Critical depth

In biological oceanography, critical depth is defined as a hypothetical surface mixing depth where phytoplankton growth is precisely matched by losses of phytoplankton biomass within the depth interval. This concept is useful for understanding the initiation of phytoplankton blooms. Critical depth as an aspect of biological oceanography was introduced in 1935 by Gran and Braarud. It became prominent in 1953 when Harald Sverdrup published the "Critical Depth Hypothesis" based on observations he had made in the North Atlantic on the Weather Ship M. Sverdrup provides a simple formula based on several assumptions that relates the critical depth to plankton growth and loss rates and light levels. Under his hypothesis, net production in the mixed layer exceeds losses only if the mixed layer is shallower than the critical depth. His hypothesis has often been misconstrued to suggest that spring phytoplankton blooms are triggered when the mixed layer depth shoals to become shallower than the critical depth in the spring. In fact, this is not what Sverdrup intended. Since 1953, further investigation and research has been conducted to better define the critical depth and its role in initiating spring phytoplankton blooms. Recent analysis of satellite data suggest that the theory does not explain all spring blooms, particularly the North Atlantic spring bloom. Several papers have appeared recently that suggest a different relationship between the mixed layer depth and spring bloom timing. Sverdrup defines the critical depth at which integrated photosynthesis equals integrated respiration. This can also be described as the depth at which the integral of net growth rate over the water column becomes zero. The net growth rate equals the gross photosynthetic rate minus loss terms. Gross photosynthesis exponentially decays from a maximum near the surface to approach zero with depth. It is affected by the amount and angle of solar radiation and the clarity of the water. The loss rate is the sum of cellular respiration, grazing, sinking, advection, viral lysis, and mortality.