Silica Production Rates Research Articles

Drivers of diatom production and the legacy of eutrophication in two river plume regions of the northern Gulf of Mexico

In the northern Gulf of Mexico (nGoM), the Louisiana Shelf (LS) and Mississippi Bight (MB) subregions are influenced by eutrophication to varying degrees. Despite recognition that dissolved silicon may regulate diatom productivity in the nGoM, there is only one published data set reporting biogenic silica (bSiO2) production rates for each subregion. We report that bSiO2 production rates on the LS and MB are high and appear to be controlled by different nutrients among seasons. Despite exceptional upper trophic level biomass regionally, which suggests significant primary production by diatoms (as in other systems), gross euphotic-zone integrated bSiO2 production rates are lower than major bSiO2 producing regions (e.g. upwelling systems). However, when normalizing to the depth of the euphotic zone, the bSiO2 production rates on the LS are like normalized rates in upwelling systems. We suggest local river-plume influenced hydrography concentrates diatom productivity within shallow euphotic zones, making production more accessible to higher trophic organisms. Comparison of rates between the LS and MB suggest that the fluvial nitrate within the LS stimulates bSiO2 production above that in the MB, which has a smaller watershed and is less eutrophic (relatively). Beyond understanding the factors controlling regional bSiO2 production, these data offer the most comprehensive Si-cycle baseline to date as the LS and MB will likely exchange freely in the mid to late century due to land subsidence of the Mississippi River delta and/or sea-level rise.

Biogenic matter content in marine sediments in the vicinity of the Antarctic Peninsula: Recent sedimentary conditions under a diverse environment of production, transport, selective preservation and accumulation

Burial fluxes of organic carbon and biogenic silica were determined in 17 continental shelf sediment cores collected from the northern Weddell Sea, the Bransfield Strait, and the southern Drake Passage. Coring sites included open-shelf stations as well as slope and glacial trough environments, with water depths varying from 220 to 760 m. Apparent 210Pb accumulation rates from these cores ranged from 0.04 g m−2y−1 to 0.21 g m−2y−1 (1 to 3 mm y−1), with organic carbon burial rates ranging from 3 to 15 g OC m−2y−1 and biogenic silica accumulation rates ranging from 15 to 126 g SiO2 m−2y−1. OC contents below the surface mixed layer ranged from 0.26 to 1.51 wt. % (avg. 0.64 %). Biogenic silica contents at depth ranged from 2.3 to 11.2 wt. % (avg. 7.5%), with an average bSi/OC ratio (wt. %/wt. %) at depth of 12. Annual OC primary production rates and biogenic silica production rates in the euphotic zone were estimated from satellite chlorophyll-a data in the literature and from a seasonal model for biogenic particle export from surface waters. Based on these biogeochemical data, preservation efficiencies (i.e., mass burial rate/water column production rate) were calculated for organic carbon and biogenic silica. These preservation efficiency values ranged from 2 to 18% (avg. 9%) for OC and 8 to 106% (avg. 54%) for bSi. These relatively high preservation efficiencies resulted from extensive lateral sediment focusing (210Pb Psi (Ψ) values [burial flux/water column production rate] ranging from 2 to 33; avg. of 16), cold bottom water temperatures (2 to −2°C), and relatively high biogenic Si and OC production rates in the euphotic zone. The enhanced preservation efficiency for bSi relative to OC (i.e., 54% vs. 9%) in these Antarctic settings is consistent with the change in the phytoplankton bSi/OC (wt. %/wt. %) value of 2 for this area to the burial bSi/OC value of 12. Excess 210Pb activities in surface sediments varied from 4 to 47 dpm g−1. The surface mixed layer in the seabed varied in thickness from 0 to 4 cm. The penetration of excess 210Pb into these Antarctic Peninsula sediments varied from 6 to 28 cm (avg. 18 cm). The inventory of excess 210Pb in the seabed varied from 13 to 230 dpm cm−2 (avg. 110 dpm cm−2). Although 210Pb was the only radionuclide measured in this study, “apparent” 210Pb sediment accumulation rate (SAR) values from these 17 cores (assuming that deep bioturbation is negligible) are believed to be accurate SAR values because of good agreement between 210Pb and 14C chronologies from nearby cores reported in the literature.

