Hydrogeomorphic Zones Research Articles

Differences in the vertical accretion of sediment among mangrove species with different aerial root types

As a type of zonal vegetation along tropical and subtropical coastlines, mangrove species have developed aerial root systems to adapt to their environment. However, little is known about the differences in the surface vertical accretion (SVA) of sediment among aerial root types. In this study, three typical mangrove species with different aerial root systems (Avicennia marina with pneumatophores, Bruguiera gymnorhiza with root knees, and Kandelia candel with plank roots) were investigated in Zhenzhu Bay of the Beibu Gulf, China. Vertical accretion and the total volume of aerial roots (VM) were monitored and calculated in three communities of these species along the intertidal gradient over three growing seasons (2017–2019). The results were as follows: (1) The differences in SVA and the accretion rate among the three mangrove species were significant (p < 0.01), and the highest accretion rate was detected in plank roots (1.51 cm year−1), followed by root knees (1.05 cm year−1) and pneumatophores (0.63 cm year−1). (2) The values of VM and annual increment in VM were highest for plank roots, followed by root knees and pneumatophores, and the differences in VM and the corresponding rate of increase among mangrove species were significant (p < 0.01). (3) SVA, the surface accretion rate and VM differed among hydrogeomorphic zones, with the highest values found in the seaward fringe, followed by the interior and the landward fringe. (4) Close linear relationships between VM and SVA and between VM annual increment and SVA rate were detected regardless of root type and hydrogeomorphic zone. We conclude that significant differences in SVA and the accretion rate exist among the three mangrove species, and that SVA and VM and VM annual increment and SVA rate exhibit a linear relationship.

Aboveground biomass distributions and vegetation composition changes in Louisiana's Wax Lake Delta

The Wax Lake Delta (WLD) is an actively prograding delta in the Mississippi River Delta Plain that is otherwise experiencing widespread degradation and submergence of its coastal wetlands. The WLD is actively accumulating mineral and organic sediment that increases soil surface elevation, changing emergent wetland communities as the young delta develops. There is uncertainty in how the dynamics of community composition respond to net changes in soil elevation and determine aboveground biomass and carbon storage. This study utilizes high resolution imaging spectrometer data captured on October 17, 2016, to map the delta's dominant vegetation species and wetland types. We validated this vegetation map (overall accuracy = 77.62%, Kappa = 0.72) and compared it with a published species map that used WorldView-2 data collected five years earlier on October 16, 2011. This allowed us to map changes resulting from five years of delta development to determine changes in wetland forest species (Salix nigra), two dominant herbaceous wetland species (Colocasia esculenta and Polygonum punctatum), and various grass species. Results show an increase in C. esculenta and a marginal increase in forested wetlands (S. nigra). C. esculenta's expansion occurred largely from the delta island heads toward the fringes into wetland area previously occupied by P. punctatum, which saw a corresponding decline. Additionally, this study leveraged these species distributions with a published aboveground biomass (AGB) dataset to examine the dominant plant types' growth patterns across elevational gradients. We characterized variability in AGB by marsh platform elevation and across different elevational zones categorized by hydroperiod, or hydrogeomorphic zones. We found that the herbaceous plant species peak in AGB in the low intertidal zone and decrease with elevation before increasing slightly in the higher elevations, consistent with previous field-based mesocosm experiments for C. esculenta. With vegetation distributions, succession, and growth patterns in the WLD characterized, this study may inform future restoration efforts throughout the Mississippi River Delta Plain regarding the changing vegetation composition that may emerge following changes in surface elevation with sediment deposition.

