Low River Discharge Research Articles

Variability of tidal signals in the Brent Delta Front: New observations on the Rannoch Formation, northern North Sea

Detailed observations on the Rannoch Formation in several deep Viking Graben wells indicate that the ‘classical’ wave-dominated Brent delta-front shows coupled storm–tide processes. The tidal signals are of three types: I): alternations of thick cross-laminated sandstone and thin mud-draped sandstone, whereby double mud drapes are prominent but discretely distributed, II): a few tidal bundles within bottomsets and foresets of up to 10cm-thick sets cross-strata, and III): dm-thick heterolithic lamination showing multiple, well-organized sand–mud couplets.During progradation of the Brent Delta, the Rannoch shoreline system passed upward from 1) a succession dominated by clean-water, storm-event sets and cosets frequently and preferentially interbedded with type I tidal beds, and occasional types II and III tidal deposits, toward 2) very clean storm-event beds less frequently separated by types II and III tidal beds, and then into 3) a thin interval showing muddier storm-event beds mainly alternating with type II tidal beds.It is likely that those variations in preservation bias of storm and tidal beds in each facies succession result from combined effects of 1) the frequency and duration of storms; 2) river discharge; and 3) the absolute and relative strength of tides. Tidal deposits are interpreted as inter-storm, fair-weather deposits, occurred preferentially in longer intermittent fair-weather condition and periods of lower river discharge, and well-pronounced in the distal-reach of delta-front. The formation and preservation of tidal signals between storm beds, indicate that the studied Rannoch Formation was most likely a storm-dominated, tide-influenced delta front 1) near the mouth of a large Brent river, where a significant tidal prism and high tidal range might be expected, and 2) in a setting where there were relatively high sedimentation rates associated with high local subsidence rates, so that the storm waves did not completely rework the inter-storm deposits. The documentation of the unconventional Rannoch Formation contributes to our understanding of mixed-energy coastal systems.

Dynamics of upstream movements of the European eel Anguilla anguilla in an inland area of the River Meuse over the last 20 years

The dynamics of upstream movements of the yellow eel Anguilla anguilla were investigated at Lixhe on the Belgian River Meuse in an inland fish pass regularly monitored from 1992 to 2014. Based on a constant year-to-year sampling effort, we examined the abundance of ascending yellow eels and their body size, seasonal movement, and the associated water temperature and flow. Over the last 23 years, the number of ascending yellow eels has declined at an average 4.2 % per year since 1992. The abundance of eels in 2014 is estimated at 4.5 % of the ascending stock in 1992. We observed that some annual variations in eel abundance at Lixhe might be related to opening fish passes downstream of the study site. The results clearly demonstrated that long-term declining abundance of eels has resulted in increased sizes (mean increase, 4.1 mm per year since 1992) and temperatures triggering the upstream movement process (1.03 °C per decade), with earlier dates for the last eel passages reducing the difference between temperature extremes of eel passages through the fish pass during the migration season. Eel movements occurred in spring and summer at low river discharge and were mainly triggered by high-temperature events. Eels have become larger with time because of improved feeding opportunities and more growth habitats available resulting from the long-term reduction in recruitment. This study highlights the importance of investigating long time spans for a better comprehension of the changes observed in yellow eels and for the optimization of management measures and future research.

Temporal Patterns of Larval Fish Occurrence in a Large Subtropical River.

