Reduction Rates of Sedimentary Mn and Fe Oxides: An Incubation Experiment with Arctic Ocean Sediments

To test the hypothesis that manganese- and iron-reducing bacteria in marine sediments respond rapidly to seasonal pulses of fresh organic carbon settling to the sea floor, we amended wet metal oxide?Crich and metal oxide?Cpoor sediments from the Beaufort Sea, Canadian Arctic, with organic carbon in the form of shrimp powder and incubated them at room temperature. Neither Mn nor Fe was released to the aqueous phase from unamended metal oxide?Crich sediment during a 41-day incubation, but both elements were released from sediment aliquots amended with organic carbon. Dissolved Mn appeared in the aqueous phase after a lag period of 2 days or less and reached levels as high as 600 ??mol l^?1 before levelling out. The release of dissolved Mn was accompanied by a decrease in the concentration of solid-phase reducible Mn. Dissolved Fe did not appear until 2 weeks into the incubation and only after the concentration of dissolved Mn had levelled out. For low concentrations of amended organic carbon (0.3%), the kinetics of Mn reduction fit a second-order rate law with a rate constant k = 2 × 10^?3 g ??mol^?1 day^?1, but at intermediate and high organic carbon concentrations (0.7 and 1.3%), the reduction kinetics was better described by a pseudo-first-order rate law with a rate constant k?? = 1.6 × 10^?1 day^?1. A pulse of organic carbon settling to the sea floor can trigger reduction of Mn and Fe oxides within a few days in strongly seasonal sedimentary environments, such as in the Arctic.     

Comparative geochemistry of cadmium, rhenium, uranium, and molybdenum in continental margin sediments

The concentrations of authigenic phases of Cd, Re, U, and Mo increase with depth in four 45-cm-long sediment box cores collected along the axis of the Laurentian Trough, Gulf of St. Lawrence. Average authigenic accumulation rates, estimated from element inventories, are similar to rates in other continental margin environments. Strong regional variations in sediment accumulation rate and sulfide concentration have little influence on the accumulation rates of Cd and Re. This suggests that slow precipitation kinetics controls the accumulation of Cd and Re in these sediments. The accumulation rate of authigenic U is more variable; it may be tied to the kinetics of microbially mediated U reduction and be controlled by the availability of reactive organic matter. Authigenic Mo is distinguished by a sharp subsurface concentration minimum, above which Mo cycles with manganese. Mo released to pore water upon reduction of Mn oxides diffuses downward and enriches the subsurface sediment. Mo accumulates most rapidly in the sediment with the highest sulfide content. Slow conversion of molybdate to thiomolybdate may explain the much slower Mo accumulation rate in the less sulfidic sediments. A component of authigenic Mo accumulates with pyrite in an approximately constant Mo:Fe ratio. The accumulation rate of pyrite and associated Mo is insensitive to AVS abundance. Pyrite formation may be limited by the reactivity of iron oxide minerals.     

Step zero for fisheries co-management: What precedes implementation

In this paper we argue that what precedes the implementation of co-management is as important as what happens later in the process. To a selected group of fisheries and coastal co-management practitioners in various parts of the world, we posed questions about how the idea was conceived, who participated in the initial discussion, and the preparation required before implementation. Responses received suggest a wide range of possible beginnings. In some cases (e.g., Barbados, Mozambique, and Zambia/Lake Kariba), the government spearheaded the process, while in others (e.g., Brazil and Malawi), the process was initiated by local entrepreneurs. In other instances, the initial discussion took place between communities and researchers (e.g., South Africa) or environmental groups (e.g., the Philippines). Learning about the conditions and actions taken prior to co-management implementation can assist us in evaluating the likelihood of success. In addition, by taking the time to understand situations in a local context, researchers can avoid premature and hasty attempts to embrace co-management schemes.     

Effects of hypoxia on benthic macrofauna and bioturbation in the Estuary and Gulf of St. Lawrence,Canada

The bottom water in the >300 m deep Lower St. Lawrence Estuary (LSLE) is persistently hypoxic in contrast to the normoxic bottom waters in the Gulf of St. Lawrence (GSL). We photographed the seabed at 11 stations in the Estuary and Gulf of St. Lawrence (EGSL) during the summers 2006 and 2007 and analysed the images to identify bioturbation traces (lebensspuren) and benthic macrofauna. The objective was to identify the environmental variables that influence the density and diversity of benthic macrofauna and bioturbation traces, and the differences that exist among regions with high, medium and low oxygen levels in the bottom water. The bottom water oxygen concentration is the variable that best explains the densities of total-traces as well as surface-traces. However, the density of these traces was higher in hypoxic regions than in well-oxygenated regions. The higher density of traces in the hypoxic region of the LSLE is mainly due to the activities of the surface deposit feeder Ophiura sp., which occurs in large numbers in this region. Possible explanations explored are stress behaviour of the organisms in response to hypoxia and different benthic macrofauna community structures between the hypoxic regions of the LSLE and the normoxic regions of the GSL. In the former, surface deposit feeders and low-oxygen tolerant species dominate over suspension feeders and low-oxygen intolerant species.     

Sulfate reduction in deep coastal marine sediments

Sulfate reduction rates in sediments of four deep stations in the Saguenay Fjord and the Laurentian Trough, Gulf of St. Lawrence, are among the lowest reported for the coastal environment. Maximum rates were 0.4–7.0 nmol cm⁻³ day⁻¹. The low rates are due to relatively low sedimentation rates and continuously low temperatures. Regional differences in both integrated and maximum sulfate reduction rates in the sediment correlate with sediment trap measurements of sedimentation rate and organic carbon flux.Sulfate reduction accounts for the degradation of 5–26% of the estimated downward flux of organic matter to these sediments. Unlike the absolute rate of sulfate reduction, the relative proportion of the carbon flux that is degraded via sulfate reduction is not directly correlated with the sedimentation rate but is a function of organic matter composition, intensity of bioturbation, and the abundance of sub-oxic electron acceptors. Thus, the lowest proportion of carbon degradation via sulfate reduction occurred at a Gulf site, where a combination of low sedimentation and bioturbation rates allowed a long residence time for organic matter near the sediment surface and, in consequence, a low flux of labile carbon into the sulfate reduction zone. The highest proportion was observed at a station with a similar organic carbon flux but with higher rates of sedimentation and bioturbation. At a third site, with the highest rates of sulfate reduction as well as the highest rates of sedimentation and bioturbation, the contribution of sulfate reduction to organic matter degradation was only intermediate. This is attributable to the exhaustion of the supply of porewater sulfate. In deep coastal environments the proportion of organic matter degraded via sulfate reduction can be highly variable both spatially and temporally.