Freshwater Tidal River Research Articles

Managed diversion lead time effects on flood response in a tidal freshwater river

AbstractRiver diversions are a commonly used water management tool throughout the world. Low‐gradient coastal rivers exhibit complex floodplain interactions and are subject to multiple uncertain drivers of future flood risk. The interacting factors make quantification of downstream diversion effects on ambient flood hazards in coastal riverine ecosystems challenging. This article analyzes the effects of a managed ecological diversion (MED) on downstream flood hazards in the flood‐prone tidal freshwater Vermilion River system situated within the Atchafalaya Basin region of Louisiana, USA. A flexible and efficient hydraulic and hydrologic (H&H) modeling setup and counterfactual approach is utilized to develop and analyze 60 simulation scenarios aimed at quantifying the effects of an existing baseflow augmentation program on downstream flood hazards under uncertain future conditions. The analysis demonstrated the significance of preemptive diversion closure on the downstream flood levels in downstream rivers. The role of wetland topography and residual floodplain storage on MED‐induced flood response was also highlighted. A variety of uncertain parameter/forcing scenarios were also evaluated to establish robustness of the findings and to extend the results to more general settings. The study provides insights on downstream flood response induced by MEDs, which are expected to become more prevalent in the future.

Impact of environmental stressors on the phytoplankton communities to assess the River Halda, Bangladesh

Halda is a 98 km-long tidal freshwater river of Bangladesh, which has a unique feature since it is the only natural breeding ground and the sole source of fertilized eggs of Indian Major Carps globally. The present study firstly summarizes the impact of environmental stressors to understand phytoplankton abundance, community, and their diversity in Halda River, Bangladesh. Environmental data as seasons (Winter, Monsoon), water depths, temperature, pH, Salinity, dissolved oxygen, and nutrients were collected and analyzed from the Halda River in January 2019 and August, 2020. Thirty-four phytoplankton genera, comprising five classes, were identified from the Halda River, Bangladesh. A two-way ANOVA was performed to analyze the effect of Monsoon and Winter on the physicochemical and biological parameters. It revealed that there was a statistically significant ((F9, 160)= 34.999, p= 0.0) interaction between Monsoon and Winter on the observed parameters. The water temperature (F 125.31, Fcrit 4.130, p= 0.0) and nutrients (F 11.118, Fcrit 2.322, p= 0.0) revealed significant change in the phytoplankton concentrations in the Halda River. As of the temperature and nutrients, water depths also significantly (F 1.790, Fcrit 1.719, p= 0.0) affect the phytoplankton communities in the Halda River showing the highest and lowest cell density in the surface and bottom water, respectively.

Corrigendum: Seasonal Changes in the Drivers of Water Physico-Chemistry Variability of a Small Freshwater Tidal River

This correction is being submitted because we recently noticed that many of the images in the figures were accidentally all shifted down one figure (the image in Figure 3 should be Figure 4, the image in Figure 4 should be Figure 5, the image in Figure 5 should be Figure 6, the image in Figure 6 should be Figure 7, and the image in Figure 7 should be Figure 3) and thus figures are matched with the wrong figure description. No changes were made to the figures, and the only change to a figure description was the removal of the sentence “Blue background delineates distinct period of storm-driven parameter behavior (greater variability) over the course of the year” that referred to an older (unpublished) version of the graph.

Forcing uncertainty and salinity response to dredging in a tidal freshwater river

ABSTRACT This study evaluates the salinity response to dredging under different forcing regimes for a 75-kilometer segment of the flood-prone Vermilion River located in coastal Louisiana, USA. The Vermilion is a tidal freshwater river affected by baseflow regulation, rapid urbanization, and hydromodification. This river also serves as major freshwater resource for the region. An efficient one-dimensional model calibrated previously for stage and water temperature is used to analyse the salinity impacts due to a large-scale flood control dredging. The analysis is based on 200 alternative scenarios based on combinations of uncertain key forcing variables for a critical 13-day intrusion period. The analysis reveals a nonlinear effect on mean salinities and position of the brackish water limit as a function of uncertain forcing variables. Dredging also increases the variability of salinity encroachment effects due to uncertainty in future forcing conditions (e.g. relative sea level rise, baseflow diversion volumes, and salinities at the river mouth). The robust analysis presented here provides a foundation for future model-based assessments of under-studied tidal freshwater rivers and provides key insights for coastal management and sustainability efforts in general.

