We formulate a strategic simulation model of a fjordic ecosystem, paying careful attention to consistency of spatial and temporal scales. Our model incorporates all the major physical and biological elements of the system but is simple enough to allow unambiguous identification of the mechanisms underlying its behavior. Despite this simplicity the model is capable of semiquantitative agreement with the observed behavior of the Killary Harbor ecosystem over the 198 1 season.The key conclusion from our analysis of the model is that because of the very high rates of tidal and freshwater flushing in Scottish sea-lochs, the persistence of such systems is ultimately determined by the balance of nutrient import and export in a manner analogous to a laboratory chemostat. Except at the peak of the spring bloom, inorganic nitrogen supply exceeds biological demand, so that primary production is controlled by physical factors (temperature and irradiance) and the activities of higher trophic levels.