Sediment Resuspension Research Articles

Geochemistry and contamination of sediments and water in rivers affected by the rupture of tailings dams (Brumadinho, Brazil)

Understanding the geochemistry and contamination of rivers affected by the rupture of tailings dams is paramount to emergency water resources management. This needs the long-term monitoring of tailings, sediments and water along the river, and the understanding of how the tailings interact physically and geochemically with the natural sediments and flowing water. An effort of this kind is uncommon because of its cost and opportunity. The purpose of this study was to help filling this gap using the rupture of B1 tailings dam (Brumadinho, Minas Gerais, Brazil, 25 January 2019) as example. To accomplish the objective, 31 parameters of tailings, sediment and water were monitored weekly for three years after the event, spanning impacted and non-impacted sectors of Paraopeba River, including a dam reservoir that is attenuating the tailings propagation downstream. Using hierarchical clustering analysis, the parameters were assembled into groups. Associations between parameters within the groups pointed to processes and drivers controlling the concentrations of Fe, Mn, Al, As, Pb and P in water. The results suggested reductive dissolution of Pb- and Mn-bearing minerals in the impacted and dam reservoir sectors as key process. In the rainy period, dissolved Mn concentrations in the impacted sector raised exponentially with temperature increase; in the dam reservoir, they showed episodic peaks triggered by thermal stratification-related pH decreases. In general, total concentrations > dissolved concentrations indicating predominant particle transport. Besides, concentrations were higher for Fe, Al and Mn, in the rainy season, and in the impacted sector. In the dry season, stream flow-controlled sediment resuspension triggered desorption of Fe and Al from sand grains raising their dissolved concentrations. In the rainy season, a pH declining trend probably related with erosion of acid soils in the watershed caused gradual increases of Fe, Al and P, desorbed from clays and silts. The main conclusion is that the B1 dam rupture left a strong fingerprint in the concentrations of Fe and Mn in the river, especially in the rainy season, which lasts and keeps the use of Paraopeba River suspended.

Storm sediment contribution to salt marsh accretion and expansion

Salt marshes are ecosystems with significant economic and environmental value. However, the accelerating rate of sea-level rise is a significant threat to these ecosystems. Storms significantly contribute to the sediment budget of salt marshes, playing a critical role in salt marsh survival to sea-level rise. There are, however, uncertainties on the extent to which storms contribute sediments to different areas of marsh platforms (e.g., outer marsh vs marsh interior) and on the sediment sources that storms draw on (e.g., offshore vs nearshore). This study uses field analyses from an eight-month field campaign in the Ribble Estuary, North-West England, to understand storms' influence on the sediment supply to different marsh areas and whether storms can deliver new material onto the salt marsh platform which would otherwise not be sourced in fair-weather conditions. Field data from sediment traps indicate that storm activity caused an increase in inorganic sediment supply to the whole salt marsh platform, especially benefitting the marsh interior. Geochemistry and particle size distribution analysis indicate that the majority of the sediment supplied to the salt marsh platform during the stormy periods was generated by an increase in erosion and resuspension of mudflat and tidal creek sediments, while only a minimal contribution was given by the sediments transported from outside the intertidal system. This suggests that, in the long term, storms will promote salt marsh vertical accretion but might simultaneously reduce the overall larger-scale sediment availability with implications for the marsh lateral retreat.

Sediment resuspension enhances nutrient exchange in intertidal mudflats

Intertidal coastal sediments are important centers for nutrient transformation, regeneration, and storage. Sediment resuspension, due to wave action or tidal currents, can induce nutrient release to the water column and fuel primary production. Storms and extreme weather events are expected to increase due to climate change in coastal areas, but little is known about their effect on nutrient release from coastal sediments. We have conducted in-situ sediment resuspension experiments, in which erosion was simulated by a stepwise increase in current velocities, while measuring nutrient uptake or release in field flumes positioned on intertidal areas of a tidal bay (Eastern Scheldt) and an estuary (Western Scheldt). In both systems, the water column concentration of ammonium (NH4+) and nitrite (NO2−) increased predictably with greater erosion as estimated from pore water dilution and erosion depth. In contrast, the phosphate (PO43−) dynamics were different between systems, and those of nitrate (NO3−) were small and variable. Notably, sediment resuspension caused a decrease in the overlying water PO43− concentration in the tidal bay, while an increase was observed in the estuarine sediments. Our observations showed that the concentration of PO43− in the water column was more intensely affected by resuspension than that of NH4+ and NO2−. The present study highlights the differential effect of sediment resuspension on nutrient exchange in two contrasting tidal coastal environments.

