Diffusive Oxygen Uptake Rates Research Articles

Abyssal fauna, benthic microbes, and organic matter quality across a range of trophic conditions in the western Pacific ocean

The abyssal plain covers more than half the Earth’s surface. The main food source to abyssal ecosystems is phytodetritus, which originates from phytoplankton in the surface ocean, and thus its variability to the seafloor is a major driver of abyssal ecosystem biomass and functioning. In this study, we conducted a comparative survey on organic matter (OM) quality and quantity in abyssal plain sediments and examined the distributions of megafauna, macrofauna, meiofauna, prokaryotes, and viruses in eutrophic (39°N), oligotrophic (1°N), and ultra-oligotrophic (12°N) areas of the western Pacific. We also analyzed stable carbon and nitrogen isotopic compositions of organisms at 39°N and 1°N to assess differences in benthic abyssal food-web structures with contrasting trophic states. Sediments collected at 39°N presented highest concentrations of total organic carbon (TOC) and labile OM, and high diffusive oxygen uptake rates. By contrast, the lowest values were found at 12°N. Vertical distributions of sediment macrofauna, meiofauna, and prokaryotes matched with labile OM profiles. There were prominent differences in abundances of macro- and megafauna among stations with different OM fluxes, whereas the abundance of meiofauna and prokaryotes showed smaller differences among stations. Such differences could be explained by higher turnover rates of smaller organisms. Food-web structures of abyssal plains are likely influenced by both the type and size of primary producers in surface ocean. Our results underscore the crucial importance of OM fluxes and their compositions to the abundances and vertical profiles of labile OM and benthic biota in abyssal ecosystems.

Early diagenesis in the Congo deep-sea fan sediments dominated by massive terrigenous deposits: Part I – Oxygen consumption and organic carbon mineralization using a micro-electrode approach

Organic matter (OM) transfer from the continent to the ocean occurs across margins which constitute a major area of OM recycling and burial. The lobe complex of the Congo deep-sea fan is connected to the river mouth by a canyon and alimented by recurrent turbidity currents, containing a large proportion of labile terrigenous OM and producing high sedimentation rates. These inputs support the development of ecosystems harboring rich assemblages of vesicomyid bivalves and bacterial mats, called Habitats. Here, we present O2 microprofiles and diffusive oxygen uptake rates (DOUs) obtained during the CONGOLOBE project at six sites of this active lobe complex by in situ and on-board methods based on micro-electrode profiling. The dataset is used to determine remineralization rates and study the biogeochemical dynamics of different ecosystems of the lobe area, in order to compare levee and background sediments to the Habitats developed on the flanks of the main turbiditic channel. Levee and background sediments are characterized by significantly higher DOUs than abyssal sediments at 5000m meters depth (2–5mmol O2m−2d−1versus 1.5–2.5mmol O2 m−2 d−1) and the Habitats are hotspots of OM remineralization with DOU values ranging between 8 and 40mmol O2m−2d−1. By comparing sites near the active channel to a site located 50km away, we show that the lobe connection to the main turbiditic channel is vital to the dense benthic communities.

Effects of fluctuating hypoxia on benthic oxygen consumption in the Black Sea (Crimean shelf)

