Periods Of Low Discharge Research Articles

Centennial Changes of Salt Marsh Area in Coastal Georgia (USA) Related to Large-Scale Sediment Dynamics by River, Waves, and Tides

Marsh lateral expansion and retreat are often attributed to sediment availability, but a causal link is difficult to establish. To shed light on this problem, we analyzed changes in salt marsh area along the ~ 200-km-long Georgia coast (USA) from the 1850s to 2010s in relation to total suspended sediment (TSS) and to proxies for river sediment input and local sediment resuspension. Marsh area is characterized by large gains and losses (up to 200 m2/m/yr), but relatively small net change (-50 to 50 m2/m/yr or -0.1 to 0.1%/yr). This has resulted in a general loss of marsh area, except close to the mouths of major rivers, where there is net gain. Net expansion rates decreased in the Savannah Estuary but increased in the Altamaha Estuary from the 1850s–1930s period to the 1930s–2010s period, which are consistent with observed decreases and likely increases in sediment discharge in the two estuaries, respectively. To explain the spatial patterns in the 1930s–2010s marsh area change, we estimated TSS from satellite measurements (2003 to 2020). Along the northern part of the Georgia coast, net marsh gain is positively correlated to the average TSS within the estuarine region. However, this correlation breaks down in more southern areas (Cumberland Sound). Coast-wide, there is a better correlation between TSS associated with new input from the rivers, estimated as the TSS difference between high-discharge (Jan–Mar) and low-discharge (Sept–Nov) months. To identify the effect of wave resuspension in the nearshore, we consider the TSS difference between high-wave, low-discharge (Sept–Nov) and low-wave, low-discharge periods (Jun–Aug). Wave resuspension is relatively uniform along the coast and does not explain spatial patterns of marsh area change. Sediment input from the nearshore is likely contributing to the estuarine sediment budget in Georgia, but it is not sufficient to prevent marsh lateral retreat. To identify the role of tidal resuspension and advection, we consider differences in TSS between low and high tide. This differential is relatively constant along most of the coast, but it is much lower in the southern part of the coast, suggesting a lower tidal action in this region. Sediment resuspended by tides is likely originating from internal recycling (i.e., erosion) within the estuary, and thus does not contribute to marsh lateral expansion. The proposed approach to partition TSS is a general demonstration and could be applied to other coastal regions.

Occurrence and environmental risk assessment of pharmaceuticals in the Mondego river (Portugal)

In this case study pharmaceuticals were analysed in the Mondego river (Portugal) and their environmental risk assessed by means of risk quotients based on an extensive retrieval of ecotoxicological data for freshwater and saltwater species. The Mondego river crosses Coimbra, the most populated city in the Portuguese Centro Region hosting a complex of regional hospitals. Environmentally relevant and prioritised pharmaceuticals were investigated in this study and their potential hazards were evaluated by conducting a separate risk assessment for the freshwater and estuary parts of the examined river section.A target analysis approach with method detection limits down to 0.01 ng L−1 was used to determine pharmaceuticals. Twenty-one prioritised target analytes out of seven therapeutical classes (antibiotics, iodinated X-ray contrast media (ICM), analgesics, lipid reducers, antiepileptics, anticonvulsants, beta-blockers) were investigated by applying ultra-high pressure liquid chromatography coupled to a triple quadrupole mass spectrometer equipped with an electrospray ionisation source.The relative pattern of pharmaceuticals along the middle to the lower Mondego showed a quite uniform picture while an approximately 40fold increase of absolute concentrations was observed downstream of the wastewater treatment plant (WWTP) discharge of Coimbra. The most frequently measured substance groups were the ICM, represented by the non-ionic ICM iopromide (βmin: 3.03 ng L−1 - βmax: 2,810 ng L−1). Environmentally more critical substances such as carbamazepine, diclofenac, and bezafibrate, with concentrations up to and 52.6 ng L−1, 59.8 ng L−1, and 10.2 ng L−1 respectively, may potentially affect aquatic wildlife. Carbamazepine revealed elevated risk quotients (RQs >1) along the middle and lower Mondego with a maximum RQ of 53 downstream of Coimbra. Especially for saltwater species, carbamazepine and clarithromycin pose high potential risks.Especially in periods of low water discharge of the Mondego river, other pharmaceuticals as diclofenac and bezafibrate may pose additional risks downstream of the WWTP.

