Silicate Depletion Research Articles

Meridional variations in N* and Si* along 57°30′E and 47°E transects in the indian sector of the Southern Ocean during austral summer 2011

The variations in nutrient utilization in the euphotic layer was studied using geochemical tracers N* (NO3- −16PO43−) and Si* (Si(OH)4 – NO3-) along two transect 57°30′E (T1) and 47°E (T2) during austral summer 2011 in the Indian Ocean sector of the Southern Ocean. Seawater samples collected from 8 depths within the euphotic zone (upper 200 m) along two meridional transects were analysed for nitrate, nitrite, silicate, phosphate and chlorophyll a (chl a). Clear meridional differences in nutrient limitation were observed along the two transects. Depletion of silicate at Antarctic Zone (AZ) stations was 1.6 times higher in T1 than in T2. Significant inverse correlations were observed for N* (p < 0.0001) and Si* (p < 0.001) values with temperature and salinity implying that redistribution of preformed nutrients was influenced by hydrography. However, significant variances in Si* (p < 0.001 at SSTF and p < 0.01 at PF-2) along the two transects could be attributed to differential uptake of silica by phytoplankton. Low Si* values were obtained at Polar Front 1 (PF1) and South Subtropical Front (SSTF) along T1 and from Sub Antarctic Front (SAF) to Polar Frontal Zone (PFZ) along T2. Although hydrography did have inverse correlation with tracers, the observed values indicate high consumption of silicate. Relatively higher Si* values in the north of SSTF coincided with lower abundance of diatoms in these waters. Conversely, high Si* values and abundance of diatoms at Antarctic Zone (AZ) indicate less consumption of Si. The biological consumption of nitrate was more clearly discerned in the northern region where nitrogen strongly limits the productivity both along T1 (39°S and the Agulhas Retroflection Front (ARF)) and T2 (40°S). Our observations suggest that much of the tracer distribution in these waters can be explained by the dominant phytoplankton groups, but influence of other biotic components on nutrient tracer distribution cannot be ruled out.

An Earth-sized exoplanet with a Mercury-like composition

The Earth, Venus, Mars, and some extrasolar terrestrial planets have a mass and radius that is consistent with a mass fraction of about 30% metallic core and 70% silicate mantle. At the inner frontier of the solar system, Mercury has a completely different composition, with a mass fraction of about 70% metallic core and 30% silicate mantle. Several formation or evolution scenarios are proposed to explain this metal-rich composition, such as a giant impact, mantle evaporation, or the depletion of silicate at the inner-edge of the proto-planetary disk. These scenarios are still strongly debated. Here we report the discovery of a multiple transiting planetary system (K2-229), in which the inner planet has a radius of 1.165+/-0.066 Rearth and a mass of 2.59+/-0.43 Mearth. This Earth-sized planet thus has a core-mass fraction that is compatible with that of Mercury, while it was expected to be similar to that of the Earth based on host-star chemistry. This larger Mercury analogue either formed with a very peculiar composition or it has evolved since, e.g. by losing part of its mantle. Further characterisation of Mercury-like exoplanets like K2-229 b will help putting the detailed in-situ observations of Mercury (with Messenger and BepiColombo) into the global context of the formation and evolution of solar and extrasolar terrestrial planets.

Bacterial and primary production in the Greenland Sea

Abstract Bacterial production rates were measured in water profiles collected in the Greenland Sea and adjacent areas. Hydrography and nutrients throughout the water column were measured along 75°N from 12°W to 10°E at 20 km distance intervals. Net primary production rates from satellite sensed data were compared with literature values from 14C incubations and used for regional and seasonal comparisons. Maximum bacterial production rates were associated with the region close to the edge of the East Greenland current, and the rates decreased gradually towards the center of the Greenland Sea central gyre. Integrated over the upper 20 m the maximum bacterial production rate was 17.9 mmol C m− 2 day− 1, and east of the center of the gyre the average integrated rate was 4.6 mmol C m− 2 day− 1. It is hypothesized that high bacterial production rates in the western Greenland Sea were sustained by organic material carried from the Arctic Ocean by the East Greenland Current. The depth profiles of nitrate and phosphate were very similar both sides of the Arctic front, with 2% higher values between 500 m and 2000 m in the Arctic domain, and a N/P ratio of 13.6. The N/Si ratio varied by depth and region, with increasing silicate depletion from 1500 m depth to the surface. The rate of depletion from 1500 m depth to surface in the Atlantic domain was twice as high as in the Arctic domain. Net primary production rates in the area between the edge of the East Greenland current and the center of the Greenland Sea gyre was 96 mmol C m− 2 day− 1 at the time of the expedition in 2006, and 78 mmol C m− 2 day− 1 east of the center including the Atlantic domain. Annual net primary production estimated from satellite data in the Greenland Sea increased substantially in the period between 2003 and 2016, and the rate of increase was lowest close to the East Greenland Current.

