Benthic Exchange Research Articles

Benthic exchange along a tropical estuarine salinity gradient during dry season: Biogeochemical and ecological implications

Benthic biogeochemistry and its coupling with pelagic ecology are less understood in tropical than in temperate estuaries, which have significant implications for global carbon and nutrient cycling. In dry season, the Mandovi Estuary of Western India develops a prominent salinity gradient (0–35 psu), and remains nutrient-limited but most productive, which stands in contrast to wet season. To understand the role of benthic exchange in the sustenance of estuarine biogeochemistry and ecosystem, we carried out biogeochemical measurements in the water column and a series of intact-core incubations along the estuarine salinity gradient, during the dry season of 2014. We observed that the lower estuary was nitrogen (N)-limited whereas the middle and upper estuary were phosphorus (P)-limited. The benthic respiration (38.6–82.87 mmol O2 m−2 d−1), organic carbon (Corg) mineralization (34.3–79.8 mmol C m−2 d−1) and nutrient exchange rates considerably varied along the salinity gradient. Variability in benthic respiration was apparently controlled by labile Corg content and faunal abundance in the sediments. Benthic exchange of NH4+ and NOx− were mainly controlled by salinity and NOx− of the water column, respectively. Benthic PO43− exchange was predominantly controlled by sedimentary Fe content and bottom water O2, whereas benthic SiO44− exchange was primarily controlled by sediment-water SiO44− gradient. Abiotic processes accounted for 37-79% of sediment O2 consumption (SOC). Benthic metabolism accounted for 34–60% of total community respiration and mineralized 43–145% equivalent of C fixed through primary production (PP). Of dissolved inorganic nitrogen (DIN) efflux (0.67–2.59 mmol m−2 d−1), NH4+ comprised 91% in the lower estuary but NOx− (NO3− + NO2−) comprised 97% in the upper estuary, and benthic nitrification apparently determined the dominant N species released from the sediments. Coupled nitrification-denitrification apparently caused the loss of 30–69% of NH4+ diffusing from the deeper sediments. The estuarine sediments were a net source of DIN and SiO44−, potentially meeting up to 31% of N and 40% of Si demand of the phytoplankton, but acted as a net PO43− sink owing to high sedimentary Fe and normoxia, and tended to buffer the estuarine water PO43− to 0.05–0.19 μM. Benthic PO43− influx (0.006–0.026 mmol m−2 d−1) apparently caused the P-limiting condition in the middle and upper estuary. Particularly, the role benthic DIN supply was crucial for the sustenance of estuarine PP as the estuary receives some PO43− from the sea and sufficient SiO44− from the river but not sufficient DIN. The sediments potentially immobilized up to 25% and 20% of riverine DIN and PO43− load, respectively. Overall, the study revealed an efficient benthic carbon recycling and a tight benthic-pelagic coupling in the estuary during the dry season.

Benthic nutrients and oxygen fluxes at the water sediment interface in a pearl farming atoll (Ahe, Tuamotu, French Polynesia)

Benthic exchanges of oxygen and nutrient at the sediment-water interface were investigated under light and dark conditions at 5 selected sites in a sub-tropical atoll. Mean oxygen fluxes were - 1316.5 ± 242.0 μmol m−2 h−1 and mean effluxes of oxygen under light conditions were 2231.7 ± 626.4 μmol m−2 h−1, presumably due to microphytobenthos present at the sediment-water interface. The consequences of this high related productivity was a systematic consumption of nutrients (DIN, PO4 and Si(OH)4) during almost all light incubations, contrasting with the effluxes of nutrients during dark incubations. Our results suggest that the sediments were net autotrophic and the oxygen balance in favor of microbenthic production when compared to community demand. Diurnal rates of gross benthic primary productivity were high (3423 ± 1192 μmol m−2 h−1) which emphasize the role of microphytobenthos in maintaining the oxygen reservoir in tropical lagoons.

