Carbon flux in the northwest Mediterranean estimated from microbial production

Spring profiles of microbial production derived from the dark incorporation of tritiated leucine and tritiated thymidine in the northwest Mediterranea n show an exponential decline with depth. Assuming this to represent a stea dy-state balance between microbial respiration and the downward flux of car bon, the downward flux is estimated as (1 - epsilon/epsilon )p/b, where p i s the microbial production, e their gross growth efficiency and b the coeff icient of exponential decline with depth. Summer profiles, ranging over abo ut 3 degrees of latitude and 4 degrees of longitude, were well fitted by a two-component exponential decline, suggesting two distinct microbial substr ates. Values of b for the more rapidly declining component varied between 0 .01 and 0.06 m(-1) according to location. In the case of the slower compone nt, b was estimated as 0.002 m(-1), and did not vary significantly over the region. Estimated fluxes of carbon at the surface are 123-335 mg m(-2) d(- 1) for the fast and 95 mg m(-2) d(-1) for the slow component. Below about 2 00 m, carbon flux is dominated by the slow component. Flux estimates are co mpatible with flux estimates from sediment traps in the same region. The ob served changes between the spring and summer profiles, combined with the ho rizontal homogeneity of the summer profiles below 200 m, are consistent wit h a downward transport of about 5-10 m d(-1), implying a significant disper sive component to the observed fluxes. (C) 2001 Elsevier Science Ltd. All r ights reserved.