Benthic Fluxes of Fluorescent Dissolved Organic Material, Salt, and Heat Measured by Multiple-Sensor Aquatic Eddy Covariance.

Aquatic eddy covariance (AEC) is an in situ technique for measuring fluxes in marine and freshwater systems that is based on the covariance of velocity and concentration measurements. To date, AEC has mainly been applied to the measurement of benthic oxygen fluxes. Here, development of a fast multiple-channel sensor enables the use of AEC for measurement of benthic fluxes of fluorescent material, salt, and heat at three distinct sites in Massachusetts, USA, including the Connecticut River, the Concord River, and Upper Mystic Lake. Benthic fluxes of salt, useful as a tracer for groundwater input (submarine groundwater discharge), were consistent with independent measurements made with seepage meters. Eddy fluxes of heat were consistent with the balance of incoming solar radiation and thermal conduction at the sediment surface. Benthic eddy fluxes of fluorescent dissolved organic material (FDOM) revealed a substantial net downward flux in the humic-rich Concord River, suggesting that microbial consumption of dissolved organic carbon in the sediment was significant. Simultaneous measurement of several fluxes expands the utility of AEC as a biogeochemical tool while enabling checks for mutual consistency among data channels.

The influence of bioturbator activity on sediment bacterial structure and function is moderated by environment.

Bioturbation in coastal sediments plays a crucial role in biogeochemical cycling. However, a key knowledge gap is the extent to which bioturbation influences bacterial community diversity and ecosystem processes, such as nitrogen cycling. This study paired bacterial diversity, bioturbation activity and in situ flux measurements of oxygen and nitrogen from bioturbated sediments at six estuaries along the East coast of Australia. Bacterial community diversity, composition and predicted functional profiles were similar across burrow and surface sediments but were significantly influenced by bioturbator activity (measured as number of burrows) at sites with higher fine grain content. Sediment oxygen demand increased with bioturbator activity but changes in nitrogen cycling (as measured by fluxes and predicted bacterial functional gene analysis) were more spatially variable and were unrelated to bioturbator activity and bacterial community shifts. This study highlights how bioturbator activity influences bacterial community structure and functioning and what implications this has for biogeochemical cycles in estuarine sediments.

Effects of seasonal deposition-erosion cycle on sedimentary organic carbon remineralization and oxygen consumption in a large-river delta-front estuary.

Seasonal sediment deposition-erosion events are dominant drivers of particle-solute dynamics in large-river delta-front estuaries (LDEs), but their influence on elemental cycles is not yet fully understood. To better constrain the role of deposition-erosion events on elemental cycling in LDEs, benthic fluxes of dissolved inorganic carbon (DIC), oxygen, and pore-water solute profiles were measured over different seasons in the Changjiang LDE. Benthic DIC efflux (23.4 ± 6.0 mmol C m-2 d-1) was greater than oxygen influx (7.5 ± 2.0 mmol O2 m-2 d-1) in summer but less in winter (7.7 ± 1.2 mmol C m-2 d-1 and 10.1 ± 1.5 mmol O2 m-2 d-1, respectively). The additional oxygen consumption in sediments in winter was likely due to the oxidation of inorganic diagenetic reductive products (IDRP) (e.g., NH4+, Fe2+, and Mn2+) in deeper sediments exposed by erosion, which resulted in the development of an "oxygen debt". Sedimentary oxygen respiration accounted for at least 48 % of total oxygen consumption (oxygen consumption in both water column and sediment) in winter and was significantly greater than in summer (∼15 %); this highlighted the importance of winter sediment erosion in oxygen depletion. In addition to IDRP oxidation, the remineralization of resuspended sedimentary organic carbon in water column also contributed to the oxygen consumption. The global dataset on benthic DIC and oxygen fluxes provides evidence that the "oxygen debt" is likely to be widespread in LDEs, exerting a significant impact on global carbon and oxygen cycling.

Dissolved organic matter distribution in the water column and sediment pore water in a highly anthropized coastal lagoon (Mar Menor, Spain): Characteristics, sources, and benthic fluxes.

