No abstract

[1] The thermohaline structure across the tidal fronts of the continental shelf off Patagonia is analyzed using historical and recent summer hydrographic sections. The near-summer tidal front location is determined on the basis of the magnitude of vertical stratification of the water column as measured by the Simpson parameter. Sea surface and air CO 2 partial pressures based on data from eleven transects collected in summer and fall from 2000 to 2004 are used to estimate CO 2 fluxes over the shelf. The near-shore waters are a source of CO 2 to the atmosphere while the midshelf region is a CO 2 sink. The transition between source and sink regions closely follows the location of tidal fronts, suggesting a link between vertical stratification of the water column and the regional CO 2 balance. The highest surface values of Chl a are associated with the strongest CO 2 sinks. The colocation of lowest CO 2 partial pressure (pCO 2 ) and highest Chl a suggests that phytoplankton blooms on the stratified side of the fronts draw the ocean's CO 2 to very low levels. The mean shelf sea-air difference in pCO 2 (DpCO 2 ) is À24 matm and rises to À29 matm if the shelf break front is included. Peaks in DpCO 2 of À110 matm, among the highest observed in the global ocean, are observed. The estimated summer mean CO 2 flux over the shelf is À4.4 mmol m À2 d À1 and rises to À5.7 mmol m À2 d À1 when the shelf break area is taken into account. Thus, during the warm season the shelf off Patagonia is a significant atmospheric CO 2 sink.

[1] Sea-air differences of CO 2 partial pressures (DpCO 2 ) and surface chlorophyll a (chl-a) concentration have been determined during 22 cruises in various seasons for 2000-2006 over the Patagonia Sea and shelf break. From spring to autumn, the nearshore waters act as a source of atmospheric CO 2 , while the midshelf and slope are a CO 2 sink, leading to highly negative areal means of sea-air CO 2 flux and DpCO 2 . The DpCO 2 and CO 2 flux in spring reach values of À67 matm and À7 Â 10 À3 mol m À2 d À1 , respectively, and are close to equilibrium in winter. Sea-air DpCO 2 and chl-a over the shelf are negatively correlated, suggesting that photosynthesis is one of the main processes responsible for the large CO 2 sequestration. The annual areal mean DpCO 2 and sea-air CO 2 flux are À31 matm and À3.7 Â 10 À3 mol m À2 d À1 , respectively, indicating that the Patagonia Sea is one of the strongest CO 2 sinks per unit area in the World Ocean.

The influence of the plankton community structure on carbon dynamics was studied in the surface waters of the Argentinean continental shelf (SW Atlantic Ocean) in summer and fall 2002, 2003 and 2004. The horizontal changes in plankton community respiration (R), net community production (NCP) and gross primary production (GPP) were (1) compared with the difference in the partial pressure of CO 2 (pCO 2 ) between the sea surface and the atmosphere (ΔpCO 2 ), (2) compared with oxygen saturation and (3) related to the microscopic phytoplankton assemblages. This area, which has recently been shown to be a CO 2 sink, had an average surface oxygen saturation of 108.1%, indicating that net photosynthesis could have played a dominant role in the CO 2 dynamics. At most stations, the production:respiration (GPP:R) ratio was greater than 1, indicating that planktonic communities were autotrophic; the average GPP:R ratio for the whole study was 2.99. Phytoplankton biomass (chlorophyll a) and NCP showed an inverse relationship with ΔpCO 2 and a direct relationship with %O 2 saturation when phytoplankton assemblages were dominated by diatoms (30% of the stations). This was not the case when small (≤5 µm) flagellates were the most abundant organisms. Although NCP was mostly positive for both groups of stations (i.e. diatom-dominated or small flagellate-dominated), other physical and biological processes are thought to modify the CO 2 dynamics when small flagellates are the prevailing phytoplankton group.