Oxygen fluxes involving the benthic micro- and macrophytic components in the Thau Lagoon under pre-anoxic conditions

Measurements of oxygen fluxes at the water-sediment interface (transparent and opaque enclosures) were made on bare sediments inhabited by microphytes on the one hand, and on sediments diversely colonized by macrophytes (macroalgae and seagrasses) on the other hand. Five stations, typical of different biotopes of a Mediterranean shallow lagoon were studied from May to July 1993 in order to observe an anoxic event which usually occurs at that time of the year. Average diurnal respiration of benthic communities ranged from 40 mg (or 1.25 mmol) O₂ m² h⁻¹ in bare sediments (31 % of the lagoon area) to 76 mg (or 2.37 mmol) O₂ m⁻² h⁻¹ in sediments with a medium coverage of macroalgae (37 % of the total area) and, finally, to 100 mg (or 3.12 mmol) O₂ m⁻²h⁻¹ in the denser macrophytic area (32 % of the total area). The highest diurnal gross production was observed in the zone colonized by macroalgae and seagrasses, and especially in corridors between shellfish-cultivation tables (300 mg O₂ m⁻² h⁻¹, or 9.37 mmol O₂m⁻² h⁻¹, equivalent to 113 mg C m⁻² h⁻¹). Overall, during this period, net bottom oxygen production was close to nil in ca. 30 %, and positive in 70 % of the lagoon area. The average net oxygen production for the whole lagoon in summer is thus in the order of 100 mg O₂ m⁻²h⁻¹. In 1993, at the end of July, an anoxic event was avoided due to a period of strong wind.     

Oxygen fluxes in the Norwegian Atlantic Current

Oxygen and phosphate measurements from two sections across the Norwegian Atlantic Current, the Gimsøy-NW section from 67.5°N 9°E to 71.5°N 1°E and the Bjørnøya-W section along 74.5°N from 7 to 15°E, are used to estimate oxygen fluxes in the surface layer and between the atmosphere and the ocean. Vertical entrainment velocities of 0.9 m day⁻¹ for the winter season and 0.1 m day⁻¹ for the summer season are found and applied to the upper 300 m. The resulting oxygen fluxes to the surface layer driven by this vertical mixing are 0.58±0.05 and 0.27±0.02 mol O₂ m⁻² year⁻¹ at the Gimsøy-NW and Bjørnøya-W sections, respectively. Oxygen fluxes to the surface layer due to phytoplankton production are 2.6 and 3.4 mol O₂ m⁻² year⁻¹, which represent the net community production at the two sections. Estimated uncertainties in these numbers are ±15%. The surface water is a sink for atmospheric oxygen during fall and winter and a source during the productive season for both sections. On an annual basis there is a net uptake of oxygen from the atmosphere, 3.4±0.4 mol O₂ m⁻² year⁻¹ at the Gimsøy-NW section and 4.9±0.5 mol O₂ m⁻² year⁻¹ at the Bjørnøya-W. A decrease in temperature of 1°C to 1.5°C seen between the Gimsøy-NW section and the Bjørnøya-W section is the main reason for the increased atmospheric flux of oxygen at the latter section. An oxygen budget made for the area bounded by the two sections gives a net advective flux of oxygen out of the area of approximately 10 mol O₂ m⁻² year⁻¹. The increased concentration of oxygen corresponding to the decrease in surface layer temperatures going northwards in the Norwegian Atlantic Current is mainly attributed to the air–sea oxygen exchange and phytoplankton production in this area.     

Oxygen dynamics in the North Atlantic subtropical gyre

Dissolved oxygen (DO) in the ocean is a tracer for most ocean biogeochemical processes including net community production and remineralization of organic matter which in turn constrains the biological carbon pump. Knowledge of oxygen dynamics in the North Atlantic Ocean is mainly derived from observations at the Bermuda Atlantic Time-series Study (BATS) site located in the western subtropical gyre which may skew our view of the biogeochemistry of the subtropical North Atlantic. This study presents and compares a 15 yr record of DO observations from ESTOC (European Station for Time-Series in the Ocean, Canary Islands) in the eastern subtropical North Atlantic with the 20 yr record at BATS. Our estimate for net community production of oxygen was 2.3±0.4 mol O₂ m⁻² yr⁻¹ and of oxygen consumption was −2.3±0.5 mol O₂ m⁻² yr⁻¹ at ESTOC, and 4 mol O₂ m⁻² yr⁻¹ and −4.4±1 mol m⁻² yr⁻¹ at BATS, respectively. These values were determined by analyzing the time-series using the Discrete Wavelet Transform (DWT) method. These flux values agree with similar estimates from in-situ observational studies but are higher than those from modeling studies. The difference in net oxygen production rates supports previous observations of a lower carbon export in the eastern compared to the western subtropical Atlantic. The inter-annual analysis showed clear annual cycles at BATS whereas longer cycles of nearly 4 years were apparent at ESTOC. The DWT analysis showed trends in DO anomalies dominated by long-term perturbations at a basin scale for the consumption zones at both sites, whereas yearly cycles dominated the production zone at BATS. The long-term perturbations found are likely associated with ventilation of the main thermocline, affecting the consumption and production zones at ESTOC.     

