Dissolved Silicate Concentrations Research Articles

Hypoxia and nutrient dynamics affected by marine aquaculture in a monsoon-regulated tropical coastal lagoon.

The Laoyehai (lagoon) is located at the east coast of Hainan Island in the South China Sea and has been subject to perturbations from human activities, notably marine aquaculture, and has eutrophic surface and hypoxic near-bottom waters. A lack of knowledge of hydrodynamic and biogeochemical processes is a challenge to the sustainable management of lagoon at the ecosystem level in science. Five field campaigns, including three during the southwest monsoon and two in the northeast monsoon periods, were carried out at the Laoyehai in 2008-2011. The aim of this study is to investigate the impacts of dynamic processes of hydrography and human activities on nutrient geochemistry and their relationships to the system eutrophication and hypoxia in the lagoon. In this coastal system, high levels of ammonium relative to nitrate are found, elevated phosphate skews the DIN/DIP relative to the Redfield ratio, and the dissolved silicate concentration is high because of submarine groundwater discharge. The organic fraction in the Laoyehai accounts for a large proportion of the total nutrients associated with the release of wastes from marine aquaculture. The hypoxia of near-bottom waters in the Laoyehai is created and maintained by heterotrophic processes that are fueled by organic matter, which are exacerbated by poor water exchange as a consequence of the geomorphology and weak tidal circulation.

Assessing the Potential of Sponges (Porifera) as Indicators of Ocean Dissolved Si Concentrations

We explore the distribution of sponges along dissolved silica (dSi) concentration gradients to test whether sponge assemblages are related to dSi and to assess the validity of fossil sponges as a palaeoecological tool for inferring dSi concentrations of the past oceans. We extracted sponge records from the publically available Global Biodiversity Information Facility (GBIF) database and linked these records with ocean physiochemical data to evaluate if there is any correspondence between dSi concentrations of the waters sponges inhabit and their distribution. Over 320,000 records of Porifera were available, of which 62,360 met strict quality control criteria. Our analyses was limited to the taxonomic levels of family, order and class. Because dSi concentration is correlated with depth in the modern ocean, we also explored sponge taxa distributions as a function of depth. We observe that while some sponge taxa appear to have dSi preferences (e.g., class Hexactinellida occurs mostly at high dSi), the overall distribution of sponge orders and families along dSi gradients is not sufficiently differentiated to unambiguously relate dSi concentrations to sponge taxa assemblages. We also observe that sponge taxa tend to be similarly distributed along a depth gradient. In other words, both dSi and/or another variable that depth is a surrogate for, may play a role in controlling sponge spatial distribution and the challenge is to distinguish between the two. We conclude that inferences about palaeo-dSi concentrations drawn from the abundance of sponges in the stratigraphic records must be treated cautiously as these animals are adapted to a great range of dSi conditions and likely other underlying variables that are related to depth. Our analysis provides a quantification of the dSi ranges of common sponge taxa, expands on previous knowledge related to their bathymetry preferences and suggest that sponge taxa assemblages are not related to particular dSi conditions. (Less)

Coupling of carbon and silicon geochemical cycles in rivers and lakes.

Carbon (C) and silicon (Si) biogeochemical cycles are important factors in the regulation of atmospheric CO2 concentrations and hence climate change. Theoretically, these elements are linked by chemical weathering and organism stoichiometry, but this coupling has not been investigated in freshwaters. Here we compiled data from global rivers and lakes in the United States of America and the United Kingdom, in order to characterize the stoichiometry between the biogeochemical cycles of C and Si. In rivers this coupling is confirmed by a significant relationship between HCO3−/Na+ and DSi/Na+, and DSi:HCO3− ratio can reflect the mineral source of chemical weathering. In lakes, however, these characteristic ratios of chemical weathering are altered by algal activity. The lacustrine Si:C atomic ratio is negative feedback regulation by phytoplankton, which may result in this ratio in algal assemblages similar to that in water column. And this regulation suggests lacustrine photosynthetic C fixation in this equilibrium state is quantitative and depends on the DSi concentration. These findings provide new insights into the role of freshwaters in global C and Si biogeochemical cycles.

