Comparison of Rhizon Sampling and Whole Round Squeezing for Marine Sediment Porewater

The collection and chemical analysis of sedimentary porewater is central to many marine studies. Porewater alkalinity,dissolved inorganic carbon (DIC), sulfate, nitrate, and other dissolved ions are used to identify and determine rates of geochemical reactions and microbial respiration pathways, such as sulfate reduction and denitrification (Froelich et al., 1979; Berner, 1980; Gieskes et al., 1986; D’Hondt et al., 2004; Schulz, 2006; Martin and Sayles, 2007). Ammonium is critical for understanding microbial respiration and the nitrogen cycle (Blackburn, 1988). Chloride is used to reconstruct ocean salinity variations, constrain flow rates, and estimate gas hydrate concentrations (Paull et al., 1996; Adkins et al., 2002; Spivack et al., 2002). Each of these studies requires the recovery of porewater that is not compromised by sampling artifacts

Complex Effects of Ecosystem Engineer Loss on Benthic Ecosystem Response to Detrital Macroalgae

Ecosystem engineers change abiotic conditions, community assembly and ecosystem functioning. Consequently, their loss may modify thresholds of ecosystem response to disturbance and undermine ecosystem stability. This study investigates how loss of the bioturbating lugworm Arenicola marina modifies the response to macroalgal detrital enrichment of sediment biogeochemical properties, microphytobenthos and macrofauna assemblages. A field manipulative experiment was done on an intertidal sandflat (Oosterschelde estuary, The Netherlands). Lugworms were deliberately excluded from 1x m sediment plots and different amounts of detrital Ulva (0, 200 or 600 g Wet Weight) were added twice. Sediment biogeochemistry changes were evaluated through benthic respiration, sediment organic carbon content and porewater inorganic carbon as well as detrital macroalgae remaining in the sediment one month after enrichment. Microalgal biomass and macrofauna composition were measured at the same time. Macroalgal carbon mineralization and transfer to the benthic consumers were also investigated during decomposition at low enrichment level (200 g WW). The interaction between lugworm exclusion and detrital enrichment did not modify sediment organic carbon or benthic respiration. Weak but significant changes were instead found for porewater inorganic carbon and microalgal biomass. Lugworm exclusion caused an increase of porewater carbon and a decrease of microalgal biomass, while detrital enrichment drove these values back to values typical of lugworm-dominated sediments. Lugworm exclusion also decreased the amount of macroalgae remaining into the sediment and accelerated detrital carbon mineralization and CO2 release to the water column. Eventually, the interaction between lugworm exclusion and detrital enrichment affected macrofauna abundance and diversity, which collapsed at high level of enrichment only when the lugworms were present. This study reveals that in nature the role of this ecosystem engineer may be variable and sometimes have no or even negative effects on stability, conversely to what it should be expected based on current research knowledge

Influence of advective bio-irrigation on carbon and nitrogen cycling in sandy sediments

In sandy sediments, the burrow ventilation activity of benthic macrofauna can generate substantial advective flows within the sediment surrounding their burrows. Here we investigated the effects of such advective bio-irrigation on carbon and nitrogen cycling in sandy sediments. To this end, we combined a range of complementary experimental and modelling approaches in a microcosm study of the lugworm Arenicola marina (Polychaeta: Annelida). Bio-irrigation rates were determined using uranine as a tracer, while benthic fluxes of oxygen (O2), total carbon dioxide (TCO2), dissolved inorganic nitrogen (NH4+, ΣNO2− + NO3−) and dinitrogen (N2) were measured in closed-core incubations containing lugworms acclimatized for a relatively short (2 d) and long (3 wk) duration. The fluxes induced by A. marina were compared to those induced by mechanical mimics that simulate the flow pattern induced by the lugworm. These mechanical mimics proved a useful tool to simulate the effect of lugworm irrigation on sediment biogeochemistry. Subsequently, reactive transport model simulations were performed to check the consistency of the measured fluxes and rates, and to construct closed mass balances for sedimentary nitrogen. This reactive transport model successfully captured the essential features of the nitrogen cycling within the sediment. Advective irrigation by both lugworm and mechanical mimics significantly stimulated the sediments O2 consumption, organic matter mineralization rate (TCO2 release), and denitrification rate (N2 production). While sedimentary O2 consumption was directly correlated to advective input of O2, increasing irrigation rates increased the importance of coupled nitrification-denitrification over the external input of nitrate from the overlying water

The role of macrofauna in the functioning of a sea floor: is there any seasonal, density or functional identity effect?

