Biogenic Silica In Sediments Research Articles

Ecological significance of biogenic silica in the East China Sea

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 290:15-26 (2005) - doi:10.3354/meps290015 Ecological significance of biogenic silica in the East China Sea S. M. Liu1,*, J. Zhang1,2, R. X. Li3 1College of Chemistry and Chemical Engineering, Ocean University of China, 5 Yushan Road, Qingdao 266003, PR China2State Key Laboratory of Estuarine and Coastal Research, East China Normal University, 3663 Zhongshan Road North,Shanghai 200062, PR China3First Institute of Oceanography, State Oceanic Administration, Qingdao 266061, PR China *Email: sumeiliu@mail.ouc.edu.cn ABSTRACT: Field observations were carried out in the East China Sea (ECS) in October 2000, May 2001 and April–May 2002. Biogenic silica (BSi) in core sediments, sediment traps, and suspended particulate matter (SPM) was measured; silicate profiles in pore waters were examined. The concentration of BSi in SPM was high in coastal areas. The content of BSi in ECS sediments is less than 1%, as the result of tremendous riverine input of terrigenous weathered materials. For the ECS Shelf, the dissolution of BSi in sediment cores is important to maintain high levels of silicate in the water column, supporting prolific primary production by diatoms. BSi burial efficiencies, that is, the silica accumulation rate divided by the sum of the silica accumulation rate and the benthic silicate flux, are estimated to be 36 to 97%, higher than in the Equatorial Pacific Ocean and higher than the global ocean average. BSi regeneration efficiency, that is, the ratio of BSi vertical flux minus the BSi accumulation rate relative to BSi vertical flux, is ca. 80% for the ECS. A silica budget was established for the ECS shelf, and the model output shows that major silicate input comes from the Kuroshio, followed by silicate from the riverine inflow and the Taiwan Strait. About 75% of gross BSi production dissolves in the water column over the ECS Shelf. KEY WORDS: Biogenic silica · Burial · Regeneration · Budget · East China Sea Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 290. Online publication date: April 13, 2005 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2005 Inter-Research.

Terrigenous flux and biogenic silica deposition at the Antarctic continental rise during the late Miocene to early Pliocene: implications for ice sheet stability and sea ice coverage

Drift sediments recovered from the East Antarctic continental rise at Ocean Drilling Program (ODP) Site 1165 are used to infer variations in East Antarctic Ice Sheet (EAIS) stability and sea ice coverage during the late Miocene and early Pliocene. A significant increase in the deposition of biogenic opal from ∼5.8 to 5.2 Ma points to an early Pliocene reduction in sea ice and a subsequent increase in biological productivity. Time intervals at ∼7.2 to 6.6 Ma and ∼5.2 to 4.8 Ma are characterized by pronounced maxima in the long-term trend of terrigenous matter accumulation (MAR ter) indicating high continental erosion rates potentially caused by ice sheet growth. A Southern Ocean wide impact of these events is suggested by similar evidence found at ODP Site 1095 (Antarctic Peninsula). Superimposed on the MAR ter maxima we observe enhanced orbital variability in iron accumulation at Site 1165 pointing to a dynamic behavior of the EAIS with waxing and waning ice masses. From the concurrence of these high amplitude ice sheet fluctuations with maximum variance in Earth's obliquity, we propose that the insolation gradient between high and low latitudes affected the delivery of moisture to Antarctica and thus controlled ice volume variations.

Trophic transition in a lake on the Virginia coastal plain.

To examine possible connections between lake trophic status and runoff from surrounding subwatersheds, we determined patterns of sediment and nutrient deposition in a hypereutrophic, 16-ha impoundment on the Virginia coastal plain. Spatial survey of nutrients in surface sediments documented a strong correlation between total P and extractable Fe (r2 = 0.53). Elevated biogenic silica concentrations up to 0.25% by weight were measured in sections of the lake receiving perennial stream discharge. Sediment C to N ratios were > 20 in those same sections, suggesting a large allochthonous contribution to organic matter deposition. Sediment cores 0.9 to 2.3 m in length, representing 70 years of deposition, were analyzed to develop vertical profiles of changes in sediment and nutrient deposition in deltas downstream from two more-developed and three less-developed subwatersheds (with 49 and 9% commercial and residential development, respectively). The average sediment weight percent +/- standard deviation of biogenic silica (0.027 +/- 0.037 vs. 0.009 +/- 0.006%) and total P (0.040 +/- 0.025 vs. 0.024 +/- 0.019%) was significantly higher downstream of more-developed subwatersheds. Using elevated P loadings and biogenic silica deposition as proxies for algal production, transition of the lake to its current hypereutrophic state appears to have occurred in the last 70 yr. Changes in trophic status as revealed by sediment analysis of this small lake on the Virginia coastal plain reflect a common pattern of eutrophication observed for the entire Chesapeake Bay drainage basin. Analysis of sediments from stream deltas appears to be a reasonable strategy for identifying and targeting subwatershed areas needing better management of nutrient runoff that otherwise would lead to eutrophication of downstream waters.

