Sediment-water oxygen and nutrient exchanges along the longitudinal axis of Chesapeake Bay: Seasonal patterns,controlling factors and ecological significance

Sediment-water oxygen and nutrient (NH₄ ⁺, NO₃ ^?+NO₂ ^?, DON, PO₄ ^3?, and DSi) fluxes were measured in three distinct regions of Chesapeake Bay at monthly intervals during 1 yr and for portions of several additional years. Examination of these data revealed strong spatial and temporal patterns. Most fluxes were greatest in the central bay (station MB), moderate in the high salinity lower bay (station SB) and reduced in the oligohaline upper bay (station NB). Sediment oxygen consumption (SOC) rates generally increased with increasing temperature until bottom water concentrations of dissolved oxygen (DO) fell below 2.5 mg l^?1, apparently limiting SOC rates. Fluxes of NH₄ ⁺ were elevated at temperatures >15°C and, when coupled with low bottom water DO concentrations (<5 mg l^?1), very large releases (>500 μmol N m^?2 h^?1) were observed. Nitrate + nitrite (NO₃ ^?+NO₂ ^?) exchanges were directed into sediments in areas where bottom water NO₃ ^?+NO₂ ^? concentrations were high (>18 μM N); sediment efflux of NO₃ ^?+NO₂ ^? occurred only in areas where bottom water NO₃ ^?+NO₂ ^? concentrations were relatively low (<11 μM N) and bottom waters well oxygenated. Phosphate fluxes were small except in areas of hypoxic and anoxic bottom waters; in those cases releases were high (50–150 μmol P m^?2 h^?1) but of short duration (2 mo). Dissolved silicate (DSi) fluxes were directed out of the sediments at all stations and appeared to be proportional to primary production in overlying waters. Dissolved organic nitrogen (DON) was released from the sediments at stations NB and SB and taken up by the sediments at station MB in summer months; DON fluxes were either small or noninterpretable during cooler months of the year. It appears that the amount and quality of organic matter reaching the sediments is of primary importance in determining the spatial variability and interannual differences in sediment nutrient fluxes along the axis of the bay. Surficial sediment chlorophyll-a, used as an indicator of labile sediment organic matter, was highly correlated with NH₄ ^?, PO₄ ^3?, and DSi fluxes but only after a temporal lag of about 1 mo was added between deposition events and sediment nutrient releases. Sediment O:N flux ratios indicated that substantial sediment nitrification-denitrification probably occurred at all sites during winter-spring but not summer-fall; N:P flux ratios were high in spring but much less than expected during summer, particularly at hypoxic and anoxic sites. Finally, a comparison of seasonal N and P demand by phytoplankton with sediment nutrient releases indicated that the sediments provide a substantial fraction of nutrients required by phytoplankton in summer, but not winter, especially in the mid bay region.     

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 (H _s). The relationship with H _s 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.     

Calcium carbonate dissolution in deep sea sediments: reconciling microelectrode, pore water and benthic flux chamber results

We report the benthic fluxes of O₂, titration alkalinity (TA), Ca²⁺, NO₃⁻, PO₄³⁻, and Si(OH)₄ from in situ benthic flux chamber incubations on the Ceara Rise and Cape Verde Plateau and compare them to previously published results. We find within analytical uncertainty that the TA flux is twice the calcium flux, suggesting that dissolution/precipitation of CaCO₃ is the principal mechanism controlling benthic TA and Ca²⁺ fluxes. At sites where the sediments contain significant (>35%) CaCO₃ and the overlying waters are supersaturated with respect to CaCO₃, the ratios of the total dissolution rate to the remineralization rate are significantly less than at all other study sites. We propose that these observations can be explained by precipitation of fresh CaCO₃ at the supersaturated sediment surface followed by redissolution deeper in the sediments because of metabolically-produced CO₂. A numerical simulation is presented to demonstrate the feasibility of this explanation. In addition, surface exchange reactions in high-CaCO₃ sediments coupled with high rates of particle mixing may also impact rates of metabolic dissolution and depress chamber-derived estimates of carbonate alkalinity and calcium benthic fluxes. These results suggest that at supersaturated, high CaCO₃ locations, previous models of sediment diagenesis may have overestimated the impact of metabolic dissolution on the preservation of CaCO₃ deposited on the sea floor.     

