Phytoplankton Photophysiology Traces Iron Stress Hotspot in the South Pacific Ocean

Abstract Marine primary production in the subtropical oceans is strongly regulated by (micro)nutrient availability, yet the types of nutrient stress in the South Pacific remain poorly resolved. Here we assessed this along a >10,000‐km transect of the subtropical South Pacific Ocean (GEOTRACES Section GP21). The transect was separated into three regimes in terms of phytoplankton photophysiology: a heterogeneous coastal margin, an eastern gyre boundary transition zone, and the oligotrophic subtropical gyre. The transition zone exhibited the lowest surface apparent photochemical efficiency ( F v / F m ; 0.09–0.26) and significant responses to experimental supply of both iron (Fe) and combined nitrogen (N) and Fe (with differences relative to controls, Δ F v / F m , of 0.07–0.11) but depressions to N supply alone (Δ F v / F m of −0.03 to −0.07), diagnosing this zone as Fe stressed with low N availability. The offshore coastal margin showed intermediate surface F v / F m (0.21–0.41) that increased after N addition but not Fe alone, suggesting prevalent N limitation and no Fe stress. In the nutrient‐depleted gyre, surface F v / F m was elevated (mean ± SD; 0.42 ± 0.07, n = 41), remained unchanged following any nutrient addition, and showed dawn/dusk peaks with relatively small nocturnal declines (∼33%), consistent with the absence of Fe stress and steady‐state N limitation. Basin‐wide, nitrate to dissolved Fe ratios best predicted surface F v / F m and thereby Fe stress status ( R 2 = 0.54). Additional observations and experiments suggested a basin‐wide absence of Fe stress at the deep chlorophyll maximum. Such findings are important for predicting ecosystem responses to climate‐driven shifts in nutrient supply.