Molecular iodine (I2) emission from two Laminaria species (Phaeophyceae) and impact of irradiance and temperature on I2 emission into air and iodide release into seawater from Laminaria digitata

Kelps of the genus Laminaria accumulate iodine at high concentrations, but the iodine retaining capacity can be affected by emersion and physiological stress. In this study, I₂ emission into the atmosphere from Laminaria digitata and Laminaria hyperborea was compared under controlled low irradiances and temperatures. The two species exhibited different I₂ emission rates as blades of L. digitata emitted I₂ at rates five times higher than those from newly-grown blades (current growth season) of L. hyperborea. I₂ emission was not detectable from old blades (previous growth season) of L. hyperborea. Additionally, effects of irradiance and temperature on both I₂ emission into air and net I⁻ release into seawater where assessed for L. digitata while monitoring photo-physiological parameters as stress indicators. Irradiances between 30 and 120 μmol photons m⁻² s⁻¹ had only marginal effects on both I₂ emission and I⁻ release rates, but physiological stress, indicated by photoinhibition, was observed. The results suggest that the irradiances applied here were not stressful enough to impact on the iodine release. By contrast, at elevated temperatures (20 °C), photoinhibition was accompanied by an increase in I₂ emission rates, but net I⁻ release rates remained similar at 10–20 °C. High I₂ emission rates into air and I⁻ release into seawater observed from L. digitata underpin the fundamental function of this kelp as mediator of coastal iodine fluxes.     

Community respiration/production and bacterial activity in the upper water column of the central Arctic Ocean

Community metabolism (respiration and production) and bacterial activity were assessed in the upper water column of the central Arctic Ocean during the SHEBA/JOIS ice camp experiment, October 1997–September 1998. In the upper 50 m, decrease in integrated dissolved oxygen (DO) stocks over a period of 124 d in mid-winter suggested a respiration rate of ∼3.3 nM O₂ h⁻¹ and a carbon demand of ∼4.5 gC m⁻². Increase in 0–50 m integrated stocks of DO during summer implied a net community production of ∼20 gC m⁻². Community respiration rates were directly measured via rate of decrease in DO in whole seawater during 72-h dark incubation experiments. Incubation-based respiration rates were on average 3-fold lower during winter (11.0±10.6 nM O₂ h⁻¹) compared to summer (35.3±24.8 nM O₂ h⁻¹). Bacterial heterotrophic activity responded strongly, without noticeable lag, to phytoplankton growth. Rate of leucine incorporation by bacteria (a proxy for protein synthesis and cell growth) increased ∼10-fold, and the cell-specific rate of leucine incorporation ∼5-fold, from winter to summer. Rates of production of bacterial biomass in the upper 50 m were, however, low compared to other oceanic regions, averaging 0.52±0.47 ngC l⁻¹ h⁻¹ during winter and 5.1±3.1 ngC l⁻¹ h⁻¹ during summer. Total carbon demand based on respiration experiments averaged 2.4±2.3 mgC m⁻³ d⁻¹ in winter and 7.8±5.5 mgC m⁻³ d⁻¹ in summer. Estimated bacterial carbon demand based on bacterial productivity and an assumed 10% gross growth efficiency was much lower, averaging about 0.12±0.12 mgC m⁻³ d⁻¹ in winter and 1.3±0.7 mgC m⁻³ d⁻¹ in summer. Our estimates of bacterial activity during summer were an order of magnitude less than rates reported from a summer 1994 study in the central Arctic Ocean, implying significant inter-annual variability of microbial processes in this region.     

