Zach vaughn

http://www.biologynews.net/archives/2010/10/07/volcano_fuels_massive_phytoplankton_bloom.html
Summary
There was a volcano that errupted and spilt a lot of iron lad ash into the North Pacific Ocean. The ash supposedly made the phytoplankton bloom in large numbers. The measure of the phytoplankton was the most it's been since the ocean surface measurements. Phytoplankton are free-floating, single-celled plants that form the base of the marine food chain. But although the volcanic ash fueled such a big phytoplankton bloom, it resulted in only a modest uptake of atmospheric CO2.

Opinion

My opinion is that is was just a chance that the phytoplankton grew after the eruption. You would think that the ash would kill the phyto plankton cells. The erruption could have possiable trigured a growth but I dont think its likely. I dont think they have a way to see if it was the ash that make it grew.