Advective Pore Water Exchange Research Articles

Using 224Ra/228Th disequilibrium to quantify carbon transformation and export from intertidal sandy and muddy sediments

The intertidal zone is an important pathway for the transport of dissolved carbon across the land–ocean interface. Quantifying carbon transformation and export from the intertidal zone remains large uncertainties due to the natural heterogeneity and various driving mechanisms. Here, we used the 224Ra/228Th disequilibrium method to quantify porewater exchange and dissolved carbon export from intertidal sandy and muddy sediments, two main intertidal wetland types in the coast. Porewater exchange fluxes from sandy sediments ranged from 1.4 ± 0.1 to 1084 ± 53 L m−2 h−1, which gradually increased in the landward direction. Porewater exchange fluxes from mangrove muddy sediments ranged from 0 to 4.4 ± 0.7 L m−2 h−1. Advective porewater exchange was recognized as the dominant solute transport process in sandy sediments, whereas bioturbation may be the key process in mangrove muddy sediments. Furthermore, marine dissolved organic carbon (DOC) was removed in sandy sediments, indicating that the sandy sediment acted as a sink of DOC, likely via aerobic remineralization. DOC consumption fluxes ranged from 3.7 ± 0.2 to 206 ± 9 mmol C m−2 d−1. The total consumption flux of DOC in the entire sandy beach was one order of magnitude higher than the globally maximum DOC removal flux in the upper ocean. Porewater-derived dissolved inorganic carbon (DIC) export fluxes from sandy sediments ranged from 5.6 ± 0.3 to 883 ± 43 mmol C m−2 d−1. Nearly 50% of DIC export originated from the consumption of DOC. The rate constants for DOC transformation ranged from 0.009 to 0.155 h−1. In comparison, porewater-derived DOC and DIC export fluxes from muddy sediments were 25 ± 4 and 206 ± 30 mmol C m−2 d−1, respectively. Porewater-derived DIC export from sandy sediments was comparable or even higher than those from muddy sediments. These results showed that sandy sediments acted as a fast lane for marine DOC remineralization. Overall, both organic-poor sandy and organic-rich mangrove muddy sediments play important roles in the delivery of dissolved carbon to the ocean.

Carbon Cycling in the World’s Mangrove Ecosystems Revisited: Significance of Non-Steady State Diagenesis and Subsurface Linkages between the Forest Floor and the Coastal Ocean

Carbon cycling within the deep mangrove forest floor is unique compared to other marine ecosystems with organic carbon input, mineralization, burial, and advective and groundwater export pathways being in non-steady-state, often oscillating in synchrony with tides, plant uptake, and release/uptake via roots and other edaphic factors in a highly dynamic and harsh environment. Rates of soil organic carbon (CORG) mineralization and belowground CORG stocks are high, with rapid diagenesis throughout the deep (>1 m) soil horizon. Pocketed with cracks, fissures, extensive roots, burrows, tubes, and drainage channels through which tidal waters percolate and drain, the forest floor sustains non-steady-state diagenesis of the soil CORG, in which decomposition processes at the soil surface are distinct from those in deeper soils. Aerobic respiration occurs within the upper 2 mm of the soil surface and within biogenic structures. On average, carbon respiration across the surface soil-air/water interface (104 mmol C m−2 d−1) equates to only 25% of the total carbon mineralized within the entire soil horizon, as nearly all respired carbon (569 mmol C m−2 d−1) is released in a dissolved form via advective porewater exchange and/or lateral transport and subsurface tidal pumping to adjacent tidal waters. A carbon budget for the world’s mangrove ecosystems indicates that subsurface respiration is the second-largest respiratory flux after canopy respiration. Dissolved carbon release is sufficient to oversaturate water-column pCO2, causing tropical coastal waters to be a source of CO2 to the atmosphere. Mangrove dissolved inorganic carbon (DIC) discharge contributes nearly 60% of DIC and 27% of dissolved organic carbon (DOC) discharge from the world’s low latitude rivers to the tropical coastal ocean. Mangroves inhabit only 0.3% of the global coastal ocean area but contribute 55% of air-sea exchange, 14% of CORG burial, 28% of DIC export, and 13% of DOC + particulate organic matter (POC) export from the world’s coastal wetlands and estuaries to the atmosphere and global coastal ocean.

