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Marine coastal waters of Bahrain are under pressure due to human activities and climate change. We used marine monitoring data (2005–2020) from 27 sites to establish baseline conditions and develop standards for assessments of water quality. Five hydrodynamic regions were identified: Oyster Beds, North, West, East, East (Coastal). Data from Oyster Beds sites, likely to be less impacted by human activities, were used to determine baseline conditions. For most parameters, candidate thresholds were based on 50 % and 100 % variation from baseline and 95th percentiles. Comparisons of data against different thresholds showed different outcomes. Overall, results indicate good water quality, with potential concerns in East (Coastal). Trend analyses showed some significant trends in all regions: downward (favourable) for some parameters (e.g. turbidity: North) and upward for others (e.g. nitrate: Oyster Beds, East and East (Coastal)). Future work requires greater understanding around optimum guidelines that protect and mitigate any adverse ecological impacts. • Monitoring data from near-pristine sites in the central Gulf were used to develop water quality guidelines for Bahrain. • Data on turbidity, TSS, nutrients and chlorophyll from other monitoring sites were compared against proposed thresholds. • This approach has provided valuable evidence for future management options and assessments of environmental status. [ABSTRACT FROM AUTHOR]