An international team of researchers has developed a new system to identify and compare episodes of sudden underwater darkness, known as marine darkwaves, which threaten light-dependent marine ecosystems. These events, triggered by storms, sediment runoff, and algae blooms, can drastically reduce light to the seafloor for days or months. The framework aims to standardize monitoring of these disruptions worldwide.
Beneath the ocean's surface, forces like sediment runoff, algae blooms, and organic debris can create intense periods of darkness that rival nighttime conditions in coastal waters. In a study published in Communications Earth & Environment, scientists from institutions including the University of California Santa Barbara and the University of Waikato have coined the term "marine darkwave" for these short-lived but severe reductions in underwater light.
The research highlights the vulnerability of photosynthetic organisms such as kelp, seagrasses, and corals, which rely on light for survival. "We have long known that light levels are critical for photosynthetic organisms—like algae, seagrasses and corals—and that factors that reduce light to the seafloor can impact them," said co-author Bob Miller, a research biologist at UC Santa Barbara's Marine Science Institute. "This study creates a framework for comparing such events, which we call darkwaves."
To develop this framework, the team analyzed extensive datasets: 16 years of measurements from the Santa Barbara Coastal Long Term Ecological Research site, 10 years of observations from New Zealand's Hauraki Gulf/Tīkapa Moana and Firth of Thames, and 21 years of satellite-derived seafloor light estimates along New Zealand's East Cape. These revealed 25 to 80 darkwave events since 2002, some lasting over two months and linked to major weather events like Cyclone Gabrielle. In extreme cases, light to the seabed was nearly eliminated.
Lead author François Thoral, a postdoctoral fellow at the University of Waikato and Earth Sciences New Zealand, emphasized the urgency: "Light is a fundamental driver of marine productivity, yet until now we have not had a consistent way to measure extreme reductions in underwater light." He added that even brief darkness can hinder photosynthesis and alter behaviors in fish, sharks, and marine mammals, with potentially significant ecological ripple effects.
This new tool complements existing monitors for marine heatwaves, acidification, and deoxygenation, aiding coastal managers in detecting acute ecosystem stress. Researchers at UC Santa Barbara plan to apply it further to study sedimentation impacts on California's kelp forests, influenced by fires and mudslides.