Seasonal and interannual variability of phytoplankton, nutrients, TCO2, pCO2, and O2 in the eastern subarctic Pacific (ocean weather station Papa)

Abstract

A coupled, one‐dimensional ecosystem/carbon flux model is used to simulate the seasonal and interannual variability of phytoplankton, nutrients, TCO2, O2, and pCO2 at ocean weather station Papa (OWS P at 50°N, 145°W). The 23‐year interannual simulation (1958–1980) is validated with available data and analyzed to extend seasonal and interannual variations beyond the limited observational records. The seasonal cycles of pCO2 and sea‐air CO2 flux are controlled by a combination of thermodynamics, winds, and biological uptake. There is ingassing of CO2 during the fall‐winter months when SSTs are colder and wind forcing is vigorous, while there is a much smaller ingassing of CO2 during the summer when sea surface temperatures are warmer and wind speeds are reduced. Biological production plays a major role in maintaining the air‐sea equilibrium. An abiotic simulation showed that OWS P would be a source of atmospheric CO2 (1.41 mol C m−2 yr−1) if the biological sink of CO2 were removed. The peak net community production in summer compensates for the increased temperature effect on pCO2, which prevents large outgassing in summer. Oxygen anomalies relative to the temperature‐determined saturation value show that there is a seasonal cycle of air‐sea flux, with ingassing in winter and outgassing in summer. The net surface oxygen flux is positive (0.8 mol m−2 yr−1), indicating that OWS P is a source of oxygen to the atmosphere. The average primary production is 167 g C m−2 yr−1. The 1960–1980 (1958 and 1959 spin‐up years removed) mean carbon flux is −1.8 mol C m−2 yr−1, indicating that the ocean at OWS P is a sink of atmospheric carbon. The sea‐air CO2 flux ranges from −1.2 to −2.3 mol C m−2 yr−1 during the 21‐year simulation period. This finding emphasizes the need for long‐term observations to accurately determine carbon flux budgets. A series of sensitivity experiments indicate that the seasonal variability and overall (21 years) mean of TCO2, pCO2, ΔpCO2, and air‐sea CO2 flux are strongly dependent on the gas transfer formulation adopted, the total alkalinity near the surface, and the bottom (350 m) value adopted for TCO2. The secular atmospheric pCO2 upward trend is manifested in the TCO2 concentration within the upper 100 m by an increase of 15 mmol m−3 in 20 years, consistent with observations at other locations [Winn et al., 1998; Bates, 2001].