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Abstract: In order to test the hypothesis that the ambient iron concentrations could regulate sulfate reducing activity (SRA) in mangrove areas, 10 cm cores were examined from test and reference sites. The test site at Diwar mangrove ecosystem is highly influenced by iron released by the movement of barges carrying iron ore during the non-monsoon seasons and the reference site at Tuvem is relatively pristine. The average iron concentrations were 17.9% (±8.06) at Diwar and 6.3% (±1.5) at Tuvem. Sulfate reducing rates (SRR) ranged from 50.21 to 698.66 nM cm−3 d−1 at Tuvem, and from 23.32 to 294.49 nM cm−3d−1 in Diwar. Pearson’s correlation coefficients between SRR and environmental parameters showed that at Tuvem, the SRR was controlled by SO4 −2 (r = 0.498, p < 0.001, n = 60) more than organic carbon (r = 0.316 p < 0.05, n = 60). At Diwar, the SRR was governed by the iron concentrations at an r-value of −0.761 (p < 0.001, n = 60), suggesting that ca.58% of the variation in SRR was influenced negatively by variations in ambient iron concentrations. This influence was more than the positive influence of TOC (r = 0.615, p < 0.001, n = 60). Laboratory experiments to check the influence of iron on SRR also supported our field observations. At an experimental manipulation of 50 ppm Fe3+ there was an increase in SRR but at 100 ppm an inhibitory effect was observed. At 1000 ppm Fe3+ there was a decrease in the SRR up to 93% of the control. Thus, our study showed that ambient iron concentrations influence SRR negatively at Diwar and counters the positive influence of organic carbon. Consequently, the influence could cascade to other biogeochemical processes in these mangrove swamps, especially the mineralization of organic matter to carbon dioxide by sulfate respiration. [Copyright &y& Elsevier]

Abstract: This study investigates the biogeochemical processes that control the benthic fluxes of dissolved nitrogen (N) species in Boknis Eck – a 28 m deep site in the Eckernförde Bay (southwestern Baltic Sea). Bottom water oxygen concentrations (O2−BW) fluctuate greatly over the year at Boknis Eck, being well-oxygenated in winter and experiencing severe bottom water hypoxia and even anoxia in late summer. The present communication addresses the winter situation (February 2010). Fluxes of ammonium (NH4 +), nitrate (NO3 −) and nitrite (NO2 −) were simulated using a benthic model that accounted for transport and biogeochemical reactions and constrained with ex situ flux measurements and sediment geochemical analysis. The sediments were a net sink for NO3 − (−0.35 mmol m−2 d−1 of NO3 −), of which 75% was ascribed to dissimilatory reduction of nitrate to ammonium (DNRA) by sulfide oxidizing bacteria, and 25% to NO3 − reduction to NO2 − by denitrifying microorganisms. NH4 + fluxes were high (1.74 mmol m−2 d−1 of NH4 +), mainly due to the degradation of organic nitrogen, and directed out of the sediment. NO2 − fluxes were negligible. The sediments in Boknis Eck are, therefore, a net source of dissolved inorganic nitrogen (DIN = NO3 − + NO2 − + NH4 +) during winter. This is in large part due to bioirrigation, which accounts for 76% of the benthic efflux of NH4 +, thus reducing the capacity for nitrification of NH4 +. The combined rate of fixed N loss by denitrification and anammox was estimated at 0.08 mmol m−2 d−1 of N2, which is at the lower end of previously reported values. A systematic sensitivity analysis revealed that denitrification and anammox respond strongly and positively to the concentration of NO3 − in the bottom water (NO3 − BW). Higher O2−BW decreases DNRA and denitrification but stimulates both anammox and the contribution of anammox to total N2 production (%Ramx). A complete mechanistic explanation of these findings is provided. Our analysis indicates that nitrification is the geochemical driving force behind the observed correlation between %Ramx and water depth in the seminal study of . Despite remaining uncertainties, the results provide a general mechanistic framework for interpreting the existing knowledge of N-turnover processes and fluxes in continental margin sediments, as well as predicting the types of environment where these reactions are expected to occur prominently. [Copyright &y& Elsevier]

