River Continuum Research Articles

Lead levels and abundance of microplastics in surface water and sediment along a rural–urban river gradient

Microplastics are commonly found in aquatic ecosystems and can pose environmental threats to aquatic organisms. While the threats of microplastic ubiquity are recognized, few studies have concomitantly quantified microplastic abundance and heavy metals along a rural–urban river continuum. In the current study, we studied changes in microplastics and heavy metals (using lead as a proxy) by collecting sediment and water samples along a rural–urban river over two seasons (temperate spring and summer) and across five sites in a North American River. Our results revealed that microplastics decreased in a downstream direction in surface water but did not change predictably in sediment samples collected along the river continuum. Regarding the relationship between microplastic abundance and lead concentrations, we found a positive relationship between microplastics in sediment samples and lead concentrations. Contrariwise, we found no discernible correlation between microplastics in surface water and lead concentrations along the river continuum. Given the presence of microplastics at every site and moderate lead pollution documented in the Wolf River, our results provide baseline data that can aid in the concurrent assessment of microplastics and heavy metals in river systems. These findings can inform environmental managers in planning pollution management strategies for waterways flowing through rural–urban areas.

Isotopic seasonality of fluvial-derived greenhouse gases implies active layer deepening

Abstract Climate change in the northern circumpolar regions is rapidly thawing organic-rich permafrost soils, releasing substantial dissolved CO2 and CH4 into fluvial systems. This mobilization impacts local ecosystems and global climate feedback loops, playing a crucial role in the Arctic carbon cycle. Here, we analyze the stable carbon (δ13C) and radiocarbon (F14C) isotopic composition of dissolved CO2 and CH4 in the Sagavanirktok and Kuparuk River watersheds on the North Slope, Alaska. By examining spatial and seasonal variations in these isotopic signatures, we identify patterns of carbon release and transport along the river continuum. We find consistent CO2 isotopic signatures across the fluvial continuum, reflecting a mixture of geogenic and biogenic sources integrated throughout the watershed. Bayesian mixing models further demonstrate a systematic depletion in 13C and 14C signatures of dissolved CO2 sources from spring to fall, indicating increasing contributions of aged carbon as the active layer deepens seasonally. This seasonal deepening allows percolating groundwater to access deeper, older soil horizons, transporting CO2 produced by aerobic and anaerobic soil respiration to streams and rivers. In contrast, we observe no clear relationships between the 13C and 14C compositions of dissolved CH4 and landscape properties. Given the reduced solubility of CH4, which facilitates outgassing and limits its transport in aquatic systems, the isotopic signatures are likely indicative of localized contributions from streambeds, adjacent water saturated soils, and lake outflows. Our study demonstrates that dissolved greenhouse gases are sensitive indicators of the release of old carbon from thawing permafrost and serve as early warning signals for permafrost carbon feedback. It establishes a crucial baseline for understanding their role in regional carbon cycling and Arctic environmental change.

River-estuary continuum highlighted by variabilities in carbon and nitrogen stable isotope ratios of the catadromous eel Anguilla japonica.

Estuaries exhibit high physicochemical variability and the properties of estuaries and the constituent segments are not yet systematically understood. This study aims to reveal the spatial heterogeneity of predominant organic sources using carbon and nitrogen stable isotope ratios (δ13C and δ15N, respectively) of Japanese eels (Anguilla japonica), one of the ideal natural samplers. In the Miyakoda River, Japan, our study site, the effectiveness of eel δ13C and δ15N values as standard indices of predominant organic sources was tested by employing the River Continuum Concept. This study then extended the application of these indices to the estuary, i.e., the Hamana Lake system, into which the Miyakoda River flows. Once in the upper estuary, eel δ13C and δ15N values became high, with the latter peaking in this river-estuary continuum, indicating that artificial labile subsidies (nutrients and organic matter) with high δ15N values were rapidly assimilated into the food web. Eel δ15N values decreased again in the middle estuary. Nevertheless, the influence of terrestrial organic subsidies extended into this segment, as evidenced by the low δ13C values of eels. These results suggest that refractory organic matter with low δ15N values, such as plant-derived ones, is slowly assimilated into the food web in the downstream estuarine segments. The higher δ13C values in the lower estuary suggested that the contribution of eelgrass or macroalgae occurred in addition to benthic microalgae. Thus, our results emphasize the need to consider the multiple energy flows to understand the estuary as a continuum.

