Scholarly article on topic 'Aerial extent, composition, bio-optics and biogeochemistry of a massive under-ice algal bloom in the Arctic'

Aerial extent, composition, bio-optics and biogeochemistry of a massive under-ice algal bloom in the Arctic Academic research paper on "Earth and related environmental sciences"

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{Coccolithophores / Calcification / Photosynthesis / Arctic / "Under-ice algal bloom" / "Calcium carbonate" / "Biogenic silica" / "Colored dissolved organic matter" / "Fluorescent dissolved organic matter" / "Geographic bounding coordinates: 71–74°N and 158.5–169°W"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — W.M. Balch, B.C. Bowler, L.C. Lubelczyk, M.W. Stevens

Abstract It has been long thought that coccolithophores are a minor component of the phytoplankton assemblage in Arctic waters, with diatoms typically being more dominant. Little is known about how the phytoplankton communities will change, however, as the Arctic warms. We participated in the 2011 Impacts of Climate on EcoSystems and Chemistry of the Arctic Pacific Environment (ICESCAPE) cruise to the western Arctic, performing a combination of discrete measurements (microscopy, calcification, particulate inorganic carbon (PIC), particulate organic carbon (POC), biogenic silica (BSi)) plus continuous surface bio-optical measurements (absorption, scattering, backscattering and acid-labile backscattering; the latter specific for coccolithophores). Here, we report bio-optical and coccolithophore observations from the massive under-ice algal bloom originally described in Arrigo et al. (2012). The most intense portions of the bloom were centered in cold Winter Water and there was evidence for nitrate drawdown in the top 10–20m with strong penetration of silicate rich water into the surface waters. Surface chlorophyll a and particulate absorption at 440nm approached 30μgL−1 and 1.0m−1, respectively. Particulate absorption of detritus (a p at 412nm) was highly correlated to a p at 440nm associated with chlorophyll a and slopes of the absorption spectrum showed that both dissolved and particulate absorption at 412nm exceeded that at 440nm, with slopes, S g, of 0.01 nm–1. Colored dissolved organic matter fluorescence (FDOM) was high in the bloom but the relative fluorescence yields were low, characteristic of phytoplankton-produced FDOM (as opposed to terrestrially-produced FDOM). Coccolithophore backscattering was elevated in the under-ice bloom, but it only accounted for 10% of the total particle backscattering, relatively low compared to typical subpolar waters further to the south. Total particle scattering was significantly elevated in the under-ice bloom (values of almost 2m−1), likely due to the high abundance of large diatoms. Backscattering probabilities in the bloom were ~1%, again characteristic of diatom-dominated populations with few calcifiers. PIC standing stock in the under-ice bloom was low but measurable while biogenic silica molar concentrations were 150 times greater. POC:PON molar ratios were 6–10, characteristic of healthy, rapidly growing phytoplankton, observations further buttressed by carbon:chlorophyll mass ratios of 50–100. Coccolithophore calcification was low but measurable, reaching 1.75mgCm−3 d−1 in the under-ice bloom, only 0.4% of the photosynthesis. However, the intrinsic carbon-specific growth rate was 0.4 per day for bulk POC and ~1 per day for bulk PIC, close to maximal growth rates expected at these temperatures. SEM and light microscopy results showed mostly diatoms in the bloom. The coccolithophore, Emiliania huxleyi, was observed, providing unequivocal evidence of the presence of coccolithophores in the under-ice algal bloom.

Academic research paper on topic "Aerial extent, composition, bio-optics and biogeochemistry of a massive under-ice algal bloom in the Arctic"

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Aerial extent, composition, bio-optics and Biogeochemistry of a massive under-ice algal bloom in the Arctic

W.M. Balch, B.C. Bowler, L.C. Lubelczyk, M.W. Stevens

St DEEP-SEA RESEARCH

a*— PART 11

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PII: S0967-0645(14)00110-6

DOI: http://dx.doi.org/10.1016/j.dsr2.2014.04.001

Reference: DSRII3633

To appear in: Deep-Sea Research II

Cite this article as: W.M. Balch, B.C. Bowler, L.C. Lubelczyk, M.W. Stevens, Aerial extent, composition, bio-optics and Biogeochemistry of a massive under-ice algal bloom in the Arctic, Deep-Sea Research II, http://dx.doi.org/ 10.1016/j.dsr2.2014.04.001

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Aerial extent, composition, bio-optics and biogeochemistry of a massive under-ice algal bloom in the Arctic

By W. M. Balcha*, B. C. Bowlera, L. C. Lubelczyka, M.W. Stevens, Jr. b aBigelow Laboratory for Ocean Sciences 60 Bigelow Drive, POB 380 East Boothbay, ME 04544 USA

bColby College Mayflower Hill Drive Waterville, ME 04901 USA

*To whom correspondence should be addressed. E-mail: bbalch@bigelow.org Key Words: Coccolithophores, Calcification, Photosynthesis, Arctic, Under-ice algal bloom, calcium carbonate, biogenic silica, colored dissolved organic matter, fluorescent dissolved organic matter, geographic bounding coordinates: 71°N to 74°N and 158.5° W to 169°W

Running head: Carbon fixation and composition of an under-ice algal bloom

Abstract

It has been long thought that coccolithophores are a minor component of the phytoplankton assemblage in Arctic waters, with diatoms typically being more dominant. Little is known about how the phytoplankton communities will change, however, as the Arctic warms. We participated in the 2011 ICESCAPE (Impacts of Climate on EcoSystems and Chemistry of the Arctic Pacific Environment) cruise to the western Arctic, performing a combination of

discrete measurements (microscopy, calcification, particulate inorganic carbon (PIC), particulate organic carbon (POC), biogenic silica (BSi) plus continuous surface bio-optical measurements (absorption, scattering, backscattering and acid-labile backscattering; the latter specific for coccolithophores). Here, we report bio-optical and coccolithophore observations from the massive under-ice algal bloom originally described in Arrigo et al.(2012). The most intense portions of the bloom were centered in cold Winter Water and there was evidence for nitrate drawdown in the top 10-20m with strong penetration of silicate rich water into the surface waters. Surface chlorophyll a and particulate absorption at 440nm approached 30^g L-1 and 1.0 m-1, respectively. Particulate absorption of detritus (ap at 412nm) was highly correlated to ap at 440nm associated with chlorophyll a and slopes of the absorption spectrum showed that both dissolved and particulate absorption at 412nm exceeded that at 440nm, with slopes, Sg, of 0.01. Colored dissolved organic matter fluorescence (FDOM) was high in the bloom but the relative fluorescence yields were low, characteristic of phytoplankton-produced FDOM (as opposed to terrestrially-produced FDOM). Coccolithophore backscattering was elevated in the under-ice bloom, but it only accounted for 10% of the total particle backscattering, relatively low compared to typical subpolar waters further to the south. Total particle scattering was significantly elevated in the under-ice bloom (values of almost 2 m-1), likely due to the high abundance of large diatoms. Backscattering probabilities in the bloom were ~1%, again characteristic of diatom-dominated populations with few calcifiers. PIC standing stock in the under-ice bloom was low but measurable while biogenic silica molar concentrations were 150 times greater. POC:PON molar ratios were 6-10, characteristic of healthy, rapidly growing phytoplankton, observations further buttressed by carbon:chlorophyll mass ratios of 50-100. Coccolithophore calcification was low but measurable, reaching 1.75mg C m-3 d-1 in the under-ice bloom, only 0.4% of the photosynthesis. However, the intrinsic carbon-specific

growth rate was 0.4 per day for bulk POC and ~1 per day for bulk PIC, close to maximal growth rates expected at these temperatures. SEM and light microscopy results showed mostly diatoms in the bloom. The coccolithophore, Emiliania huxleyi, was observed, providing unequivocal evidence of the presence of coccolithophores in the under-ice algal bloom.

1 Introduction

1.1 Polar phytoplankton and coccolithophores

Arctic waters have long been characterized by strong diatom dominance, as evidenced in the first description of diatoms in Arctic sea ice (Ehrenberg, 1841) as well as more recent accounts (Bursa, 1961; Poulin et al., 2011; Saito and Taniguchi, 1978; von Quillfeldt, 2000) that show diatoms to be the significant drivers of Arctic primary production in the upper water column (and under ice). Dinoflagellates are also regularly seen in Arctic waters but at lower biomass than the diatoms (Braarud, 1935; Horner, 1984; Poulin et al., 2011). Phaeocystis is another common phytoplankter in Arctic waters (Poulin et al., 2011; Sherr et al., 2003) as are nanoflagellates, which can contribute the majority of carbon biomass at specific times (Sherr et al., 2003).

