Silica Production Rates Research Articles

Evaluation of 32Si as a tracer for measuring silica production rates in marine waters

The radioisotope 32Si was evaluated as a tracer for measuring silica production rates in marine waters. 32Si is a weak β‐emitter that decays to 32P allowing the isotope to be used to determine simultaneously the production rate of biogenic silica and organic phosphorus. High specific activity 32Si, 42,000–52,000 Bq (µg Si)−1, was produced by the proton spallation of KCl. Reasonable tracer additions result in only nanomolar increases in the ambient silicic acid and orthophosphate concentrations during production rate experiments. Protocols for quantifying the amount of 32Si and 32P in samples by liquid scintillation counting (LSC) and gas‐flow proportional counting (GFPC) are presented. Both protocols eliminate the 4‐month wait for secular equilibrium between 32Si and 32P required by earlier methods. GFPC is especially useful as the instrumentation is lightweight and portable allowing real‐time analysis of samples at sea. Real‐time analysis by GFPC at sea and postcruise analysis by LSC gave results that agreed to within 17 and 7% for 32Si and 32P, respectively. Subsequent intercalibrations of the LSC and GFPC methods indicated that agreement to within 0.1–4% for both isotopes can be achieved. Parallel incubations of samples from Monterey Bay, California, using the radioisotope 32Si and the stable isotope 30Si gave silica production rates that generally agreed to within 30%. No significant bias was observed between rates obtained with the two tracers. The greater sensitivity and ease of analysis of the radioisotope makes 32Si the tracer of choice for future studies of silica production in the ocean.

Diatom growth and productivity in an oligo-trophic midocean gyre: A 3-yr record from the Sargasso Sea near Bermuda

We measured the rate of biogenic silica production in the upper 160 m at the Bermuda Atlantic Time-series Study (BATS) site in the western Sargasso Sea on 12 cruises between August 1991 and May 1994 using a 30Si tracer method. Previously initiated time series of particulate silica concentrations and the gravitational flux of siliceous particles were extended to produce an October 1988-December 1994 time series for biogenic silica concentrations and August 1991-December 1994 time series for lithogenic silica concentrations and fluxes of biogenic and lithogenie silica. Specific rates of biogenic silica production during nonbloom periods indicate active diatom growth, with a mean doubling time of 4.5 d for biogenic silica. The annual silica production rate is conservatively estimated to be 240 mmol Si m-2, which corresponds to a diatom productivity of -20-45 g C m-2 yr-I, -15-25% of the mean annual primary productivity. Comparison of silica production rates with sinking fluxes from the upper 150 m (26-53 mmol Si m-2 yr-I) indicates that -80% of the silica produced in the euphotic zone dissolves in the upper 150 m. Sinking of diatoms and their remains can account for -30% of the particulate organic carbon export at the BATS site on an annual basis and nearly 100% during the annual winter/spring diatom bloom. Atmospheric dust inputs each summer, indicated by July-August maxima in the concentration and vertical flux of lithogenic silica, do not stimulate either biogenic silica production or primary productivity, implying that iron availability limits neither diatom growth nor total primary production in the western‘ Sargasso Sea. Most of the primary production in the oceans occurs in midocean gyres and other oligotrophic systems-a seemingly contradictory situation that results from those systems’ enormous spatial extent (e.g. Ryther 1969). The phytoplankton in the midocean gyres is comprised mostly of small, nonsiliceous forms, with <l p,rn prokaryotes (e.g.

Silica production in the Sargasso Sea during spring 1989

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 142:39-45 (1996) - doi:10.3354/meps142039 Silica production in the Sargasso Sea during spring 1989 Brzezinski MA, Kosman CA Profiles of silicic acid, chlorophyll a, biogenic silica, and lithogenic silica concentrations and the rate of silica production were obtained from 7 depths in the upper 100 to 200 m at 8 stations in the western Sargasso Sea between May 7 and 18, 1989. Stations were distributed from the southern edge of the Gulf Stream to about 400 km south of Bermuda. An additional set of profiles was obtained at Stn S near Bermuda on March 28, 1989. Silicic acid concentrations in the euphotic zone were generally between 0.6 and 0.9 µM. Biogenic silica concentrations ranged from 7 to 1400 nmol Si l-1 with concentrations <50 nmol Si l-1 being typical of stations south of 35°N well away from the Gulf Stream and its eddies. Specific production rates of biogenic silica (Vb) averaged 0.16 d-1 (range: 0.01 to 0.67 d-1) corresponding to an average doubling time for the diatoms of 4.2 d. Values of Vb were lowest between 32 and 34°N with higher values observed both to the north and south. The doubling times of the diatom assemblages at stations south of 35°N (2 to 20 d), away from the Gulf Stream and its eddies, were in the same range as those reported for the picophytoplankton in the Sargasso Sea suggesting that diatoms can grow as fast as the more numerous picophytoplankton in oligotrophic oceans. Subsurface maxima in biogenic silica concentration, chlorophyll biomass and silica production rates were observed near the top of the nitracline at 7 of the 9 stations. Integrated silica production rates between the surface and the 0.1% light depth were generally between 0.2 and 0.7 mmol Si m-2 d-1 with an average of 47% of that production occurring within the nitracline. If diatoms taking up silicic acid within the nitracline also utilize nitrate, they may account for between 0.56 and 0.84 mol C m-2 yr-1 of new production which is 16 to 24% of the estimated annual total new production for the region. Diatoms · Nutrients · Silicic acid · Silicon · Silica production Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 142. Publication date: October 24, 1996 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1996 Inter-Research.