闽江口潮滩湿地不同植被带土壤及间隙水中硅的分布特征

PDF HTML阅读 XML下载 导出引用 引用提醒 闽江口潮滩湿地不同植被带土壤及间隙水中硅的分布特征 DOI: 10.5846/stxb201501140109 作者: 作者单位: 福建师范大学地理研究所,浙江大学环境与资源学院 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金项目（41401114）；福建省教育厅项目（JA14082） Changes in the distribution of silica in the porewaters and sediments of the intertidal zone with different plant communities in the Min River Estuary Author: Affiliation: Key Laboratory of Humid Sub-tropical Eco-geographical Process of Ministry of Education,Research Centre of Wetlands in Subtropical Region,Fujian Normal University, Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:以闽江口潮滩湿地为研究对象，由岸及海方向对芦苇、短叶茳芏、互花米草（潮沟内）和互花米草（潮沟外）4种湿地土壤生物硅和土壤间隙水氮硅营养盐含量及其随深度变化的特征进行为期1a的季度观测。结果显示：互花米草（外）、短叶茳芏、芦苇和互花米草（内）带湿地土壤生物硅的年均含量依次降低，分别为14.33、10.40、9.98、7.50 mg/g；互花米草（外）、互花米草（内）、短叶茳芏和芦苇带湿地土壤间隙水活性硅酸盐年均含量依次降低，分别为407、359、344、323 μmol/L；湿地各植被带土壤及间隙水含硅量均呈现夏秋季节高于冬春季节的趋势。统计分析表明：间隙水活性硅酸盐与土壤生物硅含量、距潮沟的距离之间的正相关性均比较显著（P<0.05），温度对土壤中硅含量的影响也有一定的正相关性，说明湿地植被、温度和潮汐作用是影响闽江口湿地硅分布的重要因素。与土著种对比，互花米草入侵在一定程度上改变了闽江口潮滩湿地土壤硅分布的格局。 Abstract:Silicon (Si) is an essential nutrient for many plants as well as an important element that controls the functions of terrestrial and aquatic ecosystems. Wetland ecosystems are considered to be very important for Si transport and transformation in nutrient biogeochemical cycles. However, information on Si cycling in wetland ecosystems is scarce. The Shanyutan wetland is the largest tidal wetland in the estuary of the Min River, southeast China. In this study, the Si distribution pattern and its influencing factors were determined by seasonally measuring the levels of dissolved silicates (DSis) in the porewater and biogenic silica (BSi) in the sediments of Phragmites australis wetland (PAW), Cyperus malaccensis wetland (CMW), and Spartina alterniflora wetland (SAW) from October 2012 to September 2013. For better comparison, three sampling sites were added in SAW since January 2013; these sites were isolated by a 30m wide tidal creek. The sampling sites near the land were marked as S. alterniflora wetland (inside; SAWI), whereas those near the sea were marked as S. alterniflora wetland (outside; SAWO). The results indicated that the average contents of BSi were 14.33 mg/g in SAWO, 10.40 mg/g in CMW, 9.98 mg/g in PAW, and 7.50 mg/g in SAWI. The average concentrations of DSis in the sediment porewater were 407 μmol/L in SAWO, 359 μmol/L in SAWI, 344 μmol/L in CMW, and 323 μmol/L in PAW. Generally, the contents of BSi in the sediments and DSis in the porewater were higher in summer and autumn, whereas they were lower in spring and winter. Statistical analysis revealed significantly positive correlations between DSi concentration in the porewater and BSi content in the sediments or the distance from the shoreside to the tidal creek (P<0.05). BSi content in the sediments was slightly positively correlated with temperature. Taken together, our findings suggest that the distribution of Si in the wetlands of the Min River estuary was correlated with vegetation, temperature, and tidal action. Unlike that of native species, the invasion of S. alterniflora, to some extent, altered the distribution patterns of Si in the wetlands. 参考文献 相似文献 引证文献

