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 Mediterranean show an exponential decline with depth. Assuming this to represent a steady-state balance between microbial respiration and the downward flux of carbon, the downward flux is estimated as (1−ε/ε)p/b, where p is the microbial production, ε their gross growth efficiency and b the coefficient of exponential decline with depth. Summer profiles, ranging over about 3° of latitude and 4° of longitude, were well fitted by a two-component exponential decline, suggesting two distinct microbial substrates. 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 component, 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 200 m, carbon flux is dominated by the slow component. Flux estimates are compatible with flux estimates from sediment traps in the same region. The observed changes between the spring and summer profiles, combined with the horizontal homogeneity of the summer profiles below 200 m, are consistent with a downward transport of about 5–10 m d–1, implying a significant dispersive component to the observed fluxes.