Macrobenthos influences rates and intensities of benthic processes. The way in which these processes are affected depends on their densities and functional characteristics in terms of sediment reworking (bioturbation and bio-irrigation). This study focuses on the importance of three different functional groups (FG) of macrobenthos in the ecosystem processes of the Western Coastal Banks area (Belgian Part of the North Sea). Macrobenthic activity depends on temperature and food availability. Therefore two lab experiments were performed: one before sedimentation of the phytoplankton bloom (spring: low food availability and temperature) and one when organic matter had been settled on the sea bottom (late summer: high food availability and higher temperatures). Single - species treatments of key-species belonging to three different functional groups were added to microcosms at three density levels (average natural, lower and very low) to account for possible density declines. These species are the bivalve Abra alba (FG: biodiffuser), the tube-building polychaete Lanice conchilega (FG: piston-pumper) andthe predatory polychaete Nephthy sp. (FG: regenerator/gallery-diffuser).In both winter - and summertime, L. conchilega had a more pronounced influence on oxygen consumption and release Nephtys sp.. Abra alba appeared to be a more effective sediment reworker than Nephtys sp. in both seasons. In addition, ecosystem functioning (as oxygen consumption by the sediment community and bioturbation) seems to be related to animal densities. As such, a decline of densities (due to anthropogenic or natural disturbances) most probably will decrease the rates of ecosystem functioning in theWestern Coastal Banks area

Restoration of estuarine tidal mudflat sediments after hypoxia

Ecosystem function recovery and benthic community recovery was investigated after experimentally induced depleted oxygen bottom water concentrations in a tidal mudflat (Paulinapolder, Westerschelde estuary). Macrofauna recovery developed through different succession stages and was structured by facilitative and inhibitive interactions: early colonizers had a positive effect on subsequent colonizers, while later succession species negatively affected the stable conditions created by the early colonizing tube-builders. Transitions between different stages were related to changes in environmental characteristics and biotic-environmental interactions (e.g. exploitation competition for food). Nematode community -and biogeochemical recovery were related to macrobenthic succession. Dense polychaete tube aggregations and the development of a fresh diatom bloom, as a result of the low grazing pressure by surface deposit feeding macrofauna during the first stage, stabilized the sediment and thereby enhanced macrobenthic and nematode recruitment success. Bioturbation impact of later succession species increased oxygen input in the sediment, resulting in an enhanced nitrification, denitrification and energy use

Control of confounding in the analysis phase – an overview for clinicians

In observational studies, control of confounding can be done in the design and analysis phases. Using examples from large health care database studies, this article provides the clinicians with an overview of standard methods in the analysis phase, such as stratification, standardization, multivariable regression analysis and propensity score (PS) methods, together with the more advanced high-dimensional propensity score (HD-PS) method. We describe the progression from simple stratification confined to the inclusion of a few potential confounders to complex modeling procedures such as the HD-PS approach by which hundreds of potential confounders are extracted from large health care databases. Stratification and standardization assist in the understanding of the data at a detailed level, while accounting for potential confounders. Incorporating several potential confounders in the analysis typically implies the choice between multivariable analysis and PS methods. Although PS methods have gained remarkable popularity in recent years, there is an ongoing discussion on the advantages and disadvantages of PS methods as compared to those of multivariable analysis. Furthermore, the HD-PS method, despite its generous inclusion of potential confounders, is also associated with potential pitfalls. All methods are dependent on the assumption of no unknown, unmeasured and residual confounding and suffer from the difficulty of identifying true confounders. Even in large health care databases, insufficient or poor data may contribute to these challenges. The trend in data collection is to compile more fine-grained data on lifestyle and severity of diseases, based on self-reporting and modern technologies. This will surely improve our ability to incorporate relevant confounders or their proxies. However, despite a remarkable development of methods that account for confounding and new data opportunities, confounding will remain a serious issue. Considering the advantages and disadvantages of different methods, we emphasize the importance of the clinical input and of the interplay between clinicians and analysts to ensure a proper analysis

Organism-sediment interactions govern post-hypoxia recovery of ecosystem functioning

