What Are “marine Darkwaves”, This New Phenomenon Which Threatens Our Oceans And Worries Scientists?

If episodes of marine heatwaves have already been documented since the 1980s with the explosion of satellite monitoring, another phenomenon, just as devastating for our oceans, is beginning to crystallize the concerns of the scientific community. Named “ dark waves “, these are periods during which the water clarity drops, maintaining the seabed in near total darkness for several weeks.

In doing so, the Sun’s rays no longer penetrate the waters, thereby threatening the marine ecosystems that depend on them to survive. Theorized for the first time in a study published on January 12 in the journal Communications Earth & Environmentthey complete an already very damaged picture of our oceans.

They have nothing to do with the browning of the oceans, a multifactorial phenomenon (accelerated melting of permafrost, increased runoff of organic matter, proliferation of opportunistic micro-algae, deforestation of riparian zones, etc.) which has accelerated since the 1990s and is spread over the long term. THE ” dark waves » are, on the contrary, sudden episodes, which can paralyze marine life within days. « Until now, we had no consistent way to measure these extreme reductions in underwater light, and this phenomenon didn’t even have a name », Specifies researcher François Thoral, lead author of the study.

A wall of opacity between the Sun and the abyss: ecosystems in apnea

How exactly do “ dark waves » ? They mainly result from environmental factors, often exacerbated by human activity and climate change. Violent storms, sudden plankton proliferation or even colossal sedimentary deposits after forest fires or landslides: the causes are multiple.

To find out their location and occurrence, the researchers behind this study analyzed twenty years of data collected off the coast of California and New Zealand. During this period, off the coast of New Zealand’s East Cape, between 25 and 80 “ dark waves » have been recorded over two decades. While their average duration varies between 5 and 15 days, some record episodes have kept the seabed in complete darkness for 64 consecutive days.

Their ecological impact is devastating, since without light, marine photosynthesis stops. The organisms most sensitive to this darkness are kelp forests, marine areas densely populated with algae, underwater meadows (bringing together many species of flowering plants) and corals, already threatened by global warming. On their own, these three are home to more than a quarter of the world’s marine biodiversityserving as a refuge and nursery for thousands of species. They are also those who are aptly called the “ blue lungs ”, because they produce a good part of the oxygen we breathe.

François Thoral emphasizes that “ even short periods of reduced light can impair photosynthesis (…) these events can also influence the behavior of fish, sharks and marine mammals “. By suffocating the bottom of the food chain, it is everyone at the top who also finds themselves threatened. Predators, disoriented by the darkness, hunt blindly and their living space becomes completely impassable. This void is immediately filled by species capable of navigation. they have sonar » via echolocation or ultra-developed olfactory faculties.

As in any ecosystem, if the base of the food chain is disruptedthe resilience of species to other anthropogenic stressors weakens at the same time. The study also underlines that the root of evil, the origin of “ dark waves », is found, without too much surprise on earth. Artificial soil erosion, boosted by rampant deforestation, fills rivers with sediment which then end their course at the bottom of the sea waters. A fog of organic matter which adds to the acidification of marine environments and their warming, thus occasionally creating, an unbearable environment for the species that live there. Fortunately, the fact of having given a name to this phenomenon and of having quantified it, will allow oceanologists to have a new theoretical working framework to, ultimately, curb it.