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The seasonal cycles of reactive silicate and suspended diatoms in the Dutch Wadden Sea
Van Bennekom, A.J.; Krijgsman-van Hartingsveld, E.; van der Veer, G.C.M.; van Voorst, H.F.J. (1974). The seasonal cycles of reactive silicate and suspended diatoms in the Dutch Wadden Sea. Neth. J. Sea Res. 8(2-3): 174-207
In: Netherlands Journal of Sea Research. Netherlands Institute for Sea Research (NIOZ): Groningen; Den Burg. ISSN 0077-7579, more
Peer reviewed article  

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  • Van Bennekom, A.J., more
  • Krijgsman-van Hartingsveld, E.
  • van der Veer, G.C.M.
  • van Voorst, H.F.J.

    The concentrations of reactive silicate in the western Wadden Sea are determined by 3 processes, each having a seasonal cycle, which for 1970 are summarized as follows: (a.) Advection has a winter maximum and is negligible in May and September. The source of nearly all reactive silicate advected into the western Wadden Sea is the river Rhine. (b.) Mineralization is important from May to December. The surface sediment in muddy, shallow regions contributes much more to mineralization than the suspended matter. The average release from the bottom is about 100 mg SiO2 m-2 day-1. This large contribution is caused by turbulent exchange of interstitial water from the surface sediment with overlying water, rather than by molecular diffusion. (c.) Periodic diatom production effects the reactive silicate concentration; the effect depends on water depth. In the Marsdiep reactive silicate is depleted down to growth-limiting levels of 0.5 µm in April, June and September. In shallow regions depletion is also nearly total in April, partial in June and non-detectable in September. The relative influence of PO4-P, NO3-N, NH4-N and reactive silicate on diatom blooms is discussed. It is concluded that reactive silicate alone limits the yield of diatoms; this unusual fact is related to the high amounts of N and P and the low amount of Si in the river Rhine. Some experimental data are given on the amount of silica in a number of diatom species; a range of 1.6 to 2.9 X 10-7 µg SiO2 per square µm geometrical outer surface was found. Estimation of the amount of diatomaceous silica, based on the number of diatoms and an overall value of 2.1 X 10-7 µg SiO2 um-2 appeared reasonable, the amount being comparable to the amount of reactive silicate, removed from the water. From the seasonal cycles of the reactive silicate and the diatoms a silicon budget is derived, based on the fresh water budget, the grazing rates of diatoms and the mineralization. The fresh water budget of the Wadden Sea is composed of 2 Rhine water types, water discharged from the IJsselmeer and water of the Rhine transported along the Dutch coast and entering from outside. The part of the Wadden Sea, covered in this work contains these fresh water types in about equal amounts, while the water has a residence time of 17 tidal periods. Removal of fresh water diatoms to the sediment started in April, in the residence time of 1 week 70 to 90% was removed. From data on the density of filter feeders and zooplankton it is inferred that removal by mussels is the most important; in early summer the contribution of zooplankton cannot be neglected. About half of the reactive silicate, present in the 2 Rhine water types is converted into diatoms, either in the Wadden Sea itself or during transport to the Wadden Sea. Less than a third of the silica from these diatoms is mineralized, production of diatoms is therefore responsible for a considerable net removal of reactive silicate in the estuary of the Rhine.

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