Importance and controls of anaerobic ammonium oxidation influenced by riverbed geology

Rivers are an important global sink for excess bioavailable nitrogen: they convert approximately 40% of terrestrial N runoff per year (∼47 Tg) to biologically unavailable N 2 gas and return it to the atmosphere. At present, riverine N 2 production is conceptualized and modelled as denitrification. Anaerobic ammonium oxidation, known as anammox, is an alternative pathway of N 2 production important in marine environments, but its contribution to riverine N 2 production is not well understood. Here we use in situ and laboratory measurements of anammox activity using 15 N tracers and molecular analyses of microbial communities to evaluate anammox in clay-, sand-and chalk-dominated river beds in the Hampshire Avon catchment, UK during summer 2013. Abundance of the hzo gene, which encodes an enzyme central to anammox metabolism, varied across the contrasting geologies. Anammox rates were similar across geologies but contributed different proportions of N 2 production because of variation in denitrification rates. In spite of requiring anoxic conditions, anammox, most likely coupled to partial nitrification, contributed up to 58% of in situ N 2 production in oxic, permeable riverbeds. In contrast, denitrification dominated in low-permeability clay-bed rivers, where anammox contributes roughly 7% to the production of N 2 gas. We conclude that anammox can represent an important nitrogen loss pathway in permeable river sediments

Stability of dissolved and soluble Fe(II) in shelf sediment pore waters and release to an oxic water column

Shelf sediments underlying temperate and oxic waters of the Celtic Sea (NW European Shelf) were found to have shallow oxygen penetrations depths from late spring to late summer (2.2–5.8 mm below seafloor) with the shallowest during/after the spring-bloom (mid-April to mid-May) when the organic carbon content was highest. Sediment porewater dissolved iron (dFe, 85%) consisted of Fe(II) and gradually increased from 0.4 to 15 μM at the sediment surface to ~100–170 µM at about 6 cm depth. During the late spring this Fe(II) was found to be mainly present as soluble Fe(II) (>85% sFe, 7 h. Iron(II) oxidation experiments in core top and bottom waters also showed removal from solution but at rates up to 5-times slower than predicted from theoretical reaction kinetics. These data imply the presence of ligands capable of complexing Fe(II) and supressing oxidation. The lower oxidation rate allows more time for the diffusion of Fe(II) from the sediments into the overlying water column. Modelling indicates significant diffusive fluxes of Fe(II) (on the order of 23–31 µmol m−2 day−1) are possible during late spring when oxygen penetration depths are shallow, and pore water Fe(II) concentrations are highest. In the water column this stabilised Fe(II) will gradually be oxidised and become part of the dFe(III) pool. Thus oxic continental shelves can supply dFe to the water column, which is enhanced during a small period of the year after phytoplankton bloom events when organic matter is transferred to the seafloor. This input is based on conservative assumptions for solute exchange (diffusion-reaction), whereas (bio)physical advection and resuspension events are likely to accelerate these solute exchanges in shelf-seas

Dissolution of biogenic silica in Southern Ocean surface sediments

Biogenic silica (BSi) is a major component in marine geochemical cycles and a suitable proxy for paleoproductivity. The Southern Ocean plays a key role in the biogeochemical cycle of silicon. For questions of opal preservation and to assess the global biogenic silica cycle it is important to understand the processes controlling BSi dissolution.The results from fitting the leaching curves, derived by wet-alkaline-extraction of biogenic silica, gives an estimate of the reactivity of biogenic silica in sediments. To get detailed information on kinetics and solubility of biogenic silica, continuously stirred flow-through experiments were performed. Use of flow through reactors allow quantification of dissolution rates and saturation concentrations under well defined conditions. Dissolution rates of sediment samples in stirred flow-through reactors were measured as a function of the degree of undersaturation by varying the silica acid concentrations or the flow rate of the inflow solution. By taking samples out of the reactors during the experiments we also get information about changes in species composition and how the shell structures dissolve. Sediment samples were selected from different regions of the Southern Ocean, e.g. Weddel Sea, Scotia Sea, Polar Front Zone.The information about BSi dissolution kinetics will be considered in a regional context. For this purpose detailed information of diatom assemblages and clay mineralogy are considered. The combination of results from laboratory measurements and regional distribution of parameters affecting the benthic silica cycle helps us decipher processes regulating the BSi burial and provides a more detailed understanding of the dissolution of BSi in surface sediments within certain regions of the Southern Ocean

Rhizon - an excellent pore water sampler for low maintenance collection and filtration of small volume samples

Rhizon samplers were originally designed as micro-tensiometers for soil science forseepage water sampling in the unsaturated zone some ten years ago. We have introducedthese samplers for the use in marine sediments and in saturated groundwaterenvironments for direct pore water sampling.Rhizons consist of a small microporous polymer tube (2.5mm diameter) supported bya stabilizing wire that is connected to a PVC-tube and a standard luer-lock connector.By attaching vacuum to this connector (syringe, vacuumtube or peristaltic pump)small volumes of pore water samples may be extracted from sediments without furthermaintenance. As a side effect of the 0.1 micron pore width of the polymer tube,the samples are automatically filtered. We successfully used these samplers for highresolution sampling from closed sediment cores. The Rhizons were inserted through3mm holes in the liner walls. By this method very high resolution pore water profilesamples may be taken without disturbance of the sediment structure. Since samplesare collected in directly attached syringes or vacuumtubes, contact with ambient oxygenis avoided for anoxic environments, thus eliminating both, the need for glove boxsampling and eliminating the need for cumbersome pressure filtration. Recently otherRhizon models with carbon fibre support and microporous tube diameters down to1mm have been presented

New Rhizon in situ sampler for pore water studies in aquatic sediments: For example nutrient input from submarine groundwater discharge in costal areas.

To investigate coastal biogeochemical cycles, especially at the sediment/water interface,improved sampling methods are necessary. For this purpose, we developed apore water in situ sampler with miniature sampling devices, so called Rhizons. Rhizonsoil moisture samplers have been used as sampling devices in unsaturated soilsfor the last ten years. In aquatic science they have been rarely used to extract porewater from sediments. This study presents a new developed Rhizon In Situ Sampler(RISS) as a non-destructive and inexpensive tool for in situ pore water sampling. Fieldexperiments, tracer studies and numerical modeling were combined to assess the suitabilityof Rhizons for pore water sampling. Our investigations show that the RISS isa very suitable alternative to classical methods for in situ sampling. Combined withan in situ benthic chamber system the RISS allows studies of benthic fluxes and porewater profiles at the same location with negligible effect on the incubated sedimentwater interface. This allows improved calculation and modeling of transport and reactionprocesses. Results of nutrient and freshwater input into surface water derivedby in situ sampling of tidal flat sediments of the Wadden Sea (Sahlenburg/Cuxhaven,Germany) are presented. Long term deployments of the RISS and repetitive pore watersampling at the same location might support future studies of seasonal variation ofbenthic processes in sediments of the coastal zone and open ocean