The effects of advective pore water exchange driven by shallow water waves on the oxygen distribution in a permeable (k ϭ 3.3 ϫ 10 Ϫ12 to 4.9 ϫ 10 Ϫ11 m 2 ) natural sediment were studied with a planar oxygen optode in a wave tank. Our experiments demonstrate that pore water flow driven by the interaction of sediment topography and oscillating boundary flow changes the spatial and temporal oxygen distribution in the upper sediment layer. Oxygenated water intruding in the ripple troughs and deep anoxic pore water drawn to the surface under the ripple crests create an undulating oxic-anoxic boundary within the upper sediment layer, mirroring the topographical features of the sediment bed. Anoxic upwelling zones under ripple crests can separate the oxic sediment areas of neighboring ripple troughs with steep horizontal oxygen concentration gradients. The optode showed that migrating wave ripples are trailed by their pore water flow field, alternately exposing sediment volumes to oxic and anoxic pore water, which can be a mechanism for remobilizing particulate oxidized metal precipitates and for promoting coupled nitrification-denitrification. More rapid ripple migration (experimental threshold ϳ20 cm h Ϫ1) produces a continuous oxic surface layer that inhibits the release of reduced substances from the bed, which under slowly moving ripples is possible through the anoxic vertical upwelling zones. Swift, dramatic changes in oxygen concentration in the upper layers of permeable seabeds because of surface gravity waves require that sediment-dwelling organisms are tolerant to anoxia or highly mobile and enhance organic matter mineralization.The dominant boundary layer flows in shallow marine environments are those generated by surface gravity waves. This dominance is reflected by the presence of wave ripples structuring large areas of shallow sandy seabeds that are abundant in coastal, estuarine, and shelf environments. Most of these sandy sediments are permeable (k Ͼ 10 Ϫ12 m 2 ) and thus allow interstitial water motion. Pressure differences at the sediment-water interface might drive interfacial solute transport through the surface layers of these beds. This advective transport can exceed transport by molecular diffusion by several orders of magnitude (Huettel and Webster 2001). In contrast, the major transport mechanisms in fine-grained muddy sediments are molecular diffusion and locally bioturbation (Berner 1980;Aller 1982).Increased fluid exchange between sediment and overlying water affects the oxygen dynamics in permeable sediments and therefore also affects biogeochemical processes. Booij 1 Corresponding author (eprecht@mpi-bremen.de). 2 Present address: Florida State University, Department of Oceanography, Tallahassee, FL 32306-4320. AcknowledgmentsBo Barker Jørgensen is acknowledged for support and constant interest in this work. Hans Røy is thanked for initial discussions, helpful comments, and help during fieldwork. For assistance with the planar oxygen optodes, Gerhard Holst and Björn Grunwald are a...
