Sediment Denitrification in Two Contrasting Tropical Shallow Lagoons

Sediment denitrification was monthly evaluated in two tropical coastal lagoons with different trophic states using the ¹⁵N isotope pairing technique. Denitrification rates were very low in both environments, always <5.0 μmol N₂ m^?2 h^?1 and were not significantly different between them. Oxygen consumption varied from 426 to 4248 μmol O₂ m^?2 h^?1 and was generally three times higher in the meso-eutrophic than the oligotrophic lagoon. The low denitrification activity was ascribed to both low water NO₃ ^? concentrations (<2.0 μM) and little nitrate supply from nitrification. There was no correlation of denitrification with nitrate or ammonium fluxes. Sediments in temperate environments with similar oxygen consumption rates usually presented a higher proportion of nitrification–denitrification rates. Sediment oxygen consumption was a good predictor of sediment denitrification in both studied lagoons.     

Spatial and Temporal Variability of Benthic Respiration in a Scottish Sea Loch Impacted by Fish Farming: A Combination of In Situ Techniques

The effects of fish farm activities on sediment biogeochemistry were investigated in Loch Creran (Western Scotland) from March to October 2006. Sediment oxygen uptake rates (SOU) were estimated along an organic matter gradient generated from an Atlantic salmon farm using a combination of in situ techniques: microelectrodes, planar optode and benthic chamber incubations. Sulphide (H₂S) and pH distributions in sediment porewater were also measured using in situ microelectrodes, and dissolved inorganic carbon (DIC) fluxes were measured in situ using benthic chambers. Relationships between benthic fluxes, vertical distribution of oxidants and reduced compounds in the sediment were examined as well as bacterial abundance and biomass. Seasonal variations in SOU were relatively low and mainly driven by seasonal temperature variations. The effect of the fish farm on sediment oxygen uptake rate was clearly identified by higher total and diffusive oxygen uptake rates (TOU and DOU, respectively) on impacted stations (TOU: 70 ± 25 mmol O₂ m^?2 day^?1; DOU: 70 ± 32 mmol O₂ m^?2 day^?1 recalculated at the summer temperature), compared with the reference station (TOU: 28.3 ± 5.5 mmol O₂ m^?2 day^?1; DOU: 21.5 ± 4.5 mmol O₂ m^?2 day^?1). At the impacted stations, planar optode images displayed high centimetre scale heterogeneity in oxygen distribution underlining the control of oxygen dynamics by small-scale processes. The organic carbon enrichment led to enhanced sulphate reduction as demonstrated by large vertical H₂S concentration gradients in the porewater (from 0 to 1,000 μM in the top 3 cm) at the most impacted site. The impact on ecosystem functions such as bioirrigation was evidenced by a decreasing TOU/DOU ratio, from 1.7 in the non-impacted sediments to 1 in the impacted zone. This trend was related to a shift in the macrofaunal assemblage and an increase in sediment bacterial population. The turnover time of the organic load of the sediment was estimated to be over 6 years.     

Decadal Change in Sediment Community Oxygen Consumption in the Abyssal Northeast Pacific

Long time-series studies are critical to assessing impacts of climate change on the marine carbon cycle. A 27-year time-series study in the abyssal northeast Pacific (Sta. M, 4000 m depth) has provided the first concurrent measurements of sinking particulate organic carbon supply (POC flux) and remineralization by the benthic community. Sediment community oxygen consumption (SCOC), an estimate of organic carbon remineralization, was measured in situ over daily to interannual periods with four different instruments. Daily averages of SCOC ranged from a low of 5.0 mg C m^?2 day^?1 in February 1991 to a high of 31.0 mg C m^?2 day^?1 in June 2012. POC flux estimated from sediment trap collections at 600 and 50 m above bottom ranged from 0.3 mg C m^?2 day^?1 in October 2013 to 32.0 mg C m^?2 day^?1 in June 2011. Monthly averages of SCOC and POC flux correlated significantly with no time lag. Over the long time series, yearly average POC flux accounted for 63 % of the estimated carbon demand of the benthic community. Long time-series studies of sediment community processes, particularly SCOC, have shown similar fluctuations with the flux of POC reaching the abyssal seafloor. SCOC quickly responds to changes in food supply and tracks POC flux. Yet, SCOC consistently exceeds POC flux as measured by sediment traps alone. The shortfall of ~37 % could be explained by sediment trap sampling artifacts over decadal scales including undersampling of large sinking particles. High-resolution measurements of SCOC are critical to developing a realistic carbon cycle model for the open ocean. Such input is essential to evaluate the impact of climate change on the oceanic carbon cycle, and the long-term influences on the sedimentation record.     