Summertime Biogenic Silica Production and Silicon Limitation in the Pacific Arctic Region From 2006 to 2016

AbstractWe present a decade of dissolved and particulate silica concentrations within five biological “hot spots” in the Pacific Arctic Region (PAR) and the first measurements of both biogenic silica production rates (ρSi) and the kinetics of silicon utilization from a period of four years at the same sites. The “hot spots” were located within the Bering and Chukchi Seas and identified as part of the Distributed Biological Observatory (DBO). Across all hot spots, the highest concentrations of silicic acid (Si(OH)4) and biogenic silica were found near the bottom of the euphotic zone and often correlated with increased ρSi. For the entire region, the average ρSi was 19 mmol m−2 day−1 and siliceous microplankton (i.e., diatoms) contributed an average of 62% to primary productivity and 82% to nitrate utilization. Irradiance and [Si(OH)4] had separate and interactive effects on ρSi. Irradiance modulated both the magnitude of ρSi and the response of diatoms to changes in Si(OH)4. Availability of Si(OH)4 limited ρSi in all hot spots in at least one of the four years. Kinetic experiments conducted in all hot spots demonstrated that the half‐saturation constant (Ks) for ρSi was 4–8 times higher than ever reported in the literature. In the southeastern Chukchi Sea, an east to west gradient in [bSiO2] and ρSi may have been driven by differences in the availability of NO3− rather than Si(OH)4. Despite strong interannual variability, we suggest that phytoplankton phenology responds to short‐term climatic changes, which can have far‐reaching effects on Arctic regions influenced by the Pacific‐origin waters flowing through the PAR.

Continuous synthesis of plate-like silica microparticles using microfluidics

The synthesis of plate-like silica particles, which are of importance for a variety of applications, are mainly based on the widely adopted method of the sol-gel reaction of silicon alkoxides in traditional batch-wise instrumentation. In this study, continuous-flow synthesis of amorphous plate-like silica particles is reported through combining droplet-based microfluidics and the sol-gel reaction of tetraethyl orthosilicate. The reaction was conducted at the surface of oil droplets, comprising tetraethyl orthosilicate (TEOS), suspended in acidic (HCl) water, resulting in silica particles on the surface of the droplets, leaving the device with outlet flow. The synthesized particles had plate-like structure with thickness less than 1 μm and microscale in two other dimensions. The influences of experimental parameters (TEOS and HCl concentrations and the flow rate of aqueous phase) on the silica production rate were examined. With increase in the TEOS and HCl concentrations, the production rate increases monotonically. With increase in the flow rate of aqueous phase, the production rate increases up to a specific flow rate. Beyond the flow rate, the production rate decreases with increase in the flow rate. A phenomenological model is proposed to address the production rate using the droplet-based microfluidic system. In addition, response surface methodology (RSM) was used to statistically model and optimize the production rate. At optimum values for the experimental parameters, the experimentally measured production rate was considerably comparable to that predicted by RSM.

Biogenic silica production and diatom dynamics in the Svalbard region during spring

Abstract. Diatoms are generally the dominant contributors to the Arctic Ocean spring bloom, which is a key event in regional food webs in terms of capacity for secondary production and organic matter export. Dissolved silicic acid is an obligate nutrient for diatoms and has been declining in the European Arctic since the early 1990s. The lack of regional silicon cycling information precludes understanding the consequences of such changes for diatom productivity during the Arctic spring bloom. This study communicates the results from a cruise in the European Arctic around Svalbard, which reports the first concurrent data on biogenic silica production and export, export of diatom cells, the degree of kinetic limitation by ambient silicic acid, and diatom contribution to primary production. Regional biogenic silica production rates were significantly lower than those achievable in the Southern Ocean and silicic acid concentration limited the biogenic silica production rate in 95 % of samples. Compared to diatoms in the Atlantic subtropical gyre, regional diatoms are less adapted for silicic acid uptake at low concentration, and at some stations during the present study, silicon kinetic limitation may have been intense enough to limit diatom growth. Thus, silicic acid can play a critical role in diatom spring bloom dynamics. The diatom contribution to primary production was variable, ranging from <10 % to ∼100 % depending on the bloom stage and phytoplankton composition. While there was agreement with previous studies regarding the export rate of diatom cells, we observed significantly elevated biogenic silica export. Such a discrepancy can be resolved if a higher fraction of the diatom material exported during our study was modified by zooplankton grazers. This study provides the most direct evidence to date suggesting the important coupling of the silicon and carbon cycles during the spring bloom in the European Arctic.

