Abandoning Sverdrup’s critical depth hypothesis on phytoplankton blooms
- Publication date
- Publisher
- 'Wiley'
Abstract
The Critical Depth Hypothesis formalized by Sverdrup in 1953 posits that
vernal phytoplankton blooms occur when surface mixing shoals to a depth shallower than a
critical depth horizon defining the point where phytoplankton growth exceeds losses. This
hypothesis has since served as a cornerstone in plankton ecology and reflects the very common
assumption that blooms are caused by enhanced growth rates in response to improved light,
temperature, and stratification conditions, not simply correlated with them. Here, a nine-year
satellite record of phytoplankton biomass in the subarctic Atlantic is used to reevaluate
seasonal plankton dynamics. Results show that (1) bloom initiation occurs in the winter when
mixed layer depths are maximum, not in the spring, (2) coupling between phytoplankton
growth (l) and losses increases during spring stratification, rather than decreases, (3) maxima
in net population growth rates (r) are as likely to occur in midwinter as in spring, and (4 ) r is
generally inversely related to l. These results are incompatible with the Critical Depth
Hypothesis as a functional framework for understanding bloom dynamics. In its place, a
‘‘Dilution–Recoupling Hypothesis’’ is described that focuses on the balance between
phytoplankton growth and grazing, and the seasonally varying physical processes influencing
this balance. This revised view derives from fundamental concepts applied during field dilution
experiments, builds upon earlier modeling results, and is compatible with observed
phytoplankton blooms in the absence of spring mixed layer shoaling.Key words: Critical Depth Hypothesis; grazing; growth rates; North Atlantic; phytoplankton blooms; remote sensing