Production and dissolution of biogenic silica in the ocean: Revised global estimates, comparison with regional data and relationship to biogenic sedimentation

We estimate the global rate of biogenic silica production in the ocean to be between 200 and 280 × 1012 mol Si yr−1. The upper limit is derived from information on the primary productivity of the oceans, the relative contribution of diatoms to primary production and diatom Si/C ratios. The lower limit is derived independently using a multi‐compartment model of nutrient transport and biogenic particle flux, and field data on the balance between silica production and dissolution in the upper ocean. Our upper limit is 30–50% lower than several previous estimates, due to new data indicating lower values for both the relative contribution of diatoms to primary productivity and their Si/C ratios. Globally, at least 50% of the silica produced by diatoms in the euphotic zone dissolves in the upper 100 m, resulting in an estimated export of 100–140 × 1012 mol Si yr−l to the deep ocean. Our estimates correspond to a global mean rate of biogenic silica production between 0.6 and 0.8 mol Si m−2 yr−1. Incubation experiments indicate that silica production rates exceed that mean by a factor of 3–12 in coastal areas and are 2–4 times less than the global average in the oligotrophic mid‐ocean gyres. The mean silica production rate in waters overlying diatomaceous sediments (approximately 10–12% of the surface area of the oceans) is 0.7–1.2 mol Si m−2 yr−1. That rate is only slightly higher than the global average, indicating that the silica produced in those regions is only 10–25% of the global total. The estimated production of biogenic silica in surface waters of the mid‐ocean gyres is approximately equal to that for all major areas of opal sediment accumulation combined. Regional comparison of silica production and accumulation rates suggests a strongly bimodal character in the efficiency of opal preservation in the sea. In waters overlying diatom‐rich sediments 15–25% of the silica produced in the surface layer accumulates in the seabed, while virtually none of the silica produced in other areas is preserved. The global burial/production ratio of ˜ 3% is a composite of those two very different systems. The mechanisms leading to more efficient opal preservation in regions of silica accumulation are presently unknown, but they have no simple relationship to primary productivity. Regional differences in opal preservation appear to be controlled by factors such as low surface temperature, selective grazing and aggregate formation, which diminish the rate of silica dissolution in surface waters and/or accelerate its transport to the seafloor.

The silica cycle in the Antarctic Ocean: is the Weddell Sea atypical?

The lowest biogenic silica production rates in the Southern Ocean (average of 2.59 mmol Si m-2 d-1) have been recorded in an area of heavy ice cover along a transect through the Weddell Sea from Joinville Island to Cap Norvegia (November-December 1990). The associated biomass was also very low (concentrations less-than-or-equal-to 0.6 mumol l-1 for biogenic silica and less-than-or-equal-to 0.8 mug l-1 for chlorophyll a). Based upon these direct measurements of biogenic silica production rates and other data available from the marginal ice zone and open ocean areas, we estimated the annual production of biogenic silica in the northern Weddell Sea to be 810 to 870 mmol m-2 yr-1. This leads to a revised estimate of the total annual biogenic silica production in the Southern Ocean of between 11 and 32 Tmol Si yr-1. Comparing our annual production estimate to previous estimates of vertical flux of opal in the Weddell Sea, we conclude that no more than 1 % of the silica produced annually by phytoplankton in the upper water column reaches a depth of 800 m. This is consistent with the general distribution of high accumulation rates of opal in Southern Ocean sediments which evidence an unexplained gap in the Weddell Sea. Thus, regarding the cycling of biogenic silica in the Southern Ocean, the Weddell Sea appears to be atypical.

