Cascading migrations and implications for vertical fluxes in pelagic ecosystems

Diel vertical migration is widespread across diverse taxa in the world's lakes and seas, yet its biogeochemical consequences are still poorly understood. The biologically mediated vertical flux of material in the ocean (also known as the biological pump) is a matter of major interest and concern, as it is thought to play an important role in regulating ocean carbon storage, and by extension, the global carbon cycle. Recent studies spanning multiple trophic levels from fish to dinoflagellates have led us to a concept of coupled vertical migrations that we refer to as cascading migrations. That is, migrations that occur at one trophic level can affect the vertical migration of the next lower trophic level, and so on throughout the food web (e.g. carnivores affect the migrations of herbivores, which in turn affect the migrations of motile phytoplankton). Here we hypothesize that greater trophic complexity in the water column leads to enhanced cascading vertical migrations that in turn increase the vertical fluxes of materials and energy through the water column, both upwards and downwards. We recommend that a combination of observational, experimental and modeling approaches be employed to explicitly test this hypothesis.