Trait‐Dependent Time Lags Amid Global Change in Marine Observed and Dark Diversity on the Dogger Bank (North Sea)

Understanding delayed biodiversity responses to global changes is crucial for anticipating future ecosystem dynamics and informing conservation. Here, we analysed a 32-year time series of benthic megafauna from the North Sea (Dogger Bank) to assess extinction debt and immigration credit using a probabilistic species pool framework. Within this framework, we generated suitability estimates for both observed diversity (i.e., species locally present) and dark diversity-set of suitable but locally absent species. We defined extinction debt as species persisting locally under declining suitability and immigration credit as suitable species remaining in dark diversity over time. We evaluated these dynamics across 101 epibenthic species (from demersal fishes to macroinvertebrates) and tested their association with sea warming and species traits such as body size and motility. Despite no net change in species number, the suitability of observed diversity decreased over time, indicating extinction debt. At the species level, 72% of species exhibited time-lagged responses, with large-bodied and motile species being more often in extinction debt, whereas smaller and less motile species more often in immigration credit. Consequently, in relation to temperature change, the North Sea's species pool is slowly shifting toward smaller-bodied and less motile taxa, with several cold-adapted and commercially important species in extinction debt. These delayed responses and potential compositional shifts may weaken trophic interactions and affect ecosystem resilience. As marine systems experience increased warming, integrating trait-based approaches and time-lag responses within the species pool framework can offer early warnings of biodiversity reorganization and therefore windows of opportunity for conservation.