Lateral Hydrological Connectivity Research Articles

Composition, divergence and variability: A comprehensive analysis of fish trait responses to connectivity

Connectivity, a fundamental concept in ecology, refers to the extent to which different habitats or ecosystems are interconnected through the movement of organisms, nutrients, and energy. Lateral hydrological connectivity (LHC) plays an especially critical role in shaping aquatic community organization in river-floodplain systems. However, a comprehensive understanding of various trait responses to LHC remains elusive. We characterized how attributes of fish community traits, specifically composition, divergence, and temporal variability respond to LHC in the Austrian-Hungarian floodplains of the Danube River using environmental DNA (eDNA) metabarcoding. Trait composition was quantified by community-level weighted means (CWM) as the degree of changes in trait responses along the LHC gradient from isolated oxbows to the main river. Divergence was measured using Rao’s quadratic functional diversity index and a null model approach to calculate standardized effect sizes (SES), with larger SES values indicating greater divergence and smaller values indicating convergence. Temporal variability, representing the degree of instability in community traits over time, was calculated using a functional beta diversity measure for multiple communities. Our findings revealed apparent compositional changes for many trait variables, highlighting the significance of LHC in shaping community functional diversity. Divergence patterns indicated that isolated habitats foster trait convergence presumably due to habitat filtering, whereas more connected areas promote trait divergence due to higher species richness and habitat availability. Temporal variability of traits associated with flow preference exhibited a hump-shaped relationship with LHC, suggesting intermediate connectivity zones are hotspots of ecological dynamism. The study suggests that examining composition, divergence, and temporal variability together provides a more complete understanding of trait responses to connectivity, stressing the importance of maintaining diverse connectivity levels in river-floodplain systems for effective conservation and ecosystem management.

Lateral connectivity maintains higher freshwater mussel biodiversity

Abstract The loss of lateral hydrological connectivity (LLHC) in freshwater ecosystems is increasingly threatening biodiversity; however, studies reporting this situation are limited, especially those assessing multi‐measure biodiversity features at large spatial scales. Here, and using one the most threatened taxonomic groups in freshwater ecosystems, we examined the possible influence of LLHC on the species diversity, density, and biomass of mussel assemblages and the genetic diversity and structure of Nodularia douglasiae (a common and widespread native species) in the Yangtze River floodplain. Results showed that the species diversity of freshwater mussels (α and β diversity) significantly differed between the connected and the disconnected areas, with higher α diversity and lower β diversity in the connected areas. At the same time, relative abundance, density, biomass, and genetic diversity of freshwater mussels were higher in the connected areas. High genetic differentiation and low gene flow were found among the 18 analysed N. douglasiae populations, indicating that LLHC is affecting genetic structure of this particular freshwater mussel species. Overall, results indicated that river–lake connectivity is affecting freshwater mussel biodiversity. In addition, this study highlights that multi‐measure biodiversity features provide distinct information about biodiversity dynamics, and are essential to evaluate the effects of floodplain disconnection. We advocate that an integrative approach to floodplain management embracing species and genetic diversity is needed for effective biodiversity conservation in large river ecosystems.

Linking hydrological connectivity with the richness and composition of aquatic invertebrates across the Paraná River floodplain, Argentina

Lateral hydrological connectivity can affect the community characteristics by promoting dispersal of organisms between the river and its floodplain. We investigated the species richness, specific diversity, and relative abundance of microcrustaceans transported by the flood and the macroinvertebrates retained by Pontederia crassipes Mart. roots in five floodplain lakes with different connectivity within the RAMSAR Site Humedales Chaco. Due to differences in the slopes of the floodplain studied, the connectivity was determined by the topographic location of each lake, rather than by the distance from the river. During the hydrological connection, there was a transverse gradient in the physical and chemical characteristics of the water, with an increase in the electrical conductivity and a decrease in dissolved oxygen and suspended solids in the more isolated sites. A total of 62 species of microcrustaceans (48 cladocerans and 14 copepods) and 63 taxa of macroinvertebrates were identified. The microcrustaceans had high species richness in the three most connected lakes, whereas the macroinvertebrates had a peak in taxa richness in the lakes with intermediate connectivity. The low beta diversity (βw) for both microcrustaceans (14.63 %) and macroinvertebrates (16.9 %) indicated that taxa richness varied little between sites with different hydrological connectivity. In both assemblages, cluster analysis based on the similarity of the relative abundance grouped the three sites more connected and separated from the less connected sites. Our results suggest that species richness followed different patterns in different assemblages across the lateral gradient and that the hydrological connectivity was more related to the relative abundance of microcrustaceans and macroinvertebrates than to the classical measures of βw. Species diversity indices did not vary between sites. The high total species richness of invertebrates is mainly a result of the spatial arrangement of the different floodplain lakes.