Anaerobic Carbon Mineralisation Through Sulphate Reduction in the Inner Shelf Sediments of Eastern Arabian Sea

Monsoon-induced coastal upwelling, land run-off, benthic and atmospheric inputs make the western Indian shelf waters biologically productive that is expected to lead to high rates of mineralisation of organic matter (OM) in the sediments. Dissimilatory sulphate reduction (SR) is a major pathway of OM mineralisation in near-shore marine sediments owing to depletion of other energetically more profitable electron acceptors (O₂, NO₃ ^?, Mn and Fe oxides) within few millimetres of the sediment-water interface. We carried out first ever study to quantify SR rates in the inner shelf sediments off Goa (central west coast of India) using the ³⁵S radiotracer technique. The highest rates were recorded in the upper 10 cm of the sediment cores and decreased gradually thereafter below detection. Despite significant SR activity in the upper ～12 to 21 cm at most of the sites, pore water sulphate concentrations generally did not show much variation with depth. The depth integrated SR rate (0.066–0.46 mol m^?2 year^?1) decreased with increasing water depth. Free sulphide was present in low concentrations (0–3 μM) in pore waters at shallow stations (depth <30 m). However, high build-up of sulphide (100–600 μM) in pore waters was observed at two deeper stations (depths 39 and 48 m), 7–11 cm below the sediment-water interface. The total iron content of the sediment decreased from ～7 to 5 % from the shallowest to the deepest station. The high pyrite content indicates that the shelf sediments act as a sink for sulphide accounting for the low free sulphide levels in pore water. In the moderately organic rich (2–3.5 %) sediments off Goa, the measured SR rates are much lower than those reported from other upwelling areas, especially off Namibia and Peru. The amount of organic carbon remineralised via sulphate reduction was ～0.52 mol m^?2 year^?1. With an estimated average organic carbon accumulation rate of ～5.6 (±0.5) mol m^?2 year^?1, it appears that the bulk of organic matter gets preserved in sediments in the study region.     

Assessment of carbonaceous aerosol over Delhi in the Indo-Gangetic Basin: characterization, sources and temporal variability

Semi-continuous measurements of organic carbon (OC) and elemental carbon (EC) and continuous measurements of black carbon (BC) and PM_2.5 aerosols were conducted simultaneously during the winter period of 2010–2011 at Delhi, one of the polluted urban megacities in western part of the Indo-Gangetic Basin region. The average mass concentrations of OC, EC, BC and PM_2.5 were about 54 ± 39, 10 ± 5, 12 ± 5 and 210 ± 146 μg m^?3, respectively. Contribution of total carbonaceous aerosol mass to PM_2.5 mass was found to be ~46 %. Average OC/EC ratio was found to be 5 ± 2 during the study period, suggesting the presence of secondary organic aerosols in the atmosphere over Delhi. Estimated mean secondary organic aerosol mass concentration was found to be 25 μg m^?3 and varied between 14.6 (February) and 37.0 μg m^?3 (December). A diurnal variation of OC and EC shows lower values during the day time and higher during the morning and night, which are highly associated with the corresponding variability in mixing layer heights. OC and EC were also found to be significantly correlated (r = 0.71) to each other, indicating their common sources. Concentrations of OC and EC were about 45 and 13 % higher during weekdays than weekends, respectively. Higher OC (67 %) and EC (53 %) were observed in the late evening during weekdays than those on weekends, which could be due to different emission sources during these two periods. The night/day ratio of EC and OC was found to be larger than 1.0, suggesting the relative accumulation of EC and OC near the surface at night hours.     

