Silica Retention Research Articles

A feasibility study of oil palm empty fruit bunch pulp as a substitution for radiata pine pulp for autoclave fibre cement composite production

The main objective of this study was to investigate the feasibility of using unbleached pulp from oil palm empty fruit bunches, produced via the Ethanolamine pulping process, to replace Radiata Pine pulp in autoclave fibre cement composite (AFCC) manufacturing. The Ethanolamine pulping process was chosen for its silica retention capability on empty fruit bunch surfaces. The research involves three key phases: an initial characterization of RPP and EFBP properties, including kappa numbers and pulp aspect ratios; subsequent AFCC production with varying RPP content levels (0%–13%) and different cement-to-silica (C/S) ratios (0.2–1.0); and the introduction of EFBP as a partial or complete substitute for RPP at a fixed C/S ratio. Results reveal RPP excels in fibre quality attributes like length, coarseness, fineness, width, and wet/dry zero span, while EFBP offers advantages in fibre count and macro fibrillation index. X-ray diffraction confirms silica presence on EFBP. The highest modulus of rupture (MOR), at 15.5 MPa, is achieved with 9% RPP and a C/S ratio of 1.0. Subsequent analysis, with 1% EFBP interval substitutions to total 13% pulp content, yields the highest MOR values under dry (16.7 MPa) and wet (9.9 MPa) conditions using recipes with 6% RPP and 3% EFBP. XRD confirms the presence of silica on EFBP, forming a crystalline flake layer post-autoclaving. Thus, EFBP proves suitable as a partial AFCC substitute.

Implications of eutrophication for biogeochemical processes in the Three Gorges Reservoir, China

Although the Three Gorges Reservoir (TGR) is the largest man-made lake in the Changjiang River, it traps only a small fraction of the nitrogen (N) and dissolved silicate (DSi) inflows. Internal dissolution processes of exogenous biogenic silica (BSi) to DSi within TGR may control the overall silica (Si) retention, while the primary diatom production plays a minor role in DSi removal. Transformations of reactive N caused an increase of the dissolved inorganic nitrogen (DIN) load by 3% during transport through the TGR, while retention of dissolved inorganic phosphorus (DIP) is enhanced by biological production. As a result, the TGR causes an increase of the molar DIN/DSi, DSi/DIP, and DIN/DIP ratios, and a decrease of DIN/RSi (reactive Si, the sum of DSi and BSi), leading to an enhanced phosphorus limitation downstream of the TGR. The overall impact of the changing stoichiometry as expressed by the Index for Coastal Eutrophication Potential (ICEP) is an excess production of 27 Tg C/year of non-diatom, potentially harmful phytoplankton. More intensive monitoring is thus needed to better understand the biogeochemical processes in the TGR and to support policy development aimed at improving the water quality in the Changjiang River.

Silica Retention by the Addition of Sodium Metaaluminate During the Impregnation Stage of Bamboo Kraft Pulping

Bamboo is the most utilized non-wood biomass in China. One of its most important applications is in the pulp and paper industry. In such an application, one of the significant challenges is decreasing the silica dissolution in the pulping process. In this study, the use of sodium metaaluminate (NaAlO2) to decrease the silica dissolution into liquor was investigated. The results showed that an alkali-resistant Si–Al coating layer was formed onto the silica particles by the addition of NaAlO2. Under the impregnation conditions, NaAlO2 reacted with Na2SiO3 and produced poly-condensated Si/Al compounds, which are insoluble under kraft pulping conditions. It was found that the silicon retention in bamboo chips was 90% with the addition of NaAlO2 under the conditions studied. This novel silica-retaining technology has a great potential for industrial applications.

