Phytoplankton processes during a mesoscale iron enrichment in the NE subarctic Pacific: Part II—Nutrient utilization

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Deep-Sea Research II 53 (2006) 2114–2130

Phytoplankton processes during a mesoscale iron enrichment in the NE subarctic Pacific: Part II—Nutrient utilization Adrian Marchettia,, Philippe Juneaub, Frank A. Whitneyc, Chi-Shing Wongd, Paul J. Harrisone a

Department of Botany, University of British Columbia, 6270 University Blvd., Vancouver B.C., Canada V6T 1Z4 b Department of Biological Sciences, TOXEN, Universite´ du Que´bec a` Montre´al C.P. 8888, Succ. Centre-Ville Montre´al Qc, Canada H3C 3P8 c Climate Chemistry, Institute of Ocean Sciences, P.O. Box 6000, 9860 West Saanich Road, Sidney B.C., Canada V8L 4B2 d Climate Chemistry, Institute of Ocean Sciences, P.O. Box 6000, 9860 West Saanich Road, Sidney B.C., Canada V8L 4B2 e Atmosphere, Marine and Coastal Environment Program, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China Received 14 September 2004; accepted 7 May 2006 Available online 4 October 2006

Abstract The subarctic Pacific is one of the three main regions in which phytoplankton productivity is limited by the availability of iron (Fe). During the Subarctic Ecosystem Response to Iron Enrichment (SERIES) experiment, the response of phytoplankton to the addition of Fe and the consequential effects on chemical and physical water properties were monitored. Over the duration of the Fe-induced phytoplankton bloom, macronutrient concentrations (nitrate (NO3), silicic acid (Si(OH)4) and phosphate (PO4)) were drawn down with Si(OH)4 being depleted to low concentrations (o1 mM) after 18 days. The dissolved Si(OH)4: NO3 ratio varied between two phases of the bloom. From days 0 to 10 (phase I), when all phytoplankton size classes increased in biomass, the dissolved Si(OH)4: NO3 ratio of the seawater in the patch increased as a result of the greater drawdown of NO3. After day 10 (phase II), when diatoms dominated the patch, a rapid decline in Si(OH)4 concentrations resulted in a sharp decrease in the Si(OH)4: NO3 ratio of the seawater. Increases in the suspended particulate biogenic silica (BSi) and particulate nitrogen (PN) resulted in a BSi: PN ratio of ca. 2 in the later stages of the Fe-induced bloom. The uptake of NO3 was enhanced due to the Fe enrichment. In the patch, absolute NO3 uptake rates increased in both large (X5 mm) and small (o5 mm) cells with the large cells accounting for 84% of total measured NO3 uptake over the duration of the experiment. Biomass-specific NO3 uptake rates also increased, but, in the small cells, the extent of the increase was largely dependent on the proxy for biomass used (PN or chlorophyll a). The photosynthetic efficiency of the phytoplankton assemblage was assessed at various stages of the bloom through the use of pulse amplitude-modulated (PAM) fluorometry. The trends in maximal and operational photochemical yields measured in the patch suggest that bloom termination resulted from a combination of Festress and Si-stress. The observed changes in nutrient utilization during SERIES demonstrate the crucial role of Fe in regulating macronutrient inventories and NO3 uptake rates by phytoplankton in Fe-limited regions such as the NE subarctic Pacific. r 2006 Elsevier Ltd. All rights reserved. Keywords: SERIES Fe enrichment; NE subarctic Pacific; Nitrate uptake; Size-fractionated; New production; Nutrient ratios

Corresponding author. Tel.: +1 206 221 7841; fax: +1 206 543 6073.

E-mail address: [email protected] (A. Marchetti). 0967-0645/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2006.05.031

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1. Introduction In large expanses of the world’s oceans, new production is low despite high ambient concentrations of NO3 in euphotic zone waters. In these regions, Fe deficiency prevents the accumulation of phytoplankton biomass and limits the extent of NO3 utilization (Martin and Fitzwater, 1988; Cullen, 1991). These areas where elevated concentrations of NO3, Si(OH4) and PO4 persist throughout the year are referred to as ‘‘high-nutrient, lowchlorophyll’’ (HNLC) regions (Minas et al., 1986). The ambient phytoplankton assemblages that reside in most of these regions are typically composed of photosynthetic prokaryotes and small flagellates that rely mainly on regenerated forms of nitrogen (N), despite low ambient concentrations of these N sources (Price et al., 1991; Cavender-Bares et al., 1999; Varela and Harrison, 1999; Boyd, 2002a). The subarctic north Pacific, the Equatorial Pacific and the Southern Ocean comprise the three main HNLC regions. Regional characteristics have further divided the subarctic north Pacific into western and eastern regions (for review, see Harrison et al., 2004). The NE domain is dominated by the anti-clockwise Alaskan Gyre. The subarctic current defines the boundary between the Alaskan Gyre and subtropical waters to the south. Due to westerly winds, this current moves eastward towards the west coast of North America (Bograd et al., 1999). The combination of this ocean circulation and physical properties of the NE subarctic Pacific results in weak upwelling of deep waters along with high stratification of surface waters (Gargett, 1991). In the oceanic domain of the NE subarctic Pacific, terrestrial influences are minimal, thus restricting macronutrient supply to the euphotic zone to winter deepwater mixing and weak upwelling. Consequently, there are persistently high macronutrient concentrations due to low Fe availability that limits algal growth (Martin and Fitzwater, 1988; Boyd et al., 1996). New sources of Fe are thought to be restricted to sporadic aeolian deposition events (Boyd et al., 1998; Bishop et al., 2002) or from the transport of Fe-rich coastal waters by slow-moving eddies (Johnson et al., 2005). Concentrations of dissolved macronutrients in the NE subarctic Pacific vary seasonally and spatially. Winter winds and solar radiation are the main factors producing annual variability in physical and chemical water properties in the upper

