Pergamon Deep-Sea Researclr II. Vol. 43, No. 2-3. pp. 347-383. 1996 0967-0645(9qooo93-3 Copyright 0 1996 Elsevin Science Ltd F’rinted in Great Britain. All rights reserved 09674645/96515.00+0.00 Seasonal and interannual variability of oceanic carbon dioxide species at the U.S. JGOFS Bermuda Atlantic Time-series Study (BATS) site NICHOLAS R. BATES,* ANTHONY F. MICHAELS* and ANTHONY H. KNAP* (Received 6 November 1994: in revisedform 23 June 1995; accepted 19 August 1995) Abstract-The seasonal and interannual dynamics of the oceanic carbon cycle and the strength of air-sea exchange of carbon dioxide are poorly known in the North Atlantic subtropical gyre. Between October 1988 and December 1993, a time series of oceanic measurements of total carbon dioxide (TCO?), alkalinity (TA) and calculated pC02 was obtained at the Bermuda Atlantic Time- series Study (BATS) site (31”5O’N, 64”lO’W) in the Sargasso Sea. These measurements constitute the most extensive set of CO2 species data collected in the oligotrophic North Atlantic. Seasonal changes in surface and water-column CO2 species were -4&50 pmol kg-’ in TCOZ, _ 20 pmol kg-’ in TA, and -90-100 patm in calculated pCOz. These large changes were driven principally by deep convective winter mixing, temperature forcing and biological activity. TA was well correlated with salinity (with the exception of a 15-25 pmol kg-’ drawdown of TA on one cruise resulting from open-ocean calcification). TCOz and pCO2 were well correlated with seasonal temperature changes (8-9°C). Other underlying processes, such as biological production, advection, gas exchange of CO2 and vertical entrainment, were important modulators of the carbon cycle, and their importance varied seasonally. Each spring-to-summer, despite the absence of measurable nutrients in the euphotic zone, a 35-40 pmol kg-’ decrease in TC02 was attributed primarily to the biological uptake of TCOz (evaporation/precipitation balance, gas exchange, and advection were also important). An increase in TCOz during the fall months was associated primarily with entrainment of higher TCOz subsurface waters. These seasonal patterns require a reassessment of the modelling of the carbon cycle using nutrient tracers and Redfield stoichiometries. Overall, the region is a weak sink (0.22-0.83 mol C m-* year-‘) for atmospheric CO*. Upper ocean TCOl levels increased between 1988 and 1993, at a rate of 5 1.7 pmol kg-’ year-‘. This increase appears to be in response to the uptake of atmospheric CO2 through gas exchange or natural variability of the subtropical gyre. Copyright 8 1996 Elsevier Science Ltd INTRODUCTION Approximately 60% of the anthropogenically produced carbon dioxide currently accumulates in the atmosphere, with the remainder transferred either to the ocean (Keeling et al., 1989; Quay et al., 1992) or terrestrial biomass (Tans et al., 1990). The magnitude of the oceanic carbon sink is uncertain. Large disparities exist between carbon flux estimates derived from surface measurements of the air-sea gradient of COz, A&O1 (Tans et al., 1990; Sarmiento and Sundquist, 1992; Sarmiento, 1993; Siegenthaler and Sarmiento, 1993), and those derived from measurements of oceanic 13C/‘*C isotope ratios of total carbon dioxide (Quay et al., 1992; Tans et al., 1993; Francey et al., 1995). Several models have predicted an increase in upper ocean total carbon dioxide (TC02) of l-3 pmol *Bermuda Biological Station for Research, 17 Biological Station Lane, Ferry Reach, GEOl, Bermuda. 347