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

Live Skeletonerna costatum (diatom) cells labeled with I4C 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 NZ. Release of I4CO2 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. DO1'C (dissolved organic carbon) release also exhibited high rates initially, but w ~t h the highest release in the anoxic cores. Degradation to ' T O 2 and D0I4C 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 I4CO2, and 2 and 13% were released as D0I4C in the oxic and anoxic cores, respectively. The porewater contained only small amounts of I4CO2 and DO1"C. 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 P0I4C (particulate organic carbon) in the sediment after 80 d. Similar amounts of dissolved carbon ( ' 4 C 0 2 + D 0 1 T ) were lost from the added algal POC under oxic and anoxic conditions, but the larger amounts of D0I4C released from the sediment in the anoxic cores ind~cated a slower anaerobic mineralization of D0I4C.