Non-Redfield C : N ratio of transparent exopolymeric particles in the northwestern Mediterranean Sea

The stoichiometric model of Redfield, which describes the elemental composi tion of marine organic matter, is generally used to link the production of new organic matter to the uptake of nitrate. The Redfield C:N molar ratio o f 6.6 is a well-established value for particulate organic matter produced i n surface waters. Yet recent studies have shown that the inorganic C:N upta ke ratio during nitrate-limiting conditions is >14. This non-Redfield behav ior during the production of new organic matter suggests that a large stand ing stock of organic matter, rich in carbon, should accumulate in the eupho tic zone from the spring bloom to late summer. This hypothetical pool of ca rbon-rich organic matter and the pool of transparent exopolymeric particles (TEP) exhibit similar seasonal distributions, which suggests that TEP may indeed be this carbon-rich pool. For this scheme to work, TEP should have a high C:N ratio. TEP distribution from an open-ocean site in the northwestern Mediterranean Sea (DYFAMED) was monitored, and TEP C:N ratio was measured from TEP produc ed in the laboratory by bubbling dissolved organic matter collected in the field. We found that the TEP pool increases during the summer season and th at the C:N ratio of TEP produced from naturally occurring dissolved organic matter, is well in excess of the Redfield ratio with an overall average C: N molar ratio of similar to 20. As a result, the production of TEP could be the main pathway of carbon overconsumption required of oligotrophic, nitra te-limiting waters, and, thus, TEP may represent an important intermediary pool in the ocean carbon cycle.