Diatom populations in an upwelling environment decrease silica content to avoid growth limitation.

A mix of adaptive strategies enable diatoms to sustain rapid growth in dynamic ocean regions, making diatoms one of the most productive primary producers in the world. We illustrate one such strategy off coastal California that facilitates continued, high, cell division rates despite silicic acid stress. Using a fluorescent dye to measure single-cell diatom silica production rates, silicification (silica per unit area) and growth rates we show diatoms decrease silicification and maintain growth rate when silicon concentration limits silica production rates. While this physiological response to silicon stress was similar across taxa, in situ silicic acid concentration limited silica production rates by varying degrees for taxa within the same community. Despite this variability among taxa, silicon stress did not alter the contribution of specific taxa to total community silica production or to community composition. Maintenance of division rate at the expense of frustule thickness decreases cell density which could affect regional biogeochemical cycles. The reduction in frustule silicification also creates an ecological tradeoff: thinner frustules increase susceptibility to predation but reducing Si quotas maximizes cell abundance for a given pulse of silicic acid, thereby favouring a larger eventual population size which facilitates diatom persistence in habitats with pulsed resource supplies.

Taxon-specific contributions to silica production in natural diatom assemblages.

The metabolic activity and growth of phytoplankton taxa drives their ecological function and contribution to biogeochemical processes. We present the first quantitative, taxon-resolved silica production rates, growth rates, and silica content estimates for co-occurring diatoms along two cross-shelf transects off the California coast using the fluorescent tracer PDMPO (2-(4-pyridyl)-5-((4-(2-dimethylaminoethylaminocarbamoyl)methoxy)phenyl)oxazole), and confocal microscopy. Taxon contribution to total diatom community silica production was predominantly a function of the surface area of new frustule that each taxon created as opposed to cell abundance or frustule thickness. The influential role of surface area made large diatoms disproportionately important to community silica production over short time scales (<1 d). In some cases, large taxa that comprised only ~15% of numerical cell abundance accounted for over 50% of total community silica production. Over longer time scales relevant to bloom dynamics, the importance of surface area declines and growth rate becomes the dominant influence on contribution to production. The relative importance of surface area and growth rate in relation to silica production was modeled as the time needed for a smaller, faster-growing taxon to create more surface area than a larger, slower-growing taxon. Differences in growth rate between the taxa effected the model outcome more than differences in surface area. Shifts in relative silica production among taxa are time restricted by finite resources that limit the duration of a bloom. These patterns offer clues as to how taxa respond to their environment and the consequences for both species succession and the potential diatom contribution to elemental cycling.

Skeletonema cf. costatum biogenic silica production rate determinated by PDMPO method

Diatoms are the only ecological phytoplankton that require silicic acid for growth. They are also the dominant contributor of ocean’s total primary productivity. Generation and circulation with silica walls, which the siliceous organisms form, is an important component of the marine biological pump. It is crucial to the study of the operational mechanisms of biological pump with different sea areas. Moreover, it is the key link to the study of global silicon cycle. This paper introduces the basic mechanism of the formation of diatom silica walls and a new way of researching silicic acid metabolism, namely the 2-(4-pyridyl)-5-((4-(2-dimethylaminoethylaminocarbamoyl)- methoxy)phenyl)oxazole (PDMPO) dyeing method. Under a fluorescence microscope after excitation with bright green fluorescence, it can combine with silicic acid to form a complex into the Si deposition within diatom cells. The advantage of this method is that it can monitor the metabolism of silicate after adding PDMPO. For experimentation and sample collection in each of the specified time points, samples were determinated through the unutilized silicic acid, silica dissoluble intracellular and Si deposition within diatom cells, not only using hot alkaline digestions method but also PDMPO dyeing method. Results showed a good linear relationship between PDMPO fluorescent value and biogenic silica concentration. It was also indicated that PDMPO had no deleterious impact on Skeletonema cf. costatum growth for 34 h and was useful for tracking newly-deposited biogenic silica in diatoms’ frustules.

