Net community production at Ocean Station Papa observed with nitrate and oxygen sensors on profiling floats

Abstract

Six profiling floats equipped with nitrate and oxygen sensors were deployed at Ocean Station P in the Gulf of Alaska. The resulting six calendar years and 10 float years of nitrate and oxygen data were used to determine an average annual cycle for net community production (NCP) in the top 35 m of the water column. NCP became positive in February as soon as the mixing activity in the surface layer began to weaken, but nearly 3 months before the traditionally defined mixed layer began to shoal from its winter time maximum. NCP displayed two maxima, one toward the end of May and another in August with a summertime minimum in June corresponding to the historical peak in mesozooplankton biomass. The average annual NCP was determined to be 1.5 ± 0.6 mol C m À2 yr À1 using nitrate and 1.5 ± 0.7 mol C m À2 yr À1 using oxygen. The results from oxygen data proved to be quite sensitive to the gas exchange model used as well as the accuracy of the oxygen measurement. Gas exchange models optimized for carbon dioxide flux generally ignore transport due to gas exchange through the injection of bubbles, and these models yield NCP values that are two to three time higher than the nitrate-based estimates. If nitrate and oxygen NCP rates are assumed to be related by the Redfield model, we show that the oxygen gas exchange model can be optimized by tuning the exchange terms to reproduce the nitrate NCP annual cycle.

Insight into chemical, biological, and physical processes in coastal waters from dissolved oxygen and inert gas tracers

“…These results indicate that gas exchange parameterizations not explicitly developed for the coastal ocean may be applicable in such environments. Recent studies have indicated that the parameterization of Liang et al (2013) is also appropriate for simulating O 2 in the northeast Pacific Ocean Plant et al, 2016). All of these studies demonstrate that a parameterization with no bubble flux and too much bubble flux will both result in errors in the calculated air-water flux for low solubility gases.…”

“…However, many gas exchange parameterizations do not explicitly include a bubblemediated flux and there is currently no consensus on the most accurate parameterization for bubble-mediated exchange (Liang et al, 2013;Emerson and Bushinsky, 2016;Plant et al, 2016). Published parameterizations predict a wide range of magnitudes for both the net bubble-mediated flux and the relative importance of bubbles that completely or partially dissolve for a given wind speed and gas saturation anomaly (Hamme and Emerson, 2006;Stanley et al, 2009b;Nicholson et al, 2011;Liang et al, 2013;Nicholson et al, 2016;Plant et al, 2016). For lower solubility gases, such as O 2 , a significant fraction of the total airwater flux is driven by bubbles, and uncertainties in the parameterization of bubble-mediated exchange directly translate to uncertainties in net community production calculated from O 2 mass balance when simultaneous noble gas measurements are not available (Plant et al, 2016).…”

“…In Monterey Bay, I showed that daily measurements of five stable noble gases can be used to evaluate gas exchange parameterizations in coastal waters and that the parameterization of Liang et al (2013) displayed the most skill in simulating the observations. Using O 2 sensors on autonomous Bio-Argo floats, moorings, and gliders, it is possible to quantify net community production throughout the oceans at low cost compared to ship-based time-series Nicholson et al, 2008;Emerson and Bushinsky, 2014;Nicholson et al, 2015;Plant et al, 2016). However, autonomous sensors for dissolved noble gases are not currently available and NCP estimates derived from O 2 alone are strongly dependent on the choice of gas exchange parameterization (Plant et al, 2016;Emerson and Bushinsky, 2016).…”

“…Using O 2 sensors on autonomous Bio-Argo floats, moorings, and gliders, it is possible to quantify net community production throughout the oceans at low cost compared to ship-based time-series Nicholson et al, 2008;Emerson and Bushinsky, 2014;Nicholson et al, 2015;Plant et al, 2016). However, autonomous sensors for dissolved noble gases are not currently available and NCP estimates derived from O 2 alone are strongly dependent on the choice of gas exchange parameterization (Plant et al, 2016;Emerson and Bushinsky, 2016). Measurements of natural abundance inert gases are an effective tool for developing and distinguishing between different gas exchange parameterizations, which can subsequently be used in studies where noble gas measurements are not available (Spitzer and Jenkins, 1989;Hamme and Emerson, 2006;Stanley et al, 2006Stanley et al, , 2009b.…”

“…All of these studies demonstrate that a parameterization with no bubble flux and too much bubble flux will both result in errors in the calculated air-water flux for low solubility gases. These and other studies provide insight into the most appropriate gas exchange parameterization to use in global biogeochemical models, and when interpreting data sets where inert gas measurements are not available, such as measurements of O 2 from moorings and BioArgo floats (Hamme and Emerson, 2006;Emerson and Stump, 2010;Plant et al, 2016).…”