Complex drivers of geomorphic response and habitat formation revealed in multiyear monitoring of Cosumnes River experimental floodplain reconnection

In this study, we examine a novel levee breach experiment that reconnected a floodplain along the Cosumnes River, California to determine the decadal impact of removing 250 meters of levee and assess the recruitment of large wood. This is the latest study in an ongoing series of investigations 40 years in the making along the largest river on the western slope of the Sierra Nevada without a major dam. We present the findings of this multi-modal investigation here by first measuring the geomorphic alteration of the floodplain surface to quantify the depletion and accretion of sediment across the excavated site. We then identify and quantify the deposition of large wood. Results indicate initiation of anastomosing channel formation and distinct areas of large wood recruitment supporting a naturally evolving lateral levee. Accretion resulted in more than 25,000 m3 of sediment deposition within the original excavation site, the development of multiple sand splays, and natural recruitment of native riparian tree species. We conclude by discussing implications following other approaches to floodplain restoration as a Nature-based Solution. In episodic flow regimes, like in California’s Mediterranean-montane hydroclimatic regime, restoring lateral hydrologic connectivity facilitates ecosystem function. Large flood pulse events drive sediment dynamics and geomorphic heterogeneity while enriching biodiversity through biogeochemical fluxes and habitat creation on reconnected floodplains that store floodwaters and reduce peak discharge. These findings support the importance of long-term monitoring efforts of floodplain restoration.

Surface and groundwater flow exchanges and lateral hydrological connectivity in environments of the Matusagaratí Wetland, Panama

The Matusagaratí wetland in the Panamanian Darien is one of the largest wetlands in Central America. These types of riverine wetlands, associated with large drainage basins, are complex hydrological environments where variations in water flows and exchanges condition the existence of different wetland habitats. The work aimed to establish the hydrological functioning of the Matusagaratí wetland in different sectors of the Balsas River, emphasizing the exchanges of surface and groundwater flows and the hydrological connectivity that exists between the different laterally linked wetland environments. For this purpose, a monitoring network for surface water and groundwater was established along transects intersecting various wetland environments in the middle and lower basin of the Balsas River. This network is complemented by measurement points for surface water located in streams and in the upper basin of the river. Data collected in sensors installed in boreholes were compared to river level and precipitation data. Continuous water level recording sensors were installed at the monitoring points, and samples were collected for the determination of major ions and stable isotopes. The results indicate that in the mangroves of the lower basin and in the cativo forests of the middle basin levee there is a strong exchange of water between the river and the shallow groundwater. This water exchange is strongly influenced by the tide which spreads from the estuary to the continent through the river. Meanwhile, in the middle basin, mixed forests and orey forests developed on the alluvial plain exhibit a hydrological functioning that depends primarily on precipitation inputs. This study provides data that could serve as a basis for the management of this large tropical wetland that, despite having protection initiatives, could be hydrologically impacted by unsustainable socio-economic practices.

