Sediment-water Exchange Research Articles

Nutrient-Replete Benthic Microalgae as a Source of Dissolved Organic Carbon to Coastal Waters

Dissolved organic carbon (DOC) flux dynamics were examined in the context of other biogeochemical cycles in intertidal sediments inhabited by benthic microalgae. In August 2003, gross oxygenic photosynthetic (GOP) rates, oxygen penetration depths, and benthic flux rates were quantified at seven sites along the Duplin River, GA, USA. Sediments contained abundant benthic microalgal (BMA) biomass with a maximum chlorophyll a concentration of 201 mg chl a m−2. Oxygen microelectrodes were used to determine GOP rates and O2 penetration depth, which were tightly correlated with light intensity. Baseline and 15N-nitrate amended benthic flux core incubations were employed to quantify benthic fluxes and to investigate the impact of BMA on sediment water exchange under nitrogen (N)-limited and N-replete conditions. Unamended sediments exhibited tight coupling between GOP and respiration and served as a sink for water column dissolved inorganic nitrogen (DIN) and a source of silicate and dissolved inorganic carbon (DIC). The BMA response to the N addition indicated sequential nutrient limitation, with N limitation followed by silicate limitation. In diel (light–dark) incubations, biological assimilation accounted for 83% to 150% of the nitrate uptake, while denitrification (DNF) and dissimilatory nitrate reduction to ammonium (DNRA) accounted for 40% of the NO3− uptake. The N addition shifted the metabolic status of the sediments from a balance of autotrophy and heterotrophy to net autotrophy under diel conditions, and the sediments served as a sink for water column DIN, silicate, and DIC but became a source of DOC, suggesting that the increased BMA production was decoupled from sediment bacterial consumption of DOC.

Effects of iron-ore mining and processing on metal bioavailability in a tropical coastal lagoon

Background, aim, and scope In water systems, water quality and geochemical properties of sediments determine the speciation of trace metals, metal transport, and sediment–water exchange, influencing metal availability and its potential effects on biota. Studies from temperate climates have shown that iron-ore mining and tailing wastewaters, besides being a source of trace metals, usually show high levels of dissolved ions and particulate suspended matter, thus having the potential of indirectly changing metal bioavailability. For the first time in the tropics, we identified the effects of iron-ore mining and processing on metal bioavailability in a coastal lagoon. With an extensive sampling scheme, we investigated the potential sources of metals; the links among metal levels in water, sediments, and invertebrates; and the contrasting effects on metal speciation and bioavailability.

Denitrification in the mainstem Neuse River and tributaries, USA

We conducted quarterly (spring, summer, fall and winter) determinations of denitrification at representative sites in the mainstem Neuse River and tributaries, USA. Denitrification measurements were made on intact sediment cores using the N 2 /Ar and isotope pairing techniques. We measured nitrate (NO 3 - -N) and ammonium (NH 4 + -N) concentrations in the water column at the time of core collection and evaluated rates of sediment-water exchange of dissolved oxygen (O 2 ) in conjunction with denitrification rate determinations. Nitrate concentrations in the tributary streams and river water varied over an order or magnitude, from about 10 to 110 λmol/L, while NH 4 + -N concentrations were lower, varying from 0.6 to 10.8 μmol/L. Sediments consistently consumed dissolved O 2 , with rates varying from 256 to 1418 pmol m -2 h -1 . Most cores actively denitrified with rates ranging to 222 μmol N 2 -N m -2 h -1 . On average, coupled nitrification-denitrification accounted for 66 % of total denitrification. Lowest rates of denitrification were generally observed during December, when the water temperature was lowest at 4 °C. No other relationship was identified between rates of denitrification and measured physicochemical variables within or among sites or dates as sampling frequency and replication were low. Our study suggests that denitrification represents a sink of about 5 % for the NO 3 - -N load to the mainstem Neuse River and tributary streams.

