Control by fiddler crabs (Uca vocans) and plant roots (Avicennia marina) on carbon, iron, and sulfur biogeochemistry in mangrove sediment

The influence of mangrove saplings (Avicennia marina) and fiddler crabs (Uca vocans) on carbon, iron, and sulfur biogeochemistry in mangrove sediment was studied using outdoor mesocosms with and without plants (21 m22) and crabs (68 m22). Saplings grew more leaves and pneumatophores in the presence of crabs. Dense microalgal mats lead to two to six times higher benthic production and about two times higher benthic respiration in the absence of crabs. Particle mixing by crabs increased the reactive oxidized iron (Fe(III)) in the upper 2 cm of the sediment, whereas oxygen leaching by roots maintained the deeper rhizosphere oxidized and enriched in Fe(III). The highest microbial activity, measured as carbon dioxide production and iron reduction, occurred within the upper 2 cm of ungrazed sediment and was fueled by the large near-surface biomass of microalgae. Leaching of dissolved organic carbon (DOC) from roots stimulated bulk sulfate reduction and caused an upward cascading reduction of the sediment as indicated by low Fe(III) and high Fe(II) between 2-cm and 6-cm depth. The effect DOC was also evident as increased microbial abundance at all depths in the sediment. Fe(III) was the most important electron acceptor for microbial carbon oxidation in ungrazed sediment (63–70%), whereas sulfate reduction was more important in grazed sediment (36–44%), particularly in the presence of plants. Aerobic respiration always accounted for ,20%. Fiddler crabs and roots of A. marina have complementary effects on the biogeochemistry of mangrove sediment. Their association seems to be mutually beneficial with respect to growth and food availability. Mangrove forests constitute a highly productive ecosystem along most tropical coastlines. They support abundant benthic communities and high rates of microbial decomposition (Alongi and Sasekumar 1992; Alongi et al. 2001; Holmer et al. 2001). Litter (e.g., leaves, propagules, flowers, etc.) from trees is usually considered the main source of carbon and nutrients for decomposer food webs in mangrove environments, but the contribution from macroalgae, benthic microalgae, and phytoplankton may in certain areas be of considerable importance (Newell et al. 1995; Alongi 1998). Accordingly, stable carbon isotopic studies have revealed that mangrove-derived litter is directly or indirectly the major food source for many species of benthic macrofauna, whereas others primarily consume algae (e.g., Bouillon et al. 2002). Crabs are generally the key structuring faunal group and the driver of the decomposer food webs in mangrove forest. Whereas sesarmid crabs (Grapsidae) are remarkable consumers of mangrove litter (Robertson et al. 1992; Thongtham and Kristensen 2005), fiddler crabs (Ocypodidae) are efficient consumers of benthic microalgae (France 1998; Reinsel 2004). Although crabs handle large amounts of detritus in mangrove environments, most of the organic carbon oxidation is usually mediated by anaerobic microbial processes (Alongi et al. 1998; Alongi et al. 2000; Kristensen et al. 2000). The role of specific carbon oxidation pathways 1 Corresponding author (ebk@biology.sdu.dk).