Dissolved Silicon Concentrations Research Articles

Biogeochemical Transformations of Inorganic Nutrients in the Mixing Zone between the Danube River and the North-western Black Sea

Biogeochemical transformations of inorganic nutrients were studied in the mixing zone between the Danube and the north-western Black Sea in July 1995 and in the winter–spring transition period of 1997. Inorganic nutrients, phytoplankton carbon-biomass and biogenic silica were analysed at 10 different locations across the entire salinity gradient in combination with parallel measurements of phytoplankton carbon and inorganic nutrient uptake rates, making use of different tracers ( 14C, 15N, 32Si and 32P). The quantitative and qualitative nutrient signature of the Danube outflow into the Black Sea varied seasonally, depending on upwards processes of consumption, remineralization, transformation and elimination. Danube DSi (dissolved silicon) and DIN concentrations decreased from winter to summer, although PO 4 remained constant for the whole season. The winter distribution of inorganic nutrients along the salinity gradient was conservative, and phytoplankton biomasses and activities were very low due to strong light limitation. However, significant phytoplankton carbon and inorganic nutrient uptake rates were measured in early spring and during summer at salinities depending on the hydrodynamics of the Danube and Black Sea mixing. Between a salinity of 0 and 10 (using the Practical Salinity Scale), phytoplankton (diatom) growth and mortality processes have been evidenced, which at first strongly lower the inputs of inorganic nutrients to the coastal zone, but also affect the balance among which nutrients are spread over the shelf. These modifications have important consequences for phytoplankton dynamics and species dominance at salinities above 10; diatom growth being apparently P-limited during spring and N-limited during summer. Surprisingly, no DSi limitation of diatom growth has been shown during this study, and a background DSi level of 2–3 μM was measured in the entire shelf area. Causes for such an observation appear to be (1) an efficient biotic and abiotic removal of PO 4 during mixing, (2) an important denitrification in the upper estuary and (3) an important recycling of Si, both in the water column and at the sediment–water interface. The negative long-term effect of the quantitative and qualitative (deficiency in DSi) changes of the Danube inputs on the functioning of the Black Sea pelagic and benthic food-webs have been clearly reported in the literature and cannot be questioned by the present data set. Rather, this study simply highlights the complexity of the mechanisms by which propagation of the Danube eutrophication beyond the river plume possibly occurs.

Role and properties of the gel formed during nuclear glass alteration: importance of gel formation conditions

A French SON 68 nuclear glass sample was experimentally altered to assess the mechanisms limiting the glass alteration kinetics, especially during the transition phase between the initial rate r 0 and the final rate under silicon saturation conditions. A glass specimen was altered at the initial rate for one week to form a silicon-depleted non-protective gel; the specimen was then leached under static conditions at a glass-surface-to-solution-volume ( S/ V) ratio of 500 m −1 and the alteration kinetics were compared with those of a pristine glass specimen altered under the same conditions. Unexpectedly, after static leaching the previously leached glass was 2.7 times as altered as the pristine specimen, and the steady-state silicon concentration was twice as high for the previously leached specimen. STEM characterization of the alteration films showed that the initial non-protective gel constituted a silicon pump with respect to the glass, and that the glass alteration kinetics were limited only when a fraction of the gel became saturated with silicon, and exhibited protective properties. This work also shows that silicon recondensation was uniform at micrometer scale: the silicon hydrolyzed at the reaction interface then diffused before recondensing over a length comparable to the gel thickness. In addition to these findings, this investigation suggests a reinterpretation of the effect of the S/ V ratio on the glass alteration kinetics and on the steady-state dissolved silicon concentration.

