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Here we evaluate a new field methodology for the deployment of Diffusive Equilibrium in Thin Films (DET, protected in stainless steel holders) in coarse riverbed sediments based on that originally developed for fine-grained sediments and soils. Concentration gradients of NO3 −, NH4+ and dissolved Mn were measured at cm resolution. We observed fine scale changes in NO3−, NH4+, and Mn concentrations in the river bed (0–30 cm) that were only evident at high resolution and compared them to profiles of NO3− and NH4+ obtained with low resolution multilevel piezometers. The range in concentrations of NO3− and NH4+measured through DET was larger than those measured at coarse resolution through pore water sampling from multilevel piezometer in the riverbed over the 30 cm depth. According to the results, high resolution profiles of redox sensitive chemical species in riverbeds could help in identifying and resolving hotspots of biogeochemical activity. Measurements of NH4+ using DET were higher than measurements in pore water collected from the multilevel samplers. Further studies are needed to establish whether there is a systematic bias associated with either procedure for the measurement of NH4+.

The relative magnitudes of, and factors controlling, denitrification and dissimilatory nitrate reduction to ammonium (DNRA) were measured in the soil of a re-connected temperate floodplain divided into four different land management zones (grazing grassland, hay meadow, fritillary meadow and a buffer zone). Soil samples were collected from each zone to measure their respective potentials for nitrate attenuation using 15N both at the surface and at depth in the soil column and additional samples were collected to measure the lability of the organic carbon. Denitrification capacity ranged between 0.4 and 4.2 (μmol N g(-1) dry soil d(-1)) across the floodplain topsoil and DNRA capacity was an order of magnitude lower (0.01-0.71 μmol N g(-1) d(-1)). Land management practice had a significant effect on denitrification but no significant effects were apparent for DNRA. In this nitrogen-rich landscape, spatial heterogeneity in denitrification was explained by differences in lability and the magnitude of organic carbon associated with different management practices (mowing and grazing). The lability of organic carbon was significantly higher in grazing grassland in comparison to other ungrazed areas of the floodplain, and consequently denitrification capacity was also highest in this area. Our results indicate that bacteria capable of DNRA do survive in frequently flooded riparian zones, and to a limited extent, compete with denitrification for nitrate, acting to retain and recycle nitrogen in the floodplain. Exponential declines in both denitrification and DNRA capacity with depth in the floodplain soils of a hay meadow and buffer zone were controlled primarily by the organic carbon content of the soils. Furthermore, grazing could be employed in re-connected, temperate floodplains to enhance the potential for nitrate removal from floodwaters via denitrification.

We investigated anaerobic ammonium oxidation (anammox) in continental shelf and slope sediments of the Irish and Celtic Seas by using anammox specific ladderane biomarker lipids. We used the presence of an intact ladderane phospholipid as a direct indicator for living anammox bacteria, and compared it with the abundance of ladderane core lipids derived from both living and dead bacterial biomass. All investigated sediments contained ladderane core lipids as well as the intact ladderane phospholipid, in agreement with 15 N-labeling experiments, which revealed anammox activity at all sites. Ladderane core lipid and intact ladderane phospholipid concentrations were significantly correlated ( R 2 = 0.957 and 0.464, respectively) with anammox activity over the transect of the continental shelf and slope sediments. In the Irish Sea (50–100 m water depth) highest abundances of the intact ladderane phospholipid were found in the upper 2 cm of the sediment, indicating a zone of active anammox. A sharp decline further down-core suggested a strong decrease in anammox biomass and rapid degradation of the intact lipids. In comparison, ladderane core lipids were 1–2 orders of magnitude higher in concentration than the intact ladderane phospholipid and accumulated as dead cell remnants with depth. In the slope sediments of the Celtic Sea both ladderane core lipids and the intact ladderane phospholipid were found in sediments at water depths ranging from 500 to 2000 m. Here, anammox seemed to be active at greater depths of the sediment (>2 cm). Mean abundances of both intact and core ladderane lipids in whole sediment cores increased downslope, indicating an increasing importance of anammox in deeper slope sediments.