Variability in the composition and export of silica in the Huanghe River Basin

Concentrations of suspended particle material (SPM), dissolved silicate (DSi), biogenic silica (BSi), phytoliths (plant produced siliceous microscopic structures), and other parameters were analyzed to examine the influence of both natural processes and human activities on silica delivery to the estuary of the Huanghe River (Yellow River). Our results indicate that the concentrations of DSi in the river decreased significantly since 1986. Approximately 34% of dissolved silica was trapped in the basin between 1986 and 2010 due to a reduction of soil erosion. Phytoliths comprised 67.2%–96.3% of BSi, with the smoothing bar type being the dominant form. Concentrations of BSi are significantly higher in the Huanghe River compared to other major rivers throughout the world due to its high sediment yield. We also found that the ratios of BSi/(BSi+DSi) and BSi/SPM were approximately 0.5 and 0.003 at Lijing near the river mouth, indicating that BSi carried in suspension by the Huanghe River was an important component of the rivers silica load. Significant amounts of BSi were also composed of phytoliths in Bohai Sea sediments near the Huanghe River estuary with the smoothing bar form again being the most abundant. The relatively high specific fluxes of BSi in the Huanghe River reflect its high turbidity and high erosion rates in the basin. The high sediment load originating on the Loess Plateau is likely responsible for the higher BSi flux, in agreement with a general trend of increasing BSi flux with increasing sediment flux in global river systems. This study demonstrates that BSi transported by rivers can be composed largely of phytoliths originating from the erosion of topsoils. The flux of phytoliths in river’s suspended sediment load may therefore represent a significant contribution to the biogeochemical cycle of silica in coastal waters.

DISSOLVED SILICATE IN COASTAL WATER OF SOUTH SULAWESI

ABSTRACT Dissolved silicate (DSi) in coastal waters plays a crucial role in phytoplankton growth particularly diatom. This study aimed to determine DSi concentration seasonally in waters of the western coast of South Sulawesi in relation to coastal water quality indicator. Water, chlorophyll-a, and diatom samples were collected from the coastal areas of the Tallo-Makassar, Maros, and Pangkep, in April 2013 (transitional season), June 2013 (dry season), and February 2014 (wet season). Factorial analysis of variance was used to identify significant seasonal and temporal variations, and linear regression was used to test the relationship of chlorophyll-a and diatom abundance to DSi concentrations. The results showed that the DSi concentration was higher in the wet season of 35.2-85.2 µM than in the other seasons (transitional season: 10.8-68.4 µM, dry season: 9.59-24.1 µM). The abundance of diatoms during the transitional season reached ~9.7x107 cell/m3 in the Pangkep river, 2.3x107 cell/m3 in the Tallo river, and 1.3 x 107 cell/m3 in the Maros river. Chaetoceros, Nitzschia, and Rhizosolenia dominated the diatom composition. The mean concentration of chlorophyll-a in the Makassar coastal waters was 4.52±4.66 mg/m3, while in the Maros and Pangkep waters of 1.40±1.06, and 2.72±1.94 mg/m3, respectively. There was no strong linear corelation between DSi and diatom abundances, nor chlorophyll-a. These results suggested that DSi become a non-limiting factor for the diatom growth and potentially reduce the water quality via eutrophication and diatom blooms. Keywords: dissolved silicate, diatom, chlorophyll-a, coastal waters, South Sulawesi

The interactive roles of temperature, nutrients, and zooplankton grazing in controlling the winter-spring phytoplankton bloom in a temperate, coastal ecosystem, Long Island Sound

