Sea ice in the Baltic Sea – A review

Estuarine, Coastal and Shelf Science 70 (2006) 145e160www.elsevier.com/locate/ecss

Sea ice in the Baltic Sea e A review

Mats Granskog a, Hermanni Kaartokallio b, Harri Kuosa c, David N. Thomas d,*, Jouni Vainio b

a Arctic Centre, University of Lapland, P.O. Box 122, FI-96101 Rovaniemi, Finlandb Finnish Institute of Marine Research, P.O. Box 2, FI-00561 Helsinki, Finland

c Tvarminne Zoological Station, FI-10900 Hanko, Finlandd School of Ocean Sciences, University of Wales-Bangor, Askew Street, Menai Bridge, Anglesey LL59 5AB, United Kingdom

Received 21 April 2006; accepted 5 June 2006

Available online 25 July 2006

Abstract

Although the seasonal ice cover of the Baltic Sea has many similarities to its oceanic counterpart in Polar Seas and Oceans, there are many uniquecharacteristics that mainly result from the brackish waters from which the ice is formed, resulting in low bulk salinities and porosities. In addition,due to the milder climate than Polar regions, the annual maximum ice extent is highly variable, and rain and freeze-melt cycles can occur throughoutwinter. Up to 35% of the sea ice mass can be composed from metamorphic snow, rather than frozen seawater, and in places snow and superimposedice can make up to 50% of the total ice thickness. There is pronounced atmospheric deposition of inorganic nutrients and heavy metals onto the ice,and in the Bothnian Bay it is estimated that 5% of the total annual flux of nitrogen and phosphorus and 20e40% of lead and cadmium may be de-posited onto the ice fields from the atmosphere. It is yet unclear whether or not the ice is simply a passive store for atmospherically deposited com-pounds, or if they are transformed through photochemical processes or biological accumulation before released at ice and snow melt.

As in Polar sea ice, the Baltic ice can harbour rich biological assemblages, both within the ice itself, and on the peripheries of the ice at theice/water interface. Much progress has been made in recent years to study the composition of these assemblages as well as measuring biogeo-chemical processes within the ice related to those in underlying waters. The high dissolved organic matter loading of Baltic waters and ice resultin the ice having quite different chemical characteristics than those known from Polar Oceans. The high dissolved organic material load is alsoresponsible in large degree to shape the optical properties of Baltic Sea ice, with high absorption of solar radiation at shorter wavelengths, a pre-requisite for active photochemistry of dissolved organic matter.

Land-fast ice in the Baltic also greatly alters the mixing characteristics of river waters flowing into coastal waters. River plumes extend underthe ice to a much greater distance, and with greater stability than in ice-free conditions. Under-ice plumes not only alter the mixing properties ofthe waters, but also result in changed ice growth dynamics, and ice biological assemblages, with the underside of the ice being encased, in theextreme case, with a frozen freshwater layer.

There is a pronounced gradient in ice types from more saline ice in the south to freshwater ice in the north. The former is characteristicallymore porous and supports more ice-associated biology than the latter. Ice conditions also vary considerably in different parts of the Baltic Sea,with ice persisting for over half a year in the northernmost part of the Baltic Sea, the Bothnian Bay. In the southern Baltic Sea, ice appears onlyduring severe winters.� 2006 Elsevier Ltd. All rights reserved.

Keywords: sea ice; Baltic Sea; biogeochemistry; plankton; seasons

* Corresponding author.

E-mail address: [email protected] (D.N. Thomas).

0272-7714/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ecss.2006.06.001

1. Introduction

The Baltic Sea is one of the world’s largest brackish waterbasins with a surface area of 422,000 km2 and volume of21,000 km3. The mean depth of the Baltic Sea is only 55 m,and in the Gulf of Finland and the Bothnian Bay less than40 m (Voipio, 1981). The surface salinity varies from 9 in

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146 M. Granskog et al. / Estuarine, Coastal and Shelf Science 70 (2006) 145e160

the southern Baltic Proper to <1 in the innermost parts of theGulf of Finland and the Bothnian Bay and in proximity oflarger rivers. The Baltic Sea is heavily influenced by river dis-charge, and the sea has a positive water balance, meaning thatriver runoff and precipitation exceed evaporation. The brack-ish nature of the sea is maintained by intermittent inflows ofsaline North Seawater through the Danish Straits (Stigebrandt,2001). Due to a strong choking effect of the shallow DanishStraits at the entrance of the Baltic Sea, tidal sea-level varia-tion is generally only 1e10 cm (Stigebrandt, 2001). The BalticSea extends over a large geographical area in the northesouthdimension, which leads to regional differences in the annualprimary production dynamics and radiant heating caused byvariations in solar radiation.

Seasonal sea ice plays an important role in heat budget inthe Baltic Sea (Omstedt et al., 2004), as well as contributingto salt and freshwater budgets. Annually, sea ice coversa mean of 40% of the Baltic Sea (Vainio, unpubl.), and the me-dian maximum ice extent for the period 1971e2000 was157,000 km2. Typical ice-covered areas are presented inFig. 1. Despite the brackish nature of the parent water, seaice in the Baltic appears to be structurally similar and compa-rable to sea ice formed in Polar waters although characteristicdifferences do exist (Kawamura et al., 2001a).

Studies related to sea ice have been performed for severaldecades in the Baltic Sea, mainly motivated by the develop-ment of winter shipping. Finland and Estonia are the only na-tions in the world where all harbours freeze every winter

Fig. 1. Chart presenting median maximum ice coverage and typically ice-

covered areas in the Baltic Sea for the period of 1971e2000. Numbers

1e11 denote the locations for ice statistics presented in Table 1. Abbrevia-

tions for sea areas as follows: BoB, Bay of Bothnia; BS, Bothnian Sea; AAS,

Aland Sea; AS, Archipelago Sea; GoF, Gulf of Finland; GoR, Gulf of Riga;

NBP, Northern Baltic Proper; CBP, Central Baltic Proper; SBP, southern

Baltic Proper. Data from the Finnish Ice Service (J. Vainio).

(Jevrejeva and Lepparanta, 2002), which partly explains thekeen interest in sea ice studies. Therefore, in the 1950s geo-physical studies of sea ice properties and climatology com-menced (Palosuo, 1961, 1963), and since then severalstudies have mainly focused on large-scale problems, suchas sea ice climatology and dynamics (Lepparanta, 1981; Haa-pala and Lepparanta, 1996, 1997; Jevrejeva et al., 2004). Thetime series of the maximum annual ice extent in the Baltic Sea(MIB), reconstructed from historical records from the early18th to early 20th centuries and thereafter based on observa-tions, is one of the world’s longest time series describing iceconditions (Seina and Palosuo, 1996). At its annual maximumextent, the ice covers on average between 190,000 and217,000 km2, but there is a large inter-annual variability inthe date freezing begins, thickness, extent, and break-up date(Seina and Palosuo, 1996; Haapala and Lepparanta, 1997;Vainio, unpubl.). The dominating feature of the MIB isa very large inter-annual variability that ranges from 10 to100% of the total Baltic Sea area (Haapala and Lepparanta,1997).

