Sediment clouds threaten deep-sea inhabitants

The helmet jellyfish (Periphylla periphylla) served as a test animal for the researchers. © Vanessa Stenvers, GEOMAR

A further risk from the planned mining of raw materials on the seabed is becoming clear: suspended matter clouds from sediment that has been stirred up or released by ships could damage the deep-sea ecosystems in the open water zone. This is what experiments on inhabitants of this habitat suggest: deep-sea jellyfish are significantly stressed by the sediment particles. Further studies should now clarify whether this also applies to other organisms and to what extent sediment pollution during deep-sea mining can be prevented, say the researchers.

Much sought-after raw materials lie dormant in the depths: In some areas of the ocean there are so-called manganese nodules and other deposits at the bottom that contain valuable metals. Some countries and mining companies are therefore planning to develop these special deposits. But so far there is still intensive discussion about the approvals and the extent of the planned deep-sea mining. Environmental protection is an important aspect because the deep seabed is also a habitat that forms part of the complex system of marine life.

Worried view of the open water zone

So far, the focus of attention has mostly been on the directly affected seabed. But as part of their study, the researchers led by Vanessa Stenvers from the GEOMAR Helmholtz Center for Ocean Research Kiel examined an aspect that also affects the open water zone: suspended matter clouds - so-called plumes, which are caused by deep-sea mining, could be the sensitive creatures in this pelagic affect the habitat mentioned. “The pelagic is crucial to the ocean’s ability to store carbon. Its inhabitants are also the main source of food for many fish, squid and marine mammals and therefore represent an important link in the marine food web,” emphasizes Stenvers.

There would be pollution from suspended solids in particular if sediment pumped up from the seabed was returned to the water column by the mining ships. Long-lasting sediment clouds could then spread over tens to hundreds of kilometers in the zone between 200 and around 4,000 meters water depth. Since there is usually hardly any sediment in the pelagic, it is to be feared that the creatures there will react sensitively to the change. “They have evolved under far more stable conditions than surface-dwelling animals and are therefore potentially more vulnerable to changing environmental conditions,” says Stenvers.

In order to obtain initial information about how sediment pollution affects inhabitants of the deep-sea open water zone, the researchers have now carried out studies on helmet jellyfish. They exposed these deep-sea inhabitants to experimental suspended matter pollution that could arise from marine mining and examined the animals' reactions. “Because determining stress in a jellyfish is not easy, we examined its response from different angles and combined the findings from its physiology, the change in gene activity and the microbial symbionts on the outside of the jellyfish,” says Stenvers.

Exhausting stress

As the team reports, the most noticeable effect was increased mucus production, which is due to the adhesion of the sediment particles to the jellyfish. This seems problematic, explains Stenvers: “Mucus production requires a lot of energy and can make up a significant part of an animal’s overall energy budget.” The fact that the sediment load caused stress in the jellyfish was also reflected in the increased activity of certain genes that are related to breathing, the immune system and wound healing, the researchers report.

As part of the study, the team also tested the effect of increasing the water temperature by up to four degrees Celsius, which global warming could lead to in extreme cases in the jellyfish's habitat. They also found signs of increased stress in the animals, but the exposure to the suspended matter was significantly more intense. According to the team, the particle clouds could lead to increased energy consumption, which these marine animals may not be able to compensate for through additional food intake. Ultimately, this could even lead to their starvation.

The scientists say that the observed sensitivity of the helmet jellyfish could at least be representative of other jellyfish. According to them, further studies on various deep-sea species should now follow. This would enable better assessments of the perhaps complex impacts of marine mining on deep-sea life. The researchers hope that this could then lead to the development of protective regulations or mining strategies that keep environmental damage as low as possible.

Source: GEOMAR Helmholtz Center for Ocean Research Kiel, specialist article: Nature Communications. doi: 10.1038/s41467-023-43023-6

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