Representative measurement of two-dimensional reactive phosphate distributions and co-distributed iron(II) and sulfide in seagrass sediment porewaters

Abstract

The high degree of heterogeneity within sediments can make interpreting one-dimensional measurements difficult. The recent development and use of in situ techniques that measure two-dimensional distributions of porewater solutes have facilitated investigation of the role of spatial heterogeneity in sediment biogeochemistry. A colourimetric diffusive equilibration in thin films method has been developed that allows two-dimensional, high-resolution measurement of reactive phosphate in sediment porewaters. A method detection limit of 0.22 μM, an effective upper limit of ∼1000 μM and relative standard deviations typically below 5% were achieved. This method was evaluated by deployment in seagrass (Zostera capricorni) colonised sediments, as part of combined probes with similar colourimetric methods for sulfide and iron(II). The two-dimensional, high resolution distributions obtained provide a highly representative measurement of the co-distributions of porewater solutes, allowing heterogeneous features and biogeochemical processes to be observed and interpreted. Microniches of high phosphate concentration >100 μM were observed throughout the distributions and were interpreted to be due to localised zones of rapid organic matter mineralisation, possibly using electron acceptors other than iron(III) oxyhydroxides (e.g. aerobic respiration) as often they did not correspond with microniches of higher Fe(II) concentration.

Introduction

Sediments have long been recognised as heterogeneous environments, with conceptual models of likely causes (e.g. animal burrows, faecal pellets) developed before chemical measurements were able to directly observe this heterogeneity (Aller, 1982). Sediment researchers often invoke these causes to explain variability in concentration profiles (Aller and Aller, 1998, Deborde et al., 2008). As one-dimensional (1D) profiles were obtained at greater resolution, such as with microelectrodes (Glud et al., 1995, Reimers, 1987), DET (diffusive equilibration in a thin film) techniques (Davison et al., 1991, Fones et al., 1998), micro-optode sensors (Klimant et al., 1995) and DGT (diffusive gradients in thin films) techniques (Fones et al., 2001, Zhang et al., 1995), a greater understanding was obtained of many sediment biogeochemical processes. Clear evidence for heterogenous features was obtained from high resolution oxygen profiles (Glud et al., 1995, Reimers, 1987). Nonetheless, interpretation of profile features obtained outside of well-defined environments (e.g. biofilms) often remained a challenge (Wenzhöfer et al., 2001).

Major breakthroughs were achieved with the modification of techniques to measure two-dimensional (2D) distributions of porewater solutes at mm resolution or better (Stockdale et al., 2009), including planar optodes (Glud et al., 1996, Hulth et al., 2002), DGT (Davison et al., 1997, Teasdale et al., 1999) and DET (Jézéquel et al., 2007, Robertson et al., 2008, Shuttleworth et al., 1999). The presence and influence of heterogeneous features such as sulfidic microniches (Devries and Wang, 2003, Widerlund and Davison, 2007), faunal burrows (Robertson et al., 2008, Zhu et al., 2006) and iron hot-spots within sulfidic sediment (Robertson et al., 2009) were now able to be highlighted. Historical concepts of sediment heterogeneity and the utility of recently developed techniques have been thoroughly reviewed (Stockdale et al., 2009). Colourimetric DET and DGT methods (Jézéquel et al., 2007, Robertson et al., 2008, Robertson et al., 2009, Teasdale et al., 1999) appear particularly promising to develop a range of 2D, high-resolution techniques. Any selective chemical reaction that produces a distinct colour change when an analyte is bound or precipitated is potentially suitable for a colourimetric method coupled to a 2D thin film method. The colour change can be quantified using an approach called computer-imaging densitometry (CID) (Teasdale et al., 1999). A description of DET and DGT methods is in the Supplementary files (SF).

The aims of this study were to develop and evaluate an in situ colourimetric DET method for reactive phosphate which would provide 2D, high-resolution distributions in sediment porewaters. The developed method was evaluated by deployment in seagrass colonised sediments, as part of combined probes with colourimetric methods for sulfide and iron(II). Seagrass sediments were chosen for this evaluation as they were expected to present a range of phosphate, sulfide and iron(II) concentrations and a high likelihood of heterogeneous features (Pedersen et al., 1998, Welsh, 2000). The 2D DET measurements obtained were evaluated to assess how representative they were of porewater phosphate distributions.