 99th percentile source contribution estimates for C4 plants and filamentous algae (both approximately 40%) were observed for high-gradient river food webs. Our results for low-gradient rivers supported predictions of the Flood Pulse Concept, whereas results for the three other landscape types supported the Riverine Productivity Model to varying degrees. Incorporation of landscape-scale hydrologic or geomorphic characteristics, such as river slope or floodplain width, may promote integration of fluvial ecosystem concepts. Expanding these models to include hydrologically impacted landscapes should lead to a more holistic understanding of ecosystem processes in large-river systems.

We used stable C and N isotope ratios of tissues from 29 fish species from a large subtropical lagoon in southern Brazil to examine spatial variability in isotopic composition and vertical trophic structure across freshwater and estuarine habitats. Nitrogen isotope ratios indicated a smooth gradation in trophic positions among species, with most fishes occupying the secondary and tertiary consumer level. Fish assemblages showed a significant shift in their carbon isotopic signatures between freshwater and estuarine sites. Depleted carbon signatures (from −24.7‰ to −17.8‰) were found in freshwater, whereas more enriched signatures (from −19.1‰ to −12.3‰) were obtained within the estuarine zone downstream. Based on our survey of the C 3 and C 4 plants and isotopic values for phytoplankton and benthic microalgae reported for ecosystems elsewhere, we hypothesized that the observed δ 13 C differences in the fish assemblage between freshwater and estuarine sites is due to a shift from assimilating organic matter ultimately derived from C 3 freshwater marsh vegetation and phytoplankton at the freshwater site ( δ 13 C ranging from −25‰ to −19‰), to C 4 salt-marsh (e.g. Spartina ) and widgeon grass ( Ruppia maritima ), benthic microalgae and marine phytoplankton at the estuarine sites (from −18‰ to −12‰). Our results suggested that fish assemblages are generally supported by autochthonous primary production. Freshwater fishes that likely were displaced downstream into the estuary during periods of high freshwater discharge had depleted δ 13 C values that were characteristic of the upper lagoon. These results suggest that spatial foodweb subsidies can occur within the lagoon.

Analyses of stable C and N isotopes were used to examine trophic characteristics of blue crabs Callinectes sapidus in relation to body size (15 to 165 mm carapace width, CW) within saltmarsh habitats of the Aransas National Wildlife Refuge, Texas. Blue crab trophic position did not change with increasing body size, and all size classes appear to consume primary consumers and primary producers in relatively equal proportions. However, blue crab δ13C values increased significantly with increasing body size. As body size increased, blue crabs assimilated greater proportions of carbon ultimately derived from Spartina alterniflora. Individuals of approximately 35 mm CW assimilated carbon almost exclusively derived from C3 plants and benthic algae, whereas larger individuals (125 mm CW) assimilated similar amounts of carbon derived from benthic algae and S. alterniflora-derived detritus. Given the complex nature of saltmarsh food webs and the omnivorous diet of blue crabs, the observed size-based trends have important management and conservation applications.

Natural and created Spartina brackish marsh habitats in the Guadalupe Estuary, adjacent to the Aransas National Wildlife Refuge, Texas, USA were surveyed during spring, summer, and fall 2004 to evaluate the equivalence of nekton assemblages in an old (>30 years) created marsh. During each season, six replicate samples were collected in each marsh type using a 1-m 2 drop sampler. Multivariate analysis revealed significant differences in nekton assemblage structure among marsh type, both within and across seasons. Species richness was significantly higher in the natural marsh in spring and summer but not in fall. Several species that were dominant in the natural marsh but rare or absent in the created marsh had strong correlations with the presence of oyster substrate that was only encountered in natural marsh samples. Although cumulative richness was greater in the natural marsh, eight species were collected only from the created marsh. Shrimp and fish biomass was significantly higher in natural marsh. Analysis of the density, biomass and size structure of three commercially important crustaceans indicated that the created marsh supported similar biomass of some species (white shrimp, blue crab); however, the size structure of some populations was variable among marshes (blue crab, brown shrimp). We conclude that lower substrate complexity (specifically oyster) and soil organic content in the created marsh reduced measures of nekton similarity and recommend that these features be addressed in future restoration efforts.

Aim To examine the roles of local and regional environmental variables and biotic interactions in determining the structure of local stream fish assemblages, and to compare results derived from analyses based on taxonomic and functional groups. Location Texas, USA. Methods Species abundance data were compiled for 157 stream fish assemblages in several river basins across Texas. Species were condensed into functional groups based on trophic and life-history characteristics. Local and regional environmental variables were either measured at each location or determined from scale maps and public-access data bases. The original taxonomic and functional group data sets were analysed using similarity indices, null models of co-occurrence, and direct and indirect ordination techniques. Results derived from taxonomic and functional group data sets are compared. Results Inferences regarding the relative roles of local and larger-scale factors in determining stream fish assemblage structure differ dramatically between analyses of taxonomic and functional groups. Taxonomic analyses suggest a prominent role of regional-scale environmental factors, and local assemblages sorted according to a biogeographic pattern. Functional group analyses suggest almost equal roles of factors representative of local and larger scales, and assemblages were distinguished by a habitat template irrespective of geographic region. Main conclusions The structure of local stream fish assemblages is determined ultimately by factors representing multiple scales, with the relative importance of each depending on the biological unit employed (species or functional groups). We suggest that analyses using functional groups can more directly infer ecological responses to environmental variation, and therefore may provide a more fruitful avenue for developing and testing ecological theory of community organization across biogeographic scales.

