Measuring primary production rates in the ocean: Enigmatic results between incubation and non‐incubation methods at Station ALOHA

Primary production (PP) rates were estimated using concurrent 14 C and 18 O bottle incubations and a non‐incubation oxygen isotope ( 17 Δ) based method during monthly cruises to the time series station ALOHA in the subtropical N. Pacific Ocean between March, 2006 and February, 2008. The mean gross oxygen production (GOP) rate in the photic layer (0–200m) at ALOHA was estimated at 103 ± 43 and 78 ± 17 mmol O 2 m −2 d −1 from the 17 Δ and 18 O methods, respectively. In comparison, the mean 14 C‐PP rate (daytime incubations) in the photic layer was 42 ± 7 mmol C m −2 d −1 (502 ± 84 mg C m −2 d −1 ). Seasonal and depth variability (% change) for GOP rate was 2–3 times that for 14 C‐PP. The non‐incubation 17 Δ‐GOP rates consistently exceeded the incubation 18 O‐GOP rates by 25–60%, and possible methodological biases were evaluated. A supersaturation of the dissolved O 2 /Ar gas ratio was measured every month yielding a mean annual value of 101.3 ± 0.1% and indicating a consistent net autotrophic condition in the mixed layer at ALOHA. The mean annual net community production (NCP) rate at ALOHA estimated from dissolved O 2 /Ar gas ratio was 14 ± 4 mmol O 2 m −2 d −1 (120 ± 33 mg C m −2 d −1 or 3.7 ± 1.0 mol C m −2 yr −1 ) for the mixed layer. A NCP/GOP ratio of 0.19 ± 0.08 determined from 17 Δ and O 2 /Ar measurements indicated that ∼20% of gross photosynthetic production was available for export and harvest.