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layers (Whitney and Freeland, 1999). At Ocean Station Papa (1451W, 501N; OSP), in the upper mixed layer, the average NO3 and Si(OH)4 concentrations are usually between 14–18 mM and 20–24 mM, respectively during winter months. Throughout the spring and summer, nutrient concentrations are reduced to 8–10 mM NO3 and 12–14 mM Si(OH)4 (Whitney and Freeland, 1999). Whitney and Freeland (1999) found the amounts of NO3 and Si(OH)4 removal have declined from 7.8 to 6.5 mM and 8.5 to 6.0 mM, respectively between February and September from the 1970s to the 1990s. This decrease in NO3 and Si(OH)4 utilizations suggests a decline in the rates of organic matter production, in particular, the contribution of silicon-requiring diatoms to new production. These observations suggest that the present waters of the NE subarctic Pacific may be experiencing more severe Fe deficiency than in the recent past. The nitrogenous nutrition of phytoplankton in the NE Pacific has been studied extensively (Wheeler and Kokkinakis, 1990; Wheeler, 1993; Varela and Harrison, 1999). At OSP, new (NO3based) production accounts for 20–30% of total production. Phytoplankton that typically dominate OSP preferentially utilize ammonium (NH4) and urea as sources of N, despite the presence of high NO3 concentrations. Although light availability and cooler temperatures could limit NO3 uptake rates in the winter, in the spring and summer, low uptake rates are a consequence of low Fe availability (Banse, 1991; Boyd et al., 1996). This is due to the role of Fe as a vital component of NO3 and NO2 reductases which are necessary for NO3 assimilation (Cardenas et al., 1974; Guerrero et al., 1981). It is estimated that cells growing on NO3 as a source of N require 60% more Fe than for growth on NH4 (Raven, 1988, 1990). Therefore, cells that are Felimited may also behave as if they are N-limited, with reduced NO3 uptake and assimilation. In previous microcosm Fe enrichment studies performed at OSP, biomass-specific NO3 uptake rates increased 4-fold for large phytoplankton cells (X5 mm), whereas small cells (o5 mm) showed little change (Boyd et al., 1996). Commonly, NO3 has been observed to be the proximal limiting nutrient, often being depleted to low or non-detectable concentrations after the addition of Fe. Yet there have also been several instances where Si-depletion in surface waters has been observed (Wong and Matear, 1999). Thus, the Fe nutritional status of the

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phytoplankton not only determines the efficiency of the biological carbon pump, but also has an immense influence on nutrient utilization and other biogeochemical properties. In the NE subarctic Pacific, studies on the effect of Fe additions on nutrient dynamics and biological rate processes have thus far been confined to ‘‘ondeck’’ incubation experiments (Martin and Fitzwater, 1988; Boyd et al., 1996; Crawford et al., 2003). These experiments are limited in their ability to extrapolate to the natural environment as they may introduce a number of artifacts or so-called ‘‘bottle effects’’ (Banse, 1991; Geider and LaRoche, 1994; Scarratt et al., this issue). In an effort to elucidate the in situ phytoplankton response to Fe enrichment in the NE Pacific, SERIES was performed in July 2002 at OSP. In this paper, we present the effects of Fe addition on macronutrient utilization and particulate organic matter production during the 26-day mesoscale Fe enrichment. In addition, the environmental factors speculated to regulate phytoplankton growth throughout the ephemeral algal bloom are discussed. 2. Materials and methods 2.1. SERIES: design and sampling strategy The SERIES in situ Fe enrichment was conducted from July 9 to August 3, 2002. Dissolved Fe was added to a 77 km2 patch of seawater located at 144.451W, 50.201N, NW of OSP in the NE subarctic Pacific Ocean. Complete details of the site selection criteria and methods for the Fe infusion are described in Law et al. (this issue). In brief, Fe in the form of FeSO4 was dissolved in seawater and added to surface waters along with the inert tracer gas, sulfur hexafluoride (SF6). The Fe-enriched waters (referred to as the IN-patch) were mapped nightly using the SF6 tracer along with surface chlorophyll a (chl a) and NO3 concentrations in the later stages of the experiment. For each day, the bottom of the patch or trace layer depth (ztl) was determined as the depth at which the SF6 concentration decreased to 50% of the average upper mixed layer SF6 concentration (for further details see Law et al., this issue). On day 7, a second Fe infusion (FeSO4 only) was performed in the center of the aging patch because dissolved Fe concentrations had returned to low levels. Sampling of the IN-patch center (determined by SF6 local maxima) commenced on day 2 of the experiment and was

performed daily between 06:00 and 10:00 Pacific standard time (PST). On days 1, 6, 11, 16 and 19, control stations (referred to as the OUT-patch) were sampled in adjacent waters outside the patch. Due to ship logistics, the location of the OUT-patch stations with reference to the center of the IN-patch varied among sampling days. Day 1 of the SERIES experiment was defined as the 24 h period starting on July 10 PST. Downwelling photosynthetically active radiation (PAR) was measured at depth at 13 specific wavelength channels using a Satlantic SeaWifs Profiling Multichannel Radiometer. Correction for changes in incident irradiance (Io) during the profiling was made by measuring incident downwelling irradiance just above the ocean surface concurrently for the same 13 wavelengths using a Satlantic Ocean Radiometer. PAR (mol quanta m2 d1) was integrated over the wavelengths ranging from 400 to 700 nm. The depth of the euphotic zone (zeu) was defined as the depth corresponding to 1% of Io. Vertical profiles of the water-column physical structure were collected using a General Oceanics MK3C/WOCE CTD. Upper mixed layer depths (zuml) were estimated from temperature and salinity data as the depth at which the change in sT was X0.02 m1. Discrete samples of seawater were collected with 10 L Niskin-type bottles mounted on a rosette frame. Samples were obtained from six depths corresponding to 100%, 33%, 10%, 3%, 1% and 0.1% of Io. Samples below the zeu (o1% Io) were taken in order to ensure the collection of water below the patch throughout the experiment. Sampling depths corresponding to all percentage irradiance levels throughout the experiment are provided in Marchetti et al. (this issue). All samples were dispensed immediately into acidcleaned, Q-H2O (18.2 mO) rinsed polyethylene bottles, via silicone tubing. At each sampling depth, two bottles were triggered to ensure true replicates were obtained. For parameters in which triplicate measurements were performed, two samples were taken from one bottle and one from the second bottle. 2.2. Dissolved inorganic nutrients Nutrient samples were collected in triplicate from each sampling depth. Suspended particulates were removed by filtering 15 mL of sample through a glass-fibre filter (0.7 mm nominal pore size) into

