Landfast Sea Ice Research Articles

Small scale vertical gradients of Arctic ice algal photophysiological properties

Photosynthetic parameters of phytoplankton and sea ice algae from landfast sea ice of the Chukchi Sea off Point Barrow, Alaska, were assessed in spring 2005 and winter through spring 2006 using Pulse Amplitude Modulated (PAM) fluorometry including estimates of maximum quantum efficiency (F(v)/F(m)), maximum relative electron transport rate (rETR(max)), photosynthetic efficiency (alpha), and the photoadaptive index (E(k)). The use of centrifuged brine samples allowed to document vertical gradients in ice algal acclimation with 5 cm vertical resolution for the first time. Bottom ice algae (0-5 cm from ice-water interface) expressed low F(v)/F(m) (0.331-0.426) and low alpha (0.098-0.130 (micromol photons m(-2)s(-1))(-1)) in December. F(v)/F(m) and alpha increased in March and May (0.468-0.588 and 0.141-0.438 (micromol photons m(-2)s(-1))(-1), respectively) indicating increased photosynthetic activity. In addition, increases in rETR(max) (3.3-16.4 a.u.) and E(k) (20-88 micromol photons m(-2) s(-1)) from December to May illustrates a higher potential for primary productivity as communities become better acclimated to under-ice light conditions. In conclusion,photosynthetic performance by ice algae (as assessed by PAM fluorometry) was tightly linked to sea ice salinity, temperature, and inorganic nutrient concentrations (mainly nitrogen).

Carbon Uptake Rates of Sea Ice Algae and Phytoplankton under Different Light Intensities in a Landfast Sea Ice Zone, Barrow, Alaska

To determine whether nitrogen or light exerts the most control for the rates of carbon production of ice algae and phytoplankton under the ice, nitrogen addition (NO3 or NH4) and light increment experiments were executed on the landfast sea ice of Barrow, Alaska, during the 2003 growing season by using a 13C-15N dual isotope tracer technique. The productivity of the bottom sea ice algae and phytoplankton at Barrow in 2003 was limited mainly by low light levels (approximately 0.3% of the surface irradiance) at the bottom under the snow-covered sea ice. The carbon and nitrate uptake rates of ice algae and phytoplankton increased as the incubation depth in the ice hole decreased and light intensity increased. In addition, under higher light conditions, the relative production of proteins of the bottom ice algae decreased, whereas the lipid proportion increased. The higher level of lipid synthesis of the ice algae might be significant to the nutrition of zooplankton and benthos because lipids are the most energy-dense biomolecules.

Frazil deposition under growing sea ice

Platelet ice may be an important component of Antarctic landfast sea ice. Typically, it is found at depth in first‐year landfast sea ice cover, near ice shelves. To explain why platelet ice is not commonly observed at shallower depths, we consider a new mechanism. Our hypothesis is that platelet ice eventually appears due to the sudden deposition of frazil ice against the fast ice‐ocean interface, providing randomly oriented nucleation sites for crystal growth. Brine rejected in plumes from landfast ice generates stirring sufficient to prevent frazil ice from attaching to the interface, forcing some of it to remain in suspension until ice growth rate and brine rejection slow to the point that frazil can stick. We calculate a brine plume velocity and match this to frazil rise velocity. We consider both laminar and turbulent environments. We find that brine plume velocities are generally powerful enough to prevent a significant range of frazil sizes from sticking in the case of laminar flow and that, in the turbulent case, there may be a critical ice thickness at which most remaining circulating frazil suddenly settles.

Sediment plume response to surface melting and supraglacial lake drainages on the Greenland ice sheet

AbstractIncreased mass losses from the Greenland ice sheet and inferred contributions to sea-level rise have heightened the need for hydrologic observations of meltwater exiting the ice sheet. We explore whether temporal variations in ice-sheet surface hydrology can be linked to the development of a downstream sediment plume in Kangerlussuaq Fjord by comparing: (1) plume area and suspended sediment concentration from Moderate Resolution Imaging Spectroradiometer (MODIS) imagery and field data; (2) ice-sheet melt extent from Special Sensor Microwave/Imager (SSM/I) passive microwave data; and (3) supraglacial lake drainage events from MODIS. Results confirm that the origin of the sediment plume is meltwater release from the ice sheet. Interannual variations in plume area reflect interannual variations in surface melting. Plumes appear almost immediately with seasonal surface-melt onset, provided the estuary is free of landfast sea ice. A seasonal hysteresis between melt extent and plume area suggests late-season exhaustion in sediment supply. Analysis of plume sensitivity to supraglacial events is less conclusive, with 69% of melt pulses and 38% of lake drainage events triggering an increase in plume area. We conclude that remote sensing of sediment plume behavior offers a novel tool for detecting the presence, timing and interannual variability of meltwater release from the ice sheet.

