Lakes and reservoirs play an important role in the carbon cycle, and therefore monitoring their metabolic rates is essential. The triple oxygen-isotope anomaly of dissolved O₂ [¹⁷Δ =  ln(1+δ¹⁷O) − 0.518  ×  ln(1 + δ¹⁸O)] offers a new, in situ, perspective on primary production, yet little is known about ¹⁷Δ from freshwater systems. We investigated the ¹⁷Δ together with the oxygen : argon ratio [Δ(O₂ ∕ Ar)] in the subtropical Feitsui Reservoir in Taiwan from June 2014 to July 2015. Here, we present the seasonal variations in ¹⁷Δ, GP (gross production), NP (net production) and the NP ∕ GP (net to gross ratio) in association with environmental parameters. The ¹⁷Δ varied with depth and season, with values ranging between 26 and 205 per meg. The GP rates were observed to be higher (702 ± 107 mg C m⁻² d⁻¹) in winter than those (303 ± 66 mg C m⁻² d⁻¹) recorded during the summer. The overall averaged GP was 220 g C m⁻² yr⁻¹ and NP was −3 g C m⁻² yr⁻¹, implying the reservoir was net heterotrophic on an annual basis. This is due to negative NP rates from October to February (−198 ± 78 mg C m⁻² d⁻¹). Comparisons between GP rates obtained from the isotope mass balance approach and ¹⁴C bottle incubation method (¹⁴C–GP) showed consistent values on the same order of magnitude with a GP ∕ ¹⁴C–GP ratio of 1.2 ± 1.1. Finally we noted that, although typhoon occurrences were scarce, higher than average ¹⁷Δ values and GP rates were recorded after typhoon events.