High throughput in situ imaging reveals widely occurring diel vertical migration among phytoplankton

Phytoplankton undertake daily vertical migration through the water column to optimize light and nutrient access whilst avoiding predators. However, diel vertical migration (DVM) patterns remain poorly characterized for many taxa due to limitations of labour-intensive traditional microscopy. Here, we employed high-throughput in situ imaging flow cytometry to investigate DVM. An Imaging FlowCytobot (IFCB) was deployed to continuously profile the vertical water column for ~10 weeks (August – October, 2016) at a location in the Skagerrak, eastern North Sea. This revealed significant DVM for several morpho-taxonomic groups, including taxa belonging to ciliates, dinoflagellates, and diatoms, shifting median depth by 2–6 meters between night and day. The analysis also revealed that DVM can be inferred from diel pulses in surface water biomass, which we leveraged to study DVM in an extensive IFCB time-series dataset from the central Baltic Sea (June – October in 2020 and 2021). Migratory taxa accounted for 77% and 79% of total phytoplankton biomass (size range < 10–150 μm) in the Skagerrak and Baltic Sea, respectively, underscoring the ecological significance of DVM. Most populations peaked near the surface at midday, although other patterns were also observed. While many taxa displayed consistent migration behaviours across both regions, others differed — likely due to population-specific traits or local environmental conditions. Seasonal changes in migration patterns suggest a role for community turnover and shifting environmental conditions. This study highlights the prevalence of DVM in phytoplankton and showcases the power of automated, high-throughput imaging technologies to advance our understanding of plankton ecology.