Pulsed nutrient supply as a factor inducing phytoplankton diversity

A numerical model was constructed to elucidate whether phytoplankton species diversity could be increased by an environmental fluctuation such as a pulsed nutrient supply using data set of phytoplankton physiology obtained experimentally. Three phytoplankton species, a diatom Skeletonema costatum, a dinoflagellate Gymnodinium catenatum, and a raphidophyte Chattonella antiqua, were introduced into the model. To simulate a heterogeneous extreme in aquatic environments, environmental conditions of the Ohta River estuary (northern Hiroshima Bay) were applied to the model. When each modeled species was included individually, they showed characteristic responses to different intervals of nutrient supply. S. costatum was the most sensitive, showing large fluctuations in cell density in response to pulses of nutrients, while G. catenatum showed preference for continuous nutrient supply mode. In simulations of mixed-species, a pulsed nutrient supply with an intermediate frequency interval led to a longer period of coexistence than continuous nutrient supply, showing consistency with the Intermediate Disturbance Hypothesis. This indicates that the temporal heterogeneity found in estuaries could be a factor maintaining species diversity. Implications of the present study are discussed: temporal heterogeneity may be important for preservation of estuarine ecosystems possessing high species diversity; hence dam construction may reduce this heterogeneity and this diversity. Also, temporal variability in nutrient supply can encourage coexistence of species limited by different nutrients, thus nutrient-reduction measures should encompass multiple nutrients.