Effect of colloidal silica nanoparticles extracted from agricultural waste on physical, mechanical and antifungal properties of wood polymer composite

Physical, mechanical and biological properties of wood polymer nanocomposite made of poplar wood were investigated in current research. Colloidal silica nanoparticles were produced from agricultural waste (rice husk ash) under alkaline digestion. Specimens were divided into five groups; control, impregnated with nano silica (NS), nano composite (NC) (monomer included nano), nano silica- styrene (NSt) (nano and monomer, respectively), and styrene (St) specimens. Impregnation was conducted using vacuum/pressure method with nano silica suspension and styrene monomer, respectively. Styrene polymerized in situ wood porous structure from styrene monomer through thermal-catalyst treatment. Nano silica retention was 2.8%. The results of weight percent gain (WPG) for styrene monomer, NSt and NC were 95.9, 106.7 and 114.8%, respectively. Samples treated with nano silica and styrene composite indicated higher density and mechanical properties, but lower water absorption and dimensional changes. Individual treatments of nano and monomer illustrated an increasing effect on the fungal resistance. Significant difference was found between the individual and combined treatments. In all properties, nanocomposite treatment by creating a smooth surface without cracks and reinforcing role of nano particles in polymer had more intensifying effect than nano silica-styrene treatment. Nano composites samples showed the lowest volumetric swelling due to decrease of water uptake capacity. Furthermore, modulus of elasticity, modulus of rupture, hardness and compression strength of nano composite compared to control samples increased 33.3, 13.1, 53.5 and 44.7%, respectively. Nanocomposite with uniform distribution and colloidal silica nanoparticles reinforcement in polymer structure increased interaction of silica nanoparticles with styrene and wood, and presented the best enhancement in all properties.

Membrane-based purification of proteins from nanoparticle dispersions: Influences of membrane type and ultrafiltration conditions

Abstract The combination of nanoparticles with proteins to form functional hybrid systems is receiving undiminished attention because of its many biotechnological and medical applications. The separation of these hybrid materials from unbound free biomolecules has posed difficult challenges to fractionation and purification. Here, a model study has been carried out by removing proteins (bovine serum albumin (BSA) or lysozyme (LYS)) from the dispersion mixtures with silica nanoparticles (nominal size 20 nm) using ultrafiltration (UF) membranes. Regenerated cellulose (RC) and polyethersulfone (PES) membranes with nominal molecular weight cut-off (NMWCO) of 100 kDa, and a PES UF membrane (NMWCO 300 kDa) functionalized with UV-grafted amphoteric polymer hydrogel layer consisting of N -[3-(dimethylamino)propyl]-acrylamide (DMAPAA) and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) and having an experimentally determined cut-off of 180 kDa (identical with the experimental data for PES 100 kDa) were studied. Membrane properties and filtration conditions, in particular pH value and flux, were selected or adapted based on data for single component feeds to achieve maximum protein transmission, complete silica retention and, hence, maximum silica/protein selectivity. Batch dead-end and continuous diafiltration processes were used for fractionation and purification. Overall, the performance of PES UF membranes was inferior compared to the other membranes because of too strong fouling. With membrane RC 100, the transmission data of LYS and BSA from the mixture with silica were 80% and 30%, respectively. With the hydrogel-functionalized PES membrane, the respective transmissions from the mixture were ∼35% and ∼15% for LYS and BSA, respectively. In both cases, quantitative rejection of silica could be achieved. Using continuous diafiltration, membrane RC 100 had better purification efficiency, removing a total of 91% of LYS using 6 diavolumes (DV) in 2.4 h and 84% of BSA using 10 DV in 5.5 h. With the hydrogel-functionalized PES membrane, 82% of LYS and 74% of BSA were removed using 6 and 10 DV within larger time, i.e. 4.0 and 6.8 h, respectively. Importantly, the retained silica nanoparticles remained stable in the dispersion, without any indication of aggregation. Overall, this study will add valuable knowledge to the most efficient use of ultrafiltration sieving properties for the removal or purification of proteins from systems comprising other colloidal particles having a size which is larger by a factor of only 3–10.

