Downstream Gradient Research Articles

A multicompartment approach--diatoms, macrophytes, benthic macroinvertebrates and fish--to assess the impact of toxic industrial releases on a small French river.

The River Luzou flows through a sandy substrate in the South West of France. According to the results of two assessment surveys, the Water Agency appraised that this river may not achieve the good ecological status by 2015 as required by the Water Framework Directive (2000/60/EC). This ecosystem is impacted by industrial effluents (organic matter, metals and aromatic compounds). In order to assess and characterize the impact, this study aimed to combine a set of taxonomic and non-taxonomic metrics for diatoms, macrophytes, macroinvertebrates and fish along the up- to downstream gradient of the river. Diversity metrics, biological indices, biological and ecological traits were determined for the four biological quality elements (BQE). Various quantitative metrics (biomass estimates) were also calculated for diatom communities. The results were compared to physicochemical analysis. Biological measurements were more informative than physicochemical analysis, in the context of the study. Biological responses indicated both the contamination of water and its intensity. Diversity metrics and biological indices strongly decreased with pollution for all BQE but diatoms. Convergent trait selection with pollution was observed among BQE: reproduction, colonization strategies, or trophic regime were clearly modified at impaired sites. Taxon size and relation to the substrate diverged among biological compartments. Multiple anthropogenic pollution calls for alternate assessment methods of rivers' health. Our study exemplifies the fact that, in the case of complex contaminations, biological indicators can be more informative for environmental risk, than a wide screening of contaminants by chemical analysis alone. The combination of diverse biological compartments provided a refined diagnostic about the nature (general mode of action) and intensity of the contamination.

Self-organising maps and correlation analysis as a tool to explore patterns in excitation-emission matrix data sets and to discriminate dissolved organic matter fluorescence components.

Dissolved organic matter (DOM) is a complex mixture of organic compounds, ubiquitous in marine and freshwater systems. Fluorescence spectroscopy, by means of Excitation-Emission Matrices (EEM), has become an indispensable tool to study DOM sources, transport and fate in aquatic ecosystems. However the statistical treatment of large and heterogeneous EEM data sets still represents an important challenge for biogeochemists. Recently, Self-Organising Maps (SOM) has been proposed as a tool to explore patterns in large EEM data sets. SOM is a pattern recognition method which clusterizes and reduces the dimensionality of input EEMs without relying on any assumption about the data structure. In this paper, we show how SOM, coupled with a correlation analysis of the component planes, can be used both to explore patterns among samples, as well as to identify individual fluorescence components. We analysed a large and heterogeneous EEM data set, including samples from a river catchment collected under a range of hydrological conditions, along a 60-km downstream gradient, and under the influence of different degrees of anthropogenic impact. According to our results, chemical industry effluents appeared to have unique and distinctive spectral characteristics. On the other hand, river samples collected under flash flood conditions showed homogeneous EEM shapes. The correlation analysis of the component planes suggested the presence of four fluorescence components, consistent with DOM components previously described in the literature. A remarkable strength of this methodology was that outlier samples appeared naturally integrated in the analysis. We conclude that SOM coupled with a correlation analysis procedure is a promising tool for studying large and heterogeneous EEM data sets.

Cytological and biochemical biomarkers in adult female perch (Perca fluviatilis) in a chronically polluted gradient in the Stockholm recipient (Sweden)

By measuring a battery of cytological and biochemical biomarkers in adult female perch (Perca fluviatilis), the city of Stockholm (Sweden) was investigated as a point source of anthropogenic aquatic pollution. The investigation included both an upstream gradient, 46km westwards through Lake Mälaren, and a downstream gradient, 84km eastwards through the Stockholm archipelago. Indeed, there was a graded response for most of the biomarkers and for the muscle concentrations of ΣPBDE, four organotin compounds and PFOS in the perch. The results indicated severe pollution in central Stockholm, with poor health of the perch, characterised by increased frequency of micronucleated erythrocytes, altered liver apoptosis, increased liver catalase activity, decreased brain aromatase activity, and decreased liver lysosomal membrane stability. Some biomarker responses were lowest in the middle archipelago and increased again eastwards, indicating a second, partly overlapping, gradient of toxic effects from the Baltic Sea.