Simulating hydrological connectivity and water age within a coastal deltaic floodplain of the Mississippi River Delta

Located at the mouth of large rivers, coastal deltaic floodplains are important ecosystems for trapping sediment and removing excess nitrogen introduced to the river through agricultural and urban runoff. Hydrological connectivity (the percent of river channel water interacting with floodplain wetlands), water age (the time elapsed since water entered the system via the main river channel) and water temperature have strong impacts on the biogeochemical processes controlling the fate of sediment and nutrients at continental margins. A Delft3D hydrodynamic model was used to explore the role of river and tide forcings, hydrogeomorphology, and vegetation roughness on hydrological connectivity, water age, and water temperature of a young, prograding river delta in coastal Louisiana, Wax Lake Delta. Across a simulation period of January to June 2015, an average of 39.9% of river flow entered the delta floodplains and water within the floodplains had an average water age of 0.8 days. In winter and spring, floodplain zones with higher sediment surface elevations (supratidal hydrogeomorphic zones) were characterized by increased water age and warmer water temperatures compared to subtidal floodplains and channels. Our experimental models indicate that the presence of wetland vegetation decreases hydrological connectivity by preventing flow of water into deltaic floodplain, but increases water age by retaining water within the floodplains. Despite small amplitudes (~30 cm), tides create hourly exchange between channels and floodplains, which decreases water age in floodplains, but has little impact on average connectivity. Our results emphasize the role of both hydrological forcings (river and tides) and ecosystem features (vegetation and sediment surface elevation) in regulating channel-floodplain interactions in deltaic systems. Evaluated against field data, which is often difficult to collect at high spatial and temporal frequency in floodplains, this model performs well, highlighting the utility of numerical models in capturing landscape-scale hydrodynamics.

Benthic fluxes of dissolved oxygen and nutrients across hydrogeomorphic zones in a coastal deltaic floodplain within the Mississippi River delta plain

We tested the hypothesis that benthic fluxes will increase spatially in a coastal deltaic floodplain as sediment organic matter increases in response to developing hydrogeomorphic zones along a chronosequence of the active Mississippi River Delta. A continuous flow-through core system was used to incubate intact sediment cores from three hydrogeomorphic zones along a chronosequence in the emerging Wax Lake Delta (WLD). Organic matter content increased from younger to older deltaic sediments from subtidal to supratidal hydrogeomorphic zones, which were coupled with increasing benthic oxygen and nitrogen fluxes. Mean net denitrification rate in spring was 100 µmol N2–N m−2 h−1 with significantly lower rates occurring in the younger intertidal zones (T4 transect, − 22 µmol N2–N m−2 h−1) and higher rates occurring in the older supratidal zones (T2 and T1 transects, 330 and 262 µmol N2–N m−2 h−1, respectively). Mean net denitrification rate in summer was 397 µmol N2–N m−2 h−1 without significant site-to-site variability except for the supratidal-T2 site (911 µmol N2–N m−2 h−1) showing higher denitrification rate than the other sites. Based on seasonal temperature and inundation time, annual rates of benthic NO3− removal varied from − 0.5 to − 3.4 mol m−2 y−1 and N2–N production rates varied from 1.0 to 3.2 mol N m−2 y−1 across WLD. The subtidal zone had the lowest fluxes associated with lower organic matter content, but was the hydrogeomorphic zone with the largest area and longest flood duration, and therefore contributed over half of N removal in WLD. The estimated annual NO3− removal of 896 Mg N y−1 in WLD accounts for 10 to 27% of total NO3− load to WLD, most of which is converted to N2 through denitrification. As a small prograding coastal deltaic floodplain under early stages of delta development, WLD is a continuously emerging ecosystem where the capacity of N removal increases by 0.2 to 2% per year prior to riverine NO3− is export to coastal ocean. These results highlight the contribution of the coastal deltaic floodplain in an active coastal basin in processing elevated riverine NO3− at continental margins with coastal ocean. The potential loss of this ecosystem service in N removal may increase in global significance as delta areas decline as result of accelerated relative sea level rise and decreased sediment loading in major river basins around the world.

