Evaluating triple oxygen isotope estimates of gross primary production at the Hawaii Ocean Time-series and Bermuda Atlantic Time-series Study sites

The triple oxygen isotopic composition of dissolved oxygen ( ¹⁷Δ) is a promising tracer of gross oxygen productivity (P) in the ocean. Recent studies have inferred a high and variable ratio of P to ¹⁴C net primary productivity (12-24 h incubations) (e.g., P:NPP( ¹⁴C) of 5-10) using the ¹⁷Δ tracer method, which implies a very low efficiency of phytoplankton growth rates relative to gross photosynthetic rates. We added oxygen isotopes to a one-dimensional mixed layer model to assess the role of physical dynamics in potentially biasing estimates of P using the ¹⁷Δ tracer method at the Bermuda Atlantic Time-series Study (BATS) and Hawaii Ocean Time-series (HOT). Model results were compared to multiyear observations at each site. Entrainment of high ¹⁷Δ thermocline water into the mixed layer was the largest source of error in estimating P from mixed layer ¹⁷Δ. At both BATS and HOT, entrainment bias was significant throughout the year and resulted in an annually averaged overestimate of mixed layer P of 60 to 80%. When the entrainment bias is corrected for, P calculated from observed ¹⁷Δ and ¹⁴C productivity incubations results in a gross:net productivity ratio of 2.6 (+0.9 -0.8) at BATS. At HOT a gross:net ratio decreasing linearly from 3.0 (+1.0 -0.8) at the surface to 1.4 (+0.6 -0.6) at depth best reproduced observations. In the seasonal thermocline at BATS, however, a significantly higher gross:net ratio or large lateral fluxes of ¹⁷Δ must be invoked to explain ¹⁷Δ field observations.