Per Hall

The effect of oxygen on release and uptake of cobalt, manganese, iron and phosphate at the sediment-water interface

The porewater of a sediment core taken at 6 m depth in Gullmarsfjorden, Sweden, was enriched in Fe, Mn, Co, and phosphate compared to the overlying bottom water. Yet, in situ measurements with a benthic flux-chamber, in which dissolved oxygen and pH were maintained near ambient values (regulated flux-chamber), showed that the sediment did not release any of these ions but instead removed Co, Mn, and Fe from the overlying water. In a parallel experiment, where dissolved oxygen and pH were not maintained but allowed to decrease as a result of benthic respiration, Co, Mn, Fe, and PO4 were released from the sediment. An accidental interruption of the stirring in the regulated chamber caused a pulse of dissolved Co, Mn, Fe, and PO4 to be released from the sediment. When the stirring was resumed, all four ions were again removed. The kinetics of the removal process was apparent first order with half-removal times of 3–5 days, similar to the removal kinetics of the radioactive tracers 59Fe and 54Mn from the water in a smaller chamber, run in parallel. The critical variable which controls the reactions at the sediment-water interface is the flux of oxygen from the water column into the sediment. When benthic chambers are used to measure fluxes of redox-sensitive ions, the oxygen flux must be maintained as close as possible to the actual in situ flux. If not, the measured fluxes may vary greatly in magnitude and even change direction.

Benthic fluxes of cadmium, copper, nickel, zinc and lead in the coastal environment

Fluxes of trace metals across the sediment-water interface were measured in situ at 6 m depth in Gullmarsfjorden, Sweden, using diver-operated stirred benthic flux-chambers. These were equipped so that dissolved oxygen and pH could be maintained near ambient seawater values (regulated chamber) or be allowed to change in response to benthic respiration (unregulated chamber). In the regulated chamber, Cd, Cu, Ni, and Zn were released from the sediment at constant rates both during a winter experiment (water temperature −1 °C) and during a fall experiment (+ 10°C). During the fall experiment, fluxes (in nmol m−2 d−1) of 13 (Cd), 118 (Cu), 209 (Ni), and 1400 (Zn) were measured. In winter, the release rates were lower by factors of 5 and 10 for Cu and Ni but not significantly different for Cd and Zn. Neither release nor uptake by the sediment could be demonstrated for Pb. The pore-water in a diver-collected core was depleted in Cd, Cu, and Zn and slightly enriched in Ni and Pb, relative to the ambient seawater. There was no correspondence between fluxes calculated from porewater profiles and actually measured fluxes; nor could the fluxes be directly related to the degradation rate of organic matter. In the unregulated chamber, initial trace metal release rates were lower than in the regulated chamber. As the oxygen concentration decreased, the metal fluxes decreased as well and were ultimately reversed as sulfide began to appear in the water. The fluxes of trace metals are sensitive to the oxygen regime in the flux chamber because the solubilization of these metals, which takes place in a thin oxic layer near the sediment surface, depends on the oxygen flux across the sediment-water interface.

Arctic sediments (Svalbard): consumption and microdistribution of oxygen

Total sediment oxygen consumption rates (TSOC or Jtot), measured during sediment-water incubations, and sediment oxygen microdistributions were studied at 16 stations in the Arctic Ocean (Svalbard area). The oxygen consumption rates ranged between 1.85 and 11.2 mmol m−2 d−1, and oxygen penetrated from 5.0 to ⩾ 59 mm into the investigated sediments. Measured TSOC exceeded the calculated diffusive oxygen fluxes (Jdiff) by 1.1–4.8 times. Diffusive fluxes across the sediment-water interface were calculated using the whole measured microprofiles, rather than the linear oxygen gradient in the top sediment layer. The lack of a significant correlation between found abundances of bioirrigating meiofauna and high JtotJdiff ratios as well as minor discrepancies in measured TSOC between replicate sediment cores, suggest molecular diffusion, not bioirrigation, to be the most important transport mechanism for oxygen across the sediment-water interface and within these sediments. The high ratios of JtotJdiff obtained for some stations were therefore suggested to be caused by topographic factors, i.e. underestimation of the actual sediment surface area when one-dimensional diffusive fluxes were calculated, or sampling artifacts during core recovery from great water depths. Measured TSOC correlated to water depth raised to the −0.4 to −0.5 power (TSOC = water depth−0.4 to −0.5) for all investigated stations, but they could be divided into two groups representing different geographical areas with different sediment oxygen consumption characteristics. The differences in TSOC between the two areas were suggested to reflect hydrographic factors (such as ice coverage and import/production of reactive particulate organic material) related to the dominating water mass (Atlantic or polar) in each of the two areas. The good correlation between TSOC and water depth−0.4 to −0.5 rules out any of the stations investigated to be topographic depressions with pronounced enhanced sediment oxygen consumption.