Centennial loss of lake wetlands in the Yangtze Plain, China: Impacts of land use changes accompanied by hydrological connectivity loss

Humans have played a fundamental role in altering lake wetland ecosystems, necessitating the use of diverse data types to accurately quantify long-term changes, identify potential drivers, and establish a baseline status. We complied high-resolution historical topographic maps and Landsat imagery to assess the dynamics of the lake wetlands in the Yangtze Plain over the past century, with special attention to land use and hydrological connectivity changes. Results showed an overall loss of 45.6 % (∼11,859.5 km2) of the lake wetlands over the past century. The number of lakes larger than 10 km2 decreased from 149 to 100 due to lake dispersion, vanishing, and shrinkage. The extent of lake wetland loss was 3.8 times larger during the 1930s–1970s than that in the 1970s–1990s. Thereafter, the lake wetland area remained relatively stable, and a net increase was observed during the 2010s–2020s in the Yangtze Plain. The significant loss of lake wetland was predominately driven by agricultural activities and urban land expansion, accounting for 81.1 % and 4.9 % of the total losses, respectively. In addition, the changes in longitudinal and lateral hydrological connectivity further exacerbated the lake wetland changes across the Yangtze Plain through isolation between lakes and the Yangtze River and within the lakes. A total of 130 lakes have been isolated from the Yangtze River due to the construction of sluices and dykes throughout the Yangtze Plain, resulting in the decrease in the proportion of floodplain marsh from 28.3 % in the 1930s to 8.0 % in the 2020s. Furthermore, over 260 sub-lakes larger than 1 km2 (with a total area of 1276.4 km2) are experiencing a loss of connectivity with their parent lakes currently. This study could provide an improved historical baseline of lake wetland changes to guide the conservation planning to wetland protection and prioritization area in the Yangtze Plain.

Peat swamp hydrological connectivity and runoff vary by hydrogeomorphic setting: Implications for carbon storage

AbstractDespite their importance in carbon cycling and catchment runoff dynamics, the hydrology of temperate peat swamps in response to changing hydrometeorological conditions is largely understudied. We examined the importance of hydrogeomorphic settings in controlling hydrological connectivity and runoff in a temperate peat swamp in southern Ontario, Canada over two consecutive growing seasons with contrasting conditions (dry and wet years). We chose two different small‐scale hydrogeomorphic settings to investigate: (i) a site with strong wetland‐stream interactions (i.e., an unconfined stream channel; unconfined) and (ii) a site with limited wetland‐stream interactions (confined).During the wet year, the confined site exhibited a consistently gaining stream, maintaining lateral hydrological connectivity and yielding high runoff ratios, while during the dry year, the confined site lost water and experienced low runoff ratios during storm events. Overland flow at the unconfined site maintained a longitudinal hydrological connectivity delivering water to its sub‐catchment outflow, as reinforced by hydrochemical observations. This connectivity was maintained in the wet year but ceased in the dry year despite consistent upstream sub‐catchment water inflow due to high depression storage. Runoff ratios were reduced because of this hydrological disconnection.We highlight the importance of small‐scale hydrogeomorphic setting on peat swamp carbon storage as facilitated by the variation of within‐site hydrological connectivity and runoff, which also has important implications for downstream water quality. The unconfined site maintained a higher water table position in both years and has much greater peat carbon stocks. We suggest peat swamp channelization either naturally or through drainage decreases carbon stocks.

Extended growing seasons and decreases in hydrologic connectivity indicate increasing water stress in humid, temperate forests

Forested headwater catchments are important sources of stable and abundant freshwater resources. Interactions between vegetation and topography influence lateral hydrologic connectivity by altering shallow subsurface flow paths. This in turn influences vegetation density along those paths, and subsequent hydrologic partitioning between localized water use and subsurface flows at catchment scales. Climate change impacts on forests, and the degree to which they reshape feedbacks between evapotranspiration (ET) and hydrologic connectivity, remain unclear. To clarify the extent and drivers of changing lateral hydrologic connectivity, we assessed relative changes in upslope to downslope vegetation density using the Normalized Difference Vegetation Index (NDVI) from 1984 – 2021 in 30,044 forested catchments across the Southern Appalachian Mountains. Increasing upslope NDVI relative to downslope NDVI was used as a proxy for decreasing lateral hydrologic connectivity. We then related changes in connectivity to climate and streamflow dynamics across 28 sub-regional reference watersheds. We found decreases in the ratio of downslope to upslope NDVI in almost half of the catchments (48.5%), primarily due to increasing upslope NDVI. This indicates increasing ET upslope and a decline in lateral hydrologic subsidy to downslope given precipitation. This was also supported by faster streamflow recession and increasing ET estimates relative to precipitation in over half of reference watersheds. The strongest predictor of decreasing connectivity was growing season minimum temperature (Tmin), which increased in 88% of catchments (Mean R2 = 0.27 +/- 0.13). While Tmin is not a dominant atmospheric driver of ET, this pattern has been closely linked to lengthened growing seasons. This suggests that alteration of lateral hydrologic connectivity is mainly driven by ecophysiological responses to changing climate rather than directly by atmospheric drivers. Our results emphasize the importance of vegetation dynamics shifting hydrologic partitioning and driving water limitations even in humid, temperate forests.

