Scientists have found that parts of the ocean can experience long periods of extreme darkness, sometimes lasting weeks. These events, called marine darkwaves, can reduce the light available to marine life and put pressure on ecosystems that depend on sunlight.
Researchers from the University of Waikato and the University of Canterbury in New Zealand have created a new way to identify and study these sudden drops in underwater light. Until now, scientists had no common term or method to describe these short but intense periods of ocean darkness.
Light is a key part of life in the sea. It fuels photosynthesis, supporting organisms from tiny phytoplankton to larger habitats such as kelp forests, seagrass meadows, and coral ecosystems. According to marine scientist François Thoral, scientists understood how important underwater light was, but they lacked a consistent way to measure extreme losses of it.
A New Way To Spot When The Ocean Goes Dark
The research team created a framework inspired by methods used to track other temporary ocean events, including marine heatwaves. Their approach looks at several details: how long the darkness lasts, how much light is lost compared with normal seasonal levels, and how deep the affected area goes.
To test their method, scientists used years of underwater light measurements from different coastal regions. They analyzed 16 years of data from the California coast and 10 years of observations from New Zealand’s Hauraki Gulf (Tikapa Moana).
Frequency and intensity characteristics of marine darkwave (MDW) events across three coastal observation sites. Credit: Communications Earth & Environment
The team also studied 21 years of satellite data from waters near New Zealand’s East Cape. According to the research published in Communications Earth & Environment, scientists identified between 25 and 80 marine darkwaves in this area from 2002 to 2023.
Most of these events lasted between five and 15 days, but the longest one continued for 64 days. The researchers say this new system will make it easier to compare darkwave events and understand how they affect different coastal environments.
Storms Turn The Ocean Dark
Many marine darkwaves were linked to storms that pushed sediment into coastal waters. When large amounts of dirt and particles remain suspended in the ocean, they can block sunlight from reaching deeper areas. A major example was seen after Cyclone Gabrielle in 2023, which was connected to a marine darkwave near New Zealand’s East Cape. Coastal monitoring stations in the Firth of Thames also recorded storm-related drops in underwater light.
Storms are only part of the story. Researchers also found links with runoff from deforestation, material carried into the sea after wildfires, and plankton blooms. Human activities such as dredging and coastal construction could also contribute to some events.
Conceptual overview of the environmental drivers associated with marine darkwaves. Credit: Communications Earth & Environment
As explained by the coastal scientist Chris Battershill from the University of Waikato, marine darkwaves show when coastal ecosystems are facing sudden pressure. The information could help scientists and conservation groups follow changes in marine habitats.
Ocean Darkness Can Ripple Through The Food Chain
The study did not directly measure the damage caused by marine darkwaves, but researchers pointed to earlier studies showing that reduced light can affect many ocean ecosystems. When sunlight becomes limited, plants and other organisms that rely on photosynthesis can be affected. Kelp forests, seagrass habitats, and coral reefs may all suffer when they receive less light than usual.
“Even short periods of reduced light can impair photosynthesis in kelp forests, seagrass, and corals,” Thoral said. He added that: “These events can also influence the behaviour of fish, sharks, and marine mammals. When darkness persists, the ecological effects can be significant.”
Illustration of how scientists identify a marine darkwave and observe its evolution. Credit: Communications Earth & Environment
The researchers also noted that every darkwave may not have the same impact. A sediment event and a phytoplankton bloom can change underwater conditions in different ways, meaning their effects on marine life may vary. The new framework gives scientists a clearer way to follow these hidden periods of ocean darkness and study how coastal ecosystems respond when the light suddenly disappears.