abstract Phytoplankton blooms are dynamic phenomena of great importance to the functioning of estuarine andcoastal ecosystems. We analysed a unique (large) collection of phytoplankton monitoring data covering86 coastal sites distributed over eight regions in North America and Europe, with the aim of investigatingcommon patterns in the seasonal timing and species composition of the blooms. The spring bloom wasthe most common seasonal pattern across all regions, typically occurring early (FebruaryeMarch) atlower latitudes and later (AprileMay) at higher latitudes. Bloom frequency, defined as the probability ofunusually high biomass, ranged from 5 to 35% between sites and followed no consistent patterns acrossgradients of latitude, temperature, salinity, water depth, stratification, tidal amplitude or nutrient con-centrations. Blooms were mostly dominated by a single species, typically diatoms (58% of the blooms)and dinoflagellates (19%). Diatom-dominated spring blooms were a common feature in most systems,although dinoflagellate spring blooms were also observed in the Baltic Sea. Blooms dominated bychlorophytes and cyanobacteria were only common in low salinity waters and occurred mostly at highertemperatures. Key bloom species across the eight regions included the diatoms Cerataulina pelagica andDactyliosolen fragilissimus and dinoflagellates Heterocapsa triquetra and Prorocentrum cordatum. Otherfrequent bloom-forming taxa were diatom genera Chaetoceros, Coscinodiscus, Skeletonema, and Tha-lassiosira. Our meta-analysis shows that these 86 estuarine-coastal sites function as diatom-producingsystems, the timing of that production varies widely, and that bloom frequency is not associated withenvironmental factors measured in monitoring programs. We end with a perspective on the limitationsof conclusions derived from meta-analyses of phytoplankton time series, and the grand challengesremaining to understand the wide range of bloom patterns and processes that select species as bloomdominants in coastal waters.© 2015 Elsevier Ltd. All rights reserved.