Assessment of Atmospheric Correction Algorithms for Sentinel-3 OLCI in the Amazon River Continuum

Water colour remote sensing is a valuable tool for assessing bio-optical and biogeochemical parameters across the vast extent of the Amazon River Continuum (ARC). However, accurate retrieval depends on selecting the best atmospheric correction (AC). Four AC processors (Acolite, Polymer, C2RCC, OC-SMART) were evaluated against in situ remote sensing reflectance (Rrs) measurements. K-means classification identified four optical water types (OWTs) that are affected by the ARC. Two OWTs showed seasonal differences in the Lower Amazon River, influenced by the increase in suspended sediment concentration with river discharge. The other OWTs in the Amazon River Plume are dominated by phytoplankton or by a mixture of optically significant constituents. The Quality Water Index Polynomial method used to assess the quality of in situ and orbital Rrs had a high failure rate when the Apparent Visible Wavelength was >580 nm for in situ Rrs. OC-SMART Rrs products showed better spectral quality compared to Rrs derived from other AC processors evaluated in this study. These results improve our understanding of remotely sensing very turbid waters, such as those in the Amazon River Continuum.

Internal transformation and damming regulate the longitudinal variation of DOM bioavailability in a large river

Understanding the spatial patterns of dissolved organic matter (DOM) and factors that influence them is crucial for maintaining river ecosystem functions and riverine health, considering the significant role of DOM in water quality and aquatic ecosystems. Nevertheless, there is limited knowledge regarding the spatial variation of DOM bioavailability and the factors driving them in large river systems. This study involved 39 sampling locations along the main stem of the Changjiang River, spanning its entire length (>5000 km) during a dry season. Spatial patterns of DOM were assessed by measurements of DOC concentrations and eight fluorescence DOM indices, namely fluorescence index (FI-A and FI-B), Trytophan/Tyrosine, Humic A, Humic C, humification indices (HIX-A and HIX-B), and Freshness index (β/α). The results revealed that the water DOM in the main stem of the Changjiang River primarily originated from terrestrial sources. A decline in DOM bioavailability was observed from the upper to the lower basin, aligning with the carbon processing prediction rather than the river continuum concept (RCC). The pure effect of physicochemical factors (25.30%) was greater than that of geographic factors (9.40%). The internal transformation processes determined the significant longitudinal decreases of DOM bioavailability. While no significant difference in DOM bioavailability was observed between reaches before and after the dams, the construction of dams was found to improve DOM bioavailability at the subsection scale and reduce the spatial autocorrelation of DOM bioavailability across the entire basin.

Assessing the impacts of rice terraces and ponds on the sediment and phosphorus loads in a typical hilly watershed of three gorge reservoir, China

Phosphorus is the primary eutrophication cause of river, lake, reservoir and estuary and it may be one of the principal pollutants for the agricultural watershed with middle and small cities. The sediment and phosphorus loads, related migration paths and contribution of source regions to receiving waters at different sections of a river basin are not clear. The modified Soil and Water Assessment Tool (SWAT) model was utilized to investigate the phosphorus loads and the relationships among water resources, sediment, and phosphorus loads in one of the important tributaries of the Three Gorge Reservoir (TGR) of China, and to investigate the retention dynamics of phosphorus in the river continuum and the influencing factors. It showed that under the scenario of rice terraces and ponds, the annual sediment loss and total phosphorus load were 125t and 2.5t, being reduced by 40%–80% compared to the natural slope scenario; the daily peak value of suspended sediment load was reduced by 30%–75%, and the relationship between the rainfall and runoff was weakened slightly; the retention rate of phosphorus sources reached 40%–78%. The results confirm that the rice terraces and ponds have altered local hydrological conditions, enhanced a substantial water, sediment, and phosphorus retention capacity that is highly consistent over time and space scales, thereby altering the predominant forms of phosphorus and improving the environmental resilience in soil and nutrition loss for the hilly watershed of the TGR.