Large CO2 release and tidal flushing in salt marsh crab burrows reduce the potential for blue carbon sequestration

AbstractAbundant crab burrows in carbon‐rich, muddy salt marsh soils act as preferential water flow conduits, potentially enhancing carbon transport across the soil–water interface. With increasing recognition of blue carbon systems (salt marshes, mangroves, and seagrass) as hotspots of soil carbon sequestration, it is important to understand drivers of soil carbon cycling and fluxes. We conducted field observations and flow modeling to assess how crab burrows drive carbon exchange over time scales of minutes to weeks in an intertidal marsh in South Carolina. Results showed that continuous advective porewater exchange between the crab burrows and the surrounding soil matrix occurs because of tidally driven hydraulic gradients. The concentrations of dissolved inorganic (DIC) and organic (DOC) carbon in crab burrow porewater differ with that in the surrounding soil matrix, implying a diffusive C flux in the low‐permeability marsh soil. Gas‐phase concentrations of CO2 in ∼ 300 crab burrows were approximately six times greater than ambient air. The estimated total C export rate via porewater exchange (1.0 ± 0.7 g C m−2 d−1) was much greater than via passive diffusion transport (6.7 ± 2 mg C m−2 d−1) and gas‐phase CO2 release (0.93 mg C m−2 d−1). The burrow‐related carbon export was comparable to the regional salt marsh DIC export, groundwater‐derived DIC export, and the net primary production previously estimated using ecosystem‐scale approaches. These insights reveal how crab burrows modify blue carbon sequestration in salt marshes and contribute to coastal carbon budgets.

Carbon outwelling and outgassing vs. burial in an estuarine tidal creek surrounded by mangrove and saltmarsh wetlands

AbstractMangrove‐ and saltmarsh‐dominated estuaries have high rates of organic carbon burial. Here, we estimate soil, pore water, and surface‐water carbon fluxes in an Australian estuarine tidal creek to assess whether (1) advective pore water exchange releases some of the soil carbon, (2) outwelling (lateral exports) represents a major carbon sequestration mechanism, and (3) methane emissions offset soil carbon sequestration. A radon (222Rn) mass balance implied tidally driven pore‐water exchange rates ranging from 5.5 ± 3.6 to 15.6 ± 8.1 cm d−1. Pore water exchange explained most of the dissolved organic carbon (DOC) and methane surface‐water fluxes but not dissolved inorganic carbon (DIC) and alkalinity. Organic carbon burial in soils derived from 239 + 240Pu dating was 11–63 g C m−2 yr−1. Methane and carbon dioxide emissions at the water–air interface were 0.27 ± 0.03 and 63 ± 166 mmol m−2 d−1, respectively. When calculated as CO2‐equivalents, aquatic CH4 emissions converted to 19–94 g C‐CO2 m−2 yr−1. Upscaling methane and soil carbon fluxes to representative areas revealed that CH4 emissions could offset < 5% of soil carbon burial. DIC outwelling (12 ± 6 mmol m−2 catchment d−1) was less than five‐fold greater than DOC and particulate organic carbon (POC) outwelling and four‐fold greater than catchment‐wide carbon burial. Because much of this DIC remains in the ocean after air–water equilibration, lateral DIC exports may represent an important long‐term carbon sink. Recent research has focused on quantifying carbon burial rates in blue carbon habitats such as saltmarshes and mangroves. We suggest that DIC outwelling and methane outgassing should also be considered when assessing the carbon sequestration capacity of these coastal vegetated habitats.