Several studies have investigated the relationship betweenlarge-scale atmospheric conditions and ice conditions in theBaltic Sea (Tinz, 1995), and there is considerable evidenceto show that large-scale atmospheric circulation patterns aresignificantly correlated with the ice conditions in the BalticSea (Jevrejeva, 2001). During average and mild winters,warm air masses associated with westerly moving cyclonesfrom the Atlantic dominate the Baltic climate, while in severewinters blocking anticyclonic patterns dominate (Jevrejevaand Moore, 2001). Climatic conditions during the wintermonths (DecembereFebruary) largely control the develop-ment of the ice cover, and the climate earlier in the year seemsto have little importance on the ice dynamics (Omstedt andChen, 2001).

Ice formation begins at the northernmost Bothnian Bay andthe easternmost Gulf of Finland in OctobereNovember. Nextto freeze are the Quark between the Bay of Bothnia and Both-nian Sea (Fig. 1), the entire Bothnian Bay and the coastal areasof the Bothnian Sea (Fig. 1 and Table 1). In average winters,the ice also covers the Bothnian Sea, the Archipelago Sea,the Gulfs of Finland and Riga as well as the northern part ofthe Baltic Proper. In severe winters, the Danish Straits andthe southern Baltic Proper are also covered with ice. Locallyice formation usually starts within sheltered bays and skerriesand the ice edge moves outward from the coasts as the winterprogresses. Ice forms first in the inner skerries and bays wherethe water is often fresher and shallower, thus has a lower heatcontent, and where the ice cover can be anchored to islandsand shoals (Fig. 2). This prevents the break-up of evena thin ice cover by winds or waves (Palosuo, 1963). Theland-fast ice cover usually extends to the outer skerries, wherethe water depth is typically between 5 and 15 m. Further off-shore the ice cover is highly dynamic (Lepparanta, 1981).

In spring, along with increasing solar irradiation, the ice be-gins to melt starting from south. The northern Baltic Proper isnormally open by the beginning of April. In early May, the iceprevails only in the Bothnian Bay, and has completely melted

147M. Granskog et al. / Estuarine, Coastal and Shelf Science 70 (2006) 145e160

Table 1

Ice winters between 1971 and 2000. Dates and values shown are medians (Data taken from Kalliosaari, 1978, 1982; Kalliosaari and Seina, 1987; Seina and

Kalliosaari, 1991; Seina et al., 1996, 2001)

Ice station Number of ice

station in Fig. 1

Date of first

freezing event

Date of permanent

ice cover

Break-up of permanent

ice cover

Final disappearance

of ice

Number of

real ice days

Ajos 1 10 November 16 November 11 May 18 May 183

Merikallat 2 4 December 30 December 5 May 25 May 154

Tankar 3 2 January 22 January 24 April 12 May 110

Valassaaret 4 16 December 16 December 30 April 6 May 138

Salgrund 5 15 December 29 December 5 April 10 April 106

Kylmapihlaja 6 26 December 16 January 4 April 12 April 94

Bogskara 7 14 February 18 February 21 March 27 March 0

Russaro 8 28 January 7 February 27 March 12 April 52

Helsingin matala 9 24 January 17 February 4 April 14 April 55

Orrengrund 10 31 December 22 January 16 April 25 April 99

Suursaari 11 21 January 27 January 6 April 21 April 88

a Seventeen winters were ice free.

by early June. Statistics for ice conditions in 11 locationsalong the coasts of Finland for years 1971e2000 are presentedin Table 1. The duration of an ice winter varies from some 20to 30 days in the northern Baltic Sea Proper to more than 6months in the northern Bothnian Bay (see also Seina andPeltonen, 1991; Jevrejeva et al., 2004).

Sea ice systems are considered to be vulnerable to climatechange, the effects of global warming are predicted to affectthe Arctic sea ice in particular (Comiso and Parkinson,2004). Jevrejeva et al. (2004) examined the evolution of iceseasons in the Baltic Sea during the 20th century. Their 100-year time series shows a general trend toward reduced ice con-ditions; the largest change being the length of ice season,which has decreased by 14e44 days during the last century.There has also been a reduction of about 8e20 days per cen-tury to earliest ice break-up which the authors relate to a warm-ing trend in winter air temperatures over Europe. Presentclimatic models also predict significant large-scale changesalso in the Baltic Sea region (Houghton et al., 2001; Meier,2002). These include changes in the water balance of the entireBaltic catchment area, and substantial increase of mean tem-peratures. The predicted changes are expected to be the most

Fig. 2. Baltic land-fast ice. The attachment to skerries and coastal land results

in a more stable ice cover than the more dynamic pack ice characteristic of

more open waters.

extensive during the cold season. Average winter temperaturesin northern Europe may have increased by several degrees bythe year 2100 (Meier, 2002). The ice-covered area in the BalticSea would decrease by about 45,000 km2 for each 1 �C in-crease in the average temperature and during mild wintersonly the northernmost and easternmost parts of the Gulf ofBothnia and the Gulf of Finland would freeze. For an air tem-perature increase of about 4 �C in the winter the Baltic Seawould become completely ice free (Omstedt and Hansson,2006). Naturally, the fact that a major cryosphere system islikely to undergo such major changes heightens the need forour understanding the role the ice currently plays within theseasonal dynamics of the Baltic Sea, and in turn the conse-quences of any predicted changes.

2. Sea ice in the Baltic Sea

There have been relatively few studies to actually study thesmall-scale physical properties of Baltic Sea ice, and in gen-eral detailed observations of ice formation in brackish watersare sparse (Weeks et al., 1990; Granskog et al., 2004a, inpress-a). In contrast, formation of sea ice in oceanic waters(salinity of around 34) is well documented, and thoroughly re-viewed by Weeks and Ackley (1982) and Eicken (2003). Thelatter also compares the differences between freshwater andsea ice. Sea ice formation in any brackish water (salinity<24.7), such as the Baltic Sea, resembles more the ice formedin freshwaters than that which takes place in the oceans. Thisis because during the cooling phase of the water column andduring initial freeze-up, the temperature of maximum densityof (brackish) seawater is reached prior to its temperature offreezing (Weeks and Ackley, 1982; Granskog et al., in press-a). Also the physical processes at the ice/water interface result-ing in the salinity distribution in the solidifying sea ice maydiffer, partly because thermal convection at the interface is re-stricted as the result of the low salinities (<24.7) of the water(Granskog et al., in press-a).

Even though the northern Baltic Sea has low surface watersalinity, the ice formed resembles that of sea ice, with pre-ferred horizontal c-axis, jagged grain boundaries, and


a substructure within the grains associated with brine layers(Palosuo, 1961; Kawamura et al., 2001a). At parent water sa-linities higher than about 0.6, the ice formed has these sea icecharacteristics, and it is only in proximity of river estuaries isthe ice basically freshwater ice (Palosuo, 1961).

2.1. Physical characteristics of Baltic Sea ice

Several studies have given us a basic knowledge about thetextural characteristics of Baltic Sea ice (Omstedt, 1985;Fransson et al., 1990; Weeks et al., 1990; Kawamura et al.,2001a; Granskog et al., 2003b, 2004a), which provide a recordof the growth history and conditions of the ice. These studieshave shown there are large variabilities in ice structure and thecontribution of different ice types to the total ice thickness.Studies in the Gulf of Bothnia pack ice imply that dynamicthickness growth, i.e. ridging and rafting, play an importantrole (Omstedt, 1985; Granskog et al., 2004b). For level icethat was possibly rafted, Weeks et al. (1990) reported thatup to 80% of the ice cover was composed of granular ice.Fransson et al. (1990) and Granskog et al. (2004b) also re-ported very variable textural properties of Bothnian Baypack ice. Recently, ice thickness measurements made fromHelicopter borne electromagnetic techniques have shownthat there is a greater amount of deformed ice, with greatermean thickness (>1 m) than has hitherto been reported in rou-tine ice charts (Haas, 2004).

