Under-ice Bloom Research Articles

In the cold light of day: the potential importance of under-ice convective mixed layers to primary producers

Temperate lakes are ice covered for much of the year; however, winter lake conditions have not been well studied and are undergoing rapid change. Using data collected during ice-on periods from 4 north-temperate water bodies, we report observations of stable surface layers, solar-induced convective mixed layers, and their potential impacts on phytoplankton. The convective mixed layer is defined as the region where the convective Richardson number (Ri) is ≤1. In the absence of a convective mixed layer, peaks in chlorophyll a were near the ice–water interface. Light conditions here seemed sufficient to support phytoplankton biomass accrual in the short-term in 50% of our measurements, although snow depths >13.5 cm may lead to light limitation. When a convective mixed layer was present, light conditions were sufficient for biomass accrual in 37.5% of cases. The frictional timescale for damping averaged 15 minutes, indicative of a lack of mixing at night. Convective mixing depths and velocity increased as snow declined, and results demonstrated the potential for rapid convective mixed layer deepening (up to 6.6 m h−1), underscoring the highly dynamic physical environment under ice. Although declining periods of ice cover have been subject to much attention, changes in snow cover may have equally important implications for primary producers and the potential for under-ice blooms. This link between physics and biology must be further explored to better understand how changing winters will affect water bodies.

Adaptations to photoautotrophy associated with seasonal ice cover in a large lake revealed by metatranscriptome analysis of a winter diatom bloom

Abstract There is growing recognition that winter is an important season for growth of photoautotrophs in ice-covered freshwater environments. Exemplifying this are expansive under-ice blooms of filamentous diatoms in Lake Erie. Here we describe a metatranscriptome constructed from a phytoplankton community dominated by filamentous diatoms. As expected, a high percentage (> 73%) of the sequences with BLAST hits to nucleotides or proteins in National Center for Biotechnology Information databases were associated with photosynthetic algae of which the majority were diatoms, mainly Aulacoseira spp. and Stephanodiscus spp. which was confirmed by analysis of 18S rRNA gene transcripts and microscopy. Consistent with the winter growth environment, psychrophilic and low-light adaptations were observed. Prominent among adaptations to cold were transcripts for genes involved in biosynthesis of unsaturated fatty acids, which were consistent with expected increased membrane fluidity at low temperatures. Reflecting the combined effect of low winter insolation and high light attenuation were an abundant complement of reads for light-harvesting antennae, mainly genes encoding fucoxanthin chlorophyll a / c proteins. The presence of virulence factors originating from oomycetes offers support for new hypotheses into the eventual decline of Lake Erie's winter diatom bloom. Whereas fungi were identified both through the metatranscriptome and by microscopy, dsRNA viruses of fungi were detected that may indirectly counter against fungal infection. This study demonstrates the utility of an environmental omics approach to yield insights underlying phototrophic life as well as the interactions of the entire microbial community in an extreme environment.

Variations in the proportions of melted sea ice and runoff in surface waters of the Chukchi Sea: A retrospective analysis, 1990–2012, and analysis of the implications of melted sea ice in an under-ice bloom

Retrospective analysis of apparent freshwater isotopic end-members through use of salinity–δ18O mixing lines for 15 research cruises from 1990 to 2012 indicates that the freshwater contributed by melted seasonal sea ice does not directly reflect the large change in seasonal sea ice extent in the Chukchi Sea observed over the past several decades. Instead the freshwater that appears to be contributed by melting sea ice relative to runoff is highly dependent upon cruise track (e.g. proximity to runoff) and sampling capabilities in sea ice (icebreakers sample waters with less melted sea ice). Although under certain circumstances, including later seasonal sampling and recurrent cruise tracks between years, increased melted sea ice in surface waters can be readily detected. This suggests a more ephemeral influence of melted sea ice on ecosystem properties despite the significant decadal changes in sea ice extent. As a recent case study, the freshwater component present in waters within an under-ice bloom in the Chukchi Sea reported by Arrigo et al. (2012), included a significant fraction (~10% or more) of freshwater, primarily from melted sea ice. These results suggest that this under-ice bloom, which extended more than 60km under solid ice still might be reasonably interpreted as being part of a continuum with other ice melt-associated blooms and not independent of sea ice retreat and dissolution.

