Agricultural runoff fuels large phytoplankton blooms in vulnerable areas of the ocean

Biological productivity in most of the world's oceans is controlled by the supply of nutrients to surface waters. The relative balance between supply and removal of nutrients--including nitrogen, iron and phosphorus--determines which nutrient limits phytoplankton growth. Although nitrogen limits productivity in much of the ocean, large portions of the tropics and subtropics are defined by extreme nitrogen depletion. In these regions, microbial...