Scientists have introduced a new framework for understanding 'marine darkwaves,' which are temporary, severe reductions in underwater light that can persist for months and significantly impact marine ecosystems.
This phenomenon is distinct from 'ocean darkening,' a long-term, gradual decrease in water clarity observed over decades. Marine darkwaves, conversely, are short but intense episodes caused by events such as storms, algal blooms, and sediment deposition, often triggered by natural occurrences like wildfires, cyclones, and mudslides.
Marine scientist François Thoral stated that while light is critical for marine productivity, a consistent method to measure extreme reductions in underwater light, and a name for this phenomenon, were previously lacking. The new framework allows for the identification of when and where these events occur.
The research adapted existing frameworks used for other episodic ocean events, like marine heatwaves, to define parameters for marine darkwaves, including minimum duration, the degree of light loss relative to a seasonal baseline, and the depth of the loss.
The framework was applied to 16 years of underwater light measurements from the California coast and 10-21 years of data from New Zealand coastal sites. Between 2002 and 2023, 25 to 80 marine darkwaves were detected off New Zealand's East Cape, typically lasting 5 to 15 days, with the longest recorded event spanning 64 days.
Identified causes for these events included storm conditions, such as Cyclone Gabrielle in 2023, topsoil pollution from deforestation, wildfire runoff, and plankton blooms. Potential contributors like dredging and coastal construction work were also noted.
Although the paper did not quantify direct effects on marine life, previous studies indicate that reduced light levels can impair photosynthesis in kelp forests, seagrass, and corals, and affect the behavior of fish, sharks, and marine mammals. Persistent darkness can lead to significant ecological consequences.
Future research will focus on distinguishing different types of events and quantifying habitat damage attributable to marine darkwaves. This framework provides a foundational basis for ongoing work and is expected to be valuable for coastal communities, Indigenous groups (iwi and hapū), and marine conservationists in guiding decision-making. The findings were published in Communications Earth & Environment.