Long-term changes in depth distribution and phenology of four North Atlantic plankton species

Plankton are highly sensitive to climate change, and understanding their shifts in behaviour, physiology, phenology, and biogeography is important for anticipating changes in ecosystem services. This study used a recently developed ecological niche model to reanalyze data from the Continuous Plankton Recorder (CPR) survey using 8 environmental variables to build, for the first time, a CPR-based 4D data set (longitude × latitude × depth × day) of plankton abundance. First, we analyzed spatio-temporal changes in the abundance of 4 plankton species daily from 1958 to 2022: 2 diatoms ( Ephemera planamembranacea and Odontella sinensis ) and 2 calanoid copepods ( Calanus finmarchicus and C. helgolandicus ). We validated our new CPR-based 4D data set with existing in situ data and found that modeled spatio-temporal changes in abundance reproduced observations well. Then, we showed pronounced phenological and biogeographical shifts for the 4 species. Long-term latitudinal shifts in C. finmarchicus and E. planamembranacea were observed and correlated with the Atlantic subarctic gyre state, the North Atlantic Oscillation, and the Atlantic Multidecadal Oscillation. Our results also revealed a prominent positive influence of temperature on long-term changes in the abundance of E. planamembranacea , O. sinensis and C. helgolandicus , whereas they identified a negative influence of temperature on C. finmarchicus . Interestingly, our modeled data suggest a rise in the abundance of C. finmarchicus at greater depths during peak seasons, although this depth adjustment in response to changing conditions is unlikely to overcome the overall surface decline of the species.