Coastal lagoons are among the most productive and biodiverse systems in the world and are important sentinels of climate change. The Mar Menor is one of the largest coastal lagoons in the Mediterranean, providing a variety of ecosystem services and resources to the community. However, in recent decades this lagoon has suffered drastic changes and degradation caused by human activities. We analyzed the concentration of dissolved organic carbon (DOC) and the optical properties of dissolved organic matter (DOM) in the water column and sediment pore water during the summer and winter of 2018 and during eighteen months from 2016 to 2018. Overall, we found that the composition of DOM is mainly related to and enhanced by anthropogenic activities and microbial metabolism. DOM enters the lagoon via urban and agricultural runoff, drainage systems, and wastewater treatment plants. Additionally, strong microbial metabolism in sediments leads to differences in DOM composition between water and sediments. In the water column, humic-like components accounted for 71 % of the total DOM, while protein-like compounds were most abundant in sediment pore water. We observed a strong seasonal variability associated with precipitation and the system collapse in 2016 (phytoplankton bloom), which resulted in the death of 80 % of macrophytes. The sediments act as a source of DOM to the overlying water, likely due to relatively high organic matter content and intense microbial activity, primarily through anaerobic pathways. Benthic fluxes of DOC ranged from 5.24 to 33.30 mmol m-2 d-1, being higher in winter than summer 2018 and decreasing from north to south, likely related to lower residence time in the northern basin, groundwater discharge and accumulation of organic matter from the dead meadows. We estimate a net flux of DOC from the Mar Menor toward the Mediterranean Sea of 1.57 × 107 mol yr-1.

Coastal aquaculture regulates phosphorus cycling in estuarine wetlands: Mobilization, kinetic resupply, and source-sink process.

Estuarine mangrove wetlands have gradually declined owing to the growing construction of aquaculture ponds. How the speciation, transition, and migration of phosphorus (P) adaptively change in the sediments of this pond-wetland ecosystem remains unclear. In this study, we used high-resolution devices to explore the contrasting P behaviors associated with the redox cycles of Fe-Mn-S-As in estuarine and pond sediments. The results showed that the construction of aquaculture ponds increased the content or percentage of the silt, organic carbon, and P fractions in sediments. Dissolved organic P (DOP) concentrations in pore water were fluctuant with depths, accounting for only 18±15% and 20±11% of total dissolved P (TDP) in estuarine and pond sediment, respectively. Furthermore, DOP was less strongly correlated with other P species, including Fe, Mn, and sulfide. The coupling of dissolved reactive P (DRP) and TDP with Fe and sulfide confirmed that P mobility is regulated by Fe redox cycling in estuarine sediments, whereas Fe(III) reduction and sulfate reduction co-regulate P remobilization in pond sediments. The apparent diffusion flux revealed all sediments acting as sources for TDP (0.04-0.1 mg m-2 d-1) to the overlying water, while mangrove sediments were sources of DOP, and pond sediments were major sources of DRP. The DIFS model overestimated the P kinetic resupply ability, which was evaluated using DRP rather than TDP. This study improves our understanding of P cycling and budget in aquaculture pond-mangrove ecosystems and has important implications for understanding water eutrophication more effectively.

Investigation on metal geochemical cycling in an anthropogenically impacted tidal river in Belgium.

The geochemical behavior of metals in water and sediment was investigated in the tidal section of the Zenne River in Belgium. Twelve-hour sampling campaigns were performed in October 2013 and March 2021 at the mouth of the Zenne River, under dry and rainy weather conditions respectively. Water samples were collected every hour while the passive samplers of Diffusive Gradients in Thin-films (DGT) were deployed continuously during a tidal cycle. In addition, bottom sediments were sampled at the tidal station and water samples were taken upstream and downstream of that station to identify the metal sources. The highest concentrations of Fe, Mn, Pb, Cr, Ni and Zn appear at low tide, indicating the Zenner River as a main source. However, for Co, Cd and Cu, other sources including upstream transport may explain their behavior during a tidal cycle. Fe, Pb and Cr are essentially transported in the particulate phase (<10 % dissolved) while the other metals in the dissolved phase (20 to 90 %). Rainfall and wind gust events also play an important role in trace metal distribution, increasing sediment resuspension and metal desorption. A good agreement was found between the time-averaged dissolved and DGT-labile metal concentrations with the exception of Cu and Fe, which form strong organic Cu complexes and Fe colloids respectively. The sediments of the tidal Zenne are contaminated by trace metals, thus acting as a secondary pollution source to the river. The reductive dissolution of Mn and Fe oxyhydroxides and the release of associated trace metals are the main mobilization mechanisms. Knowledge of the upstream and downstream levels in the water column, the benthic fluxes, which are based on turbulent diffusion, and the partitioning between dissolved and particulate phases allow to explain the metal concentration variations during the tidal cycle.