In vivo penetration of bare and lipid-coated silica nanoparticles across the human stratum corneum

Skin penetration of silica nanoparticles (NP) currently used in pharmaceutical and cosmetic products is a topic of interest not only to evaluate their possible toxicity, but also to understand their behaviour upon contact with the skin and to exploit their potential positive effects in drug or cosmetic delivery field. Therefore, the present work aimed to elucidate the in vivo mechanism by which amorphous hydrophilic silica NP enter human stratum corneum (SC) through the evaluation of the role played by the nanoparticle surface polarity and the human hair follicle density. Two silica samples, bare hydrophilic silica (B-silica, 162±51nm in size) and hydrophobic lipid-coated silica (LC-silica, 363±74nm in size) were applied on both the volar and dorsal side of volunteer forearms. Twelve repetitive stripped tapes were removed from the human skin and evaluated for elemental composition by Energy Dispersive X-ray (EDX) analysis and for silicon content by Inductively Coupled Plasma quadrupole Mass Spectrometry (ICP-MS). All the stripped tapes revealed nanosized structures generally located in the broad spaces between corneocytes and characterized by the same elemental composition (relative weight percentage of silicon and silicon to oxygen weight ratio) than that of the applied samples. However, only about 10% B-silica permeated until the deepest SC layers considered in the study indicating a silica retention in the upper layers of SC, regardless of the hair follicle density. Otherwise, the exposure to LC-silica led to a greater silica skin penetration extent into the deeper SC layers (about 42% and 18% silica following volar and dorsal forearm application, respectively) indicating that the NP surface polarity played a predominant role on that of their size in determining the route and the extent of penetration.

Silica Mass-Balance and Retention in the Riverine and Estuarine Scheldt Tidal System (Belgium/The Netherlands)

An annual budget for dissolved silica (DSi) and biogenic silica (BSi) was constructed for the Scheldt estuary and for the entire riverine and estuarine Scheldt tidal system (Belgium/The Netherlands) using previously published silica concentrations and fluxes for the period 2003–2005. The annual estuarine DSi mass-balance was established, based on seasonal fluxes estimated using measured DSi concentrations and (fully transient) model simulations of conservative transport. The annual BSi mass-balance was deduced from measured BSi contents in the suspended particulate matter and annual mud fluxes taken from the literature. The Scheldt estuary acted as a net sink not only for the BSi carried by the tidal river as well as that produced by diatoms in the estuary, but also for large amounts of BSi imported from the coastal zone. This results in the retention of dissolved and biogenic silica higher than that of DSi alone, which is in contrast with the classical consideration that rivers act as a source of BSi for the coastal zone. DSi and silica (DSi + BSi) retentions amounted to, respectively, 28 and 64 % in the estuary, and 33 and 66 % in the entire tidal system. This study highlights thus the predominant role of the estuary in the entire Scheldt tidal system when dealing with silica dynamics, as well as the importance of including BSi when investigating estuarine silica retention.

Spatio-temporal variability in benthic silica cycling in two macrotidal estuaries: Causes and consequences for local to global studies