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acid-cleaned polypropylene tubes. Nutrient samples were then frozen at 20 1C until onshore analysis. Immediately following the cruise, the dissolved macronutrients, Si(OH)4, PO4 and NO3+nitrite (NO2) were determined using a Bran+Luebbe Autoanalyzer 3. Because NO2 concentrations did not exceed 0.5 mM throughout the observation period (Whitney, unpubl. data), thereby making up only a minor portion of the NO3+NO2 pool, measurements of both nitrogen forms are referred to as NO3 in the text. For each sampling depth, the mean nutrient concentration was calculated from the triplicates. For ammonium (NH4) determination, from days 0 to 14, discrete samples from fixed depths were collected onboard the CCGS J.P. Tully and analyzed within 12 h of collection using a Technicon Autoanalyzer following the modified Technicon procedures described in Barwell-Clarke and Whitney (1996). After day 14, discrete samples were collected onboard the R.V. Kaiyo Maru, filtered and frozen for onshore analysis as described above. An intercomparison study was performed between fresh and frozen seawater samples to ensure significant contamination did not occur in the frozen samples (Saito et al., this issue). Water-column integrated dissolved nutrients were calculated down to zeu through trapezoidal integration. IN-patch average nutrient concentrations were calculated by dividing the integrated nutrient stocks inside the patch by the depth of integration (ztl). For the OUT-patch, depths of integration for average nutrient concentrations were performed to zuml. 2.3. Patch transects On days 15 and 19, samples were taken on overnight transects of the Fe-enriched waters to determine spatial variability of surface chl a and NO3 concentrations. For these surveys, all variables were measured from samples obtained by shipboard flow-through seawater collected from a maximum depth of 3 m below the bow of the ship. Surface water fluorescence was measured by a Wetlabs Wetstar flow-through fluorometer calibrated with discrete samples collected throughout the patch and measured by extracted chl a methods described in Marchetti et al. (this issue). NO3 concentrations were measured using a NAS-2E in situ nutrient analyzer calibrated with discrete surface samples analyzed for NO3 using a Bran+Luebbe Autoanalyzer 3.

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2.4. Biogenic silica and suspended particulate carbon and nitrogen Biogenic silica (BSi) was determined as outlined in Timothy et al. (this issue). For particulate carbon (PC) and nitrogen (PN), at least 1 L of seawater from each sampling depth was gently filtered (o100 mm Hg) onto a pre-combusted (450 1C for 4.5 h) glass-fibre filter. Filters were oven-dried at 50 1C for 24 h and stored in a desiccator until onshore analysis. Elemental carbon (C) and nitrogen (N) were analyzed using a Carlo Erba 1106 elemental analyzer. Replicate sampling was not performed for either BSi or PN measurements. IN-patch average particulate BSi and PN concentrations were calculated by dividing the integrated stocks inside the patch by the depth of integration. 2.5. Size-fractionated nitrate uptake Size-fractionated 15N–NO3 uptake rates were measured according to Dugdale and Goering (1967). Seawater samples were obtained from 4 depths corresponding to 100%, 33%, 10% and 1% of Io and dispensed into duplicate 1 L, acid-cleaned, Q-H2O (18.2 mO) rinsed, polyethylene bottles. Na15NO3 (99 at% 15N) was added to the samples at ca. 10% of the IN-patch surface NO3 concentrations that were determined on the CCGS J.P. Tully from the previous sampling day. Bottles were wrapped in layers of neutral density screening to achieve light intensities corresponding to the depth at which the samples were collected. Inoculated samples were incubated on-deck in Plexiglasr incubators maintained at near ambient sea surface temperatures using a seawater flow-through system. After 24 h, samples were gently filtered by gravity (polycarbonate, 5 mm pore size) and vacuum (o100 mm Hg) (glass-fibre, pre-combusted at 450 1C for 4.5 h) filtration set in a series filter cascade. This method of size-fractionation has commonly been implemented during other published studies (Marchetti et al., 2004; Boyd et al., 1996). Particulates collected on the polycarbonate filters were then rinsed onto pre-combusted glassfibre filters with an artificial saline solution. Filters were oven-dried at 50 1C for 24 h and stored in a desiccator until onshore analysis. Dried filters were analyzed for the atom % 15N and PN using a Europa Integra isotope mass spectrometer. Absolute uptake rates (r, NO3 taken up per unit time) were calculated using a constant transport model

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(Eq. (3) from Dugdale and Wilkerson, 1986). Samples for dissolved NO3 concentrations were collected at the end of the 24 h incubation to check if NO3 concentrations had been depleted. Biomassspecific NO3 uptake rates (V, NO3 taken up per unit PN or chl a per unit time) were also calculated according to the constant specific uptake model (Eq. (6) from Dugdale and Wilkerson, 1986). Biomass (chl a)-specific NO3 uptake rates (chl a-VNO3) were estimated using chl a measurements provided in Marchetti et al. (this issue). 15NO3 uptake rates were not corrected for possible losses of 15N in the form of dissolved organic nitrogen (Bronk et al., 1994). Therefore, the reported values are considered conservative estimates or net uptake. The range of replicates for size-fractionated NO3 uptake was generally less than 10% of the mean. Water-column integrated rNO3 were calculated down to zeu through trapezoidal integration. For the OUT-patch stations, some trace metal contamination may have occurred during sampling and preparation for the incubation experiments. However, as observed in previous studies, the initial biological response by phytoplankton usually takes at least 24 h after the addition of Fe (Boyd et al., 1996) and thus we do not believe our NO3 uptake rates were significantly affected by potential Fe contamination. 2.6. Photosynthetic efficiency measured by pulse amplitude modulated (PAM) fluorometry Water samples for PAM analyses were collected in 30-ml acid-rinsed, polycarbonate tubes from six depths corresponding to the percentages of incidence irradiance Io as described previously. Tubes were wrapped in layers of neutral density screening to achieve light intensities corresponding to the depth at which the samples were collected and placed in Plexiglasr incubators maintained at near ambient sea surface temperatures until PAM analysis was performed. Fluorescence induction measurements were carried out using a WaterPAM fluorometer (Heinz Walz, Germany) after dark adaptation of the phytoplankton for 20 min in order for complete reoxidation of the photosystem II (PSII) reaction centers to occur (Schreiber et al., 1995). Following dark adaptation, the constant fluorescence (FO) level was recorded under modulated light intensities (2 mmol m2 s1). The maximal fluorescence level (FM) was obtained by using a saturating flash (700 ms, 3000 mmol m2 s1) causing

all the plastoquinone pools to be in a maximum reduced state. Fluorescence induction that was dependent on PSII–PSI electron transport were measured under continuous actinic light at intensities similar to the depths at which the samples was collected. Simultaneously, saturating flashes (700 ms, 3000 mmol m2 s1) given periodically (every 30 s) provided the maximal fluorescence level at steady-state fluorescence (F 0M ). Once a steadystate fluorescence yield (FS) was obtained, the actinic light was turned off. Following a short dark period, the constant fluorescence of the lightadapted sample (F 0O ) was evaluated. For blanking purposes, the above procedure was also performed using a 0.2 mm filtered seawater sample. The maximum PSII photochemical yield (FM) and the operational PSII photochemical yield (F0 M) were calculated according to the following equations, after subtraction of the blank: FM ¼ ðF M  F O Þ=F M ¼ F V =F M ðKitajima and Butler; 1975Þ; F0 M ¼ ðF 0 M  F S Þ=F 0 M ðGenty et al:; 1989Þ;

