Sunday, March 7, 2010

The Global Biogeochemical Silicon Cycle

The Global Biogeochemical Silicon Cycle
Eric Struyf & Adriaan Smis & Stefan Van Damme &
Patrick Meire & Daniel J. Conley

Consequently, transport of continental DSi to the oceans is an important component
in oceanic primary production, a large part of which consists of diatoms [11]. Forty percent of all oceanic C sequestration (∼1.5–2.8 Gton C yr−1) can be attributed to the growth and sedimentation of diatoms [12, 13]. Although primary production through different groups of marine phytoplankton also results in a net CO2 flux towards the sea bottom (the “biological carbon pump”) [14], a crucial difference exists between diatoms and coccolithophores, an important subgroup of non-siliceous phytoplankton. Coccolithophores are characterized by calcite shells (=coccoliths); CO2 is produced
when calcium reacts with hydrogen carbonate during calcite formation (the “carbonate counter pump”) [15].

Therefore, an increased dominance of coccolithophores decreases the net sequestration of CO2 and consequently the flux of CO2 from the atmosphere towards the ocean floor
[11]. The biological carbon pump in the ocean is often referred to as the “biological Si pump” [7]. Changes in Si inputs to marine ecosystems, especially in the coastal ocean, can significantly influence the species composition of oceanic primary producers, especially the balance of production between diatoms and non-siliceous
phytoplankton [16]. It has been hypothesized that a higher contribution of diatoms to total oceanic phytoplankton biomass occurred during the Last Glacial Maximum (79%
vs. 54% today) as the result of increased eolian inputs of Si [11]. This demonstrates that a link exists between Si transport from terrestrial to oceanic systems, atmospheric CO2 concentrations and variations in global climate.

2.2 Silicon and Eutrophication of Coastal and Lake

Ecosystems Silicon plays an important role in the current eutrophication problems of numerous lacustrine, estuarine and coastal ecosystems [17, 18]. In most major rivers worldwide, concentrations of N and P have at least doubled as the result of anthropogenic inputs [19]. Whereas total algal growth is primarily regulated by the availability of N and P, the relative availability of Si and the availability of Si
relative to N and P, e.g. the Si:N and Si:P ratios, can influence the composition of the phytoplankton community [18]. The lack of Si can change aquatic ecosystems from
those dominated by diatoms to non-diatom based aquatic ecosystems usually dominated by flagellates [20]. Based on an evaluation of long-term algal blooms and nutrient
conditions in different regions, it can be concluded that decreased Si:N and Si:P ratios can give rise to Si limitation of diatoms and the reduction of diatoms in the phytoplankton community. In addition, subsequent non-diatom blooms can contain harmful algal species such as Phaeocystis sp., Gonyaulax sp., Chrysochromulina sp. [21].

Diatoms are the primary energetic source for estuarine and coastal food chains [22]. Transfer of energy to higher trophic levels is enhanced by diatoms through their higher nutritional value [23] and the limited amount of trophic steps between diatoms and higher trophic levels [24]. Nondiatom species are known to be less available to higher trophic levels [21, 25] and some non-diatom based food webs are economically undesirable [20]. Therefore, the proportion of diatoms in the phytoplankton community is of primary importance for many fisheries globally [20].

Furthermore, DSi limitation of diatoms and resultant blooms dominated by non-diatom species can result in anoxic conditions, increased water turbidity and excessive
production of toxic components [26].

Increases in diatom biomass as a result of higher N and P inputs results in increased diatom sinking rates and increased diatom burial in bottom sediments [27]. Consequently, in anthropogenically eutrophied systems that have experienced increases in N and P loading from human activities with sufficiently long hydrodynamic residence times, the aquatic DSi stock decreases and eutrophication problems are worsened [18]. A similar effect has been described for dams, e.g. the artificial lake effect [28]. Dams increase the residence time of water in river ecosystems, which stimulates phytoplankton productivity [29]. This results in the increased trapping of biogenic Si in lake sediments, and decreased transport of DSi downstream. This effect has been described for major dams worldwide, and is an important component of changed N:P:Si ratio’s in coastal ecosystems.

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