There are few detailed observations on the initial formationand temporal evolution of the ice in pack ice areas of theBaltic Sea. The above studies indicate that dynamic growthconditions may prevail, and the thickness growth and develop-ment of sea ice in the open areas of the Baltic Sea may resem-ble that of Antarctic waters, where the pancake ice cycle hasbeen suggested to be important in turbulent waters (Langeet al., 1989). In contrast the land-fast ice, which usually ex-tends to the 5e15 m isobath (Lepparanta, 1981), grows morestatically than the ice in the open sea (Granskog et al.,2003b). Dynamic thickness growth is therefore less importantand thermodynamic growth, both at the ice/water and ice/snowinterfaces, is the main contributor to ice thickness growth. Thelevel land-fast ice cover can generally be divided into a two-layer medium, with a granular upper layer and a columnarice bottom layer (Kawamura et al., 2001a; Granskog et al.,2003b). The upper layer is partly composed of snow-ice orsuperimposed ice and the remaining is frazil-ice, ice formedduring turbulent conditions in super-cooled water (Kawamuraet al., 2001a), although the contribution of frazil-ice for Balticland-fast sea ice growth seems very limited (Granskog et al.,2003b, 2004a).

Occasionally a third ice type is present, namely transitionice (or intermediate granular columnar Weeks et al., 1990;Granskog et al., 2003b). Transition ice shows very irregularhorizontal banding with crystals exhibiting a slight elongationin the vertical (growth) direction (Weeks et al., 1990). In theAntarctic, transition ice has been associated with nearby leads,ice deformation and a turbulent hydrodynamic regime (Eickenand Lange, 1989). According to Weeks et al. (1990) and

Granskog et al. (2003b) a similar explanation may also applyfor the Baltic Sea. Omstedt (1985) and Fransson et al. (1990)also found considerable amounts of transition ice layers in thepack ice of the Bothnian Bay, further support for dynamic con-ditions during ice growth in the area.

The contribution of snow-ice and/or superimposed ice tothe thickness growth of land-fast sea ice in the Baltic Seacan be substantial, and is far greater than in the high Arctic(Kawamura et al., 2001b; Granskog et al., 2003b, 2004a, inpress-b), this is exemplified by the highly negative d18O inthe surface layers (Fig. 3), which is indicative of contributionfrom meteoric ice (i.e. precipitation transformed to ice). Lim-ited samples from the Bothnian Bay indicate that the meteoricice contribution is relatively high in pack ice as well (Gran-skog et al., 2004b). Palosuo (1963), for example, studied thecontribution of white ice using a special gauge-stick, on whichzero-level mark was installed on the upper surface of the ice in

0 0.4 0.8 1.2 1.6 21

0.8

0.6

0.4

0.2

0

Salinity

norm

alis

ed d

epth

a

20 15 10 51

0.8

0.6

0.4

0.2

0

δ18O (per mil)

norm

alis

ed d

epth

b

0 2 4 6 8 101

0.8

0.6

0.4

0.2

0

Brine volume (%)

norm

alis

ed d

epth

c

Fig. 3. Typical profiles of (a) salinity, (b) d18O (per mil), and (c) brine volume

(%) versus normalized depth in the ice cores collected in land-fast sea ice in

the northern Baltic Sea in late winter or early spring. Thin solid lines, sites in

the Bothnian Bay; thin dotted lines, sites in the Bothnian Sea; thin dashed

lines, sites in the Archipelago Sea and the Gulf of Finland. Thick solid lines

with circles give average values, computed for all values in 0.2 normalized

depth unit bins. Modified from Granskog et al. (2003a).


the very beginning of the ice season. He found that almost halfof the land-fast ice cover could be composed of white ice, ormore exactly from upward (thermodynamic) growth of theice sheet. In a 3-year seasonal study of the mass-balance ofland-fast sea ice at one site in the Gulf of Finland, Granskoget al. (2004a) found that the snow contributed up to 35% ofthe total sea ice mass, however, the inter-annual variabilitywas large (0e35% depending on season and year). A signifi-cant part of the meteoric ice was thought to be superimposedice, 9e20% of the total ice thickness. Recent observations inthe Gulf of Bothnia indicated that practically the wholesnow cover was transformed to superimposed ice duringspring, the spring freeze/thaw period predominantly with day-time melt and nighttime refreeze, contributing up to 22% ofthe total ice thickness (and mass) at the time all snow had dis-appeared (Granskog et al., in press-b). Cheng (2002) alsonoted (through observations and modeling) that surface andsubsurface snow melt contributed to ice growth at the icesurface in the Gulf of Bothnia. Much snow accumulates onthe deformed pack ice fields, although knowledge on theamount and potential contribution to sea ice mass is virtuallynon-existent in the pack ice and deserve more attention.

The only oxygen isotopic observations of Baltic pack ice in-dicate meteoric ice contribution from 6 to 23% in the BothnianBay (Granskog et al., in press-b). Several studies have shownthat snow contributes significantly to the thickness growth ofsea ice in Antarctic waters (Lange et al., 1990; Eicken et al.,1994; Jeffries et al., 1994, 1997, 1998; Kawamura et al.,1997). The reported maximum snow fraction in Antarctic seaice is 14e16% of the total sea ice mass (Jeffries et al., 1997).In the seasonally ice-covered Sea of Okhotsk, the snow fractionwas estimated to be about 8% (Ukita et al., 2000), whereas inthe high Arctic the contribution of snow-ice seems negligiblefor land-fast sea ice growth (Kawamura et al., 2001b).

Observations of meteoric ice indicate a significant contribu-tion from snow to the development of Baltic Sea ice as com-pared to other ice-covered seas, especially in years when thesnow accumulation is high (Palosuo, 1963; Kawamura et al.,2001a; Granskog et al., 2004a). The relatively thin ice sheetin the Baltic Sea readily supports snow-ice formation. A rela-tively small amount of snow deposited onto the ice cover canresult in flooding of the ice cover with seawater potentially re-sulting in snow-ice formation. The mild conditions again seemto be favorable for the formation of superimposed ice layers(Kawamura et al., 2001a; Cheng, 2002; Granskog et al.,2004a, in press-b). The response of snow-ice and superim-posed ice formation in future climate scenarios needs to be ex-amined in more detailed, using models that can take intoaccount the small-scale physics involved in surface meltethaw cycles.