The relation between temperature and silica benthic exchange rates and implications for near-seabed formation of diagenetic opal

This study calculated the dissolution rates of biogenic silica deposited on the seafloor and the silicic acid benthic flux for 22 Ocean Drilling Program sites. Simple models developed for two host sediment types—siliciclastic and carbonate—were used to explain the variability of biogenic silica dissolution and recycling under present-day low (−0.3 to 2.14 °C) bottom-water temperatures. The kinetic constants describing silicic acid release and silica saturation concentration increased systematically with increasing bottom-water temperatures. When these temperature effects were incorporated into the diagenetic models, the prediction of dissolution rates and diffusive fluxes was more robust. This demonstrates that temperature acts as a primary control that decreases the relative degree of pore-water saturation with biogenic opal while increasing the silica concentration. The correlation between the dissolution rate and benthic flux with temperature was pronounced at sites where biogenic silica is hosted in surficial sediments mostly composed of biogenic carbonates. This association is because the dissolution of carbonates provides the alkalinity necessary for both silica dissolution (also its reprecipitation as opal-CT) and clay formation; thus strongly reducing the retarding influence of clays on biogenic opal dissolution. Conversely, the silica exchange rates were modified by presence of aluminosilicates, which led to a higher burial efficiency for biogenic opal in detrital- than in carbonate-dominated benthic layers. Though model prediction of first-order silica early transformation suggests likely effects from surface temperatures (0–4 °C) on opal-CT precipitation over short geological times (< 4 Ma) near seabed in the Antarctic Site 751, the relationship between silica solubility and surface-area variability in time is a more critical control. Since silica solubility and surface area decrease with time, the < 4 Ma elapsed time aged opal-A to the point that changes in specific surface area caused minor effects on solubility, allowing for formation of opal-CT at low temperature settings near the seabed.

Hydrogeological processes and near shore spatial variability of radium and radon isotopes for the characterization of submarine groundwater discharge

Inadequate characterization of variability in natural tracers of submarine groundwater discharge (SGD) introduces large errors into tracer-derived SGD estimates. To address this gap, we investigated spatial variability in the natural SGD tracers 223Ra, 224Ra, 226Ra, 228Ra and 222Rn using a high-density array of piezometers and seepage meters over a nearshore area where discharging groundwater transitioned from fresh to saline. Seepage meters and piezometers were used to sample groundwater and to quantify fluxes and salinity. A series of spatial patterns was distinguished beyond the normal salinity impact on Ra activity. The discharge in the interface between saltwater and freshwater was characterized by higher activities of the longer-lived isotopes (226Ra and 228Ra), while areas dominated by benthic exchange had higher activities of the shorter-lived isotopes (223Ra and 224Ra). Spatial differences in Ra activities were associated with variation in salinity and residence time in the aquifer and were indicative of underlying hydrogeological processes. At the freshwater-saltwater interface, fresh discharge is driven by terrestrial hydraulic gradients and saline discharge is driven by density gradients; both result in long residence times in the aquifer. Benthic exchange of saltwater has shorter residence times, much less than required to enrich long-lived isotopes and reach secular equilibrium. The highest activities of 222Rn were detected in the fresh discharge zone, and the lowest activities in areas dominated by benthic exchange. Direct sampling at discharge points allowed comparison between groundwater collected from inland wells and the water discharging to the bay, and indicated significant differences in the activities despite the short distance from wells to the discharge area. Substantial spatial variability in Ra and Rn was observed on the meter scale, with trends that reflected groundwater origin and residence time. Inadequate characterization of variability, trends, and fluxes introduces large errors into tracer-derived estimates of submarine groundwater discharge. Thus, the study of submarine groundwater discharge with radioactive tracers requires adequate hydrogeological knowledge to identify processes that have strong impacts on Ra and Rn activities and associated fluxes to the ocean.

Application of Stable Isotopes of Water to Study Coupled Submarine Groundwater Discharge and Nutrient Delivery

Submarine groundwater discharge (SGD)—including terrestrial freshwater, density-driven flow at the saltwater–freshwater interface, and benthic exchange—can deliver nutrients to coastal areas, generating a negative effect in the quality of marine water bodies. It is recognized that water stable isotopes (18O and 2H) can be helpful tracers to identify different flow paths and origins of water. Here, we show that they can be also applied when assessing sources of nutrients to coastal areas. A field site near a lagoon (Ringkøbing Fjord, Denmark) has been monitored at a metric scale to test if stable isotopes of water can be used to achieve a better understanding of the hydrochemical processes taking place in coastal aquifers, where there is a transition from freshwater to saltwater. Results show that 18O and 2H differentiate the coastal aquifer into three zones: Freshwater, shallow, and deep saline zones, which corresponded well with zones having distinct concentrations of inorganic phosphorous. The explanation is associated with three mechanisms: (1) Differences in sediment composition, (2) chemical reactions triggered by mixing of different type of fluxes, and (3) biochemical and diffusive processes in the lagoon bed. The different behaviors of nutrients in Ringkøbing Fjord need to be considered in water quality management. PO4 underneath the lagoon exceeds the groundwater concentration inland, thus demonstrating an intra-lagoon origin, while NO3, higher inland due to anthropogenic activity, is denitrified in the study area before reaching the lagoon.