Abstract: In this work, bioturbation effects by the polychaete Nereis spp. on oxygen demand, denitrification and solute fluxes in four European coastal areas (Goro lagoon in Italy, Tagus estuary in Portugal, Colne estuary in England and Tjärnö bay in Sweden) are compared. The studied areas are very heterogeneous with respect to tidal regimes, primary producers communities, sediment composition and organic matter content and nutrient concentrations in the water column. At each site, with the same methodological approach, undisturbed sediment cores and cores with different numbers of polychaetes added were incubated in the dark. Oxygen, inorganic nutrients (NH4 +, NO3 −, Dissolved Reactive Silica-SiO2 and Soluble Reactive Phosphorous-SRP) fluxes and coupled-uncoupled denitrification rates were quantified on the same set of cores. Nereis bioturbation had a considerable effect on biogeochemical processes and, at all sites, resulted in an immediate stimulation (from 1.5- to 4-fold higher) of oxygen and ammonium fluxes between the sediment and the water column; on the contrary bioturbation had site specific effects on reactive silica and phosphorus fluxes. Bioturbation also stimulated denitrification of water column nitrate (D w, from 1.3 to 3 times higher than in control sediments); this process removed a major fraction (>50%) of the nitrogen lost through denitrification. Overall, the strong relationship between benthic macrofauna activity and sediment biogeochemistry is confirmed by this study. Comparison of the relative stimulation of processes in different geographical areas reveals that the degree by which processes and fluxes (intensity and direction) are affected are site specific and influenced by initial macrofauna densities and sediment and water chemistry. [Copyright &y& Elsevier]

Abstract: Sediment denitrification rates and nitrous oxide (N2O) exchange fluxes in the Yangtze estuary intertidal zone (mudflats and salt marshes) were measured in summer-time using an acetylene inhibition technique and an in situ static closed chamber method. N2O natural production rates ranged between 0.10μmolNm−2 h−1 and 8.50μmolNm−2 h−1, and the denitrification rates ranged from 18.71μmolNm−2 h−1 to 35.87μmolNm−2 h−1. The N2O natural production rates in overlying water were low, with intertidal sediment being the source of overlying water N2O during the submerged period. Data analysis indicated that most sediment N2O was not derived from denitrification, but from several other nitrogen-cycling processes. During the low tide, the middle tidal marsh was the source of atmosphere N2O (exchange fluxes changed between −11.03μmolNm−2 h−1 and 13.17μmolNm−2 h−1). 5cm and 10cm depth ground temperatures were significant factors controlling the emission flux. At the low tidal flat, N2O emission flux rates ranged from −5.75μmolNm−2 h−1 to 0.49μmolNm−2 h−1 at the sediment-atmosphere interface. Overall, the middle tidal marsh was the source of atmospheric N2O, while the low tidal flat was a sink for atmospheric N2O. Plants of the intertidal zone (Scirpus mariqueter and benthic algae) were significant factors controlling N2O exchange flux. Photosynthesis of intertidal zone plants inhibited the emission of N2O and induced consumption, while plant respiration may enhance N2O emission from the intertidal zone. N2O emission and consumption at the intertidal zone-atmosphere interface correlated positively with the emission and consumption of CO2. [Copyright &y& Elsevier]

Abstract: Direct gas chromatographic measurement of denitrification rates via N2 fluxes from aquatic sediments can avoid some of the artifacts and complexities associated with indirect approaches and tracer techniques. However, measurement protocols have typically been determined based upon initial results or previous studies. We present a process-level study and simulation model for evaluating and optimizing N2 gas flux approaches in closed chamber incubations. Experimental manipulations and simulations of both artificial and natural sediments were used to conduct sensitivity analyses of key design parameters in N2 flux measurements. Experimental results indicated that depletion of labile organic matter during the long incubations required by common protocols (for diffusive off-gassing of porewater N2) may result in underestimates of denitrification rates in some systems. Simulations showed that the required incubation time was primarily a function of sediment thickness. The best approach found to minimize incubation time and reduce errors was to select the minimum sediment thickness necessary to include the entire depth distribution of nitrification–denitrification for a particular sediment system. Attempts to increase measurement sensitivity and shorten incubation times by reducing the headspace thickness to 1–2cm generally cause denitrification to be underestimated by 3–13% for gas headspaces, and up to 80% for water headspaces. However, errors were negligible with gas and water headspace thicknesses of 10cm and 15cm, respectively. Anaerobic cores to control for non-denitrification N2 fluxes shortened incubation time, but introduced artifacts in sediments with extensive macrofaunal irrigation. [Copyright &y& Elsevier]