Offshore ALAN islands across European seas: recurrent spatial coupling of night-time radiance and maritime presence.

Abstract

Artificial light at night (ALAN) is increasingly recognised as a global change pressure in marine systems, yet spatial assessments remain largely focused on coastal light spillover. This study investigates whether offshore ALAN across European seas is spatially associated with maritime traffic and whether these associations recur through time. Annual night-time radiance from the Visible Infrared Imaging Radiometer Suite Day/Night Band (VIIRS-DNB) and vessel-density data derived from the Automatic Identification System (AIS) were analysed from 2017 to 2023 over a 1-km offshore grid after excluding a 1-nautical-mile coastal buffer. Bivariate Local Moran's I was used to identify locations where relatively high night-time radiance was spatially associated with relatively high vessel density (High-High hotspots; p ≤ 0.001). Persistent cells were grouped into contiguous clusters of 1-km cells and summarised by Marine Strategy Framework Directive (MSFD) subregion using indicators of extent, occupancy, spatial configuration and radiance contrast. Persistent offshore ALAN islands covered 10,092 km2, corresponding to 0.136% of the analysed offshore grid. They were consistently brighter than the offshore background, with global median all-year ALAN of 0.700 nW cm-2 sr-1 compared with 0.268 nW cm-2 sr-1 for non-island offshore cells. Block-based comparisons confirmed that the contrast was spatially robust after reducing pseudo-replication. Persistent footprints were unevenly distributed across MSFD subregions, with contrasting profiles of total area, largest-island dominance, fragmentation and radiance intensity. The proposed ALAN-island framework provides a reproducible spatial screening layer for mapping recurrent offshore ALAN-related pressure proxies and prioritizing locations for further assessment. It does not quantify underwater irradiance, spectral composition, biological exposure or ecological risk; rather, it identifies areas where targeted optical measurements, source-resolved analyses and ecological assessments should be prioritized.