Knowledge of temporal patterns of larval fish occurrence is limited in south China, despite its ecological importance. This research examines the annual and seasonal patterns of fish larval presence in the large subtropical Pearl River. Data is based on samples collected every two days, from 2006 to 2013. In total, 45 taxa representing 13 families and eight orders were sampled. The dominant larval family was Cyprinidae, accounting for 27 taxa. Squaliobarbus curriculus was the most abundant species, followed by Megalobrama terminalis, Xenocypris davidi, Cirrhinus molitorella, Hemiculter leuscisculus and Squalidus argentatus. Fish larvae abundances varied significantly throughout the seasons (multivariate analyses: Cluster, SIMPROF and ANOSIM). The greatest numbers occurred between May and September, peaking from June through August, which corresponds to the reproductive season. In this study, redundancy analysis was used to describe the relationship between fish larval abundance and associated environmental factors. Mean water temperature, river discharge, atmospheric pressure, maximum temperature and precipitation play important roles in larval occurrence patterns. According to seasonal variations, fish larvae occurrence is mainly affected by water temperature. It was also noted that the occurrence of Salanx reevesii and Cyprinus carpio larvae is associated with higher dissolved oxygen (DO) concentrations, higher atmospheric pressure and lower water temperatures which occur in the spring. On the other hand, M. terminalis, X. davidi, and C. molitorella are associated with high precipitation, high river discharge, low atmospheric pressure and low DO concentrations which featured during the summer months. S. curriculus also peaks in the summer and is associated with peak water temperatures and minimum NH3–N concentrations. Rhinogobius giurinus occur when higher atmospheric pressure, lower precipitation and lower river discharges occur in the autumn. Dominant fish species stagger their spawning period to avoid intraspecific competition for food resources during early life stages; a coexistence strategy to some extent. This research outlines the environmental requirements for successful spawning for different fish species. Understanding processes such as those outlined in this research paper is the basis of conservation of fish community diversity which is a critical resource to a successful sustainable fishery in the Pearl River.

Lotic bacterioplankton and phytoplankton community changes under dissolved organic-carbon amendment: evidence for competition for nutrients

During periods of low river discharge, bacterial growth is typically limited by dissolved organic carbon (DOC) and is tightly regulated by phytoplankton production. However, import of allochthonous DOC into rivers by freshwater inflows may diminish bacterial reliance on phytoplankton-produced carbon, leading to competition for nitrogen (N) and phosphorus (P). To investigate phytoplankton–bacterial competition in response to allochthonous inputs, we conducted a mesocosm experiment, comparing microbial responses to the following two manipulation treatments: (1) addition of N and P, and (2) addition of a DOC and N and P. Measurement of chlorophyll-a estimated phytoplankton biomass and microscopic counts were performed to discriminate community change. Bacterial abundance was tracked using flow cytometry and community assemblages were characterised using automated ribosomal intergenic spacer analyses and 16S rRNA-amplicon sequencing. We found that bacterial abundance increased in the leachate addition, whereas chlorophyll-a was reduced and the bacterial community shifted to one dominated by heterotrophic genera, and autotrophic microbes including Synechococcus and Cyclotella increased significantly in the nutrient treatment. These observations indicated that DOC and nutrient inputs can lead to shifts in the competitive dynamics between bacteria and phytoplankton, reducing phytoplankton biomass, which may potentially shift the major pathway of carbon to higher trophic organisms, from the phytoplankton grazer chain to the microbial food web.

Differentiating between rain, snow, and glacier contributions to river discharge in the western Himalaya using remote-sensing data and distributed hydrological modeling

Rivers draining the southern Himalaya provide most of the water supply for the densely populated Indo-Gangetic plains. Despite the importance of water resources in light of climate change, the relative contributions of rainfall, snow and glacier melt to discharge are not well understood, due to the scarcity of ground-based data in this complex terrain. Here, we quantify discharge sources in the Sutlej Valley, western Himalaya, from 2000 to 2012 with a distributed hydrological model that is based on daily, ground-calibrated remote-sensing observation. Based on the consistently good model performance, we analyzed the spatiotemporal distribution of hydrologic components and quantified their contribution to river discharge. Our results indicate that the Sutlej River's annual discharge at the mountain front is sourced to 55% by effective rainfall (rainfall reduced by evapotranspiration), 35% by snow melt and 10% by glacier melt. In the high-elevation orogenic interior glacial runoff contributes ∼30% to annual river discharge. These glacier melt contributions are especially important during years with substantially reduced rainfall and snowmelt runoff, as during 2004, to compensate for low river discharge and ensure sustained water supply and hydropower generation. In 2004, discharge of the Sutlej River totaled only half the maximum annual discharge; with 17.3% being sourced by glacier melt. Our findings underscore the importance of calibrating remote-sensing data with ground-based data to constrain hydrological models with reasonable accuracy. For instance, we found that TRMM (Tropical Rainfall Measuring Mission) product 3B42 V7 systematically overestimates rainfall in arid regions of our study area by a factor of up to 5. By quantifying the spatiotemporal distribution of water resources we provide an important assessment of the potential impact of global warming on river discharge in the western Himalaya. Given the near-global coverage of the utilized remote-sensing datasets this hydrological modeling approach can be readily transferred to other data-sparse regions.

