Biological production rates off the Southern California coast estimated from triple O2 isotopes and O2 : Ar gas ratios

Abstract

The isotopic composition of dissolved O2 (17Δ) and the biological O2 saturation from O2 : Ar ratios were measured in the surface ocean during six cruises off the coast of southern California from November 2005 to August 2008 to determine rates of gross oxygen production (GOP), net oxygen production (NOP), and the NOP : GOP ratio (a measure of potential export efficiency). In the mixed layer, 17Δ of dissolved O2 (17Δdiss) ranged from 24 to 108 per meg and biological O2 saturation ranged from 101% to 113% for all regions and cruises. Mixed‐layer 17Δ‐GOP ranged from 49 ± 23 mmol O2 m−2 d−1 to 533 ± 185 mmol O2 m−2 d−1 with an annual mean for the California Cooperative Oceanic Fisheries Investigations (CalCOFI) grid of 151 ± 59 mmol O2 m−2 d−1. Mixed‐layer O2 : Ar‐NOP ranged from 8 ± 6 mmol O2 m−2 d−1 to 135 ± 31 mmol O2 m−2 d−1 with an annual mean of 25 ± 8 mmol O2 m−2 d−1, implying that the CalCOFI grid is autotrophic year‐round. 17Δ‐GOP estimates were consistently greater than on‐deck incubation‐based 14C‐primary production (14C‐PP) by a factor of 5.6 ± 0.4 and greater than satellite PP estimates by a factor of 3.5 ± 0.3 (mmol O2 : mmol C). The 17Δ‐GOP to 14C‐PP factor was twice the expected factor of 2.7 determined from comparisons of incubation‐based 18O‐GOP and 14C‐PP. The annual mean NOP : GOP ratio was 0.16 ± 0.06, suggesting a potential export efficiency that is surprisingly similar to the open ocean using comparable methods.