Seasonal Changes in the Drivers of Water Physico-Chemistry Variability of a Small Freshwater Tidal River

Where rivers meet the sea, tides can exert a physical and chemical influence on the lower reaches of a river. How tidal dynamics in these tidal river reaches interact with upstream hydrological drivers such as storm rainfall, which ultimately determines the quantity and composition of material transferred from watersheds to estuaries, is currently unknown. We monitored a small freshwater tidal river in the Pacific Northwest, United States in high resolution over 1 year to evaluate the relative importance of tides vs. upstream hydrological flows (i.e., base flow and precipitation events) on basic physico-chemical parameters (pH, dissolved oxygen, turbidity, specific conductivity, and temperature), and how these interactions relate to the downstream estuary. Tidal variability and diurnal cycles (i.e., solar radiation) dominated water physico-chemical variability in the summer, but the influence of these drivers was overshadowed by storm-driven sharp pulses in river physico-chemistry during the remainder of the year. Within such events, we found incidences of counterclockwise hysteresis of pH, counterclockwise hysteresis of dissolved oxygen, and clockwise hysteresis of turbidity, although systematic trends were not observed across events. The dominance of storm rainfall in the river’s physico-chemistry dynamics, and similar pulses of decreased pH observed in adjacent estuarine waters, suggest that the linkage between tidal streams and the broader system is variable throughout the year. High-frequency monitoring of tidal river biogeochemistry is therefore crucial to enable the assessment of how the relative strength of these drivers may change with future sea level rise and altered precipitation patterns to modulate biogeochemical dynamics across the land-ocean-atmosphere continuum.

Prevalence of Salmonella and Listeria monocytogenes in non-traditional irrigation waters in the Mid-Atlantic United States is affected by water type, season, and recovery method.

Irrigation water contaminated with Salmonella enterica and Listeria monocytogenes may provide a route of contamination of raw or minimally processed fruits and vegetables. While previous work has surveyed specific and singular types of agricultural irrigation water for bacterial pathogens, few studies have simultaneously surveyed different water sources repeatedly over an extended period of time. This study quantified S. enterica and L. monocytogenes levels (MPN/L) at 6 sites, including river waters: tidal freshwater river (MA04, n = 34), non-tidal freshwater river, (MA05, n = 32), one reclaimed water holding pond (MA06, n = 25), two pond water sites (MA10, n = 35; MA11, n = 34), and one produce wash water site (MA12, n = 10) from September 2016—October 2018. Overall, 50% (84/168) and 31% (53/170) of sampling events recovered S. enterica and L. monocytogenes, respectively. Results showed that river waters supported significantly (p < 0.05) greater levels of S. enterica than pond or reclaimed waters. The non-tidal river water sites (MA05) with the lowest water temperature supported significantly greater level of L. monocytogenes compared to all other sites; L. monocytogenes levels were also lower in winter and spring compared to summer seasons. Filtering 10 L of water through a modified Moore swab (MMS) was 43.5 (Odds ratio, p < 0.001) and 25.5 (p < 0.001) times more likely to recover S. enterica than filtering 1 L and 0.1 L, respectively; filtering 10 L was 4.8 (p < 0.05) and 3.9 (p < 0.05) times more likely to recover L. monocytogenes than 1L and 0.1 L, respectively. Work presented here shows that S. enterica and L. monocytogenes levels are higher in river waters compared to pond or reclaimed waters in the Mid-Atlantic region of the U.S., and quantitatively shows that analyzing 10 L water is more likely recover pathogens than smaller samples of environmental waters.