Sources of dissolved silica to the fjords of northern Patagonia (44–48°S): the importance of volcanic ash soil distribution and weathering

AbstractDissolved silica (DSi) plays an important biogeochemical role in the fjords of northern Chilean Patagonia (44–48°S), where it drives high biogenic productivity and promotes carbon burial. It is generally believed that the DSi riverine input to lakes and coastal environments is controlled by a combination of factors including lithology, climate, topography, vegetation, and meltwater input. In northern Chilean Patagonia several authors have proposed that the postglacial volcanic ash soils (andosols) may play a significant role in the high supply of DSi to the regional fjords. To assess the influence of andosols on DSi concentrations in north Patagonian rivers, we mapped andosol thickness and compared our results with river chemistry. The mineralogical and geochemical composition of three representative andosol profiles was also examined to evaluate the efficiency of weathering processes. The andosol thickness map clearly demonstrates that volcanic ash was predominantly deposited on the eastern side of the regional volcanoes, reflecting the influence of the prevailing westerly winds on the distribution of pyroclastic material. Mineralogical and geochemical results show that the andosol parent material has the typical andesitic basaltic signature of the regional volcanoes, i.e. high amounts of amorphous material, plagioclase, K‐feldspar, and pyroxene. Down‐profile variations in soil mineralogy and geochemistry indicate increased leaching of silica with depth, resulting from weathering of the volcanic parent material. For the five studied watersheds, a highly positive correlation (R2=0.98) was found between average andosol thickness and DSi concentrations, suggesting that andosol thickness is the main parameter affecting DSi concentrations in north Patagonian river systems. On seasonal timescales, increased precipitation (winter) and glacial meltwater (summer) input can significantly reduce DSi concentrations. We argue that the weathering of andosols constitutes the most important source of DSi to the lakes and fjords of northern Chilean Patagonia, explaining the particularly high regional rates of biogenic silica production. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Net biogenic silica production and the contribution of diatoms to new production and organic matter export in the Costa Rica Dome ecosystem.

We determined the net rate of biogenic silica (bSiO2) production and estimated the diatom contribution to new production and organic matter export in the Costa Rica Dome during summer 2010. The shallow thermocline significantly reduces bSiO2 dissolution rates below the mixed layer, leading to significant enhancement of bSiO2 relative to organic matter (silicate-pump condition). This may explain why deep export of bSiO2 in this region is elevated by an order of magnitude relative to comparable systems. Diatom carbon, relative to autotrophic carbon, was low (<3%); however, the contribution of diatoms to new production averaged 3 and 13% using independent approaches. The 4-old discrepancy between methods may be explained by a low average C:Si ratio (∼1.4) for the net produced diatom C relative to the net produced bSiO2. We speculate that this low production ratio is not the result of reduced C, but may arise from a significant contribution of non-diatom silicifying organisms to bSiO2 production. The contribution of diatoms to organic matter export was minor (5.7%). These results, and those of the broader project, suggest substantial food-web transformation of diatom organic matter in the euphotic zone, which creates enriched bSiO2 relative to organic matter within the exported material.

Variability in diatom contributions to biomass, organic matter production and export across a frontal gradient in the California Current Ecosystem