Last glacial sea surface temperatures and sea‐ice extent in the Southern Ocean (Atlantic‐Indian sector): A multiproxy approach

On the basis of the quantitative study of diatom, radiolarian, and planktic foraminiferal assemblages, we estimated summer sea surface temperature (SSST) and sea‐ice extent at 50 sediment core localities in the Atlantic and western Indian sector of the Southern Ocean to reconstruct the last glacial environment at the GLAMAP (18 to 15 ka, equal to 21,500 to 18,000 calendar (cal) years BP) and EPILOG (19.5 to 16.0 ka, equal to 23,000 to 19,000 cal years BP) time slices. Stratigraphic identification of the time slices was accomplished by a combination of AMS 14C measurements, benthic isotope, and siliceous microfossil abundance records. While the SSST estimates reveal greater surface water cooling than reconstructed by CLIMAP [1981], reaching a maximum in the area of the present Subantarctic Zone, the sea‐ice reconstruction indicates that CLIMAP overestimated the expansion of the Antarctic sea‐ice field, especially for austral summer. During winter the sea‐ice field was expanded by 60–70% compared to the present. Last glacial summer sea surface isotherms indicate a northward shift of the zonal bands of the Antarctic Circumpolar Current and a relative expansion of the cold water realm south of the Subantarctic Front by ∼5° in latitude. This coincides with a northward displacement of the zone of enhanced biogenic silica deposition and iceberg occurrence. As a result of northward expansion of Antarctic cold waters and a relatively small displacement of the Subtropical Front, thermal gradients were steepened during the last glacial in the area of the present Subtropical Front. The northward displacement of Antarctic cold waters and the related deflection of Southern Ocean waters along the eastern boundary of South America may have resulted in a weakened “cold water route” across the Drake Passage. In contrast, the transport of warm and salty surface water from the Indian into the Atlantic Ocean via the “warm water route” was not blocked allowing continuous but reduced import of heat into the South Atlantic.

Problems with biogenic silica measurement in marginal seas

Surface sediment samples from the Bohai, Yellow Sea, and the Pacific were used to assess biogenic silica (BSi) content and to study uncertainties in BSi measurements. The contents of BSi in the Bohai and Yellow Sea are all less than 1%. The dissolution of BSi in sediments from the Bohai and Yellow Sea is very important to maintain high levels of silicate in the water column. The non-biogenic silica from clay minerals has an obvious effect on BSi of sediment samples in the Bohai and Yellow Sea with low BSi and high clay minerals. The solid to solution ratio was found to have a great influence on BSi measurement, which can induce uncertainties up to 75%. The effect of loss by sorption and centrifugation is negligible. Interlaboratory comparison of techniques for BSi measurement by the wet alkaline extraction technique of Mortlock and Froelich [Deep-Sea Res. 36 (1989) 1415–1426] with clay correction was suggested to give no significant differences. However, differences in sediment compositions and reagent to sample ratio may limit the application of the wet alkaline extraction method.

Growth and demise of Permian biogenic chert along northwest Pangea: evidence for end-Permian collapse of thermohaline circulation

