Title:

Supply and consumption of dissolved organic phosphorus across the subtropical Indian Ocean

The Indian Ocean is an understudied region for marine phosphorus (P) biogeochemistry. Emerging evidence indicates marine phytoplankton can adapt to decreasing supplies of inorganic P, a required nutrient for growth, by alternatively utilizing organic forms of P. Here we investigate the dissolved organic phosphorus concentration ([DOP]) distribution across the southern Indian Ocean using observations collected on the I05 US GO-SHIP Cruise (2023) across ~33ºS latitude. We quantify the longitudinal variability of [DOP] in the upper 350 m of the Indian Ocean and infer its rate of biological consumption from surface waters, contributing as an organic nutrient to sustain marine autotrophs. Higher [DOP] along the eastern flank of the western gyre’s edge at 52ºE (~0.085 ± 0.03 µmol/kg) compared to the gyre’s center to the west at 40ºE (~0.026 ± 0.015 µmol/kg) suggests a biological consumption of ~0.06 µmol/kg within the upper 50 meters over a period of ~2.2 years, diagnosed from local surface drifter current velocities. The resulting autotrophic DOP consumption flux is 1.4 ± 0.05 mmol m-2 yr-1; three times higher than previous model estimates for this region. The ability of phytoplankton to utilize DOP in addition to PO43- when inorganic nutrient supply is depleted shows resilience that could be critical for sustaining ocean ecosystems undergoing warming and the associated redistribution of ocean surface currents.

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