Dissolved inorganic carbon,alkalinity, nutrient and oxygen seasonal and interannual variations at the Antarctic Ocean JGOFS-KERFIX site

JGOFS-KERFIX (KERguelen point FIXe) time-series station, located south of the polar front in the Indian sector of the Antarctic Ocean, was occupied monthly between January 1990 and March 1995. Annual cycles of dissolved inorganic carbon (DIC), total alkalinity (TALK), oxygen (O₂) and nutrients (nitrate, silicate, phosphate and ammonia) in the upper ocean are presented for this site. From seasonal drawdown of nutrients and DIC, we estimate a spring–summer net community production of 3.2±0.5 mol m⁻² and C/N/P ratios of 100/16/1. The Si/N ratio varies between 1.8 and 3, suggesting low iron concentrations. The spring–summer biogenic silicon export derived from silicate drawdown is 1.18 mol m⁻², consistent with model estimates of silicate export at this site. Seasonal and interannual variations of oxygen, nitrate and DIC due to physical and biological processes are quantified using a simple month-to-month budget formulation. From these budgets, an annual net community production of 5.7±3.3 mol m⁻² yr⁻¹ is estimated, about twice the averaged spring–summer production, indicating that, at KERFIX, there is a positive net community production throughout the year. Air–sea CO₂ fluxes show that KERFIX is a strong CO₂ sink for the atmosphere of 2.4–5.1 mol m⁻² yr⁻¹ in 1993, depending on the gas exchange formulation used. A 2.1–3.3 mol m⁻² yr⁻¹ outgassing of O₂ is observed at KERFIX except in 1993 and 1994 where a decreasing trend of temperature induces an increase of O₂ solubility.     

Spatial and temporal changes in estuarine water quality during a post-flood hypoxic event

A major fish kill occurred in the Richmond River estuary in January 2008 due to oxygen depletion following extensive overbank flooding. This paper examines spatial and temporal changes in the chemistry of main channel waters, thereby identifying the primary sources of deoxygenating water. Over 40 km of the mid- to lower estuary main channel was deoxygenated within seven days of the flood peak. Hypoxia was confined to downstream of the confluences with mid-estuary backswamp basins and occurred during the later phase of the flood recession. Water chemistry at key locations in the estuary indicated elevated concentrations of redox sensitive species associated with acid sulfate soils (ASS) during the hypoxic period. Peak concentrations of Fe²⁺ up to 18.2 μmol L⁻¹, dissolved Mn up to 4.3 μmol L⁻¹, chemical oxygen demand (COD) up to 2052 μmol L⁻¹, dissolved organic carbon (DOC) up to 960 μmol L⁻¹ and elemental S⁰ up to 4.7 μmol L⁻¹ were found in the backswamp discharge confluences and mid-estuary main channel locations. The geochemical signature of main channel floodwaters identifies anaerobic decomposition of floodplain vegetation in ASS backswamps as a primary process leading to generation of hypoxic waters. The transport of these hypoxic floodwaters to the estuary has been accelerated and prolonged by extensive floodplain drainage, thereby enhancing the magnitude and duration of estuarine deoxygenation.     

Deep penetrating benthic oxygen profiles measured in situ by oxygen optodes

For the first time in situ, deep penetrating O₂ profiles were measured in abyssal sediments in the western South Atlantic. Construction of deep penetrating O₂ optodes and adaptation to a benthic profiling lander are described. The opto-chemical oxygen sensors allow measurements to a depth of 55 cm in marine sediments. A vertical resolution of 0.5 cm was used to determine the O₂ dynamics in those oligotrophic deep sea sediments; the oxygen concentration across the sediment water interface was measured with a resolution of 100 μm. Oxygen penetration depth (OPD), diffusive oxygen uptake (DOU) and oxygen consumption rates were determined at four stations north of the Amazon fan and one at the Mid-Atlantic Ridge. Diffusive oxygen uptake rates ranged from 0.1 to 0.9 mmol m⁻² d⁻¹; the oxygen penetration depth ranged from 8 to 26 cm. Carbon consumption rates calculated from the diffusive oxygen uptake rates were in the range of 0.3–3.0 g C m⁻² a⁻¹. Comparison between in situ and laboratory DOU and OPD measurements confirmed previous findings that core recovery and warming have strong effects on the oxygen dynamics in deep sea sediments. Laboratory measurements yielded a decrease of 50–75% in OPD and consequently an increase in DOU by 1.5 and 18-times. Deep penetrating oxygen optodes provide a new tool to accurately determine oxygen dynamics (and thereby calculate carbon mineralization rates) in oligotrophic sediments. However, oxygen optodes as used in this study do not resolve the diffusive boundary layer (DBL). The data show that deep penetrating O₂ optodes in combination with high-resolution O₂ microelectrodes give a complete picture of the oxygen dynamics, including the DBL, in deep sea sediments.     

