Critical Depth

In biological oceanography, 'Critical Depth' is defined as a hypothesized surface mixing depth at which phytoplankton growth is precisely matched by losses of phytoplankton biomass within this depth interval.^[1]

History

Critical depth as an aspect of biological oceanography was introduced in 1935 by Gran and Braarud.^[2] 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'.^[3] He theorized that spring phytoplankton blooms are triggered when the mixed layer depth 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. Several papers have appeared recently that suggest a different relationship between the mixed layer depth and spring bloom timing.^[1][4][5]

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.'^[1] 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

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.^[6]

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.

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.^[7] 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.

Dilution Recoupling Hypothesis

Michael Behrenfeld proposes the "Dilution Recoupling Hypothesis" to describe the occurrence of annual spring blooms.^[6][8] 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.

Stratification Onset Hypothesis

Stephen Chiswell proposes the "Stratification Onset Hypothesis" to describe both the annual cycle of primary production and the occurrence of annual spring blooms in temperate waters.^[4] Chiswell shows that the observations made by Behrenfeld can be interpreted in a way that adheres to the conventional idea that the spring bloom represents a change from a deep-mixed regime to a shallow light-driven regime. Chiswell shows that the Critical Depth Hypothesis is flawed because its basic assumption that phytoplankton are well mixed throughout the upper mixed layer is wrong. Instead, Chiswell suggests that plankton are well mixed throughout the upper mixed layer only in autumn and winter, but in spring shallow near-surface warm layers appear with the onset of stratification. These layers support the spring bloom. In his Stratification Onset Hypothesis, Chiswell discusses two hypothetical oceans. One ocean is similar to that discussed by Behrenfeld, where total water column production can be positive in winter, but the second hypothetical ocean is one where net production in winter is negative. Chiswell thus suggests that the mechanisms of the Dilution Recoupling Hypothesis are incidental, rather than a cause of the spring bloom.

Shutdown of Turbulent Convection Hypothesis

John Taylor and Raffaele Ferrari propose that the spring bloom forms because of the shutdown of vertical mixing in the spring. This shutdown allows restratification to occur ^[5] Taylor and Ferrari suggest that during periods of strong forcing (i.e., winter), the mixed layer depth is likely to be a good proxy for the mixing depth. However, when the atmospheric forcing becomes weak in the spring, turbulence subsides rapidly while the mixed layer depth does not change much. Shoaling of deep mixed layers is the result of restratification which occurs on timescales of weeks to months. Therefore, the onset of the bloom can occur significantly prior to the time when the mixed layer restratifies beyond the critical depth.

References