Sedimentary profiles of pollution marker chemicals along a large tributary of Chesapeake Bay (mid-Atlantic USA)

Molecular markers in environmental geochemistry include natural product or pollutant chemicals in sediments that are indicative of discharge sources or emission pathways. Four classes of molecular marker compounds, including fatty acids, sterols, PCBs, and PAHs, in surficial sediments (top 2 cm) collected along a downstream transect of the Potomac River within the US mid-Atlantic region were analyzed and correlated with potential pollution discharge sources in close proximity to the sampling sites. Thirty-five surficial sediment samples were collected using a petite Ponar grab sampler along a 320-km longitudinal transect of the Potomac River (mid-Atlantic USA), a major tributary of Chesapeake Bay, ranging from the upland fluvial river to its confluence with the Bay. The sediments were collected along the transect at approximately equidistant points. The marker chemicals were extracted from sediments using microwave-assisted extraction and quantified on a dry weight basis by gas chromatography or gas chromatography-mass spectrometry. Sediment moisture, texture, and organic carbon and nitrogen content were also determined. Fatty acids and sterols were well correlated with ecological factors in the Potomac River, while the sterol ratio epi+brassicasterol/stigmasterol showed moderate spatial correlation with nearby waste treatment plants (WTPs) and city locations, especially in the upland and tidal freshwater river. The fecal sterol coprostanol also showed moderate spatial correlation with some WTPs in both the upland and tidal river. PCB and PAH concentrations were primarily correlated with urban and large military installations. PCBs in sediments appeared to be predominantly derived from Aroclors while PAHs showed a strong pyrogenic origin. Retene and perylene were unique markers for PAHs in sediment and were indicative of aged organic matter in sediments. The marker chemicals had utility in identifying pollution emission sources and pathways in the Potomac River. For PCB pollutants, sediment profiles reflected localized source emissions from industrial sites. PAHs showed a downstream plume effect derived from urban Washington, DC. Fatty acids and sterols were most useful for identifying ecological shifts (i.e., terrestrial versus aquatic origin), but showed spatial correlations with wastewater treatment plants and cities.

Polychlorinated biphenyls (PCBs) in adult and juvenile mallards (Anas platyrhynchos) from the Hudson River, New York, USA

The Hudson River, NY, USA is contaminated for over 300 km with polychlorinated biphenyls (PCBs) released from two General Electric (GE) capacitor plants. We collected adult and juvenile mallards (Anas platyrhynchos) from four different areas of the river; an area upstream of the GE plants (n = 38), two areas directly downstream of the GE plants (n = 41, n = 38), and an area more than 100 km downstream in the freshwater tidal river (n = 20). Collections occurred during July and August (2008) when ducks were flightless to ensure ducks were “resident” and exposures were local. Fat and muscle tissue were analyzed for PCBs. PCBs were detected in all samples, and mallards below the GE plant sites on the Hudson River had orders of magnitude higher concentrations of PCBs than those above the plants. Juvenile mallards from areas directly downstream of the GE plant sites tended to have higher PCB concentrations in fat than adults. The patterns of PCB congeners and homolog groups varied across the study areas, with areas directly downstream of the GE plants dominated by tetra-chloro biphenyls whereas samples from upstream and the freshwater tidal river tended towards higher chlorinated congeners. Congener patterns between male and female and juvenile and adult mallards were generally similar within study areas, with the exception of one area downstream of the GE plants where adult birds exhibited different patterns than juveniles. Evidence of PCBs from the GE plant sites was detected in the tidal Hudson River, more than 100 km downstream of the plant sites. More than 90% of the ducks collected in areas downstream of the GE plants but above the tidally influenced river exceed the USFDA tolerance level for PCBs in poultry, which should be a concern for consumers of waterfowl taken in proximity to the upper Hudson River.