With in situ and laboratory chamber incubations we demonstrate that coral mucus, an important component of particulate organic matter in reef ecosystems, is a valuable substrate for microbial communities in the water column and sandy sediments of coral reefs. The addition of coral mucus to the water of benthic chambers placed on lagoon sands in the coral cay Heron Island, Australia, resulted in a rapid and significant increase in both O 2 consumption and DIC production in the chambers. The permeable coral sands permitted the transport of mucus into the sediment with interfacial water flows, resulting in the mucus being mainly (> 90%) degraded in the sediment and not in the water column of the chambers. A low ratio of 0.48 (in situ) to 0.64 (laboratory) for O 2 consumption/DIC production after the addition of coral mucus, and high sulfate reduction rates (SRR) in natural sediments which were exposed to coral mucus, suggest a large contribution of anaerobic processes to the degradation of coral mucus. Oxygen penetrated less than 5 mm deep into these sediments. The microbial reaction to mucus addition was rapid, with a calculated in situ C turnover rate ranging from 7 to 18% h -1 . The degradation of coral mucus showed a dependency on the permeability of the carbonate sediments, with faster degradation and remineralization in coarse sands. This indicates the importance of permeable reef sediments for the trapping and degradation of organic matter. We suggest that coral mucus may have a function as a carrier of energy to the benthic microbial consumers.KEY WORDS: Coral mucus · POM · Degradation · Permeable carbonate sands · O 2 consumption · DIC production · C turnover Resale or republication not permitted without written consent of the publisher
A method is presented for the measurement of depth profiles of volumetric oxygen consumption rates in permeable sediments with high spatial resolution. When combined with in situ oxygen microprofiles measured by microsensors, areal rates of aerobic respiration in sediments can be calculated. The method is useful for characterizing sediments exposed to highly dynamic advective water exchange, such as intertidal sandy sediments. The method is based on percolating the sediment in a sampling core with aerated water and monitoring oxygen in the sediment using either an oxygen microelectrode or a planar oxygen optode. The oxygen consumption rates are determined using three approaches: (1) as the initial rate of oxygen decrease measured at discrete points after the percolation is stopped, (2) from oxygen microprofiles measured sequentially after the percolation is stopped, and (3) as a derivative of steady-state oxygen microprofiles measured during a constant percolation of the sediment. The spatial resolution of a typical 3 to 4 cm profile within a measurement time of 1 to 2 h is better with planar optodes (≈0.3 mm) then with microelectrodes (2 to 5 mm), whereas the precision of oxygen consumption rate measurements at individual points is similar (0.1 to 0.5 µmol L -1 min -1 ) for both sensing methods. The method is consistent with the established methods (interfacial gradients combined with Fick's law of diffusion, benthic-chambers), when tested on the same sediment sample under identical, diffusion-controlled conditions. * E-mail: lpolerec@mpi-bremen.de AcknowledgmentsWe would like to thank Gaby Eickert, Ines Schröder, Ingrid Dohrmann, Alfred Kutsche, Georg Herz, Volker Meyer, Paul Färber, and Harald Osmers for the preparation of oxygen microsensors and the construction of various mechanical and electronic parts necessary for the experimental setup. The crew of the vessel Verandering are thanked for providing pleasant and safe conditions on board. The valuable comments of two anonymous reviewers and especially of Clare Reimers are much appreciated. This study was supported by the Bundesministerium für Bildung und Forschung (BMBF, project number 03F0284a).
Rotation thromboelastometry (ROTEM) performed on whole-blood samples provides information on the contribution of fibrinogen and platelets to clot formation. Such measurements are believed superior to classical plasma coagulation measurements as a means of monitoring disturbed haemostasis. On-pump cardiac surgery is associated with high bleeding risk. The study objective was to obtain information on the frequency of abnormal values of ROTEM variables and to assess their value in estimating bleeding risk in such patients. We studied 150 patients undergoing elective on-pump cardiac surgery. We found a significant surgery-induced decrease in haemostatic potential, with more abnormal ROTEM values in intrinsically activated coagulation (up to 50%) than in extrinsically activated coagulation (up to 27%) or the maximum clot firmness in FIBTEM (10%), a test measuring the contribution of fibrinogen. All ROTEM variables tend to normalize within 14-18 h postoperatively. Best positive predictive values and specificity for a postoperative blood loss above 600 ml were found for the clot formation time in extrinsically activated coagulation (71%/94%) and the maximum clot firmness in FIBTEM (73%/95%); these values were superior to the activated partial thromboplastin time or prothrombin time (56%/72% and 43%/5%, respectively). There was no relation between preoperative or early postoperative ROTEM values and intraoperative bleeding. ROTEM recorded a benefit of administration of platelet concentrates or fresh-frozen plasma, particularly when given postoperatively, on haemostasis. In contrast, intraoperative administration of red blood cells impaired haemostasis. ROTEM can provide a more detailed diagnostic basis enabling a focused therapy to cardiac surgery patients with high bleeding risk.
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