Rung by Rung: A Ship in the Azores to Map Migration Where Light Can't Reach
The RV Maria S. Merian set sail from Ponta Delgada on 10 October for a four-week cruise through the Macaronesian region, combining deep-sea cameras, trawl nets, CTD profiles to 5,000 m, and hydroacoustics to test whether the diel vertical migration ladder continues below the photic zone.
Every evening, billions of tonnes of zooplankton and small fish rise from the deep ocean into the twilight zone and sink again before dawn. The mechanism is simple, well-documented, and entirely light-dependent: diel vertical migration tracks the sun. Below roughly 1,000 metres, that sun-driven clock stops. No light means no photo-avoidance cue, no diel signal to anchor a vertical rhythm to. What, if anything, organises animal movement in the hundreds of metres of water column beneath that boundary has been, until this year, largely a question. The MSM147 expedition aboard the RV Maria S. Merian, which set sail from Ponta Delgada in the Azores on 10 October 2026 and is scheduled to conclude on 5 November, is built around testing whether the migration that is visible in the upper ocean continues as a structured, multi-rung process into the deeper mesopelagic and upper bathypelagic, or whether it simply dissolves into undirected drift once the light cue disappears.
The hypothesis the team is testing — that a “ladder of vertical migration” extends below the photic zone, driven not by light but by cascading predator–prey interactions — belongs to the ERC-funded SEA-THROUGH project. Expedition leader is Dr. Jan Taucher of GEOMAR’s Plankton Imaging group, and the cruise routes the vessel through the Macaronesian island chain, taking stations from the Azores southward to Cape Verde. The announcement was issued on 12 October 2026, two days after departure.
The instrument suite is deliberately multimodal, because no single sensor can capture what is happening at those depths and across that range. The team is deploying deep-sea imaging systems to record behaviour in situ, an RMT (rolling marine trawl) to pull physical material up from the water column, a CTD rosette carrying a 24-bottle Niskin sampler down to 5,000 metres for discrete water-column chemistry and plankton counts, and a hydroacoustic array to track the vertical distribution and movement of scattering layers over time. Each method sees a different slice of the problem: images show what the animals are doing, the trawl shows what is physically there, the CTD gives the environmental backdrop at fixed depths, and the hydroacoustics provide a continuous, population-level view of where the biomass is and how it shifts through the night.
A parallel sampling effort is running under the same SEA-THROUGH umbrella from the University of Hamburg. Prof. Nicole Aberle-Malzahn and MSc candidate Hanna Robitschko are using the same RMT net and CTD rosette at a series of stations spanning the Azores, Madeira, Canaries, and Cape Verde, with the specific aim of quantifying which organism groups — copepods, shrimps, jellyfish, squids, lanternfish, bacteria, and the phyto- and microzooplankton fractions — contribute to nighttime carbon export at each depth. That is a different question from the GEOMAR team’s structural one, but they are two faces of the same problem: if the ladder continues below 1,000 metres, the carbon that sinks at night is not doing so passively but is being carried downward by organisms that move on a schedule, and the identity of those organisms determines how much of that flux is ultimately remineralised in the mesopelagic versus delivered to the seafloor.
What makes the below-1,000-metre question genuinely open, rather than a gap that existing data simply hasn’t filled yet, is that the observational record in that band is sparse in the specific way this experiment needs. Hydroacoustic surveys have shown that scattering layers do persist at depth, but a persistent layer is not the same as a migrating one. A community can sit at 1,200 metres all day and all night and register as a “layer” on an echo-sounder without ever completing a single vertical cycle. The ladder hypothesis predicts something more specific: that the depth at which a given taxon is found shifts systematically through the hours, and that the amplitude and timing of that shift change as you go deeper, tracking the decay of the original light signal through successive trophic steps rather than vanishing at the photic boundary.
The four-week window is short, and the station spacing through the Macaronesian archipelago means each transect is a snapshot rather than a time series. But the combination of simultaneous imaging, physical sampling, and hydroacoustic tracking at discrete, replicated depths is exactly the kind of co-located dataset that models of deep-ocean carbon flux have been waiting for. If the data show that the rhythm persists — even attenuated, even phase-shifted — below the depth where the sun no longer touches the water, the working model for how the biological pump operates in the deep ocean will need a new term in it.