Critical Depth - Criticisms

Criticisms

One of the greatest limitations to understanding the cycle of spring phytoplankton blooms is the inability to measure loss rates of phytoplankton in the vertical water column. Sverdrup’s Critical Depth Hypothesis was formulated with the simplifying assumption that loss rates are constant. As more becomes known about phytoplankton loss rate components, Sverdrup’s hypothesis has come under increasing criticism.

Sverdrup himself offered criticism of his model. First, he noted that under heavy grazing pressure, net growth can vary independent of gross production. He also said that advection rather than local growth could be responsible for the bloom he observed. Finally, he mentioned that the first increase in plankton biomass occurred before the shoaling of the mixed layer.

Smetacek and Passow published a criticism in 1990 that challenged the model on the basis that phytoplankton cellular respiration is not constant, but is a function of growth rate, depth, and other factors. They claimed that net growth depended on irradiation, species physiology, and grazing and parasitic pressures in addition to mixed layer depth. They also point out that Sverdrup’s model included respiration of the entire community, including zooplankton. Finally, they note that if averaged over a year instead of 24hrs, production in all water columns (into the sediment) is positive, thus the critical depth could be considered to actually lie beneath the biosphere.