Abstract. The outer western Crimean shelf of the Black Sea is a natural laboratory to investigate effects of stable oxic versus varying hypoxic conditions on seafloor biogeochemical processes and benthic community structure. Bottom-water oxygen concentrations ranged from normoxic (175 μmol O2 L−1) and hypoxic (< 63 μmol O2 L−1) or even anoxic/sulfidic conditions within a few kilometers' distance. Variations in oxygen concentrations between 160 and 10 μmol L−1 even occurred within hours close to the chemocline at 134 m water depth. Total oxygen uptake, including diffusive as well as fauna-mediated oxygen consumption, decreased from 15 mmol m−2 d−1 on average in the oxic zone, to 7 mmol m−2 d−1 on average in the hypoxic zone, correlating with changes in macrobenthos composition. Benthic diffusive oxygen uptake rates, comprising respiration of microorganisms and small meiofauna, were similar in oxic and hypoxic zones (on average 4.5 mmol m−2 d−1), but declined to 1.3 mmol m−2 d−1 in bottom waters with oxygen concentrations below 20 μmol L−1. Measurements and modeling of porewater profiles indicated that reoxidation of reduced compounds played only a minor role in diffusive oxygen uptake under the different oxygen conditions, leaving the major fraction to aerobic degradation of organic carbon. Remineralization efficiency decreased from nearly 100 % in the oxic zone, to 50 % in the oxic–hypoxic zone, to 10 % in the hypoxic–anoxic zone. Overall, the faunal remineralization rate was more important, but also more influenced by fluctuating oxygen concentrations, than microbial and geochemical oxidation processes.

Sedimentary organic carbon budget of coastal sediments and the importance of benthic-pelagic coupling off Namhae Island in the South Sea of Korea

We characterized the biogeochemical organic carbon cycles in the surface sediment layer (< 25 cm) in the coastal waters of Namhae off the South Sea of Korea. The total and diffusive sediment oxygen uptake rates were measured using an in situ benthic lander equipped with a benthic chamber and a microprofiler. The bottom water above the sediment-water interface was incubated to estimate the benthic flux of the dissolved inorganic nutrients and the total alkalinity using an in situ benthic chamber. In addition, the particulate materials vertically deposited onto the surface sediment and the sedimentation rates were quantified to calculate the sedimentary organic carbon budget. The total oxygen uptake rate was in the range 34.9 to 54.1 mmol O2 m-2d-1, which is about three times the diffusive oxygen uptake rate. An abnormal oxygen peak observed in the anoxic sediment layer suggests a higher bioirrigation activity in the sediment layer. The oxidation rate of organic carbon at the sediment surface showed a very narrow range (36 ± 7 to 37 ± 7 mmol C m-2d-1), and the burial flux into the sediment layer was in the range 3 to 13 mmol C m-2d-1, which accounted for 9% to 36% of the remineralization rate of the organic carbon. The N and P requirement fluxes for pelagic production could be supported by 29% and 42% of the benthic flux, respectively, which strongly suggests a benthic-pelagic coupling in the coastal area of the South Sea of Korea.

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.

Temporal variability of carbon recycling in coastal sediments influenced by rivers: assessing the impact of flood inputs in the Rhône River prodelta

Abstract. River deltas are particularly important in the marine carbon cycle as they represent the transition between terrestrial and marine carbon: linked to major burial zones, they are reprocessing zones where large carbon fluxes can be mineralized. In order to estimate this mineralization, sediment oxygen uptake rates were measured in continental shelf sediments and river prodelta over different seasons near the outlet of the Rhône River in the Mediterranean Sea. On a selected set of 10 stations in the river prodelta and nearby continental shelf, in situ diffusive oxygen uptake (DOU) and laboratory total oxygen uptake (TOU) measurements were performed in early spring and summer 2007 and late spring and winter 2008. In and ex situ DOU did not show any significant differences except for shallowest organic rich stations. Sediment DOU rates show highest values concentrated close to the river mouth (approx. 20 mmol O2 m−2 d−1) and decrease offshore to values around 4.5 mmol O2 m−2 d−1 with lowest gradients in a south west direction linked to the preferential transport of the finest riverine material. Core incubation TOU showed the same spatial pattern with an averaged TOU/DOU ratio of 1.2±0.4. Temporal variations of sediment DOU over different sampling periods, spring summer and late fall, were limited and benthic mineralization rates presented a stable spatial pattern. A flood of the Rhône River occurred in June 2008 and delivered up to 30 cm of new soft muddy deposit. Immediately after this flood, sediment DOU rates close to the river mouth dropped from around 15–20 mmol O2 m−2 d−1 to values close to 10 mmol O2 m−2 d−1, in response to the deposition near the river outlet of low reactivity organic matter associated to fine material. Six months later, the oxygen distribution had relaxed back to its initial stage: the initial spatial distribution was found again underlining the active microbial degradation rates involved and the role of further deposits. These results highlight the immediate response of the sediment oxygen system to flood deposit and the rapid relaxation of this system towards its initial state (6 months or less) potentially linked to further deposits of reactive material.