Mapping Topobathymetry in a Shallow Tidal Environment Using Low-Cost Technology

Detailed knowledge of nearshore topography and bathymetry is required for a wide variety of purposes, including ecosystem protection, coastal management, and flood and erosion monitoring and research, among others. Both topography and bathymetry are usually studied separately; however, many scientific questions and challenges require an integrated approach. LiDAR technology is often the preferred data source for the generation of topobathymetric models, but because of its high cost, it is necessary to exploit other data sources. In this regard, the main goal of this study was to present a methodological proposal to generate a topobathymetric model, using low-cost unmanned platforms (unmanned aerial vehicle and unmanned surface vessel) in a very shallow/shallow and turbid tidal environment (Bahía Blanca estuary, Argentina). Moreover, a cross-analysis of the topobathymetric and the tide level data was conducted, to provide a classification of hydrogeomorphic zones. As a main result, a continuous terrain model was built, with a spatial resolution of approximately 0.08 m (topography) and 0.50 m (bathymetry). Concerning the structure from motion-derived topography, the accuracy gave a root mean square error of 0.09 m for the vertical plane. The best interpolated bathymetry (inverse distance weighting method), which was aligned to the topography (as reference), showed a root mean square error of 0.18 m (in average) and a mean absolute error of 0.05 m. The final topobathymetric model showed an adequate representation of the terrain, making it well suited for examining many landforms. This study helps to confirm the potential for remote sensing of shallow tidal environments by demonstrating how the data source heterogeneity can be exploited.

Ecogeomorphology of coastal deltaic floodplains and estuaries in an active delta: Insights from the Atchafalaya Coastal Basin

We present here an integrated analysis of coastal deltaic floodplains in the active Atchafalaya Coastal Basin coupled to downstream deltaic estuaries to review how ecosystem properties self-organize around fluvial processes during river re-occupation as part of the delta cycle. The flood pulse of the river is critical to providing autogenic feedbacks between flow patterns, sediment delivery, vegetation productivity, and organic/inorganic accretion that produce spatial patterns of land elevation, habitat diversity, and estuary dynamics. Coastal deltaic floodplains form in the proximal region of an active delta as bar-shaped islands with interdistributary bays shape hydrogeomorphic zones influenced by both geophysical and ecological processes. Hydrogeomorphic zones in coastal deltaic floodplains of the proximal sedimentation zone can also be defined by time since subaerial emergence accounts for variability in vegetation community composition and soil successional development. The reduction in sedimentation and increase in both above- and belowground biomass associated with formation of hydrogeomorphic zones results in significant increase in organic matter density in soils, with higher N:P ratios reflecting the biotic feedback of ecological succession on delta floodplain development. In both the proximal and distal sedimentation regions, episodic events, such as river floods and cold fronts, control seasonal water levels, marsh platform inundation, and increase in elevation capital. In coastal deltaic floodplains, an increase in vegetation height and density has a twofold effect: it favors trapping of sediment on the islands; whereas an increase in roughness deflects water flow and sediment into the channels thus bypassing the marsh surface. There is evidence that this is in contrast to more constant positive feedback of vegetation on sedimentation in distal estuarine marsh platforms. Delta estuaries go through a transformation from a near-riverine estuary in the winter-spring season to a near-marine lagoon in the summer-fall season. Geomorphological displacement of vegetation types occurs as platform elevation increases in the proximal sedimentation zone as delta landform emerges, with specific vegetation dominating the respective subtidal, intertidal and supratidal hydrogeomorphic zones. This does not occur in the distal sedimentation zone that lack sediment input as marsh platform elevation decreases. This is due to presence of salinity and H2S that limit the capacity of biotic feedbacks to contribute to marsh stability. The growth of a coastal deltaic floodplain in the proximal sedimentation region of Atchafalaya Coastal Basin along with stable estuarine marshes in distal sedimentation region demonstrate the value of long-term riverine influence by preventing loss of wetland platform elevation.

Ecosystem Responses to Water Resource Developments in a Large Dryland River

AbstractLarge floodplain rivers in dryland regions are becoming increasingly modified through water resource developments. Identifying ecosystem responses in these systems is challenging because of their natural variability, limited data, and the myriad of ways they are modified. This study used organic samples from snail, mussel, and fish specimens obtained from museum collections for the determination of carbon and nitrogen stable isotope ratios, for the Barwon‐Darling River, Australia, between 1869 and 2005, a period of extensive water development. Three hypotheses were posed for this study: (1) The trophic status and food web character of the Barwon‐Darling River will change in response to water resource developments; (2) responses in the trophic status and food web character will differ between different hydrogeomorphic zones identified along the river; and (3) food chain lengths will increase in response to water resource developments. Substantive changes in trophic status and components of the food web were detected between before and after water developments. Three lines of evidence support the conclusion of anthropogenically driven changes in the food web of this dryland floodplain‐river system. Stable isotope ratios of fish, mussels, and snails differed between both hydrogeomorphic zones of the river and the before and after disturbance periods. Layman metrics, representing community niche space, differed between before and after disturbance periods. Also, both mean trophic position and food chain length differed between predisturbance and postdisturbance for the two functional process zones. The substantial shift in basal source contribution over time is a potential indicator of a state change and loss of resilience in this system.