(1) (2)

where FM represents the functional properties of the PSII core complex and the associated light harvesting complexes, and F0 M reflects the apparent PSII efficiency when steady-state electron transport occurs, and is thus influenced by multiple biochemical processes occurring in the cells under continuous illumination (Laza´r, 1999). The utilization of both quantum yields permits a better measure of the fitness of photosynthetic electron transport and related processes. 3. Results 3.1. Physical properties At the onset of SERIES, the IN-patch euphotic depth (zeu) exceeded 55 m (Fig. 1). Between days 5 and 7, zeu decreased to ca. 35 m and remained close to this depth until day 12 when it deepened to 43 m. From day 15 onwards, the euphotic depth shoaled, reaching the shallowest depth of 20 m on day 18. The OUT-patch zeu exceeded 45 m throughout the observation period. For reference, changes in the upper mixed layer depth (zuml) and bottom of the IN-patch (as marked by the tracer layer depth, ztl) are also presented in Fig. 1. Further details on changes in the zuml and ztl for the IN-patch are

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Fig. 1. Variations in the euphotic zone depths (zeu, 1% Io) for the IN-patch (solid circles) and OUT-patch (open circles) throughout the Fe enrichment experiment. Also plotted are changes in tracer layer depths (ztl, dotted gray line) and upper mixed layer depths (zuml, solid black line). The ztl represents the theoretical bottom depth of the IN-patch waters. The arrows indicate the days when the Fe infusions took place.

provided in Marchetti et al. (this issue). In brief, the upper mixed layer was highly variable throughout the experiment, with a minimum zuml of o10 m from days 1 to 4 and a maximum depth of 430 m on days 5–8. The initial ztl was also confined to o16 m at the start of the experiment. On day 5, the deepening of the mixed layer resulted in the mixing of Fe and SF6 down to ca. 30 m. After day 5, ztl was 425 m for the remainder of the observation period. zeu exceeded ztl for most of the experimental period, however, on days 17 and thereafter, ztl was deeper than zeu. 3.2. Nutrients IN-patch integrated macronutrient inventories in euphotic zone waters were highest at the onset of the experiment due to a combination of high nutrient concentrations and a deep euphotic zone (Fig. 2A). From days 4 to 8, integrated nutrient stocks decreased due to a shoaling of the euphotic zone and a steady decline of nutrient concentrations in the IN-patch. Between days 8 and 13, euphotic zone integrated nutrient stocks fluctuated suggesting lateral or vertical inputs of nutrient-rich seawater from outside of the patch in conjunction with nutrient removal by phytoplankton. From day 13

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onwards, macronutrient stocks decreased markedly. On day 18, minimum amounts of nutrients were present in euphotic zone waters (NO3 ¼ 91 mmol m2, Si(OH)4 ¼ 17 mmol m2 and PO4 ¼ 10 mmol m2) and were associated with high phytoplankton biomass and a shallow euphotic zone. OUT-patch integrated nutrient stocks remained high throughout the duration of the observation period. Further details on changes in macronutrient

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budgets for the IN- and OUT-patch waters are provided in Timothy et al. (this issue). At the start of the Fe enrichment experiment, the average dissolved macronutrient concentrations in the IN-patch were 9.8 mM for NO3, 11.4 mM for Si(OH)4 and 1.0 mM for PO4 (Fig. 2B). In phase I (days 0–10) (see Marchetti et al. (this issue) for and phase descriptions), NO3 and PO4 concentrations decreased concomitantly, whereas Si(OH)4 concentrations remained relatively constant, varying between 10 and 12 mM. During phase II (days 10–19) of the bloom, NO3 and PO4 concentrations decreased steadily coinciding with a rapid decline in Si(OH)4 concentrations until day 18, when Si(OH)4 concentrations in the IN-patch waters were reduced to o1 mM. On day 18, average NO3 and PO4 concentrations of 4.6 and 0.5 mM, respectively, remained in the IN-patch. OUT-patch mixed layer nutrient concentrations fluctuated marginally (NO3 ¼ 8–13 mM, Si(OH)4 ¼ 8–16 mM and PO4 ¼ 1.0–1.4 mM) depending on the location of the OUTpatch station. The initial dissolved Si(OH)4: NO3 ratio observed in the IN-patch was ca. 1.2:1 (Fig. 2C). In the NE subarctic Pacific, a Si(OH)4: NO3 ratio of 41 in surface waters is commonly observed due to winter

mixing of Si(OH)4-rich deep waters (Whitney and Freeland, 1999). In phase I of the bloom, there was an increase in the IN-patch Si(OH)4: NO3 ratio, reaching a maximum of 1.7:1 on day 10, due to an excess removal of NO3. In phase II, the Si(OH)4: NO3 ratio decreased rapidly due to the greater utilization of Si(OH)4 than NO3 until bloom termination on day 18. IN-patch dissolved NO3: PO4 ratios remained fairly constant at 9–10:1 throughout the experiment. Similarly, OUT-patch Si(OH)4: NO3 and NO3: PO4 ratios remained at ca. 1.2:1 and 11:1, respectively, throughout the observation period. Examples of the vertical distributions of macronutrients are presented in Fig. 3. In phase I of the Feinduced phytoplankton bloom, nutrient concentrations were relatively uniform at depths throughout the patch. In phase II of the bloom, macronutrient concentrations were lower near the surface of the patch. IN-patch NH4 concentrations varied markedly both spatially and temporally. At the onset of the experiment, NH4 concentrations were low in surface waters, increasing with depth to a maximum concentration of 0.4 mM at 40 m (Fig. 4). Surface NH4 remained low, whereas subsurface concentrations steadily increased throughout the bloom. Concurrent

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with this increase was a steady shoaling of the depths in which NH4 was measured at relatively high concentrations. From days 9 to 14, an increase in surface NH4 concentrations 40.5 mM coincided with Days after first Fe enrichment 0

