A paleontological study shows that the fish of the twilight zone - important carbon transporters in the sea - could become smaller in the course of global warming. In a warm phase around 800,000 to 700,000 years ago, the average size of the particularly important lanternfish decreased by up to 35 percent. This effect, which is also threatening in the future, could lead to the oceans being able to absorb less greenhouse gas from the atmosphere, say the researchers.
Many studies show that climate change affects many living things in complex ways. Particular attention is paid to those who in turn play an important role in the earth's carbon cycle. Because impairment can lead to a self-reinforcing process in the context of global warming. The organisms of the oceans are of great importance. Critical effects of water heating are already emerging. So far, however, little is known about the reactions of a very important group of creatures: the fish, which live in the twilight zone of the oceans at a depth of 200 to 1000 meters - in the so-called mesopelagial. Above all, through their daily migrations between higher and deeper water layers, they ensure that carbon is transported to areas with long-term storage potential.
Hot water effects on the trail
Previous studies have already shown that increased water temperatures can lead to fish shrinking or smaller species having an advantage. Among other things, this may be due to the fact that the metabolism of the animals is different in warmer water. Fish living in the upper water layers of the oceans have already been predicted to shrink in size as water temperatures rise. "So far, however, there have been hardly any studies that have dealt with the consequences of global warming on the deeper ocean layers, the twilight or mesopelagic zone," says Konstantina Agiadi from the University of Vienna.
She and her colleagues have now pursued the question using a paleontological approach. They focused on a period that can serve as a model for the current climate warming: Previous studies show that in a mild interglacial period in the middle Pleistocene 800,000 to 700,000 years ago there was partial warming of around four degrees Celsius in the oceans has come. The researchers have now gained insight into the body size of fish in the twilight zone during this time using tiny fossils that come from a sedimentary formation on the Greek island of Rhodes.
Decrease in size in the mirror of fossil auditory stones
The finds were so-called otoliths: tiny auditory stones from the inner ear that enable the fish to perceive sound and balance. The features of the otoliths are specific to fish species, and their dimensions directly reflect the size of the former individual. “We were able to use the ear stones to reconstruct past fish faunas,” says Agiadi. It turned out that compared to the colder times before and after, the mean size of the lanternfish in the eastern Mediterranean decreased by about 35 percent during the warm phase. This was mainly due to the increased frequency of smaller species at the time, the researchers explain. The results thus suggest that similar shifts could occur again in the course of the current climate change.
An average decrease in size could in turn be associated with feedback effects, say the researchers. Because the mesopelagic fish not only play a significant role in the marine carbon cycle because of their gigantic total biomass: "In particular, lantern fish make an important contribution to the so-called biological carbon pump - a natural mechanism for reducing atmospheric carbon dioxide," explains Agiadi. Because at night, these fish migrate hundreds of meters into the upper sea layers to eat plankton there. By then pulling back down, they then transport the carbon bound in the food into the deep sea.
"A decrease in the size of mesopelagic fish could therefore have a negative impact on the ability of the oceans to absorb atmospheric carbon dioxide in the near future - and unfortunately this is bad news in relation to the current climate warming," concludes senior author Martin Zuschin from the University of Vienna.
Source: University of Vienna, specialist article: Proceedings of the Royal Society B: Biological Sciences, doi:10.1098/rspb.2022.1994