In situ, high-resolution imaging of labile phosphorus in sediments of a large eutrophic lake

• The concentration of DGT-labile P showed strong variation mostly ranging from 0.01 to 0.35 mg L−1.
• The apparent diffusion flux of P across the SWI was calculated between −21 and 65 ng cm−2 d−1.
• Sediments were respectively source and sink of overlying water P in algal and macrophyte regions.
• Better correlation existed between overlying water P and DGT-labile P than by extracted labile P.
• In situ DGT measurement was a more reliable approach in monitoring of labile P in sediments.

Understanding the labile status of phosphorus (P) in sediments is crucial for managing a eutrophic lake, but it is hindered by lacking in situ data particularly on a catchment scale. In this study, we for the first time characterized in situ labile P in sediments with the Zr-oxide diffusive gradients in thin films (Zr-oxide DGT) technique at a two-dimensional (2D), submillimeter resolution in a large eutrophic lake (Lake Taihu, China, with an area of 2338 km2). The concentration of DGT-labile P in the sediment profiles showed strong variation mostly ranging from 0.01 to 0.35 mg L−1 with a considerable number of hotspots. The horizontal heterogeneity index of labile P varied from 0.04 to 4.5. High values appeared at the depths of 0–30 mm, likely reflecting an active layer of labile P under the sediment–water interface (SWI). Concentration gradients of labile P were observed from the high-resolution 1D DGT profiles in both the sediment and overlying water layers close to the SWI. The apparent diffusion flux of P across the SWI was calculated between −21 and 65 ng cm−2 d−1, which showed that the sediments tended to be a source and sink of overlying water P in the algal- and macrophyte-dominated regions, respectively. The DGT-labile P in the 0–30 mm active layer showed a better correlation with overlying water P than the labile P measured by ex situ chemical extraction methods. It implies that in situ, high-resolution profiling of labile P with DGT is a more reliable approach and will significantly extend our ability in in situ monitoring of the labile status of P in sediments in the field.

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