Marine darkwave as an event-based framework to assess unusual periods of reduced underwater light availability

Abstract Episodic reductions in underwater light can be a key driver of marine ecosystem degradation. Yet a consistent event-based framework describing the frequency, duration and intensity of substantial but short-term reductions in underwater light does not exist. Here, we proposed marine darkwaves as a framework for quantifying these episodic reductions of underwater light at specific depths which aligns with definitions of other episodic and extreme events. The framework was applied to long-term in situ time series of underwater irradiance from California, USA (16 years, 6.3 metres) and New Zealand (10 years, at 7 and 20 metres). We showed evidence of several intense marine darkwaves across these sites, with durations up to 64 days, cumulative light deficits reaching −105.6 mol photon·m −2 , and up to almost 100% light loss versus climatology. We extended the framework to satellite-derived seabed irradiance data across New Zealand’s East Cape region (2002–2023), using a set of 10 th percentile threshold and a minimum duration of 5 days. This revealed 25 to 80 spatially varying seabed events, and event durations of 5 to 15 days. Importantly, the framework enables local to continental-scale comparisons of the patterns and ecological consequences of episodic light reduction in marine ecosystems.