Planktonic diatoms and silica in Loch Leven, Kinross, Scotland: a one month silica budget
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Summary Diatom populations and silica concentrations were monitored at frequent intervals in the shallow, eutrophic Loch Leven over a 27‐day period (October 1972) and the influences of the inflows, outflow and the sediment were assessed. Changes in dissolved and particulate silica are accounted for by incorporating the results into a silica budget. During this period processes affecting silica within the loch were more important than those outside. The incorporation of diatom frustules into the sediments and the release of dissolved silica from the sediments appeared to be of particular importance. Evidence suggests that dissolution of the frustules of some planktonic diatom species was also important.Keywords:
Biogenic silica
Dissolved silica
Concentrations of amorphous particulate silica (biogenic silica) in the uppermost layer of the inner Oslofjord, Norway, varied between 2 and 324 µ g Si·liter −1 over a 1‐year period in 1977–1978. Most of the silica was present in frustules of living or dead diatoms but there was always a certain amount (up to about 50 µ g Si·liter −1 ) in a form that could not be recognized under the microscope. Silica was produced mainly during three large blooms of Skeletonema costatum in February–March, May, and June. At the peak of the June bloom, there was up to 20 times more Si present as diatom silica than as dissolved orthosilicic acid. Uptake experiments in June indicated depletion times of dissolved Si as short as 3 h. Although the S. costatum cells were less heavily silicified in May and June than in February–March, they showed no morphological signs of a drastic Si shortage.
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Biogenic silica (bSiO 2 ) dissolution plays an important role in the marine silicon cycle, diatom production and vertical export production. An ocean ecosystem model that incorporates dependency of bSiO 2 dissolution on water temperature was used to examine the effects of bSiO 2 dissolution on diatom production and bSiO 2 export. Diatoms dominate with higher bSiO 2 dissolution rates because of enhanced silicic acid supply for diatom production. Annual bSiO 2 export does not change with bSiO 2 dissolution rate, because of less bSiO 2 production with lower rates and greater bSiO 2 dissolution with higher rates. The ratio of bSiO 2 export to production changes seasonally, with maximum in late spring. The contribution of spring diatom bloom to the annual bSiO 2 export is 21–30%, smaller than observation‐based estimates. This suggests necessity for incorporating other factors controlling bSiO 2 dissolution besides water temperature, such as difference in organic coating protections against bSiO 2 dissolution between live and dead diatoms, in the model.
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This study aims to differentiate bio-mediated and biogenic CO3 -2 precipitation or terrestrial CO3 -2 input using onboard incubation techniques, to investigate the effects of resuspension in the coastal environment and to increase our understanding of predicted relationships between silicate releases and other biogeochemical variables in resuspension events. Relationships between dark silicate flux and BSi, CO3 -2, OrgC, Mn (manganese) according to the seasons were examined. The silica flux is controlled by the CaCO3 coating on the diatom skeletons due to the fact that diatom skeletons act as crystallization nuclei in the calcite precipitation that is biologically affected. The reduction in flux with BSi may be due to the reduction in the surface areas of larger diatom species. The negative linear relationships observed between silica fluxes and CO3 -2 is indicative of RSi fluxes constrained by bio-mediated carbonate increase. Linear relationships which are the same in their slopes but differ in their intercepts, reveal the effect of the change in diatom size on silica flux. Smaller diatoms have more surface area per unit volume, meaning an increased silica flux. On the other hand, seeing different CO3 -2 values at stations with the same orgC value have increased the confidence interval (CI) 95% in the linear relationship. The presence of different silica flux values in stations with the same carbonate value may be explained both by different orgC values and by diatoms containing different group sizes. The silica flux is controlled by the CaCO3 coating on the diatom skeletons due to the fact that diatom skeletons act as crystallization nuclei in the calcite precipitation that is biologically affected. The main mechanism controlling the reactive silica flux is carbonate precipitation. The observation of different silica flux values in stations with the same carbonate values can also be explained by OrgC.
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Abstract A transfer function has been established to quantify the dissolution of diatom silica in Southern Ocean sediments. The relationship between the amount of silica dissolution and changes in diatom species distribution is built by controlled progressive dissolution of biogenic silica in five recent sediment samples from box-core tops, each representative of a modern diatom species sediment assemblage. The amount of dissolved silica was measured for each experiment. The resulting data set of species abundances (42 samples containing 32 diatom species and 2 silicoflagellate genera) was added to the modern data base of diatom species distributed over the Southern Ocean (124 core tops). Q-mode factor analysis individualizes four factors explaining 83% of the variance. The first three factors are controlled by surface water properties (mostly temperature). The fourth factor is the only one correlated with loss of silica in the reference samples ( R = 0.900). We quantified the dissolution factor using this correlation: superficial sediments of the Southeast Indian Ocean are characterized, from low to high latitudes, by a decrease in silica loss by dissolution (from >50 to 10%) from the Subantarctic Zone (40°S) to around 55°S, followed by an increase of silica loss to values larger than 60% between 63° and 68°S. Application of the dissolution factor in two cores from the Southern Ocean (≈44° and 55°S) shows enhanced opal dissolution during the last glaciation, particularly during Emiliani's stage 3 (from 40,000 to 30,000 yr B.P.).
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Summary Diatom populations and silica concentrations were monitored at frequent intervals in the shallow, eutrophic Loch Leven over a 27‐day period (October 1972) and the influences of the inflows, outflow and the sediment were assessed. Changes in dissolved and particulate silica are accounted for by incorporating the results into a silica budget. During this period processes affecting silica within the loch were more important than those outside. The incorporation of diatom frustules into the sediments and the release of dissolved silica from the sediments appeared to be of particular importance. Evidence suggests that dissolution of the frustules of some planktonic diatom species was also important.
Biogenic silica
Dissolved silica
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Citations (48)