Phytoplankton spring blooms in the North Atlantic

Posted on by Angela M. Kuhn

Every spring, huge blooms of phytoplankton erupt across the North Atlantic and other mid- and high-latitude oceans, feeding the marine food web and helping pull carbon into the deep ocean. Scientists have long disagreed about what triggers them: one camp argues it’s driven by light, as the surface ocean layer shallows in spring; another argues it’s driven by predators, when there are simply too few grazers left to keep pace with phytoplankton growth.

I built and tuned a simple ocean ecosystem model to test both ideas directly, running controlled experiments that isolated each mechanism. The result: neither explanation fully works alone. Both light and grazing pressure matter, and what actually triggers a bloom depends on the interplay between them, a more nuanced answer than either camp’s original theory.

Sensitivity of phytoplankton to small changes in model variables using a Taylor decomposition method. Changes in the mixed layer depth (C) reorganize phytoplankton in the water column, but the integrated effect is negligible (black line in F). Changes in light (A, and yellow lines in F) and zooplankton (E, and blue lines in F) both have significant effects on phytoplankton prior to the spring bloom (Kuhn et al., 2015)
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Published by Angela M. Kuhn

I'm a biological oceanographer with interest in understanding the effects of the ocean's physical variability on the marine ecosystem at different scales. I use diverse statistical methods and numerical experiments to systematically combine information from observations, simple and complex numerical marine ecosystem models.