Relative to the other phytoplankton groups, coccolithophores have traditionally been thought to be rare (or absent) in Arctic waters (Poulin et al., 2011) and more abundant in the sub-polar, temperate, sub-tropical and tropical biogeographic zones of the world ocean (McIntyre and Be, 1967; Okada and Honjo, 1973; Winter et al., 1994; Ziveri et al., 2004). One hypothesized reason for the low abundance of coccolithophores in polar waters has been that they typically show lower growth at temperatures <8°C and in reduced solar radiation (Raitsos et al., 2006), such as in polar waters.

Despite their typically low abundance, coccolithophore blooms have been observed in ice-free polar waters using space-based remote sensing. Evidence from the AVHRR

(Advanced Very High Resolution Radiometer) satellite, suggests that the frequency of coccolithophore blooms in sub-polar and non-ice-covered polar Arctic waters has been increasing over twenty years (Smyth et al., 2004). These blooms are probably Emiliania huxleyi but it has been impossible to confirm this due to lack sea-truth data. Polar coccolithophore species besides E. huxleyi were previously described in early taxonomic studies from Resolute Bay (Northwest Passage), West Greenland and South Alaska (genera Pappomonas, Wigmamma, Turrisphaera and Papposphaera) where the water temperature was below 0°C (Manton et al., 1976a; Manton et al., 1976b; Manton et al., 1977). Recent work in the Atlantic Arctic (partially ice-covered/ice edge region north of Svalbard) demonstrated low abundance of coccolithophores (2.5 cells mL-1) with species mostly from the family Papposphaeraceae, found in waters <0°C with sub-micromolar nitrate and phosphate (Charalampopoulou et al., 2011). Coccolithophore species observed in this same study included E. huxleyi, Coccolithus pelagicus, Pappomonas sp., Papposphaera arctica and Wigmamma sp.

1.2 Arctic primary production and calcification

There is relatively little information on blooms of algae under Arctic ice, primarily due to the high reflectance of sea-ice, and the inability to see such blooms using satellite remote sensing. Observations from ships have provided some evidence that blooms can occur, however. For example, at ice station SHEBA in the western Arctic, chlorophyll concentrations reached as high as 4.3 mg m-3 under the ice during the summer melting of snow overlying the ice (Sherr et al., 2003). Typically, under-ice primary productivity has been assumed to be low due to the strong attenuation of light by ice and snow. Hill et al. (2013) and Matrai et al. (2013), examined historical 14C primary production and chlorophyll data. Surface productivity rates from regions like the northern Chukchi Sea were typically <10 mg C m-3 d-1 (Hill et al., 2013; Matrai et al., 2013). Nitrate also is seasonally drawn-

down under the ice, the extent of which can be used to estimate annual primary production (assuming a Redfield ratio of C:N in particulate matter and an f ratio of nitrate utilization) (Codispoti et al., 2012; Eppley and Peterson, 1979). Such estimates are within a factor of two of 14C measurements of net primary production (Codispoti et al., 2012; Hill et al., 2013). Elevated integrated primary productivity has been documented in waters with >90% ice, with rates as high as 60 mg C m-2 d-1 (but after snow is removed from the ice) (Gosselin et al., 1997).

There is only one previous study of coccolithophore calcification in a partially ice-covered region north of Svalbard (Charalampopoulou et al., 2011). In an ice-free fjord and the marginal ice zone, calcification was low, with a subsurface peak of 0.02-0.07 mg PIC m-3 d-1. In a partially ice-covered region, calcification showed a subsurface peak of 0.6 mg PIC m-3 d-1 (Charalampopoulou et al., 2011). Such rates are extremely low compared to rates measured in more coccolithophore-rich, lower latitude waters (Balch et al., 2007).

The goal of this study was to use a combination of continuous underway and discrete seawater measurements to document under-ice algal features during the ICESCAPE (Impacts of Climate on EcoSystems and Chemistry of the Arctic Pacific Environment) cruise to the western Arctic Ocean, July-August 2011. Moreover, we documented the hydrographic, biological and optical properties of these features and used the data to better understand: bloom magnitude, bloom size, dominant species, pigment-specific absorption, particle scattering and distribution of colored dissolved organic matter (CDOM). Discrete samples provided estimates of the standing stocks of particulate organic carbon (POC), particulate inorganic carbon (PIC), biogenic silica (BSi) plus photosynthesis and calcification rates. These observations provide a baseline for interpreting future changes in the phytoplankton standing stocks, rates and bio-optical properties of the Western Arctic as the region undergoes climate change (Arrigo et al., 2008).

2 Methods

2.1 Cruise details

The ICESCAPE 2011 expedition took place in the western Arctic (Fig. 1A) aboard the USCGC Healy (cruise #1101) departing Dutch Harbor, AK, USA on 25 June and returning to Seward, AK on 29 July 2011. A total of 173 stations were sampled during the cruise which included 9 sea ice stations. The station domain extended from the coast of Alaska westward to the US-Russian border - and between the Bering Strait and ~74°N. The work presented in this communication is focused on a portion of the cruise track of Healy 1101, from the Alaskan coast to the region of the giant under-ice algal bloom, northwest of Barrow, AK, within the region 71°N to 74°N and 158.5°W to 169°W (stations 37-123 and 157-173) (Arrigo et al., 2012). Station numbers and their location are shown in Fig. 1B.

Measurements included running a continuous underway system (focused on hydrographic and bio-optical properties) and measuring discrete water samples for a variety of biological and biogeochemical variables. CTD stations typically involved sampling water from eight depths. Of those eight bottles, seven were usually from the euphotic zone and one from deeper in the water column. We also sampled the top Niskin bottle of numerous CTD casts for calibration samples for the continuous underway system.

2.2 Underway bio-optical system

The Balch lab bio-optical underway system was run continuously over the course of the trip. It was started on 26 June, 2011 and shut down on 27 July, 2011 with shutdowns for weekly cleaning and calibration (see Section 2.3). This system has been described elsewhere (Balch et al., 2008). Briefly, the seawater source was located at 5m depth on CSCGC Healy. Water flowed through an ice separator then through insulated stainless steel pipes to the shipboard laboratory. Our flow-through system measured temperature, salinity, chlorophyll

a fluorescence, CDOM fluorescence (FDOM) and particle backscattering. Temperature and salinity were first measured with a SeaBird flow-through temperature and conductivity sensor. A WETLabs WETStar CDOM fluorometer was used to measure the fluorescence of colored dissolved organic matter (excitation = 370nm; emission = 460nm). This was plumbed into the flow path just after the temperature/salinity sensors. Next, chlorophyll fluorescence was measured with a WETLabs WETStar chlorophyll fluorometer (excitation = 460nm; emission = 695nm). Particle backscattering at 531 nm (using a WETLabs ECOVSF sensor aimed into a specially-designed container which minimized wall reflectance, hence maximizing the light scattering signal associated with marine particulate matter). First, the system measured particle backscattering of 531 nm light with raw seawater (pH=~8.1) running through the system for one minute. After 60 seconds of data collection (or whatever time period was set in order to have sufficient sample size to achieve standard errors of 0.5x10-5 m-1), the acid controller injected 0.2 ^m-filtered, 10% glacial acetic acid into the seawater stream, passing through a mixing coil to thoroughly mix it with the seawater, upstream of the ECOVSF. This reduced the pH to 5.5, below the dissociation point for various mineral forms of calcium carbonate. A pH sensor downstream of the sample chamber measured the pH constantly. Once the pH dropped to 5.5, backscattering was re-measured for an equivalent period of time after which the acid additions stopped and the pH re-equilibrated to raw seawater values and the entire cycle repeated. The difference in backscattering between raw seawater and acidified seawater represented "acid-labile backscattering" (bb'), which can be directly related to the concentration of suspended calcium carbonate (Balch et al., 1996).