Context-dependent effects of suspension feeding on intertidal ecosystem functioning

Species loss or gain can change ecosystem functions according to the ecological context (e.g. assemblage composition, habitat heterogeneity and disturbance regime). We document how the suspension-feeder cockle Cerastoderma edule (L.) affects some properties of intertidal sediments during macrofauna recovery after deliberately induced anoxia. Forty adult cockles were transplanted to each of 12 plots (25 × 25 cm) in 3 physically homogeneous patches (4 × 4 m). Two patches had recolonised for 4 or 7 mo (DJ and DM, respectively) and had a very similar number of species (DJ: 4.5 ± 0.3 and DM: 5 ± 0.0). The other patch (C) was natural sediment with a relatively high spe- cies diversity (6.5 ± 0.6). Species composition was different among the 3 patches (DJ < DM < C) and indicated an increasing number of individuals and functional groups in accordance with the duration of the recovery period (DJ < DM < C). Changes in abundance of the resident macrofauna, in total benthic mineralization rates (oxygen uptake and carbon dioxide release) and in the exchange of in- organic nitrogen at the sediment-water interface were compared between transplanted and undis- turbed plots. The functional response of the sediment ecosystem to the gain of C. edule varied among the patches. Following C. edule addition, ammonium release increased 3 to 4 times in the sediment which had been recolonising for 7 mo (DM), while uptake of nitrate decreased at similar levels in the other 2 patches (DJ and C). Effects were related to variations in the densities of 2 species, namely new recruits of the surface deposit-feeder Macoma balthica (≤5 mm) and the surface-detritivore and bio- irrigator Hediste diversicolor. Cockles caused a decrease in the densities of M. balthica (≤5 mm) and an increase in those of H. diversicolor, mainly in DM. Cockles also modified how densities of these species and of Heteromastus filiformis correlated to sediment-water exchanges of ammonium and oxygen. C. edule also enhanced oxygen uptake at all sites, mainly due to its specific respiration rate. Variability in the functional response among patches of habitat (e.g. context-dependence) resulted as a consequence of biotic interactions between the cockles and the resident infauna.

Sediment–water exchange of methylmercury determined from shipboard benthic flux chambers

Benthic mobilization is an important pathway by which organisms in aquatic systems may be exposed to monomethylmercury (MMHg) produced in sediments. We have developed a shipboard method for measuring MMHg effluxes from undisturbed sediment cores. This approach was used at multiple locations in a temperate coastal system, New York/New Jersey Harbor, during summer (August 2003) and winter conditions (February 2004). MMHg fluxes at the sediment–water interface were determined with shipboard benthic flux chambers (SBFCs), an intact core of sediment and overlying water that is stirred inductively and incubated under in situ conditions. For all locations and each period, sediments were a net source of MMHg; levels in water overlying sediments increased as a linear function of incubation time, and associated fluxes varied less than 10% relative standard error among replicate chambers with sediment from the same sampling location and period. Seasonality was evident with measured MMHg fluxes (pmol m − 2 d − 1 ) ranging from 39 to 55 among locations in February and 71–92 in August. Fluxes determined with the SBFCs were compared with diffusional effluxes estimated from gradients of MMHg in pore water of the same sediments (SBFC:diffusional flux ratio). SBFC:diffusional flux ratios ranged from 0.9 to 1.4 in August and 2.3–4.3 in February. SBFC:diffusional flux ratios, which correlate positively with dissolved oxygen (DO) in bottom waters, suggest that MMHg mobilization is largely diffusional when DO is less than about 80% saturation and enhanced at greater levels. Further, increased MMHg fluxes are likely due to increased bioirrigation. This is consistent with the observed positive relationship between macrofauna abundance and DO. Thus, benthic MMHg fluxes, which are enhanced relative to diffusion when DO in bottom waters is near saturation, may be linked to increased abundance and/or activity of infauna that irrigate sediment. Improving oxygen conditions, therefore, could increase sediment–water effluxes of MMHg in NY/NJ Harbor and comparable coastal marine systems.

Benthic fluxes of mercury species in a lagoon environment (Grado Lagoon, Northern Adriatic Sea, Italy)