Patterns of seed bank and vegetation diversity along a tidal freshwater river

Species richness and diversity may increase with spatial scale related to increased area and heterogeneity of habitat. Yet, in bidirectional hydrologically connected tidal ecosystems, secondary dispersal via hydrochory has the potential to homogenize seed banks, and both life history characteristics and tolerances to environmental conditions influence the composition of plant communities. How species richness, diversity, and composition of seed banks and vegetation change along environmental gradients and at different spatial scales is not well understood. We explored the relationships of seed bank and vegetation diversity across 135 plots along a tidal freshwater river in the Delaware Estuary, USA. Species richness and diversity were partitioned across three hierarchical spatial scales: individual plots, transects perpendicular to the tidal channel, and river kilometers. Community structure was also examined as it related to distance from the tidal channel and location along the tidal river. Species richness was 89 in the seed bank and 54 in the vegetation. Species-area relationships revealed that species richness reached a near maximum asymptote inland (20 m from channel) for the seed bank and at the edge (0 m) for the vegetation. Rare occurrences of species in the seed bank and vegetation were greatest 5 m from the channel edge. As spatial scale increased, seed bank richness increased, associated with the progressive accumulation of species. Seed bank diversity, however, was maximized within small plot areas and along the river. Diversity of the vegetation was maximized locally due to the abundance of a few common species. These findings suggest that suites of common species contributed to high localized vegetation diversity, yet large spatial scales maximized the number and diversity of species in the seed bank and vegetation through rare encounters, as well as the complexity of the landscape.

Conservation of a river for biodiversity and ecosystem services: the case of the Halda – the unique river of Chittagong, Bangladesh

ABSTRACTThe Halda is the only tidal freshwater river in the world where major Indian carps spawn naturally. The river also serves as a source of drinking and irrigation water, a fishing ground and a means of water transportation. We investigated the biodiversity and ecosystem services of the Halda in the context of the conservation of this river in cooperation with river-dependent community and challenges thereof. The primary and secondary field data have been collected from relevant government and non-governmental organizations using both structured and semi-structured questionnaires. According to our calculations, the total economic worth of benefits derived from provisioning services of the Halda was approximately BDT (Bangladesh Taka) 1753 million. Irrigation and drinking water were the major contributors with BDT 1333 and 122 million, respectively. Carp spawning has lower primary economic worth (BDT 10 million) but it has a much bigger impact, as it supplies fish fries to the whole country. The river supports temporary and permanent livelihood for egg and fry collectors (approximately 2000), fishermen (approximately 350), boatmen (approximately 250), boat building and mending (approximately 150), sand quarrying (approximately 760), which creates a cumulative worth of BDT 283 million. The sustained flow of provisioning service from the Halda explains the willingness to contribute money (54 million) and time (average 6.6 hours/month) by local communities for conserving this river. Half of the respondents, though illiterate, have shown genuine concern about the future of the river. The sustainable flow of services and enhancement in their economic output require conservation and scientific management of the Halda. The community willingness and its dependence, as unearthed by our research, provides compelling ground for community-based conservation and management of the river. Future conservation projects and management plans for this river should involve local communities to avail their willingness to conserve this river while sustaining their livelihood through enhancement of economic output from it.

Indirect and Non-use Values of Halda River- A Unique Natural Breeding Ground of Indian Carps in Bangladesh

Bangladesh is a Riverine country; however, Halda River is of special interest among them in recent time for being the only tidal freshwater River in the world that serves as a natural spawning ground for major Indian carps. Besides, this River serves some indirect and non-use values. This study aims at estimating the economic worth of the indirect and non-use value of Halda River. Both indirect and non-use values of this River were calculated by using contingent valuation method. The study found that total indirect use value per year from this River was Tk. 29.50 million. We also estimated that total non-use value was Tk. 31.46 million which comprised of bequest value of Tk. 14.85 million and option value of Tk. 16.61 million. The study suggests that this valid source of information should take into consideration for the decision makers in taking projects and programmes aimed at managing this River more wisely in the future.DOI: http://dx.doi.org/10.3329/jesnr.v6i2.22092 J. Environ. Sci. &amp; Natural Resources, 6(2): 31-36 2013

Comparison of Denitrification Between River Channels and Their Adjoining Tidal Freshwater Wetlands

Denitrification in tidal freshwater river channels and their adjoining freshwater wetlands greatly affects nitrogen export from river networks, yet the relative importance of these two habitats to nitrogen export has not been examined. Knowledge of how these habitats contribute to denitrification of the river nitrogen load is critical for improving models of nitrogen transport. Denitrification rates were measured in sediments from the channel, bank, and floodplain at upstream and downstream sites of two forested tidal freshwater zones (TFZs) in North Carolina, the New River and Newport River, using membrane inlet mass spectrometry to measure N2 production. Denitrification rates did not usually differ statistically between the channel, bank, and floodplain, although denitrification was highest on the floodplain at the upstream site in the Newport River. When these rates were extrapolated across the entire area of the TFZ, the channel contributed more to the N2 flux than the riparian zone. These results indicate that denitrification rates are comparable between the channel and riparian zone in forested TFZs, and that the importance of the channel versus the riparian zone depends on channel and floodplain morphology.

