Surveys across the world oceans have shown that phytoplankton biomass and production are dominated by small cells (picoplankton) where nutrient concentrations are low, but large cells (microplankton) dominate when nutrient-rich deep water is mixed to the surface. I analyzed phytoplankton size structure in samples collected over 25 yr in San Francisco Bay, a nutrient-rich estuary. Biomass was dominated by large cells because their biomass selectively grew during blooms. Large-cell dominance appears to be a characteristic of ecosystems at the land-sea interface, and these places may therefore function as analogs to oceanic upwelling systems. Simulations with a size-structured NPZ model showed that runs of positive net growth rate persisted long enough for biomass of large, but not small, cells to accumulate. Model experiments showed that small cells would dominate in the absence of grazing, at lower nutrient concentrations, and at elevated (158C) temperatures. Underlying these results are two fundamental scaling laws: (1) large cells are grazed more slowly than small cells, and (2) grazing rate increases with temperature faster than growth rate. The model experiments suggest testable hypotheses about phytoplankton size structure at the land-sea interface: (1) anthropogenic nutrient enrichment increases cell size; (2) this response varies with temperature and only occurs at mid-high latitudes; (3) large-cell blooms can only develop when temperature is below a critical value, around 158C; (4) cell size diminishes along temperature gradients from high to low latitudes; and (5) large-cell blooms will diminish or disappear where planetary warming increases temperature beyond their critical threshold."the biomass spectrum has an important future in marine ecology" (Platt 1985).A grand challenge of limnology and oceanography is to understand the processes that shape phytoplankton communities. Individual phytoplankton species appear while others disappear, for reasons largely unexplained, to reshape communities over time scales of weeks or even days. The challenge is visualized in Fig. 1, showing images of phytoplankton sampled at nearby locations in San Francisco Bay on different dates. The image conveys information about community variability at two levels: cell morphology that defines communities as assemblages of species, and cell size that ranges over 9 orders of magnitude (Finkel et al. 2010).The mechanisms of phytoplankton community variability at the species level remain mysterious. Experiments (Beninc a et al. 2008) and models (Huisman and Weissing 1999) demonstrate how species interactions, such as resource competition, can generate chaotic fluctuations in plankton food webs, even in the absence of environmental variability. Predictability of chaotic systems decays quickly over time and, as a result, "long-term prediction of species abundances can be fundamentally impossible" (Beninc a et al. 2008).