Algal carbon has been increasingly recognized as the primary carbon source supporting large-river food webs; however, many of the studies that support this contention have focused on lotic main channels during low-flow periods. The flow variability and habitat-heterogeneity characteristic of these systems has the potential to significantly influence food web structure and must be integrated into models of large-river webs. We used stable-isotope analysis and IsoSource software to model terrestrial and algal sources of organic carbon supporting consumer taxa in the main channel and oxbow lakes of the Brazos River, Texas, USA, during a period of frequent hydrologic connectivity between these habitat types. Standardized sampling was conducted monthly to collect production sources and consumer species used in isotopic analysis. Predictability of hydrologic connections between habitat types was based on the previous 30 years of flow data. IsoSource mixing models identified terrestrial C3 macrophytes (riparian origin) as the primary carbon source supporting virtually all consumers in the main channel and most consumers in oxbow lakes. Small-bodied consumers (<100 mm) in oxbow lakes assimilated large fractions of algal carbon whereas this pattern was not apparent in the main channel. Estimates of detritivore trophic positions based on delta15N values indicated that terrestrial material was likely assimilated via invertebrates rather than directly from detritus. High flows in the river channel influenced algal standing stock, and differences in the importance of terrestrial and algal production sources among consumers in channel vs. oxbow habitats were associated with patterns of flooding. The importance of terrestrial material contradicts the findings of recent studies of large-river food webs that have emphasized the importance of algal carbon and indicates that there can be significant spatial, temporal, and taxonomic variation in carbon sources supporting consumers in large rivers.

No abstract

Fish populations in the Brazos River, Texas, were surveyed monthly for 2 years to determine the relative influence of hydrology and habitat characteristics on the recruitment dynamics of seven species representing three divergent life history strategies. Surveys were conducted in two oxbow lakes with different flood recurrence intervals and the main river channel. The first year was relatively dry with few oxbow-river connections, whereas year 2 was relatively wet and connections between the main channel and floodplain habitats were common. Oxbow lakes supported greater juvenile abundances of most species relative to the main channel and were particularly important for nest building species with parental care. The river channel supported small species with extended reproductive periods and large, long-lived species that are able to store reproductive potential during sub-optimal periods. Hydrologic isolation was associated with greater rotifer densities in oxbows, and species with the greatest fecundity produced strong year classes during this period. Hydrologic connectivity did not increase juvenile production for most species, suggesting that recruitment dynamics in the Brazos River are similar to predictions of the low flow recruitment hypothesis (LFR). These results suggest that both hydrology and habitat heterogeneity interact with fish life history strategy to determine optimal conditions for recruitment and all three factors must be considered in restoration strategies for floodplain rivers.

Three oxbow lakes with different connection frequencies and an adjacent reach of the middle Brazos River, Texas, were surveyed quarterly from summer 1993 to summer 1996 to examine the effects of hydrology and physicochemical attributes on fish assemblage structure. During flood events, oxbows usually were colonized by about 20 fish species from the river channel, and several fluvial specialists were rarely or never sampled from oxbows. Multivariate analyses of seine samples revealed divergent patterns of assemblage structure during periods of isolation that also were associated with a gradient of maximum water depth, temperature, and conductivity. In contrast, analysis of gill-net samples that targeted large fishes revealed low between-site and temporal variation in assemblage structure among oxbow lakes but significant differences in the structure of oxbow versus channel assemblages. The shallowest oxbow (which dried out with greatest frequency) had the most variable fish assemblage across seasons and was dominated by small colonizing species, whereas deeper oxbows and the river channel tended to have higher species richness and more stable assemblages. Despite large differences in connection frequency and distance from the river channel, the deepest oxbows had the most similar fish assemblage structure across all seasons. Multiple regression analysis suggested that the timing and frequency of flood events (yielding colonization) in the most frequently connected oxbow interact with predator abundance (yielding mortality) to influence the abundance of common lotic-adapted minnows (red shiner Cyprinella lutrensis and bullhead minnow Pimephales vigilax) that periodically dominated the species assemblage of that oxbow after floods. Our results suggest that current hydrologic and geomorphologic dynamics in the middle Brazos River produce oxbow lakes with a range of physical characteristics yielding different disturbance and colonization regimes that strongly influence fish species assemblages.

Food web structure and major sources of primary production consumed by metafauna of Mad Island Marsh, a coastal saltmarsh on the NW coast of the Gulf of Mexico, were compared using stable isotopes and dietary analysis. Carbon and nitrogen isotope data were entered into a mixing model containing 5 potential production sources. Results were inconclusive due to overlapping isotopic signatures of certain sources, but nonetheless indicated that most fishes and macroinvertebrates assimilated material derived mostly from variable mixtures of macrophytes and filamentous algae. Highest estimates of percentage of material assimilated directly or indirectly from C 4 marsh grasses (ranging from 30 to 82%) were for spot Leiostomus xanthurus and Gulf killifish Fundulus grandis. Isotopic analysis could not reveal the detailed structure of predator-prey interactions at the species level; greater detail of trophic pathways was revealed by the dietary analysis. Estimates of vertical web structure (species trophic levels) by the 2 methods were largely concordant. The exceptions were 2 zooplanktivorous and detritivorous fish species and grass shrimp Palaemonetes pugio that had higher trophic levels according to nitrogen isotope ratios. For these taxa, the isotopic method more accurately indexed the number of trophic transfers than the dietary method, which depends on accurate dietary estimation for all food chain components leading to a consumer, and which assumes equal assimilation efficiencies for items found in stomach contents. The isotopic method underestimated trophic levels of several invertebrates, possibly due to inaccurate estimation of mean δ 15 N for production sources supporting these taxa and/or differential trophic fractionation. Together, stable isotope and dietary analyses provide a more accurate assessment of food web structure and dynamics of coastal marsh ecosystems than either method alone.