Influence of Ocean Circulation and Benthic Exchange on Deep Northwest Atlantic Nd Isotope Records During the Past 30,000 Years

AbstractNeodymium (Nd) isotopes extracted from authigenic sediment phases are increasingly used as a proxy for past variations in water mass provenance. To better constrain the controls of water mass provenance and nonconservative effects on the archived Nd isotope signal, we present a new depth transect of Nd isotope reconstructions from the Blake Bahama Outer Ridge along the North American continental margin covering the past 30 ka. We investigated five sediment cores that lie directly within the main flow path of the Deep Western Boundary Current, a major advection route of North Atlantic Deep Water. We found offsets between core tops and seawater Nd isotopic compositions that are observed elsewhere in the Northwest Atlantic. A possible explanation for this is the earlier suggested redistribution of sediment by nepheloid layers at intermediate as well as abyssal depths, transporting material downslope and along the continental margin. These processes potentially contributed to Nd isotope excursions recorded in Northwest Atlantic sediment cores during the Bølling‐Allerød and early Holocene. An Atlantic‐wide comparison of Nd isotope records shows that the early Holocene excursions had an additional contribution from conservative advection of unradiogenic dissolved Nd. Nevertheless, the trends of the Nd isotope records are in general agreement with previous reconstructions of water mass provenance from the entire Atlantic and also reveal millennial‐scale changes during the last deglaciation in temporal high resolution, which have rarely been reported before. Further, the new records confirm that during cold periods the Northwest Atlantic was bathed by an increased contribution of southern sourced water.

Trace Elements and Sr, Nd Isotope Compositions of Surface Sediments in the Indian Ocean: An Evaluation of Sources and Processes for Sediment Transport and Dispersal

AbstractTo decipher sediment provenance, understand dispersal pattern and its controlling factors in the Indian Ocean, we have studied major and trace element abundances and isotope compositions of Sr (87Sr/86Sr) and Nd (εNd) in silicate fractions of 38 surface sediment samples collected at ~1.5° interval from India (~10°N) to Antarctica (~70°S). The 87Sr/86Sr ratio and εNd vary from 0.70527 and 0.1 near Crozet Island to 0.77854 and −34.8 near Antarctica, respectively. Higher 87Sr/86Sr ratio and lower εNd near Antarctica indicate source of old granitic rocks, and the opposite is observed in sediments from the Central and the Southwest Indian Ridges indicating source of seafloor spreading and mid‐ocean ridge hydrothermal activity. Chondrite normalized Rare Earth Elements (REEs) pattern reveals positive Europium anomaly for Central and Southwest Indian Ridge sediments, while positive Gadolinium anomaly is observed in Indian and Antarctic coastal sediments. Sediments in the Indian sector of Southern Ocean and coastal Antarctica are highly dispersed through deep, bottom, and Antarctic circumpolar currents. The εNd spatial map generated for the entire Indian Ocean based on compilation of present study and results from literature shows dominance of terrigenous inputs from continental erosion with minor contribution from the ridges in the Indian Ocean. The εNd map together with trace elements can be employed for reconnaissance for mineral exploration associated with hydrothermal deposits. Further, comparison of detrital εNd with its counter phases, that is, authigenic fractions and modern seawater at the sampling sites, would enhance our knowledge of benthic exchange processes, with an implication in using Nd isotopes as a water mass circulation proxy.