Respiration and CO2 evasion dynamics in moving streambeds as a response to flow regimes

Streams and rivers are subject to discharge fluctuations that may be natural or man-made, which influence biogeochemical processes and the release of CO2 to the atmosphere. Fluctuations in discharge are also associated with changing streamwater velocity that directly affects the movement of bedforms in sandy streambeds. However, bedform movement is rarely considered when studying effects of flow dynamics on biogeochemical processes. We used planar optodes during moving-bed flume experiments to quantify the effects of different flow regimes on aerobic respiration dynamics and the subsequent release of CO2. The flow regimes included constant flow (C), sinusoidal flow (S), and single peak flow events (F). Flow regimes substantially influence aerobic respiration in the streambed and, hence, the distribution of O2 and CO2 in the streambed. Hyporheic exchange flux (HEF), bedform celerity, and respiration rates increased non-linearly with streamwater velocity. It was found that HEF and CO2 evasion were highest under dynamic flow regimes S and F. However, respiration rates, the total O2 consumed, and the total CO2 produced were highest under flow regime C. This is due to low reaction rates once the streambed became stationary in the low discharge periods of flow regimes S and F. Thus, the distribution of celerities controls total respiration. Our results highlight that substantial aerobic respiration occurs even in slow-moving streambeds and that it is important to consider bedform movement when studying biogeochemical reactions in the hyporheic zone or predicting greenhouse gas emissions from catchments.

Seasonal and Interannual Variability in the Distribution and Removal of Terrigenous Dissolved Organic Carbon in the Amazon River Plume

AbstractThe Amazon River is a large source of terrigenous dissolved organic carbon (tDOC) to the Atlantic Ocean. The fate of this tDOC in the ocean remains unclear despite its importance to the global carbon cycle. Here, we used two decades of satellite ocean color to describe variability in tDOC in the Amazon River plume. Our analyses showed that tDOC distribution has a distinct seasonal pattern, reaching northwest toward the Caribbean during high discharge periods, and moving eastward entrained in the North Brazil Current retroflection during low discharge periods. Elevated tDOC content extended beyond the shelfbreak in all months of the year, suggesting that cross‐shelf carbon transport occurs year‐round. Maximum variability was found at the plume core, where seasonality accounted for 40% of the total variance, while interannual variability accounted for 15% of the variance. Our results revealed a seasonal pattern in tDOC removal over the shelf with increased consumption in May when river discharge is high. Anomalies in tDOC removal over the shelf with respect to the seasonal cycle were significantly correlated with anomalies in tDOC concentration offshore of the shelfbreak with a lag of 30–40 days, so that anomalously high inshore tDOC removal was associated with anomalously low tDOC content offshore. This suggests that variability in the offshore transport of tDOC in the Amazon River plume is modulated by interannual changes in tDOC removal over the shelf.

Density gradient effect on circulation patterns and mixing processes: Observations at the convergence front of a salt wedge, microtidal estuary

