Definition
Sverdrup defines the critical depth as ‘a surface mixing depth at which phytoplankton community growth is precisely matched by losses of phytoplankton biomass within this depth interval.' 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. In his hypothesis, Sverdrup made the approximation that the loss rate for a phytoplankton community is constant at all depths and times.
The depth where the net growth rate is zero is referred to as the compensation depth (only 0.1-1% of solar radiation penetrates). Above this depth, the population is growing while below it, the population shrinks. At a certain depth below it, the total population losses equal the total population gains. This is the critical depth.
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