The response of fish and macroinvertebrate assemblages to multiple stressors: A comparative analysis of aquatic communities in a perturbed watershed (Onondaga Lake, NY)

Stream bioassessment studies usually use a single taxonomic group, most commonly fish or macroinvertebrates. However, differences in the life histories, habitat preferences, and physiological and behavioral traits of each assemblage may cause differences in their responses to environmental degradation. These factors can lead to discrepancies in overall assessments of ecological integrity. The Onondaga Lake Watershed (NY) has had a diverse history of pollution from multiple impacts, and the lake and several subsites are listed as USEPA Superfund sites. The response of fish and macroinvertebrate assemblages to environmental variables in the Onondaga Lake Watershed was assessed at 17 stream reaches in 2007, 2010, and 2011 using assemblage-specific metrics and indices of biotic integrity. Land use and substrate composition variables were significantly correlated (p<0.05, r>|0.3|) with the first two macroinvertebrate PCA axes and were related to a downstream gradient of increased habitat degradation for the major tributaries. Water chemistry variables, canopy cover, and stream width were significantly correlated (p<0.05, r>|0.3|) with the first two fish PCA axes. PCA axes for fish and macroinvertebrate assemblages were not significantly correlated, suggesting that the major environmental variables affecting fish and macroinvertebrate assemblages differed. Pearson correlations between the macroinvertebrate Biological Assessment Profile (BAP) and the fish Index of Biotic Integrity (IBI), and the macroinvertebrate Percent Model Affinity (PMA) and IBI were not significantly correlated. The macroinvertebrate metrics Percent Model Affinity (PMA) and BAP were positively and significantly correlated with Visual Habitat Assessment (VHA) scores, and the fish IBI was not significantly correlated with the VHA, further illustrating differences of fish and macroinvertebrates in their response to environmental variables. Results suggest that the use of multiple assemblages may be warranted for future assessments of stream quality and the development of effective monitoring programs in the Onondaga Lake Watershed.

Development of a Multimetric Index for Fish Assemblages in a Cold Tailwater in Tennessee

AbstractTailwaters downstream of hypolimnetic‐release hydropeaking dams exhibit a unique combination of stressors that affects the structure and function of resident fish assemblages. We developed a statistically and biologically defensible multimetric index of fish assemblages for the Caney Fork River below Center Hill Dam, Tennessee. Fish assemblages were sampled at five sites using boat‐mounted and backpack electrofishing gear from fall 2009 through summer 2011. A multivariate statistical approach was used to select metrics that best reflected the downstream gradients in abiotic variables. Five metrics derived from boat electrofishing samples and four metrics derived from backpack electrofishing samples were selected for incorporation into the index based on their high correlation with environmental data. The nine metrics demonstrated predictable patterns of increase or decrease with increasing distance downstream of the dam. The multimetric index generally exhibited a pattern of increasing scores with increasing distance from the dam, indicating a downstream recovery gradient in fish assemblage composition. The index can be used to monitor anticipated changes in the fish communities of the Caney Fork River when repairs to Center Hill Dam are completed later this decade, resulting in altered dam operations.

Health status of Largescale Sucker (Catostomus macrocheilus) collected along an organic contaminant gradient in the lower Columbia River, Oregon and Washington, USA

The health of Largescale Sucker (Catostomus macrocheilus) in the lower Columbia River (USA) was evaluated using morphometric and histopathological approaches, and its association with organic contaminants accumulated in liver was evaluated in males. Fish were sampled from three sites along a contaminant gradient. In 2009, body length and mass, condition factor, gonadosomatic index, and hematocrit were measured in males and females; liver and gonad tissue were collected from males for histological analyses; and organ composites were analyzed for contaminant content in males. In 2010, additional data were collected for males and females, including external fish condition assessment, histopathologies of spleen, kidney and gill and, for males, liver contaminant content. Multivariate analysis of variance indicated that biological traits in males, but not females, differed among sites in 2009 and 2010. Discriminant function analysis indicated that site-related differences among male populations were relatively small in 2009, but in 2010, when more variables were analyzed, males differed among sites in regards to kidney, spleen, and liver histopathologies and gill parasites. Kidney tubular hyperplasia, liver and spleen macrophage aggregations, and gill parasites were generally more severe in the downstream sites compared to the reference location. The contaminant content of male livers was also generally higher downstream, and the legacy pesticide hexachlorobenzene and flame retardants BDE-47 and BDE-154 were the primary drivers for site discrimination. However, bivariate correlations between biological variables and liver contaminants retained in the discriminant models failed to reveal associations between the two variable sets. In conclusion, whereas certain non-reproductive biological traits and liver contaminant contents of male Largescale Sucker differed according to an upstream–downstream gradient in the lower Columbia River, results from this study did not reveal the specific environmental factors responsible for the differences in health status among fish populations.