The high heterogeneity of silica cycling in coastal margins and the lack of silica data (compared to nitrogen and phosphorus) prevent the estimation of global silica retention in estuaries. In this study, the spatial and temporal variability of porewater silicic acid (Si(OH)4) profiles – that integrate benthic transport and reaction processes – was investigated at different spatial (metre, longitudinal and cross-section, intra-estuary) and temporal (tidal, seasonal) scales in two macrotidal estuaries, very close geographically but essentially differing in their shape. Studying the spatial and temporal variability of Si(OH)4 concentrations in porewaters provided evidence for the importance of transport processes, e.g. bio-irrigation, tidal pumping, resuspension and any combination of these processes, in affecting Si(OH)4 concentrations and fluxes and therefore temporary or permanent retention along the land–ocean continuum. We confirm that aSiO2 (amorphous silicate) transported by rivers and estuaries clearly needs to be better characterized as it provides an important source of reactive aSiO2 to sediments. This study allows us to: (1) interrogate spatial and temporal scales, although both are most often in complete interaction; (2) design the most appropriate sampling schemes to be representative of any given system and to extrapolate at the scale of the whole estuary; (3) quantify uncertainty associated to the estimations of Si(OH)4 stocks and fluxes in this type of ecosystem, essential for budget calculations. We showed that two adjacent small macrotidal estuaries, may exhibit different behaviours regarding Si retention. Temporary retention has been observed in the meanders of the Aulne Estuary and not along the more linear Elorn Estuary, demonstrating the importance of the morphology and hydrodynamic components of the estuarine filter. Research is needed in other systems and climatic zones, but our study suggests that the typology should not only account for the different types of land–ocean continuum (fjord, delta, mangrove…), but also incorporate the physical or biological attributes of the estuarine filter.

Global importance, patterns, and controls of dissolved silica retention in lakes and reservoirs

Lentic water bodies (lakes and reservoirs) offer favorable conditions for silica (SiO2) burial in sediments. Recent global estimates suggest that (1) lentic SiO2 trapping is a globally important SiO2 flux, and (2) through reservoir construction, humans have dramatically altered river dissolved SiO2 (DSi) transport and coastal DSi delivery. However, regional to global scale patterns and controls of DSi removal in lentic systems are poorly constrained. Here we use 27 published lake and reservoir DSi budgets to develop insights into patterns and controls of lentic DSi retention and to develop a new, spatially explicit, global model of lentic DSi removal called SiRReLa (Silica Retention in Reservoirs and Lakes). In our analysis, lentic DSi removal (kg SiO2 yr−1) was significantly and positively related to DSi loading (P < 0.0001; r2 = 0.98), and DSi removal efficiency was significantly and positively related to water residence time (P < 0.0001; r2 = 0.68). In addition, DSi settling rates were, on average, 6.5‐fold higher in eutrophic systems than in non‐eutrophic systems (median settling velocities: 11.1 and 1.7 m yr−1 for eutrophic and non‐eutrophic systems, respectively; P < 0.01). SiRReLa, which incorporates these insights, performed quite well in predicting both total DSi removal (kg SiO2 yr−1; Nash Sutcliffe Efficiency (N.S.E) = 0.88) and DSi removal efficiency (% Si removed; N.S.E. = 0.75), with no detectable bias in the model. Global application of SiRReLa confirms that lentic systems are important sinks for DSi, removing 89.1 Tg DSi yr−1 from watersheds globally, roughly 19–38% of all DSi inputs to surface waters. Small lakes and reservoirs (<50 km2) were critical in the analysis, retaining 81% (72 Tg DSi yr−1) of the globally retained total. Furthermore, although reservoirs occupy just 6% of the global lentic surface area, they retained approximately 35% of the total DSi removed by lentic systems. Regional hot spots for lentic DSi removal were identified and imply that lentic systems can remove the vast majority of DSi across a large fraction of Earth's land surface. Finally, a sensitivity analysis indicates that future improvements in DSi trapping and transport models should focus on improving estimates of DSi input to surface waters.

