Seasonal phenology and spatiotemporal persistence of fecal coliform contamination in shellfish-harvesting waters of Canada's Pacific and Atlantic coasts.

Fecal indicator bacteria remain central to coastal water-quality surveillance, yet long-term monitoring records are often summarized descriptively without reproducible quantification of seasonal phenology and contamination hotspot dynamics. Using fecal coliform (FC) observations from the Canadian Shellfish Sanitation Program (2011-2022, excluding 2020), we developed a large-scale, coast-specific spatiotemporal characterization of FC variability in shellfish-harvesting waters of Canada's Pacific and Atlantic coasts. Two coast-specific generalized additive mixed models (GAMMs) were fitted to estimate seasonal-interannual and seasonal space-time surfaces. These surfaces were translated into explicitly defined, reproducible metrics describing (i) interannual phenology (peak timing, amplitude, onset, and duration) and (ii) seasonal hotspot dynamics (footprint, persistence, and centroid trajectories), with uncertainty quantified via block bootstrap. On the Pacific coast, FC exhibited a prolonged and highly variable seasonal cycle, with a late-season maximum in mid-autumn and an earlier hotspot expansion in late spring. Recurrent hotspot activity was localized in the southeastern Strait of Georgia, the western Vancouver Island corridor, and isolated northwestern inlets. In contrast, within the monitored summer window, the Atlantic coast showed a lower-amplitude peak and comparatively stable hotspot extent, with persistent activity concentrated along the western New Brunswick coastline and the Northumberland Strait. Hotspot centers displayed greater within-season spatial reorganization on the Pacific coast than on the Atlantic coast. These findings provide a reproducible quantitative basis for translating irregular monitoring data into phenology and hotspot metrics and support more targeted, coast-specific monitoring and risk-based management to reduce microbial contamination and associated public-health risk in shellfish-harvesting waters.