The Permian Chert Event (PCE) was a 30 Ma long episode of unusual chert accumulation along the northwest margin of Pangea, and possibly worldwide. The onset of the PCE occurred at about the Sakmarian–Artinskian boundary in the Sverdrup Basin, Canadian Arctic, where it coincides with a maximum flooding event, the ending of high-frequency/high-amplitude shelf cyclicity, the onset of massive biogenic chert deposition in deep-water distal areas, and a long-term shift from warm- to cool-water carbonate sedimentation in shallow-water proximal areas. A similar and coeval shift is observed from the Barents Sea to the northwestern USA. A landward and southward expansion of silica factories occurred during the Middle and Late Permian at which time warm-water carbonate producers disappeared completely from the northwest margin of Pangea. Biotically impoverished and increasingly narrow cold-water carbonate factories (characterised by non-cemented bioclasts of sponges, bryozoans, echinoderms and brachiopods) were then progressively replaced by silica factories. By Late Permian time, little carbonate sediments accumulated in the Barents Sea and in the Sverdrup Basin, where the deep- to shallow-water sedimentary spectrum was occupied by siliceous sponge spicules. By that time, biogenic silica sedimentation was common throughout the world. Silica factories collapsed in the Late Permian, abruptly bringing the PCE to an end. In northwest Pangea, the end-Permian collapse of the PCE was associated with a major transgression and with a return to much warmer oceanic and continental climatic conditions. Chert deposition resumed in the distal oceanic areas during the early Middle Triassic (Anisian) after a 8–10 Ma interruption (Early Triassic Chert Gap). The conditions necessary for the onset, expansion and zenith of the PCE were provided by the thermohaline circulation of nutrient-rich cold waters along the northwestern and western margin of Pangea, and possibly throughout the world oceans. These conditions provided an efficient transportation mechanism that constantly replenished the supply of silica in the area, created a nutrient- and oxygen-rich environment favouring siliceous biogenic productivity, established cold sea-floor conditions, hindering silica dissolution, while increasing calcium carbonate solubility, and provided conditions adverse to organic and inorganic carbonate production. The northwest margin of Pangea was, for nearly 30 Ma, bathed by cold waters presumably derived from the seasonal melting of northern sea ice, the assumed engine for thermohaline circulation. This process started near the Sakmarian–Artinskian boundary, intensified throughout Middle and Late Permian time and ceased suddenly in latest Permian time. It led to oceanic conditions much colder than normally expected from the palaeolatitudes, and the influence of cold northerly-derived water was felt as far south southern Nevada. The demise of silica factories was caused by the rapid breakdown of these conditions and the establishment of a much warmer marine environment accompanied by sluggish circulation and perhaps a reduced input of dissolved silica to the ocean. Complete thawing of northern sea ice would have ended thermohaline circulation and led to warm and sluggish oceanic conditions inimical to the production, accumulation and preservation of biogenic silica.

Unraveling the atomic structure of biogenic silica: evidence of the structural association of Al and Si in diatom frustules

We used X-ray absorption spectroscopy at the Al K-edge to investigate the atomic structure of biogenic silica and to assess the effect of Al on its crystal chemistry. Our study provides the first direct evidence for a structural association of Al and Si in biogenic silica. In samples of cultured diatoms, Al is present exclusively in fourfold coordination. The location and relative intensity of X-ray absorption near-edge structure (XANES) features suggests the structural insertion of tetrahedral Al inside the silica framework synthesized by the organism. In diatom samples collected in the marine environment, Al is present in mixed six- and fourfold coordination. The relative intensity of XANES structures indicates the coexistence of structural Al with a clay component, which most likely reflects sample contamination by adhering mineral particles. Extended X-ray absorption fine structure spectroscopy has been used to get Al-O distances in biogenic silica of cultured diatoms, confirming a tetrahedral coordination. Because of its effect on solubility and reaction kinetics of biogenic silica, the structural association between Al and biogenic silica at the stage of biosynthesis has consequences for the use of sedimentary biogenic silica as an indicator of past environmental conditions.

Dissolution kinetics of biogenic silica from the water column to the sediments

Stirred flow-through experiments were conducted for the first time with planktonic biogenic silica (BSi). We investigated the dissolution kinetics of uncleaned and chemically cleaned BSi collected in ocean surface water, sediment traps, and sediments from the Norwegian Sea, the Southern Ocean, and the Arabian Sea. The solubility at 2°C is rather constant (1000 to 1200 μM). The dissolution rates are, however, highly variable, declining with water depth, and phytoplankton reactivity is two to three orders of magnitude higher than pure siliceous oozes. The reactivity decrease correlates well with an increase in the integrated peak intensity ratios of Si-O-Si/Si-OH measured by Fourier transform infrared (FTIR) spectroscopy. The removal of organic or inorganic coatings enhance the reactivity by at least an order of magnitude. Atomic Al/Si ratios of 0.03 to 0.08 in sedimentary diatom frustules decrease significantly to 0.02 as a result of removal of inorganic coatings and detritals present. Near equilibrium, the dissolution rates exhibit a linear dependence on the degree of undersaturation. At higher degrees of undersaturation—that is, at low concentrations of dissolved silica—the dissolution rates of uncleaned samples define a nonlinear trend.The nonlinear kinetics imply that the dissolution of natural BSi is strongly accelerated in silica-depleted surface waters. The FTIR results suggest that internal condensation reactions reduce the amount of surface reaction sites and are partly responsible for the reactivity decrease with depth. The high content of Al in sedimentary BSi is likely caused by precipitation of dissolved silica with Al dissolved from minerals in sediment. Nonbiogenic silica as coatings or detritals are partly responsible for the solubility and reactivity decrease of BSi in sediments. One order of magnitude different rate constants measured in Norwegian Sea and Southern Ocean sediment trap material support the so-called opal paradox—that is, high BSi accumulation rates in sediments in spite of low BSi production rates in surface waters of the Southern Ocean.

