Marine dissolved organic matter: can its C : N ratio explain carbon overconsumption?

Carbon overconsumption, i.e. the consumption of inorganic carbon relative t o inorganic nitrogen in excess of the Redfield ratio at the sea surface, wa s examined in relation to the dynamics of dissolved organic carbon and nitr ogen (DOC and DON) in the northeast Atlantic. We observed the presence of N -poor dissolved organic matter (DOM) in surface water during summer, requir ing the consumption of inorganic carbon and nitrogen in a ratio exceeding t he Redfield ratio. The C:N ratio of bulk DOM is not only different from the Redfield ratio but also variable, i.e. no fixed conversion factor of C and N exists where DOM is important in C and N transformations. The existence of N-poor DOM is recognized as a feature typical of oligotrophic systems. A t the same time, the C:N ratios of particles conform to Redfield stoichiome try as does deep-ocean chemistry. The implications of this finding are disc ussed, the conclusion being that, while DOM buildup contributes to CO, draw down seasonally, its impact on long-term carbon and nitrogen balance of the ocean is small. (C) 2000 Elsevier Science Ltd. All rights reserved.