Overlying Water Column Research Articles

Recent Increases in Water Column Denitrification in the Seasonally Suboxic Bottom Waters of the Santa Barbara Basin

AbstractDenitrification in the anoxic sediments of the Santa Barbara Basin has been well documented in the historic and modern record, but the regulation of and frequency with which denitrification occurs in the overlying water column are less understood. Since 2004, the magnitude and speciation of redox active nitrogen species in bottom waters have changed markedly. Most notable are periods of decreased nitrate and increased nitrite concentrations. Here we examine these changes in nitrogen cycling as recorded by the stable isotopes of dissolved nitrate from 2010–2016. When compared to previous studies, our data identify an increase in water column denitrification in the bottom waters of the basin. Observations from inside the basin as well as data from the wider California Current Ecosystem implicate a long‐term trend of decreasing oxygen concentrations as the driver for these observed changes, with ramifications for local benthic communities and regional nitrogen loss.

The formation mechanisms of sedimentary pyrite nodules determined by trace element and sulfur isotope microanalysis

Redox sensitive trace elements in pyrite, including nodules, are increasingly used to infer the chemical conditions of ancient oceans - but considerable uncertainty remains regarding the mechanism and timing of nodule formation. Resolving these uncertainties is important because pyrite nodules must form in connection with the overlying water column, rather than during late diagenesis, to reflect the composition of the global ocean. Existing models for pyrite nodule formation have been specific to pyrite textures from individual sites, and we lack a unified model that can explain the compositional and textural diversity observed in nodules from different localities. In this study we examine ten pyrite nodules from several geological periods (Neoarchean to Carboniferous) using in situ LA-ICP-MS and SHRIMP-SI analyses. We present transects of spot analyses of trace elements (As, Ag, Cu, Co, Ni, Sb and Se) and S isotope ratios for each nodule. The pyrite nodules can be classified according to three main categories: those with (1) little to no trace element or isotopic zonation of the nodule from core to margin, (2) strong zonation from core to margin, and (3) minor zonation near the core but more significant zonation near the margin of the nodule. We further illustrate this zonation with a NanoSIMS element map from an additional pyrite nodule. These results are interpreted to indicate nodule formation along a spectrum between two end-member mechanisms. We suggest that the absence of trace element or isotopic zonation reflects nodule growth by a pathway that is analogous to the pervasive growth mechanism for carbonate nodules. This model involves the production of many nucleation sites that are evenly distributed within the volume that the nodule eventually occupies. Consequently, this mechanism results in a chemically homogenous nodule. Pyrites formed this way are suitable for paleoceanographic reconstruction. The other end-member mechanism is analogous to the concentric growth model for carbonate concretions. In this scenario, the core of the nodule forms first and is followed by the addition of concentric layers - each with a progressively different trace element content and δ34S signature as diagenesis progresses. Despite having limited utility for reconstructing ancient seawater, these late forming nodules may track the evolving availability of bioessential trace elements for the subsurface biosphere with important implications for global biogeochemical cycles. Spatial trends in trace elements and S isotopes thus speak to the mechanisms of pyrite nodule formation and provide a framework for evaluating nodule suitability for a range of paleoenvironmental studies.

Coupled trace element and SIMS sulfur isotope geochemistry of sedimentary pyrite: Implications on pyrite growth of Caixiashan Pb–Zn deposit