Hydration of Cement Pastes Containing Accelerator at Various Temperatures: Application to High Early Strength Pavement Patching

AbstractAccelerators are used to accelerate cement hydration, thereby reducing setting time and increasing early age strength gain. Accelerators are typically developed for use at low temperatures; however, they may be used in other applications such as high early strength concrete mixtures for pavement patching, which requires short closure time. In this study, the effects of a commercial accelerator on the hydration of cement pastes made with Type I ordinary portland cement are investigated at four temperatures. At lower temperatures (10°C and 23°C), the accelerator decreases the time to reach the silicate peak and sulfate depletion peak time and increases the peak heights. At higher temperatures (37.5°C and 50°C), the silicate and sulfate depletion peak times or heights do not change appreciably. At high temperatures, the pastes are very likely undersulfated, as indicated from calorimetry data with added sulfate. The accelerator does not significantly increase the cumulative heat release at 72 hours in any of the pastes. The results from pastes in the laboratory are used to interpret flexural strength results obtained from field concrete. Flexural strength results indicate that accelerated concretes cured at high temperatures have lower strength as compared to concretes cured under standard curing temperatures. The addition of sulfate increases the flexural strength of accelerated concrete cured at higher temperatures. From these results, it is evident that the accelerator is not effective at higher temperatures and that the addition of accelerator may negatively impact hydration and strength gain at higher temperatures. The addition of sulfate alleviates some of the negative impact of accelerator at higher temperatures.

Causality of an extreme harmful algal bloom in Monterey Bay, California, during the 2014–2016 northeast Pacific warm anomaly

AbstractAn ecologically and economically disruptive harmful algal bloom (HAB) affected much of the northeast Pacific margin in 2015, during a prolonged oceanic warm anomaly. Caused by diatoms of the genus Pseudo‐nitzschia, this HAB produced the highest particulate concentrations of the biotoxin domoic acid (DA) ever recorded in Monterey Bay, California. Bloom inception followed strong upwelling during the spring transition, which introduced nutrients and eliminated the warm anomaly locally. Subsequently, moderate and intermittent upwelling created favorable conditions for growth and accumulation of HAB biomass, which was dominated by a highly toxigenic species, P. australis. High cellular DA concentrations were associated with available nitrogen for DA synthesis coincident with silicate exhaustion. This nutrient influence resulted from two factors: (1) disproportionate depletion of silicate in upwelling source waters during the warm anomaly, the most severe depletion observed in 24 years, and (2) silicate uptake by the dense diatom bloom.

Response of Chattonella marina (Raphidophyceae) and marine plankton to yellow clay and thiazolidinedione derivative TD49 in a mesocosm enclosure

We examined the effects of the algicide thiazolidinedione (TD49) and yellow clay on Chattonella marina and assessed their ecological risk for the entire planktonic community. Mesocosm (1000 L) exposure experiments were employed to investigate time-course responses over 9 days. The growth of C. marina was controlled at ≥0.4 μM TD49 but not inhibited in yellow clay treatments. Although the algicidal activity of the 0.4-μM TD49 + 0.4 kg t−1 yellow clay treatment for C. marina was high (72.5 % at 24 h), target alga regrowth occurred. In all treatments, inorganic nutrients such as nitrate + nitrite and phosphate decreased following commencement of the experiment but were >1 μM (limitation concentration) at days 5 and 6, even though consumption pattern of those nutrients was influenced by the TD49 concentration. Depletion of silicate in initial stages played an important role in controlling the shift from diatoms including Chaetoceros and Skeletonema spp. to cryptophytes. Zooplankton were not affected by even the highest the yellow clay treatments and TD concentration of <0.8 μM, but their abundance significantly reduced after day 1 at 0.8 μM TD49. Zooplankton nauplii gradually increased to the end of the experimental period, implying that TD49 may have a limited effect on zooplankton communities. The initial dosing concentration of each substance and the fate of nutrients following algicide application were critical in determining the timing of shifts in the phytoplankton and zooplankton species composition, as well as the algicidal effect on the target alga.