Oyster (<Emphasis Type="Italic">Crassostrea virginica</Emphasis>) Aquaculture Shifts Sediment Nitrogen Processes toward Mineralization over Denitrification

Filter-feeding bivalves, like oysters, couple pelagic primary production with benthic microbial processes by consuming plankton from the water column and depositing unassimilated material on sediment. Conceptual models suggest that at low to moderate oyster densities, this deposition can stimulate benthic denitrification by providing denitrifying bacteria with organic carbon and nitrogen (N). While enhanced denitrification has been found at oyster reefs, data from oyster aquaculture are limited and equivocal. This study measured seasonal rates of denitrification, as well as dissimilatory nitrate reduction to ammonium (DNRA), and dissolved inorganic N fluxes at a rack and bag eastern oyster (Crassostrea virginica) aquaculture farm. Consistent with models, denitrification was enhanced within the farm, with an average annual increase of 350% compared to a reference site. However, absolute denitrification rates were low relative to other coastal systems, reaching a maximum of 19.2 μmol m^?2 h^?1. Denitrification appeared to be nitrate (NO₃ ^?) limited, likely due to inhibited nitrification caused by sediment anoxia. Denitrification may also have been limited by competition for NO₃ ^? with DNRA, which accounted for an average of 76% of NO₃ ^? reduction. Consequently, direct release of ammonium (NH₄ ⁺) from mineralization to the water column was the most significant benthic N pathway, with seasonal rates exceeding 900 μmol m^?2 h^?1 within the farm. The enhanced N processes were spatially limited however, with significantly higher rates directly under oysters, compared to in between oyster racks. For commercial aquaculture farms like this, with moderate oyster densities (100–200 oysters m^?2), denitrification may be enhanced, but nonetheless limited by biodeposition-induced sediment anoxia. The resulting shift in the sediment N balance toward processes that regenerate reactive N to the water column rather than remove N is an important consideration for water quality.     

Variability in Concentrations and Fluxes of Methane in the Indian Estuaries

In order to examine the fluxes of methane (CH₄) from the Indian estuaries, measurements were carried out by collecting samples from 26 estuaries along the Indian coast during high discharge (wet) and low water discharge (dry) periods. The CH₄ concentrations in the estuaries located along the west coast of India were significantly higher (113?±?40 nM) compared to the east coast of India (27?±?6 nM) during wet and dry periods (88?±?15 and 63?±?12 nM, respectively). Supersaturation of CH₄ was observed in the Indian estuaries during both periods ((0.18 to 22.3?×?10³ %). The concentrations of CH₄ showed inverse relation with salinity indicating that freshwater is a significant source. Spatial variations in CH₄ saturation were associated with the organic matter load suggesting that its decomposition may be another source in the Indian estuaries. Fluxes of CH₄ ranged from 0.01 to 298 μmol m^?2 day^?1 (mean 13.4?±?5 μmol m^?2 day^?1) which is ~30 times lower compared to European estuaries (414 μmol m^?2 day^?1). The annual emission from Indian estuaries, including Pulicat and Adyar, amounted to 0.39?×?10¹⁰ g CH₄?year^?1 with the surface area of 0.027?×?10⁶ km² which is significantly lower than that in European estuaries (2.7?±?6.8?×?10¹⁰ g CH₄?year^?1 with the surface area of 0.03?×?10⁶ km²). This study suggests that Indian estuaries are a weak source for atmospheric CH₄ than European estuaries and such low fluxes were attributed to low residence time of water and low decomposition of organic matter within the estuary. The CH₄ fluxes from the Indian estuaries are higher than those from Indian mangroves (0.01?×?10¹⁰ g CH₄?year^?1) but lower than those from Indian inland waters (210?×?10¹⁰ g CH₄?year^?1).     

Estimation of soil organic carbon storage and its fractions in a small karst watershed

With few available soil organic carbon (SOC) profiles and the heterogeneity of those that do exist, the estimation of SOC pools in karst areas is highly uncertain. Based on the spatial heterogeneity of SOC content of 23,536 samples in a karst watershed, a modified estimation method was determined for SOC storage that exclusively applies to karst areas. The method is a “soil-type method” based on revised calculation indexes for SOC storage. In the present study, the organic carbon contents of different soil types varied greatly, but generally decreased with increasing soil depth. The organic carbon content decreased nearly linearly to a depth of 0–50 cm and then varied at depths of 50–100 cm. Because of the large spatial variability in the karst area, we were able to determine that influences of the different indexes on the estimation of SOC storage decreased as follows: soil thickness > boulder content > rock fragment content > SOC content > bulk density. Using the modified formula, the SOC content in the Houzhai watershed in Puding was estimated to range from 3.53 to 5.44 kg m^?2, with an average value of 1.24 kg m^?2 to a depth of 20 cm, and from 4.44 to 14.50 kg m^?2, with an average value of 12.12 kg m^?2 to a depth of 100 cm. The total SOC content was estimated at 5.39 × 10⁵ t.     

