Global change is pushing biodiversity towards “novel” ecological states. Across our world’s oceans, we are witnessing and forecasting widespread changes to species ranges, ecosystem functioning, and our climate – but we are yet to understand how patterns in novel species compositions may coincide with or predict understudied instances of novel functional assemblages and environmental conditions. Here, we characterise and investigate the emergence of taxonomic, functional, and environmental novelty through time for copepod assemblages of the North Atlantic Ocean between 1958 – 2019. We employ an innovative, standardised novelty detection framework to quantify novelty based on a plankton community’s species composition, distribution of organismal traits, and environmental context. We find that novelty in species compositions was observed most frequently and sometimes co-occurred with novel trait assemblages, though neither correlated strongly with environmentally novel conditions. In relatively diverse areas, novel copepod assemblages emerged from small and transient abundance shifts; in less species-rich communities, novel assemblages were dominated by previously unobserved copepod species. Our work directly tests for correlations in the occurrence of novelty among multiple community attributes to provide a holistic recent history of marine ecosystem novelty.