Oligocene biogenic siliceous deposits on the slope of the northern South China Sea

The abundance of radiolarian, diatom and sponge spicule and H4SiO4 in pore-waters increase abruptly at the boundary between Early and Late Oligocene (about 30-27.5 Ma) at Site 1148 of the northern South China Sea (SCS), indicating high biogenic silica accumulation during this time. At the same time (about 30-28 Ma), high biogenic silica deposition occurred in the central equatorial Pacific. Comparison of the biogenic silica accumulation at Site 1148 of the SCS with that at Site 929 of the Atlantic verifies that the biogenic silica accumulation between the low latitude Pacific and Atlantic oceans expresses the evident relationship of compensation during the Oligocene. Biogenic silica accumulation decreased in the Atlantic, whereas it increased in the Pacific at the boundary between the Early and Late Oligocene. It resulted from the formation and presence of North Atlantic deep water (NADW) in the Atlantic basin, indicating an intensive basin-basin fractionation. XRD analysis and SEM observation of the samples from Site 1148 demonstrate that most of radiolarian, diatom and sponge spicule have suffered from dissolution and reprecipitation, suggested by the opal-A→opal-CT transformation. As a result of the transformation, porosity increased, but dry and bulk densities decreased, reflecting the consequence of diagenesis on the physical property of sediment.

Effects of Holocene Alnus Expansion on Aquatic Productivity, Nitrogen Cycling, and Soil Development in Southwestern Alaska

Numerous pollen records provide evidence for the widespread range expansion of Alnus throughout Alaska and adjacent Canada during the middle Holocene. Because Alnus can fix atmospheric N2, this vegetational change probably had a profound effect on N availability and cycling. To assess this effect, we analyzed a sediment core from Grandfather Lake in southwestern Alaska for a suite of geochemical indicators, including elemental composition, biogenic silica (BSi) content, and carbon (C) and nitrogen (N) isotopes of organic matter. These data, in conjunction with a pollen record from the same site, are used to infer biogeochemical processes associated with the mid-Holocene Alnus expansion. The increase in Alnus pollen percentages from 10% to 70% circa 8000-7000 BP (14C years before present) suggests the rapid spread of Alnus shrub thickets on mountain slopes and riparian zones in the Grandfather Lake region. Coincident with this vegetational change, the mean value of the sediment BSi content increases from 20.4 to 106.2 mg/g, reflecting increased diatom productivity within the lake as a result of Alnus N2 fixation in the watershed soils and the associated N flux to the lake. Elevated aquatic productivity at this time is also supported by increased percentages of organic C and N, decreased C:N ratios, and decreased values of δ 13C. Furthermore, the δ 15N values of sediments increase substantially with the establishment of Alnus shrub thickets, suggesting enhanced N availability and accelerated N cycling within the lake and its watershed. Superimposed on a general trend of soil acidification throughout the postglacial period, soil acidity probably increased as a result of the Alnus expansion, as can be inferred from decreasing ratios of authigenic base cations to allogenic silica (Si) and increasing ratios of authigenic aluminum (Al) to allogenic Si. The ultimate cause of these mid-Holocene ecosystem changes was an increase in effective moisture in the region.