Late‐summer peak in sediment accumulation in two lakes with contrasting watersheds, alaska

The timing of clastic sedimentation in two glacial‐fed lakes with contrasting watersheds was monitored using sequencing sediment traps for two consecutive years at Allison Lake (Chugach Range, Alaska) and four months at Shainin Lake (Brooks Range, Alaska). Shainin Lake is a weakly stratified lake fed by distant glaciers, whereas Allison Lake is more strongly stratified and fed predominantly by proximal glaciers. At Shainin Lake, sediment accumulation started in late June and reached its maximum in mid‐August, just before lake mixing and during a period of low river discharge. The grain size of the sediment reaching the sediment trap in Shainin Lake was homogenous throughout the summer. At Allison Lake, pulsed sedimentation of coarse particles during late summer and early fall storms were superimposed on the fine‐grained sedimentation pattern similar to that observed at Shainin Lake. These storms triggered underflows that were observed in the thermal structure of the lake and deposited abundant sediment. The sequencing sediment traps reveal a lag between fluvial discharge and sediment deposition at both lakes, implying limitations to interpreting intra‐annual sedimentary features in terms of inflow discharge.

The impact of commercially treated oil and gas produced water discharges on bromide concentrations and modeled brominated trihalomethane disinfection byproducts at two downstream municipal drinking water plants in the upper Allegheny River, Pennsylvania, USA

In 2010, a dramatic increase in the levels of total trihalomethane (THM) and the relative proportion of brominated species was observed in finished water at several Pennsylvania water utilities (PDW) using the Allegheny River as their raw water supply. An increase in bromide (Br−) concentrations in the Allegheny River was implicated to be the cause of the elevated water disinfection byproducts. This study focused on quantifying the contribution of Br− from a commercial wastewater treatment facility (CWTF) that solely treats wastes from oil and gas producers and discharges into the upper reaches of the Allegheny River, and impacts on two downstream PDWs. In 2012, automated daily integrated samples were collected on the Allegheny River at six sites during three seasonal two-week sampling campaigns to characterize Br− concentrations and river dispersion characteristics during periods of high and low river discharges. The CWTF discharges resulted in significant increases in Br− compared to upstream baseline values in PDW raw drinking water intakes during periods of low river discharge. During high river discharge, the assimilative dilution capacity of the river resulted in lower absolute halide concentrations, but significant elevations Br− concentrations were still observed at the nearest downstream PDW intake over baseline river levels. On days with active CWTF effluent discharge the magnitude of bromide impact increased by 39ppb (53%) and 7ppb (22%) for low and high river discharge campaigns, respectively. Despite a declining trend in Allegheny River Br− (2009–2014), significant impacts from CWTF and coal-fired power plant discharges to Br− concentrations during the low river discharge regime at downstream PDW intakes was observed, resulting in small modeled increases in total THM (3%), and estimated positive shifts (41–47%) to more toxic brominated THM analogs. The lack of available coincident measurements of THM, precursors, and physical parameters limited the interpretation of historical trends.