An in situ study of production from diel oxygen modelling, oxygen exchange, and electron transport rate in the kelp Ecklonia radiata

Macroalgal forests provide the foundation for most shallow reef ecosystems in temperate environments; hence tools for accurately measuring primary productivity are integral for ecosystem management. This study compares estimates of production/potential production in an Ecklonia radiata kelp forest in Tasmania, Australia, using diel oxygen gross primary production (GPP) models, benthic exchange chambers, and electron transport rate in photosystem II measured using PAM fluorometry. Two approaches to modelling GPP show good fit with environmental dissolved oxygen (DO), with gross oxygen production of the kelp bed ranging between ~0-20 for one model and ~0-8 µmol O2 m-2 s-1 for the other, with total daily GPP (±SE) of 464 ± 28 and 347 ± 7 mmol O2 m-2, respectively. The oxygen production rate of E. radiata in benthic chambers ranged between 0 and 9.6 µmol O2 m-2 s-1, with total daily production as 204 ± 13 mmol O2 m-2, half that estimated from modelling DO. The peak value for maximum relative electron transport rate was 49 µmol e- m-2 s-1 at PAR of 208 µmol m-2 s-1. Oxygen evolution from benthic chambers and electron transport rates from PAM fluorometry were well correlated; however, the latter may overestimate oxygen production. Water column DO can measure GPP of the benthic communities; however, additional measurements/more sophisticated models may be necessary. Benthic exchange chambers and PAM fluorometry can potentially estimate the contribution of E. radiata to total daily production provided that the measurements can be calibrated with other methods to obtain actual productivity. Additionally, upscaling requires reliable biomass estimates.

Spatio-temporal variability in benthic exchanges at the sediment-water interface of a shallow tropical coastal lagoon (south coast of Gulf of Mexico)

The sediment in Laguna de Términos, the largest and shallowest system in the Southwest portion of the Gulf of Mexico features a broad range of ecological and hydrobiological characteristics driven by annual weather cycles (dry and wet seasons), causing large salinity gradients during the wet season due to large river discharges. Four sampling campaigns were carried out during the wet and the dry seasons in 2009 and 2010 on a selection of 13 out of 35 stations. Measurements of Sediment Oxygen Demand (SOD) and nutrient fluxes at the sediment-water interface were performed using lab incubations with 15 cm diameter sediment cores. SOD fluctuated between 1327 ± 161 and 2248 ± 359 μmol m−2 h−1 for dry and wet seasons respectively. Silicate effluxes were also significantly higher during the wet seasons (89.4 ± 15.9 μmol m−2 h−1) than during the dry season (46.5 ± 11.4 μmol m−2 h−1). PO4 fluxes were low all over the study period without seasonal trend. No significant difference was measured for DIN fluxes but there was a tendency for DIN uptake during the wet season (−2.9 ± 18.8 μmol m−2 h−1) and conversely an efflux during the dry season (24.3 ± 7.3 μmol m−2 h−1). SOD correlated to organic matter and chloropigment content of the sediments while silicate fluxes responded to enhanced chloropigments in the sediments. During both seasons, total benthic nutrient fluxes overwhelmed largely riverine inputs and benthic carbon mineralization rates approximated a significant proportion of the pelagic organic carbon production. We conclude that benthic processes in Laguna de Términos are largely driven by weather variability and that they contribute substantially to carbon and nutrient budgets in this shallow sub-tropical system.

Impact of seasonality on the nutrient concentrations in Gautami-Godavari Estuarine Mangrove Complex, Andhra Pradesh, India

Spatiotemporal variations of dissolved nutrients were studied along Gautami-Godavari mangrove ecosystem to delineate their sources and fate. Average values of nitrate (NO3−), dissolved silica (DSi) and phosphate (PO43−) is 2.09 mg/l, 12.7 mg/l and 0.16 mg/l in wet season and 0.47 mg/l, 6.96 mg/l and 0.29 mg/l in dry season respectively. In wet season river discharge has significant influence on NO3− and DSi. In dry season, NO3− and PO43− are controlled by groundwater discharge, benthic exchange and various in situ processes owing to sediment redox condition. Mixing model shows net addition of phosphate in Coringa mangroves (95%) and Lower estuary (13%) and net removal of nitrate (24.79%) in Coringa mangrove and in estuary (58.9%). Thus present mangrove acts as net source for phosphate and net sink for nitrate and DSi. Nutrient ratio shows seasonal switching between potential Phosphorus and Nitrogen limitation in wet and dry season respectively.

