Ronnie N. Glud

Pathways of organic carbon oxidation in three continental margin sediments

We have combined several different methodologies to quantify rates of organic carbon mineralization by the various electron acceptors in sediments from the coast of Denmark and Norway. Rates of NH4+ and Sigma CO2 liberation sediment incubations were used with O2 penetration depths to conclude that O2 respiration accounted for only between 3.6-17.4% of the total organic carbon oxidation. Dentrification was limited to a narrow zone just below the depth of O2 penetration, and was not a major carbon oxidation pathway. The processes of Fe reduction, Mn reduction and sulfate reduction dominated organic carbon mineralization, but their relative significance varied depending on the sediment. Where high concentrations of Mn-oxide were found (3-4 wt% Mn), only Mn reduction occurred. With lower Mn oxide concentrations more typical of coastal sediments, Fe reduction and sulfate reduction were most important and of a similar magnitude. Overall, most of the measured O2 flux into the sediment was used to oxidized reduced inorganic species and not organic carbon. We suspect that the importance of O2 respiration in many coastal sediments has been overestimated, whereas metal oxide reduction (both Fe and Mn reduction) has probably been well underestimated.

Oxygen dynamics of marine sediments

Abstract Benthic O2 availability regulates many important biogeochemical processes and has crucial implications for the biology and ecology of benthic communities. Further, the benthic O2 exchange rate represents the most widely used proxy for quantifying mineralization and primary production of marine sediments. Consequently, numerous researchers have investigated the benthic O2 dynamics in a wide range of environments. On the basis of case studies – from abyssal sediments to microbial phototrophic communities – I hereby try to review the current status on what we know about controls that interrelate with the O2 dynamics of marine sediments. This includes factors like: sedimentation rates, bottom water O2 concentrations, diffusive boundary layers, fauna activity, light, temperature, and sediment permeability. The investigation of benthic O2 dynamics represents a challenge in resolving variations on temporal and spatial scales covering several orders of magnitude. Such an effort requires the use of several complementary measuring techniques and modeling approaches. Recent technical developments (improved chamber approaches, O2 optodes, eddy-correlation, benthic observatories) and advances in diagenetic modeling have facilitated our abilities to resolve and interpret benthic O2 dynamics. However, all approaches have limitations and caveats that must be carefully evaluated during data interpretation. Much has been learned during the last decades but there are still many unanswered questions that need to be addressed in order to fully understand benthic O2 dynamics and the role of sediments for marine carbon cycling.

Diffusive and total oxygen uptake of deep-sea sediments in the eastern South Atlantic Ocean:in situ and laboratory measurements

Abstract Total O 2 uptake rates were measured by the benthic flux chamber lander ELINOR, and O 2 microprofiles were measured by the profiling lander PROFILUR in the eastern South Atlantic. Diffusive O 2 fluxes through the diffusive boundary layer and the depth distribution of O 2 consumption rates within the sediment were calculated from the obtained microprofiles. The depth integrated O 2 consumption rate agreed closely with the diffusive O 2 uptake at all stations. Total O 2 uptake was 1.2–4.2 times the diffusive O 2 uptake, and the difference correlated with the abundance of macrofauna in the sediment. Diffusive O 2 uptake and O 2 -penetration depths correlated with the organic content of the sediments and exhibited an inverse correlation with water depth. Total and diffusive rates of in situ O 2 uptake were higher than previously published data for shelf and abyssal sediments in the Atlantic, but were comparable to rates from upwelling areas in the eastern Pacific. Laboratory measurements on recovered sediment cores showed lower O 2 penetration depths and higher diffusive uptake rates than in situ measurements. The differences increased with increasing water depth. We primarily ascribe this compression of O 2 profiles to a transiently increased temperature during recovery and enhanced microbial activity in decompressed sediment cores. Total O 2 uptake rates measured in the laboratory on macrofauna-rich stations were, in contrast, lower than those measured in situ because of underrepresentation and disturbance of the macrofauna.

Microenvironmental control of photosynthesis and photosynthesis-coupled respiration in an epilithic cyanobacterial biofilm.