A more productive, but different, ocean after mitigation

AbstractReversibility studies suggest a lagged recovery of global mean sea surface temperatures after mitigation, raising the question of whether a similar lag is likely for marine net primary production (NPP). Here we assess NPP reversibility with a mitigation scenario in which projected Representative Concentration Pathway (RCP) 8.5 forcings are applied out to 2100 and then reversed over the course of the following century in a fully coupled carbon‐climate Earth System Model. In contrast to the temperature lag, we find a rapid increase in global mean NPP, including an overshoot to values above contemporary means. The enhanced NPP arises from a transient imbalance between the cooling surface ocean and continued warming in subsurface waters, which weakens upper ocean density gradients, resulting in deeper mixing and enhanced surface nitrate. We also find a marine ecosystem regime shift as persistent silicate depletion results in increased prevalence of large, non‐diatom phytoplankton.

Temperature‐dependent remineralization in a warming ocean increases surface pCO2 through changes in marine ecosystem composition

AbstractTemperature‐dependent remineralization of organic matter is, in general, not included in marine biogeochemistry models currently used for Coupled Model Intercomparison Project Phase 5 (CMIP5) climate projections. Associated feedbacks have therefore not been quantified. In this study we aim at investigating how temperature‐dependent remineralization rates (Q10 = 2) in a warming ocean impact on the marine carbon cycle, and if this may weaken the oceanic sink for anthropogenic CO2. We perturb an Earth system model used for CMIP5 with temperature‐dependent remineralization rates of organic matter using representative concentration pathway (RCP)8.5‐derived temperature anomalies for 2100. The result is a modest change of organic carbon export but also derived effects associated with feedback processes between changed nutrient concentrations and ecosystem structure. As more nutrients are recycled in the euphotic layer, increased primary production causes a depletion of silicate in the surface layer as opal is exported to depth more efficiently than particulate organic carbon. Shifts in the ecosystem occur as diatoms find less favorable conditions. Export production of calcite shells increases causing a decrease in alkalinity and higher surface pCO2. With regard to future climate projections, the results indicate a reduction of oceanic uptake of anthropogenic CO2 of about 0.2 PgC yr−1 toward the end of the 21st century in addition to reductions caused by already identified climate‐carbon cycle feedbacks. Similar shifts in the ecosystem as identified here, but driven by external forcing, have been proposed to drive glacial/interglacial changes in atmospheric pCO2. We propose a similar positive feedback between climate perturbations and the global carbon cycle but driven solely by internal biogeochemical processes.

The effect of coastal processes on phytoplankton biomass and primary production within the near-shore Subtropical Frontal Zone

This study evaluated drivers of phytoplankton net primary production (NPP) rates and chlorophyll-a (chl-a) concentrations within the coastally oriented Subtropical Frontal Zone (STFZ) off the South Island of New Zealand. Time series measurements of hydrographic parameters, macronutrients, size fractionated NPP and chl-a were conducted on a bi-monthly basis from July 2009 to November 2010. This study found that nutrient limitation in these waters is controlled by the dual influx of silicate inputs from riverine sources in coastal neritic water (NW) and oceanic inputs of nitrate from the high nutrient, low chlorophyll (HNLC) region of the offshore Sub-Antarctic Surface Waters (SASW). Total chl-a concentrations and primary production rates were perennially higher in near-shore NW and modified Subtropical waters (STW) than in the SASW, with highest indicators of biological production observed in the Austral spring and summer seasons (October to March). These periods of peak production and biomass were dominated in both parameters by microphytoplankton (>20 μm) size fractions. The coupled dominance by these large phytoplankton and the near depletion of silicate in all characterised waters within the frontal system indicate the importance of silicic diatoms as drivers of bloom production. The influence of coastal waters on the STFZ system is most pronounced with the intrusion of neritic water beyond the shelf boundary during periods of surface water thermal stratification and riverine dilution through flooding events. These two events were notably observed during the Spring 2009 sampling cruise in December 2009 and in the flood event in May 2010.

