Benthic phosphorus cycling in the Peruvian oxygen minimum zone

Ulrike Lomnitz,Andrew W Dale,Klaus Wallmann,Anna Noffke,Stefan Sommer,Christian Hensen,Carolin R Löscher

doi:10.5194/bg-13-1367-2016

Abstract

Abstract. Oxygen minimum zones (OMZs) that impinge on continental margins favor the release of phosphorus (P) from the sediments to the water column, enhancing primary productivity and the maintenance or expansion of low-oxygen waters. A comprehensive field program in the Peruvian OMZ was undertaken to identify the sources of benthic P at six stations, including the analysis of particles from the water column, surface sediments, and pore fluids, as well as in situ benthic flux measurements. A major fraction of solid-phase P was bound as particulate inorganic P (PIP) both in the water column and in sediments. Sedimentary PIP increased with depth in the sediment at the expense of particulate organic P (POP). The ratio of particulate organic carbon (POC) to POP exceeded the Redfield ratio both in the water column (202 ± 29) and in surface sediments (303 ± 77). However, the POC to total particulate P (TPP = POP + PIP) ratio was close to Redfield in the water column (103 ± 9) and in sediment samples (102 ± 15). This suggests that the relative burial efficiencies of POC and TPP are similar under low-oxygen conditions and that the sediments underlying the anoxic waters on the Peru margin are not depleted in P compared to Redfield. Benthic fluxes of dissolved P were extremely high (up to 1.04 ± 0.31 mmol m−2 d−1), however, showing that a lack of oxygen promotes the intensified release of dissolved P from sediments, whilst preserving the POC / TPP burial ratio. Benthic dissolved P fluxes were always higher than the TPP rain rate to the seabed, which is proposed to be caused by transient P release by bacterial mats that had stored P during previous periods when bottom waters were less reducing. At one station located at the lower rim of the OMZ, dissolved P was taken up by the sediments, indicating ongoing phosphorite formation. This is further supported by decreasing porewater phosphate concentrations with sediment depth, whereas solid-phase P concentrations were comparatively high.

Highlights

Phosphorus is an essential nutrient; it serves as an energy carrier for all living species and is a limiting macronutrient for marine primary production on geological timescales (Ingall and Jahnke, 1994; Föllmi, 1996; McManus et al, 1997; Filippelli, 2002; Paytan and McLaughlin, 2007; Tsandev et al, 2012; Ruttenberg, 2014)
This suggests that the relative burial efficiencies of particulate organic carbon (POC) and total particulate phosphorus (TPP) are similar under low-oxygen conditions and that the sediments underlying the anoxic waters on the Peru margin are not depleted in P compared to Redfield
Around 30 % of the P discharged to the oceans is potentially bioavailable (Compton et al, 2000) as dissolved P, inorganic P adsorbed to minerals, or associated with metal oxides and P bound within particulate organic matter

Summary

Introduction

Phosphorus is an essential nutrient; it serves as an energy carrier for all living species and is a limiting macronutrient for marine primary production on geological timescales (Ingall and Jahnke, 1994; Föllmi, 1996; McManus et al, 1997; Filippelli, 2002; Paytan and McLaughlin, 2007; Tsandev et al, 2012; Ruttenberg, 2014). Particulate and dissolved phosphorus in the ocean originates from terrestrial chemical weathering of the P containing mineral group of apatite (Filippelli, 2002). Around 30 % of the P discharged to the oceans is potentially bioavailable (Compton et al, 2000) as dissolved P, inorganic P adsorbed to minerals, or associated with metal oxides and P bound within particulate organic matter. The largest fraction of the fluvial P is trapped in estuaries or buried in continental margin sediments and thereby removed from the P cycle before it reaches the open ocean (Compton et al, 2000). The delivery of P to the sediments in the open ocean is mainly composed of organic and inorganic P associated with the export of organic detritus and other particles from the photic zone. P adsorbed to minerals such as Mn and Fe (oxyhydr)oxides (Föllmi, 1996; Delaney, 1998; Faul et al, 2005) are further sources, as well as P input from fish debris

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