The Role of Microphytobenthic Primary Production in a Mediterranean Mussel Culture Area

The production and biomass of microphytobenthos in a Mediterranean mussel farm was studied during 1991–92. Gross and net microphytobenthic production and respiration were calculated from oxygen fluxes in transparent and black bell jars at two stations; sediments under a mussel table and reference sediments, both located at 5 m depth. Net oxygen fluxes were mainly negative under the mussel tables (average −19·5 mg O₂ m⁻² h⁻¹, CV=132%), and microphytobenthos production could not meet the sediment oxygen demand; in the reference sediments, microphytobenthos production was responsible for net oxygen production (average +13·0 mg O₂ m⁻² h⁻¹, CV=118%). Benthic respiration rates were, on average, 47·3 mg O₂ m⁻² h⁻¹(CV=82%) under the tables and 27·7 mg O₂ m⁻² h⁻¹(CV=45%) in reference sediments. Aerobic respiration could remineralize less than 2% of the biodeposited carbon under the tables, implying that a large amount of organic material is accumulating under the tables, and that most of the degradation will be anaerobic. Gross microbenthic production showed sharp changes between 1991 and 1992 under the mussel tables and for reference sediments (averages 20·98 mg O₂ m⁻² h⁻¹, CV=135% and 33 mg O₂ m⁻² h⁻¹, CV=48%, respectively). Despite the negative oxygen balance in the sediments under the tables, microphytobenthos was more productive than phytoplankton in bottom waters. Per unit area, phytoplankton was more productive than microphytobenthos at both stations, especially in the area of the mussel tables, where phytoplanktonic production was enhanced by the excretion products of mussels. Microphytobenthos was composed mainly of diatoms in the sediments under the tables, while in reference sediments, the population was more diverse, with algae containing chlorophyllbalso present. Chlorophyllaconcentration in sediments under the tables was 207 mg m⁻²(CV=73%) and 95 mg m⁻²(CV=28%) in reference sediments; the stock of plant pigments was increased under the tables by biodeposition. Microphytobenthos constitutes a compartment with an important contribution in biomass, but also in oxygen production.     

Oxygen distribution and aerobic respiration in the north and south eastern tropical Pacific oxygen minimum zones

Highly sensitive STOX O₂ sensors were used for determination of in situ O₂ distribution in the eastern tropical north and south Pacific oxygen minimum zones (ETN/SP OMZs), as well as for laboratory determination of O₂ uptake rates of water masses at various depths within these OMZs. Oxygen was generally below the detection limit (few nmol L⁻¹) in the core of both OMZs, suggesting the presence of vast volumes of functionally anoxic waters in the eastern Pacific Ocean. Oxygen was often not detectable in the deep secondary chlorophyll maximum found at some locations, but other secondary maxima contained up to ~0.4 µmol L⁻¹. Directly measured respiration rates were high in surface and subsurface oxic layers of the coastal waters, reaching values up to 85 nmol L⁻¹ O₂ h⁻¹. Substantially lower values were found at the depths of the upper oxycline, where values varied from 2 to 33 nmol L⁻¹ O₂ h⁻¹. Where secondary chlorophyll maxima were found the rates were higher than in the oxic water just above. Incubation times longer than 20 h, in the all-glass containers, resulted in highly increased respiration rates. Addition of amino acids to the water from the upper oxycline did not lead to a significant initial rise in respiration rate within the first 20 h, indicating that the measurement of respiration rates in oligotrophic Ocean water may not be severely affected by low levels of organic contamination during sampling. Our measurements indicate that aerobic metabolism proceeds efficiently at extremely low oxygen concentrations with apparent half-saturation concentrations (K_m values) ranging from about 10 to about 200 nmol L⁻¹.     

Nitrous oxide and N-nutrient cycling in the oxygen minimum zone off northern Chile

