Investigating biogenic heterogeneity in coastal sediments with two-dimensional measurements of iron(II) and sulfide

Abstract

Environmental context. Microbial respiration generally occurs in distinct layers within coastal sediment, producing high porewater iron or sulfide concentrations, although this layering is dramatically modified by the activities of sediment-dwelling organisms. The present study describes use of a new technique to simultaneously measure two-dimensional concentrations of porewater iron and sulfide at millimetre resolution, allowing the patchiness of patterns of microbial respiration in sediment to be clearly observed. The measurements generally supported a conceptual model predicting the effects of animal burrows and seagrass roots on the porewater iron and sulfide distributions, although the addition of organic matter provided some unexpected observations that require further investigation. Abstract. One of the most powerful predictive tools in sediment biogeochemistry is the electron acceptor layering model, which describes the order in which oxidised compounds are reduced by successions of respiring microbial populations, and how this layering is influenced by benthic macro-organism activity. However, techniques allowing convenient determination of heterogeneous distributions of reduced substances, such as iron(II) and sulfide, have been lacking. A combined diffusive gradients in thin films–diffusive equilibrium in thin films technique was used to quantitatively measure the two-dimensional iron(II) and sulfide distributions at high resolution in the vicinity of various sediment features, including macrofauna burrows, particulate organic matter and macrophyte roots. Substantial heterogeneity was observed for both analytes in all probes, especially in the vicinity of seagrass roots and particulate organic matter. Measured distributions tended to follow the general patterns predicted by the tertiary electron acceptor layering model. However, there was unexpected overlap of sulfide and iron(II) distributions at the millimetre to centimetre scale in several samples from different sediments, notably the more complex sediments containing particulate organic matter and seagrass roots. The cause of such overlap is unclear and further study is necessary to elucidate how such distributions can occur.

Spatial oxygen heterogeneity in a Hediste diversicolor irrigated burrow

“…iron (II) and sulphide). Two-dimensional zonations of these reduced compounds have been shown to be highly heterogeneous in bioturbated sediments, particularly in the vicinity of burrows and roots (Robertson et al, 2009;Pagès et al, 2011), which could potentially drive variable fluxes of both reduced compounds and oxygen in the different part of the burrows.…”

Interactive influences of the marine yabby (Trypaea australiensis) and mangrove (Avicennia marina) leaf litter on benthic metabolism and nitrogen cycling in sandy estuarine sediment

“…Particle reworking results in the mixing of organic matter to depth and infauna burrows increase the area of the sediment-water interface favouring solute exchange with the overlying water (Welsh, 2003). Ventilation of these burrows by their residents transports oxygen-rich water to the deeper sediment, influencing the distribution of oxic, suboxic and anoxic sediment zones (Wenzhöfer & Glud, 2004;Robertson et al, 2008Robertson et al, , 2009. Therefore, burrow wall sediments and infauna can provide a substrate for colonising aerobic microbial communities, including nitrifying bacteria (Welsh & Castadelli, 2004;Laverock et al, 2010).…”

Two decades of chemical imaging of solutes in sediments and soils – a review

“…The most recent developments in 2D imaging include simple colorimetric DET techniques for measuring Fe(II) and phosphate [15,16,[35][36][37][38] as well as a 2D DET technique for the measurement of NO 3 À and N 2 isotopic composition [39]. These approaches are discussed in more detail below.…”

“…For the investigation of sediment chemistry, typical DET and DGT imaging (and 1D profile) applications were done using flat ($0.5 cm) gel housings that have a sampling window of 2-5 Â 15 cm [11,14,34,49,62]. Recently, a larger and thicker (1.4 cm) gel holder with an 8 Â 17 cm window was used for dual-layer DET-DGT studies [16,36,37]. For sampling, these gel setups are pushed into the sediment, and left for a given period for equilibration or analyte uptake, respectively.…”

“…(1) and (2) M is the mass of analyte bound to the resin gel, Dg is the diffusive layer thickness, D is the diffusion coefficient in the hydrogel, A is the sampling area, and t is the sampling time [41,60]. DGT chemical imaging has so far been used to investigate sediment biogeochemistry as affected by plant roots and animal burrows [15,[36][37][38]53,61], solute dynamics in microbial microniches in sediments [13,16,36,[47][48][49]56,[62][63][64][65] as well as for investigating nutrient and contaminant uptake and solubilisation in the vicinity of terrestrial plant roots [12,50,59]. …”