Silicon
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 0°C
and 69 mg Si l-1 at 25°C;
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.
Next Section
References
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