Toward an integrated coastal sea-ice observatory: System components and a case study at Barrow, Alaska

The morphology, stability and duration of seasonal landfast sea ice in Alaska's coastal zone is changing alongside large-scale ice thinning and retreat. The extent and complexity of change at the local level requires an integrated observing approach to assess implications of such change for coastal ecosystems and communities that rely on or make use of the sea-ice cover. Barrow, Alaska is an example of a community that experiences and utilizes a broad range of sea-ice types and conditions. The local population is increasingly forced to adapt to less stable sea ice, loss of multiyear ice and a shorter ice season. We are working toward an integrated coastal ice observatory to monitor landfast and adjacent pack ice and to maximize the usefulness of information to the community. The observatory includes: (1) satellite remote-sensing datasets distributed in near real-time; (2) a coastal sea-ice radar and webcam that monitor ice movement and evolution; (3) a mass-balance site that provides temperature profiles and thickness information for ice and snow; (4) sea-level measurements; (5) periodic ice thickness surveys using direct drilling and electromagnetic induction sounding; and (6) a program of regular, undirected observations by Iñupiat sea-ice experts. We examine two significant landfast ice breakout events off Barrow in spring of 2007. During these events, Barrow's subsistence whaling community partook in a successful hunting season observing and responding to these breakout events and their impacts on ice stability. Using local expert knowledge to parse geophysical datasets obtained from the observatory has provided deeper insight into different approaches for assessing ice stability, and integrating information on ice growth, origin, morphology, and dynamics, as well as winds, weather, and currents.

Spring time production of bottom ice algae in the landfast sea ice zone at Barrow, Alaska

The primary production of bottom ice algae is an important food source for sympagic, pelagic and benthic organisms in the Arctic Ocean as well as Antarctic Ocean. Using 13C- 15N isotope tracers, the recent ice algal production at Barrow during the spring season was lower in 2003 than three decades ago, although the maximum chlorophyll-a concentration for the bottom ice algae was similar to the values from previous studies. Estimated recent new and total production rates of the ice algae were 0.8 g C m - 2 yr - 1 and 2.0 g C m - 2 yr - 1 respectively, while the rates of water column phytoplankton were 0.2 g C m - 2 yr - 1 and 0.7 g C m - 2 yr - 1 for the spring sampling period in 2003. The ice algae contributed 74% of the pelagic primary production under the landfast sea ice at Barrow before the phytoplankton spring bloom. At the end of the season in 2003, a high carbon allocation of lipids in the ice algae was found. Three possible explanations- nutrient depletion, increasing light, and/or changes in species composition- were suggested for the high carbon incorporation into lipids. This high lipid synthesis of the bottom ice algae might be significant to zooplankton and benthic fauna grazers because lipids are the most energy dense biomolecules.

Arctic epishelf lakes as sentinel ecosystems: Past, present and future

Ice shelves are a prominent but diminishing feature of the northern coastline of Ellesmere Island in the Canadian High Arctic (latitude 82–83°N). By blocking embayments and fiords, this thick coastal ice can create epishelf lakes, which are characterized by a perennially ice‐capped water column of freshwater overlying seawater. The goal of this study was to synthesize new, archived, and published data on Arctic epishelf lakes in the context of climate change. Long‐term changes along this coastline were evaluated using historical reports, cartographic analysis, RADARSAT imagery, and field measurements. These data, including salinity‐temperature profiling records from Disraeli Fiord spanning 54 years, show the rapid decline and near disappearance of this lake type in the Arctic. Salinity‐temperature profiling of Milne Fiord, currently blocked by the Milne Ice Shelf, confirmed that it contained an epishelf lake composed of a 16‐m thick freshwater layer overlying seawater. A profiling survey along the coast showed that there was a continuum of ice‐dammed lakes from shallow systems dammed by multiyear landfast sea ice to deep epishelf lakes behind ice shelves. The climate warming recently observed in this region likely contributed to the decline of epishelf lakes over the last century, and the air temperature trend predicted for the Arctic over the next several decades implies the imminent loss of this ecosystem type. Our results underscore the distinctive properties of coastal ice‐dammed lakes and their value as sentinel ecosystems for the monitoring of regional and global climate change.