Abstract The role of the major biogeochemical processes in Hg cycling at the sediment–water interface was investigated in the Grado Lagoon (Northern Adriatic Sea). This wetland system has been extensively contaminated from the Idrija Hg Mine (Slovenia) through the Isonzo River suspended load carried by tidal fluxes. Three approaches were used to study the sediment–water exchange of total Hg (THg), methylmercury (MeHg), reactive Hg (RHg) and dissolved gaseous Hg (DGHg): (1) estimation of diffusive fluxes from porewater and overlying water concentrations, (2) measurements of benthic fluxes using a deployed light benthic chamber in situ and (3) measurements of benthic fluxes during oxic–anoxic transition with a laboratory incubation experiment. The THg solid phase, ranging between 9.5 and 14.4 μg g −1 , showed slight variability with depth and time. Conversely, MeHg contents were highest (up to 21.9 ng g −1 ) at the surface; they tended to decrease to nearly zero concentration with depth, thus suggesting that MeHg production and accumulation occur predominantly just below the sediment–water interface. Porewater MeHg concentrations (0.9–7.9 ng L −1 , 0.15–15% of THg) varied seasonally; higher contents were observed in the warmer period. The MeHg diffusive fluxes (up to 17 ng m −2 day −1 ) were similar to those in the nearby Gulf of Trieste [Covelli, S., Horvat, M., Faganeli, J., Brambati, A., 1999. Porewater distribution and benthic flux of mercury and methylmercury in the Gulf of Trieste (Northern Adriatic Sea). Estuar. Coast. Shelf Sci. 48, 415–428], although the lagoon sediments contained four-fold higher THg concentrations. Conversely, the THg diffusive fluxes in the lagoon (up to 110 ng m −2 day −1 ) were one- to two-fold higher than those previously estimated for the Gulf of Trieste. The diurnal MeHg benthic fluxes were highest in summer at both sites (41,000 and 33,000 ng m −2 day −1 at the fishfarm and in the open lagoon, respectively), thus indicating the influence of temperature on microbial processes. The diurnal variations of dissolved THg and especially MeHg were positively correlated with O 2 and inversely with DIC, suggesting an important influence of benthic photosynthetic activities on lagoon benthic Hg cycling, possibly through the production of organic matter promptly available for methylation. The results from the dark chamber incubated in the laboratory showed that the regeneration of dissolved THg was slightly affected by the oxic–anoxic transition. Conversely, the benthic flux of MeHg was up to 15-fold higher in sediments overlain by O 2 depleted waters. In the anoxic phase, the MeHg fluxes proceeded in parallel with Fe fluxes and the methylated form reached approximately 100% of dissolved THg. The MeHg is mostly released into overlying water (mean recycling efficiency of 89%) until the occurrence of sulphide inhibition, due to scavenging of the available Hg substrate for methylation. The results suggest that sediments in the Grado Lagoon, especially during anoxic events, should be considered as a primary source of MeHg for the water column.

Global diversity of oligochaetous clitellates (“Oligochaeta”; Clitellata) in freshwater

Oligochaeta sensu stricto, namely clitellates exclusive of branchiobdellids and leeches, occur in marine, estuarine, freshwater and terrestrial environs. About one-third of the almost 5,000 valid species described to date is aquatic. With the exception of some earthworm-like genera (the “megadriles”), aquatic oligochaetes are usually small, ranging from 1 mm to a few centimetres in length (the “microdriles”). Although predominantly terrestrial, 4 of the 14 described megadrile families include species that occur in aquatic or semi-aquatic habitats. The microdriles are fully aquatic, with the exception of the primarily terrestrial family Enchytraeidae, and comprise 13 families. About 1,700 valid species of aquatic oligochaetes are known to date; of these, about 1,100 are freshwater. The most speciose group is the Tubificidae with over 1,000 described species including 582 being considered as freshwater inhabitants. No fewer than 60 species of megadriles are also considered aquatic. Recent years have seen a continuous increase in the number of described species, so that any estimate of the proportion of known freshwater oligochaete species to unknown species would be very imprecise. Molecular studies have recently confirmed the long suspected paraphyly of the Oligochaeta if the group does not include branchiobdellids and leeches, so that Clitellata has become synonymous with “Oligochaeta”. The family Capilloventridae has been recently shown to represent a basal clade of Clitellata, supporting an aquatic (freshwater?) origin of the clitellates. In contrast, the adaptation to freshwater of the aquamegadriles is most likely secondary. The Palaearctic region supports the most abundant and diverse freshwater oligochaete fauna, with more than 600 valid species described to date; 80% of these are considered endemic. However, it is likely that the apparent concentration of genera and species in the Northern Hemisphere is biassed given the relatively late and still limited interest in the oligochaete fauna of the Southern Hemisphere. Ancient lakes, as well as ground waters, are important centres of endemicity but, except for Lake Baikal, they represent important knowledge gaps. Aquatic oligochaetes perform ecological functions and roles with potentially important repercussions for human health issues. These ecological values of oligochaetes include their importance in aquatic food chains; their impact on sediment structure and water-sediment exchanges; their long history of use in pollution monitoring and assessment; their potential to reduce sludge volumes in sewage treatment systems; and their role as intermediate host for several myxozoan parasites of fishes, including commercially exploited species.

