Spring Bloom
Change the light, respiration, and mixing layer depth of a water system to determine the properties for a spring bloom to occur.
The spring phytoplankton bloom is a rapid and remarkable increase in the concentration of phytoplankton that typically occurs in early spring. It is often observed in the North Atlantic, in sub-polar waters, and in temperate coastal marine waters. Understanding the timing of the spring bloom and the processes that generate this phenomenon was a major problem addressed in biological oceanography. The classic explanation for the spring bloom is Sverdrup’s (1953) critical depth concept. Despite later critiques of this concept, it remains an important theory in biological oceanography. And, because Sverdrup’s explanation integrates concepts from physical and biological oceanography, it is a great exam question in oceanography courses.
One way to define the critical depth is the depth is the depth above which depth-integrated daily gross photosynthesis equals the depth-integrated respiration, where respiration includes biomass removal by phytoplankton respiration and by grazers. Sverdrup (1953) argued that when the critical depth becomes deeper than the depth of the surface mixed layer, the daily average rate of organic-matter production will exceed its rate of destruction and a phytoplankton bloom will occur. When the critical depth is shallower than the mixed-later depth, no bloom can occur. In spring, as light availability increases, the critical depth deepens. The timing of the spring bloom, therefore, depends upon light intensity, the rate of loss of phytoplankton (respiration), and the depth of the surface mixed layer.
The following three experiments enable you to explore how light, mixing, and respiration interact to generate a spring phytoplankton bloom. In the first experiment, you can manipulate the intensity of sunlight, the respiration rate, and the mixed layer depth and identify conditions under which a phytoplankton bloom can occur. In the second experiment, light intensity is set by measurements of light intensity measured in Bellingham Bay, WA. You can manipulate the depth of mixing and respiration. The third experiment takes place in shallow water where water depth sets the mixed-layer depth. You can manipulate light and respiration to observe the conditions that lead to a phytoplankton bloom in shallow water.
Spring bloom inititation and Sverdrup’s critical-depth model PDF - Limnol.
Change the light, respiration, and mixing layer depth of a water system to determine the properties for a spring bloom to occur.
Alter the mixing layer depth and respiration levels to see a coorelation with light and the time of year a bloom may occur.
Discover the critical depth of a shallow water system where the seafloor acts as the mixed layer depth.
In the three experiments above students examine different scenarios under which a spring phytoplankton bloom may occur.
The Sverdrup critical-depth model suggests that annual phytoplankton blooms occur as a result of light supply and the depth of vertical mixing. Assuming that a given body of water has sufficient nutrient supply, phytoplankton abundance will increase rapidly, given the right levels of light, respiration, and depth of mixing.
The Sverdrup critical depth model is not withouts its challenges. Recent examinations of the timing of spring blooms in the Pacific and Atlantic are indeed consistent with the critical depth model (Obata et al., 1996). However challenges to the theory have been raised (e.g., Smetacek and Passow 1990, Behrenfeld 2010).
Sverdrup (1953) provided an implicit solution for the critical depth: Dcr/(1-exp(-kDcr)) = 1/k, where Dcr is the critical depth and k is the light attenuation coefficient. For the simulations provided here, k is assumed to be constant. The critical depth is calculated, not from Sverdrup's solution but by directly integrating the productivity and respiration profiles and picking the depth where they are equal.
If you have questions feel free to contact Dr. Shull