Behaviour and life-history strategies of zooplankton have evolved in response to seasonal cycles in food availability, predation risk and abiotic conditions. A key challenge is to understand how different activities over the year are linked. For instance, how does a change in spring activities, such as the timing or amount of egg production, influence autumn activities, for instance energy storage or migra- tion? Trade-offs viewed in relation to individual lifetime fitness consequences couple these events. The framework of optimal annual routines provides theory and methodology for consistent analyses of these temporal trade-offs. Here I describe the key parts of optimal annual routine models and how the models can be used to: (i) study phenology, life-history strategies, and population dynamics; (ii) predict responses to environmental change; and (iii) guide future zooplankton studies. I mainly discuss the adaptations of zooplankton species inhabiting high latitude oceans where the seasonal cycle and its effects are particularly strong. Empirical challenges include issues of seasonal resolution, state-dependent pro- cesses and individual variability. Two ecological problems with avenues for future work are discussed in particular detail: the role of sea ice and ice algae in the life cycle of copepods and krill, and the adaptive value and ecological consequences of semelparous versus iteroparous reproductive strategies.
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