Phosphonates and particulate organic phosphorus cycling in an anoxic marine basin

Abstract

Total particulate phosphorus (TPP), particulate inorganic P (PIP), and particulate organic P (POP) concentrations were measured in a year‐long series of sediment trap samples collected throughout the oxic‐anoxic water column (275 m, 455 m, 930 m, and 1,255 m) of the Cariaco Basin. TPP, PIP, and POP fluxes varied seasonally and decreased significantly with depth, from 65 to 19 µmol TPP m−2 d−1, 43 to 8 µmol PIP m−2 d−1, and 22 to 11 µmol POP m−2 d−1. Significant flux relationships (p < 0.001) were found between POP and particulate organic carbon (POC) and particulate organic nitrogen (PON). The lack of a relationship between POC and PIP fluxes and the large fraction of TPP associated with the PIP pool in both oxic and anoxic traps suggests that future analyses must separate PIP and POP when evaluating biological relationships between C, N, and P. The strong relationships between POC, PON, and POP also suggest that POP is not preferentially remineralized relative to PON and POC with increasing depth in this anoxic environment. P composition was also determined using solid state 31P nuclear magnetic resonance (NMR), and it was found that phosphonates, chemically and thermally inert compounds, are a significant fraction of the TPP pool. Furthermore, these compounds were preferentially removed relative to more bioavailable P esters during a low flux event. Their selective removal suggests that these compounds may be an unrecognized source of bioavailable P under anoxic conditions.