The underway bio-optical system had a separate flow loop that passed through a WETLabs ac-9, to measure spectral absorption and attenuation at nine wavelengths: 412, 440, 488, 510, 555, 630, 650, 676 and 715nm. In the flow path to the ac-9 was a solenoid

that diverted the seawater stream through a 1|m filter, then a 0.2 |m filter prior to running the water through the ac-9. Every two minutes, the solenoid would alternate between filtered and unfiltered seawater, thus providing absorption and attenuation (at 9 spectral wavelengths across the visible spectrum) for raw and filtered seawater. In turn, this allowed calculation of the absorption and attenuation of total suspended particles and dissolved organic matter. The difference between raw and dissolved ac-9 measurements represented particulate absorption and beam attenuation. Total scattering was calculated as attenuation minus absorption.

2.3 Underway system calibration

Calibrations of the complete underway system were performed just prior to departure, approximately weekly during the cruise as well as a final calibration after final shut down. These calibrations were used to estimate biofouling corrections during each operation period. The protocol was to run 0.2um filtered RO water from the ship's Milli-Q system, under pressure, through the entire flow path prior to cleaning ("a dirty calibration" which provided the endpoint for estimating the optical contribution of biofouling). Then, the system was carefully disassembled and cleaned, reassembled and a "clean calibration" performed (which represented the beginning calibration for the next operational segment, with no biofouling. Post cruise, the biofouling corrections were interpolated between the initial clean calibration and the following "dirty calibration". The backscattering signal associated with the wall of the flow-through container was also estimated by running 0.2um-filtered RO water following cleaning as well as 0.2um-filtered seawater. Daily, biofouling of the wall was estimated by first shunting the inflowing water through a separate 0.2um filter prior to passage through the system and comparing this bbp value to that of pure seawater (Mobley, 1994).

2.4 Discrete samples

For the full CTD cast (the "productivity cast"), particulate inorganic carbon (PIC) was measured on 0.2L seawater samples filtered onto 0.4pm pore-size polycarbonate filters,

rinsed with potassium tetraborate buffer (Poulton et al., 2006) and biogenic silica (BSi) was measured by filtering 0.2 L seawater onto 45mm 0.4pm polycarbonate filters, stored and measured according to Brzezinski et al. (1989). Particulate organic carbon (POC)/particulate organic nitrogen (PON) was measured using JGOFS protocols (JGOFS, 1996) while coccolithophore counts were processed ashore using polarized light microscopy (Haidar and Thierstein, 2001) (but substituting Norland #74 brand optical adhesive instead of Canada Balsam). Surface and chlorophyll maximum depths were sampled for scanning electron microscope and prepared for analysis ashore according to Goldstein et al. (2003). These same depths were sampled for "live" microscopy using the Filter Freeze Transfer technique (Hewes and Holm-Hansen, 1983), with samples filtered on 0.4um polycarbonate filters prior to transfer and then samples examined using an AO-Spencer Model 10 microscope equipped with epifluorescence and polarization optics. Nutrient samples were run on an AA3 autoanalyzer for nitrate, nitrite, ammonium, phosphate and silicate (but only nitrate and silicate results will be discussed here).

At the daily productivity cast, samples were taken for measuring primary production and calcification from the 30L Niskin samples (with Silicone O-rings). Water was sampled from 6 light depths: 38.6%, 21.1%, 11.7%, 3.5%, 1.9% and 0.3%. Estimation of those light depths was performed based on the percent light as measured by the scalar PAR sensor aboard the CTD, scaled to the above-water downwelling PAR irradiance measured from the superstructure of USCGC Healy. Given that standard depths were typically sampled (surface, 10m, 25m, 50m, 100m plus the chlorophyll fluorescence maximum), the percent of surface PAR was estimated at each standard depth, then the closest Niskin bottle to each target light depth was chosen for productivity incubation. Often, the water column was only 30-40m and the euphotic depth was shallower still. It was common that water from a single Niskin bottle would be used for more than one simulated in situ incubation sample since the

depth range sampled by the Niskin bottle encompassed several standard light depths. Water samples for incubation were transferred from Niskin bottles to incubation bottles inside the ship's enclosed hanger. Water samples for 14C carbon fixation measurements were pre-filtered through 200 pm nitex mesh to remove large grazers. Incubations were performed in 70 mL polystyrene tissue culture bottles that were previously thoroughly cleaned with 10% HCl, then ethanol, 4 rinses with ship's distilled water and finally 3 rinses of polished reverse-osmosis water, then rinsed three times with each sea water sample prior to filling. Photosynthesis and calcification were measured using the microdiffusion technique (Paasche and Brubak, 1994) with modifications by Balch et al. (2000) (see also Fabry (2010)). 14C-bicarbonate (60-100 pCi) was added to each water sample. Incubations were performed in triplicate (with an additional sample killed with 2% formalin (final concentration). Incubations were performed in simulated in situ conditions on-deck, corrected for both light quantity (using bags made of neutral-density shade cloth) and quality (spectral narrowing using layers of blue acetate as bag inserts). Bottle transfers between the CTD hanger and radioisotope van were always done in a darkened themal cooler to reduce light and temperature shock to the phytoplankton. Deck incubators consisted of a white plastic tub open to ambient sky light, chilled using surface seawater from the ship's flowing sea water system. The daily PAR was measured using the ship's PAR sensor set on top of the ship's meteorological mast. All filtrations were performed using 0.4 pm pore-size polycarbonate filters. Following the microdiffusion step, filters and sample "boats" were placed in scintillation vials with 7mL of Ecolume scintillation cocktail. Samples were counted using a Beckman-Coulter LS6500 scintillation counter with channel windows set for 14C counting with calibration checked with a sealed 14C standard. Counts were performed for sufficient time to reach 2% precision or 20 minutes for samples with lower counts. Blank 14C counts were always run for scintillation cocktail as well as the phenethylamine CO2 absorbent.

Standard equations were used for calculating primary production and calcification from the 14C counts with a 5% isotope discrimination factor assumed for the physiological fixation of 14C-HCO3 (as opposed to 12C-HCO3). Aerial integrations of carbon fixation to the base of the euphotic zone were based on the PAR attenuation measured during the CTD cast and depth integrations were performed using trapezoidal integration. Photosynthesis and calcification measurements were normalized to fluorometer-derived chlorophyll concentration. Samples for chlorophyll analysis were filtered on 25mm, GF/F filters (Whatman) then submerged in 5mL of 90% acetone, extracted for ~ 24h at 3°C. Following centrifugation, the fluorescence of the supernatant was analyzed using a Turner 10-AU fluorometer (Turner Designs, Inc.), previously calibrated with chlorophyll standard (Sigma) (Holm-Hansen et al., 1965). Intrinsic, carbon-specific growth rates for POC (pPOC) and PIC (pPIC) (units d-1) were estimated by dividing the rates of photosynthesis or calcification (in units of moles m-3 d-1) by POC or PIC concentrations (moles m-3), respectively. 3 Results

3.1 Cruise details and hydrographic observations

The general study area of Healy cruise 1101 was the western Arctic (Fig. 1A). The period that the Healy 1101 cruise was in the vicinity of the under-ice algal bloom was between calendar days 183-205. During this period, the southern extent of the ice edge receded north ~100km (Fig. 1B). Water temperatures over the top 5m showed the presence of coldest waters (<-1°C), indicative of Winter Water (Rudels et al., 1990) in the far western portion of the study area, near stations 54-57 (Fig. 1B,C). The next coldest waters were observed in the northern extent of the study area, over the Canadian Basin (Station 100; Fig. 1C). Highest salinities were observed along the southern end of the cruise track, extending (in patches) to station 67 (Fig. 1D), usually associated with waters of 2 to 5oC. Lowest salinities were found in the 0 to -1oC water of the Canada Basin (Fig. 1D).

3.2 Chlorophyll, absorption and fluorescence observations

Chlorophyll concentrations (derived from the continuous underway fluorescence measurements calibrated to discrete chlorophylls) reached greatest values of ~30ug L-1 in the western portion of the study area, where Winter Water reached the top 5m (see white contour line in Fig. 2A). This was the site of the under-ice bloom described earlier (Arrigo et al., 2012). Lowest chlorophyll a values were seen in the Canadian Basin, (~300X lower at 0.1 ug L-1 (Fig. 2A)). Using a chlorophyll concentration of >2pg L-1 as the criterion for the bloom the largest horizontal dimension measured in the under-ice algal bloom, using the continuous underway system, was ~140km (Fig. 2A).