The influence of extreme river discharge conditions on the quality of suspended particulate matter in Rivers Meuse and Rhine (The Netherlands)

As a consequence of climate change, increased precipitation in winter and longer periods of decreased precipitation in summer are expected to cause more frequent episodes of very high or very low river discharge in the Netherlands. To study the impact of such extreme river discharge conditions on water quality, toxicity profiles and pollutant profiles were determined of suspended particulate matter (SPM) collected from Rivers Meuse and Rhine. Archived (1993–2003) and fresh (2009–2011) SPM samples were selected from the Dutch annual monitoring program of the national water bodies (MWTL), representing episodes with river discharge conditions ranging from very low to regular to very high. SPM extracts were tested in a battery of in vitro bioassays for their potency to interact with the androgen receptor (AR), the estrogen receptor (ER), the arylhydrocarbon receptor (AhR), and the thyroid hormone transporter protein transthyretin (TTR). SPM extracts were further tested for their mutagenic potency (Ames assay) and their potency to inhibit bacterial respiration (Vibrio fischeri bioluminescence assay). Target-analyzed pollutant concentrations of the SPM samples and additional sample information were retrieved from a public database of MWTL results. In vitro toxicity profiles and pollutant profiles were analyzed in relation to discharge conditions and in relation to each other using correlation analysis and multivariate statistics. Compared to regular discharge conditions, composition of SPM during very high River Meuse and Rhine discharges shifted to more coarse, sandy, organic carbon (OC) poor particles. On the contrary, very low discharge led to a shift to more fine, OC rich material, probably dominated by algae. This shift was most evident in River Meuse, which is characterized by almost stagnant water conditions during episodes of drought. During such episodes, SPM extracts from River Meuse demonstrated increased potencies to inhibit bacterial respiration and to compete with thyroid hormone to bind to TTR, possibly due to the presence of fycotoxins. Meanwhile concentrations of polychlorobiphenyls (PCBs) in SPM were also increased. Very high River Meuse discharges on the other hand corresponded to increased androgenic and AhR agoniztic responses, which coincided with increased PAH levels and PAH-related in vivo risk estimates (i.e. multi-substance potentially affected fraction of species; msPAF). In River Rhine, very high discharges also corresponded to increasing androgenic potencies in SPM. Concentrations and corresponding msPAF values of PAHs (and metals), however, decreased with very high discharges in River Rhine in contrast to River Meuse. Mutagenicity was observed for SPM extracts from River Rhine collected during all discharge conditions, except during regular discharge. Aggregated toxicity index values, which were useful to identify toxicity profiles deviating from the generally observed pattern, did not correlate with river discharges, probably due to opposite effects of discharge conditions on different bioassay responses. In conclusion, SPM quality and related in vivo risk estimates changed during very low or very high discharge conditions but the changes were specific for the different toxic endpoints and pollutants in the different rivers. Moreover, bioassay responses to a series of consecutively collected samples from River Rhine during the Christmas flood of 1993 indicated that SPM quality is variable within a single episode of extreme discharge.

Upstream dispersal of an invasive crayfish aided by a fish passage facility

Fish passage facilities for reservoir dams have been used to restore habitat connectivity within riverine networks by allowing upstream passage for native species. These facilities may also support the spread of invasive species, an unintended consequence and potential downside of upstream passage structures. We documented dam passage of the invasive virile crayfish, Orconectes virilis (Hagen, 1870), at fish ladders designed for upstream passage of American eels, Anguilla rostrata (Lesueur, 1817), in the Shenandoah River drainage, USA. Ladder use and upstream passage of 11 virile crayfish occurred from 2007–2014 during periods of low river discharge (<30 m 3 s –1 ) and within a wide range of water temperatures from 9.0–28.6 °C. Virile crayfish that used the eel ladders were large adults with a mean carapace length and width of 48.0 mm and 24.1 mm, respectively. Our data demonstrated the use of species-specific fish ladders by a non-target non-native species, which has conservation and management implications for the spread of aquatic invasive species and upstream passage facilities. Specifically, managers should consider implementing long-term monitoring of fish passage facilities with emphasis on detection of invasive species, as well as methods to reduce or eliminate passage of invasive species.

Factors Contributing to Hypoxia in the Minjiang River Estuary, Southeast China.