Measurements of dissolved gases (O₂, N₂O), nutrients (NO₃⁻, NO₂⁻, PO₄³⁻), and oceanographic variables were performed off northern Chile (∼21°S) between March 2000 and July 2004, in order to characterize the existing oxygen minimum zone (OMZ) and identify processes involved in N₂O cycling. Both N₂O and NO₃⁻ displayed sharp, shallow peaks with concentrations of up to 124 nM (1370% saturation) and 26 μM, respectively, in association with a strong oxycline that impinges on the euphotic zone. NO₂⁻ accumulation below the oxycline's base reached up to 9 μM. The vertical distribution of physical and chemical parameters and the existing relationships between apparent oxygen utilization (AOU), apparent N₂O production (ΔN₂O), and NO₃⁻ revealed three main layers within the upper OMZ. The first layer, or the upper part of the oxycline, is located between the base of the mixed layer and the mid-point of the oxycline (around σ_t=25.5 kg m⁻³). There the O₂ declines from ∼250 to ∼50 μM, and strong (but opposing) O₂ and NO₃⁻ gradients and their associated AOU–ΔN₂O and AOU–NO₃⁻ relationships indicate that nitrification produces N₂O and NO₃⁻ in the presence of light. The second layer, or lower part of the oxycline, represents the upper OMZ boundary and is located between the middle and the base of the oxycline (25.9<σ_t<26.1 kg m⁻³). In this layer NO₃⁻ reduction begins at O₂ levels ranging from ∼50 to ∼11 μM and accumulation of 41–68% of the ΔN₂O pool occurs. The accumulation of N₂O (but not of NO₂⁻ or NH₄⁺) and the observed AOU–ΔN₂O and AOU–NO₃⁻ relationships (which are opposite to those of the overlying first layer) suggest that a coupling between nitrification and NO₃⁻ reduction is involved in N₂O cycling in this second layer. The third layer is the OMZ core, where the O₂ concentration remains constant (O₂<11 μM). It coincides with σ_t>26.2 kg m⁻³, which is typical of Equatorial Subsurface Water (ESSW). In this layer, N₂O and NO₃⁻ continue to decrease, but a large NO₂⁻ accumulation is observed. Considering all the data, a biogeochemical model for the upper OMZ off northern of Chile is proposed, in which nitrification and denitrification differentially mediate N₂O cycling in each layer.     

The metabolic balance between autotrophy and heterotrophy in the western Arctic Ocean

The goal of this study was to explore how net community production (NCP) is influenced by the relationship between primary production and community respiration in the western Arctic Ocean. Plankton NCP and respiration were determined by measuring changes in oxygen in light and dark bottle incubations, respectively. Rates of NCP averaged over shelf, slope and basin waters were positive in summer 2002 (57±191 mmol O₂ m⁻² d⁻¹) and spring 2004 (85±86 mmol O₂ m⁻² d⁻¹) and negative in summer 2004 (−25±176 mmol O₂ m⁻² d⁻¹). Determinations of NCP obtained from bottle incubations were similar to rates inferred from in situ changes in dissolved inorganic carbon. An examination of the spatial variability of primary production and community respiration indicated that respiration is distributed more uniformly than primary production. A spatial offset between photosynthesis and respiration from the shelf to the Arctic basin was present in spring 2004, but was not seen at other times. NCP and the potential for export appear to be dependent on an uncoupling of primary production and community respiration. NCP continued into the summer after the stock of NO₃⁻ had been depleted. Our data suggest that the uniform distribution of respiration relative to primary production is an important factor influencing NCP and the potential for export in the western Arctic.     

In vitro simulation of oxic/suboxic diagenesis in an estuarine fluid mud subjected to redox oscillations

Estuarine turbidity maxima (ETMs) are sites of intense mineralisation of land-derived particulate organic matter (OM), which occurs under oxic/suboxic oscillating conditions owing to repetitive sedimentation and resuspension cycles at tidal and neap-spring time scales. To investigate the biogeochemical processes involved in OM mineralisation in ETMs, an experimental set up was developed to simulate in vitro oxic/anoxic oscillations in turbid waters and to follow the short timescale changes in oxygen, carbon, nitrogen, and manganese concentration and speciation. We present here the results of a 27-day experiment (three oxic periods and two anoxic periods) with an estuarine fluid mud from the Gironde estuary. Time courses of chemical species throughout the experiment evidenced the occurrence of four distinct characteristic periods with very different properties. Steady oxic conditions were characterised by oxygen consumption rates between 10 and 40 μmol L⁻¹ h⁻¹, dissolved inorganic carbon (DIC) production of 9–12 μmol L⁻¹ h⁻¹, very low NH₄⁺ and Mn²⁺ concentrations, and constant NO₃⁻ production rates (0.4 - 0.7 μmol L⁻¹ h⁻¹) due to coupled ammonification and nitrification. The beginning of anoxic periods (24 h following oxic to anoxic switches) showed DIC production rates of 2.5–8.6 μmol L⁻¹ h⁻¹ and very fast NO₃⁻ consumption (5.6–6.3 μmol L⁻¹ h⁻¹) and NH₄⁺ production (1.4–1.5 μmol L⁻¹ h⁻¹). The latter rates were positively correlated to NO₃⁻ concentration and were apparently caused by the predominance of denitrification and dissimilatory nitrate reduction to ammonia. Steady anoxic periods were characterised by constant and low NO₃⁻ concentrations and DIC and NH₄⁺ productions of less than 1.3 and 0.1 μmol L⁻¹ h⁻¹, respectively. Mn²⁺ and CH₄ were produced at constant rates (respectively 0.3 and 0.015 μmol L⁻¹ h⁻¹) throughout the whole anoxic periods and in the presence of nitrate. Finally, reoxidation periods (24–36 h following anoxic to oxic switches) showed rapid NH₄⁺ and Mn²⁺ decreases to zero (1.6 and 0.8–2 μmol L⁻¹ h⁻¹, respectively) and very fast NO₃⁻ production (3 μmol L⁻¹ h⁻¹). This NO₃⁻ production, together with marked transient peaks of dissolved organic carbon a few hours after anoxic to oxic switches, suggested that particulate OM mineralisation was enhanced during these transient reoxidation periods. An analysis based on C and N mass balance suggested that redox oscillation on short time scales (day to week) enhanced OM mineralisation relative to both steady oxic and steady anoxic conditions, making ETMs efficient biogeochemical reactors for the mineralisation of refractory terrestrial OM at the land-sea interface.     

