Transport of carbon dioxide and ammonium in bioturbated (Nereis diversicolor) coastal, marine sediments

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Two identical experiments with sieved and homogenized sandy and muddy sediment were conducted to determine transport enhancement of porewater solutes (TCO2 and NH+4) by the presence of the polychaete Nereis diversicolor (1000–1500 m−2). Flux measurements showed that N. diversicolor enhanced the release of CO2 and NH+4 1.5–5 times. Accordingly, porewater concentrations of these compounds were reduced considerably in the bioturbated zone of both types of sediments. Two different diagenetic models, “effective (eddy) diffusion” and “nonlocal exchange”, were used to describe solute profiles in the bioturbated sediments. In permeable sandy sediments advective porewater movements may occur more readily than in more cohesive muddy sediments. The “effective diffusion” model (with De = 1.6–2.0 cm2 d−1) provided an excellent fit to the measured concentrations of both solutes below the bioturbated zone in permeable sandy sediment, whereas this model overestimated the concentration in the bioturbated zone. However, in the less permeable muddy sediment the “effective diffusion” model overestimated the NH+4 profile considerably at all depths. The “nonlocal exchange” model (with α = 0.17–0.29 d−1), on the other hand, provided an excellent fit in the less permeable muddy sediment, suggesting that solute profiles here were controlled by molecular diffusion, even in the presence of burrow irrigation. For the permeable sediment, the “nonlocal exchange” model (with α = 0.14 d−1) underestimated the measured NH+4 profile. Accordingly, linear slopes from plots of porewater TCO2 as a function of porewater NH+4 revealed that eddy diffusion (or advective porewater movements) was important in the bioturbated zone of this sediment type. However, combined with the generally more realistic shape of profiles derived by the “nonlocal exchange”, these evidences suggest that both eddy and molecular diffusion must operate in the bioturbated zone of permeable sediments.

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