Peter Wilson

Measuring the sediment content of acid volatile sulfides (AVS), an important determinant of coastal ecosystem functioning, is laborious and therefore rarely considered in routine coastal monitoring. Here, we describe a new approach to estimate the in situ distribution of AVS in subtidal soft sediment. Using amperometric H2S microelectrodes and a flatbed scanner in the laboratory, we first established a strong correlation (R2 = 0.95) between the AVS content (as extracted by cold 1 mol L-1 HCl) and the color intensity of sediment collected at 12 m water depth off the eastern coast of Waiheke Island, New Zealand. We then used this correlation to estimate the distribution of AVS in the upper 20 cm of this sediment from sediment profile images. These images were obtained in situ with a lightweight imaging device consisting of a modified flatbed scanner housed inside a watertight acrylic tube (SPI-Scan™, Benthic Science). We made two types of estimates from the acquired images: First, we obtained a vertical AVS concentration profile by averaging the color intensities of horizontally aligned pixels. Second, we created a two-dimensional distribution plot of AVS concentration by assigning individual pixel color intensities. Because our technique enables assessments of temporal and spatial variations in the AVS content of subtidal soft sediment, we suggest using it in routine coastal monitoring.

We investigated the sediment–seawater solute flux at five sites in the polluted Avon–Heathcote Estuary, New Zealand, to provide a point of comparison for future studies of the effects of the closure of a major wastewater outfall into the estuary. Sediments collected in winters 2007 and 2008, and summer 2008, ranked consistently in organic matter content. Microelectrode profiling and sediment-core incubations revealed (1) a dominant role of microphytes in regulating solute flux causing significant differences in the dark and light sediment O2 consumption (Rd, Rl), total sediment O2 utilisation (TOUd, TOUl), and inorganic nutrient flux, (2) consistent ranking of sites in solute flux, and (3) a clear solute-flux signature of the wastewater effluent. Sediment near the wastewater outfall exhibited the highest absolute R and TOU, the lowest ratio Rl/Rd, the highest dark efflux of dissolved reactive phosphorus and ammonium, and the highest dark and light uptake of nitrate + nitrite.

Periodic disturbance of surface sediment is a natural feature of marine environments. Following exposure to oxygenated seawater, the disturbed sediment oxidises leading to the recovery of its surface chemistry. Despite its importance for the ecology of the sediment–water interface, the dynamics of this recovery is not well known. We studied the effects of disturbance depth and seawater flow speed on the oxidation of estuarine cohesive sediment in a laboratory flume with microelectrodes. We removed surface sediment to 2 depths (5 and 50 mm) and then observed changes in sediment O2 distribution and consumption over 1 h under conditions of slow and fast flow (3.5 and 7.5 cm s–1). Measurements were repeated 1 d later. The consumption of O2 in the treated sediments reached a ‘quasi stable state’ within 7 h (50 mm depth) and 16 h (5 mm depth) characterised by very slow changes due to slow oxidation of reduced solids. Faster flow increased the rate at which sediment from 50 mm depth oxidised but not that of the sediment from 5 mm depth. After 20 to 24 h, sediments disturbed to 50 and 5 mm depths still differed in O2 distribution and consumption, both from each other and from the pre-treatment sediment. Differences in the response of pore water O2 distribution to an abrupt increase in flow speed (3.5 to 7.5 cm s–1) were also still evident at this time. Our measurements confirmed the results of previous theoretical analyses in that they indicate that the duration of the recovery of the surface sediment chemistry from disturbance and the chemical properties of the recovering sediment are controlled by the kinetics of solute and solid oxidation. Oxidation of reduced solids in disturbed sediment can result in a characteristic chemical signature at the sediment surface that lasts in the order of at least days.

Dissolved Zn, Cd, Cu, Fe, and Pb concentrations were measured along a salinity gradient in the Whau Estuary, Auckland, New Zealand. We found a mid-salinity maximum in dissolved Zn and Cd concentrations, consistent with significant loss of these metals from the particulate phase into the dissolved phase. Changes in the chemical speciation of these two metals were coupled to changes in salinity and this was the major driver for Zn and Cd loss from particulate material. Contrastingly, Cu concentrations were conservative with salinity, whereas there was significant scavenging of Fe and Pb from the dissolved phase into the particulate phase. Analysis of sediment pore-water metal concentrations indicated a peak in Zn concentration within the suboxic layer. The peak occurred at a shallower depth than those for Mn and Fe. The concentration gradient across the sediment–water interface suggests that diffusional loss of Zn from the sediment pore water into the overlying water column was occurring. Conversely, the diffusion of Cu from the water column into the sediment pore water was likely to occur because pore-water Cu concentrations were lower than the overlying water column concentrations. The results from the present study show the importance of chemical speciation and the lability of metals attached to particulate material as potentially being a critical determinant on sediment metal concentrations.

The sediment content of acid volatile sulfides (AVS), a proxy for the deposition rate of organic carbon on the seafloor, is an important determinant of coastal soft-sediment ecosystem functioning. The traditional measurement of AVS, however, is laborious and therefore rarely considered in routine coastal monitoring. We developed a rapid method to derive vertical AVS concentration profiles and two-dimensional AVS distribution maps from in situ sediment profile images. The foundation of this method is a strong correlation (R2 = 0.95) between sediment colour intensity and AVS concentration. This method relies on the assumption that all AVS compounds contribute to sediment colour and that they are quantitatively extracted by acid. Despite the strong correlation, two pools do not comply with these assumptions: some dissolved sulfide species are colourless, and some highly coloured sulfide minerals are not extracted quantitatively, if at all. The relative proportions of these pools may change temporally or spatially, which would invalidate the correlation. Therefore, it is important that we have a detailed understanding of sulfur speciation in the sediments we are studying. I will discuss our investigation into sulfur speciation, the implications this has for our technique, and its application to the monitoring of organic enrichment.

Enrichment of coastal sediments with organic carbon is of interest to coastal managers worldwide. The majority of this carbon is mineralised by sulfate reduction, a bacterially mediated reaction that leads to the production of hydrogen sulfide (H2S). H2S readily reacts with sediment iron compounds to form iron sulfides that contribute to the distinct black colouration of organic-rich sediment. Although the black iron sulfides, and therefore AVS, can serve as a proxy for organic carbon enrichment, its measurement has not been used in routine monitoring because of its laborious nature. I will explore the relationship between sediment colour and the concentration of AVS, and how this correlation can be used to develop a rapid method for assessing and monitoring the effects of organic enrichment using sediment profile imagery.

Enrichment of coastal sediment with organic carbon is of interest to coastal managers worldwide.  I will explain how acid volatile sulfide (AVS) can serve as a proxy for organic carbon enrichment and how its measurement can be used in environmental monitoring.  In my proposed research I will explore the relationship between sediment colour and the concentration of AVS to develop a rapid method for the assessment of the effects of organic enrichment using sediment profile imagery.

Sulfur, the 16th element on the periodic table, is one of the predominant chemical species in the ocean that contributes to salinity (in the form of sulfate), and the main terminal process in the anaerobic mineralisation of organic matter in marine sediments.  When marine sediments are extracted in acid, hydrogen sulfide (H2S) is produced.  The sources of the evolved H2S are known as acid volatile sulfides—AVS.  I will attempt to unravel the mysteries of sulfur’s role in marine sediments and explain why measuring AVS can give us insight into the current state of coastal environments.  I will also briefly explain how we can predict the in situ concentration and distribution of AVS with sediment profile imaging.