Mesoscale and seasonal variability of community production and respiration in the surface waters of the N.E. Atlantic Ocean

Gross community production (GCP) and dark community respiration (DCR) rates were measured in the N.E. Atlantic Basin (38–45°N and 15°20′–21°20′W) during the Programme Océanographique Multidisciplinaire Méso-Echelle (POMME). Three cruises were conducted over a one-year period (2001), and GCP and DCR were measured at 20 stations using seawater taken from 5 m. In winter, GCP is light limited whereas DCR, which is mainly due to bacteria, is limited by substrate availability. GCP and DCR were under the influence of mesoscale features, with the cyclonic structure enhancing the autotrophy. In spring, light and resource availability remain the major controlling parameters, which are constrained by north–south zonation, rather than mesoscale features. The most productive area is south of 41°N at the start of the bloom and is associated with greater DCR as autotrophs contribute to community respiration. The late-summer period is oligotrophic, which contrasts with the previous winter and spring period, characterised by a large quantity of TOC accumulated (+13.2 μM C from spring to late-summer) and with greater DCR in the southern area. The GCP/DCR ratio greatly varied seasonally, ranging from 0.6 in late summer to 1.3 in winter and 2.4 in spring. These results, even restricted to surface ecosystem metabolism (5 m), strongly suggest that the trophic status of a regional system in the open ocean should be determined over a seasonal cycle.