Dissolved oxygen (DO) is not only a fundamental parameter of coastal water quality, but also an indication of organics decomposed in water and their degree of eutrophication. There has been a concern about the deterioration of dissolved oxygen conditions in the Minjiang River Estuary, the longest river in Fujian Province, Southeast China. In this study, the syntheses effects on DO was analyzed by using a four year time series of DO concentration and ancillary parameters (river discharge, water level, and temperature) from the Fuzhou Research Academy of Environmental Sciences, at three automated stations along the Minjiang River Estuary. Hypoxia occurred exclusively in the fluvial sections of the estuary during the high temperature and low river discharge period and was remarkably more serious in the river reach near the large urban area of Fuzhou. Enhancement of respiration by temperature and discharge of domestic sewage and industrial wastewater, versus regeneration of waters and dilution of pollutant concentration with increased river discharge, which regarded as the dominant antagonist processes that controlled the appearance of seasonal hypoxia. During the high temperature and the drought period, minimal mainstream flow above 700 m3·s−1, reduction of pollutants and forbidding sediment dredging in the South Channel should be guaranteed for strong supports on water quality management and drinking water source protection.

Impact of Sediment on Agrichemical Fate and Bioavailability to Adult Female Fathead Minnows: A Field Study.

Precipitation induced runoff is an important pathway for agrichemicals to enter surface water systems and expose aquatic organisms to endocrine-disrupting compounds such as pesticides and steroid hormones. The objectives of this study were to investigate the distribution of agrichemicals between dissolved and sediment-bound phases during spring pulses of agrichemicals and to evaluate the role of suspended sediment in agrichemical bioavailability to aquatic organisms. To accomplish these objectives, suspended sediment and water samples were collected every 3 days from a field site along the Elkhorn River, located at the downstream end of a heavily agricultural watershed, and were screened for 21 pesticides and 21 steroids. Adult female fathead minnows (Pimephales promelas) were exposed in field mesocosms to river water containing varying sediment loads. Changes in organism hepatic gene expression of two estrogen-responsive genes, vitellogenin (VTG) and estrogen receptor alpha (ERα), as well as the androgen receptor (AR) were analyzed during periods of both low and high river discharge. Trends in agrichemical concentrations of both the dissolved and sediment phases as a function of time show that, while sediment may act as both a source and a sink for agrichemicals following precipitation events, the overall driver for molecular defeminization in this system is direct exposure to the sediment-associated compounds. This study suggests that endocrine disrupting effects observed in organisms in turbid water could be attributed to direct exposure of contaminated sediment.

Distribution of hypoxia and pycnocline off the Changjiang Estuary, China

Abstract The distributions of hypoxia and the pycnocline off the Changjiang Estuary were investigated by making several field observations from June 2 to 11, from July 18 to 23, from August 20 to 30, from October 3 to 13, 2006, and from August 27 to September 3, 2009. The observations from July 18 to 23, 2006, mainly focused on analyzing the relationship between hypoxia and the extension of the river plume and vertical stratification. In July, the Changjiang diluted water (CDW) was influenced by the easterly typhoon winds, causing it to extend northward rather than northeastward. By using the maximum vertical density gradient as a stratification intensity index, we found that the area of low ( 2.0 kg/m4), which indicated that the summer pycnocline can effectively block vertical DO exchange and maintain hypoxia near the bottom. The observed hypoxic area was 500 km2, which was much smaller than the hypoxic areas observed in previous studies, and occurred because of the enhanced mixing that resulted from Typhoon Bill. During the observation period of August 20–30, 2006, the maximum density gradient was weaker due to distinct low river discharge. No hypoxia was observed in the eastern and southeastern sea off the Changjiang Estuary where hypoxia often occurs. However, hypoxia occurred over a large area of 15,400 km2 in the northern observation domain where hypoxia rarely occurs. During June 2–11 and October 3–13, 2006, the maximum density gradient was weaker, and the area with low DO was smaller than in July 2006. This finding resulted from relatively low river discharge and weaker solar heating. Consequently, no hypoxia occurred in the bottom layer. The area of low DO was similar to that of the maximum vertical density gradient. From August 27 to September 3, 2009, high river discharge and strong solar heating produced a larger and more intense pycnocline. The hypoxic area reached 3735 km2 and was very similar to the area of the pycnocline, which was greater than 3.0 kg/m4. The seasonal variations of the pycnocline were consistent with those of hypoxia, and the pycnocline played an important role in preserving hypoxic conditions. The seasonal influences of biogeochemical process on hypoxia in 2006 were discussed. The residual current speeds at the bottom were small and favorable for maintaining hypoxia during the summer.