Sediment resuspension and transport processes during dense water cascading events along the continental margin of the southern Adriatic Sea (Mediterranean Sea)

The near-bottom nepheloid layer in the western margin of the Southern Adriatic Sea was monitored for 8-years by measurements acquired at two mooring sites. The two moorings, equipped with CTD probes and ADCPs, are located in the Bari Canyon and in an open slope sector along the Southern Adriatic Margin. These regions are of interest because affected by episodic dense shelf water cascading events whose dynamic has direct implications on deepwater morpho-dynamic, biogeochemical cycles and trophic networks. In this work, the sedimentation flux and its interdecadal dynamic is analysed examining in detail the sedimentary processes triggered by dense water flow through the analysis of the echo records of ADCPs.The integration of hydrodynamic, turbidity and particle grain-size data provided estimates of the sediment flux, separating phases when the flow actively erodes the seabed from phases when particles are transported to the mooring location through density flows.The frequency and velocity of dense-water cascading currents vary in time and space reflecting the capacity of sediment transport. Data analysis demonstrated that the hydrodynamic event that mostly accounts for sediment transfer to the deep basin is represented by current pulses induced by the passage of dense waters. The average annual sediment flux has been quantified and the Bari canyon shows transport more than five time larger than in the open slope sector, confirming that the canyon is the dominant pathway of sediment transfer to the deep basin. In contrast, in the open slope, albeit a minor lateral sediment advection, is impacted by currents that are able to trigger intense resuspension of seabed sediments, which can contribute over 80% of the total solid load.This study allows unravelling the role of cascading in the sediment resuspension and transport processes and is essential to support deciphering the sedimentary records in the study area. The long temporal extent of the dataset used for quantification provides a reliable contribution to the Quaternary sediment budget determination.

Monsoon-induced response of algal chlorophyll to trophic state, light availability, and morphometry in 293 temperate reservoirs

Eutrophication management is one of the greatest environmental challenges for lacustrine systems worldwide. The empirically predicted models between algal chlorophyll (CHL-a) and total phosphorus (TP) provide a basis for managing eutrophication in lakes and reservoirs, but other environmental factors influencing the empirical relations must be considered. Here, we tested the impacts of morphological and chemical variables, as well as the effect of the Asian monsoon, on the functional response of CHL-a to TP using two-year data of 293 agricultural reservoirs. This study was based on the approaches of empirical models (linear and sigmoidal), CHL-a:TP ratio, and trophic state index deviation (TSID). Algal CHL-a exhibited a strong log-linear relation with TP on the basis of 2-year average data (R2 = 0.69, p < 0.001), whereas it had a more sigmoidal relation in terms of monsoon-seasonal averages (R2 = 0.52, p < 0.001). The linear segment of the CHL-a–TP relation aligned with the gradient of TP (10 mg/L < TP < 100 mg/L) from mesotrophic to eutrophic conditions. The transfer efficiency of TP to CHL-a based on the 2-year mean CHL-a:TP was high (0.6 <) across all assessed agricultural systems. CHL-a:TP showed insignificant correlations with reservoir morphological variations, but it decreased (<0.5) in eutrophic and hypereutrophic systems during the monsoon season (July–August). Because TP and total suspended solids (TSS) have become increasingly abundant, light conditions become insufficient for algal growth during and after the monsoon season. Light-limited conditions become more prevalent in hypereutrophic systems with shallow depth and high dynamic sediment ratio (DSR) because of the intense rainfall inputs and wind-induced sediment resuspension of the post-monsoon season. TSID reflected the degree of phosphorus limitation and the reduction in underwater light corresponding to changes in reservoir water chemistry (ionic content, TSS, and TN:TP ratio), trophic state gradient, and morphological metrics (mainly mean depth and DSR). Our findings suggest that monsoon-induced changes in water chemistry and light attenuation, which are also associated with anthropogenic pollutant runoffs and reservoir morphology, are critical factors that influence the functional response of algal CHL-a to TP in temperate reservoirs. Modeling and assessing eutrophication should therefore take into account monsoon seasonality along with individual morphological features further.

