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

Abstract

Primary production (PP) rates were estimated using concurrent 14C and 18O bottle incubations and a non‐incubation oxygen isotope (17Δ) 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 O2 m−2 d−1 from the 17Δ and 18O methods, respectively. In comparison, the mean 14C‐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 14C‐PP. The non‐incubation 17Δ‐GOP rates consistently exceeded the incubation 18O‐GOP rates by 25–60%, and possible methodological biases were evaluated. A supersaturation of the dissolved O2/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 O2/Ar gas ratio was 14 ± 4 mmol O2 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 17Δ and O2/Ar measurements indicated that ∼20% of gross photosynthetic production was available for export and harvest.