Hypoxia represents one of the major causes of biodiversity and ecosystem functioning loss for coastal waters. Since eutrophication-induced hypoxic events are becoming increasingly frequent and intense, understanding the response of ecosystems to hypoxia is of primary importance to understand and predict the stability of ecosystem functioning. Such ecological stability may greatly depend on the recovery patterns of communities and the return time of the system properties associated to these patterns. Here, we have examined how the reassembly of a benthic community contributed to the recovery of ecosystem functioning following experimentally-induced hypoxia in a tidal flat. We demonstrate that organism-sediment interactions that depend on organism size and relate to mobility traits and sediment reworking capacities are generally more important than recovering species richness to set the return time of the measured sediment processes and properties. Specifically, increasing macrofauna bioturbation potential during community reassembly significantly contributed to the recovery of sediment processes and properties such as denitrification, bedload sediment transport, primary production and deep pore water ammonium concentration. Such bioturbation potential was due to the replacement of the small-sized organisms that recolonised at early stages by large-sized bioturbating organisms, which had a disproportionately stronger influence on sediment. This study suggests that the complete recovery of organism-sediment interactions is a necessary condition for ecosystem functioning recovery, and that such process requires long periods after disturbance due to the slow growth of juveniles into adult stages involved in these interactions. Consequently, repeated episodes of disturbance at intervals smaller than the time needed for the system to fully recover organism-sediment interactions may greatly impair the resilience of ecosystem functioning.

The effect of tidal forcing on biogeochemical processes in intertidal salt marsh sediments

<p>Abstract</p> <p>Background</p> <p>Early diagenetic processes involved in natural organic matter (NOM) oxidation in marine sediments have been for the most part characterized after collecting sediment cores and extracting porewaters. These techniques have proven useful for deep-sea sediments where biogeochemical processes are limited to aerobic respiration, denitrification, and manganese reduction and span over several centimeters. In coastal marine sediments, however, the concentration of NOM is so high that the spatial resolution needed to characterize these processes cannot be achieved with conventional sampling techniques. In addition, coastal sediments are influenced by tidal forcing that likely affects the processes involved in carbon oxidation.</p> <p>Results</p> <p>In this study, we used in situ voltammetry to determine the role of tidal forcing on early diagenetic processes in intertidal salt marsh sediments. We compare ex situ measurements collected seasonally, in situ profiling measurements, and in situ time series collected at several depths in the sediment during tidal cycles at two distinct stations, a small perennial creek and a mud flat. Our results indicate that the tides coupled to the salt marsh topography drastically influence the distribution of redox geochemical species and may be responsible for local differences noted year-round in the same sediments. Monitoring wells deployed to observe the effects of the tides on the vertical component of porewater transport reveal that creek sediments, because of their confinements, are exposed to much higher hydrostatic pressure gradients than mud flats.</p> <p>Conclusion</p> <p>Our study indicates that iron reduction can be sustained in intertidal creek sediments by a combination of physical forcing and chemical oxidation, while intertidal mud flat sediments are mainly subject to sulfate reduction. These processes likely allow microbial iron reduction to be an important terminal electron accepting process in intertidal coastal sediments.</p

Natural-abundance radiocarbon as a tracer of assimilation of petroleum carbon by bacteria in salt marsh sediments

Author Posting. © The Authors, 2005. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 70 (2006): 1761-1771, doi:10.1016/j.gca.2005.12.020.The natural abundance of radiocarbon (14C) provides unique insight into the source and cycling of sedimentary organic matter. Radiocarbon analysis of bacterial phospholipid lipid fatty acids (PLFAs) in salt-marsh sediments of southeast Georgia (USA) – one heavily contaminated by petroleum residues – was used to assess the fate of petroleum-derived carbon in sediments and incorporation of fossil carbon into microbial biomass. PLFAs that are common components of eubacterial cell membranes (e.g., branched C15 and C17, 10-methyl-C16) were depleted in 14C in the contaminated sediment (mean Δ14C value of +25 ± 19 ‰ for bacterial PLFAs) relative to PLFAs in uncontaminated “control” sediment (Δ14C = +101 ± 12‰). We suggest that the 14C-depletion in bacterial PLFAs at the contaminated site results from microbial metabolism of petroleum and subsequent incorporation of petroleum-derived carbon into bacterial membrane lipids. A mass balance calculation indicates that 6-10% of the carbon in bacterial PLFAs at the oiled site could derive from petroleum residues. These results demonstrate that even weathered petroleum may contain components of sufficient lability to be a carbon source for biomass production by marsh sediment microorganisms. Furthermore, a small but significant fraction of fossil carbon is assimilated even in the presence of a much larger pool of presumably more-labile and faster-cycling carbon substrates.This study was supported by Georgia Sea Grant (RR100-221/926784), the National Science Foundation (OCE-9911678) and NOSAMS (thanks to J. M. Hayes)