We examined large volumes of seawater under the microscope to explore the limits of phytoplankton diversity in a highly dynamic coastal ecosystem. Our analysis showed that conventional sample volumes severely underestimate the species richness of these phytoplankton communities. The number of species observed doubled after a 10-fold increase in sample volume, implying that estimates of phytoplankton species richness depend critically on sampling effort. The volume of sample needed to detect 90% of the species varied between 0.25 and 1 L depending on the concentration of phytoplankton biomass.