The photosynthetic performance of an epilithic cyano‐bacterial biofilm was studied in relation to the in situ light field by the use of combined microsensor measurements of O2, photosynthesis, and spectral scalar irradiance. The high density of the dominant filamentous cyanobacteria (Oscillatoria sp.) embedded in a matrix of exopolymers and bacteria resulted in a photic zone of < 0.7 mm. At the biofilm surface, the prevailing irradiance and spectral composition were significantly different from the incident light. Multiple scattering led to an intensity maximum for photic light (400–700 nm) of ca. 120% of incident quantum irradiance at the biofilm surface. At the bottom of the euphotic zone in the biofilm, light was attenuated strongly to < 5–10% of the incident surface irradiance. Strong spectral signals from chlorophyll a (440 and 675 nm) and phycobilins (phycoerythrin 540–570 nm, phycocyanin 615–625 nm) were observed as distinct maxima in the scalar irradiance attenuation spectra in the upper 0.0–0.5 mm of the biofilm. The action spectrum for photosynthesis in the cyanobacterial layer revealed peak photosynthetic activity at absorption wavelengths of phycobilins, whereas only low photosynthesis rates were induced by light absorption of carotenoids (450–550 nm).

Impacts of longline mussel farming on oxygen and nitrogen dynamics and biological communities of coastal sediments

Benthic communities and benthic mineralization were studied in two shallow coastal regions of New Zealand: Tasman Bay, a possible future site for mussel farm development, and Beatrix Bay, which already hosts several longline mussel farms. In Tasman Bay, microphytobenthic (MPB) production added significantly to the total primary production of the bay. The activity of benthic microalgae had a pronounced effect on oxic conditions, solute exchange and denitrification rates. Benthic mineralization, quantified as the dark oxygen uptake, was in the range of 675±11 μmol m−2 h−1. Denitrification rates were high and fueled entirely by nitrate produced by the nitrifying community within the sediment. Competition for inorganic nitrogen between benthic microalgae and nitrifiers/denitrifiers resulted in diel variation in nitrogen cycling and reduced the inorganic nitrogen efflux and denitrification activity in the light. Calculated in electron equivalents, denitrification accounted for 11–20% of the total carbon mineralization—one of the highest numbers reported for coastal sediments. Reduced sediments, containing low MPB biomass and few subsurface macroinvertebrate species, were observed below a mussel farm in Beatrix Bay, presumably due to the intensified sedimentation of organic matter. Oxygen consumption increased in the organic-rich sediments, and ammonium effluxes were up to 14 times higher than those of unaffected sediments 250 m away from the farm. Denitrification rates below the farm were low as the coupled nitrification–denitrification was inhibited by the presence of sulfide. The dissimilative reduction of nitrate to ammonium (DRNA) was, however, stimulated in the reduced sediment. The enhanced benthic mineralization was associated with sulfidic sediments and a lower nitrogen removal rate due to impeded benthic photosynthesis and denitrification activity. The described local conditions associated with mussel farming should be taken into account when new areas are considered for development.

Optical measurement of oxygen and temperature in microscale: strategies and biological applications

Sediments, microbial mats, biofilms and other microbial communities are characterized by steep gradients of physical and chemical parameters. Microsensors are powerful tools to measure these parameters with a sufficient spatial resolution and with a small disturbance of the micro-environment in natural systems. Recently, fiber-optical microsensors have been introduced in the field of aquatic biology as an alternative to existing electrochemical microsensors. Such micro-optodes have already been developed for high-resolution measurement of dissolved oxygen and for temperature measurements. They are easy to fabricate and show an improved long-term and storage stability. An overview is given on the development and characterization of different types of micro-optodes for oxygen and temperature. A luminescence lifetime-based device has been developed which is portable and enables microsensing both in the laboratory and under field conditions. Limitations in practical work with optical microsensors are demonstrated, and strategies to overcome them briefly discussed. A micro-optode array as well as a method for high-resolution oxygen imaging in sediments are presented as two different ways to investigate the two-dimensional oxygen distribution in heterogeneous living systems. Future applications and developments in micro-optode research will be discussed briefly.

Benthic carbon mineralization in the Atlantic: a synthesis based on in situ data from the last decade