Contrasted sensitivity of DMSP production to high light exposure in two Arctic under-ice blooms

In polar regions, low-light acclimated phytoplankton thriving under sea ice may be suddenly exposed to high irradiance when ice pack breaks or surface currents carry them into adjacent ice-free areas. Here we experimentally determined how rapid shifts in light regime affect the phytoplankton and the production of dimethylsulfoniopropionate (DMSP), an algal compound with antioxidant properties. Springtime experiments were carried out on under-ice phytoplankton blooms that exhibited different taxonomic assemblages in 2010 and 2011 within the Canadian Arctic Archipelago. Phytoplankton samples collected under-ice were incubated for 24h or less on the ice at irradiances mimicking what they would experience in an ice-free upper mixed layer. We observed contrasting responses between the two years. In 2010, exposure of the under-ice centric diatom bloom to high irradiance consistently resulted in a rapid chlorophyll a decrease accompanied by a quasi-total release of the intracellular DMSP. In 2011, only the first experiment conducted with a low biomass flagellate-dominated assemblage resulted in a similar response. During the subsequent under-ice pennate diatom bloom, neither a chlorophyll a decrease nor a consistent increase in dissolved DMSP concentrations were observed during the experiments. The results demonstrate that the response of under-ice phytoplankton to the irradiance prevailing in ice-free water may vary depending on the developmental stage and taxonomic composition of the under-ice bloom. The rapid chlorophyll a decrease during the centric diatom bloom in 2010 and the associated DMSP exudation may potentially lead to a significant increase in DMS production via the microbial pathway at the ice edge.

The influence of winter water on phytoplankton blooms in the Chukchi Sea

The flow of nutrient-rich winter water (WW) through the Chukchi Sea plays an important and previously uncharacterized role in sustaining summer phytoplankton blooms. Using hydrographic and biogeochemical data collected as part of the ICESCAPE program (June–July 2010–11), we examined phytoplankton bloom dynamics in relation to the distribution and circulation of WW (defined as water with potential temperature ≤−1.6°C) across the Chukchi shelf. Characterized by high concentrations of nitrate (mean: 12.3±5.13µmolL−1) that typically limits primary production in this region, WW was correlated with extremely high phytoplankton biomass, with mean chlorophyll a concentrations that were 3-fold higher in WW (8.64±9.75µgL−1) than in adjacent warmer water (2.79±5.58µgL−1). Maximum chlorophyll a concentrations (~30µgL−1) were typically positioned at the interface between nutrient-rich WW and shallower, warmer water with more light availability. Comparing satellite-based calculations of open water duration to phytoplankton biomass, nutrient concentrations, and oxygen saturation revealed widespread evidence of under-ice blooms prior to our sampling, with biogeochemical properties indicating that blooms had already terminated in many places where WW was no longer present. Our results suggest that summer phytoplankton blooms are sustained for a longer duration along the pathways of nutrient-rich WW and that biological hotspots in this region (e.g. the mouth of Barrow Canyon) are largely driven by the flow and confluence of these extremely productive pathways of WW that flow across the Chukchi shelf.

The influence of sea ice and snow cover and nutrient availability on the formation of massive under-ice phytoplankton blooms in the Chukchi Sea

A coupled biophysical model is used to examine the impact of changes in sea ice and snow cover and nutrient availability on the formation of massive under-ice phytoplankton blooms (MUPBs) in the Chukchi Sea of the Arctic Ocean over the period 1988–2013. The model is able to reproduce the basic features of the ICESCAPE (Impacts of Climate on EcoSystems and Chemistry of the Arctic Pacific Environment) observed MUPB during July 2011. The simulated MUPBs occur every year during 1988–2013, mainly in between mid-June and mid-July. While the simulated under-ice blooms of moderate magnitude are widespread in the Chukchi Sea, MUPBs are less so. On average, the area fraction of MUPBs in the ice-covered areas of the Chukchi Sea during June and July is about 8%, which has been increasing at a rate of 2%yr–1 over 1988–2013. The simulated increase in the area fraction as well as primary productivity and chlorophyll a biomass is linked to an increase in light availability, in response to a decrease in sea ice and snow cover, and an increase in nutrient availability in the upper 100m of the ocean, in conjunction with an intensification of ocean circulation. Simulated MUPBs are temporally sporadic and spatially patchy because of strong spatiotemporal variations of light and nutrient availability. However, as observed during ICESCAPE, there is a high likelihood that MUPBs may form at the shelf break, where the model simulates enhanced nutrient concentration that is seldom depleted between mid-June and mid-July because of generally robust shelf-break upwelling and other dynamic ocean processes. The occurrence of MUPBs at the shelf break is more frequent in the past decade than in the earlier period because of elevated light availability there. It may be even more frequent in the future if the sea ice and snow cover continues to decline such that light is more available at the shelf break to further boost the formation of MUPBs there.