The convergence front in estuaries is a crucial area influenced by dynamic mixing processes which vary from one tidal cycle to another, with density gradients playing a key role in shaping circulation patterns. This significance is particularly pronounced in salt wedge estuaries, where discharge holds greater influence than tides. Despite this, there is a limited amount of research based on high-resolution in-situ data to analyze circulation and mixing processes affected by density gradients in the convergence front of a salt wedge estuary. In this study, continuous measurements of current velocities and CTD profiles were conducted over three tidal cycles to analyze the intratidal variability of circulation patterns (u, v, w) and mixing parameters in the convergence front. Stratification parameter (ηs) and Richardson numbers (RL and Ri) were calculated to analyze the mixing/stratification variability within a tidal cycle. The observations revealed a persistent Subsurface Velocity Maximum (SVM) in the longitudinal component, coinciding with density gradient increases > 4 kg m−3 along the water column. Similarly, lateral circulation patterns were influenced by those increases in density gradient, resulting in eastward-directed currents and the formation of two circulations cells. As expected in a salt wedge estuary, stratification occurs during flood and High Water (HW), when the density gradient increases due to intrusion of the salt wedge. Notably, in the Magdalena River estuary, when the discharge is below 3800 m3 s−1, the salt wedge remains consistently present irrespective of the tidal phase, resulting in a permanently stratified water column with ηs values between 1.60 – 1.95. It is expected that the steady presence of stratified conditions during periods of low discharge leads to an increased accumulation of sediments. These findings provide valuable quantitative insights into the complex interplay of density gradients, circulation patterns, and stratification dynamics in salt wedge estuaries.

Effects of a forested state park on stream nutrient concentrations in an agriculturally dominated watershed in the U.S. Midwest

AbstractAgricultural land cover in the U.S. Midwest is a major source of nutrient pollution that has led to impairment of stream water quality. This study examines the impact of a forested state park on nutrient concentrations within an agriculturally dominated watershed. Water samples were collected over a 2‐year study period from eight stream sampling sites along four creeks and processed for total nitrogen (TN), nitrate (), total phosphorus (TP), and orthophosphate (). Hydrology, channel morphology, and remotely sensed land cover and vegetation data were also collected and analyzed within the study area. Results indicate that water quality responses to a forested state park vary between TN, , TP, and , and water quality variables are uniquely influenced by watershed and stream characteristics. The greatest water quality benefits most frequently occurred within the two smallest study streams with the greatest residence times and proportion of watershed areas within the forested state park. Overall, the greatest improvements to water quality occurred during periods of low stream discharge and when riparian vegetation was greenest. The results of this study suggest that conservation of forested areas within agriculturally dominated watersheds can provide water quality improvements in the U.S. Midwest. Targeting watersheds that drain small streams with long residence times for conservation may be most beneficial to improving water quality.

Spatial and seasonal variability of dissolved metals in a monsoonal estuarine environment

Distribution of metals in estuarine surface water reflects their respective sources and abundances in the fresh- and marine- water end-members, mixing processes and ability to take part in various geochemical reactions prevailing in the transitional zone. However, these elements tend to show complex geochemical behaviour, which varies both within and among different estuaries. This makes it difficult to delineate a universal distribution pattern. In this study, the behaviour of multiple dissolved metals (Sr, Ba, Fe, Mn, Mo, V, Co, Cr, Cu, Ni, Zn, U and Pb) have been evaluated in a tropical monsoonal estuary located on the east coast of India during summer (May, 2018), monsoon (September, 2018) and winter (February, 2019) season. Concentration of Sr, Mo, V, Cu and U increases towards the mouth of the estuary, whereas, Ba, Fe, Mn, Pb and Cr concentration shows a declining trend along increasing salinity gradient. All the elements show non-conservative behaviour in the estuary during at least one season within the studied period. Rapid removal of Fe and Mn takes place in the low salinity region, whereas, Mo, V, Pb and U get removed in the low- to mid- salinity zone of the estuary as a result of salinity-induced flocculation. A pronounced mid-estuarine maxima is exhibited by Ni and Zn. Barium shows peak concentrations in the low salinity region. Spatial variation in the occurrence of Ba maxima along the salinity gradient exists between the low and high discharge periods. Seasonally, highest concentration of Sr, Mo, V and U are found in the summer, followed by winter and monsoon. On the other hand, Ba, Fe, Mn, Co, Cr, Cu, Ni and Zn exhibit the highest concentrations during the monsoon period as a result of increased inputs from fluvial sources. The intrusion of seawater during the dry seasons and dilution during monsoon, which shows the highest freshwater discharge, are responsible for the seasonal variations. Thus, this study highlights the variability in distribution of elements as well as the differences in the behaviour of individual elements under contrasting environmental conditions within an estuary.

