Variation in the depth and morphology of burrows of the mud crab Helice crassa among different types of intertidal sediment in New Zealand

Crabs are among the larger and more active burrowers in intertidal sediment s in New Zealand, as in many other parts of the world. Abundances of mud cr abs Helice crassa and their burrows were compared among sites differing in the nature of their sediments. Differences in burrow architecture among sit es were quantified using casts of burrows made in situ. The effects of biot urbation on sediment geochemistry were also determined with respect to redo x potential and the concentrations of 2 chemical phases, acid volatile sulp hides (AVS) and iron pyrites (FeS2), that influence the bioavailability of heavy metals. The objective of this study was to identify effects of differ ences in the extent and nature of burrowing activity of the crab H. crassa among different sediment types on sediment geochemistry, particularly with respect to the bioavailability of heavy metals. Burrows were more abundant at muddy sites (average 22 to 59 burrows 20 cm diameter core(-1)) than at s andy sites (average 12 burrows core(-1)). Although not necessarily deeper a t muddy sites, burrows were more complex in structure. Average volumes of c asts were larger at muddy than sandy sites by a factor of up to 14.8 and at the most densely-burrowed site, burrows represented 14 % of the volume of the surrounding sediment. The architecture of the burrows of H, crassa is d iscussed in the context of published models of burrow structure and functio n for other crustacea. Patterns of differences in chemical variables among sites were not clear cut. Redox profiles generally showed decreases with de pth in the sediment and, among the muddy sites, potentials were highest and most variable at the site with greatest burrowing activity. Concentrations of AVS and FeS2 were lower in sandy sediments than in muddy ones. Among mu ddy sites, the concentration of AVS was lowest at the site with the greates t amount of burrowing, consistent with introduction of oxygen to the sedime nt and the consequent oxidation of AVS. Concentrations of FeS2 showed a pat tern that suggested that burrowing introduces FeS2 from deeper layers, wher e concentrations are higher, into shallower, bioturbated layers of the sedi ment, offsetting effects of oxidation due to burrowing activity. Concentrat ions of FeS2 were highest at the site with most burrowing activity.