We examined biological, chemical, and physical processes controlling the onset and demise of the winter–spring bloom in the third largest US estuary, Long Island Sound (LIS), during 2010 and 2011. The spring bloom in LIS initiated in the absence of thermal stratification and peaked in early February when water temperatures were at an annual minimum (≤1.0°C). Bloom initiation occurred when phytoplankton cellular growth exceeded zooplankton grazing and bloom demise occurred as temperatures rose and grazing exceeded algal growth. During the bloom collapse, nutrients were drawn down by >90% and the phytoplankton community was nutrient limited. A highly significant (r2 = 0.81, p = 0.001), multivariate linear regression model using the percentage of primary production consumed daily by zooplankton and dissolved silicate concentrations hind-casted the timing and magnitude of the bloom within 10% of observed values each year. Mesocosm experiments performed during the winter–spring bloom demonstrated that higher seawater temperatures (+3.0°C) increased zooplankton densities and grazing rates and decreased phytoplankton biomass. The winter–spring bloom in LIS was controlled by the interaction of phytoplankton growth and zooplankton grazing but not thermal stratification. Phytoplankton growth and zooplankton grazing were, in turn, influenced by nutrient availability and temperature. Continued climatic warming may lessen the intensity of the winter–spring bloom in this and other ecosystems in the future.

Biogenic silica dissolution in diatom aggregates: insights from reactive transport modelling

Diatom aggregates contribute significantly to the vertical sinking flux of particulate matter in the ocean. These fragile structures form a specific microhabitat for the aggregated cells but their internal chemical and physical characteristics remain largely unknown. Studies on the impact of aggregation on Si cycle led to what appears to be inconsistency. Despite a lower biogenic silica (bSiO2) dissolution rate and a diffusion of the silicic acid (dSi) similar in aggregate and in seawater, dSi surprisingly accumulate in aggregates. A reaction-diffusion model helps to clarify this incoherence by reconstructing dSi accumulation measured during batch experiments with aggregated and non-aggregated Skeletonema marinoi and Chaetoceros decipiens. The model calculates the effective bSiO2 dissolution rate as opposed to the experimental apparent bSiO2 dissolution rate, which results on the effective bSiO2 dissolution and transport of dSi out of the aggregate. In the model, dSi transport out of the aggregate is modulated considering alternatively retention -decrease of dSi diffusion constant- and adsorption -reversible chemical bounds between dSi and the aggregate matrix- processes. Modelled bSiO2 dissolution is modulated by the impact of dSi concentration inside aggregate and diatom viability, as enhanced persistence of metabolically active diatom has been observed in aggregates. Adsorption better explains dSi accumulation within and outside aggregates, raising the possible importance of dSi travelling within aggregates to the deep sea (potentially 20% of the total silica flux). Moreover, the model states that bSiO2 dissolution is effectively decreased in aggregates mainly due to higher diatom viability but also to other parameters discussed here

Impact of grazing management on silica export dynamics of Wadden Sea saltmarshes

During periods of silica limitation, its supply from tidal marshes is important for the stability of estuarine and coastal food webs. Saltmarshes are highly dynamic, grass dominated ecosystems: their large area, high salinity and location imply that they could efficiently contribute to the buffering of silica depletion events in the coastal zone. As grazing management potentially alters vegetation and sedimentation dynamics in saltmarshes, it could have an indirect impact on silica cycling in these systems. In two saltmarshes of the Wadden Sea coast, concentrations of dissolved and biogenic silica (DSi and BSi) were measured in eight creeks in four seasons under different management conditions. Export rates were calculated using simultaneous discharge measurements. Mean annual DSi concentration in the seepage water was 338 ± 112 μmol l−1. Ungrazed sites had significantly higher seepage water DSi concentrations than sites which were grazed by sheep. BSi concentrations were, in general, lower and more variable. DSi export rates from ungrazed sites (265 ± 155 μmol m−2 day−1) were twice as high as from grazed saltmarshes (126 ± 137 μmol m−2 day−1). DSi concentrations were among the highest values previously reported for saltmarshes and tidal freshwater marshes. Although differences in silica exports from grazed and ungrazed sites might be partly explained by silica uptake of benthic diatoms in the creeks, differences in hydrology appeared to be an overarching factor, controlling silica exports from Wadden Sea saltmarshes.