River plastic transport affected by tidal dynamics

Abstract. Plastic is an emerging pollutant, and the quantities in rivers and oceans are expected to increase. Rivers are assumed to transport land-based plastic into the ocean, and the fluvial and marine transport processes have been relatively well studied to date. However, the processes controlling the transport in tidal rivers and estuaries, the interface between fluvial and marine systems, remain largely unresolved. For this reason, current estimates of riverine plastic pollution and export into the ocean remain highly uncertain. Hydrodynamics in tidal rivers and estuaries are influenced by tides and freshwater discharge. As a consequence, flow velocity direction and magnitude can change diurnally. In turn, this impacts the transport dynamics of solutes and pollutants, including plastics. Plastic transport dynamics in tidal rivers and estuaries remain understudied, yet the available observations suggest that plastics can be retained here for long time periods, especially during periods of low net discharge. Additional factors such as riparian vegetation and riverbank characteristics, in combination with bi-directional flows and varying water levels, can lead to an even higher likelihood of long-term retention. Here, we provide a first observation-based estimate of net plastic transport on a daily timescale in tidal rivers. For this purpose, we developed a simple Eulerian approach using sub-hourly observations of plastic transport and discharge during full tidal cycles. We applied our method to the highly polluted Saigon River, Vietnam, throughout six full tidal cycles in May 2022. We show that the net plastic transport is about 20 %–33 % of the total plastic transport. We found that plastic transport and river discharge are positively and significantly correlated (Pearson's R2 = 0.76). The net transport of plastic is higher than the net discharge (20 %–33 % and 16 %, respectively), suggesting that plastic transport is governed by factors other than water flow. Such factors include wind, varying plastic concentrations in the water, and entrapment of plastics downstream of the measurement site. The plastic net transport rates alternate between positive (seaward) net transport and negative (landward) net transport as a result of the diurnal inequality in the tidal cycles. We found that soft and neutrally buoyant items had considerably lower net transport rates than rigid and highly buoyant items (10 %–16 % vs. 30 %–38 %), suggesting that transport dynamics strongly depend on item characteristics. Our results demonstrate the crucial role of tidal dynamics and bi-directional flows in plastic transport dynamics. With this paper we emphasize the importance of understanding fundamental transport dynamics in tidal rivers and estuaries to ultimately reduce the uncertainties of plastic emission estimates into the ocean.

Automated FerryBox monitoring reveals the first recorded river induced crude oil seep transport to the Strait of Magellan in southern Patagonia

This study presents the first documented occurrence of a natural crude oil seep plume associated with river discharge along the Strait of Magellan in southern Patagonia in modern times. Between September and December 2022, hydrocarbon signals were detected using a crude oil sensor integrated into a FerryBox system that traversed the Strait of Magellan and several channels of southern Patagonia, covering approximately 510 km. The highest levels of crude oil signals were observed in the mid-basin of the Strait of Magellan. These signals exhibited a strong negative correlation with sea surface salinity, coinciding with the water discharge from the San Juan River. Notably, during periods of high river discharge, typically exceeding 15 m3 s−1, a distinct crude oil plume was detected moving towards the Magellan Strait. Conversely, when river discharge fell below this threshold, no noticeable crude oil signal was observed. As river discharge decreased and winds intensified during the austral summer, the crude oil signal gradually dissipated. This observation suggests that the dispersion of crude oil becomes limited during periods of low river discharge, as buoyant currents remain confined close to the coast. Historical records indicate that this seep has been releasing hydrocarbons into the Strait of Magellan for at least the past 120 years, implying a long history of chronic crude oil input into this relatively isolated region of the world. This finding shows the potential contribution to the understanding of marine ecosystems dynamics and potential pollutants in poorly studied regions through the use of automated monitoring FerryBox system, enabling both spatial and temporal high-resolution surveys.