Mineralization and burial of organic carbon in sediments of the southern Weddell Sea (Antarctica)

Abstract Benthic fluxes of oxygen, alkalinity (AT), total carbonate (CT or ΣCO 2 ) and dissolved organic carbon (DOC) were measured during sediment-water incubations at 16 stations in the southern Weddell Sea (Antarctica) with water depths between 280 and 2514 m. The total sediment oxygen consumption rates (TSOC) were in general low (1.74-3.61 mmol m −2 day −1 ) and more comparable to measurements in slope and deep-sea sediments at a few thousand meters water depth. The decrease of TSOC with water depth was lower than that observed in many other seas. The mean carbon to nitrogen ratio (C/N) in the solid phase of surficial sediment was 8.3. Measured benthic fluxes of alkalinity, corrected for contributions from nitrification and denitrification, were quantitatively used to correct the fluxes of total carbonate for dissolution of solid phase carbonates. The ΣCO 2 fluxes, originating from carbonate dissolution (0.1661–1.77 mmol m −2 day −1 were 2.6–71 % of the ΣC0 2 fluxes (0.984–3.73 mmol m −2 day −1 ) resulting from organic carbon oxidation. Measured benthic fluxes of oxygen, ΣC0 2 and nitrate were, together with estimated denitrification rates and sediment C/N ratios, used to model respiration quotients (RQ) for organic carbon oxidation and estimate composition of the organic matter undergoing degradation. Modelled RQ varied roughly between 2/3 and 1 (mean 0.87). Measured fluxes of ΣC0 2 were 1.6–3.2 times higher than integrated organic C mineralization rates (measured during closed incubations of sieved, homogenized sediment), indicating macrofaunal (plus possibly meiofaunal) respiration to be important. However, low abundances of bioirrigating benthic macrofauna and small differences in benthic fluxes of oxygen, ΣCO 2 and alkalinity found between replicate sediment cores, suggested that macrofaunal respiration was quantitatively unimportant in these sediments. The higher measured fluxes of ΣC0 2 compared to the integrated mineralization rates, were therefore most likely caused by a large fraction of the respiration occurring directly on the sediment surface. This degradation of newly deposited organic matter was not reflected in the integrated organic C mineralization rates. Also, there was no obvious effect of this surficial degradation process on the pore water distributions of ΣCO 2 . Benthic mass balances of carbon revealed that benthic fluxes of DOC were 3–147% of the corrected fluxes of ΣCO 2 , and the recycling efficiencies (E) were up to 35% higher if the DOC fluxes were included in the calculations of E, rather than the inorganic ΣCO 2 flux alone. The recycling efficiencies, including the benthic flux of DOC, ranged between 57 and 88% (mean 78%). Measured rates of inorganic C accumulation (for most stations −2 day −1 were a factor of 6–7 lower than organic C accumulation rates (0.457–1.94 mmol C m −2 day −1 ).

Benthic nutrient fluxes on a basin-wide scale in the Skagerrak (north-eastern North Sea)

