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Aquatic Geochemistry (v.18, #6)
Preface to Bjørn Sundby’s Special Issue of Aquatic Geochemistry
by Cédric Magen; Bruno Lansard; Sean A. Crowe (pp. 457-460).
Sediment Response to 25 Years of Persistent Hypoxia by S. Lefort; A. Mucci; B. Sundby (pp. 461-474).
We investigated the impact of persistent hypoxia on sediment chemistry by comparing total, reactive (extractible with 1 M hydroxylamine–hydrochloride in 25 % acetic acid), and dissolved forms of the redox-sensitive elements Mn, Fe, and As in cores recovered between 1982 and 2007 at two sites in the Lower St. Lawrence Estuary (LSLE) where the bottom water has been severely hypoxic since the early 1980s. The data reveal that the concentrations and the vertical distributions of total solid-phase and dissolved Mn as well as total solid-phase Fe and As were not significantly affected by persistent hypoxia. In contrast, the composition of solid-phase Fe and As changed significantly as did the pore-water concentrations of both these elements. The relative amounts of solid-phase reactive Fe and As increased, and the abundance of pyrite and pyritic–As decreased in the sediment layer that accumulated since 1982. We propose that persistent hypoxic conditions restrict the supply of oxygen to the sediment and increase the relative contribution of alternate electron acceptors—Mn(IV), Fe(III), and sulfate—to microbial oxidation of organic matter. In marine iron-rich environments, such as the LSLE sediment, increased sulfate reduction may promote conversion of less reactive Fe phases to more reactive Fe phases which, in turn, interfere with pyrite formation. Consequently, a chalcophile element such as As, which would normally be sequestered with authigenic pyrite, remains available for recycling across the oxic–anoxic boundary in the sediment.
Keywords: Hypoxia; Sediment; Iron; Manganese; Arsenic; Pyrite
Effects of Tubificid Worm Bioturbation on Freshwater Sediment Biogeochemistry by P. Anschutz; A. Ciutat; P. Lecroart; M. Gérino; A. Boudou (pp. 475-497).
The effects of freshwater infaunal invertebrates on sediment geochemical properties were studied through an experimental approach using indoor microcosms during a 56-day experiment. The bioturbating organisms were tubificid worms, which consume sediment at depth and deposit undigested material at the sediment–water interface. Bioturbation intensity was determined using fluorescent tracers, and the distribution of redox-sensitive compounds was studied from replicate experimental units handled 7, 14, 21, 28 and 56 days after tubificid colonization. Worm activity transferred reduced particles and pore water at the sediment surface at a rate of 0.14 cm day−1. Compared to control experimental units, this recycled material represented at the end a several centimetre-thick layer enriched in water content, dissolved nitrate and sulphate, and depleted in oxygen, ammonium and dissolved Mn(II). Tubificids consumed O2 in bottom water, so that the sediment was anoxic, allowing a direct flux of dissolved reduced species into overlying water. Lower ammonium and Mn(II) concentrations and fluxes in anoxic sediment possibly resulted from a decrease in anaerobic microbial metabolism due to competition for labile organic carbon with tubificids. Higher sulphate concentration resulted from burial of surface waters with particle at the sediment surface, but not from bio-irrigation of burrows. Nitrate was produced in anoxic condition, as observed in almost every mixed modern sediments.
Keywords: Bioturbation; Sediment; Tubificids; Early diagenesis
Recolonisation by Macrobenthos Mobilises Organic Phosphorus from Reoxidised Baltic Sea Sediments by Nils Ekeroth; Magnus Lindström; Sven Blomqvist; Per O. J. Hall (pp. 499-513).
In recent decades, eutrophication has increased the extent of hypoxic and anoxic conditions in many coastal marine environments. In such conditions, the nutrient flux across the sediment–water interface is a key process controlling the biogeochemical dynamics, and thereby the level and character of biological production. In some areas, management attempts to drive the ecosystem towards phosphorus (P) limitation, which calls for reliable knowledge on the mechanisms controlling P-cycling. We report a well-controlled laboratory experiment on benthic fluxes of P, when shifting from a state of hypoxic and azoic sediments to oxic and zoic bottom conditions. Adding any of three types of macrobenthic fauna (mysid shrimp, pontoporeid amphipod and tellinid clam) to oxygenated aquarium sections resulted in benthic P fluxes that differed consistently from the azoic control sections. All species caused liberation of dissolved organically bound P (DOP) from the sediment, in contrast to the azoic systems. The shrimp and the amphipod also resuspended the sediment, which resulted in a release of P bound to particles (>0.45 μm). Dissolved inorganic phosphate (DIP) was released during hypoxic conditions, but was taken up after oxygenation, irrespective of the presence or absence of bottom fauna. In the presence of fauna, the uptake of DIP roughly equalled the release of DOP, suggesting that the benthic efflux of DOP following oxygenation and bottom fauna (re)colonisation might be considerable. This is an hitherto overlooked animal-controlled nutrient flux, which is missing from coastal marine P budgets.