Feedback Mechanisms Between Cyanobacterial Blooms,Transient Hypoxia,and Benthic Phosphorus Regeneration in Shallow Coastal Environments

We investigated the dissolved oxygen metabolism of the Curonian Lagoon (Baltic Sea) to assess the relative contributions of pelagic and benthic processes to the development of transient hypoxic conditions in shallow water habitats. Metabolism measurements along with the remote sensing-derived estimates of spatial variability in chlorophyll a were used to evaluate the risk of hypoxia at the whole lagoon level. Our data demonstrate that cyanobacterial blooms strongly inhibit light penetration, resulting in net heterotrophic conditions in which pelagic oxygen demand exceeds benthic oxygen demand by an order of magnitude. The combination of bloom conditions and reduced vertical mixing during calm periods resulted in oxygen depletion of bottom waters and greater sediment nutrient release. The peak of reactive P regeneration (nearly 30 μmol m^?2 h^?1) coincided with oxygen depletion in the water column, and resulted in a marked drop of the inorganic N:P ratio (from >40 to <5, as molar). Our results suggest a strong link between cyanobacterial blooms, pelagic respiration, hypoxia, and P regeneration, which acts as a feedback in sustaining algal blooms through internal nutrient cycling. Meteorological data and satellite-derived maps of chlorophyll a were used to show that nearly 70 % of the lagoon surface (approximately 1,000 km²) is prone to transient hypoxia development when blooms coincide with low wind speed conditions.     

Denitrification and N2O production in near-shore marine sediments

Methods were developed for determining rates of denitrification in coastal marine sediments by measuring the production of N₂ from undisturbed cores incubated in gas-tight chambers. Denitrification rates at summer temperatures (23°C) in sediment cores from Narragansett Bay, Rhode Island, were about 50μmol N₂m^?2 hr^?1. This nitrogen flux is equal to approximately one-half of the NH⁺₄flux from the sediments at this temperature and is of the magnitude necessary to account for the anomalously low N/P and anomalously high O/N ratios often reported for benthic nutrient fluxes. The loss of fixed nitrogen as N₂ during the benthic remineralization of organic matter, coupled with the importance of benthic remineralization processes in shallow coastal waters may help to explain why the availability of fixed nitrogen is a major factor limiting primary production in these areas. Narragansett Bay sediments are also a source of N₂O, but the amount of nitrogen involved was only about 0.2 μmol m^?2 hr^?1 at 23°C.     

Benthic Oxygen Fluxes Measured by Eddy Covariance in Permeable Gulf of Mexico Shallow-Water Sands

Oxygen fluxes across the sediment–water interface reflect primary production and organic matter degradation in coastal sediments and thus provide data that can be used for assessing ecosystem function, carbon cycling and the response to coastal eutrophication. In this study, the aquatic eddy covariance technique was used to measure seafloor–water column oxygen fluxes at shallow coastal sites with highly permeable sandy sediment in the northeastern Gulf of Mexico for which oxygen flux data currently are lacking. Oxygen fluxes at wave-exposed Gulf sites were compared to those at protected Bay sites over a period of 4 years and covering the four seasons. A total of 17 daytime and 14 nighttime deployments, producing 408 flux measurements (14.5 min each), were conducted. Average annual oxygen release and uptake (mean ± standard error) were 191 ± 66 and ?191 ± 45 mmol m^?2 day^?1 for the Gulf sites and 130 ± 57 and ?152 ± 64 mmol m^?2 day^?1 for the Bay sites. Seasonal variation in oxygen flux was observed, with high rates typically occurring during spring and lower rates during summer. The ratio of average oxygen release to uptake at both sites was close to 1 (Bay: 0.9, Gulf: 1.0). Close responses of the flux to changes in light, temperature, bottom current velocity, and wave action (significant wave height) documented tight physical–biological, benthic–pelagic coupling. The increase of the sedimentary oxygen uptake with increasing temperature corresponded to a Q₁₀ temperature coefficient of 1.4 ± 0.3. An increase in flow velocity resulted in increased oxygen uptake (by a factor of 1–6 for a doubling in flow), which is explained by the enhanced transport of organic matter and electron acceptors into the permeable sediment. Benthic photosynthetic production and oxygen release from the sediment was modulated by light intensity at the temporal scale (minutes) of the flux measurements. The fluxes measured in this study contribute to baseline data in a region with rapid coastal development and can be used in large-scale assessments and estimates of carbon transformations.     