Benthic nutrient fluxes in the intertidal flat within the Changjiang （Yangtze River） Estuary

In an annual cycle from March 2005 to February 2006, benthic nutrient fluxes were measured monthly in the Dongtan intertidal flat within the Changjiang （Yangtze River） Estuary. Except for NH4^＋, there always showed high fluxes from overlying water into sediment for other four nutrients. Sediments in the high and middle marshes, covered with halophyte and consisting of macrofauna, demonstrated more capabilities of assimilating nutrients from overlying water than the low marsh. Sampling seasons and nutrient concentrations in the overlying water could both exert significant effects on these fluxes. Additionally, according to the model provided by previous study, denitrification rates, that utilizing NO3- transported from overlying water （Dw） in Dongtan sediments, were estimated to be from -16 to 193 μmol·h^-1·m^-2 with an average value of 63 μmol·h^-1·m^-2 （n=18）. These estimated values are still underestimates of the in-situ rates owing to the lack of consideration of DN, i.e., denitrification supported by the local NO3^- production via nitrification.     

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

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 O₂ 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 O₂ 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.     

Benthic biogeochemistry beneath the Mississippi River plume

Biogeochemical processes occurring near the sediment-water interface of shallow (≈20 m) water sediments lying beneath the Mississippi River plume on the Louisiana shelf were studied using benthic chambers and sediment cores. Three sites were chosen with distinctly different characteristics. One was overlain by oxic water where aerobic respiration dominated organic matter remineralization. The second site was overlain by oxic water but organic matter remineralization was dominated by sulfate reduction. The third site was overlain by hypoxic water and aerobic remineralization was of minor significance. Major differences were observed in the fluxes of CO₂(17–56 mmol m⁻² d⁻¹), O₂(2–56 mmol m⁻² d⁻¹) and nutrients (e.g., NH₄ ⁺, 2.6–4.2 mmol m⁻² d⁻¹) across the sediment-water interface, and the relative importance of different electron acceptors, even though the sites were in close proximity and at nearly the same water depth. Large variations in the efficiency of organic-C burial (3%–51%) were also calculated based on a simplified model of the relationships between the fraction of organic matter remineralized by sulfate reduction and the fraction of sulfide produced that is buried as pyrite. These observations demonstrate the high degree of spatial heterogeneity of benthic biogeochemistry in this important near-deltaic environment.     

Benthic fluxes and porewater concentration profiles of dissolved organic carbon in sediments from the North Carolina continental slope

Numerous studies of marine environments show that dissolved organic carbon (DOC) concentrations in sediments are typically tenfold higher than in the overlying water. Large concentration gradients near the sediment–water interface suggest that there may be a significant flux of organic carbon from sediments to the water column. Furthermore, accumulation of DOC in the porewater may influence the burial and preservation of organic matter by promoting geopolymerization and/or adsorption reactions. We measured DOC concentration profiles (for porewater collected by centrifugation and “sipping”) and benthic fluxes (with in situ and shipboard chambers) at two sites on the North Carolina continental slope to better understand the controls on porewater DOC concentrations and quantify sediment–water exchange rates. We also measured a suite of sediment properties (e.g., sediment accumulation and bioturbation rates, organic carbon content, and mineral surface area) that allow us to examine the relationship between porewater DOC concentrations and organic carbon preservation. Sediment depth-distributions of DOC from a downslope transect (300–1000 m water depth) follow a trend consistent with other porewater constituents (ΣCO₂ and SO₄²⁻) and a tracer of modern, fine-grained sediment (fallout Pu), suggesting that DOC levels are regulated by organic matter remineralization. However, remineralization rates appear to be relatively uniform across the sediment transect. A simple diagenetic model illustrates that variations in DOC profiles at this site may be due to differences in the depth of the active remineralization zone, which in turn is largely controlled by the intensity of bioturbation. Comparison of porewater DOC concentrations, organic carbon burial efficiency, and organic matter sorption suggest that DOC levels are not a major factor in promoting organic matter preservation or loading on grain surfaces. The DOC benthic fluxes are difficult to detect, but suggest that only 2% of the dissolved organic carbon escapes remineralization in the sediments by transport across the sediment-water interface.     