Colloform pyrite with core-rim texture is commonly deposited in carbonate platforms associated with the sulfide ores such as the Caixiashan Pb–Zn deposit. However, the genesis of colloform pyrite in Pb–Zn deposits, its growth controls and their geological implication are insufficiently understood. Integration of in-situ trace element and SIMS sulfur isotopes has revealed geochemical variations among these pyrite layers. These colloform pyrite occur as residual phases of core-rim aggregates, the cores are made up of very fine-grained anhedral pyrite particles, with some rims being made up of fine-grained and poorly-crystallized pyrite, while the other rims were featured with euhedral cubic pyrite, which are cemented by fine-grained calcite and/or dolomite with minor quartz. Sulfur isotope analysis shows that some well-preserved rims have negative δ34S values (–28.12‰ to –0.49‰), whereas most of the cores and rims have positive δ34S values (>0 to +44.28‰; peak at +14.91‰). Integrating with the methane and sulfate were observed in previous fluid inclusion study, we suggest that the 34S depleted rims were initially formed by bacteria sulfate reduction (BSR), whereas the positive δ34S values were resulted from the sulfate reduction driven by anaerobic methane oxidation (AOM). The well-developed authigenic pyrite and calcite may also support the reaction of AOM. Combined with petrographic observations, trace element composition of the colloform pyrite reveals the incorporation and precipitation behavior of those high abundance elements in the pyrite: Pb and Zn were present as mineral inclusion and likely precipitated before Fe, as supported by the time-resolved Pb–Zn signal spikes in most of the analyzed pyrite grains. Other metals, such as Hg, Co and Ni, may have migrated as chloride complexes and entered the pyrite lattice. Arsenic and Sb, generally influenced by complex-forming reactions rather than substitution ones, could also enter the pyrite lattice, or slightly predate the precipitation of colloform pyrite as mineral inclusions, which are controlled by their hydrolysis constant in the ore fluids. The colloform pyrite may have grown inward from the rims. The successive BSR reaction process would enrich H232S in the overlying water column but reduce the metal content, the nucleation of these pyrite rims was featured by strongly negative sulfur isotopes. The following AOM process should be activated by deformation like the turbidity sediment of the mudstone as the sulfide deposition are associated with fault activities that caused the emission of methane migration upward and simultaneously replenishing the metal in the column. The higher AOM reaction rate and the higher metal supply (not only Fe, but with minor other metals such as Pb and Zn) caused by sediment movement enhanced the metal concentration within the pyrite lattice.

Phosphorus biogeochemical cycling in intertidal surface sediments from the Maowei Sea in the northern Beibu Gulf

Surface sediments from 23 stations within the Maowei Sea intertidal flat were sampled for phosphorus (P) species analysis in December 2016. Different P forms were separated and quantified by a sequential extraction procedure. Results showed that the concentrations of total P (TP) ranged from 4.06 to 33. 33μmol g−1, and inorganic P (IP) was the predominant chemical form of TP. Fe-bound P was the main chemical form of IP. Terrestrial inputs, biological processes, and particle size were responsible for the distribution characteristics of different P species. In addition to detrital P, regeneration and transformation occured among different P forms in the intertidal surface sediments. The principal component analysis reflected the importance of organic matter in the binding of various P species to fine-grained sediments. Sedimentary organic carbon to P ratios were > 106, and suggested that organic matter in the intertidal sediments were mainly derived from terrestrial sources or contain mostly reworked organic matter. Organic carbon to reactive P ratios were 29−90, suggesting efficient sequestration of inorganic P in sediments. Potential bioavailable P represented a significant proportion of the sedimentary P pool, accounting for 70.0−95.9% of TP in the intertidal surface sediments and potentially contributing substantial amounts of reactive P to the overlying water column, especially in high organic and reducing sedimentary environments.

Organic matter mineralization in the deep water of the Gotland Basin (Baltic Sea): Rates and oxidant demand

Abstract Time-series observations of total CO2 (CT) accumulation in the deep water of the Gotland Basin were initiated in 2003. The measurements were performed four to five times per year and were complemented by the determination of nutrient and O2/H2S concentrations. The major objective of those measurements was to study organic matter (OM) mineralization. This was facilitated during two periods of stagnation, from 2004 to 2006 and from 2007 to 2014, identified on the basis of temperature-salinity diagrams. In a follow-up to our assessment of the data from the first period, we present here an overall picture of CT accumulation in the deep water of the Gotland Basin. CT measurements were performed at five different depths between 150 m and the sediment surface at 235 m. Mineralization rates for three individual sub-layers were obtained from CT mass balances that took into account vertical mixing across the boundaries of the sub-layers. Mixing coefficients were calculated from the mass balances for salinity. The mass balance calculations yielded the source/sink strength of CT, O2, H2S, NO3− and NH4+ for each time interval between measurements. The accumulated release of CT was shown to increase linearly with time and to reflect consistent mean annual mineralization rates of ~2 mol m−2 for both stagnation periods, which indicated the absence of significant decadal variability. A dependency of the mineralization rate on the redox conditions in the overlying water column was not detected. For the first stagnation period, the sequential use of different oxidants, including O2, NO3− and SO42−, was identified. A determination of oxidant consumption with respect to OM mineralization yielded a mean molar ratio of 1.2 oxygen equivalents per carbon atom of OM, a distinct deviation of the OM composition described by classical Redfield stoichiometry.

