Oceans are made up of multiple layers. Lighter, warmer waters sit at the top while denser, cooler waters can be found at the bottom. To move heat, ocean mixing is vital.
Ocean mixing also moves oxygen, nutrients and pollutants between different layers, playing a major role in how ecosystems sustain life. A new study has shown how fish can influence ocean ecosystems by circulating nutrients and oxygen around the waters when they spawn. Prior to this study, it was not previously understood how fish contribute to ocean mixing.
A team led by the University of Southampton spent fifteen days monitoring water turbulence in the Ría de Pontevedra, a bay in the northwest coast of the Iberian Peninsula. The experts measured variations in ocean current speed and temperature over very small distances.
It is known that turbulence and mixing increases during major storm events. The results of the study showed that increased levels of turbulence and mixing occurred every night, even without a storm.
The research team used acoustic data and samples collected with small fishing nets. This allowed the team to identify fish that were gathering in the area at night. The small nets were full of recently spawned eggs of European anchovy. The anchovies’ frantic behaviour during spawning had caused the turbulence. This is referred to as ‘biological mixing’.
“We believe that biological mixing was intense in our observations because the bay is highly stratified – the temperature and other properties vary significantly at different depths,” explained Dr. Bieito Fernández Castro, who led the study.
Previous studies suggested that biological turbulence causes minimal mixing because the circular motions fish generate while swimming are too small, especially in deeper ocean water. “However, we have shown that closer to land, where the layers change over a much shorter distance, the anchovies are able to mix them together,” said Dr. Castro.
While biological mixing may influence the open ocean, the study revealed that it can be significant in coastal ecosystems. This is because thriving marine life coexists with rapid changes in the make-up of the ocean.
The ocean mixing generated by fish could impact the redistribution of temperature, nutrients and oxygen, which play a fundamental role in the functioning of the ecosystem that fish rely on. The findings highlight the capacity of living organisms to influence the environment they live in.
“The observation of how our anchovies drove mixing was totally fortuitous. We were set to study how turbulence affects marine life and we end up showing, for the first time, that marine life can influence ocean turbulence, which in turn influences marine life!” concluded Dr Fernández Castro.