Long-term observations that resolve seasonal and interannual controls on N₂O dynamics remain scarce. Here, we present a decade-long (November 2015–April 2026) weekly time series of N₂O concentrations from Bedford Basin, a seasonally stratified coastal fjord in Nova Scotia, along with concurrent measurements of dissolved oxygen, inorganic nutrients, hydrographic variability, and microbial community composition.
The time series revealed substantial interannual variability in N₂O accumulation, including two extreme N₂O events (Events I and II) characterized by contrasting biogeochemical and microbial signatures. The first event, observed from late 2019 to early 2020, coincided with elevated NH₄⁺ and NO₂⁻ concentrations, indicating enhanced N₂O production associated with nitrification. In contrast, the second event, occurring from late 2021 to early 2022, developed during a period of declining bottom-water oxygen concentrations and was characterized by decreasing NO₃⁻ and elevated NH₄⁺ concentrations, suggesting a greater contribution from nitrate-reducing pathways, potentially linked to dissimilatory nitrate reduction to ammonium (DNRA).
This decade-long record reveals contrasting N₂O production regimes within the same marine ecosystem. The observed shifts in N₂O production pathways were closely associated with changes in microbial community composition and physical mixing dynamics, which likely regulate nitrifier biomass through dilution and regrowth processes, consistent with the conceptual framework proposed by Haas et al. (2021).
 

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