Porewater inputs drive Fe redox cycling in the water column of a temperate mangrove wetland

Iron is a vital micronutrient within coastal marine ecosystems, playing an integral role in the scale and dynamics of primary production and carbon cycling in the world's oceans. We investigated the relative importance of in situ Fe(II) production from photochemical, microbial and thermal Fe reduction in the surface water column as well as advective porewater inputs in a temperate saline wetland in Australia containing mangrove and saltmarsh vegetation. The diel average concentration of Fe(II) (0.63 ± 0.21 μM, accounting for >70% of the total dissolved Fe present in surface water) was much higher than commonly reported in oxygenated marine waters despite high dissolved oxygen concentrations (81–112% saturation), pH (7.7–7.8) and salinity (33–36) that favor Fe oxidation. In situ production of Fe(II) in the surface water column was primarily driven by microbial processes rather than photochemical and thermal reduction, with a maximum production rate of 4.9 × 10−3 nM s−1. Advective porewater Fe(II) inputs to the wetland averaged over a diel cycle (3.0 × 10−1 nM s−1) were an order of magnitude greater than the combined Fe(II) production rate from autochthonous water column processes (1.0 × 10−2 nM s−1). A bottom up model based on the estimated individual fluxes was used to explain the high Fe(II) concentrations measured during a 24 h time series experiment. Combined, different lines of evidence suggest that advective porewater exchange provides significant quantities of Fe(II) to the estuarine wetland.

Carbon, nutrient and trace metal cycling in sandy sediments: A comparison of high-energy beaches and backbarrier tidal flats

In order to evaluate the importance of coastal sandy sediments and their contribution to carbon, nutrient and metal cycling we investigated two beach sites on Spiekeroog Island, southern North Sea, Germany, and a tidal flat margin, located in Spiekeroog's backbarrier area. We also analyzed seawater and fresh groundwater on Spiekeroog Island, to better define endmember concentrations, which influence our study sites.Intertidal sandy flats and beaches are characterized by pore water advection. Seawater enters the sediment during flood and pore water drains out during ebb and at low tide. This pore water circulation leads to continuous supply of fresh organic substrate to the sediments. Remineralization products of microbial degradation processes, i.e. nutrients, and dissolved trace metals from the reduction of particulate metal oxides, are enriched in the pore water compared to open seawater concentrations.The spatial distribution of dissolved organic carbon (DOC), nutrients (PO43−, NO3−, NO2−, NH4+, Si(OH)4 and total alkalinity), trace metals (dissolved Fe and Mn) as well as sulfate suggests that the exposed beach sites are subject to relatively fast pore water advection, which leads to organic matter and oxygen replenishment. Frequent pore water exchange further leads to comparatively low nutrient concentrations. Sulfate reduction does not appear to play a major role during organic matter degradation. High nitrate concentrations indicate that redox conditions are oxic within the duneward freshwater influenced section, while ammonification, denitrification, manganese and iron reduction seem to prevail in the ammonium-dominated seawater circulation zone. In contrast, the sheltered tidal flat margin site exhibits a different sedimentology (coarser beach sands versus finer tidal flat sands) and nutrients, dissolved manganese and DOC accumulate in the pore water. Ammonium is the dominant pore water nitrogen species and intense sulfate reduction leads to the formation of sulfide, which precipitates dissolved iron as iron sulfide. These findings are due to slower advective pore water exchange in the tidal flat sediments. This study illustrates how different energy regimes affect biogeochemical cycling in intertidal permeable sediments.

Carbon cycling hysteresis in permeable carbonate sands over a diel cycle: Implications for ocean acidification

Dissolved inorganic carbon, dissolved oxygen, H+, and alkalinity fluxes from permeable carbonate sediments at Heron Island (Great Barrier Reef) were measured over one diel cycle using benthic chambers designed to induce advective pore‐water exchange. A complex hysteretic pattern between carbonate precipitation and dissolution in sands and the aragonite saturation state (ΩAr) of the overlying chamber water was observed throughout the incubations. During the day, precipitation followed a hysteretic pattern based on the incidence of photosynthetically active radiation with lower precipitation rates in the morning than in the afternoon. The observed diel hysteresis seems to reflect a complex interaction between photosynthesis and respiration rather than ΩAr of the overlying water as the main driver of carbonate precipitation and dissolution within these permeable sediments. Changes in flux rates over a diel cycle demonstrate the importance of taking into account the short‐term variability of benthic metabolism when calculating net daily flux rates. Based on one diel cycle, the sediments were a net daily source of alkalinity to the water column (5.13 to 8.84 mmol m−2 d−1, depending on advection rates), and advection had a net stimulatory effect on carbonate dissolution. The enhanced alkalinity release associated with benthic metabolism and pore‐water advection may partially buffer shallow coral reef ecosystems against ocean acidification on a local scale.