The diagenetic geochemistry and contamination assessment of iron,cadmium, and lead in the sediments from the Shuangtaizi estuary,China

Sediment and pore water samples have been collected from the coastal tidal flat in the Shuangtaizi estuary, China, in order to investigate the geochemical behavior of iron, cadmium, and lead during diagenesis and to assess the degree of contamination. The calculated enrichment factors and geoaccumulation indices for separate elements show that anthropogenic activities have had no significant influence on the distribution of Fe and Pb in the study area, whereas the distribution of Cd has been closely influenced in this way. The high percentage of exchangeable Cd (average of 56.34%) suggests that Cd represents a potential hazard to benthic organisms in the estuary. The calculated diffusive fluxes of metals show that the most mobilized metal is Fe (9.22 mg m^?2 a^?1), followed by Cd (0.54 mg m^?2 a^?1) and Pb (0.42 mg m^?2 a^?1). Low Fe²⁺ contents in surface pore water, alongside high chromium-reducible sulfur contents, and low acid-volatile sulfur, and elemental sulfur contents at 0–25 cm depth in sediments show that Fe²⁺ is formed by the reduction of Fe oxides and is transformed first to a solid phase of iron monosulfides (FeS) and eventually to pyrite (FeS₂). The release of adsorbed Pb due to reductive dissolution of Fe/Mn oxides during early diagenesis could be a source of Pb²⁺ in pore water. From the relatively low total organic carbon contents measured in sediments (0.46–1.28%, with an average of 0.94%) and the vertical variation of Cd²⁺ in pore water, sulfide or Fe/Mn oxides (instead of organic matter) are presumed to exert a significant influence on carrying or releasing Cd by the sediments.     

Benthic Carbon Mineralization and Nutrient Turnover in a Scottish Sea Loch: An Integrative In Situ Study

Based on in situ microprofiles, chamber incubations and eddy covariance measurements, we investigated the benthic carbon mineralization and nutrient regeneration in a ~65-m-deep sedimentation basin of Loch Etive, UK. The sediment hosted a considerable amount of infauna that was dominated by the brittle star A. filiformis. The numerous burrows were intensively irrigated enhancing the benthic in situ O₂ uptake by ~50 %, and inducing highly variable redox conditions and O₂ distribution in the surface sediment as also documented by complementary laboratory-based planar optode measurements. The average benthic O₂ exchange as derived by chamber incubations and the eddy covariance approach were similar (14.9 ± 2.5 and 13.1 ± 9.0 mmol m^?2 day^?1) providing confidence in the two measuring approaches. Moreover, the non-invasive eddy approach revealed a flow-dependent benthic O₂ flux that was partly ascribed to enhanced ventilation of infauna burrows during periods of elevated flow rates. The ratio in exchange rates of ΣCO₂ and O₂ was close to unity, confirming that the O₂ uptake was a good proxy for the benthic carbon mineralization in this setting. The infauna activity resulted in highly dynamic redox conditions that presumably facilitated an efficient degradation of both terrestrial and marine-derived organic material. The complex O₂ dynamics of the burrow environment also concurrently stimulated nitrification and coupled denitrification rates making the sediment an efficient sink for bioavailable nitrogen. Furthermore, bioturbation mediated a high efflux of dissolved phosphorus and silicate. The study documents a high spatial and temporal variation in benthic solute exchange with important implications for benthic turnover of organic carbon and nutrients. However, more long-term in situ investigations with like approaches are required to fully understand how environmental events and spatio-temporal variations interrelate to the overall biogeochemical functioning of coastal sediments.     