Baseline characterization of aerobic hydrocarbon degrading microbial communities in deep-sea sediments of the Great Australian Bight, Australia.

Exploratory drilling for deep-sea oil and gas resources is planned for the Great Australian Bight (GAB). There is scant knowledge of the region's benthic ecosystems and no baseline information of the region's indigenous oil degrading bacteria. To address this knowledge gap, we used next generation sequencing (NGS) of three marker genes (alkB, c23o and pmoA) to detect and characterize the microbial communities capable of aerobic hydrocarbon degradation. Unique, highly novel microbial communities capable of degrading hydrocarbons occur in surface sediments at depths between 200 and 2800 m. Clustering at 97% demonstrated differences in community structure with depth, changing most markedly between 400 and 1000 m depth on the continental slope, and identified putative functional 'ecotypes' related to depth. Observed differences in community structure showed strong correlations with temperature, other physicochemical properties of the overlying water column and are further modulated by differences in sediment grain size. This study provides important baseline data on hydrocarbon degrading microbial communities prior to the start of petroleum resource extraction. Our data will inform future ecological monitoring of the GAB deep-sea ecosystem.

Bioelectricity generation by wetland plant-sediment microbial fuel cells (P-SMFC) and effects on the transformation and mobility of arsenic and heavy metals in sediment.

Two wetland plant-sediment microbial fuel cell systems (PSM1 and PSM2) and one wetland sediment microbial fuel cell system (SM) were constructed to investigate their electricity production performance and the simultaneous migration and transformation of arsenic and heavy metals in sediment and overlying water, arsenic and heavy metals uptake by plants. The bioelectricity generation was monitored for 175days, and sediment samples were collected at three time points (64, 125 and 200days) for the analysis. The results showed that plants improved the efficiency of the electricity production by the fuel cell system. The average output voltage was: PSM1 (0.32V) > PSM2 (0.28V) > SM (0.24V)(P ≤ 0.05).The electricity production of the electrodes and the introduction of plants affected the mobility and transformation of As, Zn and Cd in the sediment, which contributed to their stability in the sediment and reduced the release of these metals into the overlying water column. The bioelectricity production process affected the bioavailability of arsenic and heavy metals in the sediment and attenuated metal uptake by plants, which indicated the potential for remediation of arsenic and heavy metals pollution in sediment.

Vibrio parahaemolyticus risk assessment in the Pacific Northwest: it's not what's in the water.

The Gram-negative bacterium Vibrio parahaemolyticus (Vp) is a major cause of illness associated with the consumption of raw or undercooked seafood, primarily oysters. This species is a natural member of the bacterial community in brackish waters and is bioaccumulated by oysters through filter feeding. Only a subset of strains is thought to be pathogenic. Currently known virulence markers include the gene for the thermostable direct hemolysin (tdh). In this work we analyzed water and oysters for total Vp and strains encoding tdh from 26 oyster-growing areas of the Puget Sound and Pacific coast of Washington state in 2007 and 2008. In addition, possible plankton-associated Vp were assessed from net tow samples. The density of both total and tdh+ Vp in the water column were considerably higher in 2008 than 2007. However, the concentrations of both total and tdh+ Vp in the oyster tissue was similar for both years. A high proportion of Vp strains in the water column was found to be tdh+ in both 2007 and 2008; however, tdh+ strains were detected at much lower levels in oysters. The data show that analysis of Vp density in the oysters is a better risk assessment tool than density in the overlying water column.

