A persistent subsurface chlorophyll maximum in the Interpolar Frontal Zone south of Australia: Seasonal progression and implications for phytoplankton‐light‐nutrient interactions

Abstract

A subsurface chlorophyll maximum (SCM), composed principally of large diatoms, has been consistently observed in summer and autumn between 53° and 58°S along 140°E on Australian World Ocean Circulation Experiment and Joint Global Ocean Flux Study cruises from 1994 to 1998. This region lies in a zone of weak northward flow, between two branches of the Polar Front (hence the Interpolar Frontal Zone (IPFZ)). In the IPFZ, mixed layer nitrate concentrations are high (>24 μM) year‐round, while mixed layer silicic acid is intermediate (about 15 μM) in winter but is depleted to 2 μM or less in late summer. Dissolved Fe concentrations, only available for summer, are low (<0.2 nM). Mixed layer chlorophyll concentrations are generally <0.3 μg L−1 and surface waters thus fit the high‐nitrate low‐chlorophyll (HNLC) definition. In spring and early summer, the SCM is relatively shallow (about 60 m), intense (up to 1.5 μg chl a, L−1), and contributes 30–50% of column production. By March the SCM is deep (100 m or greater), less intense (about 0.5 μg chl a L−1), and contributes at most 20% of column production. The existence of a SCM in a HNLC region is surprising, and we consider a number of possible explanations. The SCM may be partly explained by changes in C:chl a ratios, but phytoplankton species composition in the SCM also differs from that in the mixed layer. Sinking of diatoms, mediated by Fe and/or silicic acid availability, appears to play an important role in the formation and maintenance of the SCM.