Characterizing the subsurface chlorophyll a maximum in the Chukchi Sea and Canada Basin

Throughout the Arctic Ocean, subsurface chlorophyll a (Chl a) maxima (SCM) develop every summer after the water column stratifies and surface nutrients have been exhausted. Despite its ubiquity, the SCM׳s distribution, seasonal dynamics, and productivity remain uncertain. Here we present the first in-depth analysis of the SCM in the Chukchi Sea and adjacent Canada Basin, drawing on data collected during the field program Impacts of Climate on the EcoSystems and Chemistry of the Arctic Pacific Environment (ICESCAPE). The SCM was significantly shallower on the Chukchi shelf (30m) than in the Canada Basin (56m), and in both regions was correlated with the euphotic and nitracline depths, suggesting an actively growing community maintaining its optimal position within the water column, consistent with previous work. The SCM was located significantly deeper than the net primary productivity (NPP) maximum, which averaged 15m depth. The development of the SCM on the Chukchi shelf appears tightly linked to under-ice blooms, beginning ~1 month prior to sea-ice retreat and reaching ~15m depth by the time ice retreats, beyond the range of satellite ocean-color sensors. A seasonal analysis of historical data from the region shows that the SCM deepens to ~30m by July and remains there throughout the summer, a depth that is consistent with previous studies across the pan-Arctic shelves. We employed a spectral model of light propagation through the water column to demonstrate that surface Chl a and CDOM play approximately equal roles in attenuating light, limiting euphotic depth, and therefore SCM depth, to ~30m, thus greatly limiting new production. If surface Chl a and CDOM were reduced, allowing greater light penetration, new production on Arctic shelves could potentially be 40% greater.

Large, omega-3 rich, pelagic diatoms under Arctic sea ice: sources and implications for food webs.

Pelagic primary production in Arctic seas has traditionally been viewed as biologically insignificant until after the ice breakup. There is growing evidence however, that under-ice blooms of pelagic phytoplankton may be a recurrent occurrence. During the springs of 2011 and 2012, we found substantial numbers (201–5713 cells m−3) of the large centric diatom (diameter >250 µm) Coscinodiscus centralis under the sea ice in the Canadian Arctic Archipelago near Resolute Bay, Nunavut. The highest numbers of these pelagic diatoms were observed in Barrow Strait. Spatial patterns of fatty acid profiles and stable isotopes indicated two source populations for C. centralis: a western origin with low light conditions and high nutrients, and a northern origin with lower nutrient levels and higher irradiances. Fatty acid analysis revealed that pelagic diatoms had significantly higher levels of polyunsaturated fatty acids (mean ± SD: 50.3±8.9%) compared to ice-associated producers (30.6±10.3%) in our study area. In particular, C. centralis had significantly greater proportions of the long chain omega-3 fatty acid, eicosapentaenoic acid (EPA), than ice algae (24.4±5.1% versus 13.7±5.1%, respectively). Thus, C. centralis represented a significantly higher quality food source for local herbivores than ice algae, although feeding experiments did not show clear evidence of copepod grazing on C. centralis. Our results suggest that C. centralis are able to initiate growth under pack ice in this area and provide further evidence that biological productivity in ice-covered seas may be substantially higher than previously recognized.

Massive regime shifts and high activity of heterotrophic bacteria in an ice-covered lake.

In winter 2009/10, a sudden under-ice bloom of heterotrophic bacteria occurred in the seasonally ice-covered, temperate, deep, oligotrophic Lake Stechlin (Germany). Extraordinarily high bacterial abundance and biomass were fueled by the breakdown of a massive bloom of Aphanizomenon flos-aquae after ice formation. A reduction in light resulting from snow coverage exerted a pronounced physiological stress on the cyanobacteria. Consequently, these were rapidly colonized, leading to a sudden proliferation of attached and subsequently of free-living heterotrophic bacteria. Total bacterial protein production reached 201 µg C L−1 d−1, ca. five times higher than spring-peak values that year. Fluorescence in situ hybridization and denaturing gradient gel electrophoresis at high temporal resolution showed pronounced changes in bacterial community structure coinciding with changes in the physiology of the cyanobacteria. Pyrosequencing of 16S rRNA genes revealed that during breakdown of the cyanobacterial population, the diversity of attached and free-living bacterial communities were reduced to a few dominant families. Some of these were not detectable during the early stages of the cyanobacterial bloom indicating that only specific, well adapted bacterial communities can colonize senescent cyanobacteria. Our study suggests that in winter, unlike commonly postulated, carbon rather than temperature is the limiting factor for bacterial growth. Frequent phytoplankton blooms in ice-covered systems highlight the need for year-round studies of aquatic ecosystems including the winter season to correctly understand element and energy cycling through aquatic food webs, particularly the microbial loop. On a global scale, such knowledge is required to determine climate change induced alterations in carbon budgets in polar and temperate aquatic systems.