Seasonal Oxygen Dynamics in a Warm Temperate Estuary: Effects of Hydrologic Variability on Measurements of Primary Production,Respiration, and Net Metabolism

Seasonal responses in estuarine metabolism (primary production, respiration, and net metabolism) were examined using two complementary approaches. Total ecosystem metabolism rates were calculated from dissolved oxygen time series using Odum’s open water method. Water column rates were calculated from oxygen-based bottle experiments. The study was conducted over a spring-summer season in the Pensacola Bay estuary at a shallow seagrass-dominated site and a deeper bare-bottomed site. Water column integrated gross production rates more than doubled (58.7 to 130.9 mmol O₂ m^?2 day^?1) from spring to summer, coinciding with a sharp increase in water column chlorophyll-a, and a decrease in surface salinity. As expected, ecosystem gross production rates were consistently higher than water column rates but showed a different spring-summer pattern, decreasing at the shoal site from 197 to 168 mmol O₂ m^?2 day^?1 and sharply increasing at the channel site from 93.4 to 197.4 mmol O₂ m^?2 day^?1. The consistency among approaches was evaluated by calculating residual metabolism rates (ecosystem ? water column). At the shoal site, residual gross production rates decreased from spring to summer from 176.8 to 99.1 mmol O₂ m^?2 day^?1 but were generally consistent with expectations for seagrass environments, indicating that the open water method captured both water column and benthic processes. However, at the channel site, where benthic production was strongly light-limited, residual gross production varied from 15.7 mmol O₂ m^?2 day^?1 in spring to 86.7 mmol O₂ m^?2 day^?1 in summer. The summer rates were much higher than could be realistically attributed to benthic processes and likely reflected a violation of the open water method due to water column stratification. While the use of sensors for estimating complex ecosystem processes holds promise for coastal monitoring programs, careful attention to the sampling design, and to the underlying assumptions of the methods, is critical for correctly interpreting the results. This study demonstrated how using a combination of approaches yielded a fuller understanding of the ecosystem response to hydrologic and seasonal variability.     

An Evaluation of Temporal Changes in Sediment Accumulation and Impacts on Carbon Burial in Mobile Bay,Alabama, USA

The estuarine environment can serve as either a source or sink of carbon relative to the coastal ocean carbon budget. A variety of time-dependent processes such as sedimentation, carbon supply, and productivity dictate how estuarine systems operate, and Mobile Bay is a system that has experienced both natural and anthropogenic perturbations that influenced depositional processes and carbon cycling. Sediments from eight box cores provide a record of change in bulk sediment accumulation and carbon burial over the past 110 years. Accumulation rates in the central part of the basin (0.09 g cm^?2) were 60–80 % less than those observed at the head (0.361 g cm^?2) and mouth (0.564 g cm^?2) of the bay. Sediment accumulation in the central bay decreased during the past 90 years in response to both anthropogenic (causeway construction) and natural (tropical cyclones) perturbations. Sediment accumulation inevitably increased the residence time of organic carbon in the oxic zone, as observed in modeled remineralization rates, and reduced the overall carbon burial. Such observations highlight the critical balance among sediment accumulation, carbon remineralization, and carbon burial in dynamic coastal environments. Time-series analysis based solely on short-term observation would not capture the long-term effects of changes in sedimentation on carbon cycling. Identifying these relationships over longer timescales (multi-annual to decadal) will provide a far better evaluation of coastal ocean carbon budgets.     

