Anoxic nitrification in marine sediments

Nitrate peaks are found in pore-water profiles in marine sediments at depths considerably below the conventional zone of oxic nitrification. These have been interpreted to represent nonsteady- state effects produced by the activity of nitrifying bacteria, and suggest that nitrification occurs throughout the anoxic sediment region. In this study, ΣNO3 peaks and molecular analysis of DNA and RNA extracted from anoxic sediments of Loch Duich, an organic-rich marine fjord, are consistent with nitrification occurring in the anoxic zone. Analysis of ammonia oxidiser 16S rRNA gene fragments amplified from sediment DNA indicated the abundance of autotrophic ammonia-oxidising bacteria throughout the sediment depth sampled (40 cm), while RT-PCR analysis indicated their potential activity throughout this region. A large non-steady-state pore-water ΣNO3 peak at ~21 cm correlated with discontinuities in this ammonia-oxidiser community. In addition, a subsurface nitrate peak at ~8 cm below the oxygen penetration depth, correlated with the depth of a peak in nitrification rate, assessed by transformation of 15N-labelled ammonia. The source of the oxidant required to support nitrification within the anoxic region is uncertain. It is suggested that rapid recycling of N is occurring, based on a coupled reaction involving Mn oxides (or possibly highly labile Fe oxides) buried during small-scale slumping events. However, to fully investigate this coupling, advances in the capability of high-resolution pore-water techniques are required

Tidal sands as biogeochemical reactors

Sandy sediments of continental shelves and most beaches are often thought of as geochemical deserts because they are usually poor in organic matter and other reactive substances. The present study focuses on analyses of dissolved biogenic compounds of surface seawater and pore waters of Aquitanian coastal beach sediments. To quantitatively assess the biogeochemical reactions, we collected pore waters at low tide on tidal cross-shore transects unaffected by freshwater inputs. We recorded temperature, salinity, oxygen saturation state, and nutrient concentrations. These parameters were compared to the values recorded in the seawater entering the interstitial environment during floods. Cross-shore topography and position of piezometric level at low tide were obtained from kinematics GPS records. Residence time of pore waters was estimated by a tracer approach, using dissolved silica concentration and kinetics estimate of quartz dissolution with seawater. Kinetics parameters were based on dissolved silica concentration monitoring during 20-day incubations of sediment with seawater. We found that seawater that entered the sediment during flood tides remained up to seven tidal cycles within the interstitial environment. Oxygen saturation of seawater was close to 100%, whereas it was as low as 80% in pore waters. Concentrations of dissolved nutrients were higher in pore waters than in seawater. These results suggest that aerobic respiration occurred in the sands. We propose that mineralised organic matter originated from planktonic material that infiltrated the sediment with water during flood tides. Therefore, the sandy tidal sediment of the Aquitanian coast is a biogeochemical reactor that promotes or accelerates remineralisation of coastal pelagic primary production. Mass balance calculations suggest that this single process supplies about 37 kmol of nitrate and 1.9 kmol of dissolved inorganic phosphorus (DIP) to the 250-km long Aquitanian coast during each semi-diurnal tidal cycle. It represents about 1.5% of nitrate and 5% of DIP supplied by the nearest estuary

Vertical distribution and respiration rates of benthic foraminifera: Contribution to aerobic remineralization in intertidal mudflats covered by Zostera noltei meadows

The present study investigates the influence of seagrass root systems on benthic hard-shelled meiofauna (foraminifera). In February and July 2011, sediment cores were collected at low tide at two sites in Arcachon lagoon, a vegetated site with Zostera noltei and a second site with bare sediments. We used the highly discriminative CellTracker™ Green fluorogenic probe technique to recognize living foraminifera and to describe foraminiferal density and diversity. Three dominant species of foraminifera were observed: Ammonia tepida, Haynesina germanica and Eggerella scabra. The two calcareous species, A. tepida and H. germanica, were preferentially found in the upper half to 1 cm of the sediment. At the vegetated site, these two species had a slightly deeper microhabitat. In the literature, both species have been described alive in much deeper sediment layers, possibly due to false positives from the Rose Bengal staining method. These two species also showed 1) higher densities at the site with Z. noltei, 2) a higher density in February when conditions were supposed optimal due to a microphytobenthos bloom, and 3) dissolved calcitic shells in July, probably resulting from a lower pH. The agglutinated species E. scabra was present alive down to at least 7 cm depth. E. scabra showed high densities in the anoxic part of the sediment at both the vegetated and bare sites, with a substantially higher density in summer at the site with bare sediments. Its presence at depth may be related to its trophic requirements; this species could be less dependent on labile organic matter than A. tepida and H. germanica. On this intertidal mudflat, the foraminiferal contribution to aerobic carbon remineralization, based on respiration rate measurements, can account for up to 7% of the diffusive oxygen uptake, almost five times more than the maximum contribution recorded in open marine environments (300 m depth) in the Bay of Biscay

