Dominance and Compromise in Freshwater Phytoplanktonic Flagellates: The Interaction of Behavioural Preferences for Conflicting Environmental Gradients

1. Motile phytoplanktonic flagellates display a suite of sensory-mediated behavioural preferences to single-environmental gradients. However, in nature multiple gradients of essential resources are often spatially and temporally conflicting, therefore it is important to understand how flagellate behaviours interact under these conditions. 2. Five species of these photosynthetic micro-organisms were exposed to combinations of representative physical and chemical gradients in laboratory preference chambers. Using single-factor responses as templates, the effects of multiple gradients were investigated and hierarchies of preferences constructed. 3. Three distinct interaction responses were observed: 'dominance' of one of two preferences; bisecting 'compromise' interactions, suggesting co-dominance while potentially optimizing resource acquisition; and an 'extreme response' in which preferences appeared more important than avoidance of unfavourable conditions. 4. In all species, preference for light was dominant over preference for temperature and phosphate. More complex dominance, compromise and extreme interaction responses were induced by combinations of light, oxygen and carbon dioxide gradients. 5. Construction of a hierarchy of dominant responses demonstrated that depending on species, response to light generally superseded preferences for CO2, O2and recessive preferences for temperature and phosphate. 6. Interaction responses may confer functional ecophysiological advantages, with dominant preferences influencing the spatial and temporal distribution, migration, depth regulation and succession of phytoplanktonic flagellates in freshwater ecosystems.