Nongame Fish Species Distribution and Habitat Associations in the Snake River Basin of Southern Idaho

The distribution and abundance of game fish populations are commonly monitored closely, whereas sampling of nongame species is often neglected. We used a broad-scale salmonid sampling project both to simultaneously assess the distribution and relative abundance of nongame fish species in small streams (i.e., ≤15 m wetted width) in the Snake River basin of southern Idaho and to relate the distribution and abundance of nongame species to abiotic and biotic stream conditions in the study area. Of the 1738 reaches surveyed, 34% were dry or contained too little water to support fish, and an additional 21% had flowing water but were still absent of fish. At least one species of nongame fish was captured at 30% of the reaches surveyed, and all 18 native nongame fish species believed present in study area streams were captured. The most widely distributed species was speckled dace Rhinichthys osculus (present in 21% of surveyed reaches, excluding dry and nearly dry reaches), followed by bridgelip sucker Catostomus columbianus (19%), Paiute sculpin Cottus beldingi (15%), and redside shiner Richardsonius balteatus (13%). The species least often present (≤ 1%) were leopard dace Rhinichthys falcatus and peamouth Mylocheilus caurinus. Common carp Cyprinus carpio (captured at 3 locations) and oriental weatherfish Misgurnus anguillicaudatus (2 locations) were the only nonnative nongame species encountered. Catostomids and cyprinids generally formed a composite fish assemblage that was associated with higher stream order (i.e., farther from headwaters), lower elevation, and lower-gradient reaches where streams were wider and deeper. Stream order was the strongest loading factor for canonical correlations constructed for study reaches both above and below Shoshone Falls, suggesting that the increased abundance of catostomids and cyprinids generally occurred on a longitudinal downstream gradient. Biologists sampling game fish populations in streams should also record data on nongame species to more closely monitor their status through time.

Do stream fish track climate change? Assessing distribution shifts in recent decades

Understanding the ability of species to shift their distribution ranges in response to climate change is crucial for conservation biologists and resources managers. Although freshwater ecosystems include some of the most imperilled fauna worldwide, such range shifts have been poorly documented in streams and rivers and have never been compared to the current velocity of climate change. Based on national monitoring data, we examined the distributional changes of 32 stream fish species in France and quantified potential time lags in species responses, providing a unique opportunity to analyze range shifts over recent decades of warming in freshwater environments. A multi‐facetted approach, based on several range measures along spatial gradients, allowed us to quantify range shifts of numerous species across the whole hydrographic network between an initial period (1980–1992) and a contemporary one (2003–2009), and to contrast them to the rates of isotherm shift in elevation and stream distance. Our results highlight systematic species shifts towards higher elevation and upstream, with mean shifts in range centre of 13.7 m decade−1 and 0.6 km decade−1, respectively. Fish species displayed dispersal‐driven expansions along the altitudinal gradient at their upper range limit (61.5 m decade−1), while substantial range contractions at the lower limit (6.3 km decade−1) were documented for most species along the upstream–downstream gradient. Despite being consistent with the geographic variation in climate change velocities, these patterns reveal that the majority of stream fish have not shifted at a pace sufficient to track changing climate, in particular at their range centre where range shifts lag far behind expectation. Our study provides evidence that stream fish are currently responding to recent climate warming at a greater rate than many terrestrial organisms, although not as much as needed to cope with future climate modifications.