In situ oxygen uptake rates by coastal sediments under the influence of the Rhône River (NW Mediterranean Sea)

The influence of riverine inputs on biogeochemical cycling and organic matter recycling in sediments on the continental shelf off the Rhône River mouth (NW Mediterranean Sea) was investigated by measuring sediment oxygen uptake rates using a combination of in situ and laboratory techniques. Four stations were investigated during two cruises in June 2001 and June 2002, with depths ranging from 9 to 192 m and over a distance to the Rhône River mouth ranging from 4 to 36 km. Diffusive oxygen uptake (DOU) rates were determined using an in situ sediment microprofiler and total oxygen uptake (TOU) rates were measured using sediment core incubations. There was good agreement between these two techniques which indicates that the non-diffusive fraction of the oxygen flux was minimal at the investigated stations. DOU rates ranged from 3.7±0.4 mmol O 2 m −2 d −1 at the continental shelf break to 19.3±0.5 mmol O 2 m −2 d −1 in front of the Rhône River mouth. Sediment oxygen uptake rates mostly decreased with increasing depth and with distance from the Rhône mouth. The highest oxygen uptake rate was observed at 63 m on the Rhône prodelta, corresponding to intense remineralization of organic matter. This oxygen uptake rate was much larger than expected for the increasing bathymetry, which indicates that biogeochemical cycles and benthic deposition are largely influenced by the Rhône River inputs. This functioning was also supported by the detailed spatial distribution of total organic carbon (TOC), total nitrogen (TN) and C/N atomic ratio in surficial sediments. Sediments of the Rhône prodelta are enriched in organic carbon (2–2.2%) relative to the continental shelf sediments (<1%) and showed C/N ratios exceeding Redfield stoichiometry for fresh marine organic matter. A positive exponential correlation was found between DOU and TOC contents ( r 2=0.98, n=4). South-westward of the Rhône River mouth, sediments contained highly degraded organic matter of both terrestrial and marine origin, due to direct inputs from the Rhône River, sedimentation of marine organic matter and organic material redeposition after resuspension events.

Oxygen distribution and bioirrigation in Arctic fjord sediments (Svalbard, Barents Sea)

The penetration of oxygen into silty fjord sediments from northern Norway and SW Svalbard was studied at 6 sites during a research cruise to the northern Barents Sea. Profiles of oxy- gen were measured by microelectrodes on retrieved sediment cores in a thermostated flow aquarium and used to develop composite, 2-dimensional images of the oxygen distribution. Oxygen pene- trations ranged from 3 to 11 mm with a mean depth of 6 to 8 mm. The mean diffusive oxygen uptake rates across the sediment-water interface ranged from 2.8 to 13.4 mmol O2 m -2 d -1 . The diffusive flux accounted for 60 to 95% of the total oxygen uptake of the sediments as measured in situ by a flux chamber lander. The sediments were densely populated by fauna such as tube-dwelling polychaetes. Inhabited and relict tubes of 1 to 2.5 mm diameter reached densities of up to 1 cm -2 , and about 15% of all oxygen microprofiles showed evidence of advective oxygen flow through the tubes. Based on oxygen microprofiles and data on burrow geometry and density, burrows extending down into the anoxic sediment extended the oxic sediment volume by 2 to 10% and thereby enhanced the oxygen flux. Anoxic microenvironments were not detected, but during inactivity of polychaetes their tubes became depleted in oxygen relative to the surrounding sediment.