Holocene palaeoceanographic changes in Barrow Strait,Canadian Arctic: foraminiferal evidence

Holocene changes in the benthic and planktic foraminiferal fauna (>63 µm) from a marine sediment core (ARC‐3 Canadian Arctic Archipelago, 74° 16.050′ N, 91° 06.380′ W, water depth 347 m) show that significant environmental and palaeoceanographic variations occurred during the last 10 ka. Foraminiferal assemblages are restricted to the ca. 4.5–10 ka interval as younger samples are mostly barren of foraminifera due to intense carbonate dissolution after ca. 4.5 ka. Foraminiferal assemblages in the ca. 4.5–10 ka interval are dominated by the benthic species Islandiella helenae and Cassidulina reniforme (57% of total), with Elphidium clavatum, Cibicides lobatulus and Buccella frigida also being common in this interval. The dominance of these species indicates a seasonal sea ice regime which is consistent with the occurrence of the sea ice diatom‐derived organic geochemical biomarker IP₂₅ throughout the core. The abundances of C. reniforme and E. clavatum decline upcore; consistent with more frequent mixing of the Barrow Strait water column during the early Holocene. It is likely that the influence of CO₂‐rich Arctic surface water masses have caused an increase in bottom water corrosivity after ca. 8.5 ka, and dissolution has been further enhanced by sea ice‐related processes after ca. 6 ka, concomitant with increased IP₂₅ fluxes. Dissolution is strongest when IP₂₅ fluxes are highest, suggesting a link between the sea ice and benthic systems. Copyright © 2010 John Wiley & Sons, Ltd.     

Sediment community metabolism associated with continental shelf hypoxia, Northern Gulf of Mexico

Net fluxes of respiratory metabolites (O₂, dissolved inorganic carbon (DIC), NH₄ ⁺, NO₃ ^?, and NO₂ ^?) across the sediment-water interface were measured using in-situ benthic incubation chambers in the area of intermittent seasonal hypoxia associated with the Mississippi River plume. Sulfate reduction was measured in sediments incubated with trace levels of³⁵S-labeled sulfate. Heterotrophic remineralization, measured as nutrient regeneration, sediment community oxygen consumption (SOC), sulfate reduction, or DIC production, varied positively as a function of temperature. SOC was inversely related to oxygen concentration of the bottom water. The DIC fluxes were more than 2 times higher than SOC alone, under hypoxic conditions, suggesting that oxygen uptake alone cannot be used to estimate total community remineralization under conditions of low oxygen concentration in the water column. A carbon budget is constructed that compares sources, stocks, transformations, and sinks of carbon in the top meter of sediment. A comparison of remineralization processes within the sediments implicates sulfate reduction as most important, followed by aerobic respiration and denitrification. Bacteria accounted for more than 90% of the total community biomass, compared to the metazoan invertebrates, due presumably to hypoxic stress.     

Pore-water response on seasonal environmental changes in intertidal sediments of the Weser Estuary,Germany

In order to determine time-dependent changes in estuarine pore-water chemistry and flux variations across the sediment-water interface, sediment cores of an intertidal mud flat in the Weser Estuary were taken monthly over a one-year period. Sediment temperature, pH, Eh, Cl^–, O₂, NO ₃ ^– , and SO ₄ ^2– pore-water concentrations were measured and showed variations that relate to the changes of surface temperature and estuarine water composition. Fick's first law was applied to quantify diffusive fluxes from concentration gradients in the diffusive boundary layer and in the pore water. Total nitrate fluxes were calculated from flux chamber experiments. Diffusive oxygen fluxes increased from 5 mmol m^–2 d^–1 in winter to 18 mmol m^–2 d^–1 in early summer, while nitrate fluxes into the sediment increased from 3 mmol m^–2 d^–1 in winter to 60 mmol m^–2 d^–1 in early summer. Oxygen and nitrate fluxes into the sediment correlated linearly to sediment temperature. Sulfate fluxes increased from 0.5 mmol m^–2 d^–1 in winter to 10 mmol m^–2 d^–1 in August and September. Converted into carbon fluxes, the sum of these oxidants ranged from 10 mmol m^–2 d^–1 in winter to 80 mmol m^–2 d^–1 in summer. An estimation of the upper limit of the annual nitrate flux into the sediment showed that about 10% of the 250,000 t of nitrate discharged annually by the river may be decomposed within the inner Weser Estuary.     