Demystifying the particulate black carbon conundrum in aquatic systems

Particulate black carbon (PBC) constitutes a notable fraction of riverine particulate carbon (C), and is considered to be a potent sink of C due to its refractory nature in the environment. However, its potential to regulate global climate has been challenged with recent arguments of transformation of BC in the environment. In this study, we explore the transport and transformation of PBC along the continuum of six river basins in western India. Using the CTO-375 method and measuring the concentration and stable isotopic composition of the soot fraction of the BC spectrum, we examined the reasons for its variation in the river continuum. Our investigation suggested that changes in concentration and isotopic composition were due to allochthonous inputs and degradation, implying that BC may not be as recalcitrant as previously thought. We also estimated riverine export fluxes of PBC to the Arabian Sea, which was a minor fraction of the current global flux estimates.

Longitudinal changes on ecological diversity of Neotropical fish along a 1700 km river gradient show declines induced by dams

In naturally flowing waters, communities are expected to increase their taxonomic and functional diversity as well as increase the complexity of food web architecture along the longitudinal gradient from headwaters to mouth. However, these theories do not necessarily apply to dammed rivers. We analysed fish communities at 12 locations along the 1700 km stretch of the transnational Uruguay River. We determined species richness, beta-diversity and its components (turnover and nestedness), trophic diversity, and isotopic niches at community level, with particular focus on areas upstream and downstream of the four existing dams. We estimated trophic diversity metrics based on diet analysis, and isotopic niche breadth based on Bayesian isotopic niche models. We detected consistent longitudinal changes, supporting predictions from the River Continuum Concept. However, taxonomic richness, trophic diversity, and isotopic niche decreased abruptly immediately downstream of dams, and the component contributing the most to beta diversity was turnover, with the highest values occurring mainly after the cascade of dams in the upper Uruguay River, fitting expectations from the Serial Discontinuity Concept. The dam-free stretches of the Uruguay River allowed the recovery of ecological diversity as fish richness and trophic and isotopic niche increased. The negative impacts of dams should urgently be considered in the Neotropics, where many large dams are projected.

Greenhouse gas emissions (CO2–CH4–N2O) along a large reservoir‐downstream river continuum: The role of seasonal hypoxia

AbstractRecent studies suggest that hypolimnetic respiration may be responsible for greenhouse gas (GHG) emissions from deep reservoirs. Currently, quantitative evaluation of aerobic vs. anaerobic processes and priming (enhanced processing of organic matter due to the addition of labile carbon) in regulating GHG production and emissions across the reservoir‐downstream continuum remains largely unknown. High‐resolution, annual time‐series observations in a large, subtropical reservoir (Shuikou) experiencing seasonal hypoxia in southeast China indicate that aerobic hypolimnetic CO2 production dominated in most periods of the stratified spring/summer with higher rates at higher temperatures. In addition, anaerobic production of hypolimnetic CO2 occurred in the late stratified spring/summer period, which stimulated hypolimnetic production of CH4 and N2O. Incubation experiments showed that priming in spring enhanced both aerobic and anaerobic production of excess GHGs. A late spring flood event generated the highest daily efflux of CO2 through the flushing of GHG‐enriched hypolimnion waters. Turbine degassing contributed 59%, 93%, and 63% of annual CO2, CH4, and N2O effluxes, respectively. Moreover, annual downstream GHG emissions were similar to those in the transition/lacustrine zone of the Shuikou reservoir. Diurnal variation observations revealed net CO2 emissions even during algal bloom seasons. The reservoir‐downstream river continuum was a year‐round source of GHGs (218.5 ± 18.9 Gg CO2‐equivalent yr−1; CO2 contributed 91%). However, the loss of oxygen also leads to increased production and storage of recalcitrant dissolved organic carbon (RDOC). Thus, identifying mechanisms controlling both GHG emissions and RDOC production is crucial to constrain the carbon neutrality issue of hydroelectric reservoirs in the context of climate change mitigation strategies.