AbstractIn the offshore waters of Southern California, submesoscale processes associated with fronts may stimulate phytoplankton blooms and lead to biomass shifts at multiple trophic levels. Here we report the results of a study on the cycling of biogenic silica (bSiO2) with estimates of the contributions of diatoms to primary and new production in water masses adjacent to (i.e., coastal or oceanic) and within an offshore front in the Southern California Current Ecosystem (CCE). The coastal and oceanic water were sampled in cyclonic and anticyclonic eddies, respectively, with the frontal water being an interaction region between the eddy types. Concentrations of bSiO2 varied by 25‐fold across the front, with concentrations in frontal waters 20–25% of those in coastal waters. Rates of biogenic silica production spanned an equally large range, with rates within the frontal region that were half those in the coastal regions. Contributions of diatoms to primary and new production were disproportionately higher than their contribution to autotrophic biomass in all areas, ranging from 5–8%, 19–30%, and 32–43% for both processes in the oceanic, frontal and coastal waters, respectively. Across the frontal area, diatoms could account for &lt;1.0%, 6–8%, and 44–72% of organic matter export in the oceanic, frontal and coastal waters, respectively. The results suggest that the regions of frontal interactions between eddies in the southern CCE can account for variability in diatom biomass, productivity and export over very short spatial scales that is comparable to the variability observed across the Pacific basin.

Biogenic silica cycling during summer phytoplankton blooms in the North Pacific subtropical gyre

Biogenic silica (bSiO2) cycling, diatom abundance and floristics were examined within summer-period diatom blooms in the North Pacific Subtropical Gyre (NPSG) in 2008 and 2009. Hemiaulus hauckii was the most abundant diatom observed in an expansive (100,000km2) bloom near the subtropical front in the northeastern NPSG in 2008 and the small pennate diatom Mastogloia woodiana dominated a smaller (30,000km2) bloom sampled in 2009 in the gyre interior. In both blooms, the bSiO2 stock and production rates were up to an order of magnitude higher relative to non-bloom areas. Remnants of a bSiO2 export event was sampled in the H. hauckii bloom area where the export rate at 300m exceeded that at 150m, and was among the highest values recorded in the NPSG. The M. woodiana bloom was very active with specific bSiO2 production rates of 0.50–0.75d−1 and net bSiO2 production rates were among the highest observed in any subtropical-gyre diatom bloom to date. Net silica production rates in the euphotic zone were strongly positive within blooms and near zero outside of blooms, consistent with an important role for blooms in bSiO2 export. The difference in the areal extent of the H. hauckii and M. woodiana blooms was consistent with remote-sensing observations that blooms in the northeastern portion of the NPSG, near the subtropical front, are typically more extensive than those in the gyre interior near Hawaii Ocean Time-series station ALOHA. Initial estimates suggest that blooms in the northeast region produced 3–25 times more bSiO2 in 2008 and 2009, respectively, than did blooms in the gyre interior; and due to the large areal extent these blooms, their area-integrated production of bSiO2 is similar to intense diatom blooms coastal upwelling systems (e.g. Monterey Bay, Santa Barbara Channel) despite significantly lower production rates and standing stock.

Mechanisms and Kinetics of Liquid Silicon Oxidation During Industrial Refining

During oxidative ladle refining (OLR) of silicon, the metal surface is partly oxidized, resulting in the formation of a condensed silica fume (SiO2). This fugitive emission of silica represents a potential health hazard to the workers in the silicon and ferrosilicon industry. In the current work, industrial measurement campaigns aimed at recording the fume generation during OLR were performed at the Elkem Salten plant in Norway. The measured amounts of silica produced were 2.5–5.1 kg/h, depending on the gas flow rate in the refining process. The rate of silica production correlates with the total flow rate and amount of air in the purge gas, and increases as the flow rate increases. The results of this work suggest that fume generation during OLR primarily results from oxidation of the exposed metal surface, with oxygen transport from the surrounding atmosphere to the metal surface being the limiting factor. Other identified mechanisms of SiO2 formation were splashing of the metal and/or oxidation of SiO gas carried with the refining purge gas.