Citizen scientists filling knowledge gaps of phosphate pollution dynamics in rural areas

In situ monitoring is fundamental to manage eutrophication in rivers and streams. However, in recent decades, the frequency and spatial coverage of regulatory monitoring have often been reduced due to funding and infrastructure limitations. This reduction has made it impossible to provide adequate coverage for most water bodies. In this study, trained citizen scientists filled spatial and temporal gaps in agency monitoring across a major catchment in rural England. By integrating data from citizen scientists, regulatory agencies, and the local water company, it was possible to demonstrate the opportunities for hypothesis-based citizen scientist monitoring to identify continuous and event-driven sources of phosphate pollution. Local citizen scientists effectively covered important spatial gaps, investigating river conditions both upstream and downstream of suspected pollution point sources, improving the identification of their temporal dynamics. When combined with long-term monitoring data from regulatory agencies, it became possible to identify areas within the catchment that exhibited increased phosphate concentrations during periods of low river discharge (summer). Inter-annual trends and anomaly detection suggested that continuous pollution sources dominated over event-driven sources in many sub-basins, allowing for the prioritisation of mitigation actions. This study highlights the opportunity for citizen scientists to fill gaps in regulatory monitoring efforts and contribute to the improved management of eutrophication in rural catchments.

Microplastic patterns in riverine waters and leaf litter: Leaf bag technique to investigate the microplastic accumulation trends in lotic ecosystems

Microplastics (MPs) are one of the major ecological concerns of the last years and despite the increasing interest and the rise of many studies regarding freshwater habitats, many aspects about distribution patterns, transport pathways and impacts of MPs in those systems need to be investigated. The present study characterizes the temporal trends of MP concentrations in waters of a riverine stretch of the northeastern Italy, subject to flow rate variations and investigates the MP accumulations patterns in the leaf litter, simulated in situ via leaf bag technique. MP concentrations in the water were significantly and negatively correlated to the flow rate regimes, with higher concentrations observed during low discharge periods. MPs accumulation in leaf bags agreed with trends observed in the water and the presence of wastewater discharge points positively affects the levels of MP contaminations within the leaf bags. These findings seem to suggest that the maintenance of a hydrological regime at relatively high levels in the investigated system could allow to maintain the self-purifying riverine processes and the disposal of microplastics like any other polluting substance. The use of leaf bag technique for the purpose to investigate MP accumulation trends on field provided useful information, is easy to modulate in terms of time periods and allow to record the evolution of the MP patterns also in relation to high flow rate episodes. Our results suggest that the method can be employed in new a perspective, to improve the knowledge about one of the major threats of the Anthropocene.

Drought-Induced Salinity Intrusion Affects Nitrogen Removal in a Deltaic Ecosystem (Po River Delta, Northern Italy)

In the summer of 2022, the Po River Delta (Northern Italy), a eutrophication hotspot, was severely affected by high temperatures, exceptional lack of rainfall and saline water intrusion. The effect of saline intrusion on benthic nitrogen dynamics, and in particular the N removal capacity, was investigated during extreme drought conditions. Laboratory incubations of intact sediment cores were used to determine denitrification and DNRA rates at three sites along a salinity gradient in the Po di Goro, an arm of the Po River Delta. Denitrification was found to be the main process responsible for nitrate reduction in freshwater and slightly saline sites, whereas DNRA predominated in the most saline site, highlighting a switch in N cycling between removal and recycling. These results provide evidence that salinity is a key factor in regulating benthic N metabolism in transitional environments. In a climate change scenario, salinity intrusion, resulting from long periods of low river discharge, may become an unrecognized driver of coastal eutrophication by promoting the dissimilatory nitrate reduction to ammonium and N recycling of bioactive nitrogen within the ecosystem, rather than its permanent removal by denitrification.