A millennial‐scale record of Arctic Ocean sea ice variability and the demise of the Ellesmere Island ice shelves

Sea‐ice ice shelves, at the apex of North America (>80° N), constitute the oldest sea ice in the Northern Hemisphere. We document the establishment and subsequent stability of the Ward Hunt Ice Shelf, and multiyear landfast sea ice in adjacent fiords, using 69 radiocarbon dates obtained on Holocene driftwood deposited prior to coastal blockage. These dates (47 of which are new) record a hiatus in driftwood deposition beginning ∼5500 cal yr BP, marking the inception of widespread multiyear landfast sea ice across northern Ellesmere Island. This chronology, together with historical observations of ice shelf breakup (∼1950 to present), provides the only millennial‐scale record of Arctic Ocean sea ice variability to which the past three decades of satellite surveillance can be compared. Removal of the remaining ice shelves would be unprecedented in the last 5500 years. This highlights the impact of ongoing 20th and 21st century climate warming that continues to break up the remaining ice shelves and soon may cause historically ice‐filled fiords nearby to open seasonally.

Inference of optical properties from radiation profiles within melting landfast sea ice

Vertical in‐ice spectral radiation profiles were measured within melting 1.5‐ to 1.7‐m‐thick landfast sea ice in western Hudson Bay on 25 April 2005. Because the surface ice was subject to extensive melting and refreezing, the sea ice had fractioned into two main types, i.e., areas of more reflective white ice and less reflective blue ice. The shortwave albedo was about 0.69 for white ice and 0.47 for bare blue ice. The corresponding shortwave transmittance through the ice cover was about 0.02 and 0.09, respectively. The inherent optical properties of the sea ice were inferred by tying the input and output of radiative transfer simulations to the radiation profiles and the ice physical properties, as well as to the irradiance measurements above and below the ice cover. To explain observed spectral albedo and transmittance simultaneously, the ice/snow above the interior ice was divided into three layers on the basis of the following observations: snow (white ice) or a thin soot containing layer (blue ice), drained ice above and saturated ice below the waterline. Similarly, the bottom portion was divided on the basis of the presence of a living ice algae layer adjacent to the seawater interface and a layer extending 30 cm above the bottom containing mostly detrital matter. The interior of the ice, i.e., roughly 20–40 cm from boundaries, was well‐represented by a single layer of pure sea ice as the radiation field was nearly asymptotic and the absorption spectra showed little evidence of impurities. Representative values for the scattering coefficient ranged 600–800 m−1, with a Henyey‐Greenstein asymmetry parameter of 0.995. Observations within white ice suggest that about 40% of the energy responsible of the internal melting was provided directly by shortwave radiation, while the rest is due to heat conduction.

Bio‐optical and structural properties inferred from irradiance measurements within the bottommost layers in an Arctic landfast sea ice cover

Irradiance spectra were measured at vertical increments within the bottommost layers of landfast sea ice with the aid of divers in Franklin Bay, Canada, in an effort to obtain input parameters for bio‐optical modeling of sea ice. The study took place between 22 April and 9 May 2004 during the overwintering stage of CASES (Canadian Arctic Shelf Exchange Study). The ice was about 1.8 m thick with a snow cover of variable thickness (∼0.04 to 0.4 m). Ice surface temperatures increased from about −12° to −6.4°C during the sampling period, while ice temperatures within the bottommost portion under study ranged from −3.0° to −1.2°C. Ice algae were visible within the bottommost centimeters of the sea ice. This algae layer had a marked effect on the spectral distribution of transmitted irradiance beneath the ice. Particulate absorption spectra, ap(λ), measured from melted ice samples showed evidence of chloroplastic pigment degradation and could not fully explain the shape of the in situ diffuse attenuation coefficient, Kd(λ), for the algal layer. Interior ice layers, however, did show absorption curves similar to ap(λ) from samples, suggesting the presence of degraded algal pigments within these layers. The discrete ordinates radiative transfer (DISORT) code was iterated in an inverse approach to estimate ap(λ) and the scattering coefficient, btot, from the irradiance profiles. For the bottom 0.1 m of the sea ice, btot was around 400 m−1, while at the 0.1‐ to 0.2‐m layer from the ice bottom it decreased to 165 m−1. Using ap(λ) combined with wavelength independent btot as inputs to DISORT seem to adequately explain the radiative transfer near the bottom of first‐year sea ice provided that adjustments were made to the brine volume fraction.