Dynamics and stoichiometry of nutrients and phytoplankton in waters influenced by the oxygen minimum zone in the eastern tropical Pacific

The tropical South East Pacific is characterized by strong coastal upwelling on the narrow continental shelf and an intense oxygen minimum zone (OMZ) in the intermediate water layer. These hydrographic properties are responsible for a permanent supply of intermediate water masses to the surface rich in nutrients and with a remarkably low inorganic N:P stoichiometry. To investigate the impact of OMZ-influenced upwelling waters on phytoplankton growth, elemental and taxonomical composition we measured hydrographic and biogeochemical parameters along an east–west transect at 10°S in the tropical South East Pacific, stretching from the upwelling region above the narrow continental shelf to the well-stratified oceanic section of the eastern boundary regime. New production in the area of coastal upwelling was driven by large-sized phytoplankton (e.g. diatoms) with generally low N:P ratios (<16:1). While nitrate and phosphate concentrations were at levels not limiting phytoplankton growth along the entire transect, silicate depletion prohibited diatom growth further off-shore. A deep chlorophyll a maximum consisting of pico-/nano- (Synechococcus, flagellates) and microphytoplankton occurred within a pronounced thermocline in subsurface waters above the shelf break and showed intermediate N:P ratios close to Redfield proportions. High PON:POP (>20:1) ratios were observed in the stratified open ocean section of the transect, coinciding with the abundance of two strains of the pico-cyanobacterium Prochlorococcus; a high-light adapted strain in the surface layer and a low-light adapted strain occurring along the oxic-anoxic transition zone below the thermocline. Excess phosphate present along the entire transect did not appear to stimulate growth of nitrogen-fixing phytoplankton, as pigment fingerprinting did not indicate the presence of diazotrophic cyanobacteria at any of our sampling stations. Instead, a large fraction of the excess phosphate generated within the oxygen minimum zone was consumed by non-Redfield production of large phytoplankton in shelf surface waters.

Export and mesopelagic particle flux during a North Atlantic spring diatom bloom

Spring diatom blooms are important for sequestering atmospheric CO 2 below the permanent thermocline in the form of particulate organic carbon (POC). We measured downward POC flux during a sub-polar North Atlantic spring bloom at 100 m using thorium-234 ( 234Th) disequilibria, and below 100 m using neutrally buoyant drifting sediment traps. The cruise followed a Lagrangian float, and a pronounced diatom bloom occurred in a 600 km 2 area around the float. Particle flux was low during the first three weeks of the bloom, between 10 and 30 mg POC m −2 d −1. Then, nearly 20 days after the bloom had started, export as diagnosed from 234Th rose to 360–620 mg POC m −2 d −1, co-incident with silicate depletion in the surface mixed layer. Sediment traps at 600 and 750 m depth collected 160 and 150 mg POC m −2 d −1, with a settled volume of particles of 1000–1500 mL m −2 d −1. This implies that 25–43% of the 100 m POC export sank below 750 m. The sinking particles were ungrazed diatom aggregates that contained transparent exopolymer particles (TEP). We conclude that diatom blooms can lead to substantial particle export that is transferred efficiently through the mesopelagic. We also present an improved method of calibrating the Alcian Blue solution against Gum Xanthan for TEP measurements.

A mixing analysis of surface water in the Oyashio region: Its implications and application to variations of the spring bloom

Abstract We analyzed the near-daily hydrographic data collected from ships at a station southeast of Hokkaido Japan during April 2007. We calculated mixing ratios of Coastal Oyashio Water (COW), Oyashio Water (OYW), and modified Kuroshio Water (MKW). The COW mixing ratio tended to be high in depths shallower than 30 m during April, while the OYW ratio was high at the deeper depths. Diapycnal mixing due to the strong wind was suggested between these layers. It is suggested that stratification of low-salinity COW initiates the spring bloom in early-April, but that it is not important for temporal changes of chlorophyll at each depth during and after the bloom. Despite the intense water exchange at the station, the COW ratio correlated significantly with chlorophyll for the overall data set during April and for the data in 2–4-day blocks. Silicic acid, nitrate, and phosphate were also significantly correlated with the COW ratio for each 2–4-day block. This implies that each of these variables changed simultaneously at a constant COW ratio within the temporal and spatial scales of the phytoplankton bloom, were it not for the occurrence of vigorous vertical mixing and large-scale advective events. Using the linear regression equations for each 2–4-day block, we calculated time-series of chlorophyll and nutrients at the mean COW ratio (46%). Their rates of change in these series reveal the spring bloom dynamics in 2007 as follows: The spring bloom ended due to silicate depletion and grazing by large zooplankton in mid-April. In late-April, the nutrients increased, but chlorophyll decreased, indicating little nutrient utilization.