Assessment of the genetic diversity of Chinese freshwater mussels and refuge areas in the Yangtze River floodplain

Abstract Freshwater mussels are one of the most diverse and endangered faunas worldwide. Owing to the continuing global biodiversity loss, it is essential to identify and protect populations that are not severely affected by human disturbances and ensure that natural genetic diversity is not inevitably lost. The generation of baseline genetic diversity data will help guide species conservation and restoration plans in the Yangtze River Basin where freshwater mussels are facing multiple threats and where strong declines have been observed. The genetic diversity of freshwater mussels was assessed using 818 mtDNA cytochrome c oxidase subunit 1 (COI), 565 28S rRNA and 509 COI + 28S datasets, and refuge areas in the Yangtze River floodplain were predicted using a Maxent habitat suitability model. The results showed that the genetic diversity of Aculamprotula spp., Sinosolenaia carinata, Cuneopsis rufescens, Lamprotula cornuumlunae, Lanceolaria lanceolata, Ptychorhynchus pfisteri, Schistodesmus spp. and Sinohyriopsis cumingii was lower than that of other freshwater mussels, which indicated that the conservation status of these species should be prioritized. The habitat suitability modelling showed that Poyang Lake and Dongting Lake basins (lakes and tributary rivers connected to the Yangtze River mainstem) present the most suitable habitats for freshwater mussels, indicating that lateral hydrological connectivity may be the most important factor in maintaining the genetic diversity of Chinese freshwater mussels. Consequently, Poyang Lake and Dongting Lake basins should be established as protected areas. This study provides valuable information for the conservation and management of freshwater mussel biodiversity in the Yangtze River basin, which will not only help conserve this vulnerable group but will also provide wider benefits to freshwater ecosystems.

Geomorphic Landscape Features Affect Plant Invasion in Salt Marshes: The Role of Tidal Channel Meanders

The invasion of smooth cordgrass (Spartina alterniflora Loisel.) has greatly threatened China's coastal salt marshes, yet how geomorphic landscape features, such as tidal channel meanders, influence cordgrass invasion processes remains unclear. We found that lower hydrodynamic disturbance, bed shear stress, and higher propagule pressure triggered by eddies due to the convex structure of channel meanders facilitated the seedling establishment and subsequent growth, whereas the concave side considerably inhibited the cordgrass invasion. Our findings suggest that the meandering geomorphology of tidal channels could act as stepping stones to greatly facilitate cordgrass landward spread along tidal channels by creating invasion windows. Photo 1. Invasion range of S. alterniflora is promptly extending landward via tidal channels in salt marshes, as shown in this photograph (taken in the Yellow River Delta, China). As a consequence, the drainage density of tidal channels, the lateral hydrological connectivity between tidal channels and their served marsh flats, and bio-geomorphic landscape formation could be considerably altered. Photo credit: Zhonghua Ning. Photo 2. UAV aerial image presents the process and pattern of S. alterniflora (green and green-yellow color in tidal channel margins) landward expansion attached to the meandering geomorphic structure of tidal channels, particularly the convex sides of channel meanders, which could provide consecutive spatial “windows of opportunity” for S. alterniflora establishment. Photo credit: Zhonghua Ning. Photo 3. As shown in this photograph, S. alterniflora seedlings preferentially colonize on the convex side of tidal channel meanders. Compared with the concave side, the eddies and reduced flows generally exist on the convex side, thereby providing suitable physical environments (lower hydrodynamic disturbance, bed shear stress, and salinity-moisture stress) for seed stranding, seedling establishment, and subsequent growth of S. alterniflora. Photo credit: Zhonghua Ning. These photographs illustrate the article “Tidal channel meanders serve as stepping-stones to facilitate cordgrass landward spread by creating invasion windows” by Zhonghua Ning, Baoshan Cui, Cong Chen, Tian Xie, Weilun Gao, Youzheng Zhang, Zhenchang Zhu, Dongdong Shao, Dongxue Li, and Junhong Bai, published in Ecological Applications. https://doi.org/10.1002/eap.2813.