Cycling of biogenic silica within the upper water column of the Ross Sea

We determined the distributions of biogenic particulate silica and dissolved silicic acid in the upper 80 m over the continental shelf of the southern Ross Sea, Antarctica, during two occupations of an east-west transect at 76°30′S in mid-January and early February 1990. There was a persistent (at least 3½ weeks in duration) diatom bloom within a surface meltwater lens extending 100–150 km seaward from the edge of the receding pack ice, with biogenic silica concentrations frequently exceeding 20 μmol l −1 in the upper 15 m. There were also other significant maxima in biogenic silica, with concentrations greater than 7 μmol l −1 at distances of 250–500 km seaward of the ice edge. These maxima were apparently unrelated to meltwater effects. 30Si tracer experiments to measure the production and dissolution rates of biogenic silica as a function of depth within the upper 50 m at three stations within the ice-edge diatom bloom indicate that the specific production rate (i.e. the rate per unit of biogenic silica present) ranged from 0.05 to 0.12 day −1 ( 0.07–0.17 doublings day −1). The resulting vertically integrated silica production rates ranged from 27 to 50 mmol m −2 day −1, with a mean of 34. Vertically integrated rates of biogenic silica dissolution ranged from 16 to 28 mmol m −2 day −1, with a mean of 22, or 64% of the mean silica production rate. The mean resulting net rate of biogenic silica production within the bloom, 12 mmol m −2 day −1, leads to an estimated net annual silica production rate of 1.0 mol m −2 in the upper 50 m of the southwestern Ross Sea. A revised silica budget for the western Ross Sea, incorporating the most recent estimates of production, redissolution, accumulation in the sediments and efflux from the seabed, indicates that virtually all silica exported from the upper 50 m must reach the sea-floor unless advective transport from the east is significant.

Nutrient release rates from the sediments of Saginaw Bay, Lake Huron

Direct measurements of net production rates and pore water profiles of solutes in the fine-grained sediments of Saginaw Bay, imply corresponding steady-state fluxes to the overlying water of 1.1–1.3 (I), 450–1010 (NH4+), 1250–2650 (Si(OH)4), 3000–3400 (Ca2+), 440–1330 (Mg2+), 1.5–728 (Fe2+), and 179–281 (Mn2+) μmoles/m2/day and 11.0–11.8 (alkalinity) meq/m2/day at 17.5 °C. Silica production rates in sediments apparently follow first order kinetics with a rate coefficient of ∼0.09/day and a steady-state silica concentration of 1.2 mM at 23.5°C. The remaining solutes follow kinetics approximately independent of solute concentration over the range of concentrations observed. Measured solute production rates are consistent with observed solute profiles only if lateral diffusion gradients are maintained in the sediments by the burrowing and irrigation activity of benthic organisms such asChironomous, the dominant burrower in Saginaw Bay. Assuming that solute fluxes from Saginaw Bay are representative of all of the post-glacial sediments of Lake Huron, the iodine flux from sediments is comparable to the total fluvial input of iodine. The extrapolated silica fluxes from Lake Huron sediments balance the estimated biogenic silica flux to the sediments.

Phytoplankton bloom dynamics of the western Ross Sea ice edge—II. Mesoscale cycling of nitrogen and silicon

In the austral summer of 1983 we examined the spatial distributions of nutrients and phytoplankton biomass, nutrient assimilation rates and phytoplankton growth rates in an intense diatom bloom associated with a receding ice edge in the western Ross Sea. There was significant near-surface depletion of nitrate, phosphate and silicic acid within the bloom, but at almost all stations their concentrations remained higher than those generally considered limiting to phytoplankton growth. Ammonium concentrations within the bloom were frequently below detection limits (>0.1 μM in this study), and nearly always >0.25 μM. Rates of nitrate uptake were high enough to satisfy approximately 65% of the phytoplankton nitrogen demand estimated from 14C-primary productivity and particulate organic carbon: nitrogen ratios. Growth rates estimated for the overall phytoplankton assemblage based on 15N uptake, and the diatom assemblage based on 30Si uptake, were generally between 0.07 and 0.4 divisions per day. The mean rate of biogenic silica production within the bloom was 38 mmol Si m −2 day −1, which is an order of magnitude higher than previous estimates of silica production in the Southern Ocean, and comparable to rates observed in coastal upwelling systems at low latitudes. Accumulation of biogenic silica within the surface layer very nearly matched the depletion of dissolved silicic acid (i.e. the sum of these two quantities was very nearly conservative). The observed high rates of nitrate uptake and biogenic silica production imply that ice-edge phytoplankton blooms are a quantitatively important net source of organic and siliceous biogenic material within the Ross Sea.