Nutrient early diagenesis in the sandy sediments of the dogger bank area, north sea: Pore water results

Abstract

Nutrient pore water profiles from the upper 100 mm of the sandy and organic-poor sediments of 12 stations in the Dogger Bank area (North Sea) were obtained during July – August 1988. O₂ penetration, as measured with microelectrodes, varied between 4 and 15 mm. Silicate, ammonium, nitrate, and phosphate concentrations were low compared with values reported in the literature for other marine interstitial waters. Nitrate showed clear nitrification peaks in the upper 10 mm of the sediment, while the other constituents reached their maximum levels between 20 and 40 mm. In some cores the latter concentrations declined again below these depths. Averaged over all stations, benthic respiration, measured in whole-core incubations, was 12 mmol(O₂)·m⁻²·day⁻¹, while measured nutrient sediment-water exchange rates were 333 (Si), 167 (NH₄⁺), 161 (NO₃⁻) and 32 (P) μmol·m⁻²·day⁻¹. Compared with the daily phytoplankton nutrient requirements, these fluxes amount to less than 10% for Si, ∼2% for N, and ∼ 3% for P. This suggests that nutrient regeneration largely occurs in the water-column. The pore-water profiles were evaluated quantitatively, using one-dimensional steady-state diagenetical models in which the apparent diffusion coefficient is calibrated against experimentally obtained fluxes. Results indicate that apparent diffusion is 1 to 6 times molecular diffusion.

For Si the first-order dissolution rate estimated from the Si model is 0.2 to 9.3·10⁻⁶·s⁻¹ at the sediment surface, declining to less than 5% of these values at 7.5 to 37.5 mm. The combined ammonium - nitrate model revealed that mineralization of organic nitrogen into ammonium is restricted to the upper 6 to 59 mm. First-order nitrification and denitrification rates were estimated at 0.3 to 16.5·10⁻⁴ and 0.06 to 5.5·10⁻⁴·s⁻¹, respectively. Total nitrification was calculated at 155 to 1930 μmol·m⁻²·day⁻¹, total denitrification at 54 to 546 μmol·m⁻²·day⁻¹. Denitrification is completely fuelled by nitrate produced during nitrification in the upper mm's. According to the model, 70 to 80% of the organic N being mineralized within the sediment is released to the water column, largely as nitrate. The P model includes both first-order sorption in the upper oxidized sediment layer and first-order phosphate removal in the zone below. Oxic sorption rate is estimated at 1 to 120·10⁻⁶·s⁻¹, sorption equilibrium concentration is ∼1.7 mmol·m⁻³. For the removal rate below oxygen penetration, two situations were observed. At 8 stations, removal rate in the reduced zone was 3 orders of magnitude less than oxic sorption. At 4 stations, both rates and equilibria were equal to those in the oxic layers, probably indicating the absence of substantial Fe(III) reduction in the upper cm's at these stations. Average benthic P-mineralization was calculated at 80 μmol·m⁻²·day⁻¹, of which ∼ 60 μmol·m⁻²·day⁻¹ was at least temporarily removed from the pore water by sorption. This implies that ∼25% of the organic P mineralized within the sediment is directly regenerated to the water column.