Entrainment induced by near-inertial drift of sea ice and its impact on under-ice biogeochemical processes in marginal ice zones

Mooring observation of hydrography, hydrodynamics and suspended particles distribution under a drifting sea ice revealed the mixing and entrainment pattern in the upper mixed layer (ML) of the marginal ice zone. The ice floe where the mooring system was installed drifted as near-inertial motion with approximately 12-h cycle. The mixing pattern induced by this near-inertial drift can be divided into two distinct regimes. First, simple entrainment (upward) fluxes from the seasonal pycnocline to sea ice–water boundary are induced by shear across ML and seasonal pycnocline during the period when ice floes drift toward pack ice. The entrainment speed was in the range of 0.25–2mh−1, which matches well with thickening and thinning of the ML during a near-inertial period. Turbulent wakes on the boundary between sea ice and open water occurred behind the advancing edge of ice. In the second regime, when ice floes drift toward open ocean, the turbulent wakes at the advancing edge of ice are combined with the entrainment caused by near-inertial motion, which results in a complex mixing pattern of both upward and downward fluxes in the ML. The echo intensity observed by the acoustic Doppler current profiler and beam attenuation from transmissometer revealed the elevated concentration of suspended particulate materials in the ML, which can be direct evidence visualizing the mixing pattern. Results suggest that the mixing and entrainment found in our study sustain particulate matters in suspension within upper ML for a few months. This may provide a potential mechanism to sustain abundant organic particulates in the ML and upper pycnocline for months after under-ice bloom. Under strong wind events like storms, the entrainment induced by near-inertial motion may also get enhanced, which causes elevated supply of nutrients from the deeper, permanent pycnocline to the ML.

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

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 (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 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−3d−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.

Role of shelfbreak upwelling in the formation of a massive under-ice bloom in the Chukchi Sea

In the summer of 2011, an oceanographic survey carried out by the Impacts of Climate on EcoSystems and Chemistry of the Arctic Pacific Environment (ICESCAPE) program revealed the presence of a massive phytoplankton bloom under the ice near the shelfbreak in the central Chukchi Sea. For most of the month preceding the measurements there were relatively strong easterly winds, providing upwelling favorable conditions along the shelfbreak. Analysis of similar hydrographic data from summer 2002, in which there were no persistent easterly winds, found no evidence of upwelling near the shelfbreak. A two-dimensional ocean circulation model is used to show that sufficiently strong winds can result not only in upwelling of high nutrient water from offshore onto the shelf, but it can also transport the water out of the bottom boundary layer into the surface Ekman layer at the shelf edge. The extent of upwelling is determined by the degree of overlap between the surface Ekman layer and the bottom boundary layer on the outer shelf. Once in the Ekman layer, this high nutrient water is further transported to the surface through mechanical mixing driven by the surface stress. Two model tracers, a nutrient tracer and a chlorophyll tracer, reveal distributions very similar to that observed in the data. These results suggest that the biomass maximum near the shelfbreak during the massive bloom in summer 2011 resulted from an enhanced supply of nutrients upwelled from the halocline seaward of the shelf. The decade long trend in summertime surface winds suggests that easterly winds in this region are increasing in strength and that such bloom events will become more common.

Phytoplankton assemblage structure in and around a massive under-ice bloom in the Chukchi Sea

Standard and imaging flow cytometry were used to examine the composition of phytoplankton assemblages in and around a massive under-ice bloom in the Chukchi Sea in 2011. In the core of this bloom, roughly 100km northwest of Hanna Shoal, diatoms represented roughly 87% of the water column carbon-specific biomass of phytoplankton, while nanophytoplankton contributed ~9%. Picoeukaryotes were also observed in this bloom, as were phycoerythrin-containing cells consistent with Synechococcus spp., but picophytoplankton, dinoflagellates, and prymnesiophytes each represented only ~1% of the bloom׳s phytoplankton biomass. More broadly along this part of the Chukchi shelf, nanophytoplankton typically comprised a larger fraction of phytoplankton biomass in the water column, 22% on average but up to 82% at certain locations. Dinoflagellates and prymnesiophytes contributed at most 2% of water column biomass at any location and were most abundant in the deeper slope stations northeast of Hanna Shoal, east of the bloom. Picophytoplankton were most abundant in these deeper slope stations as well, and also in recently ice-free areas to the south around Hanna Shoal. These cell-derived estimates of phytoplankton carbon biomass, which were computed from imaging and standard cytometric observations of phytoplankton cell sizes and from published carbon:volume relationships, agree well with independent measurements of particulate organic carbon concentration from traditional biochemical assays.