The role of biomineralization in the origin of sepiolite and dolomite

Sepiolite is an important industrial mineral whose mechanism of formation and detailed origin are still not clear. Sepiolite is frequently found with dolomite, Mg-smectites or palygorskite in the Intermediate Unit of Miocene sediments of the Madrid basin. Herein, we describe mineralogy, texture and mineral chemistry of sepiolite-rich layers (up to 90% concentration) in marl-clay sediments, either with gypsum, chert (micro-crystalline quartz), or dolomite, that are up to 2 to 3 m thick. The materials studied were formed in shallow lake environments or mudflats. These sediments contain dolomite aggregates resembling mineralized microorganisms (biomorphs) with ovoid morphologies and spherical to tubular internal voids. The regular size and relatively uniform shape of the aggregates are likely controlled by the shape and size of the microorganisms. The paper describes the presence of organic debris, silicification processes and potassium silicate depletion related to the basal sediments that preceded sepiolite rich layers. Potassium depletion, as a nutrient, in detrital components and the occurrence of organic and reduced sediment (U and sulfide concentration) at the interface with sepiolite also can be linked to biosignatures related to the origin of sepiolite. In addition, the occurrence of dolomite with carbon that has δ¹³C PDB values of &lt; −7.5 permil confirms the possibility of an organic carbon source. High concentrations of sepiolite are observed where the amount of dolomite is small. The remaining dolomite in virtually pure sepiolite layers is present as partially dissolved biomorphs or fully recrystallized isolated crystals. Thus, a general process for dolomite dissolution and recrystallization is associated with sepiolite differentiation. In addition, the formation of sepiolite related to the biomineralization of dolomite is likely. The potential role of biomineralization in the formation of these high-grade sepiolite deposits depends on two factors: (1) specific organic-inorganic interactions and (2) the highly porous nature of the sepiolite precursor dolomite during the diagenetic evolution of these sediments. The confirmation of these processes awaits the future development of sepiolite synthesis strategies that incorporate the appropriate biogeochemical reactions.

Effects of Water Temperature and Silicate on the Winter Blooming Diatom Stephanodiscus hantzschii (Bacillariophyceae) Growing in Eutrophic Conditions in the Lower Han River, South Korea

ABSTRACT We investigated the relationships between fluctuations in environmental factors and blooms of the diatom Stephanodiscus hantzschii by bi-weekly monitoring from February 2001 to February 2002 and from February 2004 to February 2005, in the eutrophic lower Han River (LHR), South Korea. With the exception of the summer rainy season when nutrients were lower, diatoms were abundant. Initial development of S. hantzschii populations (the most dominant diatom species in the LHR) started in late autumn, peaked in winter to early spring, and then gradually decreased in mid-spring. The peak of S. hantzschii abundance occurred at low water temperatures (approximately 5–10°C) and caused a significant decrease in silicate concentration, whereas other nutrients remained at eutrophic levels. The dynamics of S. hantzschii under the perennially eutrophic conditions of the LHR appears to be primarily affected by changes in water temperature, but growth of the diatom may be restricted by depletion of silicate.