Spatial factors overcome seasonality regulating the consumption of allochthonous food resources by fishes from tropical lotic ecosystems

Abstract In lotic ecosystems, a higher availability of allochthonous food resources is expected during floods and rainfall events, which may yield a higher consumption of these resources by consumers. However, allochthonous input and seasonality in environmental conditions are locally dependent. Thus, dietary responses of freshwater consumers to seasonality should also be locally dependent. Herein, we applied a meta‐analytical approach to identify the spatial and seasonal environmental variables associated with the relative consumption of allochthonous food resources by fishes from tropical lotic ecosystems in different seasons. We gathered 566 observations of the diet of fish populations (i.e., restricted to the same sampling event) from 38 studies conducted in 70 localities across the tropics during flood and drought periods (hereafter, high‐water and low‐water periods, respectively). We retrieved latitude, seasonal channel width, elevation and two proxies for allochthonous input for each locality: terrestrial biomass (without considering seasonality) and the normalised difference vegetation index (NDVI; considering seasonality). Then, we applied meta‐regression models using these environmental variables as predictors and the proportion of allochthonous food resources in each consumer's diet (%Allo) as the response variable. Our models revealed that the diet of fishes from tropical lotic ecosystems is mostly autochthonous‐based, independent of the hydrological period (high‐water or low‐water), and aquatic invertebrates were the most consumed food items. Terrestrial vegetation biomass and elevation predicted increasing allochthony in the diet of tropical fishes for high‐water, but only terrestrial biomass predicted allochthony at low‐water periods. We found no relationship between latitude, channel width or NDVI with allochthony. Our study highlights the importance of considering the hierarchical level of biological organisation and fish trophic guilds for understanding seasonal patterns of relative consumption of allochthonous and autochthonous food resources by fishes. We also argue that variations in seasonality across the tropics are likely to have undermined its influence on allochthony in fish diets. The absence of a non‐relationship between channel width and allochthony provides evidence that the River Continuum Concept is better suited to temperate lotic ecosystems. Our results provide novel insights and the big‐picture of seasonal pattern in the diets of tropical fishes for use in future local empirical studies, improving forecasting of the relationship between the factors underpinning the availability of allochthonous and autochthonous food resources, and the diet of freshwater fishes.

Urban and agricultural land use led to niche differentiation of AOA, AOB and comammox along the Beiyun River continuum

River ecological health has been severely threatened by anthropogenic land-use pressures. Here, by combining remote sensing and molecular biology methods, we evaluated the impact of land-use activities on nitrification, a fundamental ecological process in rivers, which is conducted by ammonia-oxidising archaea (AOA) and ammonia-oxidising bacteria (AOB), or the newly discovered complete ammonia oxidisers (comammox). We explored the relationships of the abundance, activity, and diversity of AOA, AOB, and comammox in river sediments with land-use pressure by proposing a quantitative land use pattern index (LPI) over a 184 km continuum along the Beiyun River in North China. We found that comammox dominated nitrification in the forestry upstream (67.07 % in summer, 56.40 % in winter), while AOB became the major player in the urban middle (56.51 % in summer, 53.08 % in winter) and agricultural downstream reaches (62.98 % in summer, 50.74 % in winter). In addition, urban and agricultural land use lowered the α diversity of AOA and comammox, as well as simplified their co-occurrence networks, but promoted AOB diversity and complicated their networks. The structural equation model illustrated that the key drivers affecting the key taxa and activities were ammonia, and C/N for AOB, and total organic matter, and pH for comammox. We thus conclude that watershed urban and agricultural land use drive the niche differentiation of AOA, AOB, and comammox, specifically leading to a robust AOB community but weakened AOA and comammox communities. Our study connects the macro and micro worlds and provides a new paradigm for studying the variation in microbial communities as well as the potential ecological consequences under the increased anthropogenic land-use pressures in the Anthropocene.