Silicon Retention in River Basins: Far-reaching Effects on Biogeochemistry and Aquatic Food Webs in Coastal Marine Environments

Abstract Regulation of rivers by damming as well as eutrophication in river basins has substantially reduced dissolved silicon (DSi) loads to the Black Sea and the Baltic Sea. Whereas removal of N and P in lakes and reservoirs can be compensated for by anthropogenic inputs in the drainage basins, no such compensation occurs for DSi. The resulting changes in the nutrient composition (DSi:N:P ratio) of river discharges seem to be responsible for dramatic shifts in phytoplankton species composition in the Black Sea. In the Baltic Sea, DSi concentrations and the DSi:N ratio have been decreasing since the end of the 1960s, and there are indications that the proportion of diatoms in the spring bloom has decreased while flagellates have increased. The effects on coastal biogeochemical cycles and food web structure observed in the Black Sea and the Baltic Sea may be far reaching, because it appears that the reductions in DSi delivery by rivers are probably occurring worldwide with the ever increasing construction...

Apparent Solubility Limit of Nuclear Glass

ABSTRACTMost nuclear glass alteration models are based on a first-order kinetic law of the general type r = r0 (1–Csi/C*) or r = r0 (1–Q/K). It is generally assumed that the establishment of quasi steady-state concentrations in solution corresponds to an intrinsic glass solubility limit that may be expressed either in terms of the dissolved silicon concentration C* or as an ion activity product at saturation K. Experimental research over the last five years in France has shown that the quasi steady-state concentrations observed in solution do not correspond to a thermodynamic solubility limit with respect to the glass. This assertion is supported by a large body of experimental evidence that is discussed in this paper. The signification of the C* parameter is therefore reconsidered, and its implications on long-term behavior modeling are discussed.

Significance of salinity and silicon levels for growth of a formerly estuarine eelgrass ( Zostera marina ) population (Lake Grevelingen, The Netherlands)

Since the early 1980s, the eelgrass, Zostera marina L., population in the saline Lake Gevelingen, The Netherlands, is rapidly declining. An earlier study, in which long-term data on eelgrass coverage in this former estuary were correlated with several environmental variables, showed only one significant correlation: coverage was positively related to water column silicon levels. In addition, a negative correlation with salinity was observed, but this was not significant. In the present study, the effect of silicon and the effect of salinity on the development of Z. marina were investigated experimentally. Enhancement of dissolved silicon concentrations in the water did not stimulate Z. marina above-ground production or an increase in final above- and below-ground biomass. The highly significant correlation between eelgrass coverage and water column silicon levels, thus, remains to be explained. The results of the growth experiments did, however, demonstrate a clear effect of salinity on Z. marina growth. Plants cultured at 22 psu showed a higher production of shoots and leaves, resulting in more above-ground biomass, than plants grown at 32 psu. In addition, below-ground biomass was also higher at 22 psu. Measurements of chlorophyll a fluorescence, performed with a PAM-fluorometer, indicated a reduction of photosynthesis in the high-salinity treatments. Thus, low salinity stimulates development of Z. marina from Lake Grevelingen. Eelgrass from such a historically estuarine area may be more sensitive to high salinities than other, more marine populations. Recovery of the autochthonous eelgrass population is expected to be favoured when the estuarine conditions of the seagrass area are re-established, or when restoration programmes are carried out with allochthonous ecotypes that are less sensitive to high salinities.

The Effects of a Developing Biofilm on Chemical Changes across the Sediment‐Water Interface in a Freshwater Environment

AbstractThe development of a diatom biofilm on a river sediment was studied using a fluvarium channel with intensive investigations over a total duration of 16 days. The overlying solution was monitored for dissolved calcium, silicon and soluble reactive phosphorus, pH, dissolved oxygen and temperature. The surface of the sediment was sampled for chlorophyll a, algal cell density and porewater profile measurements of calcium concentration, pH and dissolved oxygen were made using microelectrodes. At the end of the experiment the sediment was longitudinally sectioned and porewaters isolated and analysed.A diatom biofilm developed within approximately 5 days leading to a decrease in the concentrations of dissolved silicon and phosphorus in the overlying solution. After approximately 270 hours, the dissolved silicon concentration remained low (average of 6.7 μM). As the diatom numbers increased, photosynthetic activity was evident from increases in dissolved oxygen and pH at the interface. By the end of the experiment diurnal changes in the overlying solution of dissolved calcium, alkalinity and soluble reactive phosphorus were evident. Vertical concentration gradients in dissolved calcium, phosphorus and silicon in the sediment porewater were found at the end of the experiment.The results are consistent with the development of a photosynthetically active diatom biofilm that acted as a barrier to the diffusion of silicon from the porewater. It also induced precipitation and dissolution of calcite and co‐precipitation of phosphate with calcite. Chemical fluxes of silicon, calcium and phosphorus were estimated from concentration gradients in the sediment and found to be much smaller than fluxes measured from changes in the bulk solution indicating that processes at the sediment‐water interface and biofilm mainly control the flux to the overlying solution.

