Dynamic performances of phytoplankton weaker in natural environments than those in culture are attributable to the greater variabilites in temperature, photoperiod and incident light intensity obtaining. Commonly, a critical shortage of supply of an essential resource depresses the attainable rate of cell replication. In nature, phytoplankton populations in nature experience dynamic losses (to death, to pathogens, to irretrievable settlement and to phagotrophic consumers), together combining so that even growing populations may experience net decline in standing-crop numbers. High frequency of sampling and quantification are necessary to account for fluctuating recruitment rates. Species-specific populations may perform as well as they do in cultures, at least for short periods, but they may eventually be susceptible to subsaturating resource levels (‘limiting’ factors). Light limitation may owe to low average insolation (for instance, in a deep-mixed or turbid surface layer) or to a curtailed aggregate daily photoperiod. Depletion of essential nutrient resources in the water impacts on all species, usually in a way that is predictable on the basis of the stoichiometries of supply, demand and intracellular reserves. In the short term, inadequate supplies of inorganic carbon can be problematic and species with efficient carbon-concentrating mechanisms may maintain the relatively fastest rates of growth.