Determinants of fish assemblage structure in Mount Itoupé mountain streams (French Guiana)

Fish assemblages inhabiting the mountain streams of the Guiana shield have scarcely been studied. Here we investigated fish assemblages in eight headwater mountain streams of the Mount Itoupe (French Guiana), and tested how local freshwater fish assemblages are shaped by environmental characteristics. We show that Mount Itoupe streams host uneven species assemblages, characterized by low species richness (less than ten species per site) and a high proportion of small Siluriform species. Differences in fish assemblage composition between the sites were mainly due to basin identity, but regional climate and position of the site in the river gradient also had a significant effect, although of lower magnitude. In contrast, local habitat hardly affected fish richness, abundances or assemblage composition. Species richness increased along the upstream–downstream gradient, whereas fish abundance depended mainly on the slope orientation of Mount Itoupe. Although these results need confirmation on a larger amount of sampling sites, located on other Guianese mountains, this preliminary study shows that the mountain streams of the Guiana Shield, although rarely investigated, host not only uneven species assemblages, but also present an original combination of environmental determinants shaping fish assemblage structure. Those rare and original ecosystems hence deserve more attention and should be preserved from human disturbances.

A multibiomarker approach on the Atlantic tomcod (Microgadus tomcod) in the St. Lawrence Estuary

A multibiomarker approach was developed on juvenile Atlantic tomcod (Microgadus tomcod) to evaluate the pertinence of this approach for low-cost screening assessment of the environmental quality of various coastal sites within estuaries. Several biometric indices and biomarkers (ethoxyresorufin-O-deethylase (EROD) activity, metallothionein concentration, and immune responses) were investigated on immature and maturing tomcods (≤ 31 months) collected in four environmentally contrasted sites in the St. Lawrence Estuary (SLE). Simultaneous examination of various age classes provides the opportunity to detect short-term responses in sensitive young-of-the-year fish (e.g., EROD induction) and longer-time effects associated with chronic exposure and bioaccumulation (e.g., metallothionein induction). Principal component analysis was helpful to discriminate between responses possibly related to contaminant exposure (EROD, metallothionein) and responses that could be affected by upstream-downstream gradient (immune response, biometric indices). Measurement of a battery of biomarkers in young tomcods at several sites along the shore of the SLE is a low-cost screening investigation useful to identify hot spots requiring further investigation with chemical analysis and additional reference sites.

Study on Social Risk Evaluation Index System for Tailings Pond Dam-Break Based on the Vulnerability Theory

In allusion to the status quo of deficient in evaluation theory and methods on the social risk of dam-break, this paper established 24 index systems for the evaluation of dam-break social risk composed by the dam-break hazard intensity, accident probability, hazard-affected body inherent vulnerability and disaster relief capacity. The quantitative calculation formula has also been put forward for evaluation indexes. In line with the Delphi method, a judgment matrix on the social risk caused by dam-break has been constructed, and has obtained the weight of each index by virtue of analytic hierarchy process. The results show the internal friction angle, the downstream main ditch longitudinal gradient, the current dam height and the monitoring &amp; forecasting capacity are the key indicators that affect the social risks of dam-break. The results of this study help to further enhance and improve the technical level of risk evaluation for the tailings pond.

Mollusc communities along upstream–downstream gradients in small coastal basins of the south-western Iberian Peninsula

Freshwater mollusc communities readily respond to various human-induced stressors, and thus are appropriate models for studying the effects of such stressors on the structure and dynamics of stream macroinvertebrate communities. This paper examined the distribution of freshwater molluscs in 35 stream reaches of 18 small coastal basins in the south-western Iberian Peninsula. Using several multivariate techniques, I showed that mollusc distribution mainly responded to gradients in drainage area, water availability, pollution and salinity. Upstream and downstream communities were clearly differentiated, with the former dominated by freshwater species (Bithynia tentaculata, Galba truncatula, Radix balthica, Ancylus fluviatilis and Planorbarius metidjensis) and the latter by species typical of brackish waters (Peringia ulvae and Myosotellamyosotis). There was a clear decrease in species richness from upper to lower reaches. The conservation of mollusc communities of these small basins requires a deeper understanding of their ecological requirements, effective control of urban discharges and an analysis of their interaction with invasive species.