The annual silica cycle of the North Pacific subtropical gyre

Silica cycling in the upper 175 m of the North Pacific Subtropical Gyre was examined over a two year period (January 2008–December 2009) at the Hawaii Ocean Time-series (HOT) station ALOHA. Silicic acid concentrations in surface waters ranged from 0.6 to 1.6 μM, exhibiting no clear seasonal trends. Biogenic silica concentrations and silica production rates increased by an order of magnitude each summer following stratification of the upper 50 m reaching values of 157 nmol Si L −1 and 81 nmol Si L −1 d −1, in 2008 and 2009, respectively. Sea surface height anomalies together with analyses of variability in isothermal surfaces at 150–175 m indicated that the summer periods of elevated biogenic silica were associated with anticyclonic mesoscale features during both years. Lithogenic silica concentrations increased in the spring during the known period of maximum atmospheric dust concentrations with maximum values of 36 nmol Si L −1 in the upper 10 m. Dust deposition would enhance levels of dissolved iron in surface waters, but there was no response of diatom biomass or silica production to increases in near-surface ocean lithogenic silica concentrations suggesting iron sufficiency of diatom silica production rates. Low ambient silicic acid concentrations restricted silica production rates to an average of 43% of maximum potential rates. Si sufficiency only occurred during the summer period when diatom biomass was elevated suggesting that bloom diatoms are adapted to exploit low silicic acid concentrations. Annual silica production at HOT is estimated to be 63 mmol Si m −2 a −1 with summer blooms contributing 29% of the annual total. Diatoms are estimated to account for 3–7% of total phytoplankton primary productivity, but 9–20% of organic carbon export confirming past suggestions that diatoms are relatively minor contributors to primary productivity and autotrophic biomass, but important contributors to new and export production in oligotrophic open-ocean ecosystems. Annual silica production at HOT is nearly 4-fold lower than estimates at the Bermuda Atlantic Time-series Study (BATS) site in the Sargasso Sea from the 1990s, but annual silica export at the base of the euphotic zone is similar between the two gyres indicating very different balances between silica production and its loss in surface waters. On a relative basis, BATS is a more productive system with respect to silica, where biogenic silica is recycled with high efficiency in surface waters; in contrast the NPSG is a lower productivity system with respect to silica, but where lower recycling efficiency leads to a much higher fraction of new silica production. The two gyres show contrasting long-term trends in diatom biomass as biogenic silica concentrations at HOT have been increasing since 1997, but they have been decreasing at BATS suggesting very different forcing of decadal trends in the contribution of diatoms in carbon cycling between these gyres. Combining the data from both gyres indicates that globally subtropical gyres produce 13 Tmol Si a −1, which is only 51% of previous estimates reducing the contribution of subtropical gyres to 5–7% of global annual marine silica production.

Application of low-level beta counting of 32Si for the measurement of silica production rates in aquatic environments

Silicon is an important element for certain plankton groups in aquatic systems. In marine systems there is a strong coupling between the biogeochemical cycles of carbon and nitrogen with that of silicon due to the high proportion of primary productivity carried out by diatoms. Currently 32Si is the most used radiotracer for the measurement of biogenic silica production rates in aquatic systems with isotope activity typically quantified by Cherenkov or liquid scintillation counting. Here we describe a new method for quantifying 32Si activity using a gas-flow proportional counter optimized for low-level beta radiation analysis. The background activity is two-orders of magnitude lower and more stable than that of liquid scintillation counters, leading to consumption of 67–75% less isotope per sample. Sample preparation is non-destructive enabling reanalysis of samples indefinitely. Methods for counting samples under conditions of secular equilibrium and non-equilibrium between 32Si and its daughter isotope 32P are described. Analysis of equilibrated samples yields the highest sensitivity and accuracy, but requires that sample analysis be delayed for 120 days for secular equilibrium. Non-equilibrium methods are less accurate, but provide results within 2 days enabling experimental designs and protocols to be evaluated and optimized in near real time.

Mechanisms controlling silicon isotope distribution in the Eastern Equatorial Pacific

The distribution of silicon isotopes along a meridional transect at 140°W longitude in the Eastern Equatorial Pacific was used to test the hypothesis that δ 30Si of silicic acid in surface waters should correlate with net silica production rates (gross silica production minus silica dissolution) rather than rates of gross silica production due to the opposing Si isotope fractionations associated with silica production and silica dissolution. Variations in δ 30Si appeared significantly correlated with net silica production rates in equatorial surface waters and not with gross production rates. Around the Equator, values of δ 30Si as low as deep water values occurred in the upper mesopelagic in a zone of net silica dissolution and high detrital biogenic silica content, where the release of low δ 30Si silicic acid from opal dissolution would be expected to decrease δ 30Si. The δ 30Si of the deep water at 140°W appears constant for depths >2000 m and is similar to the deep water at 110°W. This study brings to light the importance of considering Si fractionation during diatom silica dissolution, the biological fractionation during silica production and physical factors such as currents and mixing with adjacent water masses when interpreting silicon isotope distributions.