Benthic Phosphorus Dynamics in the Gulf of Finland, Baltic Sea

Benthic fluxes of soluble reactive phosphorus (SRP) and dissolved inorganic carbon (DIC) were measured in situ using autonomous landers in the Gulf of Finland in the Baltic Sea, on four expeditions between 2002 and 2005. These measurements together with model estimates of bottom water oxygen conditions were used to compute the magnitude of the yearly integrated benthic SRP flux (also called internal phosphorus load). The yearly integrated benthic SRP flux was found to be almost 10 times larger than the external (river and land sources) phosphorus load. The average SRP flux was 1.25?±?0.56?mmol?m^?2?d^?1 on anoxic bottoms, and ?0.01?±?0.08?mmol?m^?2?d^?1 on oxic bottoms. The bottom water oxygen conditions determined whether the SRP flux was in a high or low regime, and degradation of organic matter (as estimated from benthic DIC fluxes) correlated positively with SRP fluxes on anoxic bottoms. From this correlation, we estimated a potential increase in phosphorus flux of 0.69?±?0.26?mmol?m^?2?d^?1 from presently oxic bottoms, if they would turn anoxic. An almost full annual data set of in situ bottom water oxygen measurements showed high variability of oxygen concentration. Because of this, an estimate of the time which the sediments were exposed to oxygenated overlying bottom water was computed using a coupled thermohydrodynamic ocean?Csea and ecosystem model. Total phosphorus burial rates were calculated from vertical profiles of total phosphorus in sediment and sediment accumulation rates. Recycling and burial efficiencies for phosphorus of 97 and 3%, respectively, were estimated for anoxic accumulation bottoms from a benthic mass balance, which was based on the measured effluxes and burial rates.     

Spatial and Temporal Variability of Sediment Organic Matter Recycling in Two Temperate Eutrophicated Estuaries

This paper deals with the spatial and seasonal recycling of organic matter in sediments of two temperate small estuaries (Elorn and Aulne, France). The spatio-temporal distribution of oxygen, nutrient and metal concentrations as well as the organic carbon and nitrogen contents in surficial sediments were determined and diffusive oxygen fluxes were calculated. In order to assess the source of organic carbon (OC) in the two estuaries, the isotopic composition of carbon (δ ¹³C) was also measured. The temporal variation of organic matter recycling was studied during four seasons in order to understand the driving forces of sediment mineralization and storage in these temperate estuaries. Low spatial variability of vertical profiles of oxygen, nutrient, and metal concentrations and diffusive oxygen fluxes were monitored at the station scale (within meters of the exact location) and cross-section scale. We observed diffusive oxygen fluxes around 15 mmol m^?2 day^?1 in the Elorn estuary and 10 mmol m^?2 day^?1 in the Aulne estuary. The outer (marine) stations of the two estuaries displayed similar diffusive O₂ fluxes. Suboxic and anoxic mineralization was large in the sediments from the two estuaries as shown by the rapid removal of very high bottom water concentrations of NO _x ^? (>200 μM) and the large NH₄ ⁺ increase at depth at all stations. OC contents and C/N ratios were high in upstream sediments (11–15 % d.w. and 4–6, respectively) and decreased downstream to values around 2 % d.w. and C/N ≤ 10. δ ¹³C values show that the organic matter has different origins in the two watersheds as exemplified by lower δ ¹³C values in the Aulne watershed. A high increase of δ ¹³C and C/N values was visible in the two estuaries from upstream to downstream indicating a progressive mixing of terrestrial with marine organic matter. The Elorn estuary is influenced by human activities in its watershed (urban area, animal farming) which suggest the input of labile organic matter, whereas the Aulne estuary displays larger river primary production which can be either mineralized in the water column or transferred to the lower estuary, thus leaving a lower mineralization in Aulne than Elorn estuary. This study highlights that (1) meter scale heterogeneity of benthic biogeochemical properties can be low in small and linear macrotidal estuaries, (2) two estuaries that are geographically close can show different pattern of organic matter origin and recycling related to human activities on watersheds, (3) small estuaries can have an important role in recycling and retention of organic matter.     

The Role of CaCO<Subscript>3</Subscript> Reactions in the Contemporary Oceanic CO<Subscript>2</Subscript> Cycle