Droughts and dead zones on the rise

Environmental Science![Figure][1] Lake Erie as seen from space PHOTO: NASA/JEFF SCHMALTZ, LANCE/EOSDIS MODIS RAPID RESPONSE Nutrient-rich agricultural runoff or wastewater discharge can lead to the formation of harmful algal blooms or oxygen-depleted (hypoxic) “dead zones” in surface waters. But now, Zhou et al. show that low tributary river discharge was the largest contributor to a record-breaking hypoxic event in Lake Erie in 2012. Drought conditions across much of North America that year decreased water flow into the lake, exacerbating the effects of persistent nutrient runoff. Because drought and other extreme climate events are expected to increase with climate change, management strategies need to consider all factors that may degrade future water quality. Environ. Sci. Technol. 10.1021/es503981n (2014). [1]: pending:yes

Winter storm-induced hydrodynamics and morphological response of a shallow transgressive shoal complex: Northern Gulf of Mexico

Using extended deployments during seasons of low and high discharge from the Atchafalaya River, meteorological, hydrodynamic and bottom boundary layer parameters were monitored from Tiger and Trinity Shoal complex, off Louisiana coast, USA. During winter storms, the surface current speed measured at both shoals exceeded 0.5 m/s and the entire water column followed the prevailing wind direction. The current speed close to the bottom exceeded 0.3 m/s during high energy northerly winds. The mean water level in the shoal complex increased during southerly winds and decreased during northerly winds, such that the difference between wind set-up and set-down exceeded 0.7 m in Tiger Shoal and 0.6 m in Trinity Shoal during high energy frontal passages. The swell height was inversely correlated with mean water level, and increased during pre-frontal phase and decreased during post-frontal phase of winter storms. The sea (short waves) height responded quickly to wind direction and speed; and within a few hours after the wind shifted and blowing from the north, the sea height increased during both deployments. Bimodal wave frequency spectrum was observed during wind veering from southerly to northerly, when both sea and swell intensities were significant. The Tiger Shoal bed sediment texture transformed drastically, from mud to shell and shell hash assemblage, within a period of two weeks during the December 2008 deployment. Backscatter signal intensity from a Pulse Coherent Acoustic Doppler Profiler (PCADP) and its velocity estimates were used to determine the vertical extend and timing of mud resuspension and their eventual flushing out from the shoal environment, when exposed to high energy winter storm passages. The computed time frame for a total transformation of bottom sediment texture (from muddy bottom to shell and shell hash assemblage) was supported by the combined wave and bottom current induced shear stress at shoal bed. The bed samples collected from Tiger Shoal before and after the deployment in spring 2009 consisted of more than 80% shell and shell hash, which again confirmed a stable bottom as predicted from the PCADP data. However, the fine sand and mud dominated bed at Trinity Shoal was highly dynamic and experienced a few cm of ephemeral sediment deposition during the passage of each cold front, as revealed from the analysis of acoustic backscatter data from the PCADP. Suspended sediment concentration estimated from Optical Backscatterance Sensors (OBS) and PCADP were in good agreement during low river discharge events in December 2008; but significantly diverged during the spring 2009 deployment, when a high suspended sediment load was discharged into the shelf from the Atchafalaya River, and subsequently pushed farther offshore into the deployment sites by wind-induced strong currents during the passage of cold fronts.

Diagnóstico da qualidade de água ao longo do estuário do rio Itajaí-Açu, SC.