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the rapid decline in nanophytoplankton standing stocks (Marchetti et al., this issue). Throughout the bloom, subsurface (430 m) NH4 concentrations increased yet never exceeded 1.6 mM. Mapping of IN-patch surface chl a and NO3 concentrations were performed on days 15 and 19 of the experiment (Fig. 5). Horizontal displacement of chl a from the IN-patch between transect periods indicated the movement of the patch waters in a NW direction. Waters to the west of the IN-patch were distinct in measured properties from waters to the east of the patch. To the west of the IN-patch, OUT-patch waters had higher NO3 concentrations and were denser than OUT-patch waters to the east of the IN-patch. On day 15, patch size was approximately 621 km2 positioned on a SW to NE axis (Law et al., this issue). Chl a and NO3 concentrations varied markedly in the IN-patch with isolated pockets of relatively high or low biomass accumulation. Maximum surface chl a concentrations at this time were approximately

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3 mg m3 with corresponding surface NO3 concentrations of 5 mM. On day 19, patch size had increased to 934 km2 (Law et al., this issue). The patch had elongated horizontally and was now positioned on a W–E axis. Maximum chl a concentrations were approximately 4–5 mg m3 and surface NO3 concentrations were drawn down to 3 mM in some sections of the patch. Highest chl a concentrations were confined to three pockets comprising less than 10% of the total patch area. Suspended BSi and PN concentrations increased in the IN-patch when compared to the OUT-patch (Fig. 6A). From days 1 to 18, patch-averaged PN concentrations increased 2-fold, from 2.6 to 5.3 mM. OUT-patch PN also varied considerably, ranging between 1.4 to 3.2 mM. Between days 1 and 17, BSi concentrations increased 10-fold, from 0.9 to

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PN BSi

BSi or PN (μM)

8

6

4

2

0 (B)

IN-patch OUT-patch

BSi : PN (molar)

1.6

1.2

0.8

0.4

0.0 0

2

4

6

8

10

12

14

16

18

20

Days after first Fe enrichment Fig. 6. Temporal variations of (A) patch-averaged BSi and PN concentrations, and (B) BSi: PN molar ratios in the IN-patch (solid symbols) and OUT-patch (open symbols) throughout the Fe enrichment experiment. The dashed line in panel B represents a BSI: PN ratio of 1.

9.1 mM. OUT-patch BSi concentrations remained consistently low (o1.5 mM) throughout the observation period. The resulting BSi: PN molar ratio for the IN-patch exhibited marked changes during each phase of the phytoplankton bloom (Fig. 6B). During phase I, the BSi: PN ratio was o1, ranging from 0.4:1 on day 1 to 0.7:1 on day 9. In phase II, the BSi: PN ratio increased rapidly to 41, reaching a maximum ratio of 1.9 on day 17. 3.3. Nitrate uptake rates Changes in NO3 uptake rates in response to Fe enrichment differed between the X5 and o5 mm size-fraction. In the IN-patch surface waters, absolute rates of NO3 uptake (rNO3) for the X5 mm size-fraction increased 40-fold compared to the OUT-patch and reached a maximum uptake rate of 1.3 mM NO3 d1 on day 15 (Fig. 7A). For the o5 mm size-fraction, rNO3 increased 15-fold compared to the OUT-patch, reaching a maximum rate of 0.20 mM d1 on day 17. Absolute NO3 uptake rates for both size-fractions decreased vertically with irradiance in the euphotic zone. OUT-patch rNO3 remained low throughout the observation period. The maximum water-column integrated rNO3 (20.3 mmol m2 d1) was reached on day 13 in the IN-patch, representing a 27-fold increase relative to the mean OUT-patch value (Table 1). Based on the summed totals of water-column integrated rNO3, the X5 mm size-fraction accounted for 84% of the NO3 uptake over the duration of the Fe enrichment experiment. Biomass (PN)-specific NO3 uptake rates (PNVNO3) also increased in the IN-patch relative to the OUT-patch (Fig. 7B). In surface waters, PN-VNO3 of the X5 mm size-fraction increased 11-fold compared to the OUT-patch. The maximum PN-VNO3 (0.32 d1) by the large size-fraction was reached on day 13, 4 days before the peak in phytoplankton chl a on day 17. Likewise, for the o5 mm size-fraction, PN-VNO3 increased 7-fold in surface waters compared to the OUT-patch. In this size-fraction, PNVNO3 displayed no clear trends and the fastest uptake rates of 0.08 d1 occurred on days 4, 10 and 15–17. Similar to rNO3, OUT-patch PN-VNO3 remained low throughout the observation period. Alternatively, specific NO3 uptake rates may be calculated using chl a as a proxy for phytoplankton biomass (chl a-VNO3). In the X5 mm size-fraction, chl a-VNO3 displayed similar trends compared to PN-VNO3 throughout the phytoplankton bloom

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0.4

(A)

1.4

0.8

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. 5m : 5m

1.0

0.6

0.3

0.5

0.8

0.4

0.2

0.6

0.3

0.4

0.2

0.1

0.1

0.2 0.0

0.0

0.0 33% Io

1.4

0.7 0.3

1.2

0.6 0.5 chl a - VNO3 (mmol N (mg chl a)-1 d-1)

1.0 0.2

0.8 0.6 PN - VNO3 (d-1)

0.4 ρNO3 (μM d-1)

(C)

0.7

Surface

1.2

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0.2 0.0 10% Io

1.4 1.2

0.1

0.0

0.3

1.0 0.2

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0.4 0.3 0.2 0.1 0.0 0.7 0.6 0.5 0.4 0.3

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0.2 0.0

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0.0 1% Io

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0.7 0.3

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0.6 0.5

1.0 0.2

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8 10 12 14 16 18 20

0

2

4

6

8 10 12 14 16 18 20

0

2

4

6

8 10 12 14 16 18 20

Days after first Fe enrichment

Fig. 7. Size-fractionated (A) absolute NO3 uptake rates, (B) PN-specific NO3 uptake rates and (C) chl a-specific NO3 uptake rates at surface (100%), 33%, 10% and 1% Io in the IN-patch (solid symbols) and OUT-patch (open symbols) throughout the Fe enrichment experiment. Shaded areas denote measurements that were below the bottom depth of the IN-patch, as indicated by ztl. Error bars represent the range associated with the mean (n ¼ 2).