Summary This research investigated the ecosystem engineering by in-stream macrophytes, dominated by Ranunculus spp., and associated suspension-feeding blackfly larvae (Diptera: Simuliidae) for five reaches in the Frome and Piddle catchments, Dorset (UK) over one annual growth cycle (2003). This paper focuses on the modification of flow velocities and the trapping of fine sediment (particles In-stream macrophyte growth was extensive, with maximum percentage cover of 80% recorded in September and October 2003. The macrophyte cover significantly altered flow patterns and flow velocities within and between the macrophyte stands. The reduced flow velocities within the plants promoted sediment trapping, reaching volumes of 0.085 m3 of fine sediment trapped per metre square of vegetation at one site. The effective particle sizes of the sediments trapped within Ranunculus stands were dominated by the 250–500 μm fraction from March to July 2003 whereas a higher proportion of smaller fractions occurred from October to December. Faecal pellets were highly abundant in the sediments trapped within Ranunculus stands (up to 2.2 × 108 faecal pellets per m2) and their dimensions (total size range 25–400 μm) fall within the dominant size fraction of the trapped sediments. Our findings demonstrate the need to consider the biogenic component of the fine sediments in chalk streams in future studies of sediment and nutrient dynamics.

Summary Mini-piezometers were used to assess surface–subsurface hydrological exchange and biogeochemical processing in different patches on the river bed (coarse gravel, fine gravel/sand, silt/sand) at two sites on the River Lambourn (Berkshire, UK). Positive vertical hydraulic gradients (VHG) dominated the riverbed, indicating potentially upwelling subsurface water. Hydraulic conductivity was highly variable in the shallow sediments, but was generally low at greater depths, suggesting that positive VHG may not translate to the rapid movement of subsurface water. Well defined areas of downwelling were not identified, although negative VHG did occur at the low/intermediate depths (∼10–20 cm). Furthermore, steep temperature gradients within the top 30 cm suggested that connectivity with the surface water was restricted to a shallow layer within the sediments. The three patch types differed in biogeochemical activity, largely as a function of their sediment size distribution, organic content and surface–subsurface exchange dynamics. Nitrate reduction was associated with hypoxic ( 30 cm) that contained oxygen >90 μM. A high background concentration of nitrous oxide (∼180 nM) occurred within the river bed, regardless of patch type and depth into the bed. In the nitrate reduction zone, N 2 O concentration was highly variable, with production and reduction of N 2 O. Ammonium concentration was higher within the river bed than in surface water and decreased with depth, particularly in the organically rich silt/sand and coarse gravels, where organic matter may get trapped. From a hydrological/biogeochemical perspective, the Lambourn differs from many other rivers. Despite being hypernutrified (surface and subsurface nitrate >400 μM), the bed of the Lambourn is predominantly aerobic and does not serve as a significant nitrate sink. In addition, despite the permeable geology of the catchment, biogeochemical processing of nutrients may be restricted to the thin, but biologically productive, layer in the shallow river bed sediments.

Sediment characteristics, sediment respiration (oxygen uptake and sulphate reduction) and sediment–water nutrient exchange, in conjunction with water column structure and phytoplankton biomass were measured at five stations across the western Irish Sea front in August 2000. The transition from thermally stratified (surface to bottom temperature difference of 2.3 °C) to isothermal water (14.3 °C) occurred over a distance of 13 km. The influence of the front on phytoplankton biomass was limited to a small region of elevated near surface chlorophyll (2.23 mg m −3 ; 50% > biomass in mixed waters). The front clearly marked the boundary between depositional sediments (silt/clays) with elevated sediment pigment levels (≈60 mg m −2 ) on the western side, to pigment impoverished ( −2 ) sand, through to coarse sand and shell fragments on the eastern side. Maximal rates of sedimentary respiration on the western stratified side of the front e.g. oxygen uptake S2 (852 μmol O 2 m −2 h −1 ) and sulphate reduction at S1 (149 μmol SO 4 2− m −2 h −1 ), coupled to significant efflux of nitrate and silicate at the western stations indicate closer benthic–pelagic coupling in the western Irish Sea. Whether this simply reflects the input of phytodetritus from the overlying water column or entrapment and settlement of pelagic production from other regions of the Irish Sea cannot yet be resolved.