The influence of channel deepening on estuarine turbidity levels and dynamics, as exemplified by the Ems estuary

Active deepening of tidal channels usually results in the alteration of the vertical and the horizontal tide. This may lead to concurrent significant increases in mean suspended matter concentrations (SPM) in coastal plain estuaries, the turbidity maximum (ETM) included. This is exemplified by an extensive analysis of the Ems estuary, a prototypical heavily stressed estuary in the Dutch-German border area. Measurements show that the SPM concentrations in the lower reaches of the estuary have increased an average of 2- to 3-fold between 1954 and 2005, with a 10-fold increase observed in the upper estuary (tidal river). Longitudinal profiles of surface SPM demonstrate that the ETM has moved upstream by up to 25 km and has broadened into a zone 30 km in length which extends into the freshwater tidal river. On an annual scale, variations in freshwater discharge significantly influence the formation and breakdown of the ETM: during low river discharge the ETM approaches equilibrium over 2–3 months, whilst elevated river discharges relocate the ETM downstream over several weeks.An exploratory, semi-analytical model is calibrated to simulate the equilibrium SPM distribution in the upper estuary during five time periods from 1965 to 2005, using archival bathymetric and tidal data. Results suggest that the deepening of tidal channels and a reduction in hydraulic drag have most likely resulted in a landward shift of the SPM trapping location. The measured increase in SPM concentrations and the development of fluid mud around the 1990s likely contributed to reduced mixing and bottom drag, creating a feedback loop that further altered tidal and SPM dynamics. It is argued that the removal of some non-erodible (consolidated) layers in the lower reaches of the estuary has created new internal sediment sources that may be responsible for feeding the observed high SPM concentrations, rather than increased sediment input from the boundaries. All findings are based on and supported by measured short-term seasonal fluctuations, as well as long-term developments of yearly averaged concentrations in the longitudinal SPM distribution.

Sediment transport due to extreme events: The Hudson River estuary after tropical storms Irene and Lee

AbstractTropical Storms Irene and Lee in 2011 produced intense precipitation and flooding in the U.S. Northeast, including the Hudson River watershed. Sediment input to the Hudson River was approximately 2.7 megaton, about 5 times the long‐term annual average. Rather than the common assumption that sediment is predominantly trapped in the estuary, observations and model results indicate that approximately two thirds of the new sediment remained trapped in the tidal freshwater river more than 1 month after the storms and only about one fifth of the new sediment reached the saline estuary. High sediment concentrations were observed in the estuary, but the model results suggest that this was predominantly due to remobilization of bed sediment. Spatially localized deposits of new and remobilized sediment were consistent with longer term depositional records. The results indicate that tidal rivers can intercept (at least temporarily) delivery of terrigenous sediment to the marine environment during major flow events.

River flow controls on tides and tide‐mean water level profiles in a tidal freshwater river

Tidal rivers feature oscillatory and steady gradients in the water surface, controlled by interactions between river flow and tides. The river discharge attenuates the tidal motion, and tidal motion increases tidal‐mean friction in the river, which may act as a barrier to the river discharge. Time series of tidal water level amplitudes at five gauge stations along the River Mahakam in Indonesia, and tidal flow velocity amplitudes at a discharge monitoring station were obtained applying wavelet analysis. Temporal variations in tidal damping coefficients for quaterdiurnal, semidiurnal, and diurnal tidal species were quantified from the observed amplitude profiles. The analysis shows that tidal damping during the rising limb of a discharge wave differs from damping during the falling limb. Wavelet cross‐correlations between surface levels yielded empirical estimates of wave numbers. An empirical relation between tidal damping and river flow is derived to describe subtidal bottom friction along an idealized tidal river resembling the Mahakam. The subtidal friction is decomposed into contributions from the river flow only, from river‐tide interaction, and from tidal asymmetry. Even for high river flow and low tidal velocity, river flow enhances friction attributed to river‐tide interaction, causing subtidal water level setup. A simple multilinear regression model using subtidal bottom friction is employed to predict subtidal water levels at locations upstream, with a relatively good agreement between predictions and observations. The explicit expression shows the nonlinear dependence of subtidal friction on river flow velocity, explaining the complex behavior of tidal‐mean surface level profiles.

