The paradox of the so-called ‘high-nitrate, low-chlorophyll’ parts of the ocean is that although the most important nutrient, nitrate, is available, phytoplankton productivity remains limited. Evidence that lack of iron is responsible has been the centre of research attention for the past few years. But silicate, which diatoms use to make their shells, is now also implicated as a limiting factor — at least in that part of the eastern Pacific in which the latest studies were carried out.
