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

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

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.

Impact of fiddler crabs and plant roots on sediment biogeochemistry in a Georgia salt marsh

© Inter-Research 2003 · www.int-res.comDOI: 10.3354/meps259237The influence of macrofauna and macrophytes on sediment biogeochemistry was quantified in a Spartina alterniflora (Loisel) saltmarsh, with emphasis on sulfur and iron cycling. Vertical profiles of sediment geochemistry and rates of microbial metabolism at 3 sites with different abundances of fiddler crab Uca pugnax burrows, vegetation coverage and hydrology were supplemented with high-resolution radial profiles around burrow walls and S. alterniflora roots. Carbon oxidation was measured as sulfate reduction using the 35S technique, as total anaerobic CO2 production, and as Fe(III) reduction by monitoring Fe(II) evolution. Depth-integrated (0 to 10 cm) sulfate reduction was 25% lower, while total Fe and Fe(III) concentrations were 1.5 and 6 times higher, respectively, in bioturbated than in nonbioturbated sediment. Low sulfate-reduction rates adjacent to burrow walls (3% of those in bulk sediment) were counteracted by very high Fe(III) reduction rates. Thus, Fe(III) reduction accounted for 54 to 86% of the total carbon oxidation within 4 cm distance of burrows, decreasing in importance with distance from the burrow wall. Overall, S. alterniflora roots showed a greater impact on sediment biogeochemistry than crab burrows. Sulfate reduction was almost absent in the rhizosphere, whereas Fe(III) reduction rates (6.2 µmol Fe cm-3 d-1) were among the highest reported for marine sediments, accounting for >99% of carbon oxidation. Our results confirm the universal relationship between the contribution of Fe(III) respiration to total carbon oxidation and solid Fe(III) concentrations that has been suggested based on studies of subtidal marine sediments. The importance of Fe(III) respiration was strongly dependent on Fe(III) concentrations below levels of 30 µmol cm-3, whereas above this level almost all anaerobic respiration was mediated by Fe(III) reduction in saltmarsh sediments

Freshwater marshes as dissolved silica recyclers in an estuarine environment (Schelde estuary, Belgium)

Compared to knowledge about N and P processing in the aquatic continuum of lakes, wetlands and estuaries, knowledge concerning transport and cycling of Si is only fragmentary. Furthermore, Si research in estuaries has mainly been focused on subtidal benthic sediments and uptake and recycling by diatom communities. The biogeochemical cycling of Si in tidal wetlands, which can contain large amounts of Si, has thus far been neglected. We have conducted several whole ecosystem Si mass-balances on a freshwater marsh located in the Schelde estuary (6 tidal cycles, 2 with BSi included). Our measurements show that the freshwater marsh acts as an important source of dissolved Si to the main river (1–18% more export than import, on average 0.114 g m–2). This export is compensated by import of amorphous silica into the marsh (19–55% more import than export). The marsh was shown to act as silica recycler, resupplying biologically available dissolved Si to the estuarine ecosystem. Extrapolations show that during summer and spring months, when dissolved silica is depleted due to diatom growth, almost half of the total dissolved silica load in the main river channel could result from marsh recycling. [KEYWORDS: estuary ; dissolved silica recycling ; biogenic silica ; eutrophication ; freshwater marshes]

N and Si cycling in freshwater tidal marshes

Tidal marshes have high sediment surface areas, promoting processes affecting nutrient speciation, transformation and retention. In this project we performed whole-ecosystem 15NH4 + tracer addition experiments to quantify the fate and transport of ammonium through a tidal freshwater marsh. Combined with a mass-balance study, this approach allowed a simultaneous examination of transport and processing of nitrogen. We also performed additional studies on Si cycling in parallel to the N-labeling experiment. Our work shows that the large reactive surface of the tidal freshwater marsh vegetation is crucial for nitrogen transformation and assimilation. It clearly revealed the dominant role of microbes in initial nitrogen retention in marsh ecosystems. Parallel study of silica cycling revealed that tidal marshes act as biogenic Si recycling surfaces, importing biogenic Si while exporting dissolved Si. Export of dissolved Si is greatest during summer and spring, when dissolved Si concentrations in inundating waters are depleted by diatoms. Tidal marshes thus buffer estuarine dissolved Si in times of limitation. We can conclude that the studied marsh had an increasing effect on the Si:N ratio in flooding waters. Export of DSi and import of total dissolved nitrogen (DIN) contributed about equally to the increase of the Si:N ratio

**Phragmites australis** and silica cycling in tidal wetlands

Tidal marshes have recently been shown to be important biogenic Si recycling surfaces at the land-sea interface. The role of vegetation in this recycling process has not yet been quantified. In situ and ex situ decomposition experiments were conducted with Phragmites australis stems. In a freshwater tidal marsh, litterbags were incubated at different elevations and during both winter and summer. Biogenic Si (BSi) dissolution followed a double exponential decay model in the litterbags (from ca. 60 to 15 mg g(-1) after 133 days), irrespective of season. Si was removed much faster from the incubated plant material compared to N and C, resulting in steadily decreasing Si/N and Si/C ratios. Ex situ, decomposition experiments were conducted in estuarine water, treated with a broad-spectrum antibiotic, and compared to results from untreated incubations. The bacteria] influence on the dissolution of dissolved Si (DSi) from R australis stems was negligible. Although the rate constant for dissolved Si dissolution decreased from 0.004 to 0.003 h(-1), the eventual amount of BSi dissolved and saturation concentration in the incubation environment were similar in both treatments. P. australis contributes to and enhances dissolved Si recycling capacity of tidal marshes: in a reed-dominated small freshwater tidal marsh, more than 40% of DSi export was attributable to reed decomposition. As the relation between tidal marsh surface and secondary production in estuaries has been linked to marsh Si cycling capacity, this provides new insight in the ecological value of the common reed. (C) 2007 Elsevier B.V. All rights reserved

Decomposing curves for <i>Ulva</i> detritus in lugworm excluded (treatment A−; square symbols in the graphs) or procedural control sediments (treatment A+; triangle symbols in the graph).

<p>Symbols indicate mean (± SE) for macroalgal biomass and are relative to experiments 2.</p