Reproductive activity of seven species representing three divergent life history strategies was monitored monthly for 2 years in channel and floodplain habitats of the Brazos River, Texas, USA, to evaluate associations between reproductive activity and biotic and abiotic factors predicted by conceptual models to influence reproduction. An information-theoretic approach was used to select best approximating models for each species, and model-averaged estimates of regression coefficients were calculated. Model selection indicated that monthly flow based on the 30-year hydrograph and temperature was strongly supported as factors associated with reproductive activity of all three life history strategies. The timing of reproduction in relation to the long-term hydrograph was related to life history traits. Reproductive activity of species with large adult size and high fecundity was greatest in spring just prior to increasing flows, whereas species with small adult size and extended breeding seasons exhibited greater activity in late spring and summer when mean flow was greatest. Nest-building species with parental care were more abundant in off-channel habitats where floods were less common. Instream flow management would benefit from consideration of flow and habitat requirements needed to support the diverse life history strategies displayed by fishes in river-floodplain systems.Résumé : Nous avons suivi à chaque mois pendant 2 ans l'activité reproductive de sept espèces représentant trois stratégies de cycle biologique distinctes dans les habitats du chenal et de la plaine d'inondation de la Brazos, Texas, É.U., afin d'évaluer les associations entre l'activité reproductive et les facteurs biotiques et abiotiques qui, d'après certains modèles conceptuels, l'influencent. Nous utilisons une approche basée sur la théorie de l'information pour sélec-tionner les modèles qui s'ajustent le mieux à chaque espèce et nous calculons à l'aide des modèles les estimations moyennes des coefficients de régression. La sélection des modèles indique que le débit mensuel basé sur la courbe de 30 ans et la température sont les facteurs le plus fortement retenus pour expliquer l'activité reproductive dans les trois stratégies de cycle biologique. La détermination du moment de la reproduction en fonction de la courbe de débit à long terme dépend des caractéristiques du cycle biologique. Les espèces avec adultes de grande taille et à forte fécondité ont une activité reproductive maximale au printemps juste avant l'accroissement des débits, alors que les espèces avec des adultes de petite taille et une saison de fraye étendue montrent plus d'activité reproductive à la fin du printemps et en été quand le débit moyen est maximal. Les espèces qui construisent des nids et qui prennent soin de leurs petits sont plus abondantes dans les habitats hors du chenal où les crues sont moins fréquentes. La gestion des débits dans les cours d'eau pourrait être améliorée si on tenait compte des exigences de débit et d'habitat requises pour le maintien des...

–  The middle Brazos River, located in east‐central Texas, is a meandering lowland river with many oxbow lakes on its flood plain. Flood dynamics of the Brazos River are aseasonal, and faunal exchange during lateral connections of the main river channel and oxbows is pulse‐like and occurs only during floods that may be months or years apart. Patterns of resource use among sympatric gar species (Lepisosteus oculatus, Lepisosteus osseus and Atractosteus spatula) associated with river–flood plain connectivity were studied for a period of 2 years (May 2003 to May 2005). The first year was relatively dry yielding few lateral connections, whereas the second year was relatively wet resulting in more frequent lateral connections. This study focused on habitat and diet partitioning among the three gar species in oxbow habitats with different connection frequencies and an active river channel site. Overall, 684 gars were collected with experimental gillnets: 19 A. spatula (alligator gar), 374 L. oculatus (spotted gar) and 291 L. osseus (longnose gar). There was strong partitioning of habitat between spotted and longnose gars, in which 98% of spotted gars were captured in oxbow habitats and 84% of longnose gars were captured in the river channel. Hydrology did not appear to affect habitat partitioning, although longnose gar abundance significantly increased in oxbows during the wet year. Diet overlap was high between spotted and longnose gars. Temporal variation in diet was significantly influenced by flood pulses that connected oxbows with the river channel, and which allowed predators and their prey to move between habitats.

Fourteen years (1996–2009) of juvenile Chinook salmon, Oncorhynchus tschawytscha (Walbaum), migration data on the regulated Stanislaus River, California, USA were used to evaluate how survival, migration strategy and fish size respond to flow regime, temperature and spawner density. An information theoretic approach was used to select the best approximating models for each of four demographic metrics. Greater cumulative discharge and variance in discharge during the migration period resulted in higher survival indices and a larger proportion of juveniles migrating as pre‐smolts. The size of pre‐smolt migrants was positively associated with spawner density, whereas smolt migrant size was negatively associated with temperature and positively associated with discharge. Monte Carlo techniques indicated high certainty in relationships between flow and survival, but relationships with juvenile size were less certain and additional research is needed to elucidate causal relationships. Flow is an integral part of the habitat template many aquatic species are adapted to, and mismatches between flow and life history traits can reduce the success of migration and the diversity of migratory life history strategies. The analyses presented here can be used to assist in the development of flow schedules to support the persistence of salmon in the Stanislaus River and provide implications for populations in other regulated rivers with limited and variable water supply.