Two Kinds of Waves Causing the Resuspension of Deep-Sea Sediments: Excitation and Internal Solitary Waves

Two Kinds of Waves Causing the Resuspension of Deep-Sea Sediments: Excitation and Internal Solitary Waves

Saltwater intrusion regulates the distribution and partitioning of heavy metals in water in a dynamic estuary, South China

The mixing processes of fresh-salt water in estuarine and coastal regions have a substantial impact on the characteristics of heavy metals. A study was conducted in the Pearl River Estuary (PRE), located in South China, to examine the distribution and partitioning of heavy metals and the factors that influence their presence. Results showed that the hydrodynamic force, caused by the landward intrusion of the salt wedge, was the major contributor to the aggregation of heavy metals in the northern and western PRE. Conversely, metals were diffused seaward at lower concentrations along the plume flow in surface water. The study found that some metals, including Fe, Mn, Zn and Pb, were significantly higher in surface water than in bottom water in eastern waters, but the reverse was true in the southern offshore area, where limited mixing hindered the vertical transfer of metals in the water column. The partitioning coefficients (KD) of metals varied, with Fe exhibiting the highest KD (1038 ± 1093 L/g), followed by Zn (579 ± 482 L/g) and Mn (216 ± 224). The highest KD values of metals in surface water were observed in the west coast, while the highest KD in bottom water was found in eastern areas. Furthermore, re-suspension of sediment and the mixing of seawater and freshwater offshore, caused by seawater intrusion, resulted in the partitioning of Cu, Ni and Zn towards particulate phases in offshore waters. This study provides valuable insights into the migration and transformation of heavy metals in dynamic estuaries influenced by the interaction of freshwater and saltwater and highlights the importance of continued research in this field.

Sediment plume simulation from bottom-trawled fishery and deposition effects on rhodoliths and deep-water corals from Campos basin, Brazil

Extensive rhodoliths and deep-water coral has been described along the continental shelf and slope of the Campos Basin, but such environments can be highly vulnerable to the bottom trawling fishing. This study applied the MOHID software to simulate the sediment resuspension during the bottom trawling operation and to evaluate possible impacts of sediment deposition onto rhodoliths and deep-water coral. The sediment transport model was validated by field measurements of a bottom trawling operation and presented good accuracy. The results indicated that sediment resuspension by a double-rig trawling induced a 0.13 mm sediment layer deposition onto the rhodoliths from the continental shelf which has potential deleterious effects considered that such thickness is higher than the threshold proposed by the literature. On the contrary, a sediment resuspension/deposition by a single-rig trawling has no impact in the deep-water corals from the continental slope based on the thin layer deposition that was lower than the proposed tolerance limits.

A Turbulence Survey in the Gulf of Naples, Mediterranean Sea, during the Seasonal Destratification

The seasonality of the vertical mixing at coastal sites is not well characterized yet. Here, a time series of the dissipation rate of turbulent kinetic energy (ε) was obtained from weekly morning microstructure observations covering the destratification period (July 2015, February 2016) at a coastal site in the western Mediterranean Sea, influenced by freshwater runoffs. Estimated with bulk parameters from the public re-analyzed dataset ERA5, the Ekman layer, and the convective penetration depth scale with the mixed layer depth (MLD) with a good agreement. Below the MLD, peaks of ε are observed in the baroclinic layers that progressively overlap with the bottom layer, where repeated near-bottom turbidity peaks provide evidence of sediment resuspension, suggesting energetic processes within the bottom boundary layer. In the subsurface, moderate values (10−9 to 10−8 W kg−1) are observed, following a Burr type XII distribution. Significant correlation with ε at MLD is obtained with a model combining the effects of wind, wind–wave, and convection, highlighting a calm sea bias in our data, plus a sunrise bias when morning buoyancy fluxes are stabilizing. Another correlation, obtained from a pure-wind estimation 18 h before, suggests the role of wind in generating internal waves in the stratified layers, thus, impacting mixing intensity.

