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

Two identical experiments with sieved and homogenized sandy and muddy sediment were conducted to determine transport enhancement of porewater solutes (TCO₂ and NH₄ ⁺) by the presence of the polychaeteNereis diversicolor (1000–1500 m^–2). Flux measurements showed thatN. diversicolor enhanced the release of CO₂ and NH₄ ⁺ 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 D_e=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 NH₄ ⁺ profile considerably at all depths. The nonlocal exchange model (with=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=0.14 d^–1) underestimated the measured NH₄ ⁺ profile. Accordingly, linear slopes from plots of porewater TCO₂ as a function of porewater NH₄ ⁺ 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.