An interlaboratory comparison for the measurement of biogenic silica in sediments

Abstract An interlaboratory comparison was made to assess the variability in the measurement of biogenic silica (BSi) in sediments among aquatic scientists. The purpose of this study was to (1) assess the present state of BSi analysis by aquatic scientists, (2) establish the range of variability in the measurement of BSi across the community of aquatic scientists (paleolimnologists, paleooceanographers, limnologists, estuarine scientists, and oceanographers), and (3) to determine if patterns in the measurement were related to specific methodologies used or to treatments. This study included 30 laboratories primarily using a variety of different wet chemical extraction techniques. The six samples used in the interlaboratory comparison were from modern freshwater and coastal marine depositional environments. The samples were chosen to cover a wide range of BSi concentrations, contain no sponge spicules or radiolarians, and were easy to digest relative to relict samples. Independently measured BSi concentrations of the same sediment ranged widely. BSi concentrations determined by X-ray diffraction were significantly higher than concentrations determined by wet chemical methods. The percent standard deviation of the mean in samples analyzed by wet chemical digestion techniques was highest in the samples with the lowest BSi concentration (Table 3) with 67.5% variation in sample 6 (1.31±0.88 wt.% as SiO2), and lowest at the highest BSi concentration with 21.2% variation in sample 2 (44.3±9.38 wt.% as SiO2). An Analysis of Variance (ANOVA) was used to test for significant differences between treatments for the following variables: (1) pH of digestion solution, (2) digestion solution (Na2CO3 or NaOH), and (3) whether a mineral correction was made. Although significant differences were found, inspection of the raw data suggests that several anomalous points may be responsible for the significant differences obtained. Therefore, the ANOVA was rerun following removal of 7 laboratories from the comparison. These laboratories were removed if they measured BSi concentrations more than ±1 standard deviation from the mean in at least three of the six samples used in the intercomparison. For this analysis no significant differences were found between laboratories. Finally, no significant differences were found between the two most widely used techniques to measure BSi in sediments (DeMaster, D.J., 1979. The marine budgets of silica and 32 Si . PhD Dissertation, Yale University, 308 pp.; Mortlock, R.A. and Froelich, P.N., 1989. A simple method for the rapid determination of biogenic opal in pelagic marine sediments. Deep-Sea Res., 36, 1415–1426.). The importance of optimizing the technique for the type of sample and of making a determination of slope for the mineral correction are explored.

Kinetic and Thermodynamic Properties of the Biogenic Silica-Sea Water System: From Laboratory-Based Experiments to the Oceanic Silica Cycle

There have been numerous studies of the production, dissolution and preservation of biogenic silica in the oceans. Nonetheless, a complete mechanistic and quantitative understanding of the dissolution process is still lacking. This interferes with our ability to systematically predict silica dissolution fluxes under variable conditions and, hence, it represents a major obstacle to the use of the accumulation rate of biogenic silica as a palaeoproductivity index, and to the reconstruction of the silica cycle through geological time. Using stirred flow-through reactors, we have measured the effects of a number of critical physical and geochemical parameters on the solubility and dissolution kinetics of biogenic silica in sediments from the Southern Ocean. In this paper, we analyse whether the data set obtained can explain the reactivity and solubility of biogenic silica in a range of marine environments.

Lignin phenols in sediments of Lake Baikal, Siberia: application to paleoenvironmental studies

Sediments from three cores obtained from distinct depositional environments in Lake Baikal, Siberia were analyzed for organic carbon, total nitrogen and lignin phenol concentration and composition. Results were used to examine changes in paleoenvironmental conditions during climatic cycles of the late Quaternary (<125 ka). Average organic carbon, and total nitrogen concentrations, atomic C N ratios and organic carbon accumulation rates were significantly higher in the Holocene compared with the late Pleistocene, reflecting overall warmer temperatures and increased runoff during the Holocene. A Holocene maximum in organic carbon was observed at about 6 ka, and may represent the warmest/wettest period of the Holocene. At one site (Academician Ridge) pronounced late Pleistocene maxima in organic carbon and biogenic silica were observed at about 80–85 ka, probably indicative of an interstadial period with enhanced aquatic productivity. Total sedimentary lignin phenol contents were generally lower in the late Pleistocene compared to the Holocene, but with several peaks in concentration during the late Pleistocene. These late Pleistocene peaks in total sedimentary lignin content (dated at about 80, 50 and 30 ka) directly precede or occur during peaks in sedimentary biogenic silica contents. These periods likely represent relatively warm interstadial times, with increased precipitation producing the observed increase in terrestrial runoff and aquatic productivity. Lignin phenol ratios ( S V , C V and P V ) were used to examine changes in terrestrial vegetation type resulting from changes in paleoenvironmental conditions during the late Pleistocene. A degree of caution must be used in the interpretation of these ratios with regard to vegetation sources and paleoenvironmental conditions, because of potential compositional changes in lignin resulting from biodegradation. Nevertheless, results show that long glacial periods were characterized by terrestrial vegetation composed of a mix of non-woody angiosperm vegetation and minor gymnosperm forest. Shorter interstadial periods are defined by a change to dominant gymnosperm forest and were observed at about 80, 75, 63, 50 and 30 ka, ranging from about 2–6 kyr in duration. These interstadial periods of the late Pleistocene defined by lignin phenol ratios generally occur during longer periods of enhanced sedimentary biogenic silica content (about 10–15 ka in duration), providing corroborative evidence of these warm interstadial periods.

