Photo caption: A satellite image from Sentinel 2 following Cyclone Gabrielle on 14 February 2023 shows a Waihau Bay Raukokore river plume and marine darkwaves.
Te Whare Wānanga o Waitaha | University of Canterbury (UC), University of Waikato and Earth Science New Zealand researchers have delivered a major scientific advancement by creating the first framework to identify short-term reductions in underwater light, described as marine darkwaves.
The study, published in Communications Earth & Environment, introduces marine darkwaves; short-term but intense episodes of underwater darkness that can severely impact kelp forests, seagrass beds, and other light-dependent marine life.
Distinguished Professor David Schiel from UC's School of Biological Sciences says the research addresses a critical gap in understanding coastal environmental change.
"Degradation of many of New Zealand's coastal kelp forests is increasingly due to sediment run-off from intensified land use, which causes a highly compromised light environment compounded by the smothering of habitats," Professor Schiel says. "The marine darkwave framework provides an international standard for categorising the underwater light environment and tracking changes over time."
The research project, led by lead author and marine scientist Dr François Thoral from Waikato and Canterbury Universities, analysed 16 years of data from California, 10 years of data from New Zealand's Hauraki Gulf, and 21 years of satellite observations from New Zealand's East Cape.
The analysis revealed between 25 and 80 marine darkwaves along the East Cape since 2002, many linked to storms and major weather systems including Cyclone Gabrielle. Some events lasted from days to over two months, with some seabed areas receiving almost no light.
"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," Dr Thoral says.
Even brief periods of reduced light can impair photosynthesis in kelp forests, seagrass and corals, while also influencing the behaviour of fish, sharks and marine mammals.
The marine darkwave framework complements existing frameworks for tracking marine heatwaves, ocean acidification and deoxygenation, providing a standardised tool for coastal managers, iwi, hapū, and conservation groups to identify when marine ecosystems face acute stress.
The darkwave work is part of a large coastal research programme in the Bay of Plenty, conducted through Ministry of Business, Innovation and Employment funding aimed at understanding and reducing the sources of light degradation in coastal kelp forests, primarily through land-sourced sedimentation. The five-year programme is led by Professors Schiel (University of canterbury) and Chris Battershill (Waikato University), and Rahera Hia (Ngati Pukenga), and will expand the monitoring networks to support future research on marine darkwaves in New Zealand waters.
The paper was published in the Nature Portfolio journal Communications Earth & Environment, on 12 January (UK time).