Triacylglycerol accumulation and profiling in the model diatoms Thalassiosira pseudonana and Phaeodactylum tricornutum (Baccilariophyceae) during starvation

Although substantial economic barriers exist, marine diatoms such as Thalassiosira pseudonana and Phaeodactylum tricornutum hold promise as feedstock for biodiesel because of their ability to manufacture and store triacylglycerols (TAGs). The recent sequencing of these two marine diatom genomes by the United States Department of Energy Joint Genome Institute and the development of improved systems for genetic manipulation should allow a more systematic approach to understanding and maximizing TAG production. However, in order to best utilize these genomes and genetic tools, we must first gain a deeper understanding of the nutrient-mediated regulation of TAG anabolism. By determining both the yield and molecular species distribution of TAGs we will, in the future, be able to fully characterize the effects of genetic manipulation. Here, we lay the groundwork for understanding TAG production in T. pseudonana and P. tricornutum, as a function of nitrate and silicate depletion. Diatoms were starved of either nitrate or silicate, and TAGs were extracted with hexane from lyophilized samples taken at various time intervals following starvation. The timing of TAG production and the relative abundance of TAGs were estimated by fluorescence spectroscopy using Nile red and the total yield per biomass determined by gravimetric assay. TAGs were analyzed using thin layer chromatography, gas chromatography–mass spectrometry, and electrospray ionization mass spectrometry to identify the major TAG species produced during the growth curve. Under our conditions, the TAG yield from T. pseudonana is about 14–18% of total dry weight. The TAG yield from P. tricornutum is about 14% of total dry weight. Silicate-starved T. pseudonana accumulated an average of 24% more TAGs than those starved for nitrate; however, the chemotypes of the TAGs produced were generally similar regardless of the starvation condition employed.

Growing islands and sinking solutes: processes maintaining the endorheic Okavango Delta as a freshwater system

In the Okavango Delta 98–99% of the water from inflow and rainfall is lost to the atmosphere through evapotranspiration. As a consequence 94% of inflowing solutes are retained within the Delta landscape. This process might be expected to result in an entirely saline environment, but that is not the case: the surface waters have very low salinity, supporting a typical freshwater biota. It has been deduced that the numerous islands in the Delta (about 150,000 within an area of 13,500 km2) have been formed through evapotransporative concentration in the groundwater, of infiltrating solutes, followed by precipitation and volume increase. Evidence of this is the large amount of calcrete in island soils. These islands of 3–10 m thickness with clayey soils are underlain by fine Kalahari sand to a depth of 200–300 m, which also indicates that they are formed through surface processes. The infiltration rate of surface water from floodplains and streams into islands is very high, and is predominantly a lateral process that is unidirectional. Evapotranspiration in the riparian woodland zone cause the ground-waters in the central area of islands—with halophyte grasslands—to have very high salinities. By use of chloride as a conservative element the concentration factor between central island groundwater and surface water is calculated to be 500–1,000. This groundwater is depleted of calcium and magnesium supporting the early deductions that these elements have precipitated as calcrete. There is also a large depletion of silicate and potassium that probably have precipitated as well forming the clayey soils typical of the islands. The central island groundwater is dominated by sodium, bicarbonate and dissolved organic matter. The gradual increase of salinity here causes a periodic let off of this water through a density-driven process to deeper layers. This process together with island growth through precipitation of solutes are the two major sink processes of inflowing solutes and explains why the Okavango Delta is at present a freshwater system. The whole island complex is calculated to be 100,000–400,000 years old while some intensely studied islands may be younger: 80,000–240,000 years. The discrepancy is explained by a biassed selection of islands currently in flooded areas with better growth conditions. The uniqueness of the Okavango Delta and ideas for future research are discussed.

Nutrient environment of red tide- infested waters off south-west coast of India

The bloom-infested waters along the southwest coast of India were assessed to bring about the probable cause related to the excessive algal production. Low nitrate and silicate concentrations were concomitant with slightly higher levels of phosphate. The silicate depletion in the bloom area is possibly an indication of community succession (diatom to dinoflagellate), since it was completely utilized by the preceding diatom blooms. The dinoflagellates in this region could have been advected from the northern regions where it was noticed during the previous months.