Silica retention in the Three Gorges Reservoir

A mass balance of dissolved silica (DSi) based on daily measurements at the inflow and outflow of the Three Gorges Reservoir (TGR) in 2007 and a more precise budget, with inflow, outflow, primary production, biogenic silica (BSi) settlement, dissolution of BSi in the water column and flux of DSi at the sediment–water interface in the dry season (April) of 2007 were developed. We address the following question: How much does the Three Gorges Dam (TGD) affect silica transport in the TGR of the Changjiang River (Yangtze River)? The DSi varied from 71.1 to 141 μmol/l with an average of 108 μmol/l, and it ranged between 68.1 and 136 μmol/l, with an average of 107 μmol/l in inflow and outflow, respectively, in the TGR in 2007. The linear relationship of DSi between inflow and outflow water is significant (r = 0.87, n = 362, p < 0.01). Along the main stream of the TGR, the DSi concentration decreases with an average concentration of 84.0 μmol/l in the dry season. However, the stratification of DSi was not obvious in the main channel of the TGR in the dry season. The BSi is within the range of 0.04–5.00 μmol/l, with an average concentration of 2.1 μmol/l in the main channel of the TGR, while it is much higher in Xiangxi Bay (1.30–47.7 μmol/l, 13.1 μmol/l) than in the main stream of the TGR and the other bays. After the third filling of the TGR, approximately 3.8% of the DSi was retained by the TGR based on a 12-month monitoring scheme in 2007, which would slightly reduce nutrient fluxes of the Changjiang River to the East China Sea (2%). DSi was lost during January to June and November, whereas the additions of DSi were found during the other months in 2007. The budget results also indicate that there is a slight retention of DSi. The retention of DSi in the reservoir is approximately 2.9%, while BSi is approximately 44%. Compared with the total silica load, the retention of DSi and BSi in the reservoir is only 5.0% in the dry season. With its present storage capacity, the reservoir does not play an important role as a silica sink in the channel of the TGR. The DSi load is significantly related to discharge both in inflow and outflow waters (p < 0.01). DSi retention, to some extent, is the runoff change due to impoundment.

Influence of Soda−Air−AQ Pulping of Straw on Silica Precipitation, Paper Strength, and Performance of CPVA as a Dry Strength Additive

In this study, the impact of soda−air−anthraquinone (AQ) pulping conditions on delignification and silica precipitation onto pulp fibers and paper sheets was studied. The results showed that the yield and silica precipitation slightly increased upon application of AQ in soda−air pulping. Scanning electron microscopy (SEM) and elemental analysis confirmed the deposition of silica-related particles on fibers and on paper sheets for both soda−air and soda−air−AQ pulping. Furthermore, increasing the cooking temperature had a stronger impact on delignification and silica deposition than did increasing the cooking time for soda−air−AQ pulping. In another set of experiments, the influence of cationic poly(vinyl alcohol) (CPVA) with two different molecular weights on improving the strength properties of straw pulps produced under various soda−air−AQ pulping conditions was studied. The adsorption of CPVA onto straw pulp and its effect on silica retention for paper sheets were comprehensively investigated.

Dynamics of dissolved and biogenic silica in the freshwater reaches of a macrotidal estuary (The Scheldt, Belgium)

Temporal evolution of dissolved and biogenic silica concentrations along the Scheldt tidal river and in its tributaries was investigated during 1 year in 2003. In the tributaries, dissolved silica (DSi) concentrations remained high and biogenic silica (BSi) concentrations were low throughout the year. In the tidal river during summer, DSi was completely consumed and BSi concentrations increased. Overall, most of the BSi was associated with living diatoms during the productive period in the tidal river. Nevertheless, the detrital BSi was a significant fraction of the total BSi pool, of which less than 10% could be attributed to phytoliths. The tidal river was divided into two zones for budgeting purposes. The highest productivity was observed in the zone that received the highest water discharge, as higher riverine DSi input fluxes induced presumably a less restrictive DSi limitation, but the discharge pattern could not explain all by itself the variations in DSi consumption. Silica uptake and retention in the tidal river were important at the seasonal time-scale: from May to September, 48% of the riverine DSi was consumed and 65% of the produced BSi was deposited, leading to a silica (DSi + BSi) retention in the tidal river of 30%. However, when annual fluxes were considered, DSi uptake in the tidal river amounted to 14% of the DSi inputs and only 6% of the riverine silica (DSi + BSi) was retained in the tidal river.

Silica fluxes and trapping in two contrasting natural impoundments of the upper Mississippi River

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 phosphorus 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.