– In this study we examine trends in body condition of three Cichla species native to the Cinaruco River, Venezuela. Specifically, we characterise the potential subsidy provided by migratory Semaprochilodus kneri to Cichla by comparing condition among species, hydrologic seasons and different size-classes (based on the ability to consume S. kneri assuming gape-limitation). All three species had lowest body condition during the rising-water season (following the reproduction period). Only Cichla temensis had greater body condition during the falling-water season than low-water season. During the falling-water season, C. temensis >300 mm SL had significantly greater condition than smaller conspecifics. Annual migration of S. kneri into the river during the falling-water season provides a resource subsidy to large C. temensis. Transfer of whitewater production into nutrient-poor rivers by migratory fishes appears to be a general phenomenon in the major Neotropical river basins and may partially account for high standing stocks of large piscivores in these unproductive ecosystems.

Taim Ecological Reserve is located within the Taim Hydrological System and was created to protect a heterogeneous and productive landscape harboring exceptional biological diversity in southern Brazil. Using stable isotope ratio analyses of carbon (delta13C) and nitrogen (delta15N), we provide a preliminary description of the food web structure, including estimates of production sources supporting fish populations and vertical trophic structure, within a representative lake of this system. A total of 21 organisms (5 macrophytes, 3 mollusks and 13 adult fishes) representing 16 species were collected for isotope analysis. Fishes had delta13C values ranging from -24.30º/oo to -28.31º/oo , showing concordance with the range of values observed for macrophytes (-25.49 to -27.10º/oo), and suggesting that these plants could be a major carbon source supporting these fishes. delta13C signatures of Corbicula (-30.81º/oo) and Pomacea (-24.26º/oo) indirectly suggest that phytoplankton and benthic algae could be alternative carbon sources for some consumers. Nitrogen isotope ratios indicated approximately three consumer trophic levels. The pearl cichlid Geophagus brasiliensis was a primary consumer. Two catfishes (Trachelyopterus lucenai and Loricariichthys anus) were secondary consumers. Two congeneric pike cichclids (Crenicichla lepidota and C. punctata), a catfish (Pimelodus maculatus) and the characids Astyanax fasciatus and Oligosarcus robustus were tertiary consumers. Further studies including additional primary producers and consumers and greater sample numbers should be conducted to provide a more complete and detailed description of food web structure and dynamics within the reserve. A Estação Ecológica do Taim está inserida dentro do Sistema Hidrológico do Taim e foi criada para proteger uma região heterogênea e produtiva no sul do Brasil, abrigando uma diversidade biológica excepcional. A partir da análise de isótopos estáveis do carbono (delta13C) e nitrogênio (delta15N), esse trabalho fornece uma descrição preliminar da estrutura da teia alimentar numa lagoa representativa desse sistema, incluindo estimativas dos produtores primários que sustentam suas populações de peixes e a estrutura trófica vertical. Um total de 21 organismos (5 macrófitas, 3 moluscos e 13 peixes adultos), representando 16 espécies, foram coletados para a análise da composição isotópica. Os peixes tiveram valores de delta13C variando entre -24.30º/oo e -28.31 º/oo , com uma amplitude de variação concordante com as macrófitas (-25.49 e -27.10º/oo), sugerindo indiretamente que essas plantas poderiam ser uma fonte de carbono importante para os peixes. As assinaturas delta13C de Corbicula (-30.81º/oo) e Pomacea (-24.26º/oo) sugerem que fitoplâncton e algas bentônicas podem ser outra fonte de carbono para alguns consumidores. As razões isotópicas do nitrogênio indicaram aproximadamente três níveis tróficos de consumidores. O cará Geophagus brasiliensis foi um consumidor primário, enquanto o penharol Trachelyopterus lucenai e o cascudo-viola Loricariichthys anus foram consumidores secundários. Duas espécies congenéricas de joaninha (Crenicichla lepidota e C. punctata), o pintado Pimelodus maculatus, o lambari Astyanax fasciatus e o dentudo Oligosarcus robustus foram consumidores terciários. Novas investigações, incluindo outros produtores primários e consumidores e um maior número de amostras, deveriam ser conduzidas para propiciar uma descrição mais completa e detalhada da estrutura e dinâmica da teia alimentar no Taim.

Biotic assemblages of aquatic floodplain systems have great potential to randomly reshuffle during annual flood periods, and have been described both as stochastically and deterministically assembled. However, only a limited number of studies have been conducted in relatively few habitat types. To evaluate large-bodied fish assemblage structure of floodplain creeks, we used experimental gill nets to sample fishes at sites spaced at even intervals within three creeks in consecutive dry seasons. A total of 60 species were collected, 41 of which were collected both years. The most frequently collected species were piscivores and algivores/detritivores. Multivariate analysis suggested non-random patterns of assemblage structure in both years. Correspondence analysis (CA) of the species abundance-by-site matrix for 2001 suggests species assemblages were most similar among sites within the same creek regardless of depth or longitudinal position. Discriminant function analysis (DFA) correctly predicted 100% of samples based on creek identity, and species ordination scores revealed creek-specific species subsets. In 2002, CA and DFA did not distinguish creeks based on species assemblages. Instead, we observed a significant positive relationship between assemblage composition and site depth and position along the creek longitudinal gradient. Assemblages were most similar among sites of comparable depth and longitudinal position, regardless of creek identity. Predators occurred almost exclusively at mouth and mid-reach sites. Flood duration prior to our 2002 sampling period was prolonged due to abnormally heavy rainfall in November and December 2001 (typically the falling-water period), and may account for the observed inter-annual variation in fish assemblage structure.