The “salt wedge pump”: Convection‐driven pore‐water exchange as a source of dissolved organic and inorganic carbon and nitrogen to an estuary

Hypoxia and anoxia in coastal waters have typically been explained by the respiration of sinking organic matter associated with nutrient over‐enrichment and phytoplankton blooms. Here, we assess whether submarine groundwater discharge and seawater recirculation in sediments can explain widespread chemical anomalies, including low dissolved oxygen, in salt wedge estuaries. We rely on high‐resolution radon (a natural groundwater and pore‐water tracer), and dissolved carbon concentrations and stable isotope observations in the Yarra River estuary in Melbourne, Australia. Radon was highly enriched within the salt wedge, demonstrating enhanced pore‐water exchange at this area. We use the term “salt wedge pump” to describe convection‐driven advective pore‐water exchange at the sediment–water interface during the upstream propagation of the salt wedge. Radon‐derived convection‐driven pore‐water exchange rates within the salt wedge were estimated at 2.8 cm d−1, a value equivalent to 2.4% of the total river freshwater runoff to the estuary. Pore‐water exchange led to pulsed dissolved inorganic carbon (DIC) and ammonium fluxes ∼ 10‐fold higher than measured diffusive fluxes. In contrast, diffusive sediment oxygen uptake was 5‐fold higher than oxygen uptake related to advective pore‐water exchange. Estimated fluxes, associated with the nonconservative DIC, δ13C‐DIC, and ammonium behavior within the estuary support convective pore‐water exchange as a major source of DIC and ammonium to the estuary, but not of dissolved organic carbon, nitrate, dissolved organic nitrogen, and anoxia. Accounting for seawater recirculation in sediments may help reconcile unbalanced carbon and nitrogen budgets in several coastal systems.

Relationship between advective pore-water exchange at sediment-water interface and wave ripples

Relationship between advective pore-water exchange at sediment-water interface and wave ripples

The widespread occurrence of coupled carbonate dissolution/reprecipitation in surface sediments on the Bahamas Bank

Using two complimentary approaches (pore water advection/diffusion/ reaction modeling and stable isotope mass balance calculations) we show that carbonate dissolution/reprecipitation occurs on early diagenetic time scales across a broad range of sediments on the Great Bahamas Bank. The input of oxygen into the sediments, which strongly controls sediment carbonate dissolution, has two major sources—belowground input by seagrasses (that is, seagrass O2 pumping), and permeability-driven advective pore water exchange. The relative importance of these O2 delivery mechanisms depends on both seagrass density, and on how bottom water flow interacts with the seagrass canopy and leads to this advective exchange. Dissolution appears to involve the preferential dissolution of high-Mg calcite, and the rates of dissolution increase linearly with increasing seagrass density. Isotopic evidence of dissolution/reprecipitation is consistent with the occurrence of Ostwald ripening as the mechanism of reprecipitation, in which smaller crystals dissolve and then reprecipitate as larger crystals, with little or no change in mineralogy. Estimates of the aerially-integrated dissolution flux on the Bahamas Bank suggest that carbonate dissolution is an important loss term in the budget of shallow water carbonate sediments, and that on-bank carbonate dissolution, rather than offshore transport, may represent an important sink for gross shallow water carbonate production. Dissolution in carbonate bank and bay sediments may also be a significant alkalinity source to the surface ocean, and should be considered in global alkalinity/carbonate budget. Finally, coupled dissolution/reprecipitation may have a major impact on the stable isotope composition of carbonate sediments that are ultimately preserved in the rock record. These processes may therefore need to be considered, for example, when using carbon isotope records to obtain information on the operation of the global carbon cycle during the Phanerozoic.

