Abstract
Nitrogen-fixing microbes (diazotrophs)—support productivity in marine ecosystems but their dynamics in temperate coastal waters remain poorly characterised. Using monthly niƒH amplicon sequencing and environmental data collected over nine years (2011-2020) in Buzzards Bay, diazotroph diversity and community composition were assessed in the context of environmental drivers. Diazotroph Alpha diversity had limited seasonal variation in Richness but higher Shannon diversity and Evenness in winter compared to summer. Community composition displayed clear seasonal variation, with winter and summer communities most distinct. Non-cyanobacterial diazotrophs were consistently prominent and exhibited marked seasonal shifts, with sulphate reducing taxa dominating. The cyanobacterial, algal symbiont UCYN-A (Candidatus Atelocyanobacterium thalassa) peaked consistently in late summer–early autumn, and was overwhelmingly dominated by clade A2, with minor contributions from clades A1 and A4.UCYN-A relative abundance had a significant increase in the fall communities over the study period, suggesting a progressive strengthening of ecological conditions that favour UCYN-A in fall, while Desulphuromusa kysingii-related Amplicon Sequence Variants (ASVs) had a decreasing trend during spring, summer, and fall. Temperature (°C) was the primary factor correlating with community composition, with turbidity and nutrients also contributing. Diazotroph dynamics were broadly similar between the coastal and mid-bay stations; however, significantly higher richness at the coastal station compared to mid-bay indicates nearshore or sedimentary influence playing a role in diazotroph community assembly. Overall, seasonal environmental filtering shapes diazotroph community structure in this temperate estuarine system, with niche differentiation among dominant taxa underpinning the temporal stability of the potential for nitrogen-fixation capacity.