Biogenic silica recycling in surficial sediments across the Polar Front of the Southern Ocean (Indian Sector)

The processes controlling preservation and recycling of particulate biogenic silica in superficial sediments must be understood before one uses biogenic silica as a proxy in paleooceanographic studies, and in order to compute oceanic mass balances for silica. In this respect, the Antarctic Ocean is certainly a key region due to its high productivity and export of biogenic silica. In order to quantify sedimentary fluxes and identify crucial processes that allow the preservation of biogenic silica, pore water and solid phase silica profiles were performed on sediment cores from the Southern Ocean (Indian Sector) during the ANTARES 1 cruise. In combination with solubility data reported by Van Cappellen and Qiu (1997a), a process model representing the early diagenesis of silica was developed. In this model, a dependence with depth of the kinetic constant was introduced to allow the preservation of biogenic silica in sediment porewater undersaturated with respect to that phase. Using this steady-state model, it is proposed that a proportionality of the reactivity of the biogenic silica with its settling flux is necessary to explain the observed profiles. It is then shown using this model that the preservation of biogenic silica is not a linear function of the deposited flux. Using a modified version of this model containing an explicit term of reprecipitation, we hypothesize that reprecipitation alone cannot counterbalance dissolution and that its effect is certainly related to a decrease in either surface solubility or kinetics of dissolution.

Late pleistocene sedimentation: a case study of the central Indian Ocean Basin

Concentration-depth profiles of excess 230Th have been measured in five sediment cores from the central Indian Ocean Basin to determine their accumulation rates and excess 230Th flux to the depositional environment. Sedimentation accumulation rates ranged between 1 and 5 mm ky −1. The excess 230Th inventories in the cores range from 44 to 481 dpm cm −2, considerably lower than that expected from its production in the overlying water column. These low excess 230Th inventories (relative to its production) have been attributed to the effects of winnowing by bottom currents. The absence of 230Th ex in core (SK-176) has been attributed to the lack of sediment deposition in this area caused by the influence of Antarctic Bottom Water (AABW). This study shows that the deposition of biogenic silica and lithogenic components exhibited orders of magnitude variations durining Late Pleistocene. The biogenic flux variation are attributed to productivity changes while the episodic variations in the non-biogenic flux could result from major changes in the influx of terrigenous materials via the Ganges-Brahmaputra.

The Mechanism Controlling Plant Nutrient Concentrations in the Northern Adriatic Sea

AbstractA 20 year data set for the northern Adriatic was analyzed and the factors establishing the nutrient environment identified. Concentrations ranged widely (TIN 0.0–78, PO2 0.01–1.1, and SiO4 0.0–59 mmol m−3). In early winter remineralization increased concentrations. Characteristic winter, late spring and fall phytoplankton blooms alternately decreased and increased concentrations, as modified by river input. In summer nutrients were minimal under a semi‐closed circulation pattern and high vertical stability, due to closely coupled nitrogen and phosphorus assimilation‐regeneration processes and biogenic silica sedimentation. “New” primary production supported mainly by river input of “new” nutrients approximated “regenerated” primary production supported by regenerated nutrients, making the ecosystem especially sensitive to eutrophication pressure from anthropogenic increases in the Po River nutrient load.

