A high-resolution modeling study on diel and seasonal vertical migrations of high-latitude copepods

Despite diel and seasonal vertical migrations (DVM and SVM) of high-latitude zooplankton have been studied since the late-19th century, questions still remain about the influence of environmental seasonality on vertical migration, and the combined influence of DVM and SVM on zooplankton fitness. Toward addressing these, we developed a model for simulating DVM and SVM of high-latitude herbivorous copepods in high spatio-temporal resolution. In the model, a unique timing and amplitude of DVM and SVM and its ontogenetic trajectory were defined as a vertical strategy. Growth, survival and reproductive performances of numerous vertical strategies hardwired to copepods spawned in different times of the year were assessed by a fitness estimate, which was heuristically maximized by a Genetic Algorithm to derive the optimal vertical strategy for a given model environment. The modelled food concentration, temperature and visual predation risk had a significant influence on the observed vertical strategies. Under low visual predation risk, DVM was less pronounced, and SVM and reproduction occurred earlier in the season, where capital breeding played a significant role. Reproduction was delayed by higher visual predation risk, and copepods that spawned later in the season used the higher food concentrations and temperatures to attain higher growth, which was efficiently traded off for survival through DVM. Consequently, the timing of SVM did not change much from that predicted under lower visual predation risk, but the body and reserve sizes of overwintering stages and the importance of capital breeding diminished. Altogether, these findings emphasize the significance of DVM in environments with elevated visual predation risk and shows its contrasting influence on the phenology of reproduction and SVM, and moreover highlights the importance of conducting field and modeling work to study these migratory strategies in concert.