Primary production modeling identifies restoration targets for shifting shallow, eutrophic lakes to clear-water regimes

Benthic primary production (BPP) plays an important functional role in lakes, improving water quality by stabilizing clear-water regimes. Shallow, eutrophic lakes lacking BPP communities can be difficult to restore because self-stabilizing feedbacks of phytoplankton dominance can impede the establishment of BPP. BPP in lakes is light limited, and ecosystem models can provide guidance for determining the water clarity necessary to re-establish BPP dominance. We developed a multi-tiered framework for the restoration of shallow, eutrophic lakes that incorporates multiple turbidity sources, lake morphometry, and water-level fluctuations to determine water-clarity thresholds above which BPP dominance may be established. We present a case study applying this restoration target framework to a large, shallow lake (Utah Lake, Utah, USA), where water clarity was greatly impeded by sediment resuspension and high algal biomass. Our analysis, which used commonly available lake-monitoring data, indicated that a return to BPP dominance is possible in Utah Lake, particularly if external nutrient loading to its shallow, wind-protected bays is reduced. Our novel framework incorporates regime shift theory to improve shallow, eutrophic lake restoration efforts. By incorporating ecological feedbacks when identifying the restoration targets necessary for returning a lake to a self-stabilizing, clear-water regime, this restoration target framework offers economical and logistical advantages over strategies that focus solely on phytoplankton management or fish biomanipulation.

Distribution, sources and dispersion of polybrominated diphenyl ethers in the water column of the Strait of Georgia, British Columbia, Canada

Dissolved and particulate polybrominated diphenyl ether (PBDE) concentrations were measured in the water column of the Strait of Georgia (SoG), Haro Strait, Juan de Fuca Strait, Burrard Inlet, and the Fraser River to assess their sources and dispersion. Total PBDE concentrations in the water column of the southern basin of the SoG are surprisingly high (similar to the load reported for coastal zones heavily impacted by human activities). Moreover, the dissolved fraction (i.e. passing through a 2.2 μm pore size filter) accounts for >95 % of the total load, which is unlike what is more typically found in other coastal zones, where particulate PBDEs generally dominate. Decreasing concentrations away from the southern SoG, eventually reaching typical open ocean values in Juan de Fuca Strait, point to the Vancouver metropolitan area as the main proximal source of PBDEs. About half of the direct PBDE input comes from wastewater treatment plants, with atmospheric deposition and the Fraser river accounting for most of the rest. However, these direct sources alone cannot explain the high dissolved PBDE load observed in the water column of southern SoG. PBDE scavenging rates estimated from concentration gradients and water transit times imply a PBDE flux to the seafloor which largely exceeds the measured burial rates of PBDEs in sediments. To reconcile these observations and explain the dominance of the dissolved fraction in the water column of the southern SoG, we invoke and provide supporting evidence for the release of colloidal PBDE from the resuspension of PBDE-contaminated sediments by bottom currents. If confirmed, this continued PBDE exchange between sediments and the water column would maintain high levels of PBDEs, and possibly other hydrophobic and persistent organic contaminants, in the water column of the southern SoG until the contaminated sediments are buried below the sediment mixed layer.

Seabed Dynamic Responses Induced by Nonlinear Internal Waves: New Insights and Future Directions

Strong nonlinear internal waves generate a significant pressure force on the seafloor and induce a pore-pressure response penetrated in the seabed and are thus an important driver of sediment resuspension and a potential trigger of seabed failure. The following provides an overview of the seabed responses induced by nonlinear internal waves and the theory, models, and limited observations that have provided our present knowledge. The pressure disturbance is generated by the combined effect of interface displacement and near-bottom acceleration by the nonlinear internal waves. Recent observations in the South China Sea have shown that the pressure magnitudes up to 4 kPa, which is the largest known disturbance. Intense pore-pressure changes in roughly the top 1 m of the weakly conductive seabed are expected during the shoaling and breaking of the nonlinear internal waves and lead to 2 cm sediments of the local seabed appearing in transient liquefaction. Since the fluid seepage reduces the specific weight of the bed, results show that the contribution of vertical seepage on sediment resuspension is estimated at 11% for a seabed saturation of 0.97. Finally, in situ observations are needed to confirm theoretical knowledge and to help improve our ability to model the multiscale interaction process between the seabed and internal waves in the future.