Characterization and growth response to temperature and salinity of psychrophilic, halotolerant Chlamydomonas sp. ARC isolated from Chukchi Sea ice

Sea ice provides a habitat for a diverse community of microorganisms, which comprise a substantial portion of primary production in ice-covered seas. Organisms immured in sea ice have to withstand strong changes in temperature and salinity. We report on the growth rate response to salinity and temperature of the chlorophyte Chlamydomonas sp. ARC, isolated from land-fast sea ice in the Chukchi Sea, Alaska. We found it to be a euryhaline psychrophile capable of growth at tem- peratures as low as -5°C and at salinities from 2.5 to 100 ‰. The maximum growth rate of 0.41 d -1 (±0.027) was found at 5°C and a salinity of 30‰. The salinity growth range of this organism indicates that it is well adapted to the variable salinity environment associated with brine channels in sea ice, as well as the hypotonic conditions associated with melting ice. Based upon morphology and molec- ular phylogenetic reconstructions using the 18S ribosomal ribonucleic acid (rRNA) gene and the ribu- lose 1,5-bisphosphate carboxylase/oxygenase large subunit (rbcL) gene, this Arctic Chlamydomonas falls into a distinct clade containing other as yet unassigned psychrophilic Chlamydomonas strains isolated from Arctic and Antarctic environments, pointing to a bi-polar distribution of this clade. It is also very closely related to the brackish water mesophile Chlamydomonas kuwadae Gerloff, and is capable of growth above the psychrophilic range in low-salinity medium, indicating that it may represent an intermediate between mesophilic and psychrophilic lifestyles.

Rapid loss of the Ayles Ice Shelf, Ellesmere Island, Canada

On August 13, 2005, almost the entire Ayles Ice Shelf (87.1 km2) calved off within an hour and created a new 66.4 km2 ice island in the Arctic Ocean. This loss of one of the six remaining Ellesmere Island ice shelves reduced their overall area by ∼7.5%. The ice shelf was likely weakened prior to calving by a long‐term negative mass balance related to an increase in mean annual temperatures over the past 50+ years. The weakened ice shelf then calved during the warmest summer on record in a period of high winds, record low sea ice conditions and the loss of a semi‐permanent landfast sea ice fringe. Climate reanalysis suggests that a threshold of >200 positive degree days year−1 is important in determining when ice shelf calving events occur on N. Ellesmere Island.

On the link between SAR‐derived sea ice melt and development of the summer upper ocean mixed layer in the North Open Water Polynya

We examine the ability of SAR‐derived landfast sea ice thermodynamic state surrounding the North Water Polynya to predict the timing of a shallow, stably stratified summer ocean mixed layer in the region. Radarsat‐1 ScanSAR is used to derive melt and pond onset dates over landfast sea ice surrounding the polynya, describing its thermodynamic evolution. Salinity and temperature profiles, connected in time and space, are complemented by potential density profiles and mixed layer depths, which describe the evolution of the upper ocean mixed layer between 4 April and 21 July 1998. Surface salinity driven potential density stratification and summer mixed layer depths are temporally coincident. Sea ice–ocean connection is first described at the local scale, then regionally. At the local scale, a reduction in surface salinity occurs between the melt and pond onsets over landfast sea ice. At the polynya scale, interpolated pond onset date is highly effective at predicting the timing of a thin, stably stratified summer ocean mixed layer; this has consequences for the potential prediction of timing of maximum ecosystem productivity.

Biogas (CO2, O2, dimethylsulfide) dynamics in spring Antarctic fast ice

We studied the temporal variations of CO2, O2, and dimethylsulfide (DMS) concentrations within three environments (sea‐ice brine, platelet ice‐like layer, and underlying water) in the coastal area of Adeélie Land, Antarctica, during spring 1999 before ice breakup. Temporal changes were different among the three environments, while similar temporal trends were observed within each environment at all stations. The underlying water was always undersaturated in O2 (around 85%) and oversaturated in CO2 at the deepest stations. O2 concentrations increased in sea‐ice brine as it melted, reaching oversaturation up to 160% due to the primary production by the sea‐ice algae community (chlorophyll a in the bottom ice reached concentrations up to 160 µg L−1 of bulk ice). In parallel, DMS concentrations increased up to 60 nmol L21 within sea‐ice brine and the platelet ice‐like layer. High biological activity consumed CO2 and promoted the decrease of partial pressure of CO2 (pCO2). In addition, melting of pure ice crystals and calcium carbonate (CaCO3) dissolution promoted the shift from a state of CO2 oversaturation to a state of marked CO2 undersaturation (pCO2 < 30 dPa). On the whole, our results suggest that late spring land fast sea ice can potentially act as a sink of CO2 and a source of DMS for the neighbouring environments, i.e., the underlying water or/and the atmosphere.