Phytoplankton blooms beneath the sea ice in the Chukchi sea

In the Arctic Ocean, phytoplankton blooms on continental shelves are often limited by light availability, and are therefore thought to be restricted to waters free of sea ice. During July 2011 in the Chukchi Sea, a large phytoplankton bloom was observed beneath fully consolidated pack ice and extended from the ice edge to >100km into the pack. The bloom was composed primarily of diatoms, with biomass reaching 1291mg chlorophyll am−2 and rates of carbon fixation as high as 3.7gCm−2d−1. Although the sea ice where the bloom was observed was near 100% concentration and 0.8–1.2m thick, 30–40% of its surface was covered by melt ponds that transmitted 4-fold more light than adjacent areas of bare ice, providing sufficient light for phytoplankton to bloom. Phytoplankton growth rates associated with the under-ice bloom averaged 0.9d−1 and were as high as 1.6d−1. We argue that a thinning sea ice cover with more numerous melt ponds over the past decade has enhanced light penetration through the sea ice into the upper water column, favoring the development of these blooms. These observations, coupled with additional biogeochemical evidence, suggest that phytoplankton blooms are currently widespread on nutrient-rich Arctic continental shelves and that satellite-based estimates of annual primary production in waters where under-ice blooms develop are ~10-fold too low. These massive phytoplankton blooms represent a marked shift in our understanding of Arctic marine ecosystems.

Impacts of sea ice retreat, thinning, and melt-pond proliferation on the summer phytoplankton bloom in the Chukchi Sea, Arctic Ocean

In 2011, a massive phytoplankton bloom was observed in the Chukchi Sea under first-year sea ice (FYI), an environment in which primary productivity (PP) has historically been low. In this paper, we use a 1-D biological model of the Chukchi shelf ecosystem, in conjunction with in situ chemical and physiological data, to better understand the conditions that facilitated the development of such an unprecedented bloom. In addition, to assess the effects of changing Arctic environmental conditions on net PP (NPP), we perform model runs with varying sea ice and snow thickness, timing of melt, melt ponds, and biological parameters. Results from model runs with conditions similar to 2011 indicate that first-year ice (FYI) with at least 10% melt pond coverage transmits sufficient light to support the growth of shade-adapted Arctic phytoplankton. Increasing pond fraction by 20% enhanced peak under-ice NPP by 26% and produced rates more comparable to those measured during the 2011 bloom, but there was no effect of further increasing pond fraction. One of the important consequences of large under-ice blooms is that they consume a substantial fraction of surface nutrients such that NPP is greatly diminished in the marginal ice zone (MIZ) following ice retreat, where NPP has historically been the highest. In contrast, in model runs with <10% ponds, no under-ice bloom formed, and although peak MIZ NPP increased by 18–30%, this did not result in higher total annual NPP. This suggests that under-ice blooms contribute importantly to total annual NPP. Indeed, in all runs exhibiting under-ice blooms, total annual NPP was higher than in runs with the majority of NPP based in open water. Consistent with this, in model runs where ice melted one month earlier, peak under-ice NPP decreased 30%, and annual NPP was lower as well. The only exception was the case with no sea ice in the region: a weak bloom in early May was followed by low but sustained NPP throughout the entire growth season (almost all of which occurred in deep, subsurface layers), resulting in higher total annual NPP than in cases with sea ice present. Our results also show that both ultraviolet radiation and zooplankton grazers reduce peak open water NPP but have little impact on under-ice NPP, which has important implications for the relative proportion of NPP concentrated in pelagic vs. benthic food webs. Finally, the shift in the relative amount of NPP occurring in under-ice vs. open-water environments may affect total ecosystem productivity.

