The role of calcareous and biosiliceous organisms in the organic carbon export rates of the NW Iberian coastal upwelling system

Diana Zuniga1, Celia Santos2, Maria Frójan3, Emilia Salgueiro4, Catarina Dinis Cavaleiro5, Fernando Alonso-Perez3, Alexandra Silva2, José-Abel Flores6, Francisco Figueiras3, Fatima F G Abrantes4 and Carmen G Castro7, (1)University of Vigo, Physical Oceanography, Vigo, Spain, (2)IPMA, Divisão de Geologia e Georecursos Marinhos, Lison, Portugal, (3)IIM-CSIC, Instituto de Investigaciónes Mariñas – CSIC, Vigo, Spain, (4)Instituto Port Mar e Atmosfera, Lisbon, Portugal, (5)MARUM - University of Bremen, Bremen, Germany, (6)University of Salamanca, Department of Geology, Salamanca, Spain, (7)Instituto de Investigacións Mariñas – CSIC, Vigo, Spain
Abstract:
Understanding ocean carbon cycling must include determining the efficiency of the biological pump, expressed as the fraction of the total carbon produced through photosynthesis that is exported from the surface layer.

To assess the organic carbon export rates in the highly productive NW Iberian coastal upwelling system, a mooring line dotted with an automated PPS 4/3 sediment trap was deployed off Cape Silleiro at the base of the photic zone. The samples were collected from November 2008 through June 2012 to determine major biogenic compounds (organic carbon (OC), calcium carbonate (CaCO3) and biogenic silica (bSiO2)) of the total mass flux. In addition, water column samples for phytoplankton counting were also recovered during monthly cruises.

Strong positive correlations between OC fluxes and both CaCO3 and bSiO2 fluxes (r=0.96 and r=0.95, respectively) point that OC export rates in this coastal upwelling system are seasonally controlled by siliceous (diatoms) and calcareous (coccolitophorids) phytoplankton blooms. The higher contribution of CaCO3 (10 ± 6%) to the vertical fluxes compared to bSiO2 share (6 ± 2%) mirrored the higher abundances of coccolitophorids respect to diatoms observed in the upper layer, highlighting the role of carbonate organisms as ballast minerals for OC. Otherwise, in terms of carbon units, OC preferentially co-sedimented with bSiO2 as occurred in diatom dominated production regimes, where stronger seasonality and a more event-driven export or pulsed sedimentation occurred. In this regard, we observed how the evolution of environmental conditions during the productive upwelling seasons modified the diatom community structure in the upper water column with the subsequent alteration of organic carbon export. Overall, we found a succession from well adapted to turbulent conditions Chaetoceros sp. to Leptocylindrus danicus blooms that mainly occurred under stratified water column conditions.

Back to: Physical and Biogeochemical Processes and the Support of Shelf Sea Primary Productivity and Carbon Cycling I