Lateral Hydrological Connectivity Driven by Tidal Flooding Regulates Range-Expansion of Invasive Spartina alterniflora in Tidal Channel-Salt Marsh Systems

Understanding how hydrological features affect habitat invasibility is crucial for predicting whether variations of such hydrological features may act as important inducement regulating range-expansion of invasive species in tidal channel-salt marsh systems. Although lateral hydrological connectivity (LC), or the hydrological connections between tidal channels and adjacent marsh flats, is an important hydrological feature, its effect on plant invasion has rarely been studied in depth. Here, we examined the effects of lateral hydrological connectivity on range-expansion of Spartina alterniflora ( S. alterniflora ) in tidal channel margins of a typical tidal channel-salt marsh system, in the Yellow River Delta, China. Field surveys and transplanting experiments showed that high LC greatly favors S. alterniflora to expand its invasion ranges along tidal channel margins by mediating such habitat physical forms of stress as soil salinity, soil moisture and soil hardness. In contrast, low LC, exacerbates those forms of stress, thereby significantly checking the lateral expansion of S. alterniflora . Moreover, human-made and naturally formed geomorphic structures in salt marshes (e.g., artificial ditches, pools, and hollow microtopography), particularly at high elevations, could potentially enhance LC over time, thereby making such sites prone to invasion by S. alterniflora . These results highlight the importance of lateral hydrological connectivity as an essential driver to regulate S. alterniflora lateral expansion along with the tidal channels. Our results imply that considering the relationships between hydrological processes and spread processes of exotic species should be incorporated into future management frameworks for risk assessment and ecological control of invasive plant species.

Hydrologic connectivity and source heterogeneity control concentration–discharge relationships

AbstractChanges in streamwater chemistry have frequently been used to understand the storage and release of water and solutes at the catchment scale. Streamwater chemistry typically varies in space and time, depending on sources, mobilization mechanisms, and pathways of water and solutes. However, less is known about the role of lateral hydrologic connectivity and how it may influence streamwater chemistry and solute export patterns under different wetness conditions. This study analyses long‐term low‐frequency data from four UK catchments using antecedent catchment wetness as proxy for lateral hydrologic connectivity. We demonstrate that solute mobilization mechanisms can vary depending on catchment wetness, as different catchment areas become hydrologically connected to or disconnected from streams. We show that flow and streamwater chemistry are mostly decoupled under dry conditions, leading to stronger impacts of the heterogeneity in solute sources on mobilization patterns during dry conditions compared to wet conditions. Our results demonstrate that the lateral and vertical distributions of solutes need to be integrated and considered together with the temporally variable hydrologic connectivity of these lateral areas to the stream when assessing streamwater chemistry. This combined analysis thus enables inferences regarding the lateral distribution of solutes throughout the catchment; it also indicates that a better understanding of the relationship between lateral hydrologic connectivity and the lateral and vertical distributions of solute concentrations can help to identify particularly vulnerable points in the catchment and their potential polluting effects on streams.