N2O Production, Nitrification and Denitrification in an Estuarine Sediment

The mechanisms regulating N₂O production in an estuarine sediment (Tama Estuary, Japan) were studied by comparing the change in N₂O production with those in nitrification and denitrification using an experimental continuous-flow sediment–water system with¹⁵N tracer (¹⁵N-NO−3 addition). From Feburary to May, both nitrification and denitrification in the sediment increased (246 to 716 μmol N m⁻² h⁻¹and 214 to 1260 μmol N m⁻² h⁻¹, respectively), while benthic N₂O evolution decreased slightly (1560 to 1250 nmol N m⁻² h⁻¹). Apparent diffusion coefficients of inorganic nitrogen compounds and O₂at the sediment–water interface, calculated from the respective concentration gradients and benthic fluxes, were close to the molecular diffusion coefficients (0·68–2·0 times) in February. However, they increased to 8·8–52 times in May except for that of NO−2, suggesting that the enhanced NO−3 and O₂supply from the overlying water by benthic irrigation likely stimulated nitrification and denitrification. Since the progress of anoxic condition by the rise of temperature from February to May (9 to 16 °C) presumably accelerated N₂O production through nitrification, the observed decrease in sedimentary N₂O production seems to be attributed to the decrease in N₂O production/occurrence of its consumption by denitrification. In addition to the activities of both nitrification and denitrification, the change in N₂O metabolism during denitrification by the balance between total demand of the electron acceptor and supply of NO−3+NO−2 can be an important factor regulating N₂O production in nearshore sediments.     

Organic carbon flux and remineralization in surface sediments from the northern North Atlantic derived from pore-water oxygen microprofiles

Organic carbon fluxes through the sediment/water interface in the high-latitude North Atlantic were calculated from oxygen microprofiles. A wire-operated in situ oxygen bottom profiler was deployed, and oxygen profiles were also measured onboard (ex situ). Diffusive oxygen fluxes, obtained by fitting exponential functions to the oxygen profiles, were translated into organic carbon fluxes and organic carbon degradation rates. The mean C_org input to the abyssal plain sediments of the Norwegian and Greenland Seas was found to be 1.9 mg C m⁻² d⁻¹. Typical values at the seasonally ice-covered East Greenland continental margin are between 1.3 and 10.9 mg C m⁻² d⁻¹ (mean 3.7 mg C m⁻² d⁻¹), whereas fluxes on the East Greenland shelf are considerably higher, 9.1–22.5 mg C m⁻² d⁻¹. On the Norwegian continental slope C_org fluxes of 3.3–13.9 mg C m⁻² d⁻¹ (mean 6.5 mg C m⁻² d⁻¹) were found. Fluxes are considerably higher here compared to stations on the East Greenland slope at similar water depths. By repeated occupation of three sites off southern Norway in 1997 the temporal variability of diffusive O₂ fluxes was found to be quite low. The seasonal signal of primary and export production from the upper water column appears to be strongly damped at the seafloor. Degradation rates of 0.004–1.1 mg C cm⁻³ a⁻¹ at the sediment surface were calculated from the oxygen profiles. First-order degradation constants, obtained from C_org degradation rates and sediment organic carbon content, are in the range 0.03–0.6 a⁻¹. Thus, the corresponding mean lifetime of organic carbon lies between 1.7 and 33.2 years, which also suggests that seasonal variations in C_org flux are small. The data presented here characterize the Norwegian and Greenland Seas as oligotrophic and relatively low organic carbon deep-sea environments.     