Recent water-level fluctuations, future trends and their eco-environmental impacts on Lake Qinghai

The water level of Lake Qinghai, the largest lake on the Qinghai–Tibetan Plateau, has increased continuously, at an average speed of 0.21 m per year since 2005, causing a rapid expansion of the lake area. We investigated the hydrological processes of Lake Qinghai and the surrounding watershed that have influenced water level and lake area from 1956 to 2019. Relationships among water level, climate change and human activities were also assessed. Water level and lake area were positively correlated with precipitation and runoff into the lake, and negatively correlated with evaporation. Climate change factors including precipitation and runoff were the primary causes of lake level change, whereas human activities, including variation in a human footprint index, land use, and grassland irrigation, were secondary factors. A time series model forecasted that from 2020 to 2050 water levels will increase further by 2.45 m. Although this increase in water level may have some benefits, such as reduced local desertification, the expansion of lake area will continue to flood low beaches, pasture lands, near shore infrastructure and roads, and impact tourism locations. However, continued water level rise may also have negative ecological effects, such as reduce habitat of seasonal birds and reduced water quality due to erosion and sediment resuspension in shallow nearshore lake areas. Local stakeholders, government authorities, and scientists should give greater attention to anticipated changes in water level, and further ecological studies and infrastructure adaptation measures should be implemented.

End-member characteristics of sediment grain size in modern Yellow River delta sediments and its environmental significance

IntroductionThe sediments of the Yellow River delta record the environmental changes in the Yellow River basin on a long time scale and are sensitive to the river diversion and sedimentary environment evolution.MethodsIn this study, cores YDC and YDG in the delta near the estuary of the Yellow River since 1976 were taken as the research object, and the parameterized end-member analysis model was used to analyze the grain-size data of the cores.ResultsThe results show that: The main sediments were silty and sand (58% and 77.9%). The grain-size parameters showed opposite changes with wide grain-size ranges and mixed sizes obviously. End member 1 (EM1) and end member 2 (EM2) were composed of clay and fine silt with fine particle sizes of 0.04 mm and 9.8 mm, respectively, and were deposited under weak hydrodynamic conditions during long-distance transport along the Yellow River. End member 3 (EM3) and end member 4 (EM4) were coarse silt with mode particle sizes of 40.14 mm and 66.89 mm, respectively. These were deposited by waves and tidal currents under strong dynamic conditions.DiscussionAccording to the channel changes and the sedimentary facies data of the modern Yellow River delta, before the Yellow River was diverted in 1996, the cores YDC and YDG were both in the estuary-front bar sedimentary environment. In 1996, the Yellow River was diverted to the Qing 8 course. Subsequently, the core YDC changed to the delta-plain sedimentary facies, and the core YDG changed to the delta-front facies. The sediment sources of the two cores changed from the Yellow River sediments to coastal sediment erosion and resuspension. The sedimentary environment changed from siltation to erosion. The results of this study are of great practical significance for estuary management and coastal engineering construction in the modern Yellow River delta and can provide a scientific basis for ecological protection and high-quality development of the Yellow River basin.

Hydrodynamic effects on the ability of bottom traps to capture nutrient-rich sediments in shallow eutrophic lakes