Tidal geomorphology affects phytoplankton at the transition from forested streams to tidal rivers

Summary1. Coastal rivers can have long tidally influenced reaches that are affected by tides but do not contain saline water. These tidal freshwater reaches have steep geomorphic gradients where the river transitions from narrow, heavily shaded streams to wide, unshaded channels. The influence of these gradients on river ecosystem production is poorly understood.2. We characterised gradients in irradiance, geomorphology, water clarity and chlorophyll a along 9‐ to 16‐km tidal freshwater reaches of the Newport and White Oak Rivers in North Carolina, USA, and examined the effect of nutrient enrichment on phytoplankton growth in the Newport River. Underwater irradiance was modelled at 2–4 week intervals along both rivers using measurements of the above‐canopy irradiance, canopy cover, water column light attenuation (Kd) and water depth. Suspended material (TSS), dissolved organic carbon (DOC) and chlorophyll a were sampled at 2‐week interval at five sites on the Newport River and on four dates at four sites on the White Oak River over the course of one year.3. Phytoplankton nutrient limitation was assessed at three locations along the tidal gradient. River water was collected during March, April, June and October, and incubated in 10‐L plastic outdoor containers under ambient water temperature and sunlight. Additions of inorganic nitrogen and phosphorus served as treatments; growth rate during the 4 days of incubation was calculated from the change in chlorophyll a concentration over time.4. Canopy cover decreased from more than 90% to &lt;10% over the length of both tidal freshwater rivers. Water column irradiance and phytoplankton biomass increased as tree canopy cover decreased and channel width increased. Channel width exceeded predictions for non‐tidal rivers by threefold because of tidal influence. TSS and DOC decreased significantly along the length of the Newport River, but no significant gradients were observed in the White Oak River. Kd did not vary along the tidal gradient of either river.5. Mesocosm experiments indicated that inorganic nitrogen and phosphorus enhanced the growth of phytoplankton advected from the non‐tidal river into the tidal freshwater river during spring and summer. Phytoplankton in the tidal freshwater reach were generally not nutrient limited.6. Tidal hydrology (in the absence of saltwater) directly affected the morphology of the channel and indirectly affected biological growth and production. The significant increase in river width, irradiance and phytoplankton biomass distinguished these tidal freshwater ecosystems from their upstream (non‐tidal fluvial) counterparts, while the strong influence of riparian shading distinguished them from the saline estuaries downstream. Future development of ecosystem and biogeochemical models for tidal freshwater rivers will benefit from the linkages between geomorphology and biological processes identified here.

The role of a freshwater tidal area with controlled reduced tide as feeding habitat for European eel (Anguilla anguilla, L.)

Summary Implementation of the Controlled Reduced Tide (CRT) technique could increase the total surface of tidal freshwater marshes in Europe and ease implementation of restoration projects in coastal defense and riverine ecosystems. The goal was to determine whether a regularly flooded area connected to a freshwater tidal river could act as an important foraging area for European eel, and if so, to what extent the diet of eels in this flooding area differed from that of eels foraging in the river itself. The stomach contents of eels from the River Schelde were compared with eels from the Lippenbroek, an adjacent CRT area. Prey diversity (H′) of individual eels was about four times higher in the Lippenbroek than in the River Schelde. Moreover, 12 prey categories in eel stomachs from the Lippenbroek were found whereas only three categories were retrieved from eels in the River Schelde. In the Lippenbroek, eels fed on terrestrial organisms (lumbricids, caterpillars and other insects), but also on fish and fish eggs and to a lesser extent on other aquatic prey (Lumbricullidae, chironomids and Hirudinea). In contrast, eels from the main river fed mainly on tubificids, fish, and some gammarids. Consequently, eels in the Schelde estuary are opportunistic feeders, but with a preference for large benthic prey. The number and weight of aquatic organisms ingested by eels in the Lippenbroek is not significantly different from the River Schelde. However, eels foraging in the Lippenbroek area had consumed significantly more terrestrial prey. Furthermore the total caloric value estimated for the ingested prey of eels from the Lippenbroek (derived from the literature) was about twice as high as that for eels from the River Schelde. While the condition index remained inconclusive, an Ancova revealed that eels captured in the Lippenbroek were significantly heavier for a given length than eels captured in the Schelde. The study showed that with a controlled reduced tide to restore lateral connectivity of large tidal rivers with their adjacent floodplains, high quality habitats for the European eel are created. These measures could significantly contribute to the production of eels in better condition, which have better chances to reproduce successfully. Hence, wetland restoration could enhance the recovery of the European eel stocks.