Persistent disconnect between flow restoration and restoration of river ecosystem functions after the removal of a large dam on the Sélune River

The removal of the two dams on the Sélune River since 2019 has led to profound changes in the aquatic ecosystem. Lentic habitats bordered by forest had shifted to new conditions (running water and sparsely vegetated riverbanks) therefore shaping organisms’ assemblies. We studied how the reestablishment of aquatic lotic habitats in interaction with riparian vegetation could mediate the restoration of important ecological functions in the new river. Six stations located along the river continuum were surveyed for 3 years after dam removal: two control stations upstream the former reservoir, three restored stations within the former reservoir, and one control station downstream. We monitored physico-chemical characteristics, phytobenthos biomass and the river’s benthic metabolism, and assessed the functional composition of macroinvertebrate communities. We compared the recorded variables among upstream, downstream and restored sampling stations. We observed a rapid recolonization by invertebrates, but a still low phytobenthic primary production in restored stations. Such a low primary productivity was also reflected in the functional composition of invertebrate communities. Three years after dam removal, there was still a significant time lag between communities recovery and expected ecosystem functioning restoration. We observed a quick colonization by aquatic running-water invertebrate communities of new lotic reaches, but a slower recovery of important ecological functions rates such as those observed in control stations.

Size‐dependent community patterns differ between microbial eukaryotes and bacteria in a permafrost lake–river–sea continuum

AbstractMicrobial communities play a crucial role in ecosystem functioning, with contributions that can vary among taxonomic domains and size fractions. However, microbial assembly processes for bacteria and eukaryotes are seldom characterized together using size fractionation, especially in flowing waters. Here, we used amplicon sequencing combined with physicochemical measurements to determine how size fractionated (small fraction 0.22–3 μm; large fraction > 3 μm) community structure and diversity varied over a subarctic river continuum. We sampled the Sheldrake River, a 25 km river flowing through degrading discontinuous permafrost, from its lacustrine source through subarctic forest shrub tundra to its discharge plume in eastern Hudson Bay (Nunavik, Canada). Microbial community structure differed by size fraction and among habitats, with differences in the variables potentially driving community structure among size fractions and microbial domains. For the small size fraction, colored dissolved organic matter was a significant covariate of community variation for both bacteria and eukaryotes, consistent with the influence of landscape gradients. There were contrasting diversity patterns along the lake–river transect between bacterial size fractions. An abundance‐based approach indicated that for all communities, assembly processes were dominated by homogeneous selection, while an incidence‐based method showed dominance of heterogeneous selection for bacteria and homogenizing dispersal for microbial eukaryotes. Our findings show how different components of riverine microbial communities can have divergent patterns along the downstream continuum to the sea.

Application of Tesla Valve to Bidirectional Open-Channel Flows

Dams hinder water circulation and consequently disrupt the river continuum which adversely affects aquatic organisms and degrades water quality. An alternative new concept for a dam has been proposed with an optimal design that prevents seawater from flowing upstream while maintaining the continuity of the river–estuary ecosystem. A Tesla valve was used because its asymmetric geometry increases the flow resistance only in a single direction. The numerical model setup aimed to simulate wave-induced open-channel flows in OpenFOAM using the volume of fluid method and investigate the capability of the channel design to enhance the asymmetric flow resistance, and the model was validated by laboratory experiments. Various combinations, scales, shapes, and arrangements of the structures were considered for deployment. Subsequently, these designs were evaluated based on the difference in flow discharges in each flow direction. The optimal choice included a 30° angle between the sidewalls and the structures, and streamwise spacing between the structures at 5/3 times the channel width. Further, longer main structures with substructures and a slightly offset design can help increase the flow asymmetry in bidirectional open-channel flows. Although the present study has limitations in terms of methodology and complex structural factors, the proposed design suggests the basic design of a new concept for Tesla valve-like geometry.

Spatial Distribution Patterns of Zooplankton and Macroinvertebrates in a Small River under Strong Anthropogenic Pressure

The main objective of this study was to examine the spatial distribution patterns of aquatic invertebrates in an environment characterized by significant anthropogenic stress. During the entire research period, at all the sites, we noticed 72 taxa of zooplankton and 30 taxa of macroinvertebrates. Variation partitioning analysis reveals that the assemblages of planktonic organisms are much more determined by the time of sampling rather than by the site, in contrary to macroinvertebrates where the sampling time was only slightly more responsible for the composition of the benthic communities than the site. Spatial distribution of aquatic organisms in a small lowland river under strong anthropopressure shows significant deviations from the expectations of the River Continuum Concept (RCC). The benthic macroinvertebrate and littoral zooplankton communities exhibited a strong association with local site conditions, while, in contrast, pelagic zooplankton exhibited a strong dependency on drift and its production in the upper reaches of the river, leading to relatively consistent compositions downstream despite the highly altered river environment. To improve the biodiversity values and ecological state of a river, restoration treatments of bed and shore zones are required.