Benthic oxygen uptake rates quantified by the use of microsensors and flux chambers over a period of approx. 10 yr were compiled and used to assess the organic carbon mineralization in the central and South Atlantic (351N–501S). Measurements were performed in situ and in the laboratory on recovered sediment cores. In contrast to the laboratory data, both the in situ diffusive (DOU) and total oxygen uptake (TOU) decreased with increasing water depth. The data demonstrated that sediment recovery alter the O2 microdistribution and affect the measured O2 uptake rates. The ratio between TOU and DOU, a measure of the benthic fauna-mediated oxygen uptake, decreased from 3 to 4 in shallow and productive areas to around 1 at the deeper sites. The in situ oxygen uptake rates (both diffusive and total) also correlated with the oceanic primary production. Based on the compiled in situ measurements an empirical relation between the surface water primary production (PP, g C m � 2 yr � 1 ), water depth (z; m) and benthic mineralization deduced from the TOU and DOU was established (C-DOU=PP 0:7358 z � 0:3306 (g C m � 2 yr � 1 ); C-TOU=PP 1:0466 z � 0:4922 (g C m � 2 yr � 1 )). These equations were extrapolated to the entire investigated area of the Atlantic. The mineralization mimicked the surface water primary production, with high consumption rates in the upwelling areas. For the entire area (water depthX1000 m) the benthic carbon mineralization was between 134 and 168 � 10 12 gC yr � 1 (from C-DOU and C-TOU, respectively), which equals 1.7–2.1% of the surface water primary production. These rates are higher than previous estimates of benthic carbon mineralization in deep-sea sediments. Integrated for the investigated area of the Atlantic the benthic fauna-mediated carbon mineralization accounted for 35 � 10 12 gC yr � 1 (or 21% of the total mineralization rate). Using our relations to calculate the organic carbon flux through the 1000 m depth horizon revealed that between 212 and 333 � 10 12 gC yr � 1 sink below this depth horizon, of which 63% and 51% is remineralized in the sediments. Particulate organic carbon fluxes obtained from sediment trap data cannot support either the measured or extrapolated benthic mineralization. The areal distribution of the oxygen penetration depth (OPD) for the investigated area of the Atlantic was estimated from the relation between the in situ C-DOU and OPD measurements

Benthic chamber and profiling landers in oceanography - A review of design, technical solutions and functioning

We review and evaluate the design and operation of twenty-seven known autonomous benthic chamber and profiling lander instruments. We have made a detailed comparison of the different existing lander designs and discuss the relative strengths and weaknesses of each. Every aspect of a lander deployment, from preparation and launch to recovery and sample treatment is presented and compared. It is our intention that this publication will make it easier for future lander builders to choose a design suitable for their needs and to avoid unnecessary mistakes.

Manganese oxidation and in situ manganese fluxes from a coastal sediment

Abstract Manganese fluxes from the seafloor were measured in situ in Aarhus Bay, Denmark, with a free operating benthic flux-chamber lander (ELINOR). Constant effluxes were observed during 3 h incubations. The benthic Mn flux resulted in bottom water concentrations of dissolved Mn up to 0.6 μM, whereas concentrations above the pycnocline were about 0.1 μM. Similar fluxes were observed from sediment cores incubated in the laboratory under in situ conditions. A large variation between cores was attributed to the small area covered by each core. Manganese reduction in the upper 0–1 cm of the sediment supported steep porewater gradients of Mn towards the surface. However, calculated diffusive Mn fluxes towards the sediment surface were 3–16 times higher than the benthic effluxes. This demonstrated high rates of Mn oxidation in the 1–2 mm thin oxic surface layer with turnover times of 2 h or less. Model calculations including measured microdistributions of O2 and pH yielded rate constants more than 1000 times higher than those reported for abiotic Mn oxidation implying that Mn oxidation in the sediment was microbially mediated. The rapid oxidation was, as an internal source of oxidized Mn, essential to the intense redox cycling of Mn in the surface sediment.

PHOTOSYNTHESIS AND PHOTOSYNTHESIS‐COUPLED RESPIRATION IN NATURAL BIOFILMS QUANTIFIED WITH OXYGEN MICROSENSORS1

Photosynthesis and respiration were analyzed in natural biofilms by use of O2 microsensors. Depth profiles of gross photosynthesis were obtained from the rate of decrease in O2 concentration during the first few seconds following extinction of light, and net photosynthesis of the photic zone was calculated from O2 concentration gradients measured at steady state. Respiration within the photic zone was calculated as the difference between gross and net photosynthesis. Two types of biofilms were investigated: one dominated by diatoms, and one dominated by cyanobacteria. High O2/CO2 ratios caused increased respiration especially within the diatom biofilm, which could indicate that photorespiration was a dominant O2‐consuming process. The rate of respiration was constant within both biofilms during the first 4.6 s following extinction of light, even when respiration was stimulated by high O2/CO2 ratio. The assumption of a constant rate of respiration during the dark period is an essential one for the determination of gross photosynthetic activity by use of O2 microsensors. We here present the first evidence to substantiate this assumption. The results strongly suggest that gross photosynthesis as measured by use of O2 microsensors may include carbon equivalents that are subsequently lost through photorespiration. Computer modeling of photosynthesis profiles measured after 1.1, 1.6, and 2.6 s of dark incubation illustrated how the actual photosynthesis profile could have appeared if it had been possible to do the determination at time 0. Diffusion of O2 during the up to 4.6‐s long dark incubations did not affect gross photosynthetic rate when integrated over all depths, but the apparent vertical distribution of the photosynthetic activity was strongly affected.