Benthic fluxes in a tropical Estuary and their role in the ecosystem

In-situ measurements of benthic fluxes of oxygen and nutrients were made in the subtidal region of the Mandovi estuary during premonsoon and monsoon seasons to understand the role of sediment–water exchange processes in the estuarine ecosystem. The Mandovi estuary is a shallow, highly dynamic, macrotidal estuary which experiences marine condition in the premonsoon season and nearly fresh water condition in the monsoon season. The benthic flux of nutrients exhibited strong seasonality, being higher in the premonsoon compared to the monsoon season which explains the higher ecosystem productivity in the dry season in spite of negligible riverine nutrient input. NH₄⁺ was the major form of released N comprising 70–100% of DIN flux. The benthic respiration rate varied from −98.91 to −35.13 mmol m⁻² d⁻¹, NH₄⁺ flux from 5.15 to 0.836 mmol m⁻² d⁻¹, NO₃⁻ + NO₂⁻ from 0.06 to −1.06 mmol m⁻² d⁻¹, DIP from 0.12 to 0.23 mmol m⁻² d⁻¹ and SiO₄⁴⁻ from 5.78 to 0.41 mmol m⁻² d⁻¹ between premonsoon to monsoon period. The estuarine sediment acted as a net source of DIN in the premonsoon season, but changed to a net sink in the monsoon season. Variation in salinity seemed to control NH₄⁺ flux considerably. Macrofaunal activities, especially bioturbation, enhanced the fluxes 2–25 times. The estuarine sediment was observed to be a huge reservoir of NH₄⁺, PO₄³⁻ and SiO₄⁴⁻ and acted as a net sink of combined N because of the high rate of benthic denitrification as it could remove 22% of riverine DIN influx thereby protecting the eco system from eutrophication and consequent degradation. The estuarine sediment was responsible for ∼30–50% of the total community respiration in the estuary. The benthic supply of DIN, PO₄³⁻ and SiO₄⁴⁻ can potentially meet 49%, 25% and 55% of algal N, P and Si demand, respectively, in the estuary. Based on these observations we hypothesize that it is mainly benthic NH₄⁺ efflux that sustains high estuarine productivity in the NO₃⁻ depleted dry season.     

Microbial photosynthesis in coral reef sediments (Heron Reef, Australia)

We investigated microphytobenthic photosynthesis at four stations in the coral reef sediments at Heron Reef, Australia. The microphytobenthos was dominated by diatoms, dinoflagellates and cyanobacteria, as indicated by biomarker pigment analysis. Conspicuous algae firmly attached to the sand grains (ca. 100 μm in diameter, surrounded by a hard transparent wall) were rich in peridinin, a marker pigment for dinoflagellates, but also showed a high diversity based on cyanobacterial 16S rDNA gene sequence analysis. Specimens of these algae that were buried below the photic zone exhibited an unexpected stimulation of respiration by light, resulting in an increase of local oxygen concentrations upon darkening. Net photosynthesis of the sediments varied between 1.9 and 8.5 mmol O₂ m⁻² h⁻¹ and was strongly correlated with Chl a content, which lay between 31 and 84 mg m⁻². An estimate based on our spatially limited dataset indicates that the microphytobenthic production for the entire reef is in the order of magnitude of the production estimated for corals. Photosynthesis stimulated calcification at all investigated sites (0.2–1.0 mmol Ca²⁺ m⁻² h⁻¹). The sediments of at least three stations were net calcifying. Sedimentary N₂-fixation rates (measured by acetylene reduction assays at two sites) ranged between 0.9 to 3.9 mmol N₂ m⁻² h⁻¹ and were highest in the light, indicating the importance of heterocystous cyanobacteria. In coral fingers no N₂-fixation was measurable, which stresses the importance of the sediment compartment for reef nitrogen cycling.     