Loss of lateral hydrological connectivity impacts multiple facets of molluscan biodiversity in floodplain lakes

Lateral hydrological connectivity (LHC) is a key process in maintaining aquatic biodiversity in river floodplain ecosystems. However, the effects of LHC loss on aquatic biodiversity are rarely studied. Here, we evaluated, for the first time, the responses of multiple facets (i.e., taxonomic, functional and phylogenetic) of alpha and beta diversity of freshwater molluscs to the LHC loss in 23 floodplain lakes in the Yangtze River Basin in China. Our results showed that taxonomic and functional alpha diversities were all significantly higher in connected lakes (CLs) than in disconnected lakes (DLs), whereas phylogenetic alpha diversity (Δ+) was lower in CLs than in DLs. For beta diversity facets, taxonomic (Tβsor) and phylogenetic (Pβsor) dissimilarities were slightly more contributed by the turnover component or equally contributed by the turnover and nestedness-resultant components both in CLs and DLs. Instead, functional beta diversity (Fβsor), generally showing much lower values than Tβsor and Pβsor, was mainly contributed by the nestedness-resultant component (76.6–84.0%), especially in DLs. We found that only functional dissimilarities were significantly higher in DLs than CLs, indicating a high level of functional diversity loss without replacement of species possessing traits sensitive to hydrological disconnection (i.e., large body size, lamellibranch body form, filter feeding, ovoviviparity and burrowing habits). In general, lake area, hydrological connectivity, aquatic vegetation coverage and nutrient levels (TN and TP) played important roles in structuring variation in molluscan alpha and beta diversities, although the three diversity facets responded to different environmental factors. Our results suggest that loss of connectivity to the mainstem river has negative impacts on molluscan assemblages in floodplain lakes. More importantly, as taxonomic, functional and phylogenetic diversities responded somewhat differently to the loss of hydrological connectivity, all of these biodiversity facets should be better incorporated into aquatic biodiversity assessment and conservation programs in large river floodplains.

Impact of river flow modification on wetland hydrological and morphological characters.

A good number of researchers investigated the impact of flow modification on hydrological, ecological, and geomorphological conditions in a river. A few works also focused on hydrological modification on wetland with some parameters but as far the knowledge is concerned, linking river flow modification to wetland hydrological and morphological transformation following an integrated modeling approach is often lacking. The current study aimed to explore the degree of hydrological alteration in the river and its effect on downstream riparian wetlands by adopting advanced modeling approaches. After damming, maximally 67 to 95% hydrological alteration was recorded for maximum, minimum, and average discharges. Wavelet transformation analysis figured out a strong power spectrum after 2012 (damming year). Due to attenuation of flow, the active inundation area was reduced by 66.2%. After damming, 524.03 km2 (48.9% of total pre-dam wetland) was completely obliterated. Hydrological strength (HS) modeling also reported areas under high HS declined by 14% after post-dam condition. Wetland hydrological security state (WSS) and HS matrix, a new approach, are used to explore wetland characteristics of inundation connectivity and hydrological security state. WSS was defined based on lateral hydrological connectivity. HS under critical and stress WWS zones deteriorated in the post-dam period. The morphological transformation was also well recognized showing an increase in area under the patch, edge, and a decrease in the area under the large core area. All these findings established a clear linkage between river flow modification and wetland transformation, and they provided a good clue for managing wetlands.

Long-term analysis of aquatic macrophyte diversity and structure in the Paraguay river ecological corridor, Brazilian Pantanal wetland

Spatial and temporal variation in limnological variables, as well as local changes in the hydrological pattern, may affect inundation hydrology and alter biotic interactions and functional diversity of aquatic macrophyte communities in the Pantanal. The objectives of this research in the floodplain along the Paraguay River were to: 1) examine changes in the diversity of aquatic macrophyte communities over a ten-year period based on surveys taken in 2008 and 2018; and 2) evaluate the possible indirect relation of changes in rainfall patterns, which in turn affect lateral hydrological connectivity and limnological variables that directly affect the composition and productivity of the aquatic macrophyte community. Comparison of data taken in 2008 and 2018 revealed a change in the timing and a reduction in the number of rainy days. These changes in rainfall patterns coincided with limnological changes, including increases in transparency, pH, dissolved oxygen, ammonium, nitrate, total nitrogen, orthophosphate, and total phosphorus, as well as decreases in water temperature and connectivity. Species richness of aquatic macrophyte communities in three ecological zones of the Paraguay River floodplain increased between 2008 and 2018, with increases in emergent grasses, emergent non-grasses, and amphibious life forms, showing a trend that favors species that are more adapted to drought conditions. The seasonal pattern of inundation and desiccation, as well as dissolved oxygen and temperature, where correlated with macrophytes communities variations. The colonization and expansion of rooted emergent macrophytes over this period could be a response to local anthropogenic activities, hydrological trends, or may reflect interannual rainfall variability.

