Abstract
Recycling of phosphorus (P) from sediments contributes to the development of bottom-water hypoxia in many coastal systems. Here, we present results of a year-long assessment of P dynamics in sediments of a seasonally hypoxic coastal marine basin (Lake Grevelingen, the Netherlands) in 2012. Sequential phosphorus extractions (SEDEX) and X-ray absorption spectroscopy (XAS) indicate that P was adsorbed to Fe-(III)-(oxyhydr)oxides when cable bacteria were active in the surface sediments in spring. With the onset of summer hypoxia, sulphide-induced dissolution of the Fe-(III)-(oxyhydr)oxides led to P release to the pore water and overlying water. The similarity in authigenic Ca-P concentrations in the sediment and suspended matter suggest that Ca-P is not formed in situ. The P burial efficiency was ≤ 32%. Hypoxia-driven sedimentary P recycling had a major impact on the water-column chemistry in the basin in 2012. Water-column monitoring data indicate up to ninefold higher surface water concentrations of phosphate in the basin in the late 1970s and a stronger hypoxia-driven seasonal P release from the sediment. The amplified release of P from the sediment in the past is attributed to the presence of a larger pool of Fe-bound P in the basin prior to the first onset of hypoxia. Given that P is not limiting, primary production in the basin has not been affected by the decadal changes in P availability and recycling over the past 40 years. The changes in P dynamics on decadal time scales were not recorded in sediment profiles of total P or organic C/total P.
Highlights
Phosphorus (P) is an essential nutrient for marine organisms (Arrigo 2005)
A seasonal succession in the dominant type of sulphur-oxidising bacteria in the sediments was observed from 2011 to 2015 with cable bacteria dominating in spring and Beggiatoaceae in autumn (Seitaj et al 2015)
In spring, when cable bacteria are active, an Fe-(III)-(oxyhydr)oxiderich surface layer is formed from upward diffusing Fe2+ released through FeS dissolution (Fig. 5)
Summary
Phosphorus (P) is an essential nutrient for marine organisms (Arrigo 2005). Human activities on land, such as the widespread use of fertiliser and the release of waste water to streams and rivers have led to increased inputs of nutrients to many coastal environments since the 1950s (Beusen et al 2016; Diaz and Rosenberg 2008; Rabalais et al 2010; Ruttenberg 2003). The number of coastal environments suffering from hypoxia is still increasing worldwide (Diaz and Rosenberg 2008; Rabalais et al 2014). In sediments overlain by oxic bottom waters, phosphate released to the pore water from degrading sediment organic matter can be bound to iron (Fe)(oxyhydr)oxides (Anschutz et al 1998; Mort et al 2010; Estuaries and Coasts (2018) 41:921–939
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