The effects of biogenic silica detritus, zooplankton grazing, and diatom size structure on silicon cycling in the euphotic zone of the eastern equatorial Pacific

Two novel and independent couplings of field data sets suggest that only ~10‐20% of the biogenic silica (bSi) in the surface waters of the eastern equatorial Pacific upwelling zone is associated with living diatoms. Accounting for the ~80‐90% contribution of detrital bSi reconciles discrepancies between estimates of diatom growth rates obtained by different methods. Approximately 50% of the bSi associated with living diatoms was found to be in cells whose greatest linear dimension was &gt; 40 µm. The contribution of that &gt; 40‐µm fraction to bSi production was less than its contribution to biomass, as smaller diatoms, between 10 and 40 µm, contributed 59‐100% of bSi production. A steady‐state model, parameterized using field data, appears to explain the origin of the large detrital bSi pool, relative to the living fraction. Because specific rates of diatom growth are several times higher than those of detrital bSi dissolution, bSi must be predominantly detrital to balance gross rates of silica production and dissolution. Zooplankton grazing is the dominant process creating the detrital bSi pool, and the balance between diatom growth and zooplankton grazing rates maintains a living bSi pool that is a small fraction of the total bSi. Our results show near equivalence between the gross rates of detrital silica production by mesozooplankton grazing and bSi export from the euphotic zone, and also between the rates of detrital silica production by microzooplankton grazing and bSi dissolution, suggesting distinct roles for the two zooplankton size classes in regional Si biogeochemistry.

Biogenic silica production and the diatom contribution to primary production and nitrate uptake in the eastern equatorial Pacific Ocean

To quantify biogenic silica production rates and estimate diatom contribution to primary production and nitrate uptake we examined the distribution of silicic acid, biogenic silica and silica production along zonal and meridional transects between 110°W and 140°W in the upwelling zone of the eastern equatorial Pacific during December, 2004, and September, 2005. Silicic acid concentrations [Si(OH) 4] in the upper 75 m were consistently between 2.5 to 4.0 μM but were twice as high near the equator at 110°W, compared to 140°W, consistent with a strong eastward shoaling of the nutricline. Euphotic-zone integrated biogenic silica concentrations [bSiO 2] had a narrow range, averaging 9.8±1.4 mmol Si m −2 during 2004 and 10.4±4.0 mmol Si m −2 in 2005. Minima in [bSiO 2] occurred within the equatorial undercurrent on all transects; creating atypical profiles where [bSiO 2] decreased with depth from the surface and then increased below the equatorial undercurrent. Specific production rates of biogenic silica (V b) were highest (∼0.2-0.3 d −1) in the upper 50 m, decreased to <0.05 d −1 at the base of euphotic zone, and were typically near zero at 150 m. Unlike other open ocean systems, there was a strong diel cycle in silica production in the euphotic zone as daytime rates were consistently 2-3 times higher than night rates. V b was significantly correlated with ambient [Si(OH) 4], but only when rate data were separated by the fraction of photosynthetically active radiation penetrating the sea surface that was present at each sampling depth (%I 0). This interaction between [Si(OH) 4] and light on V b was highly systematic, with the extent of Si limitation of V b being a linear function of %I 0. Consistent with these effects, the average V b at stations within one degree of the equator doubled from 0.11 to 0.21 d −1 between 140°W and 110°W paralleling the doubling of [Si(OH) 4]. Vertically-integrated silica production rates (per day) tended to be higher within the zone of active upwelling (within±1° latitude of the equator), but high rates were also observed south of 2°S. Variability in silica production rates along zonal transects showed spatial coherence with changes in the meridional velocity flow field forced by tropical instability waves. Despite such perturbations the mean integrated rate of silica production was very similar between cruises, 1.6±0.6 mmol Si m −2 d −1 in 2004 and 1.3±0.7 mmol Si m −2 d −1 in 2005; consistent with the idea that phytoplankton rate processes are much less variable in the equatorial Pacific than in other systems. Comparison of silica production rates to rates of C and N use indicates that diatoms were responsible for ∼18% and ∼13-18% of total primary production and nitrate uptake, respectively. Our results suggest that diatoms, despite their minor contribution to the autotrophic community biomass (∼6%), have a disproportionally large impact on organic matter cycling in the eastern equatorial Pacific.