Sulfate enrichment in estuaries of the northwestern Gulf of Mexico: The potential effect of sulfide oxidation on carbonate chemistry under a changing climate

AbstractWater quality parameters from 2000 to 2020 were used to identify the spatial and temporal sulfate variations in estuaries of the northwestern Gulf of Mexico. Sulfate enrichment relative to conservative mixing was found to be associated with a low river discharge period from 2012 to 2014 in all estuaries. Based on reaction stoichiometry, sedimentary sulfide oxidation holds significant potential for reducing the alkalinity in estuarine waters. However, during this extreme drought, alkalinity enrichment was also occasionally observed in some of the southern estuaries along with sulfate enrichment, and when alkalinity depletion occurred, the magnitude of depletion was usually much less than what would be expected based on sulfide oxidation alone. This discrepancy can be partially explained by carbonate dissolution and other proton removal pathways (e.g., Fe‐oxide dissolution), and by the uncertainties in the model used to estimate alkalinity enrichment/depletion. Under a changing climate, the close coupling between river discharge variation and estuarine sulfate dynamics will significantly impact estuarine carbonate chemistry.

The benthic food web connects the estuarine habitat mosaic to adjacent ecosystems

Energy flows from land to sea and between pelagic and benthic environments have the potential to increase the connectivity between estuaries and adjacent ecosystems as well as among estuarine habitats. To identify such energy flows and the main trophic pathways of energy transfer in the Minho River estuary, we investigated the spatial and temporal fluctuations of carbon and nitrogen stable isotope ratios in benthic (and their potential food sources) and epibenthic consumers. Sampling was conducted along the estuarine salinity gradient from winter to summer of 2011. We found that the carbon (δ13C = 13C/12C) and nitrogen (δ15N = 15N/14N) stable isotope ratios of the most abundant benthic and epibenthic consumers varied along the salinity gradient. The δ13C values increased seaward, whereas the opposite pattern was found for the δ15N, especially during the summer. The stable isotope ratios revealed two trophic pathways in the Minho estuary food web. The first pathway is supported by phytoplankton and represented by filter feeders such as zooplankton and some deposit feeders, particularly amphipods and polychaetes. The second pathway is supported by detritus and composed essentially of deposit feeders, which by being consumed, allow detritus to be incorporated into higher trophic levels. Spatial and temporal feeding variations in the estuarine benthic food web are driven by hydrology and proximity to adjacent ecosystems (terrestrial, marine). During high river discharge periods, the δ13CPOC (ca. -28‰) and C:NPOM (>10) values suggested an increase of terrestrial-derived OM to the particulate OM pool, which was then used by suspension feeders. During low river discharge periods, marine intrusion increased upriver, which was reflected in benthic consumers' 13C-enriched stable isotope values. No relationship was found between food quality (phytoplankton vs. detritus) and food chain length because the lowest and highest values were associated with freshwater and saltmarsh areas, respectively, both dominated by the detrital pathway. This study demonstrates that benthic consumers enhance the connectivity between estuaries and its adjacent ecosystems by utilizing subsidies of terrestrial and marine origin and that benthic-pelagic coupling is an important energy transfer mechanism to the benthic food web.

COMPARISON ON STUDY OF LITHIUM ION AND LEAD ACID CHARGING AND DISCHARGING CHARACTERISTICS

A lithium-ion or Li-ion battery is a type of rechargeable battery which uses the reversible reduction of lithium ions to store energy. It is the predominant battery type used in portable consumer electronics and electric vehicles. Energy density: 250–693 Wh/L (0.90–2.49 MJ/L) Specific energy: 100–265 Wh/kg (0.360–0.954 MJ/kg) Charge/discharge efficiency: 80–90% Cycle durability: 400–1,200 cycles Self-discharge rate: 0.35% to 2.5% per month depending on state of charge Specific power: c. 250–340 W/kg Lithium batteries, as opposed to alkaline, are capable of giving off a strong energy surge after a long period of low discharge. This makes them ideal for fire alarms. Alkaline batteries provide good, long-term power, but they lose strength over time. On the other side , the Lead acid battery are inexpensive compared to newer technologies, lead–acid batteries are widely used even when surge current is not important and other designs could provide higher energy densities. Energy density: 80–90 Wh/L Nominal cell voltage: 2.1 V Self-discharge rate: 3%–20%/month Specific energy: 35–40 Wh/kg Charge temperature interval: Min. −35°C, max. 45°C Charge/discharge efficiency: 50%–95% Cycle durability: <350 cycle In order to observe the characteristic nature of Lead acid & Lithium ion battery , a review is made on the subject using Matlab Simulink and Simscape.