Particle absorption was also highest in the under-ice algal bloom, with elevated values of ~1m-1, reaching 100km from the ice edge and lowest values in the Canadian Basin (north of station 95; Fig. 2B). Absorption of colored dissolved organic matter (CDOM; ag4i2) was elevated within the under-ice bloom and lowest in the Canada Basin (Fig. 2B). Absorption of both CDOM plus detrital matter (agp412) was elevated in the under-ice bloom, twice the magnitude of ag412 (Fig. 2C). Values of agp412 were also elevated near shore (Fig. 2D). The proportion of total absorption at 412 nm contributed by the dissolved (<0.2um) fraction was generally 70- 90% over the study area except in the under-ice bloom where only 30-50% of the total absorption was contributed by dissolved materials (Fig. 2F).

Chlorophyll-specific absorption (Fig. 2E) were calculated by first subtracting dissolved absorption from the total particulate and dissolved absorption at all wavelengths, in order to estimate particulate absorption. The particulate absorption was then calculated at each wavelength, subtracting the residual absorption at 715nm to correct for scattering effects (Bricaud et al., 1988).The absorption cross section of chlorophyll at 440nm (a*p440) was calculated by dividing the particulate absorption (m-1) by the chlorophyll concentration (units mg m-3). Average values of a*p440 were 0.025 m2 (mg Chl)-1 in the western Winter Water as

well as in the cold waters of the Canadian Basin (Fig. 2E). Highest absorption cross-sections were seen in the warmest, high salinity waters near the Alaskan coast.

Both ap440 and ap4i2 were well correlated to chlorophyll biomass. The plot of particulate absorption at 440nm (ap440) versus chlorophyll concentration (Fig. 3A) had a Y intercept of 0.002 m-1, barely above zero (Table 1), indicating that particulate absorption of phytoplankton was virtually all associated with viable, chlorophyll-containing phytoplankton, not detritus. Further, ap412 (which would normally be expected to be representative of particulate detritus) was highly correlated to chlorophyll with a slope of 0.025 m2 (mg Chl)-1 and Y intercept of 0.005 m-1 (Fig. 3B; Table 1). The high correlation between ap412 and ap440 can be seen in Fig. 3C, with an r2 = 0.975 and slope of 0.926 (Table 1). Thus, ap412 was as good proxy of chlorophyll a as ap440, not detritus. Values of dissolved absorption at 412nm (ag412) had a positive but far reduced correlation with chlorophyll a, however, with only a factor of two increase in ag412 observed over >2 orders of magnitude of chlorophyll (Fig. 3D). The relation was still statistically-significant (Table 1).

The slope of the absorption spectrum of dissolved material between 412 and 440 nm (Fig. 4A), Sg (nm-1) , was calculated according to Stedmon and Markager (2001)as:

Sg=((Ln(ag412/ ag440)/(440-412)) A comparable slope for the detrital and particulate absorption, Spg (nm-1), was also calculated by substituting apg412 and apg440 in place of ag412 and ag440, respectively, in the above equation (Fig. 4B). Note that positive values for these slopes indicate that 412nm absorption >440nm absorption and negative slopes indicate 412nm absorption <440nm absorption. The results show strikingly similar patterns of Sg and SPg, with positive values near shore and in the under-ice bloom and negative values in the Canada Basin.

CDOM fluorescence (FDOM) was most elevated in the coldest water of the under-ice algal bloom and over Hannah Shoals. Lowest values were observed near the coast of Alaska

and in the Canada Basin (Fig. 4C). The relative fluorescent yield of the combined dissolved/detrital material was calculated as the FDOM (from raw, unfiltered seawater) divided by the ag412. The term "relative" is used here because FDOM excitation wavelength (370nm) did not match the absorption wavelength measured by the ac-9 (412nm).

The lowest relative FDOM fluorescent yield was observed in the under-ice algal bloom while highest values were observed in the Canada Basin region. Relatively low values were also seen near the coastline of Alaska (Fig. 4D). Highest concentrations of FDOM were in the under-ice bloom, (likely produced by the intense phytoplankton growth) but this FDOM had low relative fluorescent yields (Fig. 4D). The nature of this FDOM can be evaluated through its relation to other bio-optical variables. For example, FDOM was significantly correlated with CDOM (as ag412) but the dynamic range in FDOM was less than a factor of 2 over a 10X variation in ag412 (and the squared coefficient of correlation was only ~0.3; Fig. 5A). FDOM was better correlated to the chlorophyll a concentration than ag412 (Fig. 5B). The best-fit power function to those results accounted for almost 60% of the variance (Table 1). The relative FDOM fluorescence yield also was inversely correlated with the chlorophyll concentration (Fig. 5C) such that the under-ice bloom showed the lowest fluorescent yields, accounting for about 25% of the variance. However, relative FDOM fluorescence yield was strongly inversely correlated to Sg (Fig. 5D) suggesting that the most weakly-colored CDOM and detritus (low Sg) had the highest relative fluorescent yield. Note, negative Sg values as shown in Fig. 5D indicate that agp412<agp440 (which only occurred in the clearest, most oligotrophic waters with extremely low chlorophyll and low suspended particulate matter, such as in the Canada Basin). 3.3 Optical scattering measurements

Optical scattering properties were elevated in the under-ice algal bloom. For example, the acid-labile backscattering-- that backscattering associated with suspended

calcium carbonate-- while generally low, had the most elevated values in the under-ice algal bloom (Fig. 6A). Total particulate backscattering (Fig. 6B) was also elevated within the under-ice algal bloom, such that bb' only represented, at most, 10% of the total particulate backscattering (Fig. 6C). Total scattering in the under-ice algal bloom reached values as high as 2m-1 with an order of magnitude decrease in the Canada Basin (Fig. 6D). Backscattering probability (b~b = bbp/bp; indicative of all minerogenic scattering, but not just for calcium carbonate) had values of 1% in the under-ice bloom and values up to 3-4% in the Canada Basin and in the open, warm waters south of the ice margin. Low values were seen in the southeastern portion of the study area (Fig. 6E). Waters with highest particle scattering (Fig. 6D) also had highest particle beam attenuation (Fig. 6F). Indeed, particle backscattering, particle scattering and particulate attenuation all showed similar patterns (compare relative patterns in Figs. 6B, D and F). 3.4 Chemical and biogeochemical observations

Vertical sections of PIC, POC and BSi through the under-ice algal bloom all were elevated in regions where the Winter Water reached closest to the surface (Fig. 7). PIC showed elevated values just above the sediments at about 50m, near the shelf break at the most northwesterly position of the cruise, as well as in the region close to the coast of Alaska. Ice-free waters away from the ice edge had low PIC concentrations and elevated POC and BSi. The most elevated PIC in surface waters was seen in the shallowest part of the sections, in ice-free waters, for both legs shown in the section. (Fig. 7A). POC and BSi were highly elevated under the ice, and had a subsurface peak which extended southeast of the ice edge, in the same area where PIC was low (Fig. 7A-C). Deepest waters along the section had lowest values of POC and BSi.

Ratios of PIC:POC were extremely low (~0.25%) in surface waters at the ice edge and within the under-ice algal bloom whereas the ice-free waters over Hannah Shoals (with

elevated PIC; Fig. 7A) had PIC:POC ratios of 1.5-2% (Fig. 8A). Highest PIC:POC ratios were found at 100-150m depth at the shelf break. POC:PON molar ratios of the particulate material in the under-ice algal bloom were elevated above Redfield (10-15) (Fig. 8B). POC:Chl a ratios in the under-ice algal bloom were generally low (50-100 except at the northwest corner of the survey area where there was a region with clearly elevated POC:Chl a ratios (Fig. 8C). Highest POC:Chla ratios in surface waters were found off the NW coast of Alaska.

The nitrate section through the under-ice bloom showed clear evidence of drawdown in the top10-20m as well as evidence of elevated nitrate at the shelf break which was associated with cold Winter Water (Fig. 9A). Silicate drawdown in surface waters also occurred in the under-ice bloom but concentrations of 30 pM silicate were observed at the surface at station 54 (Fig. 9B). Residual nitrate (defined as the nitrate concentration minus the silicate concentration) (Townsend et al., 2010) showed negative values of -20 to -40pM under the ice, emphasizing the strong reduction of nitrate relative to silicate (Fig. 9C).