(Fig. 7C). Maximum chl a-VNO3 (0.55 mmol N mg chl a1 d1) in surface waters was reached on day 13, representing a 4-fold increase when compared to the average OUT-patch chl a-VNO3. In the o5 mm size-fraction, chl a-VNO3 differed markedly from PN-VNO3, particularly in the

latter stages of the bloom. From days 13 to 17, chl a-VN03 in the o5 mm size-fraction increased rapidly. Maximum chl a-VNO3 (0.71 mmol N mg chl a–1 d1) was reached on day 17, representing a 9-fold increase from the average OUT-patch chl a-VNO3.

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Table 1 Water-column integrated absolute NO3 uptake rates (rNO3, mmol m2 d1) during SERIES Day

Patch

2 4a 5 7 8 9 10 12 13 15 17 18 19

IN

1 6 11 16

OUT

o5 mm rNO3

X5 mm rNO3

Summed total rNO3

1.74 1.46 1.08 1.78 1.62 0.77 1.43 0.83 1.74 2.25 2.20 1.08 1.04

2.29 1.71 1.19 3.48 4.77 3.78 8.50 9.66 18.59 17.91 13.94 7.33 5.99

4.03 3.17 2.27 5.26 6.40 4.55 9.93 10.49 20.34 20.16 16.14 8.41 7.03

0.38 0.09 0.21 0.53

0.56 0.14 0.32 0.82

0.94 0.23 0.53 1.35

Integrations are performed to the bottom of the euphotic zone (1% Io). a Rates are integrated down to the 3% Io light depth.

Table 2 Maximal and operational PSII photochemical yields (FM and F0 M) at the surface and the bottom of the IN- and OUT-patch during SERIES Day

7 12 13 15 19 9

Patch

IN IN IN IN IN OUT

F0 M

FM zs

zb

zs

zb

0.52 0.57 0.47 0.35 0.29 0.32

0.48 0.69 0.53 0.57 0.56 0.40

0.37 0.48 0.40 0.15 0.17 0.16

0.53 0.62 0.50 0.48 0.42 0.28

zs ¼ Surface IN-patch depths corresponding to 100% Io. zb ¼ Bottom IN-patch depths corresponding to 10% or 3% Io.

first ecological phase of the phytoplankton bloom began with an initial increase in growth of all phytoplankton size classes, whereas in the second phase, a decrease in pico- and nanophytoplankton biomass coincided with the continued exponentiallike growth of diatoms. The resulting phytoplankton bloom and subsequent transitions in algal composition largely impacted macronutrient inventories and utilization rates within the patch.

3.4. Photosynthetic efficiency 4.1. Vertical mixing and patch heterogeneity Both measures of photosynthetic efficiency were high (X0.4) inside the patch during days 7–13 and declined thereafter (Table 2). For the OUT-patch, values of FM and F0 M were, 0.32 and 0.16 on day 9. The FM of the IN-patch surface phytoplankton assemblage peaked at 0.57 on day 12. On day 13, FM started to decrease and reached the OUT-patch reference value on day 19 (ca. 0.30). Similar to FM, the F0 M reached its maximal value of 0.48 on day 12, and then started to decrease and reached minimum values of 0.15–0.17 on days 15–19. When the photosynthetic efficiency was measured at depth (3–10% Io) both FM and F0 M values were higher than at the surface with peak values of 0.69 and 0.62, respectively, being achieved on day 12. 4. Discussion Results from SERIES show that the alleviation of Fe limitation resulted in an increase in phytoplankton biomass and an assemblage shift from pico- and nanophytoplankton to large diatoms (Boyd et al., 2004; Marchetti et al., this issue). Based on observed shifts in dominant phytoplankton size classes, the bloom was divided into two ecological phases. The

The development of the Fe-induced algal bloom was strongly influenced by physical processes. The dilution of the upper mixed layer by wind events and horizontal mixing processes throughout the Fe enrichment experiment had a clear influence on macronutrient concentrations in the patch. At the onset of SERIES, an unusually shallow mixed layer restricted the Fe dispersion to the upper 10 m. Wind events on days 5 and 12 increased the upper mixed layer depth to 430 m. The subsequent increases in patch volume resulted in lateral and vertical influxes of macronutrient-rich seawater which diluted the accumulating phytoplankton biomass. The ongoing horizontal dispersion of Fe also resulted in a 10-fold increase in patch area during days 1–20, incorporating waters adjacent to the patch (Law et al., this issue). This constant supply of macronutrients around the periphery confined the maximum drawdown of NO3, PO4 and Si(OH)4 to isolated pockets in the center region of the patch coinciding with maximum phytoplankton biomass (Fig. 5). Because daily sampling of the IN-patch at the precise locations of maximum phytoplankton biomass was not always achieved, patch heterogeneity is likely

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the cause of day-to-day variability observed in the measured parameters.

4.2. Iron and macronutrient dynamics Initial macronutrient concentrations in the Feenriched patch were similar to levels previously measured in HNLC waters of the NE subarctic Pacific (Whitney and Freeland, 1999). A summary of changes in dissolved and particulate nutrient concentrations and ratios in the patch throughout the algal bloom is provided in Table 3. All dissolved macronutrient concentrations were reduced inside the patch as a result of the Fe-induced phytoplankton growth. The resulting stoichiometry of Si: N requirements for phytoplankton (as inferred through the drawdown of dissolved Si(OH)4 and NO3 and production of particulate BSi and PN) increased from o1 to ca. 2 between phases I and II of the bloom. Because the changes in dissolved Si(OH)4: NO3 ratios were largely reflected by those in BSi: PN ratios, differential export between Si and N did not appear to influence the stoichiometry of the nutrient drawdown. This differential drawdown of nutrients during SERIES is similar to observations made in other Fe-enrichment studies. For example, during IronEx II, performed in the Equatorial Pacific, Fe-mediated Si(OH)4: NO3 drawdown ratios increased from ca. 0.5 to ca. 2 between days 5 and 10, synchronous with the

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decrease in soluble Fe concentrations in the patch (Wells, 2003). The enhancement in inferred molar Si: N utilization ratios of phytoplankton during SERIES is likely a consequence of shifts in dominant phytoplankton taxa. In phase I (days 0–10), an increase in all size classes of phytoplankton (pico-, nano-, and microphytoplankton) resulted in an enhanced removal of NO3 and PO4 relative to Si(OH)4. During phase II (days 10–18), the rapid increase in the abundance of diatoms resulted in a greater demand for Si(OH)4, subsequently reducing Si(OH)4 to growth-limiting concentrations by day 18. The proportion of silicon-requiring to non-silicon requiring phytoplankton present during bloom evolution would strongly influence the Si(OH)4: NO3 drawdown ratios. Additionally, although the mean molar Si: N composition ratio for marine diatoms is ca. 1:1, this ratio is species specific and can range from as low as 0.4:1 to as high as 4:1 (Brzezinski, 1985). Similar to other phytoplankton groups, there were shifts in the abundance of different diatom species at various stages of the bloom during SERIES (Marchetti et al., this issue). Therefore, the observed nutrient depletion and utilization rates are likely a reflection of the specific requirements of the dominant phytoplankton species. Changes in phytoplankton nutritional requirement ratios observed during SERIES may have also, in part, been due to the return to Fe-limitation in the later stages of the bloom. As inferred through

