Iron fertilization studies conducted at repeated intervals over the span of a week have produced a larger biological response than a single fertilization event. The biological response size tends to depend on a site’s biological, chemical, and physical characteristics. In the Equatorial and North Pacific, silica is thought to constrain additional production after iron fertilization, while light limits additional production in the Southern Ocean. Native, smaller phytoplankton were initial responders to increased iron, but were quickly outcompeted by larger, coastal phytoplankton such as diatoms. The large bloom response and community shift has led to environmental concerns about fertilizing large sections of HNLC regions. One study suggests that diatoms grow preferentially during fertilization experiments. Some diatoms, such as pseudo-nitzschia, release the neurotoxin domoic acid, poisoning grazing fish. If diatoms grow preferentially during iron fertilization experiments, sustained fertilizations could enhance domoic acid poisoning in the marine food web near fertilized patches.