Iron‐Phosphorus Feedbacks Drive Multidecadal Oscillations in Baltic Sea Hypoxia

Abstract

Marginal marine basins such as the Baltic Sea, Black Sea and many estuaries are naturally susceptible to hypoxia due to stratification and isolation of deep water masses (Conley et al., 2009;Eckert et al., 2013;Lefort et al., 2012). Hypoxia has been further exacerbated in modern coastal systems by anthropogenic nutrient inputs, leading to an expansion of marine 'dead zones' worldwide (Rabalais et al., 2010). Positive feedbacks in the phosphorus (P) cycle play a key role in intensifying hypoxic conditions in aquatic systems. Phosphorus release from sediments accelerates under low-oxygen conditions, due to the dissolution of iron oxide-bound P (Fe-P; Mortimer, 1941) and the preferential regeneration of P from organic matter (Ingall et al., 1993). Enhanced sedimentary P release, in turn, sustains high productivity and oxygen demand (Vahtera et al., 2007;Van Cappellen & Ingall, 1994).An important caveat to the positive feedback between P cycling and hypoxia is that in spatially finite systems, the release of Fe-P from sediments cannot proceed indefinitely. Low-oxygen conditions at any given location may rapidly exhaust the local sedimentary inventory of Fe-P (Jilbert et al., 2011;Reed et al., 2011), while the

Contrasting distribution and speciation of sedimentary organic phosphorus among different basins of the Baltic Sea

“…Deoxygenation, in turn, can mobilize stored phosphate due to reductive dissolution of P bearing Fe and Mn (oxy)(hydr)oxides (Jilbert et al 2011) and preferential degradation of poly‐P (Brock and Schulz‐Vogt 2011; Jones et al 2016), and can then be recycled to the bottom water. This process terminates quickly and cannot be sustained without the presence of poly‐P or P bearing Fe and Mn minerals to perpetuate benthic P release (Jones et al 2016; Jilbert et al 2021).…”