Variation in stream metabolism and benthic invertebrate composition along longitudinal profiles of two contrasting river systems

Our study aimed to determine drivers of longitudinal variation in stream metabolism and benthic macroinvertebrate (BMI) composition and assess concordance of these ecological measures for two Canadian rivers (Rat River and Tobacco Creek). Gross primary production was associated with longitudinal position in both rivers but also with the percentage of the watershed used for agriculture and hydrogeomorphic zone. However, within- and among-zone differences in stream metabolism indicated that longitudinal variation followed a staircase pattern rather than a clinal pattern. BMI composition was associated with network position in both rivers, but hydrogeomorphic zones were only important in Tobacco Creek. Among-zone differences in BMI communities in Tobacco Creek depended on season. Concordance between stream metabolism and BMI composition was not observed within either river despite metabolism and BMI composition being associated with longitudinal position. For these rivers, segment-scale hydrogeomorphic conditions appear to be important modifiers of longitudinal patterns observed at the whole river scale. The lack of concordance between stream metabolism and BMI composition suggests reach-scale processes are driving ecological differences within sampling sites.

Arsenic distribution along different hydrogeomorphic zones in parts of the Brahmaputra River Valley, Assam (India)

The spatial distribution of arsenic (As) concentrations along three classified hydrogeomorphological zones in the Brahmaputra River Valley in Assam (India) have been investigated: zone I, comprising the piedmont and alluvial fans; zone II, comprising the runoff areas; and zone III, comprising the discharge zones. Groundwater (150 samples) from shallow hand-pumped and public water supply wells (2–60 m in depth) was analysed for chemical composition to examine the geochemical processes controlling As mobilization. As concentrations up to 0.134 mg/L were recorded, with concentrations below the World Health Organization and the Bureau of Indian Standards drinking-water limits of 0.01 mg/L being found mainly in the proximal recharge areas. Eh and other redox indicators (i.e., dissolved oxygen, Fe, Mn and As) indicate that, except for samples taken in the recharge zone, groundwater is reducing and exhibits a systematic decrease in redox conditions along the runoff and discharge zones. Hydrogeochemical evaluation indicated that zone I, located along the proximal recharge areas, is characterized by low As concentration, while zones II and III are areas with high and moderate concentrations, respectively. Systematic changes in As concentrations along the three zones support the view that areas of active recharge with high hydraulic gradient are potential areas hosting low-As aquifers.

Hydrogeomorphic zones characterize riverbed sediment patterns within a river network

Hydrogeomorphic zones characterize riverbed sediment patterns within a river network

Spatial and temporal patterns of carbon flow in a temperate, large river food web

Using natural abundances of stable carbon (δ13C) and nitrogen (δ15N) isotopes, we quantified spatial and temporal patterns of carbon flow through the main channel food web in the lowland section of New Zealand’s longest river, the Waikato River. The study was undertaken with the objective of determining whether the Waikato River conforms to contemporary theoretical concepts regarding carbon flow in large river food webs. Potential organic carbon sources and invertebrate and fish consumers were sampled from three different hydrogeomorphic zones on six occasions, representing a range of seasonal and flow conditions. In line with the predictions of the riverine productivity model and riverine ecosystem synthesis, autochthonous algae and biofilms were the most important basal carbon source contributing to consumer biomass. These were often supported by C3 aquatic macrophytes and allochthonous C3 riparian plants. The relative importance of organic carbon sources differed between zones and appeared to change depending on season, presumably in response to water temperature and flow, particularly in the unconstrained zone of the lower river. We also demonstrate that to draw robust conclusions, consideration must be given to quantifying the isotopic signatures of organisms lower in the food web, as these can change significantly between sampling times and hydrogeomorphic zones.