Primary production and calcification showed highest values within the under-ice bloom. While the primary production rates were high on any standard (~400 mg C m-3 d-1; Fig. 9A; Table 2), the calcification rates were only 0.4% of the primary production values (Fig. 10B). Carbon fixation dropped off rapidly in the ice free waters, as well. Primary production and calcification both attenuated with depth. Integrated primary productivity rates in the bloom approached 3g m-2 d-1 whereas integrated calcification was ~10 mg m-2 d-1 (Table 2). The C:P ratio in the bloom averaged 0.33% over the water column. Chlorophyll-normalized primary production was 5 gC (g Chl)-1 d-1 (Table 2). Integrated calcification normalized by integrated chlorophyll was also low, 0.02 gC (g Chl)-1 d-1 (Table 2). Intrinsic, carbon-specific growth rates for POC (ppoc) approached 0.4 d-1 (Fig. 10C) while ppic

approached 1d-1 (Fig. 10D). Integrated chlorophyll biomass in the center of the under-ice bloom was 490 mg m-2 (Table 2). 3.5 Microscopy

Scanning electron microscopy results from the under-ice algal bloom showed strong dominance by diatoms, with Chaetocerous sp, Fragilariopsis sp. and Thallasiosira sp. (Fig. 11A-F). Coccoliths of the coccolithophore, Emiliania huxleyi were also observed. While the coccoliths were >4um in diameter (which typically is a trait more characteristic of the type B morphotype) (Poulton et al., 2011), there were traits that align with Type A morphotypes--the distal shield was larger than the proximal shield, the radial elements were robust, and the elements in the central area were curved (Fig. 11G, H) (Poulton et al., 2011). 4 Discussion

4.1 Size of bloom based on continuous underway measurements

The hydrographic measurements made by our surface underway system clearly showed the coolest waters (-1.6°C under the ice with salinities of 30-31), characteristic of Arctic Winter Water (Coachman and Aagaard, 1974; Coachman and Barnes, 1961; Rudels et al., 1990; Rudels et al., 2004). Based on the continuous surface hydrographic data, the maximum horizontal length-scale of the Winter Water mass was about 150km (Fig. 1), close to the length of the elevated chlorophyll concentration for the bloom (~140km; Fig. 2A).

4.2 Interpreting the absorption properties of the under-ice algal bloom

The particulate absorption at 440nm showed similar trends to the chlorophyll concentration, as expected (Fig. 2B), however, the chlorophyll specific absorption at 440nm

-5 2 -1

(the absorption cross section, a*p440) averaged 0.027(SE = ±9.6x10" ) m (mg Chl) over the study region (Figs. 2E; 10A), well within the range observed for phytoplankton (Bricaud et al., 1983), in particular diatoms (Bricaud et al., 1988; Sathyendranath et al., 1987). Such variability is known to be a function of pigment composition, cell size and internal

chlorophyll concentration. The predominance of low values of the absorption cross section (ap*440; Fig. 2E) suggest that the pigments were highly packaged, characteristic of large diatoms. However, the ap*440 values observed near the coast (0.10-0.23 m2 (mg Chl)-1) were far higher than expected for phytoplankton and these may have resulted from other sources of absorbing particulate matter, or the presence of photoprotective pigments. It should be noted that the two cruise legs with such high ap*440, southeast of Hanna Shoals were performed at the end of the cruise (calendar day 204-205; July 23-24), almost one month after the earlier section through the under-ice bloom, and water temperatures had warmed 3.5-4°C and light levels would have been higher, making phytoplankton cells more high-light-adapted.

The shape of the particulate absorption spectrum contains information on phytoplankton size. Ciotti et al. (2002) normalized the spectral absorption at a given wavelength, X (aph(X); m-1), by the mean absorption across the visible spectrum (<aph>) and demonstrated that, for 440nm light, the closer the value of aph(440)/<aph> to 1.5, the greater the proportion of microplankton in the sample and alternatively, the closer the value to 3, the larger proportion of smaller phytoplankton. Using this technique, they were able to discriminate between picophytoplankton (<0.2pm), ultraphytoplankton (2-5pm), nanophytoplankton (5-20pm) and microphytoplankton (>20pm). Moreover, they could model the normalized phytoplankton absorption of any assemblage using combinations of just the micro- and pico-phytoplankton spectra. Ciotti et al. (2002) used the methanol extraction technique (Kishino et al., 1984) to unequivocally measure the spectral absorption of particulate detritus which they then subtracted from the total particulate absorption spectrum to calculate phytoplankton absorption (aph(X)).

Unfortunately, we had no methanol extraction data so we had to use other means to ascertain if ap(X) approximated aph(X). In over half of the study area, >95% of absorption at 412nm was from dissolved material, hence absorption by particulate detritus was minimal

(thus, at 440 nm, ap was probably close to aph). Particulate absorption at 412nm was only significant in the under-ice algal bloom (see Fig. 2F where ag4i2/apg4i2 was 20-50%) as well as close to the Alaskan coast. However, the carbon:chlorophyll ratio in the bloom was ~50 (Fig. 8C), more representative of actively growing phytoplankton than assemblages dominated by particulate detritus (Geider, 1987). Further, the plots of ap440 and ap412 versus chlorophyll showed that particulate absorption of phytoplankton was virtually all associated with viable, chlorophyll-containing phytoplankton, not detritus (Fig. 3; Table 1). The reduced correlation between chlorophyll a and ag412 (Fig. 3D) is consistent with other sources of ag412 than just phytoplankton, such as terrestrial sources.

In short, while the ap412 was elevated in the bloom, it strongly covaried with chlorophyll, indicative of minimum amounts of particulate detritus. We conclude that detrital absorption at 440nm was negligible in the bloom such that ap(440) would have approximated aph(440). This allowed calculation of aph(440)/<aph> (Ciotti et al., 2002) along the cruise track (Fig. 12A) as well as the resultant fraction of picoplankton that would have been expected in the assemblages (Sf; Fig. 12B). Values of aph(440)/<aph> varied from 1.5-2, suggesting that the entire study area was strongly dominated by microplankton (Ciotti and Bricaud, 2006; Ciotti et al., 2002). This conclusion was entirely consistent with the scanning electron microscopy results, as well (Fig. 11). 4.3 CDOM, FDOM andfluorescence yield

These results suggest that under-ice phytoplankton were an important source of FDOM and that FDOM fluorescence accounted for only ~30% of the variance in CDOM. The change in dissolved absorption between 412 and 440nm, normalized by the change in wavelength, Sg (Roesler and Perry, 1989) has been suggested to vary as a function of the source of CDOM. Steeper slopes typically are more representative of lignin-rich, terrestrially- derived materials (Stedmon and Markager, 2001). In this study, Sg values of

0.01 in the bloom were more representative of low-colored, autochthonous, marine CDOM (Carder et al., 1989), as opposed to highly-colored, terrestrially-derived CDOM (Stedmon and Markager, 2001)(Fig. 4A). In the Canada Basin, absorption at 440nm was greater than at 412nm, likely due the extremely low CDOM concentrations there.