Biogeochemical and contaminant cycling in sediments from a human-impacted coastal lagoon â Introduction and summary

International audienceFrom 2001 to 2003, the Microbent project (ââBiogeochemical processes at the water sediment interface in eutrophic environmentââ) was carried out within the framework of the Programme National Environnement CoËtier, the French contribution to LandeOcean Interaction in the Coastal Zone (LOICZ). The Microbent programme was focused on the study of sediment biogeochemical cycles of carbon, oxygen, sulphur, iron, nitrogen, and phosphorus in relation to the faunal activity in the sediment and their relation with the mobility of metallic contaminants at the sedimentewater interface (SWI) in a Mediterranean coastal lagoon (Thau lagoon, France; Fig. 1). The aim of Microbent was to set up an interdisciplinary study bringing together geochemists, sedimentologists, and biologists in order to understand and quantify the main reaction pathways, and the fluxes of contaminants at the SWI, including those related to benthic fauna. Work was focused on the processes which generate contaminant fluxes: (1) early diagenetic processes, which generate the chemical conditions of the environment; (2) processes leading to the transfer of contaminants from particles toward biofilms, water column, and organisms; and (3) processes of sediment mixing by organisms and sediment accumulation

Variability of Black Carbon Deposition to the East Antarctic Plateau, 1800-2000 AD

Refractory black carbon aerosols (rBC) from biomass burning and fossil fuel combustion are deposited to the Antarctic ice sheet and preserve a history of emissions and long-range transport from low- and mid-latitudes. Antarctic ice core rBC records may thus provide information with respect to past combustion aerosol emissions and atmospheric circulation. Here, we present six East Antarctic ice core records of rBC concentrations and fluxes covering the last two centuries with approximately annual resolution (cal. yr. 1800 to 2000). The ice cores were drilled in disparate regions of the high East Antarctic ice sheet, at different elevations and net snow accumulation rates. Annual rBC concentrations were log-normally distributed and geometric means of annual concentrations ranged from 0.10 to 0.18 m cro-g/kg. Average rBC fluxes were determined over the time periods 1800 to 2000 and 1963 to 2000 and ranged from 3.4 to 15.5 m /a and 3.6 to 21.8 micro-g/sq m/a, respectively. Geometric mean concentrations spanning 1800 to 2000 increased linearly with elevation at a rate of 0.025 micro-g/kg/500 m. Spectral analysis of the records revealed significant decadal-scale variability, which at several sites was comparable to decadal ENSO variability

Terrestrial groundwater and nutrient discharge along the 240-km-long Aquitanian coast

We collected samples from sea water, runnel water, beach pore waters, water from the unconfined surficial aquifer discharging at the beach face, groundwater, and rainwater from the Aquitanian coast in order to determine the flux of dissolved inorganic nitrogen (DIN), phosphorus and silica from terrestrial submarine groundwater discharge (SGD). The flux of fresh groundwater was obtained from a water balance calculation based on precipitation and evapotranspiration and assessment of the coastal watershed from hydrograph separation. Waters with intermediate salinities between sea water and freshwaters are found all along the 240-km-long coast, indicating that SGD is ubiquitous. The estimated fresh water flux is 2.25 m3 d− 1 m− 1 longshore. Terrestrial SGD provides a DIN flux of 9·106 mol each year to the adjacent coastal zone. This flux is about four times lower than the release of DIN due to tidally driven saline SGD. The freshwater DIN flux is low because the upland land use consists almost exclusively of pine forest. Dissolved organic nitrogen represents more than 60% of the total dissolved nitrogen flux. Dissolved iron, phosphorus and silica have much higher concentrations in the anoxic forest aquifer than in the fresh-water end-member of the subterranean estuary sampled in the upper beach aquifer. This suggests that the salinity gradient of the estuary does not correspond to a redox gradient. The redox front between anoxic groundwater and fresh oxic waters occurs below the soil-depleted foredune/yellow dune. Anoxic P- and Si-rich waters seep directly on the beach face only in the north Gironde, where the foredunes are eroded. This study reveals the role of the sandy foredune aquifer in biogeochemical fluxes from SGD, which is to dilute and oxidize waters from the unconfined surficial upland aquifer