Table 3 Dissolved and particulate macronutrient concentrations and ratios during phase I (days 0–10) and phase II (days 10–19) of SERIES

NO3 (mM) Si(OH)4 (mM) PO4 (mM) NH4 (mM) PN (mM) BSi (mM) Si(OH)4: NO3 NO3: PO4 DSi(OH)4: DNO3a DNO3: DPO4a BSi: PN

Phase I

Phase II

Ti

Tt

Tf

9.5370.06 11.0070.17 0.9870.01 0.01 2.60 0.90 1.1570.04 9.7670.26

5.8770.68 9.7471.05 0.6070.10 0.30 3.88 3.61 1.6670.27 9.7271.95

4.7670.80 0.8870.19 0.5470.08 0.61 5.32 9.12 0.1970.05 8.8171.97

0.7770.10 9.7370.57 0.35

1.7170.20 11.2170.47 0.93

1.82

All concentrations are patch-integrated amounts normalized to patch depth (ztl). Where provided, error bars represent 71 standard deviation associated with the mean (n ¼ 3). Ti ¼ initial (OUT-patch, day 1), Tt ¼ transition from phase I to phase II (IN-patch, day 10) and Tf ¼ final (IN-patch, day 19). All ratios are mol:mol. a Dissolved nutrient drawdown ratios were estimated using the linear regressions of Si(OH)4 versus NO3 concentrations or NO3 versus PO4 concentrations measured in the IN-patch throughout each particular phase. All regressions were significant (p50.001).

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reduced photosynthetic efficiencies after day 12, low Fe concentrations in phase II of the algal bloom resulted in cells re-entering a state of Fe-stressed growth. In both laboratory and field experiments, elevated Si(OH)4: NO3 uptake ratios have been observed in Fe-limited diatoms. Of these studies, most have suggested that the enhanced Si: N requirements of Fe-limited diatoms is a result of an increase in Si cellular quotas as cells become more heavily silicified (Hutchins and Bruland, 1998; Takeda, 1998; Leynaert et al., 2004). Alternatively, elevated Si: N and Si: C ratios in Fe-limited phytoplankton could also result from a reduction in the N and C cellular quotas with little or no change in Si (Takeda, 1998; Marchetti, submitted). Moreover, multiple field studies have elucidated that elevated Si(OH)4: NO3 uptake ratios in low Fe regions are due to reductions in NO3 utilization rather than an enhancement in Si(OH)4 utilization (Franck et al., 2003; Firme et al., 2003). Unfortunately, during SERIES, Si(OH)4 uptake rates were not measured. Therefore it is difficult to discern whether changes in either Si(OH)4 or NO3 assimilation were responsible for the enhanced Si(OH)4: NO3 drawdown ratio during phase II of the bloom. Consequently, the lack of increase in the biomassspecific (PN and chl a) NO3 uptake rates after day 13 and the decline in these rates within the Fe-stressed, diatom-dominated assemblage after day 15 may suggest a decline in NO3 utilization relative to Si(OH)4. The addition of Fe to HNLC waters of the NE subarctic Pacific stimulated NO3 uptake in phytoplankton, with large cells (X5 mm) responsible for the bulk of NO3 consumption. Similarly, during IronEx II, large cells were also responsible for the enhanced NO3 utilization caused by Fe enrichment, constituting 85–98% of the total NO3 uptake at the peak of the phytoplankton bloom (Coale et al., 1996). Other studies assessing the role of Fe in regulating the use of NO3 in Fe-limited regions observed increases in NO3 uptake rates in larger cells, with little or no change in NO3 utilization by the initially dominant, smaller cells (Price et al., 1994; Boyd et al., 1996; Cochlan et al., 2002). The lack of response by small cells to Fe addition led these investigators to conclude that Fe did not regulate NO3 consumption by the indigenous phytoplankton assemblage. In contrast, during SERIES, although large cells were responsible for the majority of absolute NO3 uptake, there were marked increases in uptake rates of small cells

(o5 mm) upon the alleviation of Fe limitation, suggesting an increased capacity for growth using NO3 as a source of nitrogen. The enhancement in absolute NO3 uptake rates of phytoplankton was due to a combination of increased biomass-specific NO3 uptake rates and phytoplankton biomass. Maximum PN-VNO3 (X5 mm ¼ 0.32 d1 and o5 mm ¼ 0.08 d1) achieved during SERIES were in fair agreement with previous measurements from Fe enrichment bioassay experiments performed at OSP (X5 mm ¼ 0.24 d1 and o5 mm ¼ 0.12 d1) (Boyd et al., 1996). During SOFeX, the maximum PN-VNO3 (converted to daily rates from short term, hourly uptake rates) were slower (north patch ¼ 0.19 d1 and south patch ¼ 0.14 d1; Cochlan, pers. comm.1), whereas PN-VNO3 measured in the Equatorial Pacific during IronEx II were considerably faster (1.2 d1; Cochlan and Kudela, 1996) than those measured during SERIES. This range in Fe-amended biomass-specific NO3 uptake rates may be attributed to differences in ambient temperatures from tropical to polar waters. The importance of temperature in regulating the extent of increase in Fe-amended biological rate processes and phytoplankton growth should not be overlooked, particularly when comparing the ecosystem responses to Fe enrichment among HNLC regions (Noiri et al., 2005; Boyd, 2002b). To specifically quantify nutrient assimilation rates by phytoplankton, investigators who use 15N to measure N uptake commonly normalize their uptake values to chl a rather than PN, thus excluding non-algal biomass such as heterotrophic bacteria and zooplankton that contributes to PN (Dugdale and Wilkerson, 1991). During SERIES, trends in chl a-VNO3 in the large cells were similar to PN-VNO3, suggesting the majority of the PN collected in the X5 mm size-fraction was composed of phytoplankton biomass. In contrast, chl a-VNO3 of the small cells increased markedly in the later stages of the algal bloom, whereas there was no concurrent increase in PN-VNO3 for this sizefraction. The observed increase in chl a-VNO3 for the small cells could result from NO3 consumption by heterotrophic bacteria (Kirchman et al., 1992). An increase in bacterial NO3 utilization without the concomitant increase in chl a biomass of small cells would lead to an overestimate in VNO3. Previous measurements performed at OSP estimated that 1

Values of PN-VNO3 reported in Coale et al. (2004) are incorrect due to clerical/editorial error.