The CDOM fluorometer used here had excitation/emission peaks of 370 and 460nm, respectively. These correspond roughly to the red-shifted, "Peak C", humic-like, CDOM fluorophore originally described by Coble (1996). FDOM can be produced by a variety of different compounds, and the fluorescence yields can be affected by a multitude of physical and chemical factors (including pH, temperature, hydrogen bonding, metal binding, etc.). Biology is also involved since bacteria (Rochelle-Newall and Fisher, 2002) and phytoplankton (Romera-Castillo et al., 2010) are both sources of FDOM. Our results showed no relation of relative fluorescence yield to water temperature (results not show). Moreover, the data support the conclusions of Romera-Castillo et al. (2010) that phytoplankton are producers of FDOM (Fig. 5B). The strong linear relation between relative fluorescent yield and Sg was not expected but shows a highly predictable continuum of relative fluorescence yield across these under-ice waters. With such a strong relationship, these results would suggest an alternative way to predict Sg using FDOM fluorescence yield. 4.4 Significance of bloom magnitude

The levels of productivity found in the under-ice bloom represent some of the highest levels found in nature (Balch et al., 1992; Morel and Maritorena, 2001). Integrated primary production rates in the bloom center (2.86g m-2 d-1) were well above under-ice rates observed previously (Gosselin et al., 1997), yet the water column assimilation efficiency of 5.8 gC (g chl)-1 d-1 was still well below the theoretical maximum for phytoplankton (Falkowski, 1981). In an integrated sense, calcification rates represented only 0.33% of the integrated productivity (Table 2). Similarly, in a pure coccolithophore culture (or dense

coccolithophore blooms in nature), one would expect a chlorophyll-normalized calcification of ~1.19 gPIC fixed (g chl)-1 h-1 (or 28.6 gPIC fixed (g chl)-1 d-1)(Balch et al, 2007). The chlorophyll normalized values observed in this study (0.02 gPIC fixed (g chl)-1 d-1; Table 2) were three orders of magnitude less than this, simply due to the dominance of diatoms (Table 2).

4.5 Coccolithophores were present in the under-ice algal bloom

Optical scattering, PIC, SEM and calcification results demonstrated the presence of coccolithophores in the under-ice algal bloom but not as a dominant part of the community. Values of acid-labile backscattering, compared to subpolar or subtropical waters, were low, approaching the sensitivity of the technique and certainly indicative of a non-bloom, background population of coccolithophores (Balch et al., 2004; Balch et al., 2011; Balch et al., 2005; Balch et al., 1996). As a percentage of the total backscattering (Fig. 6C; 4-6%) this is lower than the typical percentage of backscattering typically attributed to coccolithophores, even in oligotrophic gyres (Balch et al., 2010). PIC:POC ratios were characterized by low values (<0.2%) in the under-ice bloom, too, again suggestive of low biogeochemical impact of coccolithophores in this under-ice bloom.

In general, the distribution and fixation of PIC mirrored that of POC, and it appeared that coccolithophores were responding in the same manner to increased light penetration through the ice as were the diatoms and other algal groups (Figs. 10). Given the elevated nutrients found the Arctic Winter Water, all groups were released from light limitation together. Barber and Hiscock (2006) described algal communities in which all the phytoplankton groups responded with increased growth rate to enhanced iron, they also observed that the picoplankton response was more muted than that of the diatoms because picoplankton were selectively grazed down by the fast-responding microzooplankton (Landry, 2002). In the case of the under-ice algal bloom, it is possible that enhanced growth

of coccolithophores (by release from light limitation) was also muted by grazing by fast-responding protistan predators. Indeed, standing stocks of POC and BSi showed high-covariance under the ice (Fig. 7); both showed a subsurface "tongue" that extended out from under the ice on the northern part of the under-ice algal bloom while PIC, on the other hand, was actually reduced in that feature and greatest PIC concentrations were observed almost 100km away from the ice edge, in ice-free water of 3-4oC (Figs. 1 and 5). The appearance of E. huxleyi in waters of the Southern Ocean also occurs at such temperatures (Balch et al., 2011; Cubillos et al., 2007; Gravalosa et al., 2008; Holligan et al., 2010; Mohan et al., 2008). Elevated concentrations of PIC and BSi also were observed just above the sediments near the shelf break, at 40-60m depth, suggesting that resuspension also may have been important source of these biogenic mineral particles. This could be seen in the PIC:POC ratios which were extremely low in the under-ice algal bloom (0-0.2%) but elevated with depth, with highest values observed in the 100-150m-deep, northwest portion of the under-ice algal bloom (at 3%; Fig. 8A). As noted above, resuspension may have influenced this ratio at the shelf break. Alternatively, preferential remineralization of POC over PIC could also have produced this pattern (Honjo et al., 2008). The saturation states for calcite (Qcalcite) and aragonite (Qaragonite) showed that waters were saturated for calcite and aragonite in this region (Bates personal communication), thus dissolution of calcite and aragonite would have been unlikely.

4.6 Nutrient limitation and a mismatch in carbon standing stocks versus rates of carbon fixation

Molar ratios of C:N were, for the most part, greater than the 6.6 C:N Redfield ratio (Redfield et al., 1963) in the top 20m of the under-ice feature (Fig. 8B) but given that nitrate levels under the ice had been depleted to micromolar levels (Fig. 9A), then it would be expected that the populations would have shown signs of nitrogen limitation. This

interpretation was buttressed by the low POC:Chlorophyll ratios in under-ice algal bloom waters where nitrate depletion had only occurred in the upper 10m of the water column. Indeed, regions of elevated POC:PON corresponded to high POC:Chlorophyll, for example at the northwest corner of the study area. Deeper Winter Water under the ice was rich in nitrate and silicate as evidenced by the covariance of the -1.6 isotherm and isolines of silicate and nitrate (Fig. 9A and B). Moreover, Winter Water was elevated in silicate relative to nitrate (by 40pm at 60m depth; Fig. 9C). In regions where chlorophyll and POC were highest, the residual nitrate was closer to zero, suggesting that the phytoplankton community uniformly drew down nitrate and silicate towards zero. This could have resulted from either adjustments of diatom cell quotas as they consumed the silicate (Baines et al., 2011) or depletion of nitrate by the majority diatom assemblage and minority, non-diatom phytoplankton plus silicate depletion by just the diatoms, in such a way that both were depleted together. What seems clear is that where physical processes brought Winter Water upwards under the ice, the release of the phytoplankton from light limitation by melt ponds then allowed nutrient drawdown to occur such that all algae began to show signs of both nitrogen limitation (increased C:N) and silicate limitation (reduced silicate with increasing BSi).

Overall, the history of the bloom formation caused a mismatch in the standing stocks and rates of fixation of particulate organic and inorganic carbon. Highest carbon fixation (for photosynthesis and calcification) was observed further into the ice, in regions where the standing stocks of POC and chlorophyll were not the highest. Elevated standing stocks of PIC and POC were found in waters where nutrients had already been depleted whereas highest productivity rates were seen where nutrients had not yet been depleted.

4.7 Conclusion- Under-ice coccolithophores and global change

The aforementioned observations demonstrate that coccolithophores were present in the under-ice algal bloom but that their relative contribution to carbon cycling was minor compared to the carbon fixation by the diatom-dominated assemblages. Coccolithophore presence was observed analytically (ICP-OES), optically and microscopically. The low numbers of coccolithophores in the under-ice bloom is consistent with previous observations of algal communities in ice-covered waters.

A comparison of the calcification rates in the under-ice algal bloom of ICESCAPE with those rates measured on polar coccolithophores by Charalampopoulou et al. is informative. Highest total calcification rates at their ice edge station was ~0.6 mg C m-3 d-1 at 20m depth, some 40% of what we observed for the under-ice algal bloom (1.5 mg C m-3 d-1). Charalampopoulou et al. (2011) also measured calcification in the marginal ice zone and a Svalbard fjord and found calcification rates 20-75 times lower than the calcification rates in Chukchi Sea under-ice algal bloom (0.02-0.07 mg C m-3 d-1).

This data set also provides the opportunity to compare the growth rates of the phytoplankton (^poc in Fig. 10C, clearly dominated by diatoms) to rates predicted in the classic treatise by Epply (1972) on the effects of temperature and phytoplankton growth. Using his equation 1 (or 1a), the predicted maximal growth rate of phytoplankton in Winter Water of -1.6°C would have been 0.77 doublings d-1 (= 1.11 d-1 specific growth rate). The highest POC-specific growth rates that we observed (~0.35 d-1) were ~30% of the maximum growth rate predicted by the Eppley (1972) equation, possibly reflecting the effect of the previous nitrate draw-down in the surface waters. Moreover, if the intrinsic rates of increase of PIC were coupled to coccolithophore growth rates, as shown previously (Fritz and Balch, 1996), then the observed PIC-specific growth rates (0.9 d-1) would have been much closer to

the maximal growth rates predicted by Eppley (1972). This would have been expected anyhow given the well-known observations that coccolithophores such as E. huxleyi can maintain higher growth rates at lower nitrate concentrations than, for example, diatoms, due to their significantly lower half saturation constants for nitrate uptake (Eppley et al., 1969; Margalef, 1978).