Molecular characteristics of sedimentary organic matter in controlling mercury (Hg) and elemental mercury (Hg0) distribution in tropical estuarine sediments

Sedimentary organic matter (SOM) plays an important role in hosting and reducing HgII in marine/estuarine sediment. This study provides a better understanding on the influence of nature of SOM, in regulating sedimentary mercury (Hg) and elemental mercury (Hg0) distribution, and speciation in the Zuari and Mandovi Estuaries that are representative of monsoon fed tropical estuaries, located in the central west coast of India. Salinity of the overlying water column controlled the physical and chemical characteristics of SOM in the estuarine systems. The high molecular weight (MW) SOM dominated at the mid and upstream (low salinity region) of the estuaries, whereas, the low MW SOM prevailed at the downstream (high salinity region). Sediment Hg showed more affinity towards the SOM of high MW. Increasing MW of SOM increased total sedimentary HgT in both the estuaries. SOM with low MW in the estuarine sediment displayed a negative relationship with the sediment Hg concentration. Distribution of Hg0 concentration in the estuarine sediment suggests that reduction of HgII in presence low MW SOM was a dominant process. It was also found that distribution and speciation of Hg0 in the estuarine sediment depends on the quantity, quality of the SOM, and the total sediment Hg loading. This study demonstrated that the competition between Hg-SOM complexation and HgII reduction by SOM controls HgII/Hg0 distribution in tropical estuarine sediment systems.

Phosphorus cycling in Lake Cadagno, Switzerland: A low sulfate euxinic ocean analogue

Low sulfate, euxinic water-column conditions were a common feature of many Precambrian and Phanerozoic periods of ocean anoxia. The cycling of phosphorus in anoxic marine environments exerts a fundamental control on primary productivity, organic carbon production and burial, and hence ultimately oxygen production, but the dynamics of the phosphorus cycle in low sulfate, euxinic settings are largely unknown. Here, we provide a detailed geochemical investigation of phosphorus cycling in the low sulfate, euxinic Lake Cadagno, Switzerland, which is considered a prime analogue for ancient euxinic oceans. We find evidence for extensive recycling of phosphorus from the sediments back to the water column, stimulated by the microbial release of phosphorus from organic matter and Fe (oxyhydr)oxide minerals. Consistent with previous studies of modern and ancient anoxic settings, this regenerated flux maintains high concentrations of phosphorus in the water column, thus promoting a positive productivity feedback. However, the low-sulfate condition of the overlying water column, combined with the rapid removal of sulfide (as pyrite) from porewaters, promotes formation of Fe(II) phosphate minerals (e.g., vivianite) close to the sediment-water interface. This, in turn, modulates the extent of phosphorus recycling back to the water column, and contrasts with modern fully marine euxinic settings, where the higher concentrations of dissolved sulfate promote sulfide formation to greater depths, thus limiting Fe(II) phosphate formation close to the sediment-water interface. The prevalence of low-sulfate conditions during past euxinic episodes suggests that the operation of this near-surface sedimentary trap for recycled phosphorus would have limited the positive P-driven productivity feedback, promoting only a moderate degree of P recycling. Furthermore, the precise magnitude of this recycled P flux would, on a global scale, have been dependent on changes in the size of Earth’s marine sulfate reservoir through time. Thus our findings have major implications for rates of P-driven productivity and organic carbon burial in ancient euxinic settings, which have not previously been factored into reconstructions of Earth’s oxygenation history.

Sediment potentially controls in-lake phosphorus cycling and harmful cyanobacteria in shallow, eutrophic Utah Lake.