Hydrodynamics are the key factor influencing sediment resuspension, transport and nutrient release in large shallow lakes. However, the hydrodynamic responses of bottom traps during pollutant capture remain unclear. In this study, a large eutrophic shallow lake was selected to carry out a field test of deep traps at the lake bottom. Based on observations of the lake current around the trap, the sedimentation rate of the particles in the trap and the nutrient content of the captured sediments, combined with the numerical simulation of the waves outside the trap, the effects of lake currents and waves on the sediment deposition in the traps were studied, and the response of the nutrient content of the sediments captured in the deep traps to the changes in lake currents and waves was analyzed. The results showed that the strong hydrodynamic force significantly promoted sediment deposition in the trap and enhanced the ability of the trap to capture sediments with high nutrient contents. The influences of waves and lake currents on the bottom trap capture of polluted sediments varied among different periods. Waves played the leading role in winter and spring, accelerating sediment capture in the bottom traps near the southern shore of eastern Lake Chaohu during this period. In summer, the lake current was the main dynamic factor contributing to the rapid deposition of particulate matter and the capture of sediments with high nutrient contents in the bottom traps of western Lake Chaohu. The multiple stepwise linear regression model based on lake current and wave data explained 37.6 % of the sediment deposition in the trap, and the model built for a single bottom trap explained more than 80 % of the deposition. After correcting the sediment deposition thickness in the trap by considering the water content of the sediment, the quantitative relation yield better inversion results for the sediment deposition process, and different thicknesses in the bottom trap were linked to different sediment deposition periods. According to the hydrodynamic strength in 2020, the thickness of the highly contaminated sediments captured by traps CC1-CC5 was calculated to be 1.09–1.93 m, and the corresponding TN and TP were 26.66–68.53 kg and 6.84–19.89 kg, respectively. This study provides a scientific analysis and guidance for the evaluation of endogenous nutrients captured by lake bottom trap methods.

Suspended sediment dynamics and influencing factors during typhoons in Hangzhou Bay, China

Hangzhou Bay is located in China on the south side of the Changjiang Estuary and is vulnerable to extreme weather, such as typhoons in the summer and autumn. In this study, a three dimensional suspended sediment numerical model was developed that considers the dynamic factors of advection, mixing, wave, and sediment-induced stratification to simulate and analyze the effect of typhoons on water and sediment transport in Hangzhou Bay. The model validations show that the model can sufficiently reproduce the variability of the suspended sediment concentration (SSC) during typhoon conditions. The simulation results show that the high SSC in the bottom layer was mainly distributed in the leading edge of the south coast, and generally exceeded 10 kg·m−3. During typhoons, the water and suspended sediment transport in Hangzhou Bay presented a pattern of "north-landward and south-seaward" circulation, which promoted the convergence of suspended sediment in the center part of the bay. During Typhoon Rumbia in 2018, the water and sediment flux across the section from Nanhui Cape to Qiqu Archipelago (NQ section) increased by 18.13% and 265.75%, respectively, compared with those before the typhoon. The wave-induced bottom shear stress during typhoons has a very significant impact on the bottom SSC. The sensitivity experiments show that the wave-induced bottom shear stress greatly promotes the sediment resuspension during typhoons, which indirectly makes the sediment-induced stratification stronger than the direct effect of waves on the vertical mixing. The strong winds brought by typhoons mainly enhanced the vertical mixing, which has a stronger effect on surface SSC than waves. The suppression of vertical mixing by sediment-induced stratification during typhoons should not be ignored, especially for high turbidity coastal waters, such as Hangzhou Bay.

Source apportionment of suspended sediment using grain-size end-member analysis

End-members mixing analysis (EMMA) was performed on in situ suspended sediment grain-size distribution (GSD) data from Haizhou Bay to estimate suspended sediment provenance and the relative contributions of different sediment sources to total seawater suspended sediment. The results show that the suspended sediment in Haizhou Bay can be expressed as a mixture of three EMs. End-member 1 represents fine-grained suspended sediment that was transported over long distances by advection; this shows a genetic connection with the old Huanghe River delta. End-member 2 represents resuspension of local sediment, while EM3 represents riverine suspended sediment. On average, advection, resuspension, and riverine input contributed 41.92%, 37.83%, and 20.25% of the total suspended sediment, respectively. Temporal changes in advection and resuspension caused considerable variability in these relative contributions. The application of EMMA herein demonstrates that it is a feasible approach to explore suspended sediment sources and their mixing processes using suspended sediment GSD data.