Constraining denitrification in permeable wave-influenced marine sediment using linked hydrodynamic and biogeochemical modeling

Permeable marine sediments are ubiquitous complex environments, the biogeochemistry of which are strongly coupled to hydrodynamic process above and within the sediment. The biogeochemical processes in these settings have global scale implications but are poorly understood and challenging to quantify. We present the first simulation of linked turbulent-oscillatory flow of the water column, porous media flow, and solute transport in the sediment with oxygen consumption, nitrification, denitrification, and ammonification, informed by field- and/or experimentally-derived parameters. Nitrification and denitrification were significantly impacted by advective pore water exchange between the sediment and the water column. Denitrification rates showed a maximum at intermediate permeabilities, and were negligible at high permeabilities. Denitrification rates were low, with only ∼ 15% of total N mineralized being denitrified, although this may be increased temporarily following sediment resuspension events. Our model-estimated denitrification rates are about half of previous estimates which do not consider solute advection through the sediment. Given the critical role of sediment permeability, topography, and bottom currents in controlling denitrification rates, an improved knowledge of these factors is vital for obtaining better estimates of denitrification taking place on shelf sediment. Broad application of our approach to myriad conditions will lead to improved predictive capacity, better informed experimental and sampling design, and more holistic understanding of the biogeochemistry of permeable sediment.

Characterization of Nitrifying, Denitrifying, and Overall Bacterial Communities in Permeable Marine Sediments of the Northeastern Gulf of Mexico

Sandy or permeable sediment deposits cover the majority of the shallow ocean seafloor, and yet the associated bacterial communities remain poorly described. The objective of this study was to expand the characterization of bacterial community diversity in permeable sediment impacted by advective pore water exchange and to assess effects of spatial, temporal, hydrodynamic, and geochemical gradients. Terminal restriction fragment length polymorphism (TRFLP) was used to analyze nearly 100 sediment samples collected from two northeastern Gulf of Mexico subtidal sites that primarily differed in their hydrodynamic conditions. Communities were described across multiple taxonomic levels using universal bacterial small subunit (SSU) rRNA targets (RNA- and DNA-based) and functional markers for nitrification (amoA) and denitrification (nosZ). Clonal analysis of SSU rRNA targets identified several taxa not previously detected in sandy sediments (i.e., Acidobacteria, Actinobacteria, Chloroflexi, Cyanobacteria, and Firmicutes). Sequence diversity was high among the overall bacterial and denitrifying communities, with members of the Alphaproteobacteria predominant in both. Diversity of bacterial nitrifiers (amoA) remained comparatively low and did not covary with the other gene targets. TRFLP fingerprinting revealed changes in sequence diversity from the family to species level across sediment depth and study site. The high diversity of facultative denitrifiers was consistent with the high permeability, deeper oxygen penetration, and high rates of aerobic respiration determined in these sediments. The high relative abundance of Gammaproteobacteria in RNA clone libraries suggests that this group may be poised to respond to short-term periodic pulses of growth substrates, and this observation warrants further investigation.

Nutrient fluxes from deep sediment support nutrient budget in the oligotrophic waters of the Gulf of Aqaba

The role of deep sediment in supporting nutrient budget in the Gulf of Aqaba has been investigated by estimating the flux of inorganic nitrogen, phosphate and sili- cate. Fluxes were calculated directly by pore water profiles and indirectly by cham- ber incubations carried out onboard the RV Meteor cruise. The results showed that maximum potential fluxes calculated by chamber incubations were higher than those calculated by porewater profiles for all nutrients (6.4-28.5 fold). This has been attributed to the additional flux due to bioturbation and flux from advective porewater exchange in the case of chamber incubation, while porewater fluxes represent diffu- sive ones. Using a rough estimation considering flux results in addition to the sedi- ment area and water mass of the Gulf of Aqaba, we estimate that 3.3 × 10 5 , 6.4 × 10 4 and 6.5 × ×× ×× 10 6 kg year -1 of inorganic nitrogen, phosphate and silicate respectively are effused from deep sediments to the water column. This quanitity would certainly sup- port the primary productivity in the oligotrophic water in the Gulf of Aqaba.

