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Representative measurement of two-dimensional reactive phosphate distributions and co-distributed iron(II) and sulfide in seagrass sediment porewaters
Authors:Pagès Anaïs  Teasdale Peter R  Robertson David  Bennett William W  Schäfer Jörg  Welsh David T
Institution:a Environmental Futures Centre, Griffith University, Gold Coast Campus, QLD 4222, Australia
b Université de Bordeaux UMR 5805 EPOC, Avenue des Facultés, 33405 Talence, France
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.
Keywords:Diffusive equilibration in a thin film (DET)  Diffusive gradients in a thin film (DGT)  Computer imaging densitometry  High resolution  Sediment heterogeneity  In situ measurement
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