Importance of stirring in the development of an iron-fertilized phytoplankton bloom

The growth of populations is known to be influenced by dispersal, which has often been described as purely diffusive(1,2). In the open ocean, however, the tendrils and filaments of phytoplankton populations provide evidence f or dispersal by stirring(3,4). Despite the apparent importance of horizonta l stirring for plankton ecology, this process remains poorly characterized. Here we investigate the development of a discrete phytoplankton bloom, whi ch was initiated by the iron fertilization of a patch of water (7 km in dia meter) in the Southern Ocean(5). Satellite images show a striking, 150-km-l ong bloom near the experimental site, six weeks after the initial fertiliza tion. We argue that the ribbon-like bloom was produced from the fertilized patch through stirring, growth and diffusion, and we derive an estimate of the stirring rate. In this case, stirring acts as an important control on b loom development, mixing phytoplankton and iron out of the patch, but also entraining silicate. This may have prevented the onset of silicate limitati on, and so allowed the bloom to continue for as long as there was sufficien t iron. Stirring in the ocean is likely to be variable, so blooms that are initially similar may develop very differently.