Effect of burrowing by the crab Helice crassa on chemistry of intertidal muddy sediments

The chemical environment was measured in vertical and horizontal profiles o f cores taken from intertidal sediments that are extensively burrowed by th e mud crab Helice crassa. The crab burrows folded the thin (2-5 mm) oxic la yer into the sediment, and the burrows appeared to have a strong influence on the concentrations of acid volatile sulfide (AVS), acid-extractable Fe-I I (probably FeCO3 and FeS), Fe-III (probably predominantly hydrous ferric o xide FeOOH), and Mn-II,Mn-III,Mn-IV and a modest effect on FeS2 but no effe ct on total organic carbon, total organic nitrogen, or acid-extractable zin c concentrations. The oxic layer was thinner in the burrows than on the sed iment surface and showed some minor differences in solid-state chemistry, w ith higher FeOOH and lower FeS2 concentrations in the burrow walls. Acid vo latile sulfide, FeCO3, and FeS2 were found in the oxic layers, presumably b ecause of deposits from crab excavations of deeper anoxic sediments. The ch emistry of the bioturbated profile was highly variable, not only because of existing burrows but also because of infilled abandoned burrows. The color s of the sediment profile were strongly related to the concentrations of Fe -II, AVS, Fe-III, Mn, and FeS2. The implications of the observed sediment c hemistry to the fate and bioavailability of contaminants is discussed.