Spatio-temporal distribution of the dominant Diatom and Dinoflagellate species in the Bay of Tunis (SW Mediterranean Sea)

Microphytoplankton composition and its relationships with hydrology and nutrient distributions were investigated over 24 months (December 1993 - November 1995) in the Bay of Tunis ( SW Mediterranean Sea). A new index, the ‘Specific Preference Index’ (SPI) obtained by computing the median value of each parameter weighed by the numerical value of each species density was developed. Using this index, the relationships between each species and temperature, salinity and major nutrients were analysed. The distribution of chlorophyll a did not show a clear correlation with microplankton abundance suggesting that other factors contribute to chlorophyll concentration, such as smaller phytoplankton size fractions or detritus. The winter-spring diatom blooms did not show a regular pattern during both years. High nutrient inputs in late summer, associated with mild meteorological conditions, contributed to the development of a large diatom bloom in autumn 1995 where significant silicate depletion was witnessed. Generally, diatoms were more stenotherm than dinoflagellates in the Bay, whereas dinoflagellates were more stenohaline than diatoms. The statistical analyses showed that the two species, Bellerochea horologicalis , and Lithodesmioides polymorpha, var., tunisiense, appeared in a wide range of environmental conditions. An excess of phosphateversus nitrate appeared to be associated with red tides of, Gymnodinium spp, whereas Peridinium quinquecorne, showed the opposite. Phosphate concentrations appear to be crucial in this coastal environment, where diatom blooms are often limited by low silicate availability.

The uptake and export of silicon and nitrogen in HNLC waters of the NE Pacific Ocean

The high-nitrate, low-chlorophyll (HNLC) waters of the Gulf of Alaska tend towards silicate rather than nitrate depletion as phytoplankton utilize nutrients during summer. This tendency is enhanced when iron supply is elevated through natural inputs such as from coastally generated mesoscale eddies or through artificial enrichment as was carried out in an in situ experiment in July 2002. However, ship-board incubations with iron enrichment demonstrate nitrate rather than silicate depletion for these waters. The difference between in situ and in vitro experiments occurs at least in part because deck incubations do not allow export of particulate Si and N. Due to the more efficient recycling of nitrogen and carbon, export favours the removal of silicon from the upper ocean (the Si pump). Previous measurements at Ocean Station Papa (50°N, 145°W) show that ∼25% of the Si, but only ∼7% of the C and ∼4% of the N utilized during spring growth, is exported to a depth of 200 m. These results in the Gulf of Alaska agree with the present understanding of phytoplankton controls in other HNLC regions and show that any estimates of carbon export from iron enrichment should be based on Si- rather than N-limitation.

Physical control of chlorophyll a, POC, and TPN distributions in the pack ice of the Ross Sea, Antarctica

The pack ice ecosystem of the Ross Sea was investigated along a 1470‐km north‐south transect during the spring 1998 oceanographic program Research on Ocean‐Atmosphere Variability and Ecosystem Response in the Ross Sea (ROAVERRS). Snow and sea ice thickness along the transect varied latitudinally, with thinner snow and ice at the northern ice edge and thin new ice in the vicinity of the Ross Sea polynya. Relative to springtime observations in other sea ice regions, algal chlorophyll a (Chl a) concentrations were low. In contrast, particulate organic carbon (POC), total particulate nitrogen (TPN), and POC:Chl a were all high, indicating either that the ice contained substantial amounts of detritus or nonphotosynthetic organisms, or that the algae had a high POC:Chl a ratio. The abundance of Chl a, POC, and TPN in the sea ice was related to ice age and thickness, as well as to snow depth: older ice had thinner snow cover and contained higher algal biomass while new ice in the polynya had lower biomass. Older pack ice was dominated by diatoms (particularly Fragilariopsis cylindrus) and had vertical distributions of Chl a, POC, and TPN that were related to salinity, with higher biomass at the ice‐water interface. Fluorescence‐based measurements of photosynthetic competence (Fv/Fm) were higher at ice‐water interfaces, and photosynthesis‐irradiance characteristics measured for bottom ice algae were comparable to those measured in pack ice communities of other regions. Nutrient concentrations in extracted sea ice brines showed depletion of silicate and nitrate, depletion or regeneration of phosphate and nitrite, and production of ammonium when normalized to seawater salinity; however, concentrations of dissolved inorganic nitrogen, phosphorous, and silica were typically above levels likely to limit algal growth. In areas where pack ice and snow cover were thickest, light levels could be limiting to algal photosynthesis. Enrichment of δ13C‐POC in the sea ice was correlated with the accumulation of POC, suggesting that carbon sources for photosynthesis might shift in response to decreasing CO2 supply. Comparisons between new ice and underlying waters showed similar algal species dominance (Phaeocystis antarctica) implying incorporation of phytoplankton, with substantially higher POC and TPN concentrations in the ice.