Pore‐water advection and solute fluxes in permeable marine sediments (I): Calibration and performance of the novel benthic chamber system Sandy

This contribution introduces the benthic chamber system Sandy that was developed for studying in situ solute fluxes in permeable sandy sediments where advective pore‐water transport can dominate solute exchange across the sediment‐water interface. The Sandy system can be deployed at the seafloor, where it autonomously performs measurements of sediment‐water solute fluxes. The innovative features include an insertion mechanism that permits gentle and deep penetration of the chamber into hard consolidated sands with minimum disturbance, and an adjustable stirrer that generates a rotationally symmetric pressure gradient between the center and the circumference of the enclosed sediment surface. In contrast to similar systems, Sandy takes advective pore‐water exchange into account. Through adjustment of the stirring rate, defined pressure gradients can be established that are similar in shape and magnitude to natural pressure gradients that develop at topographical structures of current‐exposed sediment surfaces. Solute fluxes measured in the chamber at a specific “advective” stirrer setting can be compared with those obtained at reduced stirring, where pressure gradients are absent and solute exchange therefore is restricted to diffusion and bioirrigation. Rates of pore‐water exchange that were determined by means of tracer experiments at different stirrer settings in natural coarse‐grained North Sea deposits demonstrate the feasibility of this approach. This permits, for the first time, an evaluation of the contribution of advective exchange to the total interfacial solute flux in situ and makes Sandy a valuable tool to study advection‐related processes in permeable shelf sediments.

Pore‐water advection and solute fluxes in permeable marine sediments (II): Benthic respiration at three sandy sites with different permeabilities (German Bight, North Sea)

This contribution presents total oxygen uptake (TOU) rates and nutrient fluxes of organically poor permeable shelf sands of the German Bight. Measurements have been made in situ with the novel autonomous benthic chamber system Sandy under controlled conditions of advective pore‐water exchange. Average oxygen consumption rates of 31.3 ± 18.2 mmol m−2 d−1 measured in this study were relatively high as compared with rates reported from shelf sediments with much higher organic contents. TOU of highly permeable medium and coarse grained sands was substantially enhanced in the presence of advection. This indicates that advective oxygen supply contributed significantly to respiration in these sediments and that advection has to be considered when assessing oxygen consumption and organic matter mineralization in shelf areas. In fine‐grained, less permeable sands, no effect of advection could be measured. A lower advective oxygen supply in these sediments is in agreement with a release of ammonium instead of nitrate and a shallower oxygen penetration depth. Scaled up to the entire German Bight, the results imply that in 40% of the area an effect of advection on benthic oxygen uptake and other advectionrelated processes can be largely excluded, while in the remaining 60% significant pore‐water advection potentially takes place. However, because permeabilities of the sediments investigated in this study were widely spaced, a significant effect on oxygen supply was only verified for highly permeable sands that are likely to cover approximately 3% of the area.