2.2. Salinity and brine volume of Baltic Sea ice

Bulk salinity is a fundamental and routinely measured sea iceproperty. The brine trapped into the sea ice lattice is importantfor several reasons. The volume of the liquid brine, which de-pends on the bulk salinity and temperature, governs the

permeability of the ice cover, and is important for the geophys-ics, biology and remote sensing of sea ice covers (Golden et al.,1998). The thermal conductivity, mechanical, electrical, opti-cal, and acoustical properties are in many cases a function ofthe sea ice porosity, i.e. brine volume. This in turn is associatedwith the amount and distribution of brine in the sea ice (Cottieret al., 1999). Several processes have been considered to governthe salinity of sea ice at a given time, including; initial brine en-trapment and desalination processes such as brine pocket migra-tion, brine expulsion, gravity drainage and flushing (e.g. Weeksand Ackley, 1982). Desalination processes can be classified intotwo types, those in cold ice during the growth period and thosein warm ice (Eicken, 2003). Detailed accounts on desalinationprocesses for sea ice formed at oceanic salinities are given byWeeks and Ackley (1982, 1986), Weeks (1998) and Eicken(2003). Whenever seawater freezes, most impurities are re-jected from the ice lattice, resulting in plates of pure ice. Theplates originate as dendrites with tips protruding into the seawa-ter. It is between these tips that brine is trapped. The plate widthcan vary from a few tenths of a millimeter to 1 millimeter and isdependent on the growth rate. With increased warming, discon-nected brine inclusions coalesce into vertical channels that canlead to redistribution, drainage and desalination of the ice(Meese, 1989; Weeks, 1998; Eicken, 2003).

Initial brine (salt) entrapment at the salinities of ocean wa-ters is described by Cox and Weeks (1975, 1988) and Nakawoand Sinha (1981). Experiments and observations have shownthat initial brine entrapment is related to the growth rate andis directly proportional to the seawater salinity. However,detailed studies on the relation of growth rate and segregationfor Baltic Sea ice are rare (Granskog et al., in press-a). Recentobservations indicate that the segregation of salt differslightly from the models obtained for Arctic sea ice (c.f. Na-kawo and Sinha, 1981), this again is likely to be caused bythe difference of the morphology (geometry) of the ice/waterinterface in comparison to the freezing of more saline solu-tions (Granskog et al., in press-a) Deviations of observed sa-linity profiles from that predicted by the growth rate andsegregation model are due to desalination processes duringageing of the ice. In general the desalination processes (orthe most efficient ones) are dependent on the porosity ofthe ice, and the porosity of the Baltic Sea ice expectedlydiffer from that of more saline ice (Meiners et al., 2002).However, quantitative information on desalination processesin Baltic Sea ice is virtually non-existent (Granskog et al.,in press-a). Such information would be vital towards thedevelopment of a Baltic Sea ice thermodynamic-salinitymodel that could also be used for development of an ecolog-ical sea ice model to resolve the potential consequences ofchanges in ambient conditions in the sea ice habitat.

Due to its fundamental importance (often as an explanatoryvariable), the salinity of Baltic Sea ice has been measured rou-tinely, during geophysical as well as ecological sea ice inves-tigations. However, this information is very sporadic both intime and space, except for a few recent investigations (Gran-skog et al., 2004a, in press-a). Baltic Sea ice reflects the lowwater salinities: the bulk salinities in the northern Baltic Sea


are generally less than 2 (Fig. 3a), and even lower dependingon the ambient water salinity, growth conditions and thermalhistory of the ice (Palosuo, 1963; Fransson et al., 1990; Kawa-mura et al., 2001a; Granskog et al., 2004a, in press-a).

The most systematic study on ice salinity and its develop-ment in the Baltic Sea is that by Palosuo (1963), even thoughthe vertical resolution was rather coarse. The ice salinity wasmonitored on a weekly basis at seven sites along the Finnishcoast, parent water salinities ranged from almost freshwaterup to salinities between 5 and 6. Salinity profiles of BalticSea ice have also been reported in several other studies (e.g.Fransson et al., 1990; Weeks et al., 1990; Kawamura et al.,2001a; Granskog et al., 2003b, 2004a, in press-a). Quite oftenthe salinity profiles found in Baltic Sea ice does not have theso typical C-shaped appearance (Fig. 3a) encountered in Polarregions (Granskog et al., in press-a). Usually the highest salin-ities are found in the uppermost parts of the ice cover, presum-ably because of rapid growth, flooding and snow-ice formation,while often salinity tends to decrease towards the bottom(Fig. 3a, Granskog et al., 2004b, in press-a). For land-fast seaice this might be caused by slower growth rates or also insome cases by under-ice flow and mixing of fresher river watersor changes in the morphology of the ice/water interface withchanges in growth rate (Granskog et al., 2005a,c, in press-a).

The low ice salinity is also reflected by relatively low brinevolumes (Fig. 3c, Meiners et al., 2002), despite the quite highice temperatures. This may affect the habitability of sea ice byorganisms in the Baltic Sea (Krembs et al., 2001, and refer-ences therein). The permeability of the ice is affected by theinteraction of temperature, brine salinity and brine volume.According to the law of fives (Golden et al., 1998) at a temper-ature of �5 �C and salinity of 5 sea ice has a brine volume of5%, which makes it permeable because brine pockets and in-clusions are then assumed to be interconnected. However, inthe Baltic Sea the law of fives does not obviously hold true,since the sea ice salinity very seldom reaches 5, except inthe southernmost parts (Mock et al., 1997). Using the formu-lations of Lepparanta and Manninen (1988), one can calculatethat for ice with a bulk salinity of 1 a temperature as high as�1 �C is needed for brine volumes large enough for BalticSea ice to become permeable. That is, if the relation betweenice permeability and porosity holds true for the low saline Bal-tic Sea ice as it does for more saline sea ice. Low brine vol-umes obviously affect transfer across the ice/water interface,which is important for processes such as nutrient replenish-ment for ice-associated algae and for convective heat transportthrough the sea ice (Lytle and Ackley, 1996), as well as for de-salination processes of Baltic Sea ice. However, it should al-ways be kept in mind that hitherto the data sets arepredominantly from land-fast ice, and information from thepack ice, except in the Bothnian Bay is very sporadic.

2.3. Baltic Sea ice as a habitat

In addition to ice-associated micro-organisms, ice can alsobe important for marine mammals as a breeding ground. In theBaltic Sea, the Baltic grey seal (Halichoerus grypus) and

Baltic ringed seal (Phoca hispida botnica) use sea ice as theirprimary breeding habitat. The grey seal can also breed suc-cessfully on land but ringed seal is dependent on ice for breed-ing (Meier et al., 2004). Anticipated climate change induceddiminishing of Baltic Sea ice cover may lead to survival ofBaltic ringed seal in the Bothnian Bay only and thus posesa major threat to all southern populations of Baltic ringedseal (Meier et al., 2004).

Though a northward shift in range margins has been ob-served on many bird species in Finland, probably due to cli-mate change (Brommer, 2004), very little is known on theeffect of decreasing ice cover to Baltic Sea seabirds. The fre-quency of mild winters is considered to be one of the factorsleading to recent increases of some bird species: in the Gulfof Finland, at least mute swan (Cygnus olor), cormorant (Pha-lacrocorax carbo), barnacle goose (Branta leucopsii) andgreylag goose (Anser anser) have become more common.

