Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 94

Weathering is achieved by a combination of mechanical (physical processes of wind
or ice) and chemical processes (reactions with acidic and oxidizing substances). The
rate of chemical weathering varies with the physical conditions of temperature and
rainfall amount, and the mineral composition of the rocks. Rainwater, springs, and
the leachate from soils are all high in carbon dioxide (carbonic acid) that weathers
rocks to release soluble silica. 23
The ratios of the nutrients present and the availability of dissolved silicate (DSi)
can regulate the species composition of phytoplankton assemblages. When concen-
trations of DSi become low, other types of algae that do not require DSi can dominate
algal community composition and decrease the relative importance of diatoms in
phytoplankton communities. 47 There is a considerable concern that altered nutrient
ratios in coastal waters may favor blooms of nuisance flagellate species that could
replace the normal spring and autumn bloom of siliceous diatoms. 49,50 Large-scale
hydrologic alterations on land, such as river damming and river diversion, can cause
reductions in silicate inputs to the sea. 49 This has already been observed in the Black
and Baltic Seas. Changes in the nutrient composition of river discharges seem to be
responsible for dramatic shifts in phytoplankton species composition in the Black
Sea. 51,52 In the Baltic Sea, DSi concentrations and the DSi:N ratio have been decreas-
ing since the end of the 1960s, and there are indications that the proportion of
diatoms in the spring bloom has decreased while flagellates have increased. Changing
phytoplankton species composition can have repercussions on the entire food web,
and might have enormous economic impacts. 51
Diatom phytoplankton populations are the usual food for zooplankton and filter-
feeding fishes, and contribute directly to the large fishable populations in coastal
zones. Diatoms grow very rapidly, have short lifetimes, are grazed heavily and are
rarely a nuisance. 46 They tend to dominate ecosystems whenever silicate is abundant
(concentrations greater than 2 M). 53 Diatoms account for 60% of the world's

primary production. 51 The diatom need for silicate is for the construction of their
cell walls (known as frustules). This contrasts with other algae that construct their
cell walls from organic material or from calcium carbonate.
Silicon does not have a gaseous phase, and its cycle is relatively simple because
it involves only inorganic forms. In contrast to other nutrients, particulate and
sedimentary silicon decay directly to dissolved inorganic silicon rather than passing
through a dissolved organic phase ( Figure 4.4) .

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4.1.3.1 Uptake of Silicon

In the silicon cycle, organisms utilize dissolved silicon (H 4 SiO 4 ) to produce their
skeletons, and this skeletal material dissolves following the death of the organ-
isms. 25 The half-saturation constant for growth of several diatom species is about
0.5-5.0 M. 46 Maximum in situ growth rates between 2-4 d −1 have been repeatedly

measured for diatoms, whereas the observed maximum growth rates for dinoflagel-
late, microflagellate, and eukaryotic nonmotile ultraplankton species or assem-
blages have generally been below 2.5 d −1 . 53
Diatoms use the dissolved silicon together with nitrogen and phosphorus in an
average Si:N:P ratio of 16:16:1. 27,46,54

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