Fate of organic carbon added as diatom cells to oxic and anoxic marine sediment microcosms

Live Skeletonema costatum (diatom) cells labeled with C-14 Were added to the sediment surface in 10 cores containing intertidal sediment from Saanich Inlet, Vancouver Island, Canada. The water overlying the sediment in 5 cores was sparged with atmospheric air, whereas the other 5 cores were sparged with N-2. Release of (CO2)-C-14 Started immediately in the oxic cores, reaching a maximum at Days 3 to 4, whereas release in the anoxic cores was slower with a maximum after 6 to 9 d. (DOC)-C-14 (dissolved organic carbon) release also exhibited high rates initially, but with the highest release in the anoxic cores. Degradation to (CO2)-C-14 and (DOC)-C-14 could be described by 2 successive exponential decays representing 2 decomposable fractions. After 80 d, 58 and 42% of the added label had been released as (CO2)-C-14, and 2 and 13% were released as (DOC)-C-14 in the oxic and anoxic cores, respectively. The porewater contained only small amounts of (CO2)-C-14 and (DOC)-C-14. However, oxic cores showed the lowest concentrations, indicating faunally enhanced solute fluxes in these cores. Between 31 and 37% of the added label remained as (POC)-C-14 (particulate organic carbon) in the sediment after 80 d. Similar amounts of dissolved carbon ((CO2)-C-14+(DOC)-C-14) were lost from the added algal POC under oxic and anoxic conditions, but the larger amounts of (DOC)-C-14 released from the sediment in the anoxic cores indicated a slower anaerobic mineralization of (DOC)-C-14.