Effects of bioturbation and plant roots on salt marsh biogeochemistry: a mesocosm study

The impact of benthic macrofauna (Nereis diversicolor) and macrophytes (Spartina anglica) on the dynamics of microbial mineralization rates and pathways in saltmarsh sediment was studied over 1 growing season. Measurements were conducted in 4 saltmarsh mesocosms subjected to a natural tidal regime, and inhabited by either flora, fauna or both,or kept without the presence of, macroorganisms. Total benthic mineralization was measured during spring, summer and fall as sediment oxygen uptake and carbon dioxide release under air-exposed as well as inundated conditions, while anaerobic mineralization was quantified as sulfate reduction. Porewater concentration profiles of total carbon dioxide (TCO2), dissolved organic carbon (DOC), SO42-, Cl- and Fe2+, were determined monthly. The presence of fauna had minor effects on plant growth, while sediment vegetated by S. anglica renders the habitat unsuitable for N. diversicolor, possibly due to food limitation. Without the presence of macroorganisms, a dense algal mat (Vaucheria sp.) developed on the sediment surface. As a result, the sediment was greatly reduced with benthic respiration completely dominated by sulfate reduction. Grazing by fauna (N. diversicolor) kept the sediment surface free of epibenthic microalgae, and irrigation oxidized the top 15 to 20 cm of the sediment by enhancing porewater exchange with overlying water. Porewater TCO2, DOC and Fe 21 only accumulated below 15 to 20 cm depth. Sulfate reduction was still the dominating degradation pathway, accounting for 50 to 60% of the total microbial mineralization. The presence of flora (S. anglica) enhanced total microbial mineralization but lowered the importance of sulfate reduction (22 to 54%). Porewater constituents such as DOC and TCO2 were kept low and relatively evenly distributed with depth. The oxidizing capacity of S. anglica was evident throughout the sediment column. The presence of both flora and fauna produced the largest increase in oxic as well as total microbial mineralization rates. In conclusion, the mesocosm approach simulated natural salt marsh conditions well, and the results provide good evidence for the interactions between flora and fauna as well as their impact on sediment geochemistry.