Removal of the algal toxin microcystin‐LR in permeable coastal sediments: Physical and numerical models

AbstractMicrocystin is one of the most common toxins associated with freshwater harmful algal blooms, but little is known about microcystin fate in the aquatic environment. Laboratory wave tank experiments were performed to determine whether exchange of surface water and pore water (benthic exchange) removes and dilutes microcystin‐LR (MC‐LR) at environmentally relevant concentrations in coastal waters overlying permeable sediments. Over the 100 h experiment, 60% of MC‐LR mass was removed due to interaction with sediment (via adsorption and/or biodegradation), while only 20% was removed in an experiment without sediment. The observed fate and transport of MC‐LR in sediments was adequately described with a one‐dimensional reactive transport model that uses an enhanced diffusion coefficient to represent benthic exchange of solutes. Numerical sensitivity studies showed that MC‐LR removal increases with hydraulic conductivity of sediment and wave height and decreases with water depth. For MC‐LR concentration at the WHO recreational guideline (20 ppb), sandy sediments can remove the equivalent MC‐LR mass in 1 m of surface water under typical nearshore wave conditions within tens of hours. In open water at large depths above a silty bed, removal times are much longer (on the order of weeks). Wave‐driven benthic exchange is therefore an important control on MC‐LR fate in energetic coastal areas but not in deep or calm settings where sediment–water interactions are greatly reduced. The nearshore fate of algal toxins is important to human health and socioeconomic vitality, since recreational activities and direct human exposures are concentrated along coasts.

Variability in Benthic Exchange Rate, Depth, and Residence Time Beneath a Shallow Coastal Estuary

AbstractHydrodynamically driven benthic exchange of water between the water column and shallow seabed aquifer is a significant and dynamic component of coastal and estuarine fluid budgets. Associated exchange of solutes promotes ecologically important chemical reactions, so quantifying benthic exchange rates, depths, and residence times constrains coastal chemical cycling estimates. We present the first combined field, numerical, and analytical modeling investigation of wave‐induced exchange. Temporal variability of exchange was calculated with data collected by instruments deployed in a shallow estuary for 11 days. Differential pressure sensors recorded pressure gradients across the seabed, and up‐ and down‐looking ADCPs recorded currents and pressures to determine wave parameters, surface‐water currents, and water depth. Wave‐induced exchange was calculated (1) directly from differential pressure measurements, and indirectly with an analytical model based on wave parameters from (2) ADCP and (3) wind data. Wave‐induced exchange from pressure measurements and ADCP‐measured wave parameters matched well, but both exceeded wind‐based values. Exchange induced by tidal pumping and current‐bed form interaction—the other primary drivers in shallow coastal waters were calculated from tidal stage variation and ADCP‐measured currents. Exchange from waves (mean = 20.0 cm/d; range = 1.75–92.3 cm/d) greatly exceeded exchange due to tides (mean = 3.7 cm/d) and current‐bed form interaction (mean = 6.5 × 10−2 cm/d). Groundwater flow models showed aquifer properties affect wave‐driven benthic exchange: residence time and depth increased and exchange rates decreased with increasing hydraulic diffusivity (ratio of aquifer permeability to compressibility). This new understanding of benthic exchange will help managers assess its control over chemical fluxes to marine systems.

Trace metal behavior in sediments of Jiulong River Estuary and implication for benthic exchange fluxes

Severe metal pollution due to industrial effluents releases has been documented in Jiulong River estuary, Southern China. However, integrated understanding of trace metal behavior in the sediment is lacking. In the present study, DGT (diffusive gradients in thin films) technique was employed together with sediment cores to study the porewater dynamics of trace metals as well as the benthic exchange fluxes from four sampling sites over three different months. The sedimentary environment showed distinct spatial and temporal variations due to effluent discharge and biological activities. Metal behavior was controlled by early diagenetic reactions below the interface, in suboxic layer and in deeper sediment. Precipitation as sulfides and adsorption onto Mn/Fe (hydr)oxides were important in scavenging trace metals. Estimated exchange fluxes at sediment-water interface in this estuary indicated that the overlying water was a major source for trace metals, whereas sediments could also be the source if surface remobilization (Mn/Fe reduction) dominated. Our results highlighted the impacts of both natural and anthropogenic processes on the source, fate and transformation of trace metals in this dynamic system.

