The open ocean is never truly still. Beyond the broad sweep of currents and gyres, it teems with vast, swirling whirlpools that can stretch hundreds of kilometres and endure for months. Born from meandering currents or internal instabilities, these mesoscale eddies continually mix and transport water, nutrients, and marine organisms. Though often overlooked, eddies influence everything from regional ecosystems to the ocean’s exchange of carbon with the atmosphere.

A new study led by Dr Guiyan Han, conducted during her placement at Plymouth Marine Laboratory (PML) and co-authored by PML scientist Dr Graham Quartly, sheds fresh light on how these oceanic spirals affect zooplankton – the tiny animals that underpin marine food webs and drive a major portion of global carbon cycling.

Drawing on decades of observations from the Continuous Plankton Recorder (CPR) survey, paired with satellite measurements, the team examined how zooplankton communities respond to both cyclonic and anticyclonic eddies across a wide sweep of the North Atlantic.

To capture subtle shifts in both abundance and diversity, the researchers developed a new Abundance Index, providing an improved lens through which to track biological change.

Their analysis uncovered a striking pattern, that the influence of eddies on zooplankton varies dramatically by latitude.

In the northern and southern reaches of the basin, cyclonic eddies – known for pulling nutrient-rich waters towards the surface – tended to host higher zooplankton concentrations, supporting plentiful phytoplankton blooms. But in mid-latitudes, the trend flipped. Here, anticyclonic eddies appeared to offer warmer, more stable conditions that may encourage zooplankton growth, movement, and reproduction.

The team also found that food supply, indicated by surface chlorophyll levels, exerted a stronger influence on zooplankton abundance than temperature, particularly in northern waters – highlighting the primacy of productivity in shaping life near the surface.

Long-term CPR records revealed another key shift. Since around 2005, the mid-latitude North Atlantic has experienced a notable rise in zooplankton abundance. The authors suggest this may reflect changing climate conditions that are altering the balance between eddy types and the ecological environments they create.

Dr Quartly likened the rotating features to natural ocean experiments: “These spinning eddies are like miniature laboratories in the sea, each with its own conditions that can either nurture or reduce marine life.

“By combining satellite data with the remarkable CPR time series, we can see how these processes differ across the North Atlantic and how they may be evolving in a changing climate.”

Lead author Dr Han added: “In the marine food chain, zooplankton serve as a bridge linking phytoplankton and nekton. Investigating the responses of zooplankton communities to mesoscale eddies will not only enhance our understanding of how eddy-affected marine environments impact zooplankton but also provide insights for future research on eddy – fish interactions – how these large, motile, and behaviourally flexible organisms actively select eddy habitats is undoubtedly of great interest.”

The study underscores the complexity of physical–biological interactions in the ocean and highlights the critical importance of long-term, basin-wide monitoring for understanding and anticipating ecosystem change in a rapidly shifting climate.

The study – Latitudinal transitions of eddy-affected zooplankton abundance in the mid-latitude North Atlantic – is now available to read in full at Science Direct.

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