AN INVADER WITHIN AN ALTERED LANDSCAPE: ONE CATFISH, TWO RIVERS AND AN INTER‐BASIN WATER TRANSFER SCHEME

ABSTRACTAfrican sharptooth catfish Clarias gariepinus has become established as a non‐native invasive species in Eastern Cape, South Africa, where it was translocated primarily through an inter‐basin water transfer scheme into the Great Fish and Sundays rivers. This study examined the patterns in catfish distribution and abundance, and compared trophic niches in relation to the ichthyofauna of the two rivers. Correspondence analysis revealed upstream to downstream gradients associated with the spatial distribution in species richness for most species within the mainstream and mainstream to tributary gradients that were associated mostly with the spatial distribution of native minnows in both rivers. Catfish was predicted to occur widely within the mainstream habitats and to decrease progressively from mainstreams to tributaries. Based on classification and regression trees, the physico‐chemical environment was found to be a good proxy for predicting the occurrence and abundance of catfish. Although non‐significant relationships were observed between catfish and other native fish species abundances, the study suggests potential impact due to predation and interference in habitats where the invader co‐occurs with other fishes. Comparisons of trophic niches indicated higher trophic diversity for the mainstream ichthyofauna than the tributary communities in both rivers, suggesting an upstream to downstream continuum in community structure and resource availability. Catfish within the invaded mainstream had comparable trophic niches and similar dispersion patterns among individuals for both rivers, but indicated differences in shapes of scatter. This suggests that the catfish exhibited a differential response, probably in relation to resource availability, that may be indicative of its dietary plasticity. The study suggests the proliferation of catfish and its probable impact within the mainstream flow‐altered habitats where invasion resistance was possibly reduced. Comparisons of trophic niches provided information on its probable impact at different scales and the potential risk of invasion of tributaries inhabited by native minnow species. Copyright © 2012 John Wiley &amp; Sons, Ltd.

ARE TWO SYSTEMIC FISH ASSEMBLAGE SAMPLING PROGRAMMES ON THE UPPER MISSISSIPPI RIVER TELLING US THE SAME THING?

ABSTRACTWe applied an Index of Biotic Integrity (IBI) used on Wisconsin/Minnesota waters of the upper Mississippi River (UMR) to compare data from two systemic sampling programmes. Ability to use data from multiple sampling programmes could extend spatial and temporal coverage of river assessment and monitoring efforts. We normalized for effort and tested fish community data collected by the Environmental Monitoring and Assessment Program‐Great Rivers Ecosystems (EMAP‐GRE) 2004–2006 and the Long Term Resource Monitoring Program (LTRMP) 1993–2006. Each programme used daytime electrofishing along main channel borders but with some methodological and design differences. EMAP‐GRE, designed for baseline and, eventually, compliance monitoring, used a probabilistic, continuous design. LTRMP, designed primarily for baseline and trend monitoring, used a stratified random design in five discrete study reaches. Analysis of similarity indicated no significant difference between EMAP‐GRE and LTRMP IBI scores (n = 238; Global R = −0.052; significance level = 0.972). Both datasets distinguished clear differences only between ‘Fair’ and ‘Poor’ condition categories, potentially supporting a ‘pass–fail’ assessment strategy. Thirteen years of LTRMP data demonstrated stable IBI scores through time in four of five reaches sampled. LTRMP and EMAP‐GRE IBI scores correlated along the UMR's upstream to downstream gradient (df [3, 25]; F = 1.61; p = 0.22). A decline in IBI scores from upstream to downstream was consistent with UMR fish community studies and a previous, empirically modelled human disturbance gradient. Comparability between EMAP‐GRE (best upstream to downstream coverage) and LTRMP data (best coverage over time and across the floodplain) supports a next step of developing and testing a systemic, multi‐metric fish index on the UMR that both approaches could inform. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Stable isotope analysis reveals food web structure and watershed impacts along the fluvial gradient of a Mesoamerican coastal river

AbstractEcosystem 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 δ15N 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 δ15N at sites draining impacted watersheds was enriched compared to values from forested reference sites. Assessment of primary consumer δ15N 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 &amp; Sons, Ltd.

Hydrologic and geomorphic considerations in restoration of river-floodplain connectivity in a highly altered river system, Lower Missouri River, USA

Planning for restoration of river-floodplain systems requires understanding how often and how much of a floodplain may be inundated, and how likely the floodplain is to retain the water once flooded. These factors depend fundamentally on hydrology and geomorphology of the channel and floodplain. We discuss application of an index of river-floodplain connectivity, the Land Capability Potential Index (LCPI), to regional-scale restoration planning along 600 km of the Lower Missouri River. The LCPI integrates modeled water-surface elevations, floodplain topography, and soils to index relative wetness of floodplain patches. Geomorphic adjustment of the Lower Missouri River to impoundment and channel engineering has altered the natural relations among hydrology, geomorphology, and floodplain soils, and has resulted in a regional upstream to downstream gradient in connectivity potential. As a result, flow-regime management is limited in its capacity to restore floodplain ecosystems. The LCPI provides a tool for identifying and mapping floodplain restoration potential, accounting for the geomorphic adjustment. Using simple criteria, we illustrate the utility of LCPI-like approaches in regional planning for restoration of plains cottonwood (Populus deltoides) communities, hydrologically connected floodplain wetlands, and seasonal floodplain wetlands.