2.4. Sea ice assemblages associated with Baltic Sea

Compared with the relatively well established research intosea ice physical features the study of the biota and assem-blages living within and under the ice in the Baltic Sea onlystarted in the mid 1980s (Huttunen and Niemi, 1986), andmore systematically only from mid 1990s (Norrman and An-dersson, 1994; Laamanen, 1996; Ikavalko and Thomsen,1996, 1997). This again contrasts with that known from Polarregions where extensive research has been conducted on seaice assemblages more than 100 years (reviews by Brierleyand Thomas, 2002; Thomas and Dieckmann, 2002a; Lizotte,2003). To date, the biological studies in Baltic Sea ice havebeen mostly restricted to research conducted in land-fast icein the Gulf of Bothnia (Quark area, Norrman and Andersson,1994; Haecky et al., 1998; Haecky and Andersson, 1999;Kaartokallio, 2001) and Gulf of Finland (Tvarminne area,Granskog et al., 2003a; Kaartokallio, 2004; Werner andAuel, 2004; Kuosa and Kaartokallio, 2006; Rintala et al.,2006), and only occasionally in the Kiel Bight (southern Bal-tic) and pack ice of the Gulf of Bothnia (Mock et al., 1997;Petri and Imhoff, 2001; Meiners et al., 2002; Ikavalko et al.,2004).

During the open-water period, sub-basins of the Baltic Seadiffer significantly in terms of salinity and nutrient regime, to-tal productivity as well as structure and functioning of theplanktonic food webs (Hagstrom et al., 2001). Currently it isunclear to what extent analogous differences exist in sea iceand the biology it supports. While spatial variability of seaice properties has been investigated by several studies in theArctic and Antarctic, the topic has been addressed only re-cently in the Baltic Sea (Granskog et al., 2005b; Steffenset al., in press). There appears not only sub-basin scale varia-tions in the sea ice properties (Meiners et al., 2002; Granskoget al, 2003a), but also regional scale variation in coastal sea iceproperties (Granskog et al., 2005b), largely controlled by sub-basin differences and inshoreeoffshore gradients in salinity.

Seasonal averaged biomass of ice organisms (Granskoget al., 2003a) and the ice algal community composition varies


between Gulf of Bothnia and Gulf of Finland (Huttunen andNiemi, 1986; Ikavalko and Thomsen, 1997) Typical valuesfor chlorophyll-a in the Gulf of Finland ice are in the rangeof 0.3e5.5 mg Chla m�2 (Kaartokallio, 2004; Granskog et al.,2005b,c; Kuosa and Kaartokallio, 2006) and for the Gulf ofBothnia ice 0.2e2.2 mg Chla m�2 (Haecky et al., 1998; Haeckyand Andersson, 1999; Kaartokallio, 2001; Meiners et al., 2002;Granskog et al., 2003a; Steffens et al., in press). Existing, rela-tively sparse data suggest that there are also differences in seaice food web structure and nutrient dynamics between sub-basins. Auto- and heterotrophic flagellates seem to be moredominant in food webs in ice from the Gulf of Bothnia com-pared with ice from the Gulf of Finland (Haecky and Ander-sson, 1999; Meiners et al., 2002; Kaartokallio, 2004).

In Polar sea ice, the most important organism group withregard to biomass and production are the ice algae, most oftendominated by pennate diatoms, although this may be simplya reflection that they are more easily preserved than less robustautotrophs (Lizotte, 2003). The main heterotrophic organismgroups in the Polar sea ice are bacteria, diverse heterotrophicflagellates and ciliates, which are the most diverse groupamong the non-algal protists in sea ice (Lizotte, 2003). Thepresence of organisms living in the Baltic Sea ice is knownfor some decades (Grøntved, 1950). However, the knowledgeon their species composition and distribution is still largely un-systematically gathered. Specifically, we lack information onthe heterotrophic components of the ice-related food webs,and we also know very little on the overwintering of cyano-bacteria, which are so typical to the Baltic (Laamanen,1996). The only comprehensive publication is that of Ikavalkoand Thomsen (1997). However, even the limited data set con-firms that the Baltic Sea ice consists of taxonomically diversecommunity of protists (Table 2).

Pennate diatoms are also, in terms of biomass, the dominantalgae in the sea ice of the different sub-basins of the Baltic Sea(Huttunen and Niemi, 1986; Ikavalko and Thomsen, 1997;Haecky et al., 1998; Haecky and Andersson, 1999). Other im-portant algal groups in the Baltic Sea ice are small autotrophicflagellates (Crypto-, Hapto-, Chryso- and Prasinophytes, Ika-valko and Thomsen, 1997; Ikavalko, 1998) and dinoflagel-lates, the latter occasionally even being the dominant algalgroup (Kuosa and Kaartokallio, 2006; Kaartokallio et al., inpress). The main heterotrophic organisms in Baltic Sea iceare diverse heterotrophic flagellates and ciliates of varioussizes (Ikavalko and Thomsen, 1997; Haecky and Andersson,1999; Meiners et al., 2002; Kaartokallio, 2004; Kaartokallioet al., in press) and heterotrophic bacteria (Mock et al.,1997; Meiners et al., 2002; Kaartokallio, 2004).

Ciliates are a diverse organism group in the Baltic Sea iceand have complex food web interactions, including grazing,mixo- and autotrophy. The ciliates in Baltic Sea ice are mainlyoligotrichs found also in the water column (Kaartokallio et al.,in press). The scarcity of present data does not allow a full re-view, but the genus Strombidium appears to be dominant inmany samples. The ciliate community consists of small(20e80 mm) cells, which gives an indication on their role asgrazers of small particles, possibly including bacteria.

Mixotrophic species have not been recorded with the excep-tion of Mesodinium rubrum (Kaartokallio et al., in press). Anotable feature of the ice-associated ciliate fauna is the growthof large-sized ciliates under the ice (Kaartokallio et al., inpress). These ciliates obviously grazed on dinoflagellates,which is not common among Baltic Sea ciliates.

The under-ice food web dynamics requires much more at-tention in future research. However, it appears that under-icealgal blooms dominated by dinoflagellates are a common fea-ture in the Gulf of Finland (Larsen et al., 1995). Chlorophyllvalues as high as 1500 mg l�1 have been recorded under clearice (Kuosa, unpubl.). However, these blooms concentrate onthe top centimeters of the water column, the reason being ei-ther the active swimming of the dinoflagellates or vertical sa-linity gradients, which are very common near shoreline duringwinter in the Baltic Sea. There is only sparse data on under-iceblooms in the Gulf of Bothnia (Muller-Haeckel, 1981).

Heterotrophic bacteria are the most abundant group of pro-karyotic organisms in Polar sea ice and also important with re-gard to biomass (Lizotte, 2003; Mock and Thomas, 2005).Studies which include bacterial production estimates or focussolely on Baltic Sea ice bacterial assemblages are scarce andhave been done in the Quark area (Norrman and Andersson,1994; Haecky and Andersson, 1999) and Kiel Bight (Mocket al., 1997; Petri and Imhoff, 2001) and recently also in theGulf of Finland (Kaartokallio, 2004; Kuosa and Kaartokallio,2006; Kaartokallio et al., in press). Up to date, there are onlytwo published studies including data on composition of BalticSea ice bacterial communities. A study by Kaartokallio et al.(2005), although based on a limited data set, found phylotypessimilar to those in sea ice in both Polar areas. Presence of b-proteobacteria suggest that Baltic Sea ice bacterial communitycomposition may be closer to Arctic sea ice, which would beconsistent with geographical location and the high freshwaterinfluence in both areas. An earlier study from the Kiel Bightreported that the ice bacterial community had little resem-blance to Polar sea ice counterparts (Petri and Imhoff,2001). The reason for observed differences in bacterial com-munity composition remains unclear. One explanation wouldbe the sporadic ice occurrence in the southern Baltic Seathat would affect the presence of ice-related psychrophiles inthe parent water bacterial populations. Interestingly, Petriand Imhoff (2001) report the occurrence of 16S rDNA se-quences related to fermenting bacteria and anoxygenicphototrophic purple sulphur bacteria, which implies theoccurrence of anoxic microzones in the sea ice environment.Results indicating active denitrification in Baltic Sea ice(Kaartokallio, 2001) and in Arctic sea ice (Rysgaard andGlud, 2004) further support the hypothesis on the existenceof such microzones.