Bar-built estuary as a buffer for riverine silicate discharge to the coastal ocean

River discharge is a primary source of silicates to the ocean. In this study, we show that a bar-built estuary can buffer this silicate input, which may influence the onset of the diatom spring bloom and have a leveling effect on coastal diatom production dynamics. Primary production in the northeastern Gulf of Mexico is strongly affected by the outflow of nutrient-rich water from the Apalachicola River that enters into the Apalachicola Bay. We compared the seasonal dynamics of dissolved silicate (DSi), dissolvable particulate biogenic silicate (BSi), and the total of dissolved and dissolvable silicate (DSi+BSi) in the water column and pore water at two sandy sublittoral sites located in Apalachicola Bay and the adjacent coastal Gulf of Mexico in order to determine the influence of the Bay on silicate transfer to the coastal Gulf. The one-year time series measurements in the Bay water column showed that DSi was strongly influenced by the riverine outflow, while the DSi concentrations in the nearby coastal Gulf water followed a different trend and appeared to be dominated by biological processes. The temporal changes in pore water DSi, BSi, and DSi+BSi were similar at both study sites suggesting that larger scale processes, i.e., increased sediment flushing by increased wave action in winter, controlled these changes. The sediments acted as a continuous source of DSi to the overlying water, and when the river discharge was lower during early winter, the magnitude of area-integrated DSi flux from the Bay sediments was similar to the riverine DSi input to the Bay. The Bay sediments thus functioned as a significant source of DSi for the Bay and the adjacent Gulf at the onset of the diatom spring bloom and before Apalachicola River discharge reached maximum discharge in late winter/early spring.

2009년 강원 연안의 계절별 수괴 분포 및 영양염의 시공간적 특성

우리나라 동해 강원 연안에서의 계절 변화에 따른 수괴의 분포와 화학적 특성을 규명하기 위하여 2009년 2월, 5월, 8월 그리고 11월에 고성, 속초, 양양, 강릉, 동해 연안의 5개 지점에서 CTD 관측과 동시에 영양염의 분포 특성을 조사하였다. 수온과 염분 분포를 통하여 연구해역에서의 수괴는 대마난류표층수, 대마난류중층수, 북한한류수, 동해고유수와 각 수괴들이 혼합된 혼합수로 구분되었다. 계절 변화에 따른 수괴의 분포를 보면 2월은 수직혼합이 활발하였으며 표층수의 대부분이 대마난류중층수에 기인하였고, 5월에 대마난류표층수는 강원 해역까지 진출하지 못하여 관측되지 않았다. 8월은 표층에 대마난류표층수가 유입되었으며, 11월에는 대마난류의 영향이 서서히 줄어들고 북한한류가 다시 강화되어 나타났다. 영양염 농도의 수직적 분포는 계절변화에 관계없이 표층에서 가장 낮은 농도를 보이다가 수온약층 부근에서 급격히 증가하였고, 이후 수심 200 m 이하에서 질산염과 인산염은 수심이 증가함에 따라 대체로 일정한 값을 보였으나 규산염은 증가하는 경향을 보였다. 이는 규산염의 재생산 속도가 다른 영양염의 재생산 속도에 비해 느리기 때문에 나타나는 현상이다. 연구해역에서의 수괴별 영양염의 농도 분포는 4계절 모두 동해고유수에서 가장 높았고, 그 다음으로 북한한류수, 대마난류중층수, 대마난류표층수의 순이었다. 연구해역에서의 수괴별 N/P ratio는 2월, 5월 그리고 11월은 대부분의 수괴에서 Redfield ratio보다 높거나 비슷한 수치를 나타내고 있어 대체로 이 기간 동안의 표층에서 질산염이 식물플랑크톤 성장의 제한인자로 작용하고 있지 않음을 지시한다. 반면에 8월은 대마난류표층수에서의 N/P ratio가 Redfield ratio보다 크게 낮게 나타났으며 표층에서 질산염 평균 농도가 0.86 μM을 나타내어, 본 연구해역에서는 8월의 표층에서 질산염이 식물플랑크톤 성장의 제한인자로 작용하고 있음을 보여주고 있다.

