Spatio-temporal modulation of deep pass and shallow channel oceanic inflows on the lagoon renewal time of three pearl farming atolls (French Polynesia).

Ocean-lagoon exchanges are fundamental processes for the physical, biogeochemical and ecological functioning of atoll lagoons. Although lagoon renewal has been investigated using multiple metrics, none explicitly distinguish the water masses pathways from the ocean to the lagoon. Here, we combine high-resolution hydrodynamic modelling and numerical tracers to map the source-specific contributions and spatial footprints of inflows (through passes and shallow reef-flat channels) to lagoon water renewal in three pearl-farming atolls of the Tuamotu Archipelago (French Polynesia): Ahe, Takaroa and Raroia. Climatological simulations were used to quantify seasonal variability in renewal patterns. Despite similar atmospheric and oceanic forcings, all atolls exhibited markedly different renewal behaviour. Bulk renewal time was fastest (∼ 20-30 days) in the largest lagoon and slowest (exceeding 100 days) for the smallest one, highlighting that lagoon size alone does not control renewal efficiency. While approximately half of the incoming oceanic waters enter through the pass in all atolls, the spatial influence of pass-incoming waters renewal differed markedly between atolls and seasons (from 4% to 94% of the surface of the lagoons). Seasonally, winter conditions strengthened residual circulation and reduced lagoon-averaged flushing times by up to 24% compared with summer. By coupling renewal timescales with the origin and spatial footprint of renewing waters, our approach provides a more mechanistic description of ocean-lagoon exchanges, offering new perspectives for understanding ecosystem functioning, improving pearl-farming management, and assessing the vulnerability of atoll lagoons to environmental disturbances.