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30% of total NO3 uptake could be accounted for by heterotrophic bacteria (Kirchman and Wheeler, 1998). Additionally, the 5-fold increase in bacterial abundance measured between days 14 and 16 during SERIES (Hale et al., this issue) coincides with the increase in chl a-VNO3 in small cells. During this period, PC: chl a ratios for small cells also increased markedly, suggesting the presence of large amounts of non-algal associated PC (Marchetti et al., this issue). Moreover, at peak abundances (days 16–18), heterotrophic bacteria accounted for as much as 20–25% of the PN in the o5 mm sizefraction (calculated using heterotrophic bacterial C estimates from Hale et al., this issue). The role of heterotrophic bacteria as potential consumers of NO3 in Fe enrichment experiments warrants further investigation, particularly with respect to the possible effects they may have on estimates of new production. 4.3. Bottom-up factors influencing phytoplankton growth Both maximal and operational PSII photochemical yields were elevated in the Fe-enriched waters when compared to outside the patch suggesting an enhancement in photochemical energy conversion efficiency upon the alleviation of Fe-stress. The maximum FM values achieved on day 12 (0.57 at 100% Io and 0.69 at 10% Io) using the PAM technique are in close agreement to the value of ca. 0.65 reported for nutrient-replete phytoplankton (Tings and Owens, 1992; Buchel and Wilhelm, 1993; Juneau and Harrison, 2005). The high value of FM measured inside the patch during SERIES was also similar to the value measured during IronEx II using fast repetition rate fluorometry (FRRF) (Kolber et al., 1994). The decrease in FM after day 12 suggests that the phytoplankton may have had reduced photosynthetic efficiency well before bloom termination on day 18. From days 13 to 15, the sharp reduction in F0 M when compared to the gradual decrease in FM may suggest a co-limitation between Fe and Si(OH)4. Indeed, although Fe nutritional status might influence both photochemical yields, the dramatic decline in F0 M could have resulted from an additive effect due to Si(OH)4 limitation. This is due to Fe limitation directly affecting the PSII reaction centers, for example, by reducing the chlorophyll content (Terry and Abadia, 1986) whereas Si limitation affects other less PSII-specific biochemical reactions in the cell, such

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as protein synthesis or photorespiration (Lippemeier et al., 1999). Thus, based on their various modes of action, Si limitation might affect F0 M more than FM (Lippemeier et al., 1999). Between phase I and phase II of the algal bloom, distinct shifts in the phytoplankton species composition also occurred (Marchetti et al., this issue), which may be partially responsible for the changes in the photosynthetic efficiency measured during the experiment (Juneau and Harrison, 2005). Light may have also played an important role in regulating phytoplankton growth inside the patch, particularly at depth during the peak of the phytoplankton bloom. In the NE subarctic Pacific, light is commonly limiting to phytoplankton growth in winter months when irradiances are less than what is required to maintain maximum rates of production (Welschmeyer et al., 1993; Maldonado et al., 1999; Varela and Harrison, 1999). In summer months, the euphotic depth (1% Io) is commonly well below the upper mixed layer depth (Harrison et al., 2004). In contrast, by day 17 of SERIES, the accumulation of algal biomass in the patch decreased the euphotic depth to o20 m, which was shallower than the bottom depth of the patch (ca. 35 m). Additionally, when phytoplankton grow at depth under lower light intensities, the cellular demands for Fe increase (Sunda and Huntsman, 1997). Thus, in combination with low Si(OH)4 and Fe concentrations in the later stages of the algal bloom, decreased irradiance penetration due to extensive phytoplankton biomass accumulation would have likely influenced phytoplankton nutrient utilization rates in the patch. 4.4. Summary Our results clearly indicate that during summer, Fe ultimately limits the extent of macronutrient utilization and regulates the amount of new production in the NE subarctic Pacific. Upon the alleviation of Fe limitation, absolute and biomassspecific NO3 uptake rates increased in both the large and small cells with a concomitant drawdown of all macronutrients and an increase in phytoplankton biomass. Si(OH)4 concentrations were nearly depleted and likely limiting to diatom growth 18 days after the initial Fe infusion. Measurements of photosynthetic efficiency performed using PAM techniques suggest that the phytoplankton were growing optimally between days 7 and 12, and experienced Fe or Fe- and Si-stress thereafter. The

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elevated Si(OH)4: NO3 drawdown and suspended particulate BSi: PN ratios (41:1) observed in phase II of the algal bloom may be attributed to shifts in elemental requirements of dominant phytoplankton species and/or Fe-stressed growth by diatoms. Acknowledgements The authors would like to thank the scientists on board the CCGS J.P Tully, the El Puma and the Kaiyo Maru who were instrumental in carrying out sampling and data analyses during SERIES. In particular, we would like to thank J. Barwell-Clarke from the Institute of Ocean Sciences (IOS), Sidney, British Columbia and M. Guo from the University of British Columbia for nutrient analysis. We are grateful to Y. Nojiri from the National Institute of Environmental Studies, Japan for use of BSi data as well as C. Law from the National Institute of Water and Atmospheric Research, New Zealand and M. Arychuk from IOS for use of tracer layer depths. Light data were kindly provided by L. Ziolkowski from Dalhousie University. We would also like to thank R. Riel from Heinz Walz Company for the loan of the Water-PAM system and the officers and crew of the CCGS J.P. Tully, the El Puma and the Kaiyo Maru for their efforts at sea. This work is a contribution of the Canadian-SOLAS Network (Surface Ocean—Lower Atmosphere Study) funded by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canadian Foundation for Climate and Atmospheric Sciences (CFCAS), the Department of Fisheries and Oceans Canada, and the Department of Environment Canada. P. Juneau also received support from a NSERC postdoctoral fellowship. CSW was supported by shiptime, salary and Science Strategic Funds from DFO and the Panel of Energy Research & Development of NRCan under project 52540 and the C-SOLAS project of ‘‘Iron Fertilization’’.

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