Influence of Iron, Cobalt, and Vitamin B₁₂ Supply on Phytoplankton Growth in the Tropical East Pacific During the 2015 El Niño

Abstract

Abstract Iron (Fe), cobalt (Co), and vitamin B 12 addition experiments were performed in the eastern Equatorial Pacific/Peruvian upwelling zone during the 2015 El Niño event. Near the Peruvian coastline, apparent photosystem II photochemical efficiencies ( F v / F m ) were unchanged by nutrient addition and chlorophyll a tripled in untreated controls over 2 days, indicating nutrient replete conditions. Conversely, Fe amendment further away from the coastline in the high nitrate, low Fe zone significantly increased F v / F m and chlorophyll a concentrations. Mean chlorophyll a was further enhanced following supply of Fe + Co and Fe + B 12 relative to Fe alone, but this was not statistically significant; further offshore, reported Co depletion relative to Fe could enhance responses. The persistence of Fe limitation in this system under a developing El Niño, as previously demonstrated under non‐El Niño conditions, suggests that diminished upwelled Fe is likely an important factor driving reductions in offshore phytoplankton productivity during these events.