淀川流域における複合循環系のケイ素輸送特性

It is pointed out that dissolved silicate is an essential matter for diatom and an important factor affecting ecosystems in estuaries. We made field measurements and analyses for water quality and material flow, especially dissolved silicate in the Yodo River Basin with sewerage systems and waterworks systems. Dissolved silicate concentration in Uji River is about 1 to 2 mg/L, and the spring-blooming in Lake Biwa causes a significant decrease in the concentration. The dissolved silicate concentration in water and sewage plants is hardly changing in the treating process. Moreover, underground water pumped up for tap water contains rich dissolved silicate, and is greatest source of dissolved silicate in the basin.

Spatial and Temporal Trends in Water Quality in Response to Sewage Discharge in Masan and Hangam Bays, Korea

Abstract This study investigated the spatial and seasonal variations of nutrients, chlorophyll-a (Chl-a), particulate organic carbon, and dissolved oxygen at four stations around Masan Bay from February 2004 to February 2006. Nutrients showed large spatial and seasonal variations in the study area. Dissolved inorganic nitrogen and dissolved silicate concentrations were highest at inner Masan Bay, which was ascribed to the largest inputs of both nutrients from the lands among four stations. Dissolved inorganic phosphorus concentrations are highest at Hangam Bay, where a fertilizer plant caused serious soil pollution with the by-products of phosphate rocks. The extremely high Chl-a concentrations (>60 µg L−1) were observed at inner Masan Bay in July and attributed to the plentiful nutrients, high water stability, and longer residence time at this area during summer. Surface Chl-a concentrations displayed large seasonal variations at all stations, with high values in summer and low values in fall and winter....

Tidal variability in benthic silicic acid fluxes and microphytobenthos uptake in intertidal sediment

Silicic acid (DSi) benthic fluxes play a major role in the benthic–pelagic coupling of coastal ecosystems. They can sustain microphytobenthos (MPB) development at the water–sediment interface and support pelagic diatoms when river DSi inputs decrease. DSi benthic fluxes have been studied at the seasonal scale but little is known about their dial variations. This study measured the amplitude of such variations in an intertidal area over an entire tidal cycle by following the alteration of DSi pore water concentrations at regular intervals over the flood/ebb period. Furthermore we independently estimated the potential DSi uptake by benthic diatoms and compared it to the variations of DSi pore water concentrations and fluxes. The microphytobenthos DSi demand was estimated from primary production measurements on cells extracted from the sediment. There were large changes in DSi pore water concentration and a prominent effect of tidal pumping: the DSi flushed out from the sediment at rising tide, occurs in a very short period of time, but plays a far more important role in fueling the ecosystem (800 μmol-Si m −2 d −1), than diffusive fluxes occurring throughout the rest of the tidal cycle (2 μmol-Si m −2 d −1). This process is not, to our knowledge, currently considered when describing the DSi cycling of intertidal sediments. Moreover, there was a large potential MPB requirement for DSi (812 μmol-Si m −2 d −1), similar to the advective flow periodically pumped by the incoming tide, and largely exceeded benthic diffusive fluxes. However, this DSi uptake by benthic diatoms is almost undetectable given the variation of DSi concentration profiles within the sediment.

