Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 95

respiration

dissolution

Particulate
Biogenic Si

Particulate
Mineral Si

Dissolved Si

uptake

settling

settling diffusion

WATER

SEDIMENT

Particulate
Biogenic Si

Alumino
Silicate
Clay

Detrital
Quartz
Silicon

Dissolved Si

dissolution dissolution

FIGURE 4.4 Silicon cycle.

4.1.3.2 Settling of Diatoms

The mineral content of diatoms increases their specific gravity, which causes negative
buoyancy. Diatoms, therefore, tend to sink. 7 Increased diatom production results in
increased deposition of silica into sediment. Preservation of deposited material in
sediment depends upon a variety of factors, including pH, salinity, temperature, type
of sediment, and bulk sedimentation rate. In general, accumulation of biogenic silica
in sediment mimics overlying water column productivity. 47

4.1.3.3 Dissolution of Silica

Silicon is released by dissolution of diatom tests (skeletons) rather than by micro-
bially mediated decomposition. It has been suggested by Officer and Ryther 46 and
Conley et al. 47 that dissolution rates for particulate silicon are slow relative to
regeneration of both nitrogen and phosphorus. Regeneration of biogenic silica is
primarily a chemical phenomenon, whereas grazers and bacteria biologically medi-
ate the regeneration of nitrogen and phosphorus. Both nitrogen and phosphorus
will be recycled faster and reused on shorter time scales than Si. This could mean
that midsummer shifts in algal species domination from diatoms to flagellates
might result from silicon limitation due to relatively slow regeneration. However,
Day et al. 27 report that measurements of benthic nutrient regeneration indicate that
summer rates of silicon recycling are comparable to those for nitrogen. Nixon
et al. 55 measured silica fluxes over 1 mmol m −2 h −1 from the sediment during
summer in Narragansett Bay, U.S.A. This flux was higher than commonly supposed.
Salinity is a factor that directly affects the dissolution of siliceous minerals. The
rate of dissolution of biogenic silica increases by a factor of 2 by changing the
salinity from 1 to 5%. Thus, dissolution of siliceous minerals is more rapid in marine
waters. 47