Benthic ammonium, nitrite, nitrate, phosphate and silicate fluxes were measured on a basin-wide scale (14 locations) in the open Skagerrak. Fluxes were measured in situ at two of the locations using a benthic chamber lander. The benthic flux measurements revealed patterns of geographic variation of nutrient fluxed in the Skagerrak. Nitrate fluxes generally reflected sediment deposition patterns and were mainly directed into the sediment in the high accumulation areas, and out of the sediment in areas with relatively little import of allochtonous organic matter. The nitrate fluxes were related to C/N-ratios of sediments. Low C/N-ratios were associated with high (in relative terms) nitrate effluxes and high C/N-ratios were associated with high nitrate fluxes into the sediment, suggesting that the fastest net regeneration (and nitrification) of nitrogen occurred in nitrogen-rich (low C/N) sediments. The nitrate flux into nitrogen-poor (high C/N) sediments appeared to be due to denitrification in the main sediment deposition areas, where mainly allochtonous organic matter is thought to accumulate. Areas of net benthic nitrification were thus on the Danish shelf and in the western part of the Norwegian Trench. Areas of net denitrification were in the eastern, northeastern and also central deep part of the Skagerrak. In these areas virtually all of the dissolved inorganic nitrogen fluxes were directed into the sediment and they are suggested to constitute considerable sinks for non-gaseous nitrogen. Phosphate fluxes were highest in the central deep part and lower on the margins of the Skagerrak. They appeared to be positively related to clay contents of sediments. Silicate fluxes varied little: almost all fluxes were between 1 and 2 mmol·m −2·d−1. Sediment oxygen uptake did not correlate any of the nutrient fluxes except with those of silicate. It is suggested that benthic silicate fluxes reflected the deposition of a large proportion of the fast-sinking, autochtonously produced diatoms on the sea-floor of the Skagerrak, whereas a great deal of other fresh in situ produced algal material may be flushed out of this sea. There was no relation between either organic carbon contents of sediments or water depths and benthic fluxes of any of the nutrients. The phosphate and silicate fluxes measured did not correlate with any of the other nutrient fluxes, nor with each other. The ammonium fluxes were, however, generally inversely related to the nitrate fluxes with high (in relative terms) influxes of ammonium correlating with high effluxes of nitrate. We suggest this was due to nitrification, and that the nitrifying bacteria could not meet their ammonium demand with what was regenerated in the sediment, so that additional ammonium had to be taken up from the overlying water. High nitrite influxes were associated with both high nitrate influxes and effluxes. Uptake of nitrate from the overlying water in association with nitrate effluxes is interpreted as a consumption of nitrate during nitrification, and uptake of nitrate in association with nitrate uptake was probably mainly due to nitrite consumption during denitrification. The observed relation between nitrite and nitrate fluxes indicates that the rate-limiting step in benthic nitrification was the oxidation of ammonium to nitrite, and in dentrification it probably was the reduction of nitrate to nitrite.

On the sulphur chemistry of a super-anoxic fjord, Framvaren, South Norway

Abstract The concentrations of total carbonate (C t ), sulphate, sulphide, thiols and oxygen, the ratio between the stable sulphur isotopes 34 S and 32 S in sulphate and sulphide, and the density (used to calculate salinity) were determined on samples from the water column of Framvaren, a superanoxic fjord in southern Norway. From a depth of 18m (the oxic-anoxic boundary) the initial sulphate concentration, ([SO 4 ] init ), as calculated from salinity, is significantly higher than the sum of the measured sulphur species. This is attributed to a loss of sulphur from the water column. The amount of total carbonate produced, corrected for the initial concentration (C t - 2.4 Sal/35) is found to be proportional to the amount of sulphate consumed, ([SO 4 ] init - [SO 4 ]), according to the following relation C t - 2.4 Sal/35 = 1.84 ([SO 4 ] init - [SO 4 ]). Isotopic fractionation caused by bacterial sulphate reduction in the anoxic part of the water column produces sulphide with a δ 34 S ∼ 40‰ lower than the δ 34 S for sulphate at corresponding depths. The isotopic fractionation also results in δ 34 S value for the remaining sulphate at depths below 80 m being considerably higher than the mean value for ocean water, which is close to + 20‰. The δ 34 S values for sulphate at depths between 10 and 50 m were lower than + 20‰ which indicates oxidation of sulphide, which follows upon diffusion of sulphide from deeper parts of the water column and inflow of oxygenated seawater over the sill into the anoxic water of the fjord. A conclusive scenario of the Framvaren sulphur chemistry is presented.

