Fluctuating environments and phytoplankton community structure: A stochastic model

Spatial heterogeneity in organism and resource distributions can generate t emporal heterogeneity in resource access for simple organisms like phytopla nkton. The role of temporal heterogeneity as a structuring force for simple communities is investigated via models of phytoplankton with contrasting l ife histories competing for a single fluctuating resource. A stochastic mod el in which environmental and demographic stochasticity are treated separat ely is compared with a model with deterministic resource variation to asses s the importance of stochasticity. When compared with the deterministic mod el, the stochastic model allows for coexistence over a wider range of param eter values (or life-history types). The model suggests that demographic st ochasticity alone is far more important in increasing the possibility of co existence than environmental stochasticity alone. However, the combined eff ects of both types of stochasticity produce the largest likelihood of coexi stence. Finally, the influence of relative nutrient levels and nutrient pul se frequency on these results is addressed. We relate our findings to varia ble environment theory with evidence for both relative nonlinearity and the storage effect acting in this model. We show for the first time that tempo ral dynamics generated by demographic stochasticity may operate like the st orage effect at particular spatial scales.