|Abandoning Sverdrup's Critical Depth Hypothesis on phytoplankton blooms|
Behrenfeld, M.J. (2010). Abandoning Sverdrup's Critical Depth Hypothesis on phytoplankton blooms. Ecology 91(4): 977-989
In: Ecology. Ecological Society of America: Brooklyn, NY. ISSN 0012-9658, more
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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.