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Around 30% of oceanic waters are high nitrate low chlorophyll (HNLC) where low iron levels limit algal growth. HNLC waters have mainly been studied using shipboard and lab experiments. Since 1997, remote‐sensing of phytoplankton via SeaWiFS Ocean Color has permitted monitoring of the constancy of this “HNLC condition,” i.e., spatial homogeneity and low temporal variability of chlorophyll over annual cycles. These trends can be exploited, as episodic iron inputs should be conspicuous by subsequent expression as iron‐elevated algal stocks. Subantarctic (SA) waters near New Zealand are HNLC, and the proximity of the arid Australian landmass, and the iron‐rich Subtropical Front, provide natural laboratories to detect episodic atmospheric and oceanic iron supply, respectively. Two approaches were used: Oceanic supply was inferred from episodic increases in chlorophyll concentrations in SA waters …