“Marine Darkwaves”: an event-based framework to assess frequency, duration and intensity of extreme periods of reduced underwater light availabilityExtreme Events

François Thoral^1,2, Mads Thomsen³, Matthew Pinkerton², Shinae Montie⁴, Daniel Reed⁵, Robert Miller⁵, Spencer Virgin³, Christopher Battershill¹, Leigh Tait^2,3, David Schiel³

¹University of Waikato, New Zealand - ²NIWA, New Zealand - ³University of Canterbury, New Zealand - ⁴University of Western Australia, Australia - ⁵University of California, USA

Long-term reductions in coastal water clarity (‘coastal darkening’) threaten the maintenance of ecological services provided by marine ecosystems. Shallow-water benthic autotroph communities like coral reefs, seagrass meadows, macroalgal forests, and microphytobenthic assemblages, are particularly at risk. On short time scales, episodic events such as sediment delivery and resuspension following intense rain and large waves or pelagic phytoplankton blooms, combined with greater cloud cover, cause acute reductions in water clarity and underwater light availability. Such discrete events are increasingly seen as key drivers that can deteriorate shallow benthic ecosystems, yet a consistent event-based framework for reduction of underwater light does not exist. Here we propose “Marine Darkwaves” (MDWs) as a general framework for quantifying extreme and discrete periods of reduced underwater light, which complements the ‘coastal darkening’ concept and draws from definitions of marine heatwaves and cold-spells. We present a sensitivity analysis on the effects of various minimum durations and climatological threshold values on the properties of detected MDWs. Then we define a MDW as an event when underwater light intensities are below a long-term threshold (10th percentile) for a minimum duration of 5 days. We show the utility of such framework by comparing long-term time series of underwater irradiance from in situ and satellite sensing in three case studies from New Zealand and the coast of California, USA. These show marine darkwave events of varying durations, up to 64 days, and intensities, between -4.1 to -105.6 mol photon.m-2 of light deficit (cumulative intensity) across events. We recommend adopting the MDW framework to enable global comparisons and advance our understanding of extreme events of light reduction in marine ecosystems. This novel and ambitious framework can be used to connect land catchments (or watersheds) to the marine environment and the light requirements of key biogenic species and will be instrumental in managing intensifying land uses that do not impair coastal ecosystems functioning and productivity.