The present study consisted in the evaluation of spatial and temporal quality of the water in the Itajai-Acu river estuary, from 2003 to 2011. We evaluated physicochemical parameters, dissolved inorganic nutrients, POC, BOD, Cla-a, SS and fecal coliforms. The estuary was divided in three segments (high, medium and low). From the spatial analysis, high values of SS(82.7±114 mg/l), POC (203.8 ± 230 µM), BOD (2.5 ± 0.9 mg/l) were associated with regions with high population and agricultural area., these parameters being found at higher concentrations in the high estuary. The high levels of NH4 +(31,7±11,2 µM) suggest an input of organic material in the lower estuary. In temporal analysis a strong influence of the river discharge (315± 343 m³/s) on the system was found, strongly related to SS and POC. By comparing these values with CONAMA Resolution 357/05, our data suggest that the main non-conformities were observed in the lower estuary during periods of low river discharge

Unanticipated effect of climate change on an aquatic top predator of the Atlantic rainforest

Abstract From a biodiversity conservation viewpoint it is crucial to estimate the sensitivity of species and populations to climate change, especially of key species such as top predators. Here, the El Niño‐Southern Oscillation phenomenon (ENSO) was used as a natural experiment to estimate the sensitivity of a population of the rainforest‐dwelling broad‐snouted caiman, Caiman latirostris, to extreme floods. Caiman abundance declined by 35% annually between 1996 and 1998, and then increased, without reaching 1996 levels, indicating a marked effect of the 1997 El Niño. Accordingly, the Southern Oscillation Index was positively correlated with caiman abundance, indicating lower caiman abundance with warm ENSO events. The relationship between the relative annual mean abundance of caimans and the maximum daily annual discharge of the Iguazú River was analysed. This relationship was parabolic, with caiman abundance increasing at discharges from 500 to 1500 m3 s‐1 and then decreasing at discharges from 1500 to 2500 m3 s‐1, indicating an adverse effect of both extreme low and high river discharge. No effect of illegal hunting was measurable. This study evaluated whether the negative effect of extremely high water levels on caiman abundance could be due to a decrease in the availability of the habitat more commonly used by small (&lt;60 cm total length, TL) and medium (60–120 cm TL) caimans. Small and medium‐sized caimans used herbaceous/shrub habitats more frequently than large caimans (&gt;120 cm TL), i.e. the type of habitat flooded during extreme floods. An increase in extreme floods, as forecast for the Atlantic rainforest owing to climate change, may seriously affect the population of rainforest caimans through the reduction of adequate habitat for juveniles. This counter‐intuitive result, in which an excess of water reduces the abundance of an aquatic top predator, should be considered in conservation plans of rainforest‐dwelling crocodilians. Copyright © 2014 John Wiley &amp; Sons, Ltd.

The effect of an artificial reef system on the transient fish assemblages – south-eastern coast of Brazil

Artificial reefs (ARs) are often used to improve fishing and, consequently, the economy of a region. However, the way in which the species use the reefs may vary between fish assemblages. An assessment was made of the influence of an AR complex on the transient fish population off the northern coast of Rio de Janeiro state and, therefore, two control areas were sampled. Gillnets were used to capture individual fish in six sampling surveys. Cumulative abundance and biomass curves (ABC) were used to assess the possible effects of the reefs on the community's functional structure. In the dry season, during which the influence of the Paraíba do Sul River is smaller, a larger richness of r-strategy species and juveniles of K-strategy species was observed in the reef area compared with the control areas, suggesting that the AR acts as a protective environment for these species. During the lower river discharge period the results indicated a potential disturbance in the functional structure of the AR fish community and, therefore, a less stable environment relative to the control areas. This ‘instability’ warrants a positive connotation, as it indicates that the artificial reefs are harbouring species that are particularly sensitive to predation, making the reef a powerful tool for maintaining these populations on the northern coast of Rio de Janeiro.