Variation in water-mediated connectivity influences patch distributions of total N, total P, and TN:TP ratios in the Upper Mississippi River, USA

AbstractFloodplain rivers have been conceptualized as patchwork mosaics of hydrogeomorphic zones that differ in hydrological, physiochemical, and ecological conditions. However, few investigators have empirically examined the extent to which basic ecological properties of large rivers, such as nutrient concentrations, are patchily distributed or the relationship between such distributions and water-mediated connectivity. We used global and local measures of spatial autocorrelation in 5 reaches of the Upper Mississippi River from 1994–2008 in spring (higher discharge) and summer (lower discharge) to examine distributions of total N (TN), total P (TP), and TN∶TP for evidence of patchiness. TN was distributed as discrete patches regardless of season, but the patches of TP and TN∶TP were more distinct during summer low-flow than spring high-flow conditions. The occurrence of patches of high TN, low TP, and high TN∶TP was highly predictable, generally occurring in backwater, low-velocity (0.1 m/s) environments...

Characteristics of downed wood across headwater riparian ecotones: integrating the stream with the riparian area

Although downed wood is a significant structural component of riparian ecotones, most studies of downed wood dynamics are conducted only within the stream bankfull channel or on the riparian forest floor. We examined downed wood structural characteristics (e.g., length, volume, and decay class) across the entire riparian ecotone of seven headwater streams in mature deciduous forests of northeastern Ohio to understand the structural relationships between downed wood and riparian ecotones. Sampling was stratified by hydrogeomorphic zones that were defined by lateral position within the riparian ecotone and correspond to differing fluvial geomorphology. Most downed wood pieces sampled were small (&lt;1 m) and with a high decay class. Downed wood pieces within the baseflow zone (within the wetted channel) were smaller and more abundant than those within the transitional zone (within the bankfull channel excluding the wetted channel). Surface contact was inversely related to aquatic influence across the riparian ecotone, with downed wood in Zone 3 (riparian area beyond the bankfull channel) having significantly higher contact than wood within the other zones. The ecotonal approach and the incorporation of hydrogeomorphic influences is an alternative approach that moves beyond traditional approaches to more holistic management of riparian forests.

Surface Elevation Change and Susceptibility of Different Mangrove Zones to Sea-Level Rise on Pacific High Islands of Micronesia

Mangroves on Pacific high islands offer a number of important ecosystem services to both natural ecological communities and human societies. High islands are subjected to constant erosion over geologic time, which establishes an important source of terrigeneous sediment for nearby marine communities. Many of these sediments are deposited in mangrove forests and offer mangroves a potentially important means for adjusting surface elevation with rising sea level. In this study, we investigated sedimentation and elevation dynamics of mangrove forests in three hydrogeomorphic settings on the islands of Kosrae and Pohnpei, Federated States of Micronesia (FSM). Surface accretion rates ranged from 2.9 to 20.8 mm y−1, and are high for naturally occurring mangroves. Although mangrove forests in Micronesian high islands appear to have a strong capacity to offset elevation losses by way of sedimentation, elevation change over 6½ years ranged from −3.2 to 4.1 mm y−1, depending on the location. Mangrove surface elevation change also varied by hydrogeomorphic setting and river, and suggested differential, and not uniformly bleak, susceptibilities among Pacific high island mangroves to sea-level rise. Fringe, riverine, and interior settings registered elevation changes of −1.30, 0.46, and 1.56 mm y−1, respectively, with the greatest elevation deficit (−3.2 mm y−1) from a fringe zone on Pohnpei and the highest rate of elevation gain (4.1 mm y−1) from an interior zone on Kosrae. Relative to sea-level rise estimates for FSM (0.8–1.8 mm y−1) and assuming a consistent linear trend in these estimates, soil elevations in mangroves on Kosrae and Pohnpei are experiencing between an annual deficit of 4.95 mm and an annual surplus of 3.28 mm. Although natural disturbances are important in mediating elevation gain in some situations, constant allochthonous sediment deposition probably matters most on these Pacific high islands, and is especially helpful in certain hydrogeomorphic zones. Fringe mangrove forests are most susceptible to sea-level rise, such that protection of these outer zones from anthropogenic disturbances (for example, harvesting) may slow the rate at which these zones convert to open water.