Characteristics of nutrients and heavy metals and potential influence of their benthic fluxes in the Pearl River Estuary, South China.

Nutrient and heavy metal concentrations in porewater/overlying water and their benthic fluxes were investigated to study their accumulation and transport at the sediment-water interface and the influences of sediment in the Pearl River Estuary, China. Results revealed that distribution of nutrients and metals reflected the effects of terrestrial inputs and some physicochemical processes. Benthic fluxes also suggested that nutrients and heavy metals Pb, Zn and Cd diffused from sediment to overlying water, but not for As, Co, Cr, Fe, Mn and Ni. Exchange capacities showed that 106-108 mol nutrients and 105-107 g Pb, Zn and Cd released from sediment to overlying water annually, indicating their potential ecological threat. However, 105-109 g metals As, Co, Cr, Fe, Mn and Ni were deposited annually, which may reduce the pollution pressure caused by anthropogenic activities. This study will provide references for the potential influence of benthic fluxes on estuarine environment globally.

Benthic trace metal fluxes in a heavily contaminated bay in China: Does the sediment become a source of metals to the water column?

Over three different seasons, seawater, porewater and sediment samples were collected from Jinzhou Bay, a previously heavily contaminated bay, to quantitatively assess the benthic flux of trace metals after a reduction in fluvial/sewage discharge for almost three decades. The spatial distribution patterns of trace metals in seawater, surface sediment, as well as the vertical distribution patterns of metals in porewater and solid phases in short sediment cores were reported. Metal concentrations in seawater and sediment all showed much higher Cd and Zn concentrations inside the Jinzhou Bay compared to the rest of Bohai Sea area. Zn, Ni, Pb and Co all had average benthic fluxes coming out of the sediments to the water column, contributing about 0.5%, 0.3%, 1.4% and 14% to their current standing stock in Jinzhou Bay. Seasonal difference was also identified in seawater and porewater, as well as in the benthic fluxes. In general, benthic fluxes and porewater concentrations all tended to be higher in summer, implying a close relationship between benthic flux and the temperature-dependent organic matter degradation process at the sediment-water interface. Currently, there are clearly still other sources, possibly fluvial/sewage discharge, as the main source of trace metals in Jinzhou Bay waters. For Cd and Cu, concentrations in the water column remain high on an annual basis indicating that sediment still acts as a sink. Conversely, for Pb, Zn, Co, and Ni, the sediment is beginning to act as a source to the water column. Although this may not yet be significant, it will become more and more important with time, and can last for hundreds to thousands of years.

Less metal fluxes than expected from fibrous marine sediments.

Deposits of fibrous sediment, which include fiberbanks and fiber-rich sediments, are known to exist on the Swedish seafloor adjacent to coastally located former pulp and paper industries. These deposits contain concentrations of hazardous substances that exceed national background levels and contravene national environmental quality objectives (EQOs). In this study of metal fluxes from fibrous sediments using benthic flux chamber measurements (BFC) in situ we obtained detected fluxes of Co, Mo, Ni and Zn, but no fluxes of Pb, Hg and Cr. The absence of fluxes of some of the analyzed metals indicates particle bound transport of Pb, Cr and Hg from fiberbanks even though Hg might become methylated under anoxic conditions and, in that case, may enter the food chain. We found less metal fluxes than expected and thus emphasize the importance of in-situ flux measurements as a compliment to sediment metal concentrations within risk assessments of contaminated sediments.

Benthic nutrient cycling at the sediment-water interface in a lagoon fish farming system (northern Adriatic Sea, Italy).