Investigating the role of hydrological connectivity on the processing of organic carbon in tropical aquatic ecosystems

Inland waters are highways of carbon and nutrient flows between the land and ocean. Aquatic environments integrate multiple sources and processes over space and time that influence ecosystem functionality. The complexity of these systems and their multiple interactions with the surrounding environment are conceptualised, but often lack empirical scrutiny that allows further understanding of how inland waters mobilise, transport, and utilise carbon and nutrients. This is particularly evident in tropical waters. Here, we apply advanced geochemical analyses of dissolved organic matter (DOM) composition in conjunction with algal pigment biomarkers, to determine the seasonal variability of organic matter production, processing and export for a tropical, floodpulse wetland, Tasik Chini (Malaysia). We identify two phases in the hydrological cycle: Phase 1 signifying a transition from the wet season with high suspended sediment and dissolved organic carbon concentrations. DOM is composed of humic substances, building blocks and lower molecular weight compounds. Towards the end this phase then are periods of increased water clarity and algal productivity. This is followed by Phase 2, which has a greater contribution of autochthonous DOM, composed of proteinaceous material, concomitant with lower dissolved nutrient concentrations, increased mixotrophic algae and emergent vegetation. Based on this framework, we highlight the role of such tropical wetland lakes as hydrological “bottlenecks,” through a lentic/lotic switch that shifts aquatic transport of carbon and nutrients from lateral river continuum supply to flood pulses. We highlight the need to consider inherent biases of spatial and temporal scaling when examining freshwater ecosystems along the land-ocean aquatic continuum.

Effects of large dams on the aquatic food web along a coastal stream with high sediment loads

The contribution of two basal energy sources – detrital organic matter and primary producers – as part of aquatic food webs varies typically along river continua. A host of barriers to river flow increase the water residence time and sediment and nutrient retention in reservoirs worldwide, and potentially alter the balance between detritus-based and algae-based energy pathways in the downstream food webs. We explored this issue on the Sélune River (Normandy, France), a small coastal stream that drains an agricultural catchment with high sediment runoff. Seasonal measurements of the following parameters were compared upstream and downstream of the reservoirs of two large dams (16 m and 36 m high): sediment fluxes, nutrient and chlorophyll a concentrations, algal communities in the epilithic biofilm (taxonomic composition, biomass and growth), and benthic invertebrate communities (abundance and trophic guild structure). As anticipated, annual sediment fluxes were much lower downstream of the reservoirs, where significant decreases in water turbidity, phosphate and silicate concentrations were recorded. A higher chlorophyll a concentration in water and a higher contribution of pelagic algae taxa to the photosynthetic biofilm suggested drifting and deposition of reservoir-borne phytoplankton downriver. Photosynthetic biofilm growth was higher downstream of the reservoirs in spring and fall, and so was the abundance of herbivores in the invertebrate community, notably scrapers and algae eaters. Energy pathways within riverine food webs were traced using stable isotope analyses of carbon (C) and nitrogen in the tissues of aquatic consumers (invertebrates and fish). Mixing models revealed a discontinuity in the origin of the C entering the food webs along the river continuum, confirming a greater contribution of algal C to aquatic consumers downstream of the reservoirs. These results illustrate mechanisms whereby large reservoirs can modulate C flow in food webs along a small coastal river with high sediment loads, and make it possible to anticipate the effects of dam removal on the future river ecosystem.

Prokaryote communities along a source-to-estuary river continuum in the Brazilian Atlantic Forest.