Impact of an exceptional winter flood on benthic oxygen and nutrient fluxes in a temperate macrotidal estuary: Potential consequences on summer deoxygenation

Despite 20 years of control on eutrophication, episodes of summer hypoxia still occur in the Loire estuary, impacting water quality and posing a key scientific and management challenge. This work aimed to quantify the contribution of the benthic compartment to hypoxia in the Loire estuary by direct measurement of water–sediment fluxes and an in-depth understanding of the seasonal variations of oxides and phosphorus stocks. During the summer’s low-discharge period, results show that the iron oxide-rich deposit is stable under hypoxic conditions, limiting the release of dissolved phosphorus into the overlying water column. The high nitrate content of the water column appears to be an important oxidizer of iron during hypoxic periods, limiting dissolved phosphorus leakage and aggravation of hypoxia. During the exceptional winter flood, significant sediment erosion associated with bubbling phenomena (attributed to methane efflux) created severe fractures in the sediment and stimulated water–sediment exchange. During the following months, these fractures were progressively filled, which decreased the intensity of benthic fluxes. However, due to the high residence time in the water during the summer period, a simple model demonstrated that benthic contributions were sufficient to directly (by direct oxygen consumption) or indirectly (by promoting ammonia oxidation) affect the oxygen stock in the water column during the low-discharge period. Our study demonstrates the importance of the benthic compartment in the occurrence of hypoxia and the obvious lack of knowledge to illustrate and model the biogeochemical functioning of the estuary.

Synoptic and Seasonal Variability of Small River Plumes in the Northeastern Part of the Black Sea

Small river plumes are typical features at many coastal regions in the World Ocean. These water masses have relatively small areas and volumes; however, due to their energetic dynamics localized in a thin surface layer, they strongly affect coastal circulation, water quality, and ocean-atmosphere interaction. In this study, we investigate external factors, which govern synoptic and seasonal variability of small river plumes, and, therefore, affect land-ocean fluxes of fluvial water and biogeochemically important material. We use numerical modeling to simulate small river plumes at the northeastern part of the Black Sea. We describe the response time of small river plumes to changes in river discharge and wind forcing conditions, which determines variability of river plumes at different time scales. We reveal that the influence of river plumes on coastal processes depends not only on total annual river discharge volume, but also on temporal distribution of high-discharge and low-discharge periods. Seasonal and synoptic features of local atmospheric circulation could strongly modify the relation between river plume characteristics and river discharge rate. The results obtained in this study are important for better assessment of delivery and fate of river-borne suspended and dissolved matter, as well as floating litter in coastal areas.

