Has Sverdrup's critical depth hypothesis been tested? Mixed layers vs. turbulent layers

Author(s): Franks, PJS | Abstract: © 2014 International Council for the Exploration of the Sea. All rights reserved. Sverdrup (1953. On conditions for the vernal blooming of phytoplankton. Journal du Conseil International pour l'Exploration de la Mer, 18: 287-295) was quite careful in formulating his critical depth hypothesis, specifying a "thoroughly mixed top layer" with mixing "strong enough to distribute the plankton organisms evenly through the layer". With a few notable exceptions, most subsequent tests of the critical depth hypothesis have ignored those assumptions, using estimates of a hydrographically defined mixed-layer depth as a proxy for the actual turbulence-driven movement of the phytoplankton. However, a closer examination of the sources of turbulence and stratification in turbulent layers shows that active turbulence is highly variable over time scales of hours, vertical scales of metres, and horizontal scales of kilometres. Furthermore, the mixed layer as defined by temperature or density gradients is a poor indicator of the depth or intensity of active turbulence. Without time series of coincident, in situ measurements of turbulence and phytoplankton rates, it is not possible to properly test Sverdrup's critical depth hypothesis.