Phosphorus Dynamics in a Small Eutrophic Italian Lake

Phosphorous dynamics within Lake Sirio (NW Italy) were investigated, considering both water and sediments. The total phosphorus (TP) concentration in the water is about 79 μg l−1 after the winter mixing, that is in homogeneous conditions; then TP content increases up to an average of 360 μg l−1 in late autumn in the deep hypolimnium (30–45 m). This deep lake portion accounts for only 1/12 of the water volume. Close to the water-sediment interface, TP concentrations up to 530 μg l−1 are observed. Sediment sampled at depths of 20 and 33 m contains less than 2,000 mg kg−1 of TP, whereas cores from the deepest sediments (46 m) display TP values of 2,000–4,000 mg kg−1 at the water-sediment interface, increasing with depth to 16,000 mg kg−1 at about 60–100 cm. In these deep sediments the main chemical form is the Al–Fe–Mn bound P (about 90% in the high TP cores) and Fe and Mn are also highly enriched (3 and 9 times more than in the shallow sediments respectively). The P–Fe association is confirmed by SEM-EDS and XRD analyses. The vertical distribution of the P content in the water column is consistent with its release from sediments, but in this hypothesis an unrealistic P release rate from 8.1 to 3.0 g m−2y−1 was estimated. A more complex model is therefore proposed, involving a process of P concentration in the sediments of the central (deepest) part of the lake, and a short term sediment-water exchange. The TP vertical variability and speciation in the cores suggests a change in the sediment retention capacity, connected to the lake shift to more eutrophic conditions.

Sediment-Water Exchange of Selected Heavy Metals at the Backwaters of Makupa Creek, Kenya

In situ benthic studies using incubation chambers were conducted at the backwaters of Makupa Creek (a peripheral creek located next to a municipal dumpsite) to directly measure fluxes of dissolved Mn, Fe, Ni, Cd, Cu and Pb. Physicochemical variations of the water overlying the chambers were also monitored and sediment cores were collected to measure their pH and Eh profiles. The sediment pH decreased markedly just below the water column at about the 0-2 cm segment, the decrease attributed to oxidation of organic matter and other inorganic chemical species. Organic matter oxidation contributed to the mobilization of heavy metals, demonstrating the significant effect of atmospheric oxidation on heavy metals dynamics at the Makupa creek backwaters during low tide. Humic and fluvic acid mobilization did not have a significant effect in the mobilization of Fe (Pearson correlation coefficient Fe-pH r = -0.34). However, Cu, Ni, Mn and Pb were to some extent mobilized due to solubalization of humic and fluvic acids at high pH. Ni and Cu were mobilized the most (Ni-pH r = 0.40 and Cu-pH r = 0.42), with both benthic chambers producing a pH peak at the 6th hour, which corresponded with a peak in dissolved Cu and Ni. Directly measured benthic fluxes ranged from +1680 to +8000 &#961; moles cm-2 h-1 for Fe, - 599 to +370 &#961; moles cm-2 h-1 for Mn, -1.1 to -0.0045 &#961; moles cm-2 h-1 for Cd, -33 to +1.2&#961; moles cm-2 h-1 for Pb, +174 to +432 &#961; moles cm-2 h-1 for Cu and +74 to +361 &#961; moles cm-2 h-2 for Ni. Keywords: Heavy metals, benthic flux, pollution, Kenya West Indian Ocean Journal of Marine Science Vol. 5 (2) 2006: pp. 153-162

Experimental research on the impact of Corbicula fluminea on DIN exchange at a tidal flat sediment-water interface

Based on a simulative experiment and a comparison analysis, the effect of bivalve Corbicula fluminea activity on sediment-water exchange of dissolved inorganic nitrogen (DIN) is studied. The areas included three intertidal flat sites of the Changjiang (Yangtze) River estuary in China. The interface exchange flux of ammonium, nitrate and nitrite in the short experiment (6 h) was 46.4-40, 74.8-929.1 and 2.5-14.6 μmol/(m^2•h), respectively. It was found that the burrowing activities of C. fluminea increased NH(superscript + subscript 4) and NO(superscript - subscript 3) release from sediments to overlying water in the short-term experiment. During long-term incubation, NH(superscript + subscript 4) and NO(superscript - subscript 3) released in turn from the sediments. At the beginning of incubation, bioturbation by C. fluminea could accelerate NH(superscript + subscript 4) release from sediments 2-17 times in different sites, resulting in stronger nitrification and increased NO(superscript - subscript 3) concentrations in the overlying water. Sediment profile analysis post-incubation shows that organic matter mineralization and sediment-water NH(superscript + subscript 4) exchange had been stimulated by C. fluminea bioturbation and bioirrigation during the experiment. Therefore, C. fluminea activities such as excretion, burrowing, irrigation and turbation can effectively alter nitrogen dynamics and accelerate and stimulate nitrogen exchange and cycling at the sediment-water interface.

