Mesoscale variability of primary production in the deep convection region of the NW Mediterranean

Abstract

The phytoplankton bloom in the Liguro-Provençal deep convection region represents one of the main fertilization mechanisms in the Mediterranean. This communication examines microphytoplankton observations and primary production data obtained during four cruises covering the seasonal changes between mid-March and September in the region. Our aims were to constrain primary production estimates and to ascertain the importance of short-term environmental variability on the photosynthetic response of phytoplankton assemblages in the area. Seasonal trends from winter to late summer were clear, with stratification increasing and Chl a concentration, primary production and dissolved inorganic nitrogen and phosphate fluxes decreasing. Chl a at 5 m depth reached a maximum of 7 mg m-3 on 25 March 2005 and average surface values ranged from approximately 2 mg m-3 in the March cruises to 0.1 mg m-3 in September. Vertically integrated (0-80 m) primary production attained 1800 mg C m-2 in March 2009, one of the highest values measured in the region, with an average of approximately 1000 mg C m-2, and decreased to a mean of 145 mg C m-2 in September 2009. However, during the bloom and post-bloom periods, both phytoplankton biomass and primary production showed considerable variability at the scale of days, in connection with the mesoscale heterogeneity in the region and with the physiological and ecological history of the planktonic communities associated with each present water body. These observations have implications for regional estimates of primary production based on modeling and remote sensing