Dissolved Silica Retention and Its Impact on Eutrophication in a Complex of Mountain Reservoirs

This paper reports on studies conducted during 2005 in the ecosystem of the Solina–Myczkowce mountain complex of mesotrophic reservoirs on the San River, SE Poland. Of the 1,950 t of dissolved silica calculated to flow into the reservoirs in the course of the year, c. 20% of the load was retained in the reservoirs. However, most of this retention took place in the lower Myczkowce Reservoir. Far-reaching depletion to below 10 μM L−1 of silicate was noted during the summer in the epilimnetic waters of the two reservoirs. In turn, the hypolimnion was seen to go through an enrichment process connected with sedimentation and releases from sediment. The observed depletion causes a decrease in the DSi:DIP ratio in the euphotic zone of the reservoirs, with simultaneous growth of non-siliceous algae expressed as chl a concentration.

The effectiveness of silica treatments against wood-boring invertebrates

Abstract Timber specimens were impregnated with the organo-silicate tetraethyl orthosilicate (TEOS) in an effort to deposit hard silica granules and improve resistance to wood-borers. Trials were conducted against marine borers (teredinids and Limnoria), the termite Coptotermes acinaciformis, and the wood-boring beetle Lyctus brunneus. A 14-week laboratory bioassay against C. acinaciformis showed that treated Pinus radiata containing 16.7 wt.% silica was as readily attacked as untreated timber. However, a 3-year laboratory trial of treated Castanospermum australe showed that attack by L. brunneus was prevented by 10.3 wt.% silica, and reduced by 0.7 and 3.4 wt.% silica. A trial of wood treated with copper-chromium-arsenic followed by silicon was conducted in the sea at Townsville, Australia for 7 years. Double treatment with 6.7 or 19.2 wt.% silica prevented attack in P. radiata by teredinids, while for CCA alone some replicates failed. In the same trial, double-treated Corymbia maculata with lower silica retention failed. Silica granules may overwhelm the food and waste-sorting mechanisms in teredinids and lyctine larvae, whereas borers, requiring less intimate contact with granules (Limnoria and termites), or those that do not ingest wood for food (Sphaeroma and Martesia), are little affected.

Silica retention in the Iron Gate I reservoir on the Danube River: the role of side bays as nutrient sinks

AbstractThere are longstanding concerns about the environmental impacts of super‐dams such as Iron Gate I, the Danube River's largest hydropower scheme. Iron Gate I is suspected of trapping up to 80% (∼590 000 tons per year) of dissolved silica in the form of sedimenting diatom frustules and 30 000 000 tons per year of suspended solids. This study, however, indicates that (i) conditions are unfavorable for primary production in Iron Gate I except for the small quiescent center of Orsova Bay, and the diatom production is much too low for the suspected silica uptake; (ii) Orsova Bay is the most important sediment trap as resuspension does not occur, with ∼1% (82 000 tons per year) suspended solids retention, and (iii) also the only significant silica trap, with ∼0.2% (1000 tons per year) retention. It is most conservatively estimated that no more than 5% of dissolved silica can be retained by the Iron Gate I reservoir, and therefore the earlier estimate of the huge retention can definitely be ruled out. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Is the Iron Gate I reservoir on the Danube River a sink for dissolved silica?

Damming rivers changes sediment and nutrient cycles downstream of a dam in many direct and indirect ways. The Iron Gates I reservoir on the Yugoslavian-Romanian border is the largest impoundment by volume on the Danube River holding 3.2 billion m3 of water. Silica retention within the reservoir in the form of diatom frustules was postulated to be as high as 600 kt year−1 in previous studies using indirect methods. This amount of dissolved silicate was not delivered to the coastal Black Sea, and presumably caused a shift in the phytoplankton community there, and subsequent drastic decline in fishery. We directly quantified the amount of dissolved silicate (DSi) entering and leaving the reservoir for 11 continuous months. The budget based on these data reveals two important facts: (1) only about 4% of incoming DSi was retained in the reservoir; (2) the DSi concentrations were relatively low in the rivers upstream of the reservoir compared to regional and global averages. Thus damming the Danube at the Iron Gates could not have caused the decline in DSi concentrations documented downstream of the impoundment. Rather, this change in DSi must have occurred in the headwaters of the Danube River. Potential reasons include the construction of many dams upstream of the Iron Gates, hydrologic changes resulting in lower groundwater levels, and clogging of the riverbed limiting groundwater–river exchange.