The hypothesis central to the formation of the under-ice algal bloom is that melt ponds on top of the ice allowed light to penetrate the meter-thick ice, thus releasing under-ice algae from severe light limitation (Arrigo et al., 2012). One can then ask how long it would have taken for phytoplankton with chlorophyll concentrations below the limit of detection to grow to the levels seen in the under-ice bloom. This also provides insights whether the bloom could have formed in place or was somehow advected and concentrated there. Assuming a background concentration of chlorophyll of 0.01 mg m-3 prior to the bloom (0.04 mg m-3 is typically used as the limit of detection for the fluorometric chlorophyll technique using standard practices and volumes (Parsons et al., 1984)), then using the logistic growth equation, and the above pPOC of 0.35 d-1 , then it would have taken ~23d to reach a chlorophyll concentration of 30mg m-3 (i.e. time (days) = ln[30/0.01]/0.35), assuming no grazing or other loss terms. Thus, for the above hypothesis to be consistent with our observations, than the melt ponds would have had to be present for at least three weeks prior to our measurements for there to be sufficient time for the high chlorophyll levels to form. Moreover, it is assumed that once nitrogen became limiting, then the diatom growth would have slowed (alternatively, grazing might have also reduced the net growth below maximal growth rates).

This same calculation can be done for PIC, however, in this case, the estimated intrinsic calcification rate, pPIC , was greater (0.9 d-1; Fig. 10). Even beginning with just 10 E. huxleyi cells L-1 (each containing15 coccoliths of 0.2pgPIC (Balch, 1991; Balch et al., 1991)

(or 0.25pmoles PIC/cell) for a total of 2.5pmoles PIC L-1), then after 23d, the water would have contained several mmoles PIC L-1. Such was not the case, however, with PIC levels only 0.15 pmols L-1 in surface waters of the under-ice algal bloom (Fig. 7), thus suggesting other forces were acting on the coccoliths (sinking, grazing and/or dissolution) to keep concentrations low, or simply that such PIC-specific growth rates of coccolithophores were not sustained for 23 d (most likely).

The under-ice bloom observations first described by Arrigo et al. (2012) were unique regarding the spatial scale and magnitude of such blooms that could exist under 1 meter-thick ice. Results presented here represent the first observations showing the presence of low, but measurable, numbers of coccolithophores, >100km into the thick polar ice cap in a massive under-ice feature. Remote sensing results of Smythe (2004) suggest that large scale coccolithophore blooms are becoming more abundant in ice-free polar waters. What has been missing is whether there are populations far into the ice sheet, capable of growth given the proper conditions of nutrients and light. Our results unequivocally show that there is a resident population of coccolithophores such as E. huxleyi that are poised to grow and calcify given their release from light limitation as melt ponds form and the polar ice cap melts. Given intrinsic calcification rates, it is not known why there are not more coccoliths present in these waters. It has been shown that pH has a strong influence on coccolithophores in polar waters (Charalampopoulou et al., 2011). Ocean acidification will cause the largest decline in carbonate saturation states in high latitude, polar waters (Feely et al., 2009), especially after the polar ice cap melts, allowing more efficient air-sea gas equilibration. A key point, however, will be the balance that warming/release from light limitation will play in encouraging coccolithophore growth in Arctic waters versus the inhibitory role that increasing ocean acidification will have on coccolithophore production and growth in polar waters, in the face of climate change.

Acknowledgements

We wish to thank the captain and crew of the USCGC Healy for their expert ship handling in acquiring the data described herein. Gary Lain (Scripps Inst. of Oceanography, La Jolla, CA) expedited the handling of isotopes before and after the cruise for the photosynthesis and calcification measurements. Quincy Allison, Sue Tolley and Mike Gaunce (Earth Science Project Office, NASA AMES) oversaw the transportation of all scientists, equipment and chemicals to the ship. The Scripps Shipboard Technical Support Group ran nutrient samples and provided logistical support in shipboard sampling (Jim Swift, Susan Becker and Scott Hiller). Dave Drapeau (Bigelow Laboratory for Ocean Sciences) provided logistical support pre- and post cruise. Kevin Arrigo (Stanford University) performed the chlorophyll analyses used here. Ocean Data View was used to analyze the data and plot the figures (Schlitzer, R., Ocean Data View, http://odv.awi.de, 2014). Colby College provided travel support to M.W.S to participate in the cruise. Support for this research was provided by the Ocean Biology and Biogeochemistry Division at NASA (NNX10AT67G and NNX11AO72G and NNX11AL93G) to W.M.B. We wish to thank A. Ciotti (Centro de Biologia Marinha, Universidade de Sao Paulo, Sao Sebastiao, SP, Brasil) plus two anonymous reviewers who read and commented on an earlier version of the manuscript.

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Fig.1- (A) General study area of cruise #1101 of the USCGC Healy 1101. (B) Cruise track for Healy #1101 in vicinity of under-ice bloom. Color of data points represents calendar day during 2011 that positions were occupied. Station numbers for every fifth station are shown in yellow. Blue dashed lines show southerly extent of ice sheet as estimated from MODIS imagery, as given by Arrigo (2012). Blue numbers next to blue dashed lines give the specific calendar day that the ice edge was estimated. (C) Along-track temperature (°C) from ship's flow-through seawater system (depth = 5m) directed through Balch flow-through system. The position of the ice edge on day 189 (July 8, 2011) is designated with the blue dashed line. Isopleths of temperature calculated using Ocean Data View are shown in black contour lines. The white contour line is the position of the -1°C isotherm, the approximate location where Winter Water reached a depth of 5m. The position of the ice edge on day 182 is shown with the blue dashed line for reference. (D) Along-track surface salinity data with same temperature and ice edge isopleths as shown in panel C.

Fig. 2- Underway chlorophyll a, absorption and fluorescence properties of the under-ice algal bloom. (A) Chlorophyll a calculated from underway chlorophyll fluorescence, calibrated using shipboard discrete chlorophyll measurements (ug L-1); (B) Particulate absorption at 440nm (ap 440; m-1) calculated as the difference between total absorption and absorption following filtration of <2pm diameter particles; (C) Chlorophyll-specific particulate absorption at 440nm, a*p440 [m2(mg Chl. a) -1] calculated by normalizing values of ap440 by the chlorophyll concentration; (D) Dissolved (<0.2um) absorption at 412nm normalized to total absorption (particulate and dissolved material) at 412 nm (a g412/a Pg412); (E) FDOM fluorescence (calibrated to quinine sulfate); (F) Relative FDOM fluorescence yield calculated as the FDOM fluorescence normalized to absorption of particulate plus dissolved matter at

412nm. In all panels, isopleths of temperature are shown in black contour lines. The white contour line is the position of the -1°C isotherm, the approximate location where Winter Water reached a depth of 5m. All panels also show the position of the ice edge on day 182 as a blue dashed line for reference.

Fig. 3- (A) Particle absorption at 440nm (m-1; calculated as the difference between total absorption - dissolved absorption) plotted against the concentration of chlorophyll a (mg m-3). (B) Particulate absorption at 412nm plotted against the concentration of chlorophyll a (mg m-3). (C) Particulate absorption at 412nm plotted against particulate absorption at 440nm. (D) Dissolved absorption at 412nm plotted against the concentration of chlorophyll a (mg m-3). Least-squares linear regression lines and fit equations given in panels. Values in square brackets represent the standard error of each of the fitted coefficients. Degrees of freedom (DF), F statistic for the least-squares fit line and probability (P) of estimating this value of F by chance is also given. Statistics also summarized in Table 1.

Fig. 4- Optical properties of the study area. (A) Slope of the absorption spectrum between 412nm and 440nm for dissolved material (<0.2um; Sg (per nm)). (B) Slope of the absorption spectrum between 412nm and 440nm for dissolved plus particulate material (Spg (per nm)). (C) CDOM fluorescence (FDOM; QSU). (D) Fluorescence yield of FDOM (calculated as FDOM/ag412; relative units). In all panels, isopleths of temperature and the ice edge on day 182 are shown for reference as in Fig. 1C.

Fig. 5- Variability in FDOM and fluorescence yield. (A) FDOM variability as a function of ag412. (B) FDOM variability as a function of chlorophyll concentration. (C) Relative fluorescence yield of FDOM and detritus as a function of the concentration of chlorophyll a.

