Researchers have discovered that certain ocean regions can undergo extended spells of intense darkness lasting several weeks. These phenomena, termed marine darkwaves, significantly diminish the amount of light reaching marine organisms, posing challenges to ecosystems reliant on sunlight.
Collaborating scientists from the University of Waikato and the University of Canterbury in New Zealand have developed a novel framework to detect and analyze these abrupt declines in underwater illumination. Prior to this, the scientific community lacked a unified terminology and approach to study these brief but profound darkening episodes.
Sunlight is vital in the ocean, driving photosynthesis that sustains life from microscopic phytoplankton to expansive environments such as kelp forests, seagrass beds, and coral habitats. Marine scientist François Thoral notes that while the significance of underwater light was known, a standardized method to quantify sharp reductions had been missing.
Introducing a Method to Detect Oceanic Darkness Spells
The team designed an approach inspired by tools used to monitor short-term marine events like marine heatwaves. Their technique evaluates factors including the darkness duration, the magnitude of light loss relative to typical seasonal conditions, and the depth affected.
To validate this system, they analyzed long-term underwater light data from various coastal zones. Their study encompassed 16 years of measurements along California’s coastline and 10 years of data gathered from New Zealand’s Hauraki Gulf (Tikapa Moana).
Additionally, they examined 21 years of satellite observations from the waters near New Zealand’s East Cape. The research, published in Communications Earth & Environment, reported finding between 25 and 80 marine darkwaves in this region over the period from 2002 to 2023.
The majority of these darkwaves persisted for five to 15 days, although the longest recorded event lasted 64 days. Researchers believe this classification system will enhance the ability to compare darkwave occurrences and their ecological impacts in diverse coastal settings.
Storm-Induced Sediment Clouds Trigger Ocean Darkness
Many marine darkwaves have been associated with storms that stir up sediment in coastal waters. When suspended particles accumulate, they obstruct sunlight penetration into deeper layers. One notable incident followed Cyclone Gabrielle in 2023, linked to a marine darkwave near New Zealand’s East Cape. Coastal stations in the Firth of Thames also recorded significant light reductions linked to storm activity.
However, storms are not the sole cause. The study highlights connections to runoff from deforestation, ash and debris entering the ocean after wildfires, and plankton blooms. Human activities like dredging and coastal infrastructure development may also play roles in some events.
Coastal scientist Chris Battershill from the University of Waikato explains that marine darkwaves reveal sudden stress on coastal ecosystems. Understanding these events can assist researchers and conservationists in tracking habitat changes below the ocean surface.
Underwater Darkness Influences Marine Food Webs
The investigation did not measure direct harm from marine darkwaves, but prior research indicates that lowered light availability can disrupt numerous ocean habitats. Reduced sunlight can hinder photosynthetic species like those forming kelp forests, seagrass areas, and coral reefs, potentially affecting the entire ecosystem.
“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.”
The authors emphasize that marine darkwaves may affect ecosystems differently depending on their origin — sediment suspensions versus phytoplankton blooms alter light conditions in distinct ways. This new analytical framework offers a robust foundation for identifying these hidden periods of darkness and studying their influence on coastal marine communities.
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