Riparian zone denitrification affects nitrogen flux through a tidal freshwater river

Tidal freshwater zones (TFZ) of coastal rivers link terrestrial watersheds to the ocean and are characterized by large, regularly inundated riparian zones. We investigated the effect of riparian denitrification on nitrogen flux in the TFZ Newport River, North Carolina (U.S.A.) by developing an empirical model of denitrification and parameterizing it using measured denitrification rates, sediment oxidation-reduction potential dynamics, and riparian topography. Denitrification rates were measured monthly in laboratory-incubated sediment cores by using a membrane inlet mass spectrometer to assess net water-borne N2 flux from the cores. Annual average rates of denitrification in three intertidal riparian habitats, emergent marsh, mudflat, and hardwood forest, were 1864, 1956, and 2018 μg m−2 h−1, respectively. Laboratory experiments and in-situ monitoring revealed that the temporal lag between tidal inundation and reduced, denitrifying conditions was 4–5 h. Field measurements and remotely sensed data showed that the inundated surface area during high tide was three times greater than that at low tide. By combining data on denitrification, oxidation-reduction potential, and topography, the model predicted that the daily denitrification flux constituted 2–15% of the daily riverine nitrate flux during most of the year and >100% during low discharge periods. Current regional and global nitrogen budgets thus may overestimate nitrogen delivery to the ocean by not accounting for the TFZ denitrification.

Environmental turbulent mixing controls on air‐water gas exchange in marine and aquatic systems

Air‐water gas transfer influences CO2 and other climatically important trace gas fluxes on regional and global scales, yet the magnitude of the transfer is not well known. Widely used models of gas exchange rates are based on empirical relationships linked to wind speed, even though physical processes other than wind are known to play important roles. Here the first field investigations are described supporting a new mechanistic model based on surface water turbulence that predicts gas exchange for a range of aquatic and marine processes. Findings indicate that the gas transfer rate varies linearly with the turbulent dissipation rate to the power in a range of systems with different types of forcing ‐ in the coastal ocean, in a macro‐tidal river estuary, in a large tidal freshwater river, and in a model (i.e., artificial) ocean. These results have important implications for understanding carbon cycling.

FORAGING ECOLOGY OF BALD EAGLES IN A FRESHWATER TIDAL SYSTEM

We studied the summer foraging ecology of resident and migrant bald eagles (Haliaeetus leucocephalus) along the lower Hudson River, New York, from 1998 to 2001. In this area the Hudson is a freshwater tidal river with 1- to 2-m tidal ranges. Eagles foraged most often in the open channel (35%), where success was lowest (68% capture rate). When compared to landscape availability, eagles foraged in tidal mudflats devoid of aquatic vegetation more often than expected, and they avoided areas of deep water (>3 m). Eagles foraged more often during ebb tides with foraging activity peaking just before low tide. Eagles avoided areas of high human activity but also preferentially selected areas of low to moderate activity. Fish were the most important source of food and comprised 91% of prey identified. Over 50% of the observed prey captures consisted of 3 species: American eel (Anguilla rostrata), gizzard shad (Dorosoma cepedianum), and white catfish (Ictalurus catus). Our data indicate that unvegetated tidal mudflats that were isolated from intensive human activity provided the highest quality foraging habitat. Future loss of tidal mudflats through exotic plant invasions or shoreline development may limit eagle foraging opportunities and population growth.