Spatial patterns in the composition of shallow‐water macroinvertebrate communities of a large New Zealand river

Identifying the environmental factors influencing biotic patterns in large rivers will assist with extrapolating biological monitoring results to broader scale conclusions about river condition. In the present study, we collected macroinvertebrates and physico‐chemical data at 47 shallow‐water (<1‐m deep) sites, including nine sites at major tributary junctions, during summer along the lower Waikato River, North Island, New Zealand. Macroinvertebrate communities were dominated by a few relatively abundant and widespread taxa. Upper site samples were characterised by high relative abundances of Diptera, but the significance of this group declined further downstream where Crustacea became more dominant. Overall, more taxa (36) were found at tributary junctions than at mainstem sites within four hydrogeomorphic zones (22–31 taxa per zone). Significant differences among faunal groups identified in a cluster analysis on relative abundance data were detected for the percentage of wood sampled, and for water conductivity which increased downstream at mainstem sites and was high at some junction sites. Non‐metric multidimensional scaling of percentage abundance data revealed differences in community composition among zones, and among some mainstem and tributary junction sites. Geographic position (easting and northing) was significantly correlated with taxa richness and community evenness (Pielou) at mainstem sites (excluding tributary junctions), reflecting an increase in sample diversity and less equitable taxonomic dominance with distance down river. Overall, these results point to an interplay between habitat patchiness and successional and hydrogeom orphic processes influencing macroinvertebrate community composition in the lower Waikato River. Such multiscale variations need to be accounted for in the design of invertebrate biomonitoring programmes if they are to represent the ecological condition of large river environments.

Seasonal reversals of upland‐riparian diversity gradients in the Sonoran Desert

ABSTRACTFlood disturbance and water resource availability vary sharply over time and space along arid‐region rivers and can interact in complex fashion to shape diversity patterns. Plant diversity showed spatial patterning along a topogradient from the floodplain of the San Pedro River (Arizona, USA) to the arid upland, but the patterns shifted temporally as the suite of limiting factors changed. During two of three sampling times, spatial diversity patterns were shaped primarily by gradients of water availability, the regional limiting factor. In the summer dry season, microscale diversity (species richness per 1 m2) and mesoscale diversity (cumulative species and functional types in 20, 1‐m2 plots) of herbaceous plants decreased along the topogradient from floodplain to upland, reflecting the greater water availability on the low surfaces. During a summer wet season with moderate rains and flooding, diversity increased in all hydrogeomorphic zones (floodplain, terrace, upland), but the spatial pattern along the topogradient persisted. Following a very wet winter, patterns along the topogradient reversed: scour from large floods limited diversity on the floodplain and competitive exclusion limited the diversity on undisturbed river terrace, while abundant rains allowed for high microscale diversity in the upland. Disturbance and resource availability thus interacted to influence plant species diversity in a fashion consistent with the dynamic‐equilibrium model of species diversity. In contrast to the microscale patterns, mesoscale diversity of species and functional types remained high in the floodplain during all sampling times, with 58% more plant species and 90% more functional types sampled in low floodplain than arid upland for the year as a whole. Species with a wide range of moisture and temperature affinities were present in the floodplain, and seasonal turnover of species was high in this zone. The floodplain zone of a perennial to intermittent‐flow river thus had greater plant diversity than arid Sonoran Desert upland, as measured at temporal scales that capture seasonal variance in resource and disturbance pulses and at spatial scales that capture the environmental heterogeneity of floodplains. Although periodically limited by intense flood disturbance, diversity remains high in the floodplain because of the combination of moderate resource levels (groundwater, seasonal flood water) and persistent effects of flood disturbance (high spatial heterogeneity, absence of competitive exclusion), in concert with the same climatic factors that produce seasonally high diversity in the region (temporally variable pulses of rainfall).