The Dynamics of Benthic Respiration at a Mid-Shelf Station Off Oregon

Mid-shelf sediments off the Oregon coast are characterized as fine sands that trap and remineralize phytodetritus leading to the consumption of significant quantities of dissolved oxygen. Sediment oxygen consumption (SOC) can be delayed from seasonal organic matter inputs because of a transient buildup of reduced constituents during periods of quiescent physical processes. Between 2009 and 2013, benthic oxygen exchange rates were measured using the noninvasive eddy covariance (EC) method five separate times at a single 80-m station. Ancillary measurements included in situ microprofiles of oxygen at the sediment–water interface, and concentration profiles of pore water nutrients and trace metals, and solid-phase organic C and sulfide minerals from cores. Sediment cores were also incubated to derive anaerobic respiration rates. The EC measurements were made during spring, summer, and fall conditions, and they produced average benthic oxygen flux estimates that varied between −2 and −15 mmol m−2 d−1. The EC oxygen fluxes were most highly correlated with bottom-sensed, significant wave heights (Hs). The relationship with Hs was used with an annual record of deepwater swell heights to predict an integrated oxygen consumption rate for the mid-shelf of 1.5 mol m−2 for the upwelling season (May–September) and 6.8 mol m−2 y−1. The annual prediction requires that SOC rates are enhanced in the winter because of sand filtering and pore water advection under large waves, and it counters budgets that assume a dominance of organic matter export from the shelf. Refined budgets will require winter flux measurements and observations from cross-shelf transects over multiple years.

The Benthic Exchange of O2, N2 and Dissolved Nutrients Using Small Core Incubations.

The measurement of sediment-water exchange of gases and solutes in aquatic sediments provides data valuable for understanding the role of sediments in nutrient and gas cycles. After cores with intact sediment-water interfaces are collected, they are submerged in incubation tanks and kept under aerobic conditions at in situ temperatures. To initiate a time course of overlying water chemistry, cores are sealed without bubbles using a top cap with a suspended stirrer. Time courses of 4-7 sample points are used to determine the rate of sediment water exchange. Artificial illumination simulates day-time conditions for shallow photosynthetic sediments, and in conjunction with dark incubations can provide net exchanges on a daily basis. The net measurement of N2 is made possible by sampling a time course of dissolved gas concentrations, with high precision mass spectrometric analysis of N2:Ar ratios providing a means to measure N2 concentrations. We have successfully applied this approach to lakes, reservoirs, estuaries, wetlands and storm water ponds, and with care, this approach provides valuable information on biogeochemical balances in aquatic ecosystems.

Linking nutrient loading and oxygen in the coastal ocean: A new global scale model

AbstractRecent decades have witnessed an exponential spread of low‐oxygen regions in the coastal ocean due at least in‐part to enhanced terrestrial nutrient inputs. As oxygen deprivation is a major stressor on marine ecosystems, there is a great need to quantitatively link shifts in nutrient loading with changes in oxygen concentrations. To this end, we have developed and here describe, evaluate, and apply the Coastal Ocean Oxygen Linked to Benthic Exchange And Nutrient Supply (COOLBEANS) model, a first‐of‐its‐kind, spatially explicit (with 152 coastal segments) model, global model of coastal oxygen and nutrient dynamics. In COOLBEANS, benthic oxygen demand (BOD) is calculated using empirical models for aerobic respiration, iron reduction, and sulfate reduction, while oxygen supply is represented by a simple parameterization of exchange between surface and bottom waters. A nutrient cycling component translates shifts in riverine nutrient inputs into changes in organic matter delivery to sediments and, ultimately, oxygen uptake. Modeled BOD reproduces observations reasonably well (Nash‐Sutcliffe efficiency = 0.71), and estimates of exchange between surface and bottom waters correlate with stratification. The model examines sensitivity of bottom water oxygen to changes in nutrient inputs and vertical exchange between surface and bottom waters, highlighting the importance of this vertical exchange in defining the susceptibility of a system to oxygen depletion. These sensitivities along with estimated maximum hypoxic areas that are supported by present day nutrient loads are consistent with existing hypoxic regions. Sensitivities are put into context by applying historic changes in nitrogen loading observed in the Gulf of Mexico to the global coastal ocean, demonstrating that such loads would drive many systems anoxic or even sulfidic.

Benthic Exchange and Biogeochemical Cycling in Permeable Sediments

The sandy sediments that blanket the inner shelf are situated in a zone where nutrient input from land and strong mixing produce maximum primary production and tight coupling between water column and sedimentary processes. The high permeability of the shelf sands renders them susceptible to pressure gradients generated by hydrodynamic and biological forces that modulate spatial and temporal patterns of water circulation through these sediments. The resulting dynamic three-dimensional patterns of particle and solute distribution generate a broad spectrum of biogeochemical reaction zones that facilitate effective decomposition of the pelagic and benthic primary production products. The intricate coupling between the water column and sediment makes it challenging to quantify the production and decomposition processes and the resultant fluxes in permeable shelf sands. Recent technical developments have led to insights into the high biogeochemical and biological activity of these permeable sediments and their role in the global cycles of matter.