Metabolism and carbon, oxygen, and nutrient fluxes (DIC, DOC, DO2, NO2-, NO3-, NH4+, PO43- and SiO44-) were studied during three surveys at two sites (VN1 and VN3) located at a fish farm at the Marano and Grado Lagoon (northern Adriatic Sea), using an in situ benthic chamber. Field experiments were conducted in July and October 2015 and March 2016 at a depth of approximately 2 m along the main channels of the fish farm. Water samples were collected by a scuba diver every 2 h in order to investigate daily fluxes of solutes across the sediment-water interface (SWI). Regarding the solid phase, Corg/Ntot and Corg/Porg molar ratios suggested an autochthonous marine origin of the organic matter and a minor preservation of P in the sediments, respectively; high values of sulphur (Stot) were also encountered (0.8-2%). The conditions at VN3 were mostly anoxic with high NH4+ levels (30-1027 µM) and the absence of NO3-. Substantial daily patterns of all solutes occurred especially in autumn and winter. On the contrary, fluxes at VN1 were less pronounced. Usually, inverse correlations appeared between dissolved O2 and DIC trends, but in our system this was observed only at VN3 in autumn and accomplished by a parallel increase in NH4+, PO43- and SiO44- during intense nutrient regeneration. These results are significantly different than those reported for open lagoon environments, where nutrient regeneration at the SWI and in surface sediments is the primary source of nutrients available for assimilation processes, especially during the warmer period of the year when the natural nutrient input by fresh water inflows is limited. Due to the importance of this site for aquaculture, biodiversity and ecosystem services, useful suggestions have been provided from this study in order to improve the quality of this unique aquatic system.

Dynamics of dissolved organic matter in riverine sediments affected by weir impoundments: Production, benthic flux, and environmental implications.

In order to understand the characteristics and dynamics of dissolved organic matter (DOM) in the sediment of rivers affected by impoundments, we examined the vertical profiles and the benthic fluxes of DOM in four different core sediments located at upstream sites of weirs in major rivers of South Korea. In three out of four sites, exponential accumulation of dissolved organic carbon (DOC) with depth was observed with the signature of seasonal variability. Except for the site displaying a below-detection limit of Fe(II), the general accumulation trends of DOC with depth was concurrent with the increases of Fe(II) and NH4+ and the decrease of PO43-, signifying a close linkage of the DOM dynamics with anaerobic respiration via iron reduction, an important early diagenesis pathway. The estimated benthic fluxes from the cores revealed that the sediments likely serve as DOC, chromophoric DOM (CDOM), and fluorescent DOM (FDOM) sources to the overlying water. The benthic effluxes based on DOC were comparable to the ranges previously reported in lake and coastal areas, and those of CDOM and FDOM showed even higher levels. These findings imply that impoundment-affected river systems would change the DOM composition of the overlying water, ultimately influencing the subsequent water treatment processes such as disinfection byproducts production and membrane fouling. A simple mass balance model indicated that the impoundment-affected river sediments may operate as a net carbon sink in the environments due to a greater extent of sedimentation compared to the estimated benthic efflux and sediment biological respiration.

Metal contaminant fluxes across the sediment water interface.

To date, most estimates of contaminant fluxes across the sediment/water interface in risk assessments have been done using diffusive flux models. However, the reliability of these is limited as the overall flux from the sediment may have contributions caused by advection and bioturbation. We found through a comparison of modelled fluxes versus measured fluxes, that the methods Benthic Flux Chamber and surface leaching tests in a risk assessment context showed similar magnitude while calculated fluxes deviated at least by a factor of 100 from measured fluxes. This may be explained by the flux contribution in connection with bioturbation. The chamber-measured fluxes of copper were low compared to those of zinc and cobalt, but this is consistent with leaching tests that indicated copper to be more strongly bound. Risk assessments based on total concentrations may be misleading.

Porewater dynamics of silver, lead and copper in coastal sediments and implications for benthic metal fluxes.

