Silicon

Entry to the marine environment

Recorded levels in the marine environment

Fate and behaviour in the marine environment

Effects on the marine environment

Potential effects on interest features of European marine sites

Entry to the marine environment

Silicon is an essential nutrient for diatoms, radiolaria and sponges (Head 1985, Kennish 1986), but of these taxonomic groups, it is the diatoms which have the largest effect on Si cycling and levels, and which, conversely, are most affected by silicon, low levels of which are sometimes responsible for the crash of spring diatom blooms (Conley et al 1992).

Silicon in rivers is derived from the weathering of soils and rocks, primarily feldspars, but in marine waters, the main sources are dissolution of clay minerals and detrital quartz. However, the recycling rate of silicon is much slower than that of N and P

Recorded levels in the marine environment

Silicon in tidal waters is derived overwhelmingly from natural sources, over which there is no effective control. Only in very rare cases does an anthropogenic source of Si (e.g. detergent manufacture) appear to make a large contribution to the Si budget of a localised tidal area.

Marine waters are always undersaturated with regard to silicon, with saturation levels of about 28 mg Si l-1 at 0C and 69 mg Si l-1 at 25C; reported concentrations range between 0 and 10 mg Si l-1 (Bruland 1983, Burton and Liss 1976, Riley and Chester 1971). MPMMG (1998) reported concentrations of silicate at estuarine and coastal waters sites around the UK as part of the National Monitoring Programme.

Fate and behaviour in the marine environment

Cycling of silicate in the marine environment involves assimilation by diatoms where it is incorporated into the cell wall or frustule. When the diatom dies, the frustules are deposited onto the sediment and a proportion of the silicate is returned to the water column through a process of dissolution.

Effects on the marine environment

The effects of non-toxic substances, such as silicon, on the marine environment can be sub-divided into direct effects (those organisms directly affected by changes in the concentrations of silicon) and secondary effects (those arising in the ecosystem as a result of changes in the organisms directly affected).

Direct effects

For phytoplankton, the N:P:Si ratio principally controls what species grow, rather than how much. High silicon levels can be expected to result in prolonged or more severe spring diatom blooms than in otherwise similar waters and, conversely, reduced availability of silicon can reduce the abundance and productivity of diatoms. Some other phytoplankton species also require silicon as a macronutrient, notably some toxic dinoflagellates, but the availability of silicon for these species does not appear to be a limiting factor, since they do not usually bloom during spring (the only time of year when silicon is usually limiting to growth (due to uptake by diatoms)).

No information is available on the effects of elevated or reduced levels of silicon levels on benthic fauna or flora, but the relatively high levels of Si in interstitial water infer that lack of availability is unlikely to be a problem

Indirect effects

The indirect effects of increasing silicon concentrations are associated with the effects of a bloom of diatoms in the spring or summer where productivity is not limited by light availability. Indirect consequences of the bloom are similar to those described for nitrogen but are unlikely to be as severe due to the limited availability of silicon.

The diatom Chaetoceros has long spines and blooms of this genus can damage fish by physically clogging gills which has led to fish kills in cage-reared salmon and other species (Environment Agency 1998).

Potential effects on interest features of European marine sites

Potential effects include:

  • stimulation of diatom blooms where silicon concentrations are increasing and inhibition of diatom growth and productivity where silicon is limiting.
  • blooms of Chaetoceros can result in damage to fish populations by clogging and damaging gills.

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