Carbon Cycling in Floodplain Ecosystems: Out-Gassing and Photosynthesis Transmit Soil δ13C Gradient Through Stream Food Webs

Natural braided river floodplains typically possess high groundwater–surface water exchange, which is vital to the overall function and structure of these complex ecosystems. Spring-fed streams on the floodplain are also hotspots of benthic invertebrate diversity and productivity. The sources of carbon that drive these productive spring-fed systems are not well-known. We conducted field assessments and a manipulation, modeling, and a laboratory experiment to address this issue. Initially δ13C values of both dissolved inorganic carbon (DIC) and food-web components of five springs were used to assess the sources of carbon to spring food webs. Partial pressures of CO2 in upwelling water ranged from 2 to 7 times atmospheric pressure, but rapidly approached equilibrium with the atmosphere downstream commensurate with 13C enrichment of DIC. Speciation modeling and a laboratory out-gassing experiment suggested that downstream changes in pH could be explained solely by CO2 out-gassing. However, field results indicated that both out-gassing and photosynthetic drawdown by aquatic plants controlled the net flux of CO2. A whole stream manipulation indicated out-gassing was the primary effect at the spring source, which was confirmed by invariant diel pH. At 1296 m downstream from the spring source a large diel shift in pH indicated a plant effect on CO2 concentration which would contribute to the overall downstream gradient in δ13C DIC. Within the first 1296 m the gradient in δ13 DIC was transmitted through three trophic levels of the spring food web. These findings indicate dependency on groundwater inorganic carbon by spring stream food webs and strong hydrologically mediated linkages connecting terrestrial, subsurface, and aquatic components of the floodplain.

Comparing latitudinal and upstream–downstream gradients: life history traits of invasive mosquitofish

AbstractAim Variation of life history traits along spatial gradients is poorly understood in invasive species and particularly in freshwater fish. We aimed to examine life history variation in a highly invasive fish (Gambusia holbrooki) along latitudinal and upstream–downstream river gradients and to assess the effects of age on this variation. We hypothesized similar responses in populations inhabiting environments more favourable to this species (lower latitudes and lower reaches of rivers).Location European rivers from southern Spain to southern France.Methods We sampled mosquitofish from the lowest reaches of ten river basins along 6° of latitude in the Mediterranean region and seven sites along the upstream–downstream gradient in three of the rivers. We examined abundance, population structure, size‐at‐age and other life history traits along these gradients.Results As hypothesized, lower reaches and lower latitudes both resulted in higher reproductive effort and lower body condition of mosquitofish. However, these patterns explained low per cent variation, were nonlinear and strongly depended on fish age. Independently of fish size, age groups differed in reproductive effort, in the gonadal weight–size relationship and its variation along spatial gradients. Mean size‐at‐age (or overall body size) did not vary with latitude (so the intra‐specific version of Bergmann’s rule or its converse does not apply) and in contrast increased upstream in rivers.Main conclusions Our findings suggest that for life history traits of freshwater organisms, river longitudinal variation plays a role as important as climate, with often differential effects. Our results also illustrate the poor knowledge of spatial variation of many life history traits, which precludes the understanding and prediction of biological invasions in a rapidly changing world.

The Pleistocene terrace staircase of the River Thame, central-southern England, and its significance for regional stratigraphic correlation, drainage development, and vertical crustal motions

