Toward ecosystem-based strategic environmental assessments for marine spatial planning: Enhancing cumulative effects assessments with a four-dimensional marine baseline

Understanding when and where human activities overlap in the marine environment is critical for anticipating cumulative effects, yet Strategic Environmental Assessment (SEA) baselines rarely capture this variability in sufficient spatial or temporal detail. This study develops a practical, four-dimensional (4D) cumulative activity (CA) marine baseline that integrates seasonal and depth dimensions to strengthen the foundation for Cumulative Effects Assessment (CEA) within Marine Spatial Planning (MSP). Twenty marine activities were harmonised and standardised using a Geographical Information Systems (GIS) approach, weighted by seasonal operational intensity, and aggregated to produce spatially explicit CA maps across depth zones in Irish coastal waters. Results show that CA intensity and spatial footprint vary markedly with season and depth, with higher levels in spring and summer and lower levels in winter and autumn. The shallow euphotic zone emerges as the most seasonally active and multi-use layer, with CA intensity progressively decreasing with depth. Seasonal variation further shapes disturbance orientation, with activity more strongly associated with benthic-oriented activities in winter and autumn and relatively greater pelagic-oriented activity in spring and summer. Together, these seasonal and depth-resolved disturbance patterns provide insight into where and when different types of pressure pathways may be more likely to occur, even in the absence of direct pressure data. By explicitly representing how human activities vary across space, time, and depth, the findings support more targeted and proportionate policy and decision-making within MSP, providing a structured foundation for future CEA in SEA practice contexts.