Representative measurement of two-dimensional reactive phosphate distributions and co-distributed iron(II) and sulfide in seagrass sediment porewaters
Highlights
► A new colourimetric DET method for measurement of reactive phosphate is described. ► Colourimetric measurements provide 2D, high resolution distributions of phosphate. ► Colourimetric iron(II) and sulfide characterise sediment biogeochemistry. ► A new perspective of phosphate release in sediment is suggested.
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.
Section snippets
Materials and methods
The preparation of polyacrylamide hydrogels for DET and DGT, the preparation of sediment probes and the colourimetric determination of sulfide and iron(II) concentrations have all been described previously (Robertson et al., 2008). Further details and general methods are provided in the SF.
Development of a colourimetric DET for phosphate
A detailed description of the optimisation of the colourimetric DET for iron(II) has been provided (Robertson et al., 2008). These optimised conditions were used as the starting point for the development of a colourimetric DET for phosphate, but using a staining solution combining reagents from the standard molybdate–ascorbic acid colourimetric method (Eaton and Franson, 2005). Preliminary evaluation of these reagents in a gel layer gave an intense blue colour after 20 min, when laid over
Acknowledgements
This research was supported by a Grant from the Australian Research Council’s Discovery Projects scheme (Project No. DP0559935) and a donation from the Gold Coast City Council, Catchment Management Unit. A. Pagès was also supported with a scholarship from the Griffith Centre for Coastal Management.
References (35)
- et al.
Iron, sulphur and phosphorus cycling in the rhizosphere sediments of a eutrophic Ruppia cirrhosa meadow (Valle Smarlacca, Italy)
J. Sea Res.
(2001) - et al.
Development of constrained DET for measurements of dissolved iron in surface sediments at sub-mm resolution
Sci. Total Environ.
(1998) - et al.
Diffusivity in surficial sediments and benthic mats determined by use of a combined N2O–O2 microsensor
Geochim. Cosmochim. Acta
(1995) - et al.
Two-dimensional determination of dissolved iron and sulfur species in marine sediment pore-waters by thin-film based imaging Thau lagoon (France)
Estuar. Coast. Shelf S.
(2007) - et al.
Micro-scale biogeochemical heterogeneity in sediments: a review of available technology and observed evidence
Earth-Sci. Rev.
(2009) - et al.
Pore water sampling with sediment peepers
TRAC Trends Anal. Chem.
(1995) - et al.
Deep penetrating benthic oxygen profiles measured in situ by oxygen optodes
Deep-Sea Res. I
(2001) - et al.
In situ high resolution measurements of fluxes of Ni, Cu, Fe, and Mn and concentrations of Zn and Cd in porewaters by DGT
Geochim. Cosmochim. Acta
(1995) - et al.
A new ratiometric, planar fluorosensor for measuring high resolution, two-dimensional pCO2 distributions in marine sediments
Mar. Chem.
(2006) - Aller, R. C., 1982. The Effects of Macrobenthos on Chemical Properties of Marine Sediment and Overlying Water, in...