Underestimated effect of cumulative muscle power: According to a study, fish apparently contribute more to the mixing of water layers than previously thought. Researchers have found that schools of spawning anchovies on the Spanish coast cause turbulence in the water similar to storms. As a result, the fish distribute nutrients and oxygen and thus probably make a significant contribution to the formation of the marine ecosystem, the scientists explain.
Warm, light water forms the upper layers, while cooler and therefore denser water forms the lower layers: Following this basic principle, structures are formed in the oceans. However, it is important for life that these layer systems are broken up at least to a certain extent: the mixing in the oceans ensures that heat, oxygen and nutrients are transported between the different layers. This exchange forms the basis of life for many living beings in the various marine ecosystems. Physical processes play the most important role: tides, flow systems, wind and weather cause water movements and turbulence, which lead to mixing.
What influence do living beings have?
To what extent the kinetic energy of creatures such as fish can also make a contribution was previously unclear. A rather small effect was assumed and the proof of corresponding processes was considered difficult. The so-called biomixing was therefore initially not the focus of the research team led by Fernández Castro from the University of Southampton. “Our discovery was a surprise in the context of our measurements: we actually wanted to study how turbulence affects marine life. But in the end we were able to document that living things can cause water movement, which in turn affects marine life,” Castro said.
The scientists conducted their investigations in a bay on the north-west coast of the Iberian Peninsula. From a boat, they lowered a measuring instrument into the water, which recorded fluctuations in current speeds and temperature with great sensitivity every half hour for two weeks. The researchers wanted to investigate the extent to which the layer system in the bay is influenced by turbulent effects such as those caused by wind and weather.
On the trail of “Stormy” turbulence
They were surprised to find that there was turbulence and mixing of the layer system in the bay every night, similar to what occurs during a storm. However, these water movements were also evident in completely calm weather. A look at the information provided by the ship’s echo sounder then provided clues as to the cause: the acoustic data showed schools of fish that appeared in the study area every night. Subsequent investigations then showed that these were shoals of European anchovies (Engraulis encrascicolus): The researchers’ study happened to coincide with the spawning season of the small shoal fish – the time when they form shoals and provide for offspring. According to the researchers, the explanation for the significant turbulence in the bay is that it was caused by the anchovies’ frantic behavior during spawning.
In the open ocean, such biomixing is probably of little importance for mixing, since the different water layers are usually very thick there, the scientists say. But the study results suggest that this could be different in some ecologically very important coastal areas. “We have shown that in coastal areas, where the layers change over much shorter distances, fish are able to mix them,” says Castro.
The vertical displacement generated by schools of fish could thus have a significant impact on the redistribution of temperature, nutrients and other important water parameters such as oxygen content. These factors are fundamental to the functioning of the ecosystem, and the fish themselves also depend on them, the researchers explain. According to them, the results therefore document the ability of living beings to influence and redesign their physical environment. In conclusion, Castro and his colleagues write: “Our results show that biologically driven turbulence can be a powerful mixing factor and prompt a re-examination of its effects on productive regions of the oceans.”
Source: University of Southampton, professional article: Nature Geoscience, doi: 10.1038/s41561-022-00916-3