The present analysis adjusts previous estimates of global ocean CaCO₃ production rates substantially upward, to 133 × 10¹² mol yr^?1 plankton production and 42 × 10¹² mol yr^?1 shelf benthos production. The plankton adjustment is consistent with recent satellite-based estimates; the benthos adjustment includes primarily an upward adjustment of CaCO₃ production on so-called carbonate-poor sedimentary shelves and secondarily pays greater attention to high CaCO₃ mass (calcimass) and turnover of shelf communities on temperate and polar shelves. Estimated CaCO₃ sediment accumulation rates remain about the same as they have been for some years: ~20 × 10¹² mol yr^?1 on shelves and 11 × 10¹² mol yr^?1 in the deep ocean. The differences between production and accumulation of calcareous materials call for dissolution of ~22 × 10¹² mol yr^?1 (~50 %) of shelf benthonic carbonate production and 122 × 10¹² mol yr^?1 (>90 %) of planktonic production. Most CaCO₃ production, whether planktonic or benthonic, is assumed to take place in water depths of <100 m, while most dissolution is assumed to occur below this depth. The molar ratio of CO₂ release to CaCO₃ precipitation (CO₂↑/CaCO₃↓) is <1.0 and varies with depth. This ratio, Ψ, is presently about 0.66 in surface seawater and 0.85 in ocean waters deeper than about 1000 m. The net flux of CO₂ associated with CaCO₃ reactions in the global ocean in late preindustrial time is estimated to be an apparent influx from the atmosphere to the ocean, of +7 × 10¹² mol C yr^?1, at a time scale of 10²–10³ years. The CaCO₃-mediated influx of CO₂ is approximately offset by CO₂ release from organic C oxidation in the water column. Continuing ocean acidification will have effects on CaCO₃ and organic C metabolic responses to the oceanic inorganic C cycle, although those responses remain poorly quantified.     

Groundwater Discharge as a Source of Dissolved Carbon and Greenhouse Gases in a Subtropical Estuary

Groundwater may be highly enriched in dissolved carbon species, but its role as a source of carbon to coastal waters is still poorly constrained. Exports of deep and shallow groundwater-derived dissolved carbon species from a small subtropical estuary (Korogoro Creek, Australia, latitude ?31.0478°, longitude 153.0649°) were quantified using a radium isotope mass balance model (²³³Ra and ²²⁴Ra, natural groundwater tracers) under two hydrological conditions. In addition, air-water exchange of carbon dioxide and methane in the estuary was estimated. The highest carbon inputs to the estuary were from deep fresh groundwater in the wet season. Most of the dissolved carbon delivered by groundwater and exported from the estuary to the coastal ocean was in the form of dissolved inorganic carbon (DIC; 687 mmol m^?2 estuary day^?1; 20 mmol m^?2 catchment day^?1, respectively), with a large export of alkalinity (23 mmol m^?2 catchment day^?1). Average water to air flux of CO₂ (869 mmol m^?2 day^?1) and CH₄ (26 mmol m^?2 day^?1) were 5- and 43-fold higher, respectively, than the average global evasion in estuaries due to the large input of CO₂- and CH₄-enriched groundwater. The groundwater discharge contribution to carbon exports from the estuary for DIC, dissolved organic carbon (DOC), alkalinity, CO₂, and CH₄ was 22, 41, 3, 75, and 100 %, respectively. The results show that CO₂ and CH₄ evasion rates from small subtropical estuaries surrounded by wetlands can be extremely high and that groundwater discharge had a major role in carbon export and evasion from the estuary and therefore should be accounted for in coastal carbon budgets.     

Quantification of methane emission from bacterial mat sites at Quepos Slide offshore Costa Rica

Seafloor methane emission from the Quepos Slide on the submarine segment of the Costa Rica fore-arc margin was estimated by extrapolating flux measurements from individual seeps to the total area covered by bacterial mats. This approach is based on the combination of detailed mapping to determine the abundance of seeps and the application of a numerical model to estimate the amount of benthic methane fluxes. Model results suggest that the majority of the studied seeps transport rather limited amount of methane (on average: ~177 μmol cm^?2 a^?1) into the water column due to moderate upward advection, allowing for intense anaerobic oxidation of methane (AOM; on average: 53 % of the methane flux is consumed). Depth-integrated AOM rates (56–1,538 μmol CH₄ cm^?2 a^?1) are comparable with values reported from other active seep sites. The overall amount of dissolved methane released into the water column from the entire area covered by bacterial mats on the Quepos Slide is estimated to be about 0.28 × 10⁶ mol a^?1. This conservative estimate which relies on rather accurate determinations of seafloor methane fluxes emphasizes the potential importance of submarine slides as sites of natural methane seepage; however, at present the global extent of methane seepage from submarine slides is largely unknown.