Comparisons Between SAR Backscattering Coefficient and Results of a Thermodynamic Snow/Ice Model for the Baltic Sea Land-Fast Sea Ice

We have compared the time series of C-band HH-polarization backscattering coefficients (sigmadeg) of the Baltic Sea land-fast level ice with results from a 1-D high-resolution thermodynamic snow/ice model (HIGHTSI). The sigmadeg time series were obtained from ENVISAT synthetic aperture radar (SAR) images. The study period was from the middle of the winter to the early melt season, February 3-April 7, 2004. Due to the large incidence angle range of the SAR images, the sigmadeg values were divided into three subseries. In general, the HIGHTSI results greatly helped to interpret the sigmadeg behavior with changing ice and weather conditions. The modeled snow-surface temperature, cases of snow melting, and evolution of snow and ice thickness were related to the changes in sigmadeg. Equally useful information could not be obtained solely on the basis of large-scale atmospheric models. Realistic forcing data for HIGHTSI were available in the form of coastal-weather observations and model results of the European Centre of Medium-Range Weather Forecasts (ECMWF). The latter make it possible to apply HIGHTSI in the interpretation of SAR data from all ice-covered seas. There were some cases where detailed ground truth, combined with theoretical sigmadeg modeling, would have been needed for interpretation of the sigmadeg trends. A very interesting observation was the large variation of level ice sigmadeg with changing weather conditions, which complicates automatic classification of the SAR images, and thus, the algorithms must be tuned for different ice conditions. The HIGHTSI model could act as an indicator of various ice conditions for algorithm development

Observations of Snow Water Equivalent Change on Landfast First-Year Sea Ice in Winter Using Synthetic Aperture Radar Data

In this paper, we examine the utility of synthetic aperture radar (SAR) backscatter data to detect a change in snow water equivalent (SWE) over landfast first-year sea ice during winter at relatively cold temperatures. We begin by reviewing the theoretical framework for linking microwave scattering from SAR to the thermodynamic and electrical properties of first-year sea ice. Previous research has demonstrated that for a given ice thickness and air-temperature change, a thick snow cover will result in a smaller change in the snow-ice interface temperature than will a thin snow cover. This small change in the interface temperature will result in a relatively small change in the brine volume at the interface and the resulting complex permittivity, thereby producing a relatively small change in scattering. A thin snow cover produces the opposite effect-a greater change in interface temperature, brine volume, permittivity, and scattering. This work is extended here to illustrate a variation of this effect over landfast first-year sea ice using in situ measurements of physical snow properties and RADARSAT-1 SAR imagery acquired during the winter of 1999 in the central Canadian Archipelago at cold (~-26degC) and moderately cold (~-14degC) snow-sea-ice interface temperatures. We utilize in situ data from five validation sites to demonstrate how the change in microwave scattering covaries and is inversely proportional with the change in the magnitude of SWE. These changes are shown to be detectable over both short (2 days) and longer (45 days) time durations

Summer landfast sea ice desalination at Point Barrow, Alaska: Modeling and observations

Landfast sea ice cores from two sites in Point Barrow, Alaska, extracted between 1999 and 2001, show a progressive desalination and a corresponding shape transition in the salinity profile starting at snowmelt onset, around 1 June. The vertical percolation of fresh surface meltwater through the permeable ice matrix (flushing) has long been supposed to control this transition. A parameterization of flushing in bare ice is formulated. It is incorporated into the semiempirical winter sea ice desalination model of Cox and Weeks (1988), coupled to the one‐dimensional thermodynamic sea ice model of Bitz and Lipscomb (1999) and forced under Point Barrow conditions. Adjustment to the original Cox and Weeks parameterization of gravity drainage was necessary to give reasonable agreement with observations in winter. With this change the model has the potential to simulate the full seasonal cycle of the Arctic salinity and mass balance of typical Arctic ice. In summer, the model salinity profile closely follows the observations. The model upper ice temperatures are slightly too cold. At the thin snow (Chukchi Sea) site, the model simulates the observed snow depth and ice thickness well. At the thick snow (Elson Lagoon) site, the snow disappears 7 days earlier than observed, which results in underestimating ice thickness. Sensitivity experiments suggest that a more realistic representation of snow and meltwater physics would significantly improve the simulation. Because of the effects of brine drainage on the sea ice mass balance and oceanic circulation, including the time dependence of the ice salinity profile would significantly improve the next generation of sea ice models.