Sediment–water exchange of Vibrio sp. and fecal indicator bacteria: Implications for persistence and transport in the Neuse River Estuary, North Carolina, USA

In estuaries, frequent resuspension and deposition of sediment complicate bacterial transport model development by transporting particle-attached bacteria and possibly inducing bacterial responses, such as growth, degradation, or changes in attachment. In order to better characterize these dynamics, observations were made in the Neuse River Estuary (NRE) using the combination of an in situ sampler to monitor the water column and sediment cores to monitor sediment concentrations. Two allochthonous bacteria, Escherichia coli (EC) and Enterococcus sp. (ENT), were selected as proxies for fecal contamination from stormwater runoff. Vibrio sp. (VIB), native to the NRE, was also observed as an autochthonous bacterial group that includes potentially pathogenic species. Two sampling periods were identified as dominated by different suspension types: runoff and resuspension. Despite this difference, several bacterial measures remained comparable between sampling periods. In bottom water, VIB concentration was correlated with salinity and ENT concentration was correlated with turbidity. Differences were observed for EC, where higher concentrations were found in hypoxic waters and sediment during the resuspension period. In the sediment, EC and VIB concentrations significantly increased following the passage of Hurricane Ophelia in September 2005. Throughout this study, all bacterial groups showed evidence of persistence in sediment, suggesting that sediment resuspension represents a significant source of bacteria to the water column.

The distribution of nitrate 15N/ 14N in marine sediments and the impact of benthic nitrogen loss on the isotopic composition of oceanic nitrate

We report 15N/ 14N ratios of porewater nitrate in sediments from the Bering Sea basin, where microbial nitrate reduction has been identified as a significant sink for fixed nitrogen (N). Strong 15N enrichment in porewater nitrate is observed as one goes deeper in the sediments and nitrate concentration [ NO 3 - ] decreases ( δ 15N generally reaches 25–35‰). Analysis of profiles with a one-dimensional diffusion-reaction model yields organism-scale isotope effects for dissimilatory nitrate reduction ( ε cell) of 11‰ to 30‰, in the same range as measured in previous studies of cultures and the marine and lacustrine water column. Estimates of ε cell, while uncertain, show a negative correlation with bottom water [O 2]; we propose that this relates to the [ NO 3 - ] at the depth of denitrification. The N isotope effect at the scale of nitrate sediment–water exchange ( ε app) is ∼0‰ in two unreactive deep sites and is typically <3‰ at more reactive sites at various depths. ε app is much lower than ε cell because nitrate consumption is nearly complete at the sediment depth of denitrification, minimizing the escape of 15N-enriched nitrate from the sediments. In reactive sediments, this is due to rapid denitrification, while in less reactive sediments, it is due to greater diffusive distances for nitrate to the depth of denitrification. The data suggest that low bottom water [O 2] tends to yield more complete expression of ε cell at the sediment–water scale, due to higher [ NO 3 - ] at the depth of denitrification. While porewater ammonium-N isotopes were not measured, our porewater model suggests that, in sediments with high organic matter supply and/or low-[O 2] bottom waters, the efflux and subsequent oxidation of ammonium enriched in 15N by incomplete nitrification can significantly enhance the total net isotope effect of sedimentary N loss ( ε sed, equivalent to ε app but including ammonium fluxes). Model analysis of representative sedimentary environments suggests a global mean ε sed of ∼4‰ (∼2‰ if restricted to seafloor below 1 km depth).