The activities of microbiomes in river sediments play an important role in sustaining ecosystem functions by driving many biogeochemical cycles. However, river ecosystems are frequently affected by anthropogenic activities, which may lead to microbial biodiversity loss and/or changes in ecosystem functions and related services. While parts of the Atlantic Forest biome stretching along much of the eastern coast of South America are protected by governmental conservation efforts, an estimated 89% of these areas in Brazil are under threat. This adds urgency to the characterization of prokaryotic communities in this vast and highly diverse biome. Here, we present prokaryotic sediment communities in the tropical Juliana River system at three sites, an upstream site near the river source in the mountains (Source) to a site in the middle reaches (Valley) and an estuarine site near the urban center of Ituberá (Mangrove). The diversity and composition of the communities were compared at these sites, along with environmental conditions, the former by using qualitative and quantitative analyses of 16S rRNA gene amplicons. While the communities included distinct populations at each site, a suite of core taxa accounted for the majority of the populations at all sites. Prokaryote diversity was highest in the sediments of the Mangrove site and lowest at the Valley site. The highest number of genera exclusive to a given site was found at the Source site, followed by the Mangrove site, which contained some archaeal genera not present at the freshwater sites. Copper (Cu) concentrations were related to differences in communities among sites, but none of the other environmental factors we determined was found to have a significant influence. This may be partly due to an urban imprint on the Mangrove site by providing organic carbon and nutrients via domestic effluents.

Along the river: Longitudinal patterns of functional and taxonomic diversity of plants in riparian forests

AbstractQuestionsThe River Continuum Concept (RCC) predicts a gradual shift of functional adaptations of organisms along the longitudinal (upstream–downstream) gradient, as well as the maximization of the biotic diversity in mid‐reaches. Although this theoretical framework was originally developed for stream macroinvertebrates, we tested whether such a pattern can also be observed in riparian plant communities.LocationThe study was conducted in the upper catchments of the San and Wisłoka Rivers in southeastern Poland. In both catchments, very well preserved riparian forests occur continuously along both rivers.MethodsWe studied the cover of plant species in riparian forests across two river networks. We analyzed the taxonomic and functional diversity indices, as well as community‐weighted means of functional traits in relation to the plots' positions in the catchments.ResultsThe functional richness and divergence displayed unimodal patterns of increasing values in the mid‐reaches. The patterns of taxonomic diversity were similar, but some plots in the lowest reaches were more diverse than expected, forming an additional increase in diversity. The observed patterns were largely in line with the predictions of RCC. We discovered a significant decrease in the specific leaf area and an increase in the height of herbaceous plants in communities along a longitudinal gradient. There was also a shift in the dispersal syndromes, toward a higher importance of zoochory in the lower reaches.ConclusionsThe study shows that plant communities in natural riparian forests show continuous species turnover along the longitudinal gradient that, along with the longitudinal shift of abiotic factors, creates patterns that are known from theoretical predictions.

A comprehensive review of the biodiversity of freshwater fish species in Valleys worldwide and in the Kingdom of Saudi Arabia.

An overview of freshwater fish variety worldwide and the variables influencing trends in variation between and within river basins are given in this review. Continental freshwater ecosystems are highly diverse and species-rich, housing nearly 18,000 species of fish (=50% of all fish species) in <0.5% of the total land area and providing a negligible (<0.01%) share of the planet's water supply. Large lowland tropical river basins such as the Amazon, Congo, and Mekong basins are home to the greatest freshwater fish diversity. Freshwater species of fish depth variation at the global magnitude is correlated with the total amount and variation of aquatic habitats and the environment's equilibrium overtime during the evolution of scales. The river continuum concept states that there is a predictable shift in fish species depth, diversity of species, and functional characteristics along gradients of environment from headwater to estuary. The ongoing trade of minerals and organic matter related to nearby floodplains is a strong factor in the number and variety of riverine fishes in most parts of the world (the flood pulse concept). Without coordinated conservation efforts, freshwater fishes will suffer significant losses in abundance and diversity due to the numerous threats they currently face worldwide. However, further development, adaptation, training, and guidance are needed. New technologies based on water conservation, suitable species, and local traditions are needed. Waste materials and local feed additives can also be used. Farmers should be provided with the necessary training and information.