Spatial distribution, sources and risk assessment of polycyclic aromatic hydrocarbons in the surficial sediments of the Egyptian Mediterranean coast

The Egyptian Mediterranean coast (EMC) receives a considerable quantity of polycyclic aromatic hydrocarbons (PAHs). PAHs from EMC sediments were assessed to understand the effects of marine and riverine currents on their distribution. The concentrations of total PAHs ranged between 13,156–34,852 ng/g dw. PAH levels have increased even in areas far from the shoreline under the influence of riverine inputs from the Nile River; this is attributed to the tidally induced riverine freshwater re-suspension of surface sediments in the shallow near-shore section and re-precipitation in the fare stations. PAH levels generally increase as one moves from the western to the eastern part of the studied area, owing to the effect of the marine current. Diagnostic ratios pointed toward different pyrogenic sources. SQGs were used to assess the probability of observing adverse biological effects in benthic organisms in sediment samples. The toxic and mutagenic equivalent quotient for carcinogenic PAHs was extremely high.

The biogeochemistry of zinc and cadmium in the Amundsen Sea, coastal Antarctica

The trace metals zinc (Zn) and cadmium (Cd) are both involved in the metabolic processes of marine phytoplankton, and as such, both metals play important roles in ocean biogeochemical cycles. In Antarctica, the Amundsen Sea (AS) experiences rapid ice shelf melting, and the Amundsen Sea polynya (ASP) hosts seasonal phytoplankton blooms in austral summer, with important implications for atmospheric carbon dioxide drawdown. However, the effects of ice melting and phytoplankton blooms on the biogeochemistry and distributions of Zn and Cd in the ASP remain poorly studied. Here, we present the first combined dataset of dissolved and particulate Zn and Cd in the AS (including the inflow and outflow to and from the Dotson and Getz ice shelves) collected as part of the GEOTRACES process study GPpr12. We use this dataset to assess the sources of both elements to the AS region and characterize the particle composition in the ASP. We find that the main source of both dissolved Zn and Cd in the AS is Circumpolar Deep Water (CDW), with an additional small flux of both metals from shelf sediments. By contrast, aerosol deposition, ice shelf melt, and sea ice melt are all deemed insignificant sources for either Zn or Cd in the AS. Labile particulate Zn and Cd dominate the total particulate pool in the surface layer, indicating that biological uptake is a predominant process for the cycling of both metals in the ASP, whereas sediment resuspension and ice shelf melt do not supply a significant amount of either particulate Zn or Cd. Additionally, we use two commonly used approaches to estimate biogenic and lithogenic particulate concentrations. We find high biogenic particulate concentrations at the surface, decreasing with depth, indicating remineralization plays an important role in the cycling of particulate metals. In contrast, lithogenic particulate metal concentrations remain low throughout the water column. We also show that the estimated uptake ratios of Zn and Cd relative to phosphate in the surface layer are lower than reported for the open Southern Ocean, likely related to the spatial and temporal variability of Fe in the AS. Overall, these new observations provide insight into the biogeochemistry of both Zn and Cd in the AS, a region that is subject to the influence of rapid climate change, which may have implications for the larger-scale cycling of trace metals in the Southern Ocean. Specifically, the amount of Zn and Cd supplied to the surface ASP will increase, given that the volume of CDW that flows towards the Dotson Ice Shelf is predicted to increase.

Coastal darkening exacerbates eutrophication symptoms through bottom‐up and top‐down control modification

AbstractCoastal eutrophication due to excessive anthropogenic nutrient loading is a major threat worldwide, and especially in estuaries and semi‐enclosed waterbodies, like the brackish Baltic Sea. In addition, coastal waters may become darker (coastal darkening) due to increased input of colored compounds from terrestrial run‐off and sediment resuspension. Still, the effects of darkening on coastal food web responses to eutrophication are unknown. In a mesocosm experiment with benthic and pelagic habitats, we manipulated nutrient loading, presence of fish and light availability to disentangle bottom‐up and top‐down control of eutrophication symptoms in ambient and darkened waters. Overall, we found higher pelagic Chlorophyll a concentrations (a proxy of algal biomass) with darkening and with nutrient enrichment in both clear and dark waters. Albeit fish had a strong impact on zooplankton and zoobenthos, they had no cascading effect on algae. We conclude that coastal darkening due to changes in land use and climate change can pose an additional challenge concerning the recovery of coastal waters from eutrophication.