Keywords: DOP; P retention; Bioturbation; Monoporeia affinis ; Macoma balthica ; Mysis mixta
Spatial and Temporal Variability of Benthic Respiration in a Scottish Sea Loch Impacted by Fish Farming: A Combination of In Situ Techniques by Cécile Cathalot; Bruno Lansard; Per O. J. Hall; Anders Tengberg; Elin Almroth-Rosell; Anna Apler; Lois Calder; Elanor Bell; Christophe Rabouille (pp. 515-541).
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.
Keywords: Fish farming; Microprofiling; Oxygen; H2S; Sediment; Benthic chamber; Sea loch; Scotland; Benthic mineralization; Organic carbon
Benthic Phosphorus Dynamics in the Gulf of Finland, Baltic Sea by Lena Viktorsson; Elin Almroth-Rosell; Anders Tengberg; Roman Vankevich; Ivan Neelov; Alexey Isaev; Victor Kravtsov; Per O. J. Hall (pp. 543-564).
Benthic fluxes of soluble reactive phosphorus (SRP) and dissolved inorganic carbon (DIC) were measured in situ using autonomous landers in the Gulf of Finland in the Baltic Sea, on four expeditions between 2002 and 2005. These measurements together with model estimates of bottom water oxygen conditions were used to compute the magnitude of the yearly integrated benthic SRP flux (also called internal phosphorus load). The yearly integrated benthic SRP flux was found to be almost 10 times larger than the external (river and land sources) phosphorus load. The average SRP flux was 1.25 ± 0.56 mmol m−2 d−1 on anoxic bottoms, and −0.01 ± 0.08 mmol m−2 d−1 on oxic bottoms. The bottom water oxygen conditions determined whether the SRP flux was in a high or low regime, and degradation of organic matter (as estimated from benthic DIC fluxes) correlated positively with SRP fluxes on anoxic bottoms. From this correlation, we estimated a potential increase in phosphorus flux of 0.69 ± 0.26 mmol m−2 d−1 from presently oxic bottoms, if they would turn anoxic. An almost full annual data set of in situ bottom water oxygen measurements showed high variability of oxygen concentration. Because of this, an estimate of the time which the sediments were exposed to oxygenated overlying bottom water was computed using a coupled thermohydrodynamic ocean–sea and ecosystem model. Total phosphorus burial rates were calculated from vertical profiles of total phosphorus in sediment and sediment accumulation rates. Recycling and burial efficiencies for phosphorus of 97 and 3%, respectively, were estimated for anoxic accumulation bottoms from a benthic mass balance, which was based on the measured effluxes and burial rates.
Keywords: Sediment; Fluxes; Burial; Oxic-anoxic; Gulf of Finland; Baltic Sea
Manganese Sources and Sinks in the Arctic Ocean with Reference to Periodic Enrichments in Basin Sediments by Robie W. Macdonald; Charles Gobeil (pp. 565-591).
Between 1990 and 2007, twenty-nine box cores were recovered within the Arctic Ocean spanning shelf, slope and basin locations, and analyzed for aluminum (Al), manganese (Mn), other inorganic components and organic carbon (COrg). Using these core data together with literature values, we have constructed budgets for Al and Mn in the Arctic Ocean. Most of the Al and Mn entering the Arctic comes from rivers or coastal erosion, and almost all of these two elements is trapped within the Arctic. Total Mn distributions in sediments reflect the recycling and loss of much of the Mn from shelf sediments with ultimate burial over the slopes and in basins. Mn enrichments observed as bands in long cores from the basins appear to co-occur with inter-glacial periods. Our Mn budget suggests that change in sea level associated with the accumulation and melting of glaciers is a likely cause for the banding. The Arctic Ocean, which presently contains as much as 50% shelf area, loses most of that when global sea level falls by ~120 m during glacial maxima. With lower sea level, Mn input from rivers and coastal erosion declines, and inputs become stored in permafrost on the sub-aerial shelves or at the shelf margin. Sea-level rise re-establishes coastal erosion and large riverine inputs at the margin and initiates the remobilization of Mn stored on shelves by turning on algal productivity, which provides the COrg required to reduce sedimentary Mn oxyhydroxides.
Keywords: Arctic Ocean; Manganese; Aluminum; Budget; Sediments