Impacts of crab burrow on exchange of inorganic and organic carbon across the interface of water column and sediments in salt marshes

Abstract

<p>Abundant crab burrows in salt marshes can act as preferential water flow conduits for solute exchange by perforating muddy sediments, yet the impact of crab burrows on modulating carbon sequestration potential are not well understood. The field observations to assess how crab burrows drive carbon exchange over time scales of minutes to weeks along an intertidal marsh transect in North Inlet, South Carolina were conducted. The field analysis found that (1) continuous pore water exchange between the crab burrows and the surrounding soil matrix occurs because of tidally driven hydraulic gradients; (2) the burrow CO<sub>2</sub> concentrations is higher than on the marsh surface because of respiration. The average gas-phase CO<sub>2</sub> concentration in the crab burrows was approximately six times greater than ambient air. The lab analysis found that the concentrations of DIC and DOC in crab burrow porewater were lower than porewaters in the surrounding soil matrix. The porewater δ<sup>13</sup>C-DIC signatures in the crab burrows were heavier than those in the soil matrix, reflecting a mixture with seawater.</p><p>Crab burrows can influence carbon export by three pathways: gas phase CO<sub>2</sub> release from burrows caused by the irrigation of surface water, hydraulic gradient-driven porewater exchange (PEX) and concentration gradient-driven passive diffusion transport (PDT). Among these pathways, PEX showed the dominance of the crab burrow-induced carbon export, which is at least an order of magnitude higher than the others. Crab burrow-induced carbon export from the whole intertidal transect was calculated as the sum of CO<sub>2</sub> release and the dissolved carbon loss by PEX and PDT transport from the surrounding soil matrix to the crab burrow. The estimated C export was extrapolated to US East Coast salt marshes, which was nearly equivalent to riverine DIC flux, half of salt marsh DIC exports. These new insights underline the ecological roles of crab burrows in salt marsh carbon budgets, especially the importance of porewater exchange between crab burrows and the surrounding soil matrix.</p>