Final Report Summary - ZOOTRAIT (Insight into the structure and function of marine pelagic food webs: traits and trade-offs in zooplankton feeding behaviour)
1) What is the cost in terms of predation risk and metabolic expenses of different feeding strategies in zooplankton? Zooplankton exhibit different small-scale motile behaviours related to feeding and mating activities. Copepods with “active” feeding behaviours (feeding-current and cruising feeders) showed significantly higher mortality from predation (~2–8 times) than similarly sized copepods with low motility feeding behaviour (ambush feeders). Copepod males, which have a more active motile behaviour than females (mate-seeking behaviour), suffered a higher predation mortality than females. However, the predation risk for mate-searching behaviour in copepods varied depending on feeding behaviour with ambush feeders consistently having the greatest difference in predation mortality between genders (~4 times higher for males than for females. Differences in C-specific respiration rates among copepods with different feeding behaviours were relatively small. Conclusions: motile behaviour is a key trait in zooplankton that significantly affects predation risk and therefore is a main determinant of distribution and composition of zooplankton communities.
2) What feeding behaviour is most efficient in plankton food webs? We determined the feeding efficiency of ambush and active feeders to evaluate the trade-off between efficiency vs. predation risk associated with foraging strategies in zooplankton. We show that efficiency is similar among feeding behaviours for motile prey but one order of magnitude lower for ambush than for active feeders towards non-motile prey. The prey size spectrum is narrower and towards relatively larger prey in ambushers compared to active feeders. Prey detection in ambushers relies on the hydrodynamic disturbances and is inefficient towards non-motile prey but most efficient for large motile prey. This compensates for the lower prey encounter velocity in ambushers compared to active feeders. Thus, the less risky ambushing strategy is more restricted in target prey than active feeders and prey perception mechanism determines the efficiency of zooplankton foraging strategies. The conflict between mate searching and feeding can cause significant difference in feeding efficiency between copepod genders in ambush feeders but not in feeding-current and cruising feeders. Conclusions: feeding efficiency is similar among zooplankton foraging strategies towards motile prey but one order of magnitude lower for ambush than for active feeders towards non-motile prey. Feeding efficiency and predation risk are correlated allowing the co-existence of different foraging strategies and promoting the trait diversity in marine plankton food webs.
Overall, our results represent valuable contributions for (i) a better understanding of the factors that govern the structure and function of plankton food webs, (ii) explaining the spatio-temporal coexistence of the different zooplankton foraging strategies in marine environments and (iii) implementing trait-based models to predict zooplankton trait distributions and seasonal succession of plankton communities in the ocean.