Net biological oxygen production in the ocean—II: Remote in situ measurements of O2 and N2 in subarctic pacific surface waters

Net community biological production in the euphotic zone of the ocean fuels organic matter and oxygen export from the upper ocean, which has a large influence on the atmospheric pressure of carbon dioxide and is the driving force for metabolite distributions in the sea. We determine the net annual biological oxygen production in the mixed layer of the northeast subarctic Pacific Ocean from in situ O₂ and N₂ measurements. Temperature, salinity, total gas pressure and O₂ were measured every 3 h for 9 months in 2007 at about 3 m depth on a surface mooring at Station P (50°N, 145°W). The concentration of nitrogen gas, N₂, determined from separate total gas pressure and pO₂ measurements, was used as an inert tracer of the physical processes that induce gas departure from thermodynamic equilibrium with the atmosphere. We use a simple model of the ocean’s mixed layer along with the nitrogen concentration to constrain the importance of bubbles, gas exchange and horizontal advection, which are then used in the oxygen mass balance to derive net biological oxygen production. The mixed-layer oxygen mass balance is dominated by exchange with the atmosphere, and we determine a mean summertime oxygen production of 24 mmol O₂ m^?2 d^?1. The annual pattern in the difference between the supersaturation of oxygen and nitrogen in the surface waters reveals very little net oxygen production during the winter at this location. The calculated annual net community production (NCP) of carbon from this new method, 2.5 mol m^?2 yr^?1, agrees to within its error of about×40% with previous determinations at this location from oxygen mass balance, NO₃^? draw down and ²³⁴Th measurements. This value is either indistinguishable from or lower than annual NCP measurements in the subtropical North Pacific, indicating that there is no experimental evidence for differences in annual NCP between the subarctic and subtropical North Pacific Ocean.     

Microbial degradation rates of small peptides and amino acids in the oxygen minimum zone of Chilean coastal waters

We found similar microbial degradation rates of labile dissolved organic matter in oxic and suboxic waters off northern Chile. Rates of peptide hydrolysis and amino acid uptake in unconcentrated water samples were not low in the water column where oxygen concentration was depleted. Hydrolysis rates ranged from 65 to 160 nmol peptide L⁻¹ h⁻¹ in the top 20 m, 8–28 nmol peptide L⁻¹ h⁻¹ between 100 and 300 m (O₂-depleted zone), and 14–19 nmol peptide L⁻¹ h⁻¹ between 600 and 800 m. Dissolved free amino acid uptake rates were 9–26, 3–17, and 6 nmol L⁻¹ h⁻¹ at similar depth intervals. Since these findings are consistent with a model of comparable potential activity of microbes in degrading labile substrates of planktonic origin, we suggest, as do other authors, that differences in decomposition rates with high and low oxygen concentrations may be a matter of substrate lability. The comparison between hydrolysis and uptake rates indicates that microbial peptide hydrolysis occurs at similar or faster rates than amino acid uptake in the water column, and that the hydrolysis of peptides is not a rate-limiting step for the complete remineralization of labile macromolecules. Low O₂ waters process about 10 tons of peptide carbon per h, double the amount processed in surface-oxygenated water. In the oxygen minimum zone, we suggest that the C balance may be affected by the low lability of the dissolved organic matter when this is upwelled to the surface. An important fraction of dissolved organic matter is processed in the oxygen minimum layer, a prominent feature of the coastal ocean in the highly productive Humboldt Current System.     

Measurement of dissolved oxygen using optodes in a FerryBox system

Optode sensors can provide detailed information on concentrations of dissolved oxygen, which in turn may be used to quantify variations in net primary productivity. Throughout 2005 and 2006 the performance of commercially available oxygen optodes was examined, one in each year. The optode was part of an autonomous measurement system (FerryBox) on a ferry operating between Portsmouth (UK) and Bilbao (Spain). On crossings during which water samples were collected manually, the optode outputs were compared to measurements of dissolved oxygen made by Winkler titrations. The optodes maintained good stability with no evidence of instrumental drift during the course of a year. Over the observed concentration range (230–330 mM m⁻³) the optode data were approximately 2% low in both years. By fitting the optode data to the Winkler data the median difference between the optode and Winkler measurements is reduced to less than 1 mM m⁻³ (0.3%) in both years. The most appropriate calibration factor for 2005 was corrected O₂ = Optode O₂ × 1.018 and for 2006 the corresponding equation is corrected O₂ = Optode O₂ × 0.884 + 36.8. The standard deviation (95%) of the difference between the individual Winkler measurements was 5 mM m⁻³ and 3 mM m⁻³ in 2005 and 2006 respectively.Calculation of the oxygen saturation anomaly is required for calculation of the air sea exchange of oxygen and net biological production. This calculation requires the use of both salinity and temperature data. Salinity is measured to better than 0.1 so the corresponding error in anomaly is less than 0.2 mM m⁻³. Distortion of the temperature data is present due to warming of the water pumped to the optode. In winter this warming at the optode may be as great as 0.4 °C. The difference in the pumped water temperature can be corrected for by reference to other measurements of sea surface temperature reducing the error to less than 1 mM m⁻³.