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."Critical depth" is an important term in ocean acoustics defining the lower limit of the full deep sound channel. That depth lies below the axis of the main duct of sound speed minimum. It is the point at which sound speed equals the maximum found above the axis in the surface layer. See SOFAR channel. This concept is useful for understanding the initiation of phytoplankton blooms.
History 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.
Definition 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 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
Assumptions Sverdrup’s critical depth hypothesis or model is built on several strict assumptions:
Turbulence in the thoroughly mixed top layer is strong enough to distribute the phytoplankton evenly throughout the mixed layer. Phytoplankton loss rate is independent of depth and of growth rate. Nutrients in the mixed layer are high enough that production is not nutrient limited Daily photosynthetic production of the phytoplankton community at any depth is proportional to the mean daily light energy at that depth.
That is, light is assumed to be the only factor that limits the growth of phytoplankton during pre-bloom months and the light a phytoplankton community is subject to be determined by the incident irradiance and the coefficient of light extinction.
Mechanism 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 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.
Regional applicability Since 1953, scientists have examined the applicability of Sverdrup's Critical Depth (SCD hereafter) theory in different regions around the world. Semina (1960) found that SCD hypothesis does not apply well in the Bering Sea near Kamchatka, where the bloom is more limited by stability, nutrients, and grazing than by light. Obata et al.(1996) concluded that SCD theory works well at middle and high latitudes of the western North Pacific and the North Atlantic, but it is not able to explain how the spring bloom occurs in the eastern North Pacific and the Southern Ocean. Siegel et al. (2002) deduced that eastern North Atlantic Basin south of 40°N is likely limited by nutrients rather than light and hence is another region where SCD hypothesis would not be well applied. Behrenfeld (2010) also reported that SCD doesn’t apply well in Subarctic Atlantic regions.
Most research used hydrographically defined mixed layer depth, which is not a good proxy for turbulence-driven movement of the phytoplankton and hence might not properly test the applicability of SCD hypothesis, as argued in Franks (2014). The variable regional applicability of SCD has motivated researchers to find alternate biological and physical mechanisms for spring boom initiation in addition to the mechanism proposed by Sverdrup.
Criticisms The main criticisms of Sverdrup's hypothesis result from its assumptions. One of the greatest limitations to understanding the cycle of spring phytoplankton blooms is the assumption that loss rates of phytoplankton in the vertical water column are constant. As more becomes known about phytoplankton loss rate components (such as grazing, respiration, and vertical export of sinking particles), Sverdrup’s hypothesis has come under increasing criticism. Smetacek and Passow published a paper 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, 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) rather than solely photosynthetic organisms.
Sverdrup himself offered criticism of his model when he stated that "a phytoplankton population may increase independently of the thickness of the mixed layer if the turbulence is moderate." He also said that advection rather than local growth could be responsible for the bloom he observed, and that the first increase in plankton biomass occurred before the shoaling of the mixed layer, hinting to more complex processes initiating the spring bloom.
Although Sverdrup pointed out that there is a difference between a uniform-density mixed layer and a turbulent layer, his theory is criticized for the lack of emphasis placed on how the intensity of turbulence could affect blooms. In 1999, using a numerical model, Huisman et al. formulated a "critical turbulence" hypothesis, based on the idea that spring blooms can occur even in deep mixed layers as long as turbulence stays below a critical value so that phytoplankton have enough time in the euphotic zone to absorb light. This hypothesis points to the difference between a mixed layer and an actively turbulent layer that could play a more important role.
Current considerations Despite criticism of Sverdrup's Critical Depth Hypothesis, it is still regularly cited due to unresolved questions surrounding the initiation of spring blooms. Since its introduction, Sverdrup’s hypothesis has provided a framework for future research, facilitating a wide range of studies that address its assumptions. With the advancement of interdisciplinary knowledge and technological capabilities, it has become easier to expand on Sverdrup’s basic theory for critical depth using methods that were not available at the time of its original publication.
Many studies seek to address the shortcomings of the theory by using modern observational and modeling approaches to explain how various biological and physical processes affect the initiation of spring blooms in addition to critical depth. This has led to several theories describing its initiation mechanisms that add complexity to the original theory. Theories involving the role of physiological characteristics, grazing, nutrient availability, and upper ocean physics are active areas of research on spring blooms.
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 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.
Critical turbulence hypothesis Another shortcoming of Sverdrup's model was that he did not consider the role of active turbulence in the mixed layer. Upper ocean turbulence can result from many sources, including thermal convection, wind, waves, and Langmuir circulation. It mixes the upper layer by overturning fluid and particles, reducing the stratification in which spring blooms thrive. Huisman et al. (1999) proposed a critical turbulence mechanism in addition to critical depth that explains how spring blooms can be initiated even in the absence of surface-warming stratification. This mechanism relies on a critical turbulence level over which vertical mixing rate due to turbulence is higher than phytoplankton growth rate.