Co-limitation of diatoms by iron and silicic acid in the equatorial Pacific

The relative roles of silicon (Si) and iron (Fe) as limiting nutrients in the eastern equatorial Pacific (EEP) were examined in a series of nine microcosm experiments conducted over two years between 110°W and 140°W longitude. Si and Fe additions had consistently different but synergistic effects on macronutrient use, phytoplankton biomass and phytoplankton community structure. Silicon addition increased silicic acid use and biogenic silica production, but had no significant effect on the use of inorganic nitrogen or orthophosphate, chlorophyll accumulation, particulate inorganic (PIC) carbon accumulation, or plankton community composition relative to controls. That result, together with observations that Si addition increased the cellular Si content of the numerically dominant diatom by ∼50%, indicates that the main effect of Si was to regulate diatom silicification. Like the effect of Si, Fe addition increased the rate of silicic acid use and biogenic silica production and had no effect on PIC production. Unlike the effect of Si, Fe addition also enhanced rates of organic matter production, had no effect on cellular Si content of diatoms, and resulted in the growth of initially rare, large (>40 μm) diatoms relative to controls, indicating that Fe limitation acts mainly through its effects on growth rate and phytoplankton community composition. A pennate diatom of the genus Pseudo-nitzschia dominated the diatom assemblage in situ, grew readily in the controls and did not show a strong growth response to either Fe or Si addition suggesting that its growth was regulated by other factors such as grazing or light. Addition of germanium, an inhibitor of diatom cell division, eliminated the effects of Fe on macronutrient use, biogenic silica production and chlorophyll accumulation and phytoplankton community composition, consistent with a predominantly diatom response to Fe addition. The lack of a response of PIC production to Fe suggests that coccolithophores were not Fe limited. Addition of Fe and Si together resulted in the greatest levels of nutrient drawdown and biomass accumulation through the effect of Fe in promoting the growth of large diatoms. The results suggest a form of co-limitation with Si regulating diatom silicification and the rate of biogenic silica production while Fe regulates the production of organic matter through limitation of phytoplankton growth rates, in particular those of large diatoms. The results argue against Si regulation of new production in the EEP under average upwelling conditions. Iron addition was necessary and sufficient to stimulate complete removal of nitrate within the equatorial upwelling zone suggesting that new production was restricted by low ambient dissolved Fe consistent with results from in situ Fe fertilization experiments conducted to the south of the equator outside of the equatorial upwelling zone.

Particulate silica and Si recycling in the surface waters of the Eastern Equatorial Pacific

The distributions of biogenic and lithogenic silica concentrations and net silica production rates in the upper 120 m of the Eastern Equatorial Pacific (EEP) were examined in December 2004, on two transects situated at 110°W (4°N to 3°S) and along the equator (110°W to 140°W). Lithogenic silica (lSiO 2) was generally <10 nmol Si l −1 with maximum concentrations reaching 25 nmol l −1 in surface waters. These low concentrations confirm low atmospheric inputs of particulate Si, consistent with reported low inputs of wind-borne material in the EEP. In spite of active upwelling of silicic acid-rich waters the biogenic silica (bSiO 2) concentrations were generally low, falling between 100 and 180 nmol Si l −1 in the upper 50 m and decreasing to less than 50 nmol Si l −1 below ∼90 m. Estimates of net bSiO 2 production rates revealed that the rate of production exceeded that of dissolution in the upper euphotic layer (0-40 m) along 110°W with net production extending somewhat deeper (60-100 m) to the west along the equator. Net production rates in the surface layer were low, ranging between 5 and 40 nmol Si l −1 d −1, consistent with previous observations that diatoms are small contributors to autotrophic biomass in the EEP. Net silica dissolution predominated in the lower euphotic layer (40-120 m), indicating active Si recycling which diminished the strength of the silica pump in this region.