Global spatial distribution of natural riverine silica inputs to the coastal zone

Abstract. Silica, SiO2, in dissolved (DSi) and particulate (PSi) form, is both a major product of continental weathering as well as an essential nutrient in terrestrial and aquatic systems. Here we present estimates of the spatial distribution of riverine silica fluxes under natural conditions, i.e. without human influence, to ~140 segments of the global coastal zone. Focussing on the construction of the DSi budget, natural DSi concentration is multiplied with discharge of rivers for each segment for documented basins and segments. Segments with no documentation available are estimated using clustered information based mainly on considerations of local lithology, climate, and lake retention. We approximate fluxes of particulate silica in various forms (PSi) from fluxes of suspended matter, calculated from existing models. Results have been established for silica fluxes, concentrations and yields for drainage basins of the different continents, oceans basins as well as coastal segment basins. For the continental surfaces actually draining into the oceans (exorheic regions, representing 114.7 million (M) km2), 371 M t y−1 of DSi and 8835 M t y−1 of PSi are transported, corresponding to a mean concentration of 9.5 mg l−1 and 226 mg l−1, and to a mean yield of 3.3 t km−2 y−1 and 77 t km−2 y−1, respectively. DSi yields exceeding 6.6 t km−2 y−1, i.e. &gt;2× the global average, represent 17.4% of the global continental ice-free exorheic area but correspond to 56.0% of DSi fluxes. Pacific catchments hold most of the hyper-active areas (&gt;5× global average), suggesting a close connection between tectonic activity and DSi fluxes resulting from silicate weathering. The macro-filters of regional and marginal seas intercept 33% and 46% of the total dissolved and particulate silica fluxes. The mass of DSi received from rivers per unit square area of various oceans ranges over more than one order of magnitude. When expressed per unit volume and when individual regional seas are considered this figure ranges over two to three orders of magnitude, an illustration of the heterogeneity of the land to sea connection.

Density and Viscosity of Hydrous Magmas and Related Fluids and their Role in Subduction Zone Processes

We have developed density^viscosity^composition (r^m^X) models for natural aqueous fluids and hydrous melts, based on experimental data for silicate þ H2O, especially for the pressure (P) and temperature (T) conditions above subduction zones. We examine hydrothermal and melt pathway systematics above subducting slabs into the Earth’s mantle, back up along the top-of-slab, and downward with the subduction. Aqueous slab fluids and hydrous mantle melts show distinct flow properties (as observed in activation energy in viscosity data) despite continuity in solute-polymerization characteristics. Buoyancy changes are small for fluids except in the localized vicinity of critical behaviour and at solidi where H2O partitions also into melt. Our model predicts dilute high-PT potassic haplogranite fluids to be less viscous than sodic varieties whereas for concentrated fluids a deep viscosity minimum occurs in mixed K/Na (c. 1:1 molar) compositions. Higher dissolved silicate concentrations increase fluid density and viscosity leading to slower less-buoyant flow with increasing PT. Thus ascent rates of slab fluid increase by about an order of magnitude (from c .1 0 � 3·5 to 10 � 4·3 ms � 1 for porous flow; c. 1 to 7 m s � 1 for flow through 1mm wide fractures)

Dissolved silicate retention and transport in cascade reservoirs in Karst area, Southwest China

Presently, the fluvial fluxes of Dissolved silicate (DSi) by many famous rivers in the world have been reported significantly decreased in the past several decades. DSi retention by river damming hence becomes a great concern. In this study, the impact on DSi retention and transport by cascade reservoirs in the Karst area were investigated. Based on the monitoring data for four seasons, budget of DSi in these reservoirs were calculated. Results showed that, only the downstream eutrophic Wujiangdu reservoir (WJD) was the sink for DSi, with a value of ca. 3.5 kT DSi trapped annually. The new Hongjiadu reservoir (HJD) in the headwater area revealed to be an important source for DSi. Both Dongfeng (DF) and Suofengyin (SFY) reservoirs also released more DSi to the downstream than entering them. The great discrepancy of DSi source and sink effects among these reservoirs indicates that, (i) DSi was obviously taken up in summer and spring, and the concentration of DSi could be dropped down to below 30 μΜ; (ii) during the period of thermal stratification, the concentration stratification of DSi is also developed along the water column. Possibly DSi releasing from inundation soil and bottom sediment was the important compensation for the silicon depletion in the epilimnion. In the newly constructed reservoirs ( e.g. HJD and SFY), the releasing flux of DSi can significantly exceed the assimilating flux by diatom; and (iii) due to the hypolimnion introducing for hydropower operation, water leaving the dam generally had high DSi content, and then masked the DSi taking up in the epilimnion. This process also decreased the ratio of DIN to DSi, mitigating the silicon limiting situation in the downstream.