Geothermal solute flux monitoring and the source and fate of solutes in the Snake River,Yellowstone National Park,WY

The combined geothermal discharge from over 10,000 features in Yellowstone National Park (YNP) can be can be estimated from the Cl flux in the Madison, Yellowstone, Falls, and Snake Rivers. Over the last 30 years, the Cl flux in YNP Rivers has been calculated using discharge measurements and Cl concentrations determined in discrete water samples and it has been determined that approximately 12% of the Cl flux exiting YNP is from the Snake River. The relationship between electrical conductivity and concentrations of Cl and other geothermal solutes was quantified at a monitoring site located downstream from the thermal inputs in the Snake River. Beginning in 2012, continuous (15 min) electrical conductivity measurements have been made at the monitoring site. Combining continuous electrical conductivity and discharge data, the Cl and other geothermal solute fluxes were determined. The 2013–2015 Cl fluxes (5.3–5.8 kt/yr) determined using electrical conductivity are comparable to historical data. In addition, synoptic water samples and discharge data were obtained from sites along the Snake River under low-flow conditions of September 2014. The synoptic water study extended 17 km upstream from the monitoring site. Surface inflows were sampled to identify sources and to quantify solute loading. The Lewis River was the primary source of Cl, Na, K, Cl, SiO₂, Rb, and As loads (50–80%) in the Snake River. The largest source of SO₄ was from the upper Snake River (50%). Most of the Ca and Mg (50–55%) originate from the Snake Hot Springs. Chloride, Ca, Mg, Na, K, SiO₂, F, HCO₃, SO₄, B, Li, Rb, and As behave conservatively in the Snake River, and therefore correlate well with conductivity (R² ≥ 0.97).     

Sediment-water oxygen and nutrient fluxes in a river-dominated estuary

Sediment oxygen uptake and net sediment-water fluxes of dissolved inorganic and organic nitrogen and phosphorus were measured at two sites in Fourleague Bay, Louisiana, from August 1981, through May 1982. This estuary is an extension of Atchafalaya Bay which receives high discharge and nutrient loading from the Atchafalaya River. Sediment O₂ uptake averaged 49 mg m^?2 h^?1. On the average, ammonium (NH₄ ⁺) was released from the sediments (mean flux =+129 μmol m^?2 h^?1), and NO₃ ^? was taken up (mean flux =?19 μmol m^?2h^?1). However, very different NO₃ ^? fluxes were observed at the two sites, with sediment uptake at the upper, river-influenced, high NO₃ ^? site (mean flux =?112 μmol m^?2 h^?1) and release at the lower, marine-influenced low NO₃ ^? site (mean flux =+79 μmol m^?2 h^?1). PO₄ ^3? fluxes were low and often negative (mean flux =?8 μmol m^?2 h^?1), while dissolved organic phosphorus fluxes were high and positive (mean flux =+124 μmol m^?2 h^?1). Dissolved organic nitrogen fluxes varied greatly, ranging from a mean of +305 μmol m^?2 h^?1 at the lower bay, to ?710 μmol m^?2 h^?1 at the upper bay. Total dissolved nitrogen and phosphorus fluxes indicated the sediments were a nitrogen (mean flux =+543 μmol m^?2 h^?1) and phosphorus source (mean flux =+30 μmol m^?2 h^?1) at the lower bay, and a nitrogen sink (mean flux =?553 μmol m^?2 h^?1) and phosphorus source (mean flux =+17 μmol m^?2 h^?1) in the upper bay. Mean annual O∶N ration of the positive inorganic sediment fluxes were 27∶1 at the upper bay and 18∶1 at the lower bay. Based on these data we hypothesize that nitrification and denitrification are important sediment processes in the upper bay. We further hypothesize that Atchafalaya River discharge affects sediment-water fluxes through seasonally high nutrient loading which leads to net nutrient uptake by sediments in the upper bay and release in the lower bay, where there is less river influnces.     

Influence of Organic Enrichment and <Emphasis Type="Italic">Spisula subtruncata</Emphasis> (da Costa, 1778) on Oxygen and Nutrient Fluxes in Fine Sand Sediments

The role of labile organic material and macrofaunal activity in benthic respiration and nutrient regeneration have been tested in sublittoral fine sand sediments from the Gulf of Valencia (northwestern Mediterranean Sea). Three experimental setups were made using benthic chambers. One experiment was performed in-situ through the annual cycle in a well-sorted fine sand community. The remaining experiments were carried out with mesocosms under laboratory conditions: one with different concentrations of organic enrichment (mussel meat and concentrated diatoms culture), and the other adding two different densities of the endofaunal bivalve Spisula subtruncata. Biochemical variables in surface sediment and changes in oxygen consumption and nutrient fluxes throughout incubation period were studied in each experiment. In the in situ incubations, dissolved oxygen (DO) fluxes showed a strong correlation with sedimentary biopolymeric fraction of organic carbon. Organic enrichment in the laboratory experiments was responsible for increased benthic respiration. However, sediment response (expressed as DO uptake and dissolved inorganic nitrogen—DIN—release) between oligotrophic and eutrophic conditions was more intense than between eutrophic and hypertrophic conditions. S. subtruncata abundances close to 400 and 850 ind m^?2 also intensified benthic metabolism. DO uptake and DIN production in mesocosms with added fauna were between 60 and 75 % and 65–100 % higher than in the control treatment respectively. The results of these three experiments suggest that the macrobenthic community may increase the benthic respiration by roughly a factor of two in these bottoms, where S. subtruncata is one of the dominant species. Both organic enrichment and macrobenthic community in general, and S. subtruncata in particular, did not seem to have a relevant role in P and Si cycles in these sediments.     

