Stefan Hulth

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. n nThere 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.

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.