有明海北東部流入河川の溶存態ケイ素濃度の予測

Dissolved silicate (Si(OH)4-Si; DSi) concentration was examined at 38 sampling sites in influent rivers of the northeastern Ariake Bay, and estimated on the basins of a numerical geological map of the sites' watershed. The multiple regression model was used for predicting DSi concentration at the sampling sites using the geological area ratio in their watersheds. The coefficient of determination (R2) obtained was 0.931, indicating that DSi concentration was strongly associated with its geological condition. The geological categories which contribute to DSi concentration in rivers are quaternary volcanic rocks (non-alkali, felsic), volcanic rocks (debris) and quaternary volcanic rocks (non-alkali, pyroclastic flows). This result suggests that the DSi concentration of an arbitrary station in an influent river of the northeastern Ariake Bay can be predicted on the basins of a geological map of the station.

Seawater-mediated interactions between diatomaceous silica and terrigenous sediments: Results from long-term incubation experiments

Reactors containing frustules of the cultured diatom Thalassiosira punctigera suspended in seawater were incubated with or without added sediment from the Mississippi River Delta or the Congo River Fan. The diatom frustules were separated from the terrigenous sediments by a dialysis membrane, thereby only allowing the exchange of dissolved species. One series of incubations was carried out in the laboratory, at room temperature (21 °C) and for a period of 10 months. Another series of reactors was deployed along a mooring in the Mozambique Channel at three water depths (500, 1250, and 2000 m), for a period of 22 months. Chemical analyses after total destruction of frustules collected at the end of the incubations showed elemental transfer from seawater (Mg and K) and the sediments (Al, Fe, Mn, P and Ca) to the frustules. In the presence of the terrigenous sediments, the dissolved silicate concentrations at the end of the incubations were systematically lower that those measured in the incubations without the sediments. In addition, electron microscopy revealed the formation of new mineral precipitates. These included amorphous deposits on the frustules containing Si, Fe, Al, Mg, K and P, as well as euhedral clay crystallites. Differences were observed between the incubations performed in the laboratory and those deployed at sea, likely as a result of differences in redox conditions, temperature and reaction time. Overall, the interactions between biogenic silica, seawater and lithogenic minerals reduce the regeneration of nutrient silicon fixed by siliceous organisms. These interactions take place on relatively short time scales (months to years), and affect not only the marine cycle of silicon, but also those of other major and minor elements, such as Al, Fe, Mn, K, and Mg.

Nutrient kinetics modeled from time series of substrate depletion and growth: dissolved silicate uptake of Baltic Sea spring diatoms

We estimated silicate uptake kinetics for 8 spring diatom species using a model based on time series measurements of the depletion of dissolved silicate (DSi) and increases in biomass. Furthermore, the carbon: nitrogen: silicate stoichiometric relationships and maximum growth rates were determined. Differences in DSi uptake kinetics and maximum growth rate were distinct among the species. All the most common diatom species (Chaetoceros wighamii, Pauliella taeniata, Skeletonema costatum and Thalassiosira baltica) were relatively lightly silicified and had variable but relatively low half-saturation constants (Ks), indicating that they are well adapted to low DSi concentrations. The less common Diatoma tenuis and Nitzschia frigida had higher Ks values, suggesting that they are more vulnerable to DSi limitation. The much used nitrogen:silicate ratio of 1 for marine diatom biomass was too low for most of the examined species, while a ratio of 2–3 seems to be more appropriate for these Baltic Sea species.