Calcite dissolution driven by benthic mineralization in the deep-sea: in situ measurements of Ca2+, pH, pCO2 and O2

In situ measured microprofiles of Ca2+, pCO2, pH and O2 were performed to quantify the CaCO3 dissolution and organic matter mineralization in marine sediments in the eastern South Atlantic. A numerical model simulating the organic matter decay with oxygen was used to estimate the calcite dissolution rate. From the oxygen microprofiles measured at four stations along a 1300-m isobath of the eastern African margin and one in front of the river Niger at a water depth of 2200 m the diffusive oxygen uptake (DOU) and oxygen penetration depth (OPD) was calculated. DOU rates were in the range of 0.3 to 3 mmol m−2 d−1 and showed a decrease with increasing water depth, corresponding to an increase in OPD. The calculated amount of degradated organic matter is in the range of 1 to 8.5 gC m−2 a−1. The metabolic CO2, released from mineralization of the organic matter drives calcite dissolution in these sediments overlain by calcite-supersaturated water. Fluxes across the sediment water interface calculated from the in situ Ca2+ microprofiles were 0.6 mmol m−2 d−1 for two stations at a water depth of 1300 m. The ratio of calcite dissolution flux and organic C degradation is 0.53 and 0.97, respectively. The microprofiles indicate that CO2 produced within the upper oxic sediment layer dissolves up to 85% of the calcite rain to the seafloor. Modeling our O2, pH and Ca2+ profiles from one station predicted a calcite dissolution rate constant for this calcite-poor site of 1000 mol kgw−1 a−1 (mol per kg water and year), which equals 95% d−1. This rate constant is at the upper end of reported in situ values.

Deep penetrating benthic oxygen profiles measured in situ by oxygen optodes

For the first time in situ, deep penetrating O 2 profiles were measured in abyssal sediments in the western South Atlantic. Construction of deep penetrating O 2 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 O 2 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 O 2 optodes in combination with high-resolution O 2 microelectrodes give a complete picture of the oxygen dynamics, including the DBL, in deep sea sediments.

Sulfate reduction in surface sediments of the southeast Atlantic continental margin between 15°38'S and 27°57'S (Angola and Namibia)

Sulfate reduction rates in the surface sediments from 17 stations from an along‐slope transect (1,300 m) and from a cross‐slope transect (855–4,766 m) were determined in the continental margin sediments of the Benguela Upwelling system. Profiles at all sites in the upwelling area showed increasing sulfate reduction rates from near zero at the surface to a peak at 2–5 cm (up to 29 nmol cm−3 d−1) and then decreasing exponentially with depth to near background rates at 10–20 cm depth (&lt;2 nmol cm−3 d−1). Depth‐integrated sulfate reduction rates were greatest at 1,300 m and decreased exponentially with water depth. Along the transect following the 1,300‐m isobath, depth‐integrated sulfate reduction rates were highest in the north Cape Basin (1.16 +/− 0.23 mmol m−2 d−1), decreased over the Walvis Ridge (0.67 +/− 0.02 mmol m−2 d−1), and were lowest in the south Angola Basin (0.31 +/− 0.23 mmol m−2 d−1). Depth‐integrated sulfate reduction rates were consistent with the known pattern of coastal upwelling intensities and were also strongly correlated with surface organic carbon concentrations. Sulfate reduction rates, both as a function of depth and in comparison with sediment trap data, indicated that lateral downslope transport of organic carbon occurs. Sulfate reduction was estimated to account for 20–90% of the published rates of total oxygen consumption for the sediments at 1,300 m depth and 3–16% of sediments from 2,000 to 3,000 m depth. Comparison of the sulfate reduction rate profiles with the published diffusive oxygen uptake rates showed that the kinetics of oxygen utilization in the surface sediments are much faster than those for anaerobic organic carbon remineralization, although the underlying cause of the difference was not clear.