Distribution and dynamics of various forms of phosphorus in seawater: insights from field observations in the Pacific Ocean and a laboratory experiment

Abstract

Particulate and dissolved phosphorus (P) measurements including particulate P (PP), soluble reactive P (SRP), dissolved organic P (DOP), and particulate and dissolved hydrophobic P were made at four stations in the Pacific Ocean. SRP concentrations in surface waters varied widely from 0.01 to 0.43 μM. In contrast, DOP exhibited relatively constant values of 0.21–0.37 μM. At oligotrophic ocean stations, DOP was the most significant P pool accounting for 70–98% of total dissolved P (TDP) or 60–90% of total P (TDP+PP) in the surface euphotic zone. DOP concentrations decreased with increasing water depth and reached levels near the detection limit below 1000 m. PP was a minor component accounting for 3–11% of total P in the euphotic zone at all stations. With depth at the oligotrophic stations PP concentrations decreased rapidly, reaching relatively constant levels of ∼0.5 nM below 500 m. Particulate hydrophobic P concentrations in surface waters ranged from 0.29 to 1.72 nM and sharply decreased to <0.1 nM below the euphotic zone. Dissolved hydrophobic P concentrations were much higher ranging from 6.6 to 17.9 nM at the surface and gradually decreased with depth. Hydrophobic P was a minor fraction of surface P pools. During biodegradation of DOP and PP below the euphotic zone, other P compounds are preferentially utilized relative to hydrophobic P. This results in an increased abundance relative to surface waters of hydrophobic P in DOP (18%) and PP (34%) in deep waters. Enrichment of hydrophobic P during biodegradation is supported by a laboratory experiment during which hydrophobic P increased as the percentage of bulk DOP and PP in a seawater sample. These results suggest that hydrophobic P plays two important roles in marine P cycling. In surface waters labile forms of hydrophobic P are a source of bioavailable P that may help sustain production in oligotrophic ocean regions. Export from surface waters of less reactive hydrophobic P fractions acts as a sink of P from marine ecosystems.