Bioturbation and bioirrigation extend the open exchange regions in permeable sediments

Large‐scale experimental exclusion of lugworms (Arenicola marina) from 400 m2 intertidal fine sand revealed significant effects of their bioturbation and bioirrigation on sediment characteristics, benthic infauna composition, and the dominant mineralization and benthic‐pelagic exchange processes in the sediment. Absence of lugworms resulted in sediment clogging with organic‐rich fine particles, an eightfold decrease in sediment permeability, low oxygen penetration depths, and accumulation of reduced mineralization products in the pore water. The sand flat inhabited by lugworms had low fine‐particle and chlorophyll contents and low sulfide and nutrient concentrations in the pore water. The effects were not limited to the vicinity of lugworm burrows but extended throughout the entire sediment down to ~20‐cm depth. Sediments with the lugworm shared the characteristic of low‐organic, advection‐driven permeable sand rather than of muddy, diffusion‐dominated sediments in the absence of lugworms. Areal oxygen uptake rates depended strongly on hydrodynamic conditions: under calm conditions, sedimentary oxygen uptake was slightly higher at the exclusion site. Experimental flushing using controlled hydrodynamic conditions showed that oxygen uptake at the lugworm site would be higher during more dynamic conditions (e.g., storms) due to significantly deeper oxygen penetration permitted by higher sediment permeability. Our results indicate an interactive effect of bioturbating organisms and hydrodynamics on water‐sediment exchange processes and highlight the importance of benthic infauna for sedimentary processes even in physically dominated systems.

Extension of coupled multispecies metal transport and speciation (TRANSPEC) model to soil

Atmospheric deposition of metals emitted from mining operations has raised metal concentrations in the surrounding soils. This repository may be remobilized and act as a source of metals to nearby surface aquatic systems. It is important to understand metal dynamics and the impact of various chemistry and fate parameters on metal movement in the soil environment in order to evaluate risk associated with metals in terrestrial ecosystems and accurately establish critical discharge limits that are protective of aquatic biota. Here we extend our previously developed coupled multispecies metal fate-TRANsport and SPECiation/complexation (TRANSPEC) model, which was applicable to surface aquatic systems. The extended TRANSPEC, termed TRANSPEC-II, estimates the partition coefficient, K d, between the soil-solid and -soluble phases using site-specific data and a semi-empirical regression model obtained from literature. A geochemical model calculates metal and species fractions in the dissolved and colloidal phases of the soil solution. The multispecies fugacity/aquivalence based fate-transport model then estimates inter-media transport rates such as leaching from soil, soil runoff, and water-sediment exchanges of each metal species. The model is illustratively applied to Ni in the Kelly Lake watershed (Sudbury, Ontario, Canada), where several mining operations are located. The model results suggest that the current atmospheric fallout supplies only 4% of Ni removed from soil through soil runoff and leaching. Soil runoff contributes about 20% of Ni entering into Kelly Lake with the rest coming from other sources. Leaching to groundwater, apart from runoff, is also a major loss process for Ni in the soil. A sensitivity analysis indicates that raising soil pH to above 6 may substantially reduce metal runoff and improve water quality of nearby water bodies that are impacted by runoff.

The cycling and oxidation pathways of organic carbon in a shallow estuary along the Texas Gulf Coast

The cycling and oxidation pathways of organic carbon were investigated at a single shallow water estuarine site in Trinity Bay, Texas, the uppermost lobe of Galveston Bay, during November 2000. Radio-isotopes were used to estimate sediment mixing and accumulation rates, and benthic chamber and pore water measurements were used to determine sediment-water exchange fluxes of oxygen, nutrients and metals, and infer carbon oxidation rates. Using 7Be and 234Th XS, the sediment-mixing coefficient ( D b) was 4.3 ± 1.8 cm 2 y −1, a value that lies at the lower limit for marine environments, indicating that mixing was not important in these sediments at this time. Sediment accumulation rates ( S a), estimated using 137Cs and 210Pb XS, were 0.16 ± 0.02 g cm −2 y −1. The supply rate of organic carbon to the sediment-water interface was 30 ± 3.9 mmol C m −2 d −1, of which ∼10% or 2.9 ± 0.44 mmol C m −2 d −1was lost from the system through burial below the 1-cm thick surface mixed layer. Measured fluxes of O 2 were 26 ± 3.8 mmol m −2 d −1 and equated to a carbon oxidation rate of 20 ± 3.3 mmol C m −2 d −1, which is an upper limit due to the potential for oxidation of additional reduced species. Using organic carbon gradients in the surface mixed layer, carbon oxidation was estimated at 2.6 ± 1.1 mmol C m −2 d −1. Independent estimates made using pore water concentration gradients of ammonium and C:N stoichiometry, equaled 2.8 ± 0.46 mmol C m −2 d −1. The flux of DOC out of the sediments (DOC efflux) was 5.6 ± 1.3 mmol C m −2 d −1. In general, while mass balance was achieved indicating the sediments were at steady state during this time, changes in environmental conditions within the bay and the surrounding area, mean this conclusion might not always hold. These results show that the majority of carbon oxidation occurred at the sediment-water interface, via O 2 reduction. This likely results from the high frequency of sediment resuspension events combined with the shallow sediment mixing zone, leaving anaerobic oxidants responsible for only ∼10–15% of the carbon oxidized in these sediments.