Lakes worldwide are impacted by eutrophication and harmful algal or cyanobacteria blooms (HABs) due to excessive nutrients, including legacy P released from sediments in shallow lakes. Utah Lake (northern Utah, USA) is a shallow lake with urban development primarily on the east side of the watershed, providing an opportunity to evaluate HABs in relation to a gradient of legacy sediment P. In this study, we investigated sediment composition and P concentrations in sediment, pore water, and the water column in relation to blooms of harmful cyanobacteria species. Sediments on the east side of the lake had P concentrations up to 1710 mg/kg, corresponding to elevated P concentrations in pore water (up to 10.8 mg/L) and overlying water column (up to 1.7 mg/L). Sediment P concentrations were positively correlated with Fe2O3, CaO, and organic matter abundance, and inversely correlated with SiO2, demonstrating the importance of sediment composition for P sorption and mineral precipitation. Although the sediment contained <3% Fe2O3 by weight, approximately half of the sediment P was associated with redox-sensitive Fe oxide/hydroxide minerals that could be released to the water column under reducing conditions. Cyanobacteria cell counts indicate that blooms of Aphanizomenon flos-aquae and Dolichospermum flosaquae species tend to occur on the east side of Utah Lake, corresponding to areas with elevated P concentrations in the sediment, pore water, and water column. Our findings suggest that shallow lake eutrophication may be a function of P in legacy sediments that contribute to observed HABs in specific locations of shallow lakes.

Characteristics of geochemical fractions of phosphorus and its bioavailability in sediments of a largest brackish water lake, South Asia

The spatio-temporal distribution of geochemical fractions of phosphorus and its bio-availability for Asia's largest brackish water lake has been carried out. One way ANOVA analysis revealed that both iron bound phosphorus (Fe-P) (p<0.05, n=360) and calcium bound phosphorus (Ca-P) (p<0.05, n=360) showed a significant variation on a spatio-temporal scale which may be attributed to pH and salinity of overlying water column. Pearson's correlation matrix revealed that sediment texture played a major role in the quantification of phosphorus species. The relationship between Ca-P and Ca was not established for the sediments of the lake Chilika, which suggested its authigenic origin. The concentration of bio-available phosphorus (BAP) in different sectors of the lake varied in the order of northern sector (45.64%)>central sector (42.22%)>outer channel (36.63%)>southern sector (29.27%) of total phosphorus (TP). This is supported by the fact that bioavailable P is comparatively higher in the freshwater zone than in the brackish water zone. The principal component analysis (PCA) revealed a good relationship between dissolved phosphate concentrations of water column with benthic population density. It showed that TP availability in the sediment of the lake is mostly bounded with Al, Fe and organic matter (OM). Positive association of total carbon (TC), total nitrogen (TN) and OM with negatively loading of Eh indicates towards the fact that, deposition of TN and TC is largely governed by OM availability in sediment.

Detection of the 2012 Havre submarine eruption plume using Argo floats and its implications for ocean dynamics

More than 85% of the Earth's volcanism and geothermal heat release occurs underwater at mid-ocean ridges, intra-plate hotspots, and oceanic island arcs. However, we lack a clear understanding of how a submarine eruption interacts with the overlying water column, the structure of the submarine eruptive plume, and the transport of volcanic products in the ocean by these plumes. We describe here the detection of eruption-derived temperature increase and salinity decrease from the 2012 Havre submarine caldera eruption by an existing array of temperature/salinity profiling floats, known as Argo. The Argo floats observed the signal, significant at >3 sigma level with respect to the background ocean variability (noise), at ocean depths of ∼1750–2000 m deeper than the eruptive vent itself. In order to understand this signal, we develop a new conceptual submarine plume model which suggests that the sub-vent level temperature and salinity anomalies are caused by large-scale ocean mixing induced by the eruptive plume, rather than heating directly from the eruption. Using results from scaling analysis and a modified numerical plume-gravity current model, we show that submarine eruptions can efficiently advect water masses from the deep ocean closer to the surface and form multiple subsurface intrusions during the process. Consequently, the submarine eruptive plumes can initiate significant ocean mixing, a process with significant implications for both long- and short-term influence of submarine volcanism on ocean chemistry, dynamics, and ecology. Finally, our model confirms that a detectable anomaly would persist out to 100s of km from the eruption location, comparable to Argo's spatial sampling. Given the global ocean coverage and a decadal baseline, the Argo floats (and other similar datasets), in concert with hydrophone and seismic arrays, can provide significant new constraints on subaqueous volcanism, eruption dynamics, and the impact on Earth systems through enhanced ocean mixing and nutrient supply, e.g. during submarine Large Igneous Provinces.