Soil Characteristics and Hydromorphological Patterns Control Denitrification at the Floodplain Scale

Nitrate pollution in aquatic ecosystems is still a major problem in Germany. There is a great potential to permanently remove nitrate from aquatic systems through denitrification as a relevant ecosystem function. However, the controlling factors and the dimension of the denitrification potential are still not fully understood due to the high complexity of the process. This study presents the combined assessment of potential soil denitrification rates, physical and chemical soil parameters, and hydrological parameters from six floodplains of four large German rivers, namely the Rhine, the Elbe, the Weser, and the Main. Based on multivariate statistics, results show that the denitrification potential of soil was almost solely controlled by soil pH. The lab assays showed mean soil denitrification potentials of 6.4–11.4 mg N m−2h−1(pH < 7) and 23.0–30.5 mg N m−2h−1(pH > 7). We contend that when upscaling these estimates to annual rates of potential denitrification, the duration of average inundation should be incorporated, as this accounts for water saturation and nutrient supply − the major controlling variables for denitrification. Results provide evidence that the denitrification potential can only be fully exploited in frequently inundated floodplains. Thus, despite favorable soil conditions for denitrification, floodplains that have suffered from anthropogenic impacts, lose their importance in nitrate removal for the river system. We conclude that pH and lateral hydrological connectivity are likely to be key factors that should be considered when estimating denitrification as an ecosystem function.

Variation of stomach content and isotopic niche of puye Galaxias maculatus (Jenyns, 1842) in large river systems of southern Chile

Abstract Understanding of trophic structure and flow of energy within river systems is essential for informed management of these ecosystems and conservation of native fish fauna and fisheries resources. Food resources used by riverine fish depend on productivity within the main stem and adjacent floodplains, terrestrial inputs, and longitudinal and lateral hydrologic connectivity. Trophic ecology of puye (Galaxias maculatus Jenyns, 1842) in Chilean river systems remains poorly understood and studies that combine short‐term (stomach content) and long‐term (stable isotope analyses) analyses have not been performed to date. In this study, we assessed spatial variability in diet of puye based on both stomach content and analyses of stable isotopes of carbon and nitrogen in 10 river systems of southern Chile (including main stem, headwater lakes, and estuaries). Fish in the main stems of more northern rivers, with predictable flow regimes, were characterised by the most diverse diets, broad niche space, and high proportion of terrestrial prey items and zooplankton in diet. Fish collected in lakes were characterised by narrow isotopic niche space and diet dominated by zooplankton. Diet of G. maculatus in Chilean river systems is associated with characteristics of riverine habitats within each river system (lentic, lotic, and estuarine) and hydrologic connectivity among them. Indeed, connectivity with floodplain habitats and direct supply from headwater lakes appeared to be primary drivers of fish diet in systems where these are present. These supplies maintain fish populations of good condition and allow freshwater recruitment. Therefore, maintaining seasonal flow dynamics and connectivity is essential to preserve the natural function of these river systems and conserve native fish populations.

