|Title||Has Sverdrup's critical depth hypothesis been tested? Mixed layers vs. turbulent layers|
|Publication Type||Journal Article|
|Year of Publication||2015|
|Journal||Ices Journal of Marine Science|
|Type of Article||Article|
|Keywords||boundary-layer; critical depth; diffusivity; dissipation; diurnal cycle; equatorial ocean; langmuir turbulence; large-eddy simulation; mixed layer; Mixing; north-atlantic; sea-surface; spring phytoplankton blooms; stratification; surface-layer; Sverdrup; turbulence; upper-ocean|
Sverdrup (1953. On conditions for the vernal blooming of phytoplankton. Journal du Conseil International pour l'Exploration de la Mer, 18: 287295) 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.
In reviewing mixed layers and turbulent layers, I have shown that oth the sources of energy that drive vertical turbulence and the sources of stratification that suppress vertical turbulence are extremely patchy in time and space. The key point, however, is that to properly test the SCD hypothesis one must obtain coincident measurements of the turbulence and the phytoplanktonic rates, as it is the turbulent water motions that move the phytoplankton through the vertical light gradient. Drawing conclusions about the movement of phytoplankton in a mixed layer whose depth is determined by vertical profiles of temperature or density is potentially very misleading.