Evidence for localized enhancement of biological associated with tube and burrow structures in deep-sea sediments at the HEEBLE site, western North Atlantic

Abstract

Relict burrows on the Nova Scotian Rise act as traps for highly reactive, relatively fresh organic matter from recent diatom blooms in surface waters. As such, the burrows are sites of enhanced biological activity and intensive decomposition in otherwise organic-poor sediment. These infilled burrows, as well as normally inhabited structures significantly influence sediment chemistry and the three-dimensional distributions and abundances of sediment microorganisms, meio- and macroinfauna. A tube dwelling of Amphicteis sp. (an ampharetid polychaete) and 5 burrows of various configurations were examined from box cores from a depth of ∼4820 m at the HEEBLE site. Bacterial abundances were significantly greater ( P < 0.05) in sediment immediately surrounding the burrows than in ambient sediment ( 12 vs 7 × 10 9 g 1 ). Organic carbon and nitrogen concentrations were also slightly higher near burrows. Meiofauna, particularly nematodes and foraminifera, double or even triple their densities around burrows compared with ambient sediment possibly responding to enhanced microorganism standing stocks. Total sediment ATP concentrations mirror the patterns of bacterial and meiofaunal abundances and show highest concentrations in the innermost radial zone around burrows. Sedimentary Mn and Fe distributions imply more reducing conditions and metabolically reactive organic matter near burrows. The existence of detectable, solid-phase Mn and Fe gradients associated with burrows indicate that these structures had been stable for periods of at least several weeks. At the HEBBLE site, 30–120 burrows of diameter 0.2–5 cm were visible per m 2 of seafloor, with substantially greater numbers of smaller-diameter burrows evident from X-radiographs. Although only ∼1% of the total exposed bottom surface is covered by the large-diameter burrows, each burrow can influence a surrounding radius of ∼3 cm, and thus affect sedimentary characteristics of 10–34% of the bottom (at steady state). Despite periodic erosion of the seafloor by strong near-bottom currents, filled and unfilled burrow structures are sufficiently long lived and the response of the associated organisms sufficiently rapid that distinct biogeochemical properties develop around them.