Denitrification rates across a temperate North Pacific estuary, Yaquina Bay, Oregon.

The extent and temporal variability of denitrification activity was measured in Yaquina Bay, Oregon, over a year using sediment cores collected approximately monthly from August 2003 through August 31, 2004. Denitrification rates in sediments from a marine-dominated intertidal sand flat near the mouth of the estuary averaged 0.181 ±0.114 mmol m-2 d-1 whereas sediments in the estuary (5 stations) and river averaged 0.626 ±0.141 mmol m-2 d-1. Sediment cores from all estuarine sites indicated denitrification activity throughout the year and were within the values reported for other temperate estuaries. Denitrification rates decreased with depth from 0.4 mmol m-2 d-1 in the upper 2 to 5 cm of sediment to 0.006 mmol m-2 d-1 at 28 cm sediment depth, indicating denitrification occurred primarily in the upper 5 cm. There was no relationship between denitrification rate and nitrate concentrations in the overlying water column (r2 = 0.16). Denitrification rates were lowest in areas with low sediment carbon content, particularly in the sandy intertidal areas at the mouth of the estuary (r2 = 0.78). The results suggest that denitrification rates in this estuary were influenced primarily by the availability of organic carbon. The amount of nitrogen removed by denitrification was estimated to be 8.7 percent of the annual Yaquina River load for August 2003 through August 2004. The relatively low percent lost via denitrification may be due to high river discharge when the nitrogen load was greatest during winter storm events and dissolved nitrogen was exported directly from the estuary into the Pacific Ocean. Stable isotopes were used to investigate the carbon source. The carbon isotope data increased from -27 δ13C in the freshwater river to -21.5 δ13C at the seawater site, reflecting a typical change from terrestrial plant vegetation to phytoplankton carbon sources. Similar values for δ13C between suspended and benthic sediments indicated resuspension and mixing occurred during tidal inflow.

The Influence of Sediment‐Derived Dissolved Organic Matter in the Vistula River Estuary/Gulf of Gdansk

AbstractDissolved organic matter (DOM) concentrations in sediment porewaters are often orders of magnitude higher than in the overlying water column resulting in a diffusive flux of DOM from sediments. The intensity and fate of this DOM flux is poorly understood. The Gulf of Gdansk in the Southern Baltic Sea is dominated by the Vistula River, one of the largest and most anthropogenically impacted rivers in the Baltic Sea catchment. The sediment characteristics of the region are varied, from mixed sandy conditions near shore to mud sediments in the Gdansk Deep. We investigated the significance of sediment‐derived DOM in the Gulf of Gdansk in comparison to that supplied by the river. Sediment‐derived DOM in the region was found to have an organic matter fluorescence signature distinct from the DOM in the water column. The visible wavelength fluorescence could be used to distinguish organic matter from near shore sediments influenced by riverine sources and organic matter from deeper offshore sediments, influenced by more pelagic sources. Ultraviolet‐A wavelength fluorescence dominated the sediment flux but was rapidly removed in bottom waters suggesting that it may contribute to bottom water oxygen consumption. While there is potential for DOM fluxes from sediments in the Gulf of Gdansk, the high background pelagic concentration of DOM in these waters and the much stronger influence of the Vistula River can mask the contributions from sediments.

The role of turbulent hydrodynamics and surface morphology on heat and mass transfer in corals.

