With the eutrophication of many freshwaters and coastal environments, phytoplankton blooms have become a common phenomenon. This article uses a reaction-diffusion model to investigate the implications of mixing processes for the dynamics and species composition of phytoplankton blooms. The model identifies four key parameters for bloom development: incident light intensity, background turbidity, water column depth, and turbulent mixing rates. The model predicts that the turbulent mixing rate is a major determinant of the species composition of phytoplankton blooms. In wellmixed environments, the species with lowest "critical light intensity" should become dominant. But at low mixing rates, the species with lowest critical light intensity is displaced if other species obtain a better position in the light gradient. Instead of a gradual change in species composition, the model predicts steep transitions between the dominance regions of the various species. The model predicts a low species diversity: phytoplankton blooms in eutrophic environments should be dominated by one or a few species only. The model predictions are consistent with laboratory competition experiments and many existing field data. We recommend examining competition in phytoplankton blooms under well-controlled laboratory conditions, and we derive scaling rules that facilitate translation from the laboratory to the field. Most phytoplankton competition studies published to date have utilized well-mixed laboratory systems (Tilman 1977;Sommer 1985;Grover 1991;Rothhaupt 1996;Ducobu et al. 1998;Huisman et al. 1999a). In contrast, the word "plankton" stems from the Greek neuter of plagkto, which means roaming or wandering (Hutchinson 1974). In many aquatic environments, phytoplankton species are not thoroughly mixed but slowly wander through the aquatic medium, often passively by turbulent diffusion and sinking and sometimes also actively by means of flagellae or buoyancy regulation (Reynolds 1984(Reynolds , 1997. The implications of slow mixing processes for phytoplankton competition are not well understood. However, field data and experiments clearly demonstrate that the intensity of mixing has a major impact on phytoplankton bloom development and on the species composition of phytoplankton blooms (Eppley et al. 1978;Reynolds et al. 1983;Viner and Kemp 1983;Steinberg and Zimmermann 1988;Jones and Gowen 1990;Cloern 1991;Visser et al. 1996;Berman and Shteinman 1998). KeywordsGeneral ecological theory predicts that incomplete mixing should promote species coexistence (Levin 1974;Atkinson and Shorrocks 1981;Powell and Richerson 1985;Hsu and Waltman 1993;Tilman 1994). Coexistence in spatially explicit competition models is usually brought about by two different mechanisms. First, environmental heterogeneity may favor one species at one location, another species at another location, and so on, so that, integrated over the entire habitat, species can coexist. Second, even in an initially homogeneous environment, incomplete mixing combined with in...