SVERDRUP REVISITED - CRITICAL DEPTHS, MAXIMUM CHLOROPHYLL LEVELS, AND THE CONTROL OF SOUTHERN-OCEAN PRODUCTIVITY BY THE IRRADIANCE-MIXING REGIME

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 (Z(c)) and mixed-layer depths (Z(m)) 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. Z(c) > Z(m) when phytoplankton biomass is low and the water clear; that when ice-edge blooms develop, Z(c) shoals to depths about equal to Z(m), 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 approximately 1.0-mu-g liter-1 in the Weddell and Scotia Seas and may be less in areas that experience stronger winds.