The seasonal <i>p</i>CO₂ cycle at 49°N/16.5°W in the northeastern Atlantic Ocean and what it tells us about biological productivity

Abstract

A 2‐year record of mixed layer measurements of CO 2 partial pressure ( p CO 2 ), nitrate, and other physical, chemical, and biological parameters at a time series site in the northeast Atlantic Ocean (49°N/16.5°W) is presented. The data show average undersaturation of surface waters with respect to atmospheric CO 2 levels by about 40 ± 15 μ atm, which gives rise to a perennial CO 2 sink of 3.2 ± 1.3 mol m −2 a −1 . The seasonal p CO 2 cycle is characterized by a summer minimum (winter maximum), which is due to the dominance of biological forcing over physical forcing. Our data document a rapid transition from deep mixing to shallow summer stratification. At the onset of shallow stratification, up to one third of the mixed layer net community production during the productive season had already been accomplished. The combination of high prestratification productivity and rapid onset of stratification appears to have caused the observed particle flux peak early in the season. Mixed layer deepening during fall and winter reventilated CO 2 from subsurface respiration of newly exported organic matter, thereby negating more than one third of the carbon drawdown by net community production in the mixed layer. Chemical signatures of both net community production and respiration are indicative of carbon overconsumption, the effects of which may be restricted, though, to the upper ocean. A comparison of the estimated net community production with satellite‐based estimates of net primary production shows fundamental discrepancies in the timing of ocean productivity.