Do temperature-food interactions matter?: Responses of production and its components in the model heterotrophic flagellate Oxyrrhis marina

The consequence of interactions between temperature and food concentration for protistan population dynamics and estimates of aquatic productivity are relatively unknown, primarily because we lack adequate parameters for models. Here, using the heterotrophic flagellate Oxyrrhis marina Dujardin, we demonstrate the importance of considering temperature and food concentration in combination, to determine the responses of grazing rate, specific growth rate, cell volume, specific production and yield. Specific growth rate and cell volume responded in different ways to temperature-food concentrations: prey concentration had greatest positive effects on specific growth rate with increasing temperature, and prey concentration had greatest positive effects on cell volume with decreasing temperature. The effect of these contrasting interactions on specific production (=specific growth rate x cell carbon) was a greater response to prey concentration at intermediate temperatures. We also observed that the threshold food concentration for growth increased with increasing temperature, but yield showed no clear thermal response. By applying iterative curve-fitting to data obtained from multiple temperature-food concentration combinations, we produced phenomenological models of grazing rate, specific growth rate, and cell volume. We then compared predictions from a simple predator-prey simulation model that applied either our derived equations or a single exponential (Q(10)) relationship to the specific growth and ingestion responses at 20 degrees C. Considerable differences in predator and prey abundance were obtained between the 2 models. Our results demonstrate the potentially complex effects of food and temperature in combination on production parameters, and we argue that these should be considered in aquatic ecosystem simulation models.