History

• Critical depth as an aspect of biological oceanography was introduced in 1935 by Gran and Braarud. It became prominent in 1955 when Harald Sverdrup
  Harald Sverdrup
  Harald Ulrik Sverdrup was a Norwegian oceanographer and meteorologist who made a number of important theoretical discoveries in these fields. Having first worked in Bergen and Leipzig he was the scientific director of the North Polar expedition of Roald Amundsen aboard the Maud from 1918 to 1925...
  
   published the “Critical Depth Hypothesis” based on observations he had made in the North Atlantic on the Weather Ship 'M'. He theorized that spring phytoplankton blooms are triggered when the mixed layer depth
  Mixed layer
  The oceanic or limnological mixed layer is a layer in which active turbulence has homogenized some range of depths. The surface mixed layer is a layer where this turbulence is generated by winds, cooling, or processes such as evaporation or sea ice formation which result in an increase in salinity...
  
   becomes shallower than the critical depth. 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. Behrenfeld and Boss presented a paper in 2010 that suggests a different relationship between the mixed layer depth and spring bloom timing.

Definition

• Sverdrup defines the critical depth as ‘a surface mixing depth at which phytoplankton community growth
  Population growth
  Population growth is the change in a population over time, and can be quantified as the change in the number of individuals of any species in a population using "per unit time" for measurement....
  
   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
  Photosynthesis
  Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
  
   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
  Cellular respiration
  Cellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products. The reactions involved in respiration are catabolic reactions that involve...
  
  , grazing
  Grazing
  Grazing generally describes a type of feeding, in which a herbivore feeds on plants , and also on other multicellular autotrophs...
  
  , sinking, advection
  Advection
  Advection, in chemistry, engineering and earth sciences, is a transport mechanism of a substance, or a conserved property, by a fluid, due to the fluid's bulk motion in a particular direction. An example of advection is the transport of pollutants or silt in a river. The motion of the water carries...
  
  , viral lysis
  Lysis
  Lysis refers to the breaking down of a cell, often by viral, enzymic, or osmotic mechanisms that compromise its integrity. A fluid containing the contents of lysed cells is called a "lysate"....
  
  , 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.

Critical Depth Hypothesis

• Sverdrup’s research results suggested that the shoaling of the mixed layer depth to a depth above the critical depth was the cause of spring blooms. When the mixed layer depth exceeds the critical depth, mixing of the water brings so much of the phytoplankton population below the compensation depth where photosynthesis is impossible that the overall population cannot increase in biomass. However, when the mixed layer becomes shallower than the critical depth, enough of the phytoplankton remain above the compensation depth to give the community a positive net growth rate. Sverdrup’s model is a cause and effect relationship between the depth of the mixed layer versus the critical depth and the bloom of phytoplankton.

• This trigger occurs in the spring due to seasonal changes in the critical depth and mixed layer depth. The critical depth deepens in the spring because of the increased amount of solar radiation and the decrease in the angle it hits the earth. During the winter, strong winds and storms vigorously mix the water, leaving a thick mixed layer to bring up nutrient-rich waters from depth. As the average winds decrease from the winter storms and the ocean is heated, the vertical water column becomes increasingly stratified
  Stratification (water)
  Water stratification occurs when water masses with different properties - salinity , oxygenation , density , temperature - form layers that act as barriers to water mixing...
  
   and the mixed layer depth decreases.

• Sverdrup’s Critical Depth Hypothesis is limited to explaining what initiates a spring bloom. It does not predict its magnitude. Additionally, it does not address any population controls after the initial bloom, such as nutrient limitation or predator-prey interaction with zooplankton
  Zooplankton
  Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word "zooplankton" is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...
  
  .

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, Svedrup’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
    Biosphere
    The biosphere is the global sum of all ecosystems. It can also be called the zone of life on Earth, a closed and self-regulating system...
    
    .

Dilution Recoupling Hypothesis

• Michael Behrenfeld proposes the “Dilution Recoupling Hypothesis” to describe the occurrence of annual spring blooms. 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 interactions. He describes this relationship as being diluted (fewer interactions) in the winter, when the mixed layer is deep and 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 as predicted by mixed depth shoaling, when the phytoplankton concentrations are more diluted. As stratification is established and the biomass of zooplankton increases, grazing increases and the phytoplankton biomass declines over time. Behrenfeld’s research also modeled respiration as being inversely proportional to phytoplankton growth (as growth rate is decreases, respiration rate increases). Behrenfeld’s model proposes the opposite relationship of phytoplankton growth rate to mixed layer depth than Sverdrup’s: that it is maximized when the layer is deepest and phytoplankton most diluted.