Triplett et al.(2008)

Triplett,L.D., Engstrom,D.R., Conley,D.J. and Schellhaass,S.M.(2008): Silica fluxes and trapping in two contrasting natural impoundments of the upper Mississippi River. Biogeochemistry, 87, 217-230.

『ミシシッピ川上流の２つの対照的な天然貯水池におけるケイ素フラックスとトラッピング』

『Abstract
　Human activities have altered riverine silica cycling and diminished the supply of silica to the oceans, but few rivers have been intensively monitored to evaluate the magnitude of these changes. In this study we measured dissolved silica (DSi) and amorphous silica (ASi) fluxes into and out of two large, culturally-impacted natural impoundments of the upper Mississippi River, Lakes St. Croix and Pepin, USA. ASi sedimentation rates and sediment-water fluxes of DSi were calculated for each lake, and a mass-balance approach was used to determine in-lake ASi production. ASi from terrestrial phytoliths in the lake sediments was determined to be only partially available to biotic recycling, and in-lake ASi dissolution was small relative to the total silica budgets. The river reaches upstream of the two lakes were found to have abundant DSi, and riverine diatom production was found to contribute significant amounts of ASi to each lake. The average total phosphorous concentration in Lake Pepin is four times that in Lake St. Croix but ASi production in Lake Pepin is only 2.3 times higher than in Lake St. Croix, indicating that diatom growth in Pepin is limited by factors such as turbidity. Lake St. Croix currently traps about 10％ of the inflowing total bioavailable silica (TSib = DSi + ASi) while Lake Pepin traps closer to 20％ of its inflowing TSib, clearly demonstrating the importance of silica retention in lakes and reservoirs along the land-ocean continuum.

Keywords: Biogenic silica; Dissolved silicate; Nutrient cycling; Mississippi River; Phytolith; Riverine lake』

Introduction
Methods
　Defining the silica pools
　Silica inflow and outflow
　River discharge
　ASi sequestration in lake sediments
　ASi from terrestrial plant phytoliths
　ASi dissolution in the lakes
Results and discussion
　Inflow and outflow Si fluxes
　ASi flux to sediment
　ASi from phytoliths
　ASi dissolution
　Modern silica mass balance
Synthesis: amorphous silica production and trapping
Acknowledgements
References

Table 2 Annual silica fluxes into and out of Lakes St. Croix and Pepin in t yr^-1 as SiO2
Lake DSiinflow^a ASiinflow^b TSib,inflow^c DSioutflow^a ASioutflow^b TSib,outflow^c

Lake St. Croix 53,000 (±10,000) 4,500 (±1,100) 58,000 (±10,000) 51,000 (±10,000) 2,900 (±700) 54,000 (±10,000)

Lake Pepin 213,000 (±37,000) 49,500 (±11,000) 263,000 (±39,000) 236,000 (±61,000) 25,000 (±7,600) 261,000 (±161,000)

^a Dissolved silica in lake indflow and outflow
^b Amorphos (particulate) silica inflow and outflow
^c Total bioavailable silica (DSi + ASi) in lake inflow and outflow

**Table 2 Annual silica fluxes into and out of Lakes St. Croix and Pepin in t yr^-1 as SiO2**
Lake	DSiinflow^a	ASiinflow^b	TSib,inflow^c	DSioutflow^a	ASioutflow^b	TSib,outflow^c
Lake St. Croix	53,000 (±10,000)	4,500 (±1,100)	58,000 (±10,000)	51,000 (±10,000)	2,900 (±700)	54,000 (±10,000)
Lake Pepin	213,000 (±37,000)	49,500 (±11,000)	263,000 (±39,000)	236,000 (±61,000)	25,000 (±7,600)	261,000 (±161,000)
^a Dissolved silica in lake indflow and outflow ^b Amorphos (particulate) silica inflow and outflow ^c Total bioavailable silica (DSi + ASi) in lake inflow and outflow

Table 3 Amorphous silica (ASi) fluxes as SiO2 for Lakes St. Croix and Pepin
Lake ASioutflow^a
(t yr^-1) ASisedimented^b
(t yr^-1) ASidissolution^c
(t yr^-1) ASiinflow^d
(t yr^-1) ASiin-lake^e
(t yr^-1) Areal ASiin-lake^e
(kg SiO2 m^-2 yr^-1)

Lake St. Croix 2,900 (±700) 5,600 (±1,000) 400 (±400) 4,500 (±1,100) 4,400 (±1,700) 0.13 (±0.05)

Lake Pepin 25,000 (±7,600) 52,000 (±9,300) 2,200 (±2,200) 49,500 (±11,000) 30,000 (±17,000) 0.30 (±0.17)

^a ASi in lake outflow
^b ASi sequestered in the sediments
^c ASi dissolved inthe lake column or sediments
^d ASi in the lake inflow
^e ASi produced in the lake (ASiin-lake = ASioutflow + ASisedimented + ASidissolution - ASiinflow)

**Table 3 Amorphous silica (ASi) fluxes as SiO2 for Lakes St. Croix and Pepin**
Lake	ASioutflow^a (t yr^-1)	ASisedimented^b (t yr^-1)	ASidissolution^c (t yr^-1)	ASiinflow^d (t yr^-1)	ASiin-lake^e (t yr^-1)	Areal ASiin-lake^e (kg SiO2 m^-2 yr^-1)
Lake St. Croix	2,900 (±700)	5,600 (±1,000)	400 (±400)	4,500 (±1,100)	4,400 (±1,700)	0.13 (±0.05)
Lake Pepin	25,000 (±7,600)	52,000 (±9,300)	2,200 (±2,200)	49,500 (±11,000)	30,000 (±17,000)	0.30 (±0.17)
^a ASi in lake outflow ^b ASi sequestered in the sediments ^c ASi dissolved inthe lake column or sediments ^d ASi in the lake inflow ^e ASi produced in the lake (ASiin-lake = ASioutflow + ASisedimented + ASidissolution - ASiinflow)

Treguer（最初のeの頭に´）,P.,Nelson,D.M., van Bennekom,A.J., DeMaster,D.J., Leynaert,A. and Queguiner,B.(1995): The silica balance in the world ocean: a reestimate. Science, 268, 375-379.
　『Abstract
　The net inputs of silicic acid (dissolved silica) to the world ocean have been revised to 6.1 ± 2.0 teramoles of silicon per year (1 teramole = 10¹² moles). The major contribution (about 80 percent) comes from rivers, whose world average silicic acid concentration is 150 micromolar. These inputs are reasonably balanced by the net ouputs of biogenic silica of 7.1 ± 1.8 teramoles of silicon per year in modern marine sediments. The gross production of biogenic silica (the transformation of dissolved silicate to particulate skeletal material) in surface waters was estimated to be 240 ± 40 teramoles of silicon per year, and the preservation ratio (opal accumulation in sediment/gross production in surface waters) averages 3 percent. In the world ocean the residence time of silicon, relative to total biological uptake in surface waters, is about 400 years.』

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