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 (I-C), the light [eve[ where photosynthetic and ecosystem community loss processes balance. However, reported I-C values have varied by an order of magnitude. Here, I-C estimates are determined using satellite and hydrographic data sets consistent with the assumptions in Sverdrup's 1953 critical depth hypothesis. Retrieved values of I-C are approximately uniform throughout much of the North Atlantic with a mean value of 1.3 mol photons meter(-2) day(-1). These community-based I-C determinations are roughly twice typical values found for phytoptankton 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.