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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.

[Image: see text] Oxygen depletion in coastal waters may lead to release of toxic sulfide from sediments. Cable bacteria can limit sulfide release by promoting iron oxide formation in sediments. Currently, it is unknown how widespread this phenomenon is. Here, we assess the abundance, activity, and biogeochemical impact of cable bacteria at 12 Baltic Sea sites. Cable bacteria were mostly absent in sediments overlain by anoxic and sulfidic bottom waters, emphasizing their dependence on oxygen or nitrate as electron acceptors. At sites that were temporarily reoxygenated, cable bacterial densities were low. At seasonally hypoxic sites, cable bacterial densities correlated linearly with the supply of sulfide. The highest densities were observed at Gulf of Finland sites with high rates of sulfate reduction. Microelectrode profiles of sulfide, oxygen, and pH indicated low or no in situ cable bacteria activity at all sites. Reactivation occurred within 5 days upon incubation of an intact sediment core from the Gulf of Finland with aerated overlying water. We found no relationship between cable bacterial densities and macrofaunal abundances, salinity, or sediment organic carbon. Our geochemical data suggest that cable bacteria promote conversion of iron monosulfides to iron oxides in the Gulf of Finland in spring, possibly explaining why bottom waters in this highly eutrophic region rarely contain sulfide in summer.

Molybdenum (Mo) enrichments in marine sediments are a common indicator of the presence of sulphide near the sediment-water interface and can thereby record historic bottom-water oxygen depletion. Here, we assess the impact of temporal changes in manganese (Mn) cycling and bottom-water oxygen on sedimentary Mo dynamics in a seasonally-hypoxic coastal marine basin (Lake Grevelingen, the Netherlands). High resolution line scans obtained with LA-ICP-MS and discrete sample analyses reveal distinct oscillations in Mo with depth in the sediment. These oscillations and high sediment Mo concentrations (up to ~ 130 ppm) are attributed to deposition of Mo-bearing Mn-oxide-rich particles from the overlying water, the release of molybdate (MoO 4 2 − ) to the pore water upon reduction of these Mn-oxides, and subsequent sequestration of Mo. The latter process only occurs in summer when sulphide concentrations near the sediment-water interface are elevated. Gravitational focussing of Mn oxides explains the observed increased input of Mo with increasing water depth. Diffusion of MoO 4 2 − from the overlying water contributes only a small amount to the sediment Mo enrichments. Cable bacteria may indirectly impact sediment Mo dynamics by dissolving Mn-carbonates and thereby enhancing the pool of Mn-oxides in the system, and by contributing to remobilisation of sediment Mo during oxic periods. A sediment record that spans the past ~ 45 years indicates that sediment Mo concentrations have increased over the past decades, despite less frequent occurrences of anoxia in the bottom waters based on oxygen measurements from water column monitoring. We suggest that the elevated Mo in recent sediments reflects both enhanced rates of sulphate reduction and sulphide production in the surface sediment as a result of increased input of organic matter into the basin from the adjacent North Sea since 1999, and an associated enhanced “Mn refluxing” in the marine lake in summer.