Long-term variation of the Bay of Brest ecosystem:benthic-pelagic coupling revisited

Observations over the last 20 yr of the development of pelagic and benthic communities of a western European coastal ecosystem (Bay of Brest) provide complementary hypotheses to the 'silicic acid pump' hypothesis. An increase in nitrogen inputs, independent of silicic acid levels, has lowered the Si:N molar ratios during the last 20 yr in river discharges. Since 1981, maximum concen- trations of chlorophyll a during the first spring bloom of the annual cycle have decreased, in contrast to the subsequent spring and summer blooms. Concomitantly, extensive spreading of an exotic gas- tropod Crepidula fornicata has modified the trophic structure of benthic communities by increasing suspension-feeder biomass. The following hypotheses on ecosystem functioning are made: (1) the decrease of chlorophyll biomass during the first spring bloom results from silicic acid limitation and increased suspension feeder activity, (2) benthic filtration and biodeposition activities enhance bio- genic silica retention at the sediment-water interface, and (3) recycling of trapped biogenic silica maintains diatom populations by providing silicic acid in summer and reduces primary production seasonality. These hypotheses suggest that benthic organisms control the export rate of biogenic silica towards the open-water ocean and thus the specific composition of secondary phytoplankton blooms in the Bay.

Sulfur vs. silica retention in oligotrophic lake sediments: Test of a simple residence time model

Limnologists have modeled solute retention (% of input) in lake sediments using the parameter: 100 k s {(z:t w +k s}−1, wherek s is an empirical mass transfer coefficient or ‘sedimentation velocity’ (m yr−1), z is lake mean depth (m), andt w is lake hydraulic residence time (yr). This flushing or residence time model was tested against sulfate and silica concentrations in lakes of the northeastern and northcentral U.S.A., using data collected during the U.S. EPA's Eastern Lake Survey. This parameter failed to predict intra-regional variations in lake sulfate at fall turnover, but was consistently negatively correlated with silica in each of the regional lake populations. However, except in the western section of the Upper Peninsula of Michigan, lake sulfate was everywhere inversely dependent on DOC [mean slope=−0.020(SE=0.03) meq mM−1]. Sulfur retention in aquatic environments appears more closely coupled to organic carbon and trophic state, than to variations in hydraulic residence time.

Nutrient metabolism (C, N, P, and Si) in the trophogenic zone of a meromictic lake

The dynamics of seston and dissolved elements in a meromictic lake with high concentrations of manganese and iron in the monimolimnion were studied through an annual cycle. This publication presents results for assimilation, sedimentation and recovery of nutrients (C, N, P, and Si) in the trophogenic zone. Phosphorus deficiency kept the productivity of the diatom dominated phytoplankton at an oligotrophic level. High concentrations of iron in influent streams and redistribution followed by precipitation of iron during periods of partial turnover removed phosphorus from the water. High concentrations of manganese and sulfate did not have the anticipated fertilizing effect, and recovery of nutrients from the depth of the lake was negligible. Mass balance calculations indicate that liberation of phosphorus from the sediments in the trophogenic zone was most important for the maintenance of primary production. 75% of carbon, 80% of nitrogen and 25% of phosphorus assimilated by the phytoplankton was mineralized in the trophogenic zone. Silica was effectively regenerated from the littoral zone during the decline of diatom blooms. Nitrogen and silica retention was 45% of the external load compared to 66% for phosphorus.