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Heteromastus filiformis; fecal pellets; permeability; advective flow; oxygen profiles; organic matter degradation
Abstract:
This study investigates how pellet deposition caused by the head-down deposit feeding polychaete Heteromastus filiformis enhances sediment surface layer permeability and thereby the degradation of organic matter contained in that layer. The permeability of pellet accumulations on the sediment surface was 2 orders of magnitude higher than that of the non-pelletized sediment surface, which also had a 30% lower porosity. Freshly deposited organic-rich pellets initially consumed approximately 10 times more O2 than an equivalent volume of the ambient, oxic, surface sediment. As a consequence of the deposition of the relatively large (400 to 500 µm) grain-like pellets in small mounds, topographical structures are generated on the originally rather flat surface, which cause advective pore water flow through the pellet mounds as soon as the latter are exposed to boundary layer flows. At a flow velocity of 70 mm s–1 at 5 mm above the sediment, pore water flow velocity through pellet accumulations ranged from 1 to 2 mm s–1, transporting oxygenated water to the pellets. Incubation experiments in flow-through columns showed that the O2 consumption of pellet accumulations increases by approximately 100 µmol (g dry mass)–1 d–1 when the flow rate is increased from 10 to 20 ml h–1. A faster degradation of reduced organic C and N contained in the fecal pellets was observed under the influence of flow (70% of C and 68% of N degraded) compared to stagnancy (44% of C and 40% of N degraded) in a 3 wk flume incubation. Degradation of organic C and N in the surrounding, flat, surface sediment, which was not flushed by water flow, was not detectable during the same time period. We conclude that pelletization enhances organic matter turnover in fine-grained deposits through generation of a high secondary permeability of the sediment surface layer that permits advective pore water exchange, enhancing mineralization of the deposited organic-rich pellets.
