Silica cycling

The availability of DSi can regulate the composition of phytoplankton species (Kilham, 1971 Officer and Ryther, 1980 Egge and Aksnes, 1992). For example, diatom growth is dependent on the availability of DSi to nondiatom phytoplankton species which are Si independent. DSi uptake by diatoms can even occur in bottom waters or turbid surface water where light may be limiting (Nelson et al., 1981 Brzezinski and Nelson, 1989). However, once the supply of DSi has diminished diatom production will decrease and diatoms will be replaced by other phytoplankton species. Limitation of DSi in Chesapeake Bay resulted in a rapid decline in diatoms and an increase in cyanobacteria (Malone et al., 1991). Similarly, an estimated 50% decline in DSi from the 1950s to the 1980s in the Mississippi River (Turner and Rabalais, 1991) resulted in DSi limitation for diatoms on the 

Louisiana coast (Dortch and Whitledge, 1992). Harmful algal blooms (HABs) have been linked with decreases in the Si N and Si P ratios (Smayda, 1990 Anderson et al., 2002 Ragueneau et al., 2005a,b), many of the more common bloom species consist of toxic dinoflagellates (Steidinger and Baden, 1984), prymnesiophytes (Lancelot et ah, 1987), and certain diatom species (e.g., Pseudo-nitzchia australis) (Scholln et ah, 2000). 

The cycling and availability of P in estuaries is largely dependent on P specia-tion. Consequently, total P has traditionally been divided into total dissolved P and total particulate P fractions, which can be further divided into dissolved and particulate organic P and dissolved and particulate inorganic P pools. Another defined fraction within the TP pool is reactive phosphorus, which has been used to describe the potentially bioavailable P. Much of the work to date has focused on the soluble reactive P, which is characterized as the P fraction that forms a phosphomolybdate complex under acidic conditions. 

Rivers are the major source of P to the ocean, via estuaries, where major chemical and biological transformations of P occur before it is delivered to the ocean. 

Inputs of atmospheric sources of P are generally considered to be insignificant to coastal systems and represent 10% of the riverine flux of reactive P. 

---

A box model fiar the marine silica cycle is presented in Figure 6.11 with respect to the processes that control DSi and BSi. An oceanic budget is provided in Table 16.3 in which site-specific contributions to oceanic outputs are given. This table illustrates that considerable uncertainty still exists in estimating the burial rate of BSi. Regardless, burial of BSi is responsible for most of the removal of the oceanic inputs of DSi, with the latter being predominantly delivered via river runoff. This demonstrates the importance of the biological silica pump in the crustal-ocean-atmosphere factory. 

Conversely, perturbations in the burial rate of BSi have the potential to alter the marine silica cycle. For example, changes in sea level affect the expanse of continental shelf Since BSi burial is more efficient in shelf sediments (because of better preservation), a topographic change that alters the spatial extent of this depositional environment has the potential to alter the size of the DSi reservoir. 

Figure 11.13 The silica cycle showing flux inputs (Tmol y 1) and burial in the global ocean. (Modified from Treguer et al., 1995.)...

Conley, D.J. (2002) Terrestrial ecosystems and the global biogeochemical silica cycle. Global Biogeochem. Cycle 16, 774—777. 

DeMaster, D.J. (2002) The accumulation and cycling of biogenic silica in the Southern Ocean revisiting the marine silica cycle. Deep-Sea Res. II, 49, 3155-3167. 

Ragueneau, O., Conley, D.J., Longphuirt, S., Slomp, C.P., and Leynaert, A. (2005a) A review of the Si biogeochemical cycle in coastal waters, I diatoms in coastal food webs and the coastal Si cycle. In Land-Ocean Nutrient Fluxes Silica Cycle, (Ittekot, V., Humborg, C., and Gamier, L., eds.), SCOPE, Linkoping, Sweden. 

Alexandre et al. (1997) found that the biogenic sihca input into the biogeochemical silica cycle from the dissolution of phytoliths is twice as large as silica input from primary silicate mineral weathering in the tropical Congo rainforest. Biogenic (opaline) silica dissolves faster than sihcate minerals. While most of the phytoliths dissolve rapidly with a mean residence time of 6 months (Alexandre et al., 1994), and the sihca is recycled by the forest, a small part (7.5%) does not dissolve and is preserved in the soil. 

Gnanadesikan A. (1999) A global model of silica cycling sensitivity to eddy parameterization and remineralization. Global Biogeochem. Cycles 13, 199—220. 

Recent studies have very clearly delineated the importance of aluminum in sediments for both the opal dissolution rate and the observed asymptotic H4Si04 concentration. The incorporation of aluminum into opal and the hypothesized reprecipitation of H4Si04 from dissolving opal suggest that the silica cycle may exert a significant influence on the cycling of major constiments... 

Schluter M., Rutgers van der Loefif M. M., Holby O., and Huhn G. (1998) Silica cycle in surface sediments of the South Atlantic. Deep-Sea Res. 145, 1085—1109. 

Siever R. (1992) The silica cycle in the Precambrian. Geochim. Cosmochim. Acta 56, 3265-3272. 

---