Corals require efficient heat and mass transfer with the overlying water column to support key biological processes, such as nutrient uptake and mitigation of thermal stress. Transfer rates are primarily determined by flow conditions, coral morphology and the physics of the resulting fluid-structure interaction, yet the relationship among these parameters is poorly understood especially for wave-dominated coral habitats. To investigate the interactive effects of these factors on fluxes of heat and mass, we measure hydrodynamic characteristics in situ over three distinct surface morphologies of massive stony corals in a Panamanian reef. Additionally, we implement a numerical model of flow and thermal transport for both current and wave conditions past a natural coral surface, as well as past three simplified coral morphologies with varying ratios of surface roughness spacing-to-height. We find oscillatory flow enhances rates of heat and mass transfer by 1.2-2.0× compared with unidirectional flow. Additionally, increases in Reynolds number and in surface roughness ratio produce up to a 3.3× and a 2.0× enhancement, respectively. However, as waves begin to dominate the flow regime relative to unidirectional currents, the underlying physical mechanisms mediating transfer rates shift from predominantly turbulence-driven to greater control by inertial accelerations, resulting in larger heat and mass transfer for small surface roughness ratios. We show that for rough corals in wave-dominated flows, novel trade-off dynamics for heat and mass transfer exist between broadly spaced roughness that enhances turbulence production versus narrowly spaced roughness that produces greater surface area. These findings have important implications for differential survivorship during heat-induced coral bleaching, particularly as thermal stress events become increasingly common with global climate change.

Spatially and temporally variable sulfur cycling in shallow-sea hydrothermal vents, Milos, Greece

Shallow-sea hydrothermal systems are ideal for studying the relative contributions to sedimentary sulfur archives from ambient sulfur-utilizing microbes and from fluxes of hydrothermally derived sulfur. Here we present data from a vent field in Palaeochori Bay, Milos, Greece using a suite of biogeochemical analytical tools that captured both spatial and temporal variability in biotic and abiotic sulfur cycling. Samples were collected along a transect from a seagrass meadow to an area of active venting. The abundance and isotopic composition of sulfide captured in situ, together with geochemistry from sedimentary porewaters and the overlying water column and solid phase sulfide minerals, record evidence of ephemeral activity of microbial sulfate reduction as well as sulfide oxidation. The sulfur and oxygen isotope composition of porewater sulfates indicate active sulfate reduction within the transition zone between the vents and seagrass, rapid recycling of biologically produced sulfide within non-vent sediments, and reoxidation of abiotic sulfide within the vent field. A phylogenetic survey of sediments also indicates the pervasive presence of a suite of putative sulfur-metabolizing bacteria, including sulfate reducers and sulfide oxidizers, many of which can utilize intermediate valence sulfur compounds. The isotopic composition of pyrite in these sediments consistently records a microbially influenced signature (δ34Spy of −4.4 to −10.8‰) relative to the hydrothermal endmember (δ34S ~ + 2.5‰), independent of distance from the vent source. The narrow range of pyrite δ34S across sediments with a highly variable hydrothermal influence suggests that physical mixing (e.g., by storm events) homogenizes the distribution of biogenic and hydrothermal Fe-sulfides throughout the region, overprinting the spatially and temporally variable interplay between biological and hydrothermal sulfur cycling in these environments.

Dinitrogen (N2) fixation rates in a subtropical seagrass meadow measured with a direct 15NN2 tracer method

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 605:87-101 (2018) - DOI: https://doi.org/10.3354/meps12755 Dinitrogen (N2) fixation rates in a subtropical seagrass meadow measured with a direct 15N-N2 tracer method Natasha L. Carlson-Perret*, Dirk V. Erler, Bradley D. Eyre Centre for Coastal Biogeochemistry, School of Environmental Science and Engineering, Southern Cross University, Lismore, NSW 2480, Australia *Corresponding author: natasha.carlson-perret@scu.edu.au ABSTRACT: This study used a direct 15N stable isotope labelling technique to measure rates of dinitrogen (N2) fixation within above- and belowground loci of a subtropical seagrass meadow (Zostera muelleri). The total rate of N2 fixation (i.e. sum of the above- and belowground rates) was ~38 µmol N2 m-2 h-1, similar to other rates measured in subtropical systems. Rates of N2 fixation were higher when the 15N-N2 label was added to the surface water compared to when it was added to the sediments. Furthermore, the lowest rates of N2 fixation were observed in the root/rhizome material regardless of whether the 15N-N2 label was added directly to the rhizosphere (0.12 µmol N2 m-2 h-1) or the overlying water column (7.3 µmol N2 m-2 h-1). These results suggest that there was active transport of fixed N from the leaves to the roots of the seagrass plants, in contrast to other studies in which N2 fixation was more active in the rhizosphere. Our study demonstrates the utility of the direct 15N-N2 tracer approach for quantifying the spatial heterogeneity of N2 fixation in complex seagrass environments. KEY WORDS: Nitrogen fixation · Stable isotope tracer · Zostera muelleri Full text in pdf format Supplementary material PreviousNextCite this article as: Carlson-Perret NL, Erler DV, Eyre BD (2018) Dinitrogen (N2) fixation rates in a subtropical seagrass meadow measured with a direct 15N-N2 tracer method. Mar Ecol Prog Ser 605:87-101. https://doi.org/10.3354/meps12755 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 605. Online publication date: October 26, 2018 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2018 Inter-Research.

