In about a third of the global ocean, the abundance of life is limited by the amount of biologically available iron. When a region is depleted of this important nutrient, algal productivity can be stimulated by added iron, resulting in a temporary boom in biological activity. For much of the surface ocean, the wind-borne transport of iron-rich dust and the upwelling of nutrient-filled water are the major sources of iron. Another potentially important source is the deposition of the iron-rich ash produced by volcanic eruptions. Though satellite observations and modeling work suggest that volcanic ash could seed life in such a way, there have been only a limited number of direct observations of the effects of ash deposition on surface ocean waters.