Sverdrup revisited: Critical depths, maximum chlorophyll levels, and the control of Southern Ocean productivity by the irradiance-mixing regime

Abstract

A reformulation of Sverdrup’s critical-depth calculation, using recent optical and physiological information, is developed and applied to data from the Southern Ocean. Comparisons between calculated critical depths (Zc) and mixed-layer depths (Zm) indicate that both the marginal ice zone and the open waters of the Antarctic Circumpolar Current provide favorable irradiance-mixing regimes for the initiation and early development of phytoplankton blooms in summer (i.e. Zc > Zm when phytoplankton biomass is low and the water clear; that when ice-edge blooms develop, Zc shoals to depths about equal to Zm implying that phytoplankton standing stocks in ice-edge blooms may be self-limiting as a result of reduced penetration of irradiance; and that the highest chlorophyll levels that can be sustained in summer in open waters not stabilized by meltwater are ∼ 1.0 µg liter‒1 in the Weddell and Scotia Seas and may be less in areas that experience stronger winds.