Modelling algal blooms in the middle vaal river: A site specific approach

In this article a multi-algal species mathematical model is presented that is aimed at assisting the South African authorities, by providing them with a tool that can both lead to a better understanding of a standing bloom situation and to also enable meaningful predictions of water quality in the short and medium time. The model is site specific and takes into account that variability of six environmental parameters (light, temperature, turbidity, dissolved silicon concentration, dissolved nitrogen and phosphorus concentrations), which are known to have a major effect on algal growth in the Vaal River. Using data relative to the Stilfontein site, the model was double-validated and a set of 14 algal categories were designed to represent the total algal community. The major genera constituting seven of the 14 categories have been identified.

Wax and wane of eelgrass Zostera marina and water column silicon levels

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 144:303-307 (1996) - doi:10.3354/meps144303 Wax and wane of eelgrass Zostera marina and water column silicon levels Herman PMJ, Hemminga MA, Nienhuis PH, Verschuure JM, Wessel EGJ Long-term observations in an enclosed estuarine branch in The Netherlands showed a remarkably strong correlation between water column levels of dissolved silicon (DSi) and standing stock of eelgrass Zostera marina. Si levels in the leaves of Z. marina varied between 0.02 and 0.66% of tissue dry weight; they were lower in the declining population than in other populations in neighbouring areas. We hypothesize that decreased concentrations of DSi, which may coincide with eutrophication of coastal waters, may be a factor in the worldwide decline of estuarine eelgrass populations whose significance has so far been ignored. Seagrass · Eutrophication · Silicate · Coastal · Estuaries · Nutrients Full text in pdf format PreviousExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 144. Publication date: December 05, 1996 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1996 Inter-Research.

On the estimation of minimum mechanical loss during an in situ biogenic silica dissolution experiment

Maximum dissolution rates of biogenic silica as a function of depth in the ocean were calculated as a function of the variables temperature, pH, dissolved silicon concentration, and the measured surface area of radiolarian skeletons of a single species. These rates when compared with observed weight losses during an in situ dissolution experiment suggest a mechanical weight loss of about 50%. The relationship between mechanical weight loss and total weight loss is linear, suggesting the usefulness of this mathematical approach to dissolution experiments as well as the need for caution in trying to apply such loss data directly to the environment.

Parabolic dissolution kinetics of silicate minerals: An artifact of nonequilibrium precipitation processes?

The rate-limiting step for the dissolution of many primary silicate minerals is thought to be a surface-controlled reaction. This should result in dissolution rates that are linear functions of time. However, the observed dissolution rates for aluminosilicate minerals have commonly been characterized as “parabolic”—that is, the apparent rates of release of silicon and of the alkali ions are linear functions of the square root of time ( t ½). Here, data are presented which suggest that this discrepancy may result in part from the precipitation of secondary aluminosilicates during the dissolution experiments. In simulated dissolution studies, the kinetics of formation and the stoichiometries of precipitates can be studied unambiguously as functions of time, component concentrations, and pH. Results of these studies indicate that significant amounts of silicon are incorporated into amorphous aluminum-rich precipitates at dissolved silicon concentrations greater than 170 μm. At higher concentrations, silicon precipitation rates increase. These precipitation reactions result in apparent rates of addition of dissolved silicon to solution which are linear functions of t ½. As a result of the type of precipitation reactions observed here, estimates for the rates of dissolution obtained for aluminum-rich silicate minerals may be low by as much as a factor of two.