Air–Water CO<Subscript>2</Subscript> Fluxes and Net Ecosystem Production Changes in a Baja California Coastal Lagoon During the Anomalous North Pacific Warm Condition

The present study examines the temporal variability of air–water CO₂ fluxes (FCO₂) and seawater carbonate chemistry in a Baja California coastal lagoon during an exceptionally warm anomaly that was developed in Northeast Pacific coasts during 2014. This oceanographic condition led to a summer-like season (weak upwelling condition) during the study period, which reached a maximum surface temperature anomaly of 2 °C in September 2014. San Quintín Bay acts as a source of CO₂ to the atmosphere in 2014 (3.3 ± 4.8 mmol C m^?2 day^?1) with the higher positive fluxes mainly observed in summer months (9.0 ± 5.3 mmol C m^?2 day^?1). Net ecosystem production (NEP) switched seasonally between net heterotrophy and net autotrophy during the study period, with an annual average of 2.2 ± 7.1 mmol C m^?2 day^?1, which indicates that San Quintín Bay was a net autotrophic system during the atypical warm oceanographic condition in 2014. This pattern of seasonal variations in the carbon balance at San Quintín Bay appears to be linked to the life cycle of benthic communities, which play an important role in the whole-ecosystem metabolism. Under the limited input from external sources coupled with an increase in seawater temperatures, the recycled benthic carbon and nutrient fluxes play a major role to sustain water-column processes within the bay. Since the upwelling condition may influence the magnitude of the air–water CO₂ fluxes, our results clearly indicated that San Quintín Bay is a net source of carbon to the atmosphere regardless of the adjacent oceanic conditions. Our study sheds light on the carbon dynamics and its metabolic implications in a shallow coastal ecosystem under a regional warm anomaly and contributes potentially relevant information in view of the likely future scenario of global climate change.     

The Contribution of Benthic Nutrient Regeneration to Primary Production in a Shallow Eutrophic Estuary,Weeks Bay,Alabama

Benthic oxygen, dinitrogen, and nutrient fluxes (NH₄⁺, NO₃⁻, and PO₄³⁻) were measured monthly during a 1-year period at two locations in Weeks Bay, a shallow (1.4 m) and eutrophic estuary in Alabama. Gross primary productivity (GPP), ecosystem respiration (R), and net ecosystem metabolism were determined from high-frequency dissolved oxygen measurements. Peak water column NO₃⁻ (55 μM) and chlorophyll a (138 μg/l) concentrations were measured during spring and fall, respectively. Sediments were a net source of NH₄⁺ (102 μmol m⁻² h⁻¹) and PO₄³⁻ (0.9 μmol m⁻² h⁻¹) but a sink for NO₃⁻ (−30 μmol m⁻² h⁻¹). Benthic N₂ fluxes indicated net N fixation (12 μmol N m⁻² h⁻¹). Sediment oxygen demand (0.55 g O₂ m⁻² day⁻¹) accounted for <10% of R (7.3 g O₂ m⁻² day⁻¹). Despite high GPP rates (4.7 g O₂ m⁻² day⁻¹), the estuary was net heterotrophic. Benthic regeneration supplied, on average, 7.5% and 4% of primary productivity N and P demands, respectively. These results contrast with the conventional view that benthic regeneration accounts for a large fraction of phytoplankton nutrient demand in shallow estuaries.     