Benthic Exchange of C, N, and P Along the Estuarine Ecotone of Lower Taylor Slough, Florida (USA): Effect of Seasonal Flows and Phosphorus Availability

The southern Everglades and Florida Bay have experienced a nearly 50 % reduction in freshwater flow resulting in increased salinity and landward expansion of mangrove forest. Given the marine end-member is a natural source of P to this region, it is necessary to understand the interactions between inflows and P availability in controlling the exchange of materials across the mangrove ecotone. From 2007 to 2008, we used sediment core incubations to quantify fluxes of dissolved inorganic N and P and dissolved organic carbon (DOC) in three ecotone areas (dwarf mangrove, pond, and bay). Experiments were repeated seasonally over 2 years involving P-enriched surface water as a factor. We saw consistent uptake of soluble reactive P (SRP), DOC, and nitrate + nitrite (N+N) by the soils/sediments and release of ammonium (NH4 +) from soils/sediments to the water column across all sites and seasons. P enrichment had no discernible effect on DIN or DOC flux, suggesting that rapid P uptake may have been more geochemically mediated. However, uptake of added P occurred across all sites and seasons, reflecting high uptake capacity in this carbonate system and the potential of the mangrove ecotone to sequester P as it becomes more available.

Dynamic response of surface water‐groundwater exchange to currents, tides, and waves in a shallow estuary

In shallow, fetch‐limited estuaries, variations in current and wave energy promote heterogeneous surface water‐groundwater mixing (benthic exchange), which influences biogeochemical activity. Here, we characterize heterogeneity in benthic exchange within the subtidal zone of the Delaware Inland Bays by linking hydrodynamic circulation models with mathematical solutions for benthic exchange forced by current‐bedform interactions, tides, and waves. Benthic fluxes oscillate over tidal cycles as fluctuating water depths alter fluid interactions with the bed. Maximum current‐driven fluxes (~1–10 cm/d) occur in channels with strong tidal currents. Maximum wave‐driven fluxes (~1–10 cm/d) occur in downwind shoals. During high‐energy storms, simulated wave pumping rates increase by orders of magnitude, demonstrating the importance of storms in solute transfer through the benthic layer. Under moderate wind conditions (~5 m/s), integrated benthic exchange rates due to wave, current, and tidal pumping are each ~1–10 m3/s, on the order of fluid contributions from runoff and fresh groundwater discharge to the estuary. Benthic exchange is thus a significant and dynamic component of an estuary's fluid budget that may influence estuarine geochemistry and ecology.

Effects of transient bottom water currents and oxygen concentrations on benthic exchange rates as assessed by eddy correlation measurements

AbstractEddy correlation (EC) measurements in the benthic boundary layer (BBL) allow estimating benthic O2 uptake from a point distant to the sediment surface. This noninvasive approach has clear advantages as it does not disturb natural hydrodynamic conditions, integrates the flux over a large foot‐print area and allows many repetitive flux measurements. A drawback is, however, that the measured flux in the bottom water is not necessarily equal to the flux across the sediment‐water interface. A fundamental assumption of the EC technique is that mean current velocities and mean O2 concentrations in the bottom water are in steady state, which is seldom the case in highly dynamic environments like coastal waters. Therefore, it is of great importance to estimate the error introduced by nonsteady state conditions. We investigated two cases of transient conditions. First, the case of transient O2 concentrations was examined using the theory of shear flow dispersion. A theoretical relationship between the change of O2 concentrations and the induced vertical O2 flux is introduced and applied to field measurements showing that changes of 5–10 μM O2 h−1 result in transient EC‐fluxes of 6–12 mmol O2 m−2 d−1, which is comparable to the O2 uptake of shelf sediments. Second, the case of transient velocities was examined with a 2D k‐ε turbulence model demonstrating that the vertical flux can be biased by 30–100% for several hours during changing current velocities from 2 to 10 cm s−1. Results are compared to field measurements and possible ways to analyze and correct EC‐flux estimates are discussed.