Contribution of resuspended sedimentary particles to dissolved iron and manganese in the ocean: An experimental study

A number of trace metals play essential roles in marine ecosystem structure and biological productivity. Until recently, it has been argued that phytoplankton access primarily dissolved iron, while particulate iron was considered a refractory material with little use biologically and limited interaction with the dissolved pool. In order to assess the transfer mechanisms between sediment-sourced particulate trace metals and the dissolved pool, we conducted a 14-month incubation that reacted resuspended sediments with natural seawater, both originating from the Kerguelen area (KEOPS cruises; Southern Ocean), in the dark, and at concentrations replicating natural conditions. Three types of sediments were investigated (named BioSi, BioSi + Ca, and Basalt), mostly composed of (i) biogenic silica (bSiO2), (ii) bSiO2 and calcite, and (iii) basaltic fragments, respectively. The release of dissolved silicon (dSi), iron (dFe) and manganese (dMn) was monitored regularly throughout the incubation, as well as living bacteria density and Fe organic ligands. Depending on the origin and composition of the sediment, unique dFe and dMn fluxes were observed, including a strong decoupling between dFe and dMn. The basaltic sediment released up to 1.09 ± 0.04 nmol L−1 of dFe and 0.28 ± 0.09 nmol L−1 of dMn, while the biogenic sediments released a higher 3.91 ± 0.04 nmol L−1 and 8.03 ± 0.42 nmol L−1 of dFe and dMn, respectively. Several factors influencing the release and removal of dFe and dMn were discernable at the temporal sampling resolution of the incubation, including the structural composition of the sediment, bacterial abundance, and the formation of manganese oxides. The regular sampling over short timescales and the extended sampling over one year proved to be critical to constrain the processes and exchanges that govern the contribution of the particulate to the dissolved pools. Overall, this incubation provides a strong basis for reassessing the role of resuspended sedimentary particles in the marine biogeochemical cycles of Fe and Mn. Indeed, we show that biogenic silica, calcite-rich and basaltic particles can contribute substantial dissolved Fe and Mn to the overlying water column. In the future, the global extent of this previously overlooked external metal source should be quantified through further process studies and biogeochemical models.This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.

Impact of an artificial structure on the benthic community composition in the southern North Sea: assessed by a morphological and molecular approach

Abstract Man-made structures in the North Sea are known to act as artificial reefs by providing a habitat for sessile epifauna in a predominantly soft sediment environment. This epifauna is hypothesized to cast a so-called “shadow” over the soft sediment ecosystem by altering the nutrient composition in the overlying water column. In addition, the structure itself could alter currents and thereby influence the deposition and erosion of the sediments in the wake of the platform. This study aims to assess the long-term effects of a gas platform in the southern North Sea on the surrounding benthic community by both morphological and molecular identification of benthic species. The species composition and a set of abiotic factors of the sediment around a gas platform were assessed along four transects. Differences for the abiotic factors were found in the closer vicinity of the platform in the direction corresponding to the predominant currents. The number of benthic fauna families found in the molecular approach were on average three times higher than for the morphological approach. Both approaches showed that small differences occurred primarily due to changes in sedimentary organic matter content. Differences in species composition were more pronounced between transects rather than between distances from the platform.