How turbulence from a tidal turbine structure affects terns

Author: Bielefeld University

Tidal energy converters use strong tidal currents to power underwater turbines in much the same way as wind turbines. Hardly any research has so far been conducted on how their turbulent wakes may impact the marine environment. A recent international study shows: the wake of a tidal energy structure influences how terns search for food. Researchers from Bielefeld University, Queen’s University Belfast (Northern Ireland), and the University of Plymouth (England) collaborated for the study. For the first time, drones have been used to simultaneously track the movements of terns and the turbulent wake of a tidal turbine structure. The research team presents their analysis of the extensive data in the journal Proceedings of the Royal Society B: Biological Sciences. 

Photo collage of Professor Dr Roland Langrock from Bielefeld University, Dr Lilian Lieber from Queen’s University Belfast, and Dr Alex Nimmo-Smith from the University of Plymouth.
They collaborated for the interdisciplinary research (from left): Professor Dr Roland Langrock from Bielefeld University, Dr Lilian Lieber from Queen’s University Belfast, and Dr Alex Nimmo-Smith from the University of Plymouth.

In their study, the researchers focused on the turbulent wake of a tidal turbine structure set in the Narrows tidal channel, Strangford Lough, Northern Ireland, where the flow reaches speeds of up to 18 kilometres per hour.

When tidal currents flow around the turbine monopile structure, powerful whirlpools of water form, also known as a von Kármán vortex street. The turbulent wake was previously identified in a study as a foraging hotspot for the terns breeding on the scattered islands across Strangford Lough: Common, Arctic and Sandwich Terns. Most terns were seen foraging in areas of pronounced turbulence in the water.

These slender-looking seabirds feed mainly on small fish near the water surface. Being plunge divers, they first inspect the water surface in a slow search flight. Characteristically, a tern hovers over the water and when it discovers a fish or other small prey items, it performs a shallow plunge-dive to pick up its prey.

The wake of the tidal power plant makes it easier for terns to catch fish

For this new study, the researchers used drones to record this search behaviour over different near-surface turbulent features (upwellings and vortices) in situ and subsequently analysed it in unprecedented detail. To track the terns’ foraging movements in the video analysis, the researchers used a combination of artificial intelligence and advanced statistical modelling. They also mapped the underlying evolving turbulence using Particle Image Velocimetry techniques. 

Photo of the SeaGen power plant and a bird.
The study shows how the SeaGen tidal power plant (pictured in the back) in a strait in Northern Ireland affects foraging by Sandwich Terns and other tern species.

‘With our analysis, we were able to show that tern foraging movements were associated with specific turbulent features in the wake,’ says Dr Roland Langrock from Bielefeld University, who is professor of statistics and data analysis and co-author of the study. He is a member of Bielefeld University’s Center for Statistics and supports scientists in various disciplines – especially biology – in the analysis of extensive data.

‘Thanks to this fantastic collaboration, we are able to show how the wake of the tidal turbine structure provided a foraging opportunity for terns,’ says marine ecologist Dr Lilian Lieber, who led the study as a research fellow at the Bryden Centre at Queen’s University Belfast (Northern Ireland). ‘We found that terns were more likely to actively forage for prey over vortices. The wake, characterized by upwellings and vortices can disorientate fish and other potential prey, bringing them to the surface and trap them there – making prey more accessible to terns in these places.’ 

Impact of tidal power plants on animal populations so far unclear

‘The drone provided a true bird’s eye view, allowing us to map dynamic turbulence features, while tracking tern foraging behaviour from a great distance without disturbing their behaviour,’ says Dr Alex Nimmo-Smith, co-author and associate professor in marine physics at the University of Plymouth. He led the development of automatic and reliable tern tracking using artificial intelligence. Roland Langrock adds: ‘Thanks to the extremely high-resolution animal movement data, we can show and study behavioural and decision-making processes in extreme detail. The project presented some statistical challenges and, in my view, is a valuable contribution to the field of movement ecology, especially because of its strongly interdisciplinary character.’

Photo of the SeaGen power plant.
Turbulent wake builds up behind the tower of the SeaGen power plant, providing a ‘conveyor belt of prey’ for the local terns.

Research to date has found that marine renewable energy structures can change how marine animals use such areas, leading to either avoidance or attraction, with as yet unknown im-pacts on population consequences. ‘Tidal turbine structures can change the occurrence, ex-tent, and intensity of turbulent features downstream—and this can influence the behaviour of seabirds,’ explains Lilian Lieber. ‘Therefore, it is important to observe and analyse potential wake interactions and the drone provided an excellent tool to understand the behavior of surface-foraging terns.’ For example, tidal and wave power plants, as well as offshore wind farms, can be preferred sites for marine life, but can also act as barriers or pose a collision risk. ‘Insights gained with the methods we are now using can provide a more holistic under-standing of the potential ecological impacts on marine life,’ says Lilian Lieber.