The role of biogeochemical processes in minimising uranium dispersion from a mine site

The Ranger Uranium Mine located in the Alligator Rivers Region of the Northern Territory lies in the tropical zone and has an annual wet-dry monsoonal climate. Following the commencement of the wet season, runoff from the waste rock dump accumulates in a retention pond (RP4). This water is permitted to discharge to the nearby Magela Creek once minimum flow of 5 m³/sec is reached and following filling of the pond. The discharge proceeds via a channel, experimental wetland and a backflow billabong (Djalkmarra Billabong) which acts as a natural wetland filter and flows out to Magela Creek.This study examines monitoring data for water releases over 3 wet seasons. I wet season with no release and 4 dry seasons. The monitoring data comprised electrical conductivity (EC). pH, Na, K. Ca, Mg. HCO⁻₃ SO²⁻₄, Cl⁻ and U (total or filtered, < 0.45 μm). Some ICP-MS scans of trace elements were also undertaken with particular reference being made to Re and U.Specific features of the sequence of water accumulation, release and reconstitution of Djalkmarra Billabong are able to show that U is effectively removed from solution, from about 50 ppb down to < 1 ppb. Soluble salts may remain in the water column and are removed by dilution following discharge to Magela Creek. Sediment levels show no increase in U concentration with time.The pH of the billabong water during releases (6.0–6.6) suggests that cationic forms of U, such as (UO₂)₃ (OH)⁵, predominate, favouring adsorption on to the humic-rich sediments of the natural wetland. The application of this principle enables U to be removed efficiently from waste water and to be contained within the mine lease.     

Methane distribution and cycling in Tomales Bay, California

Cycling of methane (CH₄) in Tomales Bay, a 28-km² temperature estuary in northern California with relatively low inputs of organic carbon, was studied over a 1-yr period. Water column CH₄ concentrations showed spatial and temporal variability (range=8–100 nM), and were supersaturated with respect to the atmosphere by a factor of 2–37. Rates of net water column CH₄ production-oxidation were determined by in situ experiments, and were not found to be significantly different from zero. Fluxes across the sediment-water interface, determined by direct measurement using benthic chambers, varied from ?0.1 μmol m^?2 d^?1 to +16 μmol m^?2 d^?1 (positive fluxes into water). Methane concentrations in the two perennial creeks feeding the bay varied annually (140–950 nM); these creeks were a significant CH₄ source to the bay during winter. In addition, mass-balance calculations indicate a significant additional inter CH₄ source, which is hypothesized to result from storm-related runoff from dairy farms adjacent to the bay. Systemwide CH₄ budgets of the 16-km² inner bay indicate benthic production (110 mol d^?1) and atmospheric evasion (110 mol d^?1) dominated during summer, while atmospheric evasion (160 mol d^?1) and runoff from dairy farms (90 mol d^?1) dominated during winter.     

Microcosm assessment of the biogeochemical development of sulfur and oxygen in oil sands fluid fine tailings

Bitumen recovery from Alberta oil sands generates fluid fine tailings, which are retained in tailings ponds where solids settle and release process water. The recovered water is recycled for bitumen extraction, while the resulting tailings are incorporated into various landforms for reclamation, with one option being conversion of tailings basins to viable end pit lakes. Tailings ponds commonly host diverse microbial communities, including SO₄-reducing prokaryotes. The highly reducing nature of the hydrogen sulfide produced by these prokaryotes may impact the biogeochemical cycling of key nutrients. However, the behavioral dynamics of hydrogen sulfide production in ponds containing fluid fine tailings remain to be clearly explained. In this study, microcosms are used as analogues of the sediment–water interface of a tailings pond undergoing reclamation to determine sulfide generation patterns and the behavior of O₂. In the microcosms, hydrogen sulfide fluxes correlated positively with biotic activity, reaching levels of over 2 × 10³ nmol cm⁻² s⁻¹, leading to Fe sulfide formation. Depth-related hydrogen sulfide profiles in the microcosms were comparable to those encountered in situ, in Syncrude’s West In-Pit, an active tailing pond. Oxygen diffusion across the fluid fine tailing sediment–water interface was controlled to different degrees by both biotic and abiotic processes. The results have implications for quantitatively estimating the impact of hydrogen sulfide production, O₂ availability, and biogeochemical cycling of key nutrients important for the success of life in fluid fine tailings-affected ecosystems. This paper shows that this production of hydrogen sulfide may be a self-limiting process, which will begin to decrease after a period of time.     

Nutrient pore water chemistry,Great Bay,New Hampshire: Benthic fluxes

Benthic fluxes of C, N, P and Si have been measured at two sites in Great Bay Estuary, New Hampshire. Higher fluxes of reactive phosphate, nitrate and reactive silicate were observed at the site where bioturbation is known to occur and the fluxes of NH₄ ⁺, PO₄ ^?3 and reactive silicate at this location were from 3 to 6 times higher than that calculated by simple pore water diffusion models. *** DIRECT SUPPORT *** A01BY019 00010