Physique du sédiment et des eaux interstitielles :stratégies de modélisation pour les besoins en environnement côtier

After a brief reminder on the sediment composition and facies, the paper reviews water-sediment exchanges that require to account for sediment transformation in Coastal Environment modelling. Physical processes (consolidation, bed armouring, liquefaction) and bio-chemical processes (organic matter mineralisation, adsorption/desorption, precipitation/dissolution, bioturbation…) are briefly reviewed, as well as usual modelling methods. The need for a guide book on the required minimal modelling, depending on the arisen questions, is put forward.

Modeling nitrogen cycling in a coastal fresh water sediment

Increased nitrogen (N) loading to coastal marine and freshwater systems is occurring worldwide as a result of human activities. Diagenetic processes in sediments can change the N availability in these systems, by supporting removal through denitrification and burial of organic N (Norg) or by enhancing N recycling. In this study, we use a reactive transport model (RTM) to examine N transformations in a coastal fresh water sediment and quantify N removal rates. We also assess the response of the sediment N cycle to environmental changes that may result from increased salinity which is planned to occur at the site as a result of an estuarine restoration project. Field results show that much of the Norg deposited on the sediment is currently remineralized to ammonium. A rapid removal of nitrate is observed in the sediment pore water, with the resulting nitrate reduction rate estimated to be 130 μmol N cm−2 yr−1. A model sensitivity study was conducted altering the distribution of nitrate reduction between dissimilatory nitrate reduction to ammonium (DNRA) and denitrification. These results show a 40% decline in sediment N removal as NO −3 reduction shifts from denitrification to DNRA. This decreased N removal leads to a shift in sediment-water exchange flux of dissolved inorganic nitrogen (DIN) from near zero with denitrification to 133 μmol N cm−2 yr−1 if DNRA is the dominant pathway. The response to salinization includes a short-term release of adsorbed ammonium. Additional changes expected to result from the estuarine restoration include: lower NO −3 concentrations and greater SO 2−4 concentrations in the bottom water, decreased nitrification rates, and increased sediment mixing. The effect of these changes on net DIN flux and N removal vary based on the distribution of DNRA versus denitrification, illustrating the need for a better understanding of factors controlling this competition.

Sediment–water inorganic nutrient exchange and nitrogen budgets in the Colne Estuary, UK

The exchange of dissolved inorganic nutrients (NO3 - , NO2 - NH4 + , PO4 3- and SiO3 - ) was measured across the sediment-water interface at 4 sites along the Colne Estuary at monthly intervals between August 1996 and March 1998 in the dark and under illumination. Fluxes across the sedi- ment-water interface were scaled up to calculate nutrient budgets for the whole estuary, accounting for the area of sediment exposed to illumination. Sediments were a net sink of -16.28 Mmol N yr -1 for oxidized inorganic nitrogen (NO3 - + NO2 - ). Mean nitrate fluxes across the sediment-water interface were -329 and -91 µmol m -2 h -1 in the dark and light respectively. Colne sediments were a source of ammonium in the dark (12.4 Mmol N yr -1 ). Illumination reduced ammonium flux into the overlying water or affected uptake due to assimilation by the microphytobenthos and a stimulation of nitrifi- cation. Mean ammonium fluxes across the sediment-water interface were 399 and -169 µmol m -2 h -1 in the dark and light, respectively. Consequently, sediments were a net sink for ammonium in the light (-0.31 Mmol N yr -1 ). Similarly, illumination affected silicate exchange across the sediment- water interface, with silicate flux into the overlying water reduced or reversed under illumination compared to the dark. The estuarine sediments were a net source of silicate (6.09 Mmol yr -1 ) and soluble reactive phosphate (1.91 Mmol yr -1 ). Uptake of inorganic oxidized nitrogen was approxi- mately balanced by adding together ammonium flux and denitrification supported by nitrate in the overlying water. This mass balance indicates that dissimilatory nitrate reduction to ammonium (DNRA) was a significant process in the sediments of the Colne Estuary. Fluxes in the light accounted for less than 6% of the total sediment-water exchange of both ammonium and nitrate. However, a large proportion of the -15.49 Mmol N yr -1 of oxidized inorganic nitrogen taken up by the sediment in the dark was cancelled out by a flux of 12.4 Mmol N yr -1 into the overlying water as ammonium. Net dissolved inorganic nitrogen (DIN) fluxes were -3.09 Mmol N yr -1 in the dark and -1.17 Mmol Ny r -1 in the light, as fluxes of all nitrogen species were negative under illumination. Therefore fluxes under illumination accounted for 28% of the net uptake of DIN by estuarine sediments in the Colne Estuary, despite the short time periods and small area of sediment under illumination. In macrotidal estuaries such as the Colne, where microphytobenthos are abundant, light plays an important role in moderating sediment-water nutrient exchange.