Whooping Crane Sightings in Nebraska, August 1994-January 1995

The first arrival at Aransas National Wildlife Refuge in southern Texas was on October 5, 1994, and the last arrivals (a family group) were on January 12, 1995, the latest date that an adult pair had ever arrived at Aransas. A total of 132 (125 adult/subadult and 7 young) Whooping Cranes are wintering at Aransas, including 3 birds that spent the summer at Aransas. A solitary chick sighted with Sandhill Cranes in Oklahoma on January 2, 1995 is the 133rd crane in the flock. Since, under optimum conditions, 148 cranes were expected to reach Aransas during the fall migration, the location and cause of summer and migrational losses are not known. The first dates for confirmed observations of migrating Whooping Cranes were August 16 in Canada and September 7 in the United States. The last sighting date was January 5 in Oklahoma. Sightings were reported from Alberta, Canada (1); Saskatchewan, Canada (18); Montana (1); North Dakota (5); South Dakota (2); Nebraska (5); Kansas (13); Oklahoma (4); and Texas (1). The migration progressed slowly, perhaps due to the mild weather during September and October. There were two or more Whooping Cranes present at Cheyenne Bottoms State Wildlife Area, Kansas, between October 13 and November 29, with a peak of 18 birds on November 1. A group of five Whooping Cranes was present at Cheyenne Bottoms between October 25 and November 29 (36 days), which is a record migration stopover time for the United States

Whooping Crane Sightings in Nebraska, August 1994-January 1995

The first arrival at Aransas National Wildlife Refuge in southern Texas was on October 5, 1994, and the last arrivals (a family group) were on January 12, 1995, the latest date that an adult pair had ever arrived at Aransas. A total of 132 (125 adult/subadult and 7 young) Whooping Cranes are wintering at Aransas, including 3 birds that spent the summer at Aransas. A solitary chick sighted with Sandhill Cranes in Oklahoma on January 2, 1995 is the 133rd crane in the flock. Since, under optimum conditions, 148 cranes were expected to reach Aransas during the fall migration, the location and cause of summer and migrational losses are not known. The first dates for confirmed observations of migrating Whooping Cranes were August 16 in Canada and September 7 in the United States. The last sighting date was January 5 in Oklahoma. Sightings were reported from Alberta, Canada (1); Saskatchewan, Canada (18); Montana (1); North Dakota (5); South Dakota (2); Nebraska (5); Kansas (13); Oklahoma (4); and Texas (1). The migration progressed slowly, perhaps due to the mild weather during September and October. There were two or more Whooping Cranes present at Cheyenne Bottoms State Wildlife Area, Kansas, between October 13 and November 29, with a peak of 18 birds on November 1. A group of five Whooping Cranes was present at Cheyenne Bottoms between October 25 and November 29 (36 days), which is a record migration stopover time for the United States

Whooping Crane Sightings during March-May 1997 Migration

During the 1996-97 winter, 158 (143 adult/subadult and 15 juvenile) Whooping Cranes stayed at the Aransas National Wildlife Refuge in Texas. Two early migrants in Nebraska were an adult-plumaged bird first observed on 9 March, and a juvenile confirmed on 19 March. Neither of these birds wintered at Aransas, so the total population was therefore 160 (144 adult/subadult and 16 juvenile). All 158 cranes wintering at Aransas were present 3 April, but by 10 April, about 103 cranes had begun to migrate, and by 24 April, only 13 cranes were still at Aransas. All cranes had migrated by 6 May