Structure, salinity and isotopic composition of multi-year landfast sea ice in Nella Fjord, Antarctica

The structural, salinity and stable oxygen-isotopic properties of multi-year landfast sea-ice samples collected from Nella Fjord, East Antarctica were used to determine the ice formation process and the fractional contribution of snow, in both snow ice and superimposed ice, to the total sea-ice thickness. The textural analysis of the sea-ice cores revealed a general structure of a granular surface layer, horizontal c-axis in the columnar ice and, in some cases, a “buried” granular-ice layer at depth between two columnar-ice layers. The textural and isotopic properties of the ice core samples indicate that thermodynamic mechanisms alone are responsible for the development of the multi-year fast ice in the region investigated. Superimposed ice contributed 5% to the ice thickness at some sites. Using a mid-range isotopic criterion to distinguish between snow ice and frazil ice in the granular layers, we estimate that snow-ice layers contributed 38% of the total ice thickness in the samples. The actual fraction of snow in the granular-ice layers in different cores was from 1% to 24% of the total sea-ice thickness.

Alaska landfast sea ice: Links with bathymetry and atmospheric circulation

Using Radarsat Synthetic Aperture Radar (SAR) imagery of northern Alaska and northwestern Canada, we calculated a mean climatology of the annual landfast ice cycle for the period 1996–2004. We also present the monthly minimum, mean, and maximum landfast ice extents throughout the study area. These data reveal where and when the landfast is most stable and which sections of the coast are susceptible to midseason breakout events. Stabilization of landfast ice is strongly related to the advance of the seaward landfast ice edge (SLIE) into waters around 18 m deep. Isobaths near this depth are a good approximation for midseason landfast ice extent. Comparison with work from the 1970s suggests a reduced presence of landfast ice in this region of the Arctic, due to later formation and earlier breakup. This will likely lead to an increase in coastal erosion and may also have profound effects upon subsistence activities, which are intimately linked to the timing of marine mammal migration patterns. Interannually, landfast ice formation correlates with the incursion of pack ice into coastal waters, suggesting that the later mean date of formation in recent years may be related to the increasingly northward location of the perennial sea ice edge. The timing of breakup correlates well with onset of thawing air temperatures. Analysis of regional data shows a multidecadal trend toward earlier thaw onset, which suggests that the observed change in breakup dates may be part of a longer‐term trend.

Role of Sea-ice Biota in Nutrient and Organic Material Cycles in the Northern Baltic Sea

This paper compiles biological and chemical sea-ice data from three areas of the Baltic Sea: the Bothnian Bay (Hailuoto, Finland), the Bothnian Sea (Norrby, Sweden), and the Gulf of Finland (Tvärminne, Finland). The data consist mainly of field measurements and experiments conducted during the BIREME project from 2003 to 2006, supplemented with relevant published data. Our main focus was to analyze whether the biological activity in Baltic Sea sea-ice shows clear regional variability. Sea-ice in the Bothnian Bay has low chlorophyll a concentrations, and the bacterial turnover rates are low. However, we have sampled mainly land-fast level first-year sea-ice and apparently missed the most active biological system, which may reside in deformed ice (such as ice ridges). Our limited data set shows high concentrations of algae in keel blocks and keel block interstitial water under the consolidated layer of the pressure ridges in the northernmost part of the Baltic Sea. In land-fast level sea-ice in the Bothnian Sea and the Gulf of Finland, the lowermost layer appears to be the center of biological activity, though elevated biomasses can also be found occasionally in the top and interior parts of the ice. Ice algae are light limited during periods of snow cover, and phosphate is generally the limiting nutrient for ice bottom algae. Bacterial growth is evidently controlled by the production of labile dissolved organic matter by algae because low growth rates were recorded in the Bothnian Bay with high concentrations of allochthonous dissolved organic matter. Bacterial communities in the Bothnian Sea and the Gulf of Finland show high turnover rates, and activities comparable with those of open water communities during plankton blooms, which implies that sea-ice bacterial communities have high capacity to process matter during the winter period.