Assessing the effects of lateral hydrological connectivity alteration on freshwater ecosystems: A meta-analysis

The alteration of lateral hydrological connectivity (LHC) is among the major causes of decline in biodiversity and ecosystem functioning and services in river-floodplain ecosystems. Yet, the effects of LHC alteration on aquatic ecosystems have not been rigorously tested and synthesized. Here, we conducted a literature review and meta-analysis to provide a global synthesis of the effects of LHC alteration on aquatic ecosystems, and to test the effectiveness of indicators for bioassessment. In our review we found that the effects of LHC alteration were disproportionally documented in terms of geographic distribution and research subject. Most studies were conducted in Europe and America on the biodiversity and abundance of macroinvertebrates and fish. In our meta-analysis, we compared effect sizes between pairs of three LHC levels, i.e. disconnected, moderately connected, and highly connected. We found that only a limited number of water quality indicators (e.g. chlorophyll a, total phosphorus, conductivity, and dissolved oxygen) and biological community indicators (e.g. fish species richness, and zooplankton abundance) showed significant differences between LHC levels. The responses of these indicators to LHC alteration differed, and sometimes in opposing directions. We found out responsive indicators and their response trends, which could not only facilitate effective biomonitoring and bioassessment with regard to engineering and restoration of LHC, but also provide scientific underpinnings of the understanding of trade-offs among the indicators. We identified knowledge gaps and provided recommendations on how to fill these gaps, which included investigating rarely documented regions, developing new indicators, and quantifying mechanistic links between LHC and ecosystem functioning and services.

Enhancement of lateral connectivity promotes the establishment of plants in saltmarshes

Several studies have shown that enhancing lateral hydrological connectivity in river systems can increase the exchange of materials and energy, and improves species diversity, which suggests that it might be a useful ecological restoration tool. The variation in elevation gradient across a saltmarsh system is small, which means that lateral hydrological connectivity in saltmarsh systems is often ignored and the ecological effects caused by the variation in lateral hydrological connectivity are seldom studied. Lateral hydrological connectivity presents when a hydrological connection between marsh plain and tidal creek occurs, as a time interval during which tidal flow occurred. This study explored the effects of enhancing lateral hydrological connectivity on the plant life history process using empirical studies. The enhancement of lateral hydrological connectivity on a temporal scale was achieved by placing hollowed microtopographic structures on the marsh. Data obtained through the high-frequency monitoring of tidal levels was used to calculate lateral hydrological connectivity enhancement, and field control experiments were used to determine the effects of lateral connectivity enhancement on seed retention, emergence, and seedling survival at each life stage. The results showed that lateral hydrological connectivity decreased with the increasing distance to sea and the lateral distance to tidal creek. The hollowed microtopographic structures significantly enhanced lateral hydrological connectivity on the marsh, increased soil moisture content, and reduced soil salinity. Furthermore, seed retention time was significantly increased during dispersal stage, and potential seed establishment was improved. During the emergence and growth stages, the enhanced soil moisture and reduced salinity facilitated the emergence and growth of seeds and seedlings. These effects benefit plant re-establishment in bare areas, especially in areas with low lateral hydrological connectivity. This information could be used to improve the restoration or recovery of vegetation on bare or degraded saltmarshes.

Artificial modification on lateral hydrological connectivity promotes range expansion of invasive Spartina alterniflora in salt marshes of the Yellow River delta, China

‘Invasibility’, or the extent to which a habitat is prone to being invaded by plants, is a measure of the resistance of that ecosystem to biological invasion: a limited extent represents abiotic conditions unsuitable for invasion by invasive species; however, human activity can change that and make a habitat prone to rapid invasion. Field surveys and greenhouse experiments were carried out to explore, using spatial analysis, how a strong invader, namely Spartina alterniflora, is assisted by such activities as constructing levees and digging trenches, ditches, and pits in a tidal salt marsh. These activities changed the lateral hydrological connectivity of a salt marsh. The invasibility was then estimated based on the probability of seed dispersal and retention using the classical probabilistic method, and the rate of seedling emergence using threshold analysis. Changes in lateral hydrological connectivity led to more seeds of the invading species being retained, especially in high marshes, and promoted the emergence of its seedlings by making the soil more moist and less saline. The results suggest that changes in the lateral hydrological connectivity in a salt marsh can make it more prone to being invaded. The results have important implications for the control of invasive plants by limiting human activity and thereby regulating lateral hydrological connectivity in coastal ecosystems.