Environmental assessment of tide-driven spatiotemporal dynamics of fecal coliform as a microbial hazard along Canada's coasts

Fecal coliforms (FC), as indicators of pathogenic microbial hazards, represent an important environmental contaminant posing significant risks to coastal water quality, public health, and shellfish safety along Canada's diverse shorelines. Tidal dynamics are a major driver of short-term variability in fecal indicator bacteria, yet their quantitative impacts remain poorly characterized across Canada's contrasting coastal regimes. This study addresses this gap using more than 10 years of national monitoring data to quantify tidal and rainfall effects on FC concentrations along Canada's west (Pacific) and east (Atlantic) coasts. We apply two Generalized Additive Mixed Models (GAMMs). Time-averaged sector models map mean effects of tide and rainfall, and dynamic GAMMs with seasonal smooths describe within-year and area-specific variations. On the west coast, tidal effects show a seasonal reversal, with flushing and dilution more evident in summer and enhanced nearshore retention in winter. On the east coast, spatial patterns vary with geomorphology and tidal range. Differences among Bay of Fundy's tidal basins, estuaries, embayments, exposed coastal areas, and semi-enclosed inland waters are associated with contrasts in flushing and FC retention. Rainfall frequently modifies these tidal relationships and amplifies contamination pulses in poorly flushed systems. These findings provide insights into coastal contamination behaviour and support the development of predictive risk assessment frameworks and adaptive management strategies that account for tidal phase and seasonality, thereby helping to protect shellfish-harvesting and recreational waters under changing climatic conditions.