Limnological investigation of biogenic silica sedimentation and silica biogeochemistry in Lake St. Moritz and Lake Zürich

Biogenic silica concentrations were determined from core samples of laminated sediments collected from Lake Zurich and Lake St. Moritz and used to calculate rates of biogenic silica sedimentation. In Lake Zurich biogenic silica sedimentation increased from 65 g SiO2·−2·yr−1 in 1894 to nearly 900 g SiO2·m−2·yr−1 in 1896 and in Lake St. Moritz biogenic silica sedimentation increased about 6-fold during the 1930s. Both periods of increased biogenic silica flux followed increased loadings of domestic sewage to the lakes. In Lake Zurich the period of increased flux only lasted for a few years whereas in Lake St. Moritz high fluxes were maintained to the top of the core. In Lake Zurich increased production and sedimentation of diatoms could be maintained only until the silca reservoir in the water mass was depleted whereas in Lake St. Moritz (0.10 year residence time and a mean depth of 25 m) diatom production and sedimentation was maintained at a high level by rapid replenishment of silica from tributary inputs. Although historical patterns of biogenic silica sedimentation differed for the two lakes, it is hypothesized that increased biogenic silica deposition in both lakes occurred because diatom production was stimulated by phosphorus enrichment of the water mass and that the silica biogeochemistry of both lakes was affected as a result.

Silica, diatoms and a freshwater productivity maximum in Atlantic Coastal Plain estuaries, Chesapeake Bay

Monthly data collected in 1981 from the James, York and Rappahannock rivers show that dissolved silica (= silicic acid) exhibits two distinct types of longitudinal distributions in these estuaries during the year. Conservative behavior was observed during winter or when river discharge was high. The second distribution occurs when uptake by planktonic diatoms reduces concentrations below the levels attributable to dilution by seawater. In all three systems, the non-conservative behavior is characterized by rapid uptake of silicic acid in the tidal freshwater portion, just upstream of the limit of salt intrusion, and corresponds to a chlorophyll maximum shown to contain bloom densities of diatoms. Silicic acid is exhausted below detectable concentrations in the vicinity of the diatom maximum, representing more than 98% removal of the riverine source. Non-conservative distributions of total silica (= silicic acid + biogenic silica) in this same region indicate deposition of biogenic silica to the sediments. At the same time the riverine source is depleted by the diatoms in the freshwater reaches, silicic acid is resupplied to the water column in the oligohaline portion of the estuaries. This resupply is shown to be significant, nearly equal to the amount of silicic acid entering the estuary in river input, suggesting complete mineralization of biogenic silica produced in the freshwater zone. The spatial distributions described above were observed regularly in all three systems, indicating that enhanced phytoplankton production and nutrient regeneration in the vicinity of the transition zone is a natural feature of these partially-mixed estuarine rivers. Possible mechanisms contributing to the accumulation of phytoplankton biomass and the enhanced cycling of silicon are discussed.

Biogeochemical silica mass balances in Lake Michigan and Lake Superior

Silica budgets for Lake Michigan and Lake Superior differ in several respects. Mass balance calculations for both lakes agree with previous studies in that permanent burial of biogenic silica in sediments may be only about 5% of the biogenic silica produced by diatoms. Because dissolution rates are large, good estimates of permanent burial of diatoms can not be obtained indirectly from the internal cycle of silica (silica uptake by diatoms and subsequent dissolution) but must be obtained from the sediment stratigraphy. The annual net production of biogenic silica in Lake Michigan requires 71% of the winter maximum silica reservoir which must be maintained primarily by internal cycling in this large lake whereas the comparable silica demand in Lake Superior is only 8.3%. The greater silica demand in Lake Michigan is the result of phosphorus enrichment which has increased diatom production. It is hypothesized that steady-state silica dynamics in Lake Michigan were disrupted by increased diatom production between 1955 and 1970 and that a new steady state based on silica-limited diatom production developed after 1970. Mass balance calculations for Lake Michigan show in contrast with previous work that the hypothesized water column silica depletion of 3.0 g · m−3 could have occurred even though 90% or more of the biogenic silica production is recycled.

Silicic acid regeneration from estuarine sediment cores

The release of silicic acid from sediments is known to play a n important role in determining the silicic acid concentration in the water columns of estuaries, but factors controlling the rate of release are poorly understood. Silicic acid efflux from sandy Chesapeake Bay sediment cores that were maintained in the dark was strongly dependent on biogenic silica deposition, temperature and salinity, but was independent of oxygen concentration in the overlying water. Silica dissolution and release from the cores accelerated rapidly with temperature above 15 C. The release rate also accelerated very rapidly with salinity increase between 10 O/m and 20 %, but was less dependent on change in salinity in the low and high ranges. These findings help account for observed late summer maxima in water-column silicic acid concentrations in mesohaline regions of Chesapeake Bay.