Marinas act as persistent introduction hubs driving the decadal accumulation of non-indigenous fouling species in an oceanic island system

Long–term monitoring of marine fouling assemblages and non‑indigenous species (NIS) has been historically scarce globally, particularly in island ecosystems, limiting the ability to distinguish transient detections from persistent populations. Using an 11–year monitoring dataset (2013–2025; 33 surveys, 131 sampling events) across four Madeira Archipelago marinas, we evaluated spatio-temporal patterns in biofouling assemblages. Across 71 recorded taxa, cumulative NIS richness increased steadily, ultimately accounting for nearly half (45–48%; 27–30 NIS per marina) of all recorded fouling species, reflecting major community transformation. Multivariate analyses identified marina identity as the primary driver structuring assemblages, maintaining distinct spatial signatures despite minimal thermal variation. Temporally, seasonal dynamics revealed a clear functional decoupling: mean observed richness per sampling event peaked during warmer periods, whereas cumulative multi-year species turnover was highest during cooler periods. This decoupling reflects seasonal thermal regimes, where summer warming creates windows that trigger reproductive pulses and space preemption by fast-growing colonial NIS, while cooler periods permit broader species turnover. Together, these findings confirm marinas as persistent introduction hubs, where repeated introductions, recruitment, and population persistence. Long–term, seasonally informed monitoring is therefore vital for early detection and marine biosecurity in island ecosystems.