Whereas the diverse metazoan assemblages of oceanic seaice are relatively well described (Schnack-Schiel, 2003), onlyrotifers and copepods have been described in Baltic Sea ice(Norrman and Andersson, 1994; Meiners et al., 2002; Ika-valko et al., 2004; Werner and Auel, 2004). It is proposedthat the small brine volumes effectively exclude largergrazers (c.f. Krembs et al., 2000; Kaartokallio, 2004). Werner


Table 2

Common ice-related organisms of the Baltic Sea

Species Citations

Algae

Dinophyta

Peridiniella catenata (Levander) Balech

(syn. Gonyaulax catenata)

Muller-Haeckel (1981), Huttunen and Niemi (1986),

Ikavalko and Thomsen (1997), Haecky et al. (1998), Meiners et al. (2002)

Scrippsiella hangoei (Schiller) Larsena Larsen et al. (1995), Meiners et al. (2002)

Haptophyta

Chrysochromulina birgerii Hallfors & Niemi Hallfors and Niemi (1974)

Chrysophyta (Diatomophyceae)

Achnanthes taeniata Grunow Huttunen and Niemi (1986), Haecky et al. (1998), Meiners et al. (2002)

Chaetoceros spp. Huttunen and Niemi (1986), Ikavalko and Thomsen (1997),

Haecky et al. (1998), Meiners et al. (2002)

Fragillariopsis cylindrus (Grunow) W. Krieger Meiners et al. (2002)

Melosira arctica (Ehrenberg) Dickie ex Ralfs Huttunen and Niemi (1986), Ikavalko and Thomsen (1997),

Haecky et al. (1998), Meiners et al. (2002)

Navicula pelagica P.T. Cleve Ikavalko and Thomsen (1997)

Navicula vanhoeffenii Gran Huttunen and Niemi (1986), Ikavalko and Thomsen (1997)

Nitzscia frigida Grunow Grøntved (1950), Huttunen and Niemi (1986), Ikavalko and

Thomsen (1997), Haecky et al. (1998), Meiners et al. (2002)

Thalassiosira spp. Huttunen and Niemi (1986), Ikavalko and Thomsen (1997),

Meiners et al. (2002)

Chlorophyta

Mantoniella squamata (Manton and Parke) Desikachary

Monoraphidium contortum (Thuret in Brebisson) Koma Ikavalko and Thomsen (1997), Meiners et al. (2002)

Oocystis spp. Ikavalko and Thomsen (1997)

Protozoa (obligate heterotrophs)

Chrysophyta

Paraphysomonas spp. Ikavalko and Thomsen (1996), Ikavalko and Thomsen (1997)

Chlorophyta

Polytoma papillatum Pascher Ikavalko and Thomsen (1997)

Choanoflagilledia

Diaphanoeca sphaerica Thomsen Ikavalko and Thomsen (1997)

Savillea microporea (Norris) Leadbeater Ikavalko and Thomsen (1997)

Incertae sedis

Cryothecomonas armigera Thomsen et al. Ikavalko and Thomsen (1997), Meiners et al. (2002)

Quadricilia rotundata (Skuja) Vørs Ikavalko and Thomsen (1997)

Protaspis spp.

Ciliata

Strombidium spp. Ikavalko and Thomsen (1997); Kuosa et al. (unpubl.)

Metazoa

Rotatoria

Keratella spp. Meiners et al. (2002), Werner and Auel (2004)

Synchaeta cf. baltica Ehrenberg Norrman and Andersson (1994), Meiners et al. (2002)

Synchaeta cf. littoralis Rousselet Kaartokallio (2004), Werner and Auel (2004)

Copepoda

Acartia bifilosa (Giesbrecht) Werner and Auel (2004)

a Includes most probably also Wolozynskia halophila (Biecheler) Elbrachter and Kremp (Kremp et al., 2005).

(pers. comm.) suggests that the sea ice in the Baltic functionsas an overwintering habitat, feeding ground for rotifers andpossibly for copepod nauplii, as well as a reservoir for rotiferresting eggs. The ice underside is also an important feedingground in winter for dominant species of calanoid copepodssuch as Acartia bifilosa. Populations of this species havebeen shown to reproduce, grow and develop under-ice cover(Werner and Auel, 2004).

In Antarctic sea ice the initial colonization during the seaice formation is typically followed by low-productive winterstage, bloom of the ice algae and finally a heterotrophy-dominated stage late in the season (Grossmann and Gleitz,1993; Gunther and Dieckmann, 1999). Biomass accumula-tion of the sea ice algae generally follows the seasonalincrease in solar radiation beginning at the transition ofwinter and spring and lasts until the onset of ice melt


(Cota et al., 1991; Norrman and Andersson, 1994; Haeckyand Andersson, 1999). Heterotrophic processes increase insignificance in late-bloom and post-bloom situations latein the sea ice season (Stoecker et al., 1993; Vezina et al.,1997). A similar succession sequence beginning witha low-productive winter stage followed by an algal bloomand a heterotrophy-dominated post-bloom situation hasbeen documented in the Baltic Sea ice as well (Fig. 4,Haecky and Andersson, 1999; Kaartokallio, 2004). Due tothe geographical location, the timing of ice algal bloom dif-fers between sub-basins, occurring in March in Gulf of Fin-land (Kaartokallio, 2004; Kuosa and Kaartokallio, 2006),and in April in Gulf of Bothnia (Norrman and Andersson,1994; Haecky et al., 1998; Haecky and Andersson, 1999).In Gulf of Finland the occurrence of another, minor algalbiomass maximum during a low-light period in Januaryhas also been reported (Kaartokallio, 2004; Kuosa and Kaar-tokallio, 2006).

Due to space limitation in the brine channels, internal seaice food webs are considered to be truncated, meaning that or-ganisms larger than the upper size limit of channels are lack-ing (Krembs et al., 2000). Clearly this may be even more thecase for low-salinity Baltic Sea ice (Meiners et al., 2002) com-pared to ice from oceanic origin. This simplifies the ecosystemby lowering the diversity of ecological relationships. Althoughsea ice food webs should thus be fairly simple to describe con-ceptually, sampling difficulties, the ephemeral nature of thesea ice system and significant interactions with the underlyingwater column severely limit our ability to quantify the sea icemicrobial food webs (Lizotte, 2003). Different ‘short circuits’in flow of energy and organic matter are typical of microbialfood webs inside the sea ice (Fig. 5). These include herbivoryby ciliates and flagellates, ciliate bacterivory and direct utiliza-tion of DOM by heterotrophic flagellates (Sherr, 1988; Gra-dinger et al., 1992; Stoecker et al., 1993; Laurion et al.,1995; Sime-Ngando et al., 1997; Vezina et al., 1997; Haeckyand Andersson, 1999). Of these, at least direct utilization ofDOM by flagellates (Haecky and Andersson, 1999) and ciliate

grazing over several size classes have been suggested to befunctional in Baltic Sea ice (Kaartokallio, 2004).