Role of pelletization in mineralization of fine-grained coastal sediments

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 291:23-33 (2005) - doi:10.3354/meps291023 Role of pelletization in mineralization of fine-grained coastal sediments Christian Wild1,3,*, Hans Røy1, Markus Huettel1,2 1Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany2Department of Oceanography, Florida State University, Tallahassee, Florida 32306-4320, USA3Present address: Ocean Sciences Section, Intergovernmental Oceanographic Comission, UNESCO, 1 rue Miollis, 75015 Paris, France *Email: c.wild@lrz.uni-muenchen.de ABSTRACT: This study investigates how pellet deposition caused by the head-down deposit feeding polychaete Heteromastus filiformis enhances sediment surface layer permeability and thereby the degradation of organic matter contained in that layer. The permeability of pellet accumulations on the sediment surface was 2 orders of magnitude higher than that of the non-pelletized sediment surface, which also had a 30% lower porosity. Freshly deposited organic-rich pellets initially consumed approximately 10 times more O2 than an equivalent volume of the ambient, oxic, surface sediment. As a consequence of the deposition of the relatively large (400 to 500 µm) grain-like pellets in small mounds, topographical structures are generated on the originally rather flat surface, which cause advective pore water flow through the pellet mounds as soon as the latter are exposed to boundary layer flows. At a flow velocity of 70 mm s–1 at 5 mm above the sediment, pore water flow velocity through pellet accumulations ranged from 1 to 2 mm s–1, transporting oxygenated water to the pellets. Incubation experiments in flow-through columns showed that the O2 consumption of pellet accumulations increases by approximately 100 µmol (g dry mass)–1 d–1 when the flow rate is increased from 10 to 20 ml h–1. A faster degradation of reduced organic C and N contained in the fecal pellets was observed under the influence of flow (70% of C and 68% of N degraded) compared to stagnancy (44% of C and 40% of N degraded) in a 3 wk flume incubation. Degradation of organic C and N in the surrounding, flat, surface sediment, which was not flushed by water flow, was not detectable during the same time period. We conclude that pelletization enhances organic matter turnover in fine-grained deposits through generation of a high secondary permeability of the sediment surface layer that permits advective pore water exchange, enhancing mineralization of the deposited organic-rich pellets. KEY WORDS: Heteromastus filiformis · Fecal pellets · Permeability · Advective flow · Oxygen profiles · Organic matter degradation Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 291. Online publication date: April 28, 2005 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2005 Inter-Research.

Oxygen dynamics in permeable sediments with wave‐driven pore water exchange

The effects of advective pore water exchange driven by shallow water waves on the oxygen distribution in a permeable (k = 3.3 x 10−12 to 4.9 x 10−11 m2) natural sediment were studied with a planar oxygen optode in a wave tank. Our experiments demonstrate that pore water flow driven by the interaction of sediment topography and oscillating boundary flow changes the spatial and temporal oxygen distribution in the upper sediment layer. Oxygenated water intruding in the ripple troughs and deep anoxic pore water drawn to the surface under the ripple crests create an undulating oxic‐anoxic boundary within the upper sediment layer, mirroring the topographical features of the sediment bed. Anoxic upwelling zones under ripple crests can separate the oxic sediment areas of neighboring ripple troughs with steep horizontal oxygen concentration gradients. The optode showed that migrating wave ripples are trailed by their pore water flow field, alternately exposing sediment volumes to oxic and anoxic pore water, which can be a mechanism for remobilizing particulate oxidized metal precipitates and for promoting coupled nitrification‐denitrification. More rapid ripple migration (experimental threshold ~20 cm h−1) produces a continuous oxic surface layer that inhibits the release of reduced substances from the bed, which under slowly moving ripples is possible through the anoxic vertical upwelling zones. Swift, dramatic changes in oxygen concentration in the upper layers of permeable seabeds because of surface gravity waves require that sediment‐dwelling organisms are tolerant to anoxia or highly mobile and enhance organic matter mineralization.

Rapid wave-driven advective pore water exchange in a permeable coastal sediment

In this study we present in-situ measurements of pore water flow velocities in a coastal sandy sediment (permeability=3.65×10 −10 m 2). The advective pore water flows were driven by the interaction of oscillating boundary flows with sediment wave ripples, (amplitude=7 cm, wavelength=30 to 50 cm). The measurements were carried out in the Mediterranean Sea at 50 to 70 cm water depth during a phase of very low wave energy (max. wave amplitude=10 cm). An optode technique is introduced that permits direct pore water flow measurements using a fluorescent tracer. Near the sediment surface (0.5 cm depth) pore water reached velocities exceeding 40 cm h −1. Thus, advective transport exceeded transport by molecular diffusion by at least 3 orders of magnitude. Based on the pore water velocity measurements and ripple spacing, we calculate that 140 L m −2 d −1 are filtered through the sediment. Pore water visualisation experiments revealed a flow field with intrusion of water in the ripple troughs and pore water release at the ripple crests. The wave-driven water flow through the sediment, thus, was directly linked to the wave-generated sediment topography, and its spatial dimensions. These results show that surface waves cause water filtration through permeable sediments at water depths smaller than half the wavelength. We conclude that surface gravity waves constitute an important hydromechanical process that may convert large areas of the continental shelves into expansive filter systems. Surface gravity waves thereby could affect suspended particle concentration and cycling of matter in the shelf.