Quantitative distribution of metazoan meiofauna in continental margin sediments of the Skagerrak (Northeastern North Sea)

Abstract A quantitative survey of metazoan meiofauna in continental-margin sediments of the Skagerrak was carried out using virtually undisturbed sediment samples collected with a multiple corer. Altogether 11 stations distributed along and across the Norwegian Trench were occupied during three cruises. Abundance ranged from 155 to 6846 ind·10 cm −2 and revealed a sharply decreasing trend with increasing water depth. The densities were high on the upper part of the Danish margin (6846 ind·10 cm −2 at 194 m depth) and low in the central part of the deep Skagerrak (155 ind·10 cm −2 at 637 m depth). Also body lengths were significantly shorter on the Danish margin then elsewhere in the Skagerrak, indicating a greater importance of juveniles in this area. We suggest that the high densities may be explained by a stimulated renewal of the fauna, possibly induced by an adequate food supply. The low abundances found in sediments from the deepest part of the Norwegian Trench cannot be attributed to any lack of oxygen. We suggest that the low meiofaunal abundances are caused by a decrease in the food supply (accentuated in this area by lower sedimentation rates) and/or by the very high concentrations of dissolved manganese in the pore water of these sediments. The metazoan meiofauna was largely dominated by nematodes. Comparison of the respiration rates of the nematode population with the total benthic respiration (0.5 to 14%) suggests that the relative importance of metazoan meiofauna decreased with water depth.

Chemical fluxes and mass balances in a marine fish cage farm. III. Silicon

Abstract Fluxes and pathways of silicon in a marine rainbow trout cage farm were studied. The measured fluxes included those carried by fish food, juveniles, harvest, fish loss (death and escape), sedimentation from the cages, and benthic release measured with flux chambers in-situ. Two different types of Si mass balances for the farm were constructed. The mass balance according to the flux method was based on the measured fluxes and constructed for each of two consecutive growing seasons. The mass balance constructed according to the accumulation method was based on the total input and removal of Si to the cages of the farm since it was started, and the recovery of Si in the sediment originating from the farm. In both types of mass balances external input was the major source of Si, contributing a minimum of 55–80% of the total biogenic silica in the material collected in sediment traps below the farm and of the material accumulated in the farm-derived sediment. The Si removed from the farm with harvest contributed a very small part, ∼0.3%, of the amount of Si supplied to the farm with fish food and juveniles. The loss of Si to the environment was 2.4 kg (1985), 2.5 kg (1986) and 2.5 kg (1980–1986) for each ton of fish produced. On a seasonal basis, about 4.5% of the sedimented biogenic Si was returned in dissolved form from the sediment to the overlying water. This constituted about 0.3% of the biogenic Si content in the farm-derived sediment. The flux of reactive silicate from sediment below the fish farm was enhanced about 2.5 times compared to nearby sediments unaffected by the farm.

Multivariate experimental methodology applied to the calibration of a Clark type oxygen sensor

The response of a Clark type oxygen sensor, designed for the study of benthic fluxes in the marine environment, was evaluated with respect to variations in oxygen concentration, temperature, pH, salinity, total pressure, fugacity ([O2]water/[O2]satwater) and stirring rate by means of an experimental design evaluated by partial least squares (PLS). Fugacity and temperature not only explained a major part of the response, but also stirring-rate and the cross term temperature∗fugacity were significant on the 95% level. With the obtained results, a model for predicting absolute oxygen concentrations from the sensor response was created. To verify the practical applicability of the model it was used to predict oxygen concentrations from sensor data obtained in situ during a cruise in the Skagerrak (north-eastern North Sea). Predicted concentrations were then compared, and found to be in accordance (within 5 %) with concentrations obtained from Winkler titrations. The results suggest that fugacity and temperature are the most important factors to consider while using this, and probably other, brands of Clark type oxygen macro sensors.

Trace Metal Concentrations in the Anoxic Bottom Water of Framvaren

The landlocked Norwegian fjord Framvaren is NE of Farsund. The depest part, our sampling station at 180 m, is situated at 58°09.27′N and 6°45.0′E. Dr. Jens Skei at the Norwegian Institute of Water Research in Oslo started in 1979 an international cooperation on a major investigation of Framvaren of our times. Our poster at the Texel meeting showed the depth profiles of in situ temperature, density, oxygentotal sulphide, alkalinity, pH, loss of sulphate, phosphate, ammonia, and the trace metals (except mercury) in unfiltered water. In addition the values of δ34S for sulphate and sulphide were shown.