The contrasting effects of the invasive Marenzelleria viridis and the native Nereis diversicolor on benthic metabolism, partitioning of reaction pathways and distribution of inorganic porewater (C and N) solutes in homogenized sandy sediment were investigated experimentally over a period of 1 mo. The 2 species were studied separately and in combination to observe possible effects and interactions. Benthic O2 uptake and total CO2 (TCO2) release were affected similarly by M. viridis, N. diversicolor and the two in combination, with roughly a doubling after 1 to 2 wk compared to defaunated sediment. Sulfate reduction after 1 mo, on the other hand, was more than twice as high in sediment inhabited by M. viridis alone than in any other treatment, even when combined with N. diversicolor. Denitrification estimated from benthic TCO2 release, porewater reaction stoichiometry and nutrient fluxes was largely unaffected by the presence of fauna. Accordingly, the partitioning of reaction pathways after 1 mo revealed that M. viridis stimulated sulfate reduction at the expense of aerobic respiration. Most of the oxygen uptake in M. viridis sediment was apparently due to enhanced oxidation resulting from an upward drifting front of sulfide as indicated by low redox and the appearance of Beggiatoa sp. near the surface. Porewater solute profiles showed that M. viridis was capable of stronger and deeper irrigation than N. diversicolor despite ∼10 times higher burrow ventilation by the latter species. This effect was caused by percolation of return water in the deep (>20 cm) I- or J-shaped burrows of M. viridis compared to the flushing of the more shallow (6 to 8 cm) U- shaped burrows of N. diversicolor. A replacement of the native N. diversicolor with the invasive M. viridis as the dominant burrow-dwelling polychaete in shallow coastal sediments will probably affect the biogeochemical functioning and ecological stability of the ecosystem. Among other things, organisms tolerant to sulfide are likely to be favored at the expense of more sensitive species. © Inter-Research 2011.

Hyporheic exchange and biogeochemical reactions are difficult to quantify because of the range in fluid-flow and sediment conditions inherent to streams, wetlands, and nearshore marine ecosystems. Field measurements of biogeochemical reactions in aquatic systems are impeded by the difficulty of measuring hyporheic flow simultaneously with chemical gradients in sediments. Simplified models of hyporheic exchange have been developed using Darcy's law generated by flow and bed topography at the sediment-water interface. However, many modes of transport are potentially involved (molecular diffusion, bioturbation, advection, shear, bed mobility, and turbulence) with even simple models being difficult to apply in complex natural systems characterized by variable sediment sizes and irregular bed geometries. In this study, we synthesize information from published hyporheic exchange investigations to develop a scaling relationship for estimating mass transfer in near-surface sediments across a range in fluid-flow and sediment conditions. Net hyporheic exchange was quantified using an effective diffusion coefficient (De) that integrates all of the various transport processes that occur simultaneously in sediments, and dimensional analysis was used to scale De to shear stress velocity, roughness height, and permeability that describe fluid-flow and sediment characteristics. We demonstrated the value of the derived scaling relationship by using it to quantify dissolved oxygen (DO) uptake rates on the basis of DO profiles in sediments and compared them to independent flux measurements. The results support a broad application of the De scaling relationship for quantifying coupled hyporheic exchange and biogeochemical reaction rates in streams and other aquatic ecosystems characterized by complex fluid-flow and sediment conditions.

Burrow ventilation of benthic infauna generates water currents that irrigate the interstices of the sediments surrounding the burrow walls. Such activities have associated effects on biogeochemical processes affecting ultimately important ecosystem processes. In this study, the ventilation and irrigation behavior of Marenzelleria viridis, an invasive polychaete species in Europe, was analyzed using different approaches. M. viridis showed to perform two types of ventilation: (1) muscular pumping of water out of the burrow and (2) cilia pumping of water into the burrow. Flowmeter measurements presented muscular pumping in time averaged rates of 0.15mlmin-1. Oxygen needle electrodes positioned above the burrow openings revealed that muscular undulation of the worm body pumps anoxic water out of the burrow. On the other hand, microscope observations of the animal showed that ventilation of oxygen-rich water in the burrow occurs by ciliary action. The volume of water irrigated by M. viridis appears to vary linearly within the first 24h incubation, with rates ranging from 0.003 to 0.01mlmin-1. From those rates we could estimate that the time averaged rate of cilia ventilation should be about 0.16mlmin-1. Since the cilia pumping into the burrow occurs in periods of 24±12min and at 50-70% of the measured time, considerable amounts of water from deeper sediments may percolate upwards to the sediment surface. This water is rich in reduced compounds and nutrients and may have important associated ecological implications in the ecosystem (e.g. affecting redox conditions, organic matter degradation, benthic recruitment and primary production). © 2010 Elsevier B.V.

The development of organic-inorganic hybrid materials for redox catalysis is key to access new energy conversion schemes and the sustainable production of dihydrogen. Here, bare and P-doped graphene arising from the pyrolysis of biomass (alginate from marine algae) has been used as a support for the growth and stabilization of ultra-small Ru/RuO2NPs through organometallic synthesis. P-doped graphene allows obtaining smaller and better dispersed NPs in hybrid electrodes of lower roughness and electroactive surface area. Electrochemical activation of the as-synthesised supported nanoparticles by reduction of the passivating RuO2layer generates excellent HER electrocatalysts under acidic conditions (η10of 29 mV and 15 mV for the bare and P-doped electrodes, respectively). P doping, identified as surface phosphates by31P solid state NMR, induces improvement of all HER benchmarking parameters studied, including overpotential and exchange and specific current densities. All studied materials show excellent long-term stability and selectivity for hydrogen generation with no sign of deactivation after 12 h under turnover conditions and almost quantitative faradaic efficiencies.