Benthic photosynthesis and oxygen consumption in permeable carbonate sediments at Heron Island, Great Barrier Reef, Australia

In order to investigate benthic photosynthesis and oxygen demand in permeable carbonate sands and the impact of benthic boundary layer flow on sedimentary oxygen consumption, in situ and laboratory chamber experiments were carried out at Heron Island, Great Barrier Reef, Australia. Total photosynthesis, net primary production and respiration were estimated to be 162.9±43.4, 98.0±40.7, and 64.9±15.0 mmol C m −2 d −1, respectively. DIN and DIP fluxes for these sands reached 0.34 and 0.06 mmol m −2 d −1, respectively. Advective pore water exchange had a strong impact on oxygen consumption in the permeable sands. Consumption rates in the chamber with larger pressure gradient (20 rpm, 1.2 Pa between centre and rim) simulating a friction velocity of 0.6 cm s −1 were approximately two-fold higher than in the chambers with slow stirring (10 rpm, 0.2 Pa between centre and rim, friction velocity of 0.3 cm s −1). In the laboratory chamber experiments with stagnant water column, oxygen consumption was eight times lower than in the chamber with fast stirring. Laboratory chamber experiments with Br − tracer revealed solute exchange rates of 2.6, 2.2, 0.7 ml cm −2 d −1 at stirring rates of 20, 10, and 0 rpm, respectively. In a laboratory experiment investigating the effect of sediment permeability on oxygen and DIC fluxes, a three-fold higher permeability resulted in two- to three-fold higher oxygen consumption and DIC release rates. These experiments demonstrate the importance of boundary flow induced flushing of the upper layer of permeable carbonate sediment on oxygen uptake in the coral sands. The high filtration and oxidation rates in the sub-tropical permeable carbonate sediments and the subsequent release of nutrients and DIC reveal the importance of these sands for the recycling of matter in this oligotrophic environment.

Impact of boundary layer flow velocity on oxygen utilisation in coastal sediments

Small pressure gradients generated by boundary flow-topography interactions cause advective pore water flows in permeable sediments. Advective pore water exchange enhances the flux of solutes between the sediment and the overlying water, thus generating conditions for an increased utilisation of oxygen. We compared a less permeable (k = 5 X 10-l2 m2) with a permeable sediment (k = 5 X 10- m') typical for coastal and shelf sediments. Total oxygen utilisation (TOU) in incubated sediment cores was measured in 10 laboratory experiments using recirculating flow tanks (33 runs). TOU was a function of flow velocity in permeable sediment where advective pore water flow occurred. TOU increased with the increasing volume of sediment flushed with oxygenated water. We found that TOU increased by 91 2 23% in coarse sand when flow increased from 3 to 14 cm S-' (38 mounds m-', height 10 to 30 mm, flow measured 8 cm above the sediment). Additlon of fresh algal material caused a stronger stimulation of TOU in the coarse sand than in the fine sand (4 additional flume runs). After the addition, intensive oxygen consumption reduced the oxygen penetration depth in the advectively flushed zone of the coarse sediment. However, counteracting this process, advectlve flow maintained an oxic sediment volume still larger than that in the less permeable sediment. Flow-enhanced oxygen utilisation is potentially effective in permeable beds of coastal and shelf regions, in contrast to the situation in cohesive sediments limited by predominantly diffusive oxygen supply.