2.5. Biogeochemistry of Baltic Sea ice

Although DOM accumulates in sea ice (Pomeroy andWiebe, 2001; Thomas and Papadimitriou, 2003), recyclingof DOM via the microbial loop is still considered to be a majorlink between primary and secondary producers (Gradingeret al., 1992). The significance of the microbial loop and micro-grazer herbivory is assumed to increase with decreasing algalproductivity (Laurion et al., 1995; Sime-Ngando et al., 1997).Sea ice bacteria are often larger than in the underlying water,both in Polar Oceans and the Baltic Sea, and their average cellsize increases along with the age of the ice (Grossmann andDieckmann, 1994; Gradinger and Zhang, 1997; Mock et al.,1997; Kaartokallio, 2001, 2004). The larger cell size is usuallyassumed to be a result of enhanced substrate availability, dueto high DOM concentration, or lowered grazing pressure in theice. Although several studies from Arctic sea ice suggest tightcoupling between bacterial biomass and DOM (Gradingeret al., 1992; Thomas et al., 1995 and references therein), thecoupling between primary production (supplying autochtho-nous DOM) and bacterial production (consuming DOM) inthe ice is not straightforward (Mock and Thomas, 2005).

Concentrations of dissolved organic carbon (DOC) in thesea ice environment are generally significantly higher than insurface waters (Thomas and Papadimitriou, 2003). In the Bal-tic Sea, however, the first measurements of ice DOC (Gran-skog et al., 2005b,c) show that DOC concentrations in iceare lower than in the underlying waters because of generallyhigh concentrations of terrestrially-derived DOC in the Balticseawater (Hagstrom et al., 2001). In the ice, the DOC isthought to mainly originate from material incorporated intothe ice during its formation (Giannelli et al., 2001; Thomaset al., 2001), as well as autochthonous matter produced by or-ganisms inhabiting the ice, the latter largely comprising carbo-hydrate-rich polysaccharides (Thomas et al., 2001; Herborg

PostbloomIce algal bloom

Algae

Bacteria Rotifers

Ciliates

M. rubrum

Het. flagellates

B C C

55

14

14

9

4

4 8387

10

7

5

4 2

26

10

30

53

1

A

Midwinter

Fig. 4. Biomass percentages of ice organism groups during the low biomass pre-bloom midwinter period (January), ice algal bloom [early March; 2 years (data

from B and C) presented] and heterotrophic post-bloom situation (late Marcheearly April) in Baltic Sea fast ice, SW coast of Finland. Numbers denote percentages

of total organism biomass, as mean values for the entire ice column. Data are taken from Kuosa and Kaartokallio (2006) (A), Kaartokallio et al. (in press) (B) and

Kaartokallio (2004) (C).


DOM

Heterotrophic Autotrophic

DOM

Heterotrophic Autotrophic

Sea ice Underice waterProbably move across the

ice-water interface

DOM

Diatoms Dinoflagellates Flagellates Bacteria Ciliates RotifersCalanoid

copepods

Fig. 5. Schematic representation of possible grazing and organismseDOM interactions in sea ice in the Gulf of Finland (left, sea ice left; right, under-ice water).

The prominent differences are the occurrence of copepod grazer (Acartia bifilosa) in the under-ice water, and the dominance of dinoflagellates in the reported

under-ice blooms. For species information see Table 1.

et al., 2001; Krembs et al., 2002). The complex origin of iceDOM may lead to uncoupled dynamics of DOC, DON andDOP (Thomas et al., 1995; Kaartokallio et al., in press). Gran-skog et al. (2005c) report accumulation of DOM derived fromalgal biomass in the lower ice layers in the Gulf of Finland.The first simultaneous measurement of DOM, primary produc-tion and bacterial production in a sea ice system (Kaartokallioet al., in press) found few significant correlations betweenDOM and bacterial parameters which may illustrate the com-plex origin of sea ice DOM in the Baltic Sea. Observed corre-lations between DON and bacterial production suggest thatDON concentration may be a useful indicator of refractoryDOM (Kaartokallio, 2004; Kaartokallio et al., in press). Basedon the observations by Kaartokallio et al. (in press), it seemsthat ice bacteria are dependent on autochthonous DOM pro-duced in the ice.

The main nutrient source in the sea ice internal habitat isthe initial nutrient entrapment during ice formation and, inolder sea ice, nutrient transport with brine movement(Dieckmann et al., 1991; Golden et al., 1998). Brine move-ment can transport nutrients across the ice/water interfacefrom the underlying water, in which dissolved nutrient concen-trations typically exceed those within the ice (Gradinger et al.,1992). Changes in ice temperature and porosity are importantfactors determining nutrient transport within the ice (Fritsenet al., 1994; Golden et al., 1998; Granskog et al., 2003a). Re-cycling of nutrients from allochthonous and autochthonousbiomass in the ice through decomposition and nutrient regen-eration by ice heterotrophs also constitutes an important nutri-ent source inside the ice (Cota et al., 1991). Ice bacteriadegrade particulate organic matter (POM) and regenerate nu-trients (Helmke and Weyland, 1995), as do phagotrophic

protists in the sea ice environment (Stoecker et al., 1993).The role of nutrients entrapped in the ice during formationand subsequent regeneration of this nutrient pool seem to bemore important in the Gulf of Bothnia than in the Gulf of Fin-land (Haecky and Andersson, 1999; Kaartokallio, 2004; Gran-skog et al., 2005c). This is probably due to the differentnutrient concentrations in the underlying water: in the Gulfof Finland maximum algal biomass is typically found in low-ermost ice section and phosphorus accumulates into the lowerice layers (Granskog et al., 2005c). In the Gulf of Bothnia icealgal spring bloom is located in the ice interior layers (Norr-man and Andersson, 1994; Haecky et al., 1998), concomi-tantly with highest nutrient regeneration activity(Kaartokallio, 2001). The first experimental results on growthlimitation of Baltic Sea algae and bacteria (Kuosa and Kaarto-kallio, 2006) show that light, nutrient and substrate limitationschange with progress of winter and the development of the seaice. Algal growth in Baltic Sea ice seems to be sequentiallylight- or nutrient-limited along with the winter progression(Haecky et al., 1998; Kuosa and Kaartokallio, 2006) as alsoshown for the Polar sea ice (Gosselin et al., 1990; Robinsonet al., 1998). Phosphorus is probably the most important singlelimiting nutrient for algal growth in ice (Haecky and Ander-sson, 1999; Granskog et al., 2005c; Kuosa and Kaartokallio,2006).

3. Atmosphereeice interactions

Snow on ice accumulates nutrients and trace elements fromthe air and atmospheric deposition onto the ice corresponds toroughly 5% of the total annual flux of nitrogen and phosphorusto the Baltic Sea, while for some heavy metals such as lead


and cadmium it can be 20e40% (Rahm et al., 1995; Granskogand Kaartokallio, 2004). Nutrients can be incorporated into theice sheet via snow-ice and superimposed ice formation, andeven transported deeper into the ice sheet when melting eventsoccur (Granskog et al., 2003a; Granskog and Kaartokallio,2004). At the time of ice/snow melt the sea ice is the majorsource of nutrients and trace elements to the surface BalticSea (Granskog and Kaartokallio, 2004).

