'Critical Depth', Formal definition: A hypothesized surface mixing depth at which phytoplankton growth is precisely matched by looses of phytoplankton biomass within this depth interval [1].

Critical depth as an aspect of biological oceanography was introduced in 1935 by Gran and Braarud. It became important in 1955 when Harald Sverdrup published the ‘“Critical Depth Hypothesis’”. From 1953 to 2010 further investigation and research has been done to better define the importance of the critical depth and its role in initiating spring algal blooms. Recent analysis of satellite data suggest that the theory is not applicable to all spring blooms, particularly the North Atlantic spring bloom. One of the greatest limitations to understanding the cycle of spring phytoplankton blooms is the inability to measure respiration in the vertical water column. Sverdrup’s Critical Depth Hypothesis was formulated with the assumption that respiration is constant with increasing depth. A more recent investigation published by Behrenfeld in 2010 models a vertical profile in which respiration is not constant with depth; conclusions from this model challenge Sverdrup’s theory, suggesting that the deepening of the mixed layer causes net population growth rate to exceed losses.

[edit] Critical Depth Hypothesis Sverdrup’s theory was based on observations he had made in the North Atlantic on the Weather Ship ‘’’M’’'. Sverdrup defines the critical depth as ‘a surface mixing depth at which phytoplankton growth is precisely matched by losses of phytoplankton biomass within this depth interval’ [1]. Sverdrup’s research results conclude that the shoaling of the ‘’’mixed layer depth’’’ to a depth above the critical depth is the cause of spring blooms. If these conditions are met, the surface wind mixing is only bringing phytoplankton as deep as 1% irradiance. When the mixed layer depth exceeds the critical depth, the mixing brings phytoplankton out of range from photosynthetic available radiation (PAR), resulting in a decrease in phytoplankton growth, and a net decrease in phytoplankton biomass. Sverdrup’s model is a cause and effect relationship between the depth of the wind mixed layer and the bloom of phytoplankton [2]. The critical depth hypothesis is displayed by Miller (2004) in theoretical water column, in which he assumes constant respiration with increasing depth. Biomass is represented by 1/P dP/dt of a depth interval, were P is photosynthesis, and t is time. When the interval is equal to zero it represents the depth in the water column were biomass gains are balanced by biomass losses- it represents the ‘’’critical depth’’’. Below the critical depth the biomass gain is outweighed by the biomass losses (respiration, grazing, parasites, virus’, sinking). Above this depth the gross photosynthesis exceeds the loss of biomass due to respiration, grazing, etc., and there is a net increase in phytoplankton biomass [2]. A relationship exists between the critical depth, the shoaling of the mixed layer depth, and the concentration of phytoplankton. The Critical depth deepens with time because of the change in the angle solar radiation hits the earth at 45 degrees N. The shoaling of the mixed layer over time is the result of a decreases winter storminess. During the intense winter mixing, while the phytoplankton growth is negligible, nutrients are brought to the surface. As the mixed layer depth decrease the water column tends to become stratified, leaving nutrient rich waters at the surface, providing optimum conditions for phytoplankton growth. The concentration of phytoplankton increases with time, but the dominate species shifts in time to zooplankton as a result of grazing [2]. When Sverdrup published his Critical Depth Hypothesis, he presented some precautions that he associated with his theory. First he noted that the population of a species is decreased with time due to losses, especially grazing. He also noted that the first biomass increase observed occurred prior the vertical stratification occurred. Sverdrup’s doubts, and his assumption that respiration was constant with depth, have been driving research to better understand the occurrence of spring blooms. Behrenfeld and Boss presented a paper in 2010 that suggests that the relationship between the critical depth and spring blooms is a coincidental rather than a cause and effect. [edit] Dilution Recoupling Hypothesis Michael Behrenfeld proposes the “Dilution Recoupling Hypothesis” to describe the occurrence of annual spring blooms [2][3]. He emphasized that phytoplankton growth is balanced by losses, and the balance is controlled by seasonally varying physical processes. He argued that the occurrence of optimum growth conditions allows for both the growth of predator and prey, which results in increased interactions between the two; it recouples predator-prey. He describes this relationship as being diluted (less interactions) in the winter, when stratification of the water column is minimal. Similar observations were described by Landry and Hassett (1982). The most prominent evidence supporting Behrenfeld's hypothesis is that phytoplankton blooms occur before optimal growth conditions, when the phytoplankton are decoupled from prey. As stratification is established and the biomass of zooplankton increases, grazing increases and the phytoplankton population declines over time. Behrenfeld’s research also modeled respiration as being inversely to proportional phytoplankton growth (as growth is decreasing, respiration is increasing, or vice versa).