Carbon and nitrogen isotopic composition of suspended particulate organic matter in Zuari Estuary, west coast of India

The spatio-temporal variations of the δ13C and δ15N of suspended particulate organic matter (SPOM), along with ancillary chemical and hydrographic parameters, have been examined monthly for one year (October 2010 to September 2011) at a total of seven stations to refine the understanding of SPOM sources and biogeochemical pathways in the tropical Zuari Estuary, west coast of India. The dynamic nature of C and N cycling in estuaries is reflected in the isotopic variation of SPOM as a function of space and time. The results exhibit marked seasonality, with the autochthonous component and the terrestrial component being the major SPOM sources during periods of low and high river discharges, respectively. Spatially, enrichments of δ13C and δ15N from the head (δ13C=−29.6‰, δ15N=1.78‰, C/N=7.09) to the mouth (δ13C=−20.9‰, δ15N=5.77‰, C/N=6.12) of the estuary indicate downstream reduction of terrigenous influence. The mean C:N ratios varied between 4 and 9 over the study period with low values during the monsoon, in spite of large terrestrial input, which can be attributed to microbial modification of terrestrial SPOM. A significant positive correlation of δ13C with salinity suggests the reliability of δ13C as a biogeochemical tracer for organic matter provenance. Carbon:chlorophyll plots reveal suppressed autochthonous production during the southwest monsoon which may be attributed to the rapid flushing time, light-limitation due to cloud cover and high water-column turbidity.

Habitat fragmentation and species extirpation in freshwater ecosystems; causes of range decline of the Indus river dolphin (Platanista gangetica minor).

Habitat fragmentation of freshwater ecosystems is increasing rapidly, however the understanding of extinction debt and species decline in riverine habitat fragments lags behind that in other ecosystems. The mighty rivers that drain the Himalaya - the Ganges, Brahmaputra, Indus, Mekong and Yangtze - are amongst the world’s most biodiverse freshwater ecosystems. Many hundreds of dams have been constructed, are under construction, or are planned on these rivers and large hydrological changes and losses of biodiversity have occurred and are expected to continue. This study examines the causes of range decline of the Indus dolphin, which inhabits one of the world’s most modified rivers, to demonstrate how we may expect other vertebrate populations to respond as planned dams and water developments come into operation. The historical range of the Indus dolphin has been fragmented into 17 river sections by diversion dams; dolphin sighting and interview surveys show that river dolphins have been extirpated from ten river sections, they persist in 6, and are of unknown status in one section. Seven potential factors influencing the temporal and spatial pattern of decline were considered in three regression model sets. Low dry-season river discharge, due to water abstraction at irrigation barrages, was the principal factor that explained the dolphin’s range decline, influencing 1) the spatial pattern of persistence, 2) the temporal pattern of subpopulation extirpation, and 3) the speed of extirpation after habitat fragmentation. Dolphins were more likely to persist in the core of the former range because water diversions are concentrated near the range periphery. Habitat fragmentation and degradation of the habitat were inextricably intertwined and in combination caused the catastrophic decline of the Indus dolphin.

River plume dispersion in response to flash flood events. Application to the Catalan shelf

The dispersal of freshwater inflow from a flood event is studied using a three dimensional flow model (ROMS). The model domain includes a small part of the Catalan shelf where the combination of local land topography with torrential rainfall can cause high local runoff during a short period but with a large impact on the receiving coastal waters. Both steady, low river discharge, typical of normal (low-discharge) conditions and a high discharge representative of post-rain conditions are considered. Simulated salinity profiles on the shelf near the river mouth are compared with records from CTD measurements with quite good correspondence. A strong correlation between local wind and plume response was observed. Local winds affect the trajectory of the freshwater plume that enters the Catalan shelf waters. During post-rain conditions, northerly and westerly winds exported the plume further away from the coast, whereas southerly and easterly winds confine the plume closer to the coast. During low discharge conditions, the plume remained closer to the coast due to the weak wind stresses. Results show that freshwater spread, shape and dilution are mainly controlled by local wind forcing at relatively short time scales.