(D)Relative fluorescence yield of FDOM and detritus shown as a function of the slope of the absorption spectrum between 412 and 440nm (for both dissolved matter, Sg). Least-squares linear, power or exponential regression lines and fit equations given in all panels. Values in square brackets represent the standard error of each of the fitted coefficients. Coefficients of correlation, degrees of freedom (DF), F statistic for the least-squares fit line and probability (P) of estimating this value of F by chance is also given. See also Table 1.

Fig. 6- Underway scattering and attenuation properties of under-ice algal bloom. (A) bb's31 (m-1); (B) bbptot531(m-1); (C) bb'531/bbptot531(unitless); (D) bp531 (m-1); (E) backscattering probability (b~b = bbp/bp; unitless); and (F) beam attenuation at 531nm (cp531; m-1). In all panels, isopleths of temperature and the ice edge on day 182 are shown for reference as in Fig. 1C.

Fig. 7- Discrete measurements of PIC and POC through under-ice algal bloom. (A) PIC (pM L-1); (B) POC (pM L-1) and (C) BSi. The -1.6°C isotherm is indicated on each section to show the position of the Winter Water. Two vertical red lines represent where the ship course changed at the offshore extremes of each transect. Blue bar over panel shows extent of ice cover during the transect.

Fig. 8- Discrete measurements of carbon and chlorophyll ratios through under-ice algal bloom. (A) PIC:POC; (B) POC/PON (molar) and (C) POC/Chlorophyll a(g:g). The -1.6°C isotherm is indicated on each section to show the position of the Winter Water. Two vertical red lines represent where the ship course changed at the offshore extremes of each transect.

Fig. 9- Nutrient sections through under-ice algal bloom. (A) Nitrate concentration (pM); (B) Silicate concentration (pM) and (C) Residual nitrate concentration (nitrate-silicate; pM). The -1.6°C isotherm is indicated on each section to show the position of the Winter Water. Two vertical red lines represent where the ship course changed at the offshore extremes of each transect.

Fig. 10- Carbon fixation and carbon-specific growth rates for particulate organic and inorganic carbon. (A) Average photosynthesis (mg m-3 d-1); (B) Average calcification (mg m-3 d-1); (C) pPOC (d-1); (D) pPIC (d-1). The -1.6°C isotherm is indicated on each section to show the position of the Winter Water. Two vertical red lines represent where the ship course changed at the offshore extremes of each transect. Blue bar over panel shows extent of ice cover during the transect.

Fig. 11- Scanning electron micrographs from under-ice algal bloom: station 56, 1.5m depth under ice: (A) Miscellaneous diatoms at low magnification, (B) Higher magnification view showing Fragilariopsis sp., Chataetocerous sp. and Thalassiosira sp. diatoms (possibly T. nordenskioeldii), (C-E) Fragilariopsis sp. plus two species of Thalasiossira sp. diatoms (possibly T. hyalina and T. nordenskioeldii), (F) Chaetocerous sp. diatom, (G, H) Emiliania huxleyi detached coccoliths. Scale bars shown for reference in each panel in lower right.

Fig. 12- (A)Particulate absorption at 440 nm normalized to mean absorption from 412-715nm calculated according to Ciotti et al. (2002) calculated according to their equation 1. (B) Fraction of phytoplankton that are picoplankton (Sf) calculated according to equation 3 of Ciotti et al. (2002) for measurements at 440nm. See text for details. In all panels, isopleths of temperature and the ice edge on day 182 are shown for reference as in Fig. 1C.

Figu re Dependent variable (X axis) Dependent variable Units Indepen dent variable (Y axis) Indepe ndent variabl e Units SE of Y predictio n Type of fit** Slope (m) SE slope intercept (b) SE intercept expon ent (c) SE expo nent r2 DF Fstat

3A Chlorophyll a mg m-3 ap440 m-1 0.0175 Linear 0.027 9.6x10-5 0.0023 4.28x10-4 na na 0.975 2072 80477*

3B Chlorophyll a mg m-3 ap412 m-1 0.0266 Linear 0.025 1.46x10-4 0.005 6.49x10-4 na na 0.933 2075 29077*

3C ap440 m-1 ap412 m-1 0.0183 Linear 0.926 3.20x10-3 na na na na 0.975 2073 80993*

3D Chlorophyll a mg m-3 ag412 m-1 0.0334 Linear 0.004 1.69x10-4 0.0966 7.92x10-4 na na 0.233 2231 676*

5A ap412 m-1 FDOM QSU 0.222 Linear 3.829 0.125 2.326 0.014 na na 0.305 2163 949*

5B Chlorophyll a mg m-3 FDOM QSU 0.0261 Power 2.796 0.004 na na 0.051 8 9.15 x10-4 0.597 2163 3208*

5C Chlorophyll a mg m-3 FDOM/a g412 QSU-m 0.112 Power 25.627 0.152 na na 0.107 2 0.00 39 0.252 2161 759*

5D Sg 412-440 nm-1 FDOM/a g412 QSU-m 0.109 Exponen tial 25.649 6.11x10-2 na na 39.55 9 0.33 4 0.867 2154 14022*

* Significance = P<0.001

** Different model fits: Linear model Y=mX +b; Power model Y=mXc; Exponential model Y = mecX where e is Euler's number, 2.71828

Table 1- Statistics of least square fits shown in Figures 10 and 12 this study.

Table 2 Integrated chlorophyll a plus mean primary production and calcification (measured with the microdiffusion technique in triplicate) for stations shown in Fig. 1B. Carbon fixation normalized to total phytoplankton biomass (as chlorophyll) is also given for both photosynthesis and coccolithophore calcification. "Zero" values for calcification indicate that calcification was statistically not different from zero._

Station Date

Year Time lay

Latitude Longitude Int.

Int. P

Int. C(Int.

Int. P Int. C

(Int. (Int.

Chl Chl a)

a)"1 1_

(hh.mm (deg. E.) (deg. W) UTC)

(mg m~2 d"1) (%)

(gC g Chl-1 di)_

46.01 7/3/2011 184.99 23:45 72 02 165.35

23.1 211 8

57.02 62.01

61.02 90.02 99.01 100.01 103.01 166 01

7/5/2011

7/6/2011

7/7/2011

7/7/2011

7/8/2011

7/9/2011

7/11/2011

7/12/2011

7/13C011

7/15/2011

7/23/2011

186.13 167.10 188.07 188.97 189.91 190.91

192.12 193.05

194.13 196.10 201.95

72.63 73.17 73.72 73 13 72.13 72.24 72 96 73.38 73.70 72.60 71 35

163.73 163.43 1BB.2B 167.25 163.79 162.29 160.72 160.06 160.28 153.34 130.13

262.2 490.4 69.7 213 7 137.6 166.6 147 3 3.5 26.1 12.9 154 5

1086.7 1.24 2659.7 9.57 182.1 1300.3 640.2

11.53% 755 0870

0.00 1 67 3.12 1.07 1 55 0.23 2.79 0.00 0 76

0.11% 0.33% 0.00% 0.13% 0.49% 0.11% 4.12°/. 0.47% 4.40% 0.00% 0.10°/.

4.14 5.B3 2.61 594 4.65 5.91 026 5.79 2.43 4.17 5 14

0.005 0.020 0.000 0008 0.023 0.006 0.011 0.027 0.107 0.000 0005

Fig- 6

ft 9005

ft 0004

ft 0003

ft 0002

0.0001

* i 0 1 fA 0

^bp tot53l(m"1)

■wl --41 M / 7 &

■ flflro i £ S F4 A,

bp tot53 i(m-1)

Ï I <

ft fti

/tfJ'W /rfirw /55°^

HL lUCa r _

Cp tot53

0°W 165°W 169*W 155°W

Longitude

70W 170°W

165°W

¡6Q*W

i55°W

PIC (jimol L-1)

0 299 409 600 800

Section Distance (km)

..... r—rr—^

VJ .. J m \ ■If 5

W , Resid N03 (nM )

0 200 400 600 800

Section Distance (km)

Fig. 10

Fig. 11

7-Í^N

2.75 IS

Sf (frac, pico)

170°W 165°W 160°W 155°W

Longitude

70° W I65°W 160°W ¡55*W

Fig. 12