The Thame is one of the principal left-bank affluents of the Thames, the largest river in southern England; it joins the Upper Thames at Dorchester, ∼20 km downstream of Oxford. Its terraces include a younger group of four, which date from the late Middle Pleistocene and Late Pleistocene, are disposed subparallel to the modern river, and represent drainage within the modern catchment. At higher levels there are three older terraces, the Three Pigeons, Tiddington and Chilworth terraces, which are assigned to MIS 16, 14 and 12. With much gentler downstream gradients, these are fragmentary remnants of much more substantial fluvial deposits, indicating a much larger river that was disrupted by the Anglian (MIS 12) glaciation. This interpretation supersedes an earlier view that the glacigenic deposits in the Thame headwaters correlate with the Blackditch terrace, the highest of the younger group, which has hitherto provided an argument that the glaciation in this region occurred in MIS 10. It is suggested that the headwaters of the pre-Anglian ‘Greater Thame’ river were located near Northampton and that the Milton Sands of that area represent an upstream counterpart of the Chilworth terrace deposits. It is envisaged that this early Middle Pleistocene drainage geometry, located between the Jurassic limestone and Chalk escarpments, developed as a result of the increase in uplift rates that followed the Mid-Pleistocene Revolution (MPR). It is suggested that before this time, including during the Early Pleistocene, the modern Thame catchment and adjacent regions drained southeastward through the Chalk escarpment, but these small rivers lacked the erosional power to cut through the Chalk in pace with the faster uplift occurring in the early Middle Pleistocene, and so became diverted to the southwest, subparallel to the Chalk escarpment, to form the pre-Anglian ‘Greater Thame’ tributary of the Upper Thames. The post-MPR uplift is estimated to decrease northwestward from 90 m in the Middle Thames to 75 m near the Thame–Thames confluence and to 65 m upstream of Oxford. The post-Anglian (post-450 ka) component of uplift decreases northward from 33 m near the Thame–Thames confluence to an estimated ∼20 m in the Northampton area; the relative stability of the latter area makes feasible the proposed correlation between the Milton Sands and the pre-Anglian River Thame. Limited post-Anglian uplift in the Northampton area is also inferred from the upstream convergence of the terraces of the modern rivers Nene and Great Ouse. These observed lateral variations in vertical crustal motions reflect lateral variations in crustal properties (including heat flow, crustal thickness, and thickness of underplating at the base of the crust) that are known independently. This study thus provides, for the first time, an integrated explanation of the Pleistocene drainage development across a large region of central-southern England.

Intrinsic controls on the range of volumes, morphologies, and dimensions of submarine lobes

Submarine lobe dimensions from six different systems are compared: 1) the exhumed Permian Fan 3 lobe complex of the Tanqua Karoo, South Africa; 2) the modern Amazon fan channel–mouth lobe complex, offshore Brazil; 3) a portion of the modern distal Zaïre fan, offshore Angola/Congo; 4) a Pleistocene fan of the Kutai basin, subsurface offshore Indonesia; 5) the modern Golo system, offshore east Corsica, France; and 6) a shallow subsurface lobe complex , offshore Nigeria. These six systems have significantly different source-to-sink configurations (shelf dimension and slope topography), sediment supply characteristics (available grain size range and supply rate), tectonic settings, (palaeo) latitude, and delivery systems. Despite these differences, lobe deposits share similar geometric and dimensional characteristics. Lobes are grouped into two distinct populations of geometries that can be related to basin floor topography. The first population corresponds to areally extensive but thin lobes (average width 14 km × length 35 km × thickness 12 m) that were deposited onto low relief basin floor areas. Examples of such systems include the Tanqua Karoo, the Amazon, and the Zaïre systems. The second population corresponds to areally smaller but thicker lobes (average width 5 km × length 8 km × thickness 30 m) that were deposited into settings with higher amplitude of relief, like in the Corsican trough, the Kutai basin, and offshore Nigeria. The two populations of lobe types, however, share similar volumes (a narrow range around 1 or 2 km 3), which suggests that there is a control to the total volume of sediment that individual lobes can reach before they shift to a new locus of deposition. This indicates that the extrinsic processes control the number of lobes deposited per unit time rather than their dimensions. Two alternative hypotheses are presented to explain the similarities in lobe volumes calculated from the six very different systems. The first states that the wide range of starting flow volume and grain size across all systems enters the basin floor as a narrow range due to slope ‘filtering’ via more overspill and intra-channel deposition in larger systems. The second hypothesis is a result of the gradual decrease in downstream gradient from the distributive channel base to the lobe top during lobe growth. This is not sustainable as the channel will start to aggrade, and when a steeper lateral gradient is present, an avulsion will occur to an adjacent depositional low, which will be used for flows to fill and build a new lobe. This analysis of submarine lobe volumes indicates that the basin floor topography influences lobe geometry, but the fact that lobe volumes have a narrow range indicates a strong influence of intrinsic processes.