Episodic enhancement of phytoplankton stocks in New Zealand subantarctic waters: contribution of atmospheric and oceanic iron supply

Abstract

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, detected using Ocean Color archives. Additional archives were used to confirm the oceanic provenance of iron supply, and identify supply mechanism(s). Atmospheric supply was assessed using data on source areas and loads for dust storms monitored in central Australia. Dust transport and its fate was assessed using air mass forward trajectories and SeaWiFS Ocean Color and Aerosol Optical Depth maps. During 1997–2001, episodic elevated chlorophyll events occurred in SA waters southeast of New Zealand. There was no evidence of these events being mediated by atmospheric iron supply; however, neither wind-driven lateral advection or vertical mixing alone could account for these episodes. Dust storms, over this period sent plumes either into high iron SubTropical (ST) waters or into SA waters in early spring, when cells are probably light- rather than iron-limited.