Variations in triple isotope composition of dissolved oxygen and primary production in a subtropical reservoir

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, but is yet to be evaluated in freshwater systems. We investigated the 17Δ together with oxygen-argon ratio (δO2/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 measured. The 17Δ varied with depth and season, with values ranging between 19 and 186 per meg. The 17Δ GP rates were lower from April to September averaging 215±93 mg C m−2 d−1 and higher from October to January averaging 523±66 mg C m−2 d−1. The estimated average annual 17Δ GP was 104 g C m−2 year−1 and the average annual NP was 22 g C m−2 year−1. Overall, the NP/ GP varied slightly between 0.02 and 0.36 and the reservoir was net autotrophic in the mixed layer. Comparisons between 17Δ GP rates and the production rates estimated by in vitro 14C bottle incubation method (14C GP) were consistent on the same order of magnitude, with the 17Δ GP/14C GP ratio of 1±0.8 throughout the study. Although typhoon occurrences were scarce, higher than average 17Δ values and 17Δ GP rates were recorded after typhoon events.