Hypoxia is common in temperate headwaters and driven by hydrological extremes

Hypoxia, or dissolved oxygen (DO) at low enough levels to impair organisms, is a particularly useful indicator of the health of freshwater ecosystems. However, due to limited sampling in headwater networks, the degree, distribution, and timing of hypoxia events are not known across the vast majority of most river networks. We thus sought to clarify the extent of hypoxia in headwater networks through three years of instrumentation of 78 sites across eight temperate, agricultural watersheds. We observed broadly distributed hypoxia, occurring 4 % of the time across 51 of the 78 sites over 20 months. The hypoxia was driven by three mechanisms: storm events, drying, and rewetting, with drying as the most common driver of hypoxia (55 % of all hypoxic event types). Drying induced hypoxia was most severe in smaller streams (Strahler orders ≤ 3), whereas storm events preferentially induced hypoxia in the larger streams (Strahler orders 3–5). A large diversity in DO trajectories towards hypoxia depended on hydrologic event type, with subsequent expected differences in mortality profiles of a sensitive species. Predictive models showed the most vulnerable sites to hypoxia were small streams with low slope, particularly during hot, low discharge periods. Despite variation among hypoxic events, there was remarkable similarity in the rate of DO drawdown during hypoxia events (ca. 1 mg O2 L−1 d−1). This drawdown similarity may be a useful rule-of-thumb for managers, and we hypothesize that it is either a signal of increasing lateral inflow of low DO water or a signal of increasing downstream oxygen demand. Overall, we posit that hypoxia is likely a common feature of most headwater networks that often goes undetected. Headwater hypoxia may become more common under increasingly dry conditions associated with climate and water resource management changes, with important implications for biological communities and biogeochemical processes.

Synoptic observations of sediment transport and exchange mechanisms in the turbid Ems Estuary: the EDoM campaign

Abstract. An extensive field campaign, the Ems-Dollard Measurements (EDoM), was executed in the Ems Estuary, bordering the Netherlands and Germany, aimed at better understanding the mechanisms that drive the exchange of water and sediments between a relatively exposed outer estuary and a hyper-turbid tidal river. More specifically, the reasons for the large up-estuary sediment accumulation rates and the role of the tidal river on the turbidity in the outer estuary were insufficiently understood. The campaign was designed to unravel the hydrodynamic and sedimentary exchange mechanisms, comprising two hydrographic surveys during contrasting environmental conditions using eight concurrently operating ships and 10 moorings measuring for at least one spring–neap tidal cycle. All survey locations were equipped with sensors measuring flow velocity, salinity, and turbidity (and with stationary ship surveys taking water samples), while some of the survey ships also measured turbulence and sediment settling properties. These observations have provided important new insights into horizontal sediment fluxes and density-driven exchange flows, both laterally and longitudinally. An integral analysis of these observations suggests that large-scale residual transport is surprisingly similar during periods of high and low discharge, with higher river discharge resulting in both higher seaward-directed fluxes near the surface and landward-directed fluxes near the bed. Sediment exchange seems to be strongly influenced by a previously undocumented lateral circulation cell driving residual transport. Vertical density-driven flows in the outer estuary are influenced by variations in river discharge, with a near-bed landward flow being most pronounced in the days following a period with elevated river discharge. The study site is more turbid during winter conditions, when the estuarine turbidity maximum (ETM) is pushed seaward by river flow, resulting in a more pronounced impact of suspended sediments on hydrodynamics. All data collected during the EDoM campaign, but also standard monitoring data (waves, water levels, discharge, turbidity, and salinity) collected by Dutch and German authorities are made publicly available at 4TU Centre for Research Data (https://doi.org/10.4121/c.6056564.v3; van Maren et al., 2022).

Causes of continuous and short-term hypoxia in rivers entering the sea: a case of Minjiang River in Fujian Province

Abstract In the last 10 years, the Minjiang River, which is the longest river in the Fujian Province in Southeast China, has been facing a downward trend of dissolved oxygen (DO) and a frequent occurrence of hypoxia. In this study, the development of the continuous and short-term presence of low DO was investigated by using the water age concept and average DO consumption concept based on a three-dimensional Environmental Fluid Dynamics Code in the Minjiang River. The results revealed that the spatial distribution of DO was affected by temperature, runoff, pollution emission, tidal advection, and hypoxic water discharge from the reservoir bottom. The continuous low DO in the water of the North Channel occurred frequently when the enough pollutants were aerobically decomposed faster than the rate of oxygen reaeration during the high temperature and low river discharge period. In addition, the water age and reaeration time decreased with a rapid increase in the water flow from the Shuikou dam when the reservoir capacity was released via drainage. The results of this study provide scientific insights on the mechanism involved in the occurrence of hypoxia and suggest countermeasures for addressing hypoxic problems in estuaries.