Distribution of mercury and methylmercury in deep-sea surficial sediments of the Mediterranean Sea

An investigation of the distribution, speciation and methylation of the mercury in deep-sea sediments of the Mediterranean Sea was performed. The samples were collected at different locations in the Western and Eastern Basins of the Mediterranean during the cruise of the Italian research vessel Urania in summer 2003 as part of MERCYMS project. Total mercury (Hg T) and methylmercury (MeHg) in pore water and sediments was determined down the sediment profile and coupled with other biogeochemical parameters. The diffusive fluxes of Hg T and MeHg were calculated to estimate the importance of sediment–water exchange as potential sources of Hg T and MeHg. In addition, isotopically enriched 199Hg was used to determine the methylation potentials. The concentrations of Hg T in sediments ranged between 0.06 and 2.23 nmol g − 1 and vary irregularly with depth, which may reflect changes or redistribution during diagenetic processes. No correlation between Hg T and organic carbon content was found, but a relatively high proportion of MeHg to Hg T of approx. 2.0% was observed. The integrated flux of Hg T was estimated to be 109 kmol year − 1 and agrees well with the mass balance calculation preformed for total underwater emissions of Hg T in the Mediterranean Sea of 80 kmol year − 1 by Rajar et al. [Rajar, R., Četina, M., Horvat, M., Žagar, D., 2007-this issue. Mass balance of mercury in the Mediterranean Sea. Mar. Chem. doi:10.1016/j.marchem.2006.10.001]. The emissions of MeHg were estimated to be 14 kmol year − 1 and indicate that deep-sea sediments could be an important source of the MeHg content in marine biota. It was found that accumulation of MeHg in surficial sediments of the Mediterranean is not directly related to the potential rates of bacterial Hg methylation. Much of the MeHg produced in the sediments are lost to the overlying water. Our results suggest that MeHg production depends on partitioning of Hg(II) influenced by the organic carbon content in the sediment. Reduction of organic carbon in sediment could increase pore water Hg(II) and enhance bacterial production of MeHg.

Spatial and temporal variability of benthic biogeochemical fluxes associated with macrophytic and macrofaunal distributions in the Thau lagoon (France)

The aim of the present study in the Thau lagoon was to determine spatial and temporal variations of biogeochemical fluxes at the sediment–water interface (oxygen, nutrients and DIC) associated with macrophytic and/or macrofaunal distributions during spring time, which is a period of major vegetal growth. The pattern of benthic fluxes was studied in April 2002 and May 2003 for four main benthic biota of the lagoon, with one site subject to biodeposition by oysters cultured in suspension, one site representative of the central part of the lagoon that was only under general sedimentation, one site densely covered by macroalgae and one site located within a seagrass meadow. The results emphasize the role of oyster cultures on benthic fluxes and the impact of spatial heterogeneity of the benthos (macrophytes and macrofauna) distribution at various spatial scales. Hypoxic conditions were shown to develop at the sediment–water interface in May 2003 in the central part of the lagoon. Oxygen and DIC fluxes emphasized benthic respiration overall, and spatio-temporal variability mainly due to the presence/absence of the macroalgal cover on the sediment. The absence of benthic macroinfaunal organisms in the stations under the influence of oyster biodeposition and facing a dense macroalgal cover highlighted anoxic conditions in the sediments. Major benthic macrofauna community changes were observed in the central zone between April 2002 and May 2003, emphasizing sediment organic matter enrichment. The analysis of benthic fluxes over a productive period of the annual cycle provided a basis for describing spatial and temporal variations of the dynamics of sediment–water exchanges in the lagoon.