The North Atlantic Spring Phytoplankton Bloom and Sverdrup's Critical Depth Hypothesis.

More than 50 years ago, Harald Sverdrup developed a simple model for the necessary conditions leading to the spring bloom of phytoplankton. Although this model has been used extensively across a variety of aquatic ecosystems, its application requires knowledge of community compensation irradiance (l[sub c]), the light level where photosynthetic and ecosystem community loss processes balance. However, reported l[sub c] values have varied by an order of magnitude. Here, l[sub c] estimates are determined using satellite and hydrographic data sets consistent with the assumptions in Sverdrup's 1953 critical depth hypothesis. Retrieved values of l[sub c] are approximately uniform throughout much of the North Atlantic with a mean value of 1.3 mol photons meter[sup −2] day[sup −1]. These community-based l[sub c] determinations are roughly twice typical values found for phytoplankton alone indicating that phytoplankton account for approximately one-half of community ecosystem losses. This work also suggests that important aspects of heterotrophic community dynamics can be assessed using satellite observations. [ABSTRACT FROM AUTHOR]