Decomposition of diatoms and nutrient dynamics in permeable North Sea sediments

This study addresses the decomposition of diatoms in different permeable North Sea sand beds. During three cruises in 2001 to the southern German Bight, the regeneration of nutrients was assessed after the experimental deposition of organic matter corresponding to a typical spring diatom bloom in in situ and on-board chamber experiments. The diatom pulse was followed by a high regeneration of nutrients during the first day: 5–10% d−1 of the added nitrogen was converted to NH4+ and up to 0.67% d−1 of the added biogenic silica was dissolved to Si(OH)4. These results are used to interpret the response of pore water nutrient concentrations in permeable North Sea sands to seasonal nutrient and phytoplankton dynamics in the water column. The rapid advective solute exchange in these permeable sediments reduces the accumulation of regenerated nutrients, and, thus pore water concentrations of Si(OH)4, PO43− and NH4+ decreased with increasing permeability. All sands were characterized by relatively high NO3− concentrations down to 10 cm sediment depth, indicating that the upper sediment layers are oxidized by advective flushing of the bed. Our results demonstrate that biogenic silica and organic matter are rapidly degraded in permeable coastal sands, revealing that these sediments are very active sites of nutrient recycling.