The nitrogen concentrations are about twice as high in snow-ice layers than in the rest of the sea ice cover (Granskog et al.,2003a). In this aspect, the Baltic Sea ice evidently differs fromsea ice in any other region. In the Antarctic, there is evidencethat flooded seawater on heavily snow-laden sea ice introducenutrients to the upper parts of the ice cover (Fritsen et al.,1994). This resembles the nutrient replenishment in the BalticSea ice, but the mechanisms are somewhat different with the at-mosphere (precipitation and dry deposition) as a key nutrientsource in the Baltic Sea (Granskog et al., 2003a; Granskogand Kaartokallio, 2004). However, the importance of the atmo-sphere on the nutrient budget of Baltic Sea ice is predicted todecrease towards the north, based on the deposition estimatesin the Baltic Sea area (Granat, 2001). Apparently, the role of at-mospheric deposition of nutrients is less significant for the nu-trient status of Arctic than Baltic Sea ice.

Temporal changes in sea ice properties, in particular tem-perature which affects the porosity of the ice, seem to stronglyaffect the transport of accumulated nutrients from the ice sur-face to the under-ice water column (Granskog et al., 2003a). Ithas been shown that an apparent increase in ice porosity led toa flushing of nutrients through the ice cover which in turn in-creased the nutrient levels in the lower parts of the ice cover(Granskog et al., 2003a). This potentially affected the growthof ice algae. Brine and melt-water transport is important forBaltic Sea organism assemblages as temperature and perme-ability evolution can even directly control ice organismbiomass (Kaartokallio, 2001, 2004) analogously to the exper-imental systems (Krembs et al., 2000).

The mild ice climate conditions in the Baltic Sea are notonly responsible for the superimposed ice formation describedearlier (which require snow melt or liquid precipitation duringthe ice growth period) but also for the rapid changes in iceproperties, such as the temperature and the salinity, andthereby in porosity (Granskog et al., 2003a). The thin ice coverresponds quickly, usually within hours, to changes in atmo-spheric conditions, whether it is changes in air temperatureor snow accumulation (Granskog et al., 2003a).

As well as affecting the nutrient status of sea ice, the atmo-sphere is a source of trace elements (Granskog and Virkanen,2001; Granskog and Kaartokallio, 2004). Lead is accumulatedduring winter in such amounts that the sea ice and snow coverare potentially important sources of lead during melt. Evi-dently trace metals are not related to the salinity as stronglyas major ions (seawater salts), presumably because active sec-ondary processes, such as atmospheric deposition and biolog-ical processes control their concentrations more effectively(Granskog et al., 2004b). During initial ice formation, some el-ements such as iron and aluminium, are found in excess in sea

ice relative to salinity (Granskog et al., 2004b). The reason forthat remains, however, open. It cannot be stressed enough thatespecially studies looking at the temporal evolution of the seaice environment need to have a holistic approach in order tounderstand the observed changes, since they are caused byan interplay between physicochemical and biologicalprocesses.

It remains to be seen whether or not the snow and ice aresimply a passive transient storage of atmospheric deposition(of both inorganic and organic compounds), or an active plat-form for transformations (Granskog and Kaartokallio, 2004).To understand such processes research into the snow coveron sea ice, as well as its seasonal development, especially dur-ing spring meltefreeze cycles, deserves more attention. Fur-ther data are relevant both for the observation ofsuperimposed ice formation, as well as development and val-idation of thermodynamic sea ice models with the formationof snow-ice and superimposed ice layers included (Cheng,2002; Granskog et al., in press-b). The inclusion of the phaseof the precipitation realistically in the sea ice models will bevital to better reflect Baltic Sea conditions.

It will also be important to include photochemical researchto look at the transformation of deposited chemicals in the sur-face snow and ice layers. Sea ice may be an oxidizing mediumowing to sunlight-driven reactions occurring within the ice andsnow. In particular transmission and albedo of solar radiationfrom ultraviolet to blue wavelengths (approx. 320e450 nm)have been shown by King et al. (2005) to be importance inOH radical production from photolysis of hydrogen peroxide(H2O2) and nitrate in Antarctic sea ice. These researchers pro-pose that ice may be an efficient medium for photochemistryand that 85% of ice photochemistry may occur in the top 1 mof sea ice. Thomas and Dieckmann (2002b), have also proposedthat photochemical reactions may be important in the transfor-mation of DOM in surface ice layers. In support of this Xie andGosselin (2005) observed photochemical production of carbonmonoxide (and carbon dioxide) from DOM in arctic first-yearsea ice. The DOM load in Baltic Sea ice is substantial, tosuch a degree that the optical properties of the sea ice is quitedifferent from its Arctic or Antarctic counterparts, especiallythe high CDOM (chromophoric dissolved organic matter)load results in high absorption at shorter wavelengths (Ehnet al., 2004). Therefore the potential for photodegradation of or-ganic matter in Baltic Sea ice is high, but on the other hand pho-tochemical reactions are only possible in surface layers wherelight levels are not attenuated too much (c.f. Lepparantaet al., 2003), and the photochemically active layer is likely tobe shallower than in the Antarctic or Arctic.

Not much is known about the role of sea ice in the oceaneair exchange of atmospherically important constituents in theBaltic Sea. Although there is evidence that sea ice may playa role in the oceaneair exchange of DMS and CO2 in the PolarSeas (Papadimitriou et al., 2004; Zemmelink et al., 2005, inpress; Trevena and Jones, 2006). However, the processes inthe low-salinity and high-temperature ice in the Baltic Seamay differ from those in Polar Seas and to our knowledgethere are no observations to show how sea ice in the Baltic


Sea affects oceaneair exchange of such constituents, and suchmeasurements should be incorporated into future sea ice work.

4. Influence of sea ice on landesea exchange

The Baltic Sea has substantial input of river water, resultingin a significant introduction of DOM and inorganic nutrientsinto the Sea. Not only is river inflow into the Baltic one ofthe main sources for freshwater, nutrients and contaminantsin coastal areas, but also the freshwater increases the stabilityof the water column thereby promoting ice formation. Sea icealso enhances the dispersal of river water since the imperme-able cover decreases water mixing and allows river waterplumes to spread underneath the ice (Granskog et al.,2005a,c), and especially the extremely weak tides promote for-mation of distinct under-ice plumes by even the smallest riversin the Baltic Sea (Granskog et al., 2005a). However, only a lim-ited number of studies describe the coastal hydrography in ice-covered coastal areas impacted by river runoff (Alasaarela andMyllymaa, 1978; Granskog et al., 2005a,c).

Processes that control the under-ice flow of river watersstrongly influence the winter oceanography and transport path-ways of river borne particulate and dissolved matter (Gran-skog et al., 2005a). Hence, DOM and inorganic nutrients areexpected to be transported further away from the river mouthin winter, than in summer when mixing is more intense. Theconsequences for the microbial processes in the coastal watersare considerable leading to a quite different seasonal influenceon biogeochemical cycles when compared with non-ice cov-ered coastal waters (Granskog et al., 2005c; Kaartokallioet al., in press). In particular the high nutrient and DOM load-ing of freshwater layers under the ice may stimulate growth atthe ice/water interface. In contrast the freezing of freshwaterice onto the underside of the sea ice may restrict build-up ofