Different responses of phytoplankton to Fe manipulation in Fe-limited waters with contrasting surface mixed layer depths in the western subarctic Pacific

In the western subarctic Pacific (WSP), iron (Fe) can enhance the biological carbon pump by stimulating phytoplankton photosynthesis. However, little is known about how Fe and light availability controls the phytoplankton photophysiology in the WSP near the Kuril Islands, where water mixing is sometimes enhanced, even in summer. Here, we conducted on-deck Fe-manipulated incubation experiments at two stations where surface mixed layer depths were distinct but showed similar initial macronutrient and dissolved Fe concentrations and phytoplankton community composition even experienced light availability for phytoplankton in the water column could be different. An Fe addition to water samples from the deeper surface-mixed layer enhanced the phytoplankton biomass and the photosynthetic competence in photosystem II ( F v / F m ), suggesting an Fe-light co-limitation, which was supported by the results from high functional absorption cross section of photosystem II (σ PSII ) and low light saturation index ( E k ). At the shallower mixed layer station, Fe amendment did not stimulate phytoplankton biomass and photosynthesis, indicating that Fe was sufficient for the phytoplankton. Although the centric diatom Chaetoceros species were predominant at both stations throughout incubation, the pennate diatoms Pseudo-nitzschia spp. and Cylindrotheca closterium significantly increased at the deeper and shallower-mixed layer stations, respectively, after Fe enrichment. The elongation of the pennate diatom Neodenticula seminae chains was observed under low Fe availability caused by a chelator. This could be related to the fact that N. seminae is abundant in sediment trap samples from the WSP under Fe-limited conditions and contributed to the high biological pump efficiency.