To determine the conditions that lead to a diffusive release of dissolved metals from coastal sediments, porewater profiles of Ag, Cu, and Pb have been collected over seven years at two contrasting coastal sites in Massachusetts, USA. The Hingham Bay (HB) site is a contaminated location in Boston Harbor, while the Massachusetts Bay (MB) site is 11 km offshore and less impacted. At both sites, the biogeochemical cycles include scavenging by Fe-oxyhydroxides and release of dissolved metals when Fe-oxyhydroxides are reduced. Important differences in the metal cycles at the two sites, however, result from different redox conditions. Porewater sulfide and seasonal variation in redox zone depth is observed at HB, but not at MB. In summer, as the conditions become more reducing at HB, trace metals are precipitated as sulfides and are no longer associated with Fe-oxyhydroxides. Sulfide precipitation close to the sediment-water interface limits the trace metal flux in summer and autumn at HB, while in winter, oxidation of the sulfide phases drives high benthic fluxes of Cu and Ag, as oxic conditions return. The annual diffusive flux of Cu at HB is found to be significant and contributes to the higher than expected water column Cu concentrations observed in Boston Harbor. At MB, due to the lower sulfide concentrations, the association of trace metals with Fe-oxyhydroxides occurs throughout the year, leading to more stable fluxes. A surface enrichment of solid phase trace metals was found at MB and is attributed to the persistent scavenging by Fe-oxyhydroxides. This process is important, particularly at sites that are less reducing, because it maintains elevated metal concentrations at the surface despite the effects of bioturbation and sediment accumulation, and because it may increase the persistence of metal contamination in surface sediments.

Differentiating transpiration from evaporation in seasonal agricultural wetlands and the link to advective fluxes in the root zone.

The current state of science and engineering related to analyzing wetlands overlooks the importance of transpiration and risks data misinterpretation. In response, we developed hydrologic and mass budgets for agricultural wetlands using electrical conductivity (EC) as a natural conservative tracer. We developed simple differential equations that quantify evaporation and transpiration rates using flow rates and tracer concentrations at wetland inflows and outflows. We used two ideal reactor model solutions, a continuous flow stirred tank reactor (CFSTR) and a plug flow reactor (PFR), to bracket real non-ideal systems. From those models, estimated transpiration ranged from 55% (CFSTR) to 74% (PFR) of total evapotranspiration (ET) rates, consistent with published values using standard methods and direct measurements. The PFR model more appropriately represents these non-ideal agricultural wetlands in which check ponds are in series. Using a flux model, we also developed an equation delineating the root zone depth at which diffusive dominated fluxes transition to advective dominated fluxes. This relationship is similar to the Peclet number that identifies the dominance of advective or diffusive fluxes in surface and groundwater transport. Using diffusion coefficients for inorganic mercury (Hg) and methylmercury (MeHg) we calculated that during high ET periods typical of summer, advective fluxes dominate root zone transport except in the top millimeters below the sediment-water interface. The transition depth has diel and seasonal trends, tracking those of ET. Neglecting this pathway has profound implications: misallocating loads along different hydrologic pathways; misinterpreting seasonal and diel water quality trends; confounding Fick's First Law calculations when determining diffusion fluxes using pore water concentration data; and misinterpreting biogeochemical mechanisms affecting dissolved constituent cycling in the root zone. In addition, our understanding of internal root zone cycling of Hg and other dissolved constituents, benthic fluxes, and biological irrigation may be greatly affected.

Benthic flux of dissolved organic matter from lake sediment at different redox conditions and the possible effects of biogeochemical processes.

The benthic fluxes of dissolved organic carbon (DOC), chromophoric and fluorescent dissolved organic matter (CDOM and FDOM) were studied for the sediment from an artificial lake, based on laboratory benthic chamber experiments. Conservative estimates for the benthic flux of DOC were 71 ± 142 and 51 ± 101 mg m(-2) day(-1) at hypoxic and oxic conditions, respectively. Two humic-like (C1 and C2), one tryptophan-like (C3), and one microbial humic-like (C4) components were identified from the samples using fluorescence excitation emission matrices and parallel factor analysis (EEM-PARAFAC). During the incubation period, C3 was removed while C4 was accumulated in the overlying water with no significant difference in the trends between the redox conditions. The humification index (HIX) increased with time. The combined results for C3, C4 and HIX suggested that microbial transformation may be an important process affecting the flux behaviors of DOM. In contrast, the overall accumulations of CDOM, C1, and C2 in the overlying water occurred only for the hypoxic condition, which was possibly explained by their enhanced photo-degradation and sorption to redox-sensitive minerals under the oxic condition. Our study demonstrated significant benthic flux of DOM in lake sediment and also the possible involvement of biogeochemical transformation in the processes, providing insight into carbon cycling in inland waters.