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permeable sediments; advection; diatoms; silicon cycle; sediment-water exchanges; benthic filtration
Abstract:
In laboratory chamber experiments we demonstrate that permeable sediments (>7×10−12 m2) exposed to boundary flows filter chain-forming coastal bloom diatoms (Skeletonema costatum and Thalassiosira rotula) from the water column, causing rapid transfer of fresh organic particulate matter into sediment layers as deep as 5 cm within 72 h. The penetration depth of the diatoms depends on the permeability of the bed and the length of the chains. Long chains were not transported as deep into the sediment as short chains or single cells. The fast advective transfer of phytoplankton cells into sandy sediments may be an important process facilitating organic matter uptake and preventing resuspension of deposited organic material in high-energy coastal environments. High advective flushing rates in medium- and coarse-grained sandy sediments enhanced the mineralisation of the trapped diatoms (2300 to 3200 μmol C m−2 d−1), stimulated benthic oxygen consumption (2300 to 3000 μmol O2 m−2 d−1), as well as nitrification (up to 20 μmol NO3− m−2 d−1), relative to sediment where diffusion dominated the solute exchange. Advective solute exchange rates that increase with increasing permeability prevent the accumulation of Si(OH)4 near the dissolving frustules and in the pore water, leading to an effective recycling of dissolved silica to the production process in the water column (95 to 101 μmol Si(OH)4 m−2 d−1). This process may also enhance dissolution rates of the deposited opal in coarse-grained sands by maintaining higher degrees of undersaturation than in fine-grained sediments. Our results suggest that advective filtration of planktonic diatoms into permeable sediments increases mineralisation and recycling of Si(OH)4 and organic matter in high energetic shelf areas.
