Beyond the Fe-P-redox connection: preferential regeneration of phosphorus from organic matter as a key control on Baltic Sea nutrient cycles

T Jilbert,C P Slomp,B G Gustafsson,W Boer

doi:10.5194/bg-8-1699-2011

Abstract

Abstract. Patterns of regeneration and burial of phosphorus (P) in the Baltic Sea are strongly dependent on redox conditions. Redox varies spatially along water depth gradients and temporally in response to the seasonal cycle and multidecadal hydrographic variability. Alongside the well-documented link between iron oxyhydroxide dissolution and release of P from Baltic Sea sediments, we show that preferential remineralization of P with respect to carbon (C) and nitrogen (N) during degradation of organic matter plays a key role in determining the surplus of bioavailable P in the water column. Preferential remineralization of P takes place both in the water column and upper sediments and its rate is shown to be redox-dependent, increasing as reducing conditions become more severe at greater water-depth in the deep basins. Existing Redfield-based biogeochemical models of the Baltic may therefore underestimate the imbalance between N and P availability for primary production, and hence the vulnerability of the Baltic to sustained eutrophication via the fixation of atmospheric N. However, burial of organic P is also shown to increase during multidecadal intervals of expanded hypoxia, due to higher net burial rates of organic matter around the margins of the deep basins. Such intervals may be characterized by basin-scale acceleration of all fluxes within the P cycle, including productivity, regeneration and burial, sustained by the relative accessibility of the water column P pool beneath a shallow halocline.

Highlights

Hypoxia has been intermittently present in the Baltic Sea throughout the Holocene (Zillen et al, 2008), but its severity and spatial extent have increased greatly over the past 100 years (Conley et al, 2009a; Fonselius and Valderrama, 2003; Jonsson et al, 1990)
Rising anthropogenic nutrient loadings in the early 20th century are believed to have provided the initial trigger to the modern eutrophication of the Baltic, with the resulting increased biological oxygen demand forcing the exacerbation of hypoxia (Savchuk et al, 2008)
The dissolution of P-bearing Fe-oxyhydroxides in surface sediments increases the rate of P regeneration during the transition into hypoxia or anoxia (Einsele, 1936; Mortimer, 1941, 1942)

Summary

Introduction

Hypoxia has been intermittently present in the Baltic Sea throughout the Holocene (Zillen et al, 2008), but its severity and spatial extent have increased greatly over the past 100 years (Conley et al, 2009a; Fonselius and Valderrama, 2003; Jonsson et al, 1990). Rising anthropogenic nutrient loadings in the early 20th century are believed to have provided the initial trigger to the modern eutrophication of the Baltic, with the resulting increased biological oxygen demand forcing the exacerbation of hypoxia (Savchuk et al, 2008). Both eutrophic conditions and hypoxia have persisted to the present day, despite a decline in fertilizer use in the catchment since the 1990s (Emeis et al, 2000).

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Results

Conclusion