Evidence of under-ice phytoplankton blooms in the Chukchi Sea from 1998 to 2012

The discovery in 2011 of a massive phytoplankton bloom underneath first-year sea ice in the western Arctic has prompted an investigation of the spatial and temporal distribution of under-ice phytoplankton blooms. Here, we explore the satellite record from years 1998 to 2012 for evidence of under-ice blooms on the Chukchi Sea shelf. Phytoplankton blooms were categorized as under-ice blooms, probable under-ice blooms, or marginal ice zone blooms, depending on bloom timing in relation to the timing of ice retreat. Annual bloom type maps reveal that under-ice phytoplankton blooms were present in every year of the satellite record. Averaged over all years, the combination of under-ice blooms and probable under-ice blooms covered a portion of the observable study area that was 2.5-fold higher than that of marginal ice zone blooms (71.5% and 28.5%, respectively). This finding strongly contradicts the traditional view that phytoplankton in seasonally ice-covered waters bloom only after ice retreat and instead indicates that blooms are initiated whenever light and nutrient availability is sufficient for photosynthesis, a condition often reached early in the season underneath first-year sea ice on nutrient-rich continental shelves. Spatial patterns in bloom type were distinguished relative to the date of ice retreat, with probable under-ice blooms dominating the nutrient-rich western Chukchi Sea and at higher latitudes where ice retreats later, while marginal ice zone blooms were most common in the southern and eastern Chukchi Sea where ice retreats earlier. Our results suggest that under-ice phytoplankton blooms are widespread in the Chukchi Sea and had been prevalent there for more than a decade prior to their discovery in 2011.

Role of environmental factors on phytoplankton bloom initiation under landfast sea ice in Resolute Passage, Canada

It has been common practice in scientific studies to assume negligible phytoplankton production when the ocean is ice-covered, due to the strong light attenuation properties of snow, sea ice, and ice algae. Recent observations of massive under-ice blooms in the Arctic challenge this concept and call for a re-evaluation of light conditions prevailing under ice during the melt period. Using hydrographic data collected under landfast ice cover in Resolute Passage, Nunavut, Canada between 9 May and 21 June 2010, we documented the exponential growth phase of a substantial under-ice phytoplankton bloom. Numerous factors appeared to influence bloom initi- ation: (1) transmitted light increased with the onset of snowmelt and termination of the ice algal bloom; (2) initial phytoplankton growth resulted in the accumulation of biomass below the devel- oping surface melt layer where nutrient concentrations were high and turbulent mixing was rela- tively low; and (3) melt pond formation rapidly increased light transmission, while spring-tidal energy helped form a surface mixed layer influenced by ice melt — both were believed to influ- ence the final rapid increase in phytoplankton growth. By the end of the study, nitrate+nitrite was depleted in the upper 10 m of the water column and the under-ice bloom had accumulated 508 mg chl a m �2 with a new production estimate of 17.5 g C m �2 over the upper 50 m of the water column. The timing of bloom initiation with melt onset suggests a strong link to climate change where sea ice is both thinning and melting earlier, the implication being an earlier and more ubiquitous phytoplankton bloom in Arctic ice-covered regions.

Temporal and spatial variations of phytoplankton from Boeckella Lake (Hope Bay, Antarctic Peninsula)

The main water body at Hope Bay, Boeckella Lake, was sampled at four sites for phytoplankton during summer 1991 to assess the influence of nutrients from nearby penguin rookeries on both phytoplankton density and composition. The site located at the base of the rookeries had total phosphorus values comparable to those reported from the most eutrophic Antarctic lakes. During the ice-free period most of the Chlorophyceae and Cyanophyceae recorded were concentrated at this site. Phytoplankton density increased strongly in the area opposite to the rookeries where ice began to form; an under-ice bloom of Ochromonas aff. ovalis (Chrysophyceae) was observed in this area.

Physical Control of Phytoplankton Production under Sea Ice (Manitounuk Sound, Hudson Bay)

In polar and subpolar seas, there are numerous accounts of phytoplankton blooms in the upper water column under the ice. Various mechanisms have been invoked to explain these blooms: the seeding of the underlying surface water by algal cells (epontic flora) released from the melting ice, the optimization of light utilization by the cells, and the stabilization of the upper water column by the low-salinity melting water. From studies conducted in Manitounuk Sound (Hudson Bay), it is proposed that phytoplankton blooms under the ice probably result from the simultaneous deepening of both the photic layer (seasonal light increase) and the stratified layer (low-salinity melting water). In ice-covered seas, the release of ice algae superimposes itself on the phytoplankton bloom, resulting in the observed algal increase under melting ice.Key words: phytoplankton, under-ice blooms, ice flora, stability, nutrients, Hudson Bay