Contribution of combined carbohydrates to dissolved and particulate organic carbon after the spring bloom in the northern Bay of Biscay (North-Eastern Atlantic Ocean)

Two cruises were conducted after the diatom spring bloom in the northern Bay of Biscay (2006, 2007), to assess the contribution of combined carbohydrates to organic carbon partitioning. Partitioning of total organic carbon (TOC) into particulate organic carbon (POC) and dissolved organic carbon (DOC) differed between the two years, particularly for depths above 60 m, and was related to the vernal development of the system: a post spring-bloom system in 2007, and a more stratified summer system with higher coccolithophore abundance in 2006. In general, contribution of POC to TOC ranged between 4 and 28% and decreased with depth. Concentration of high molecular weight (>1 kDa) dissolved combined carbohydrates (dCCHO) ranged from 0.6 to 1.4 µmol L−1 and contributed between 4 and 11% to DOC. Concentration of particulate combined carbohydrates (pCCHO) varied between 0.03 and 1.3 µmol L−1. A high contribution of pCCHO to POC was observed in 2007, i.e. 22–60% C compared to 3–10% C in 2006, and coincided with a higher abundance of transparent exopolymer particles (TEP). TEP accounted for 0.4–2.0 µmol C L−1 in 2007 and 0.5–1.5 µmol C L−1 in 2006. Above 60 m, differences in contribution of TEP-C to POC were most pronounced yielding 15.4±3.0% in 2007 compared to relatively low 4.8±1.4%, in 2006. TEP-C could explain about 60% in 2007 and about 40% of pCCHO-C in 2006. Hence, TEP were identified as a substantial component of pCCHO and POC, particularly in the wake of the spring bloom. Molecular composition of CCHO, i.e. HMW-dCCHO+pCCHO, revealed little difference between the years but strong variation over depth. Uronic acids (URA) were identified as a major component of CCHO (20–40%). Our study indicates that the distribution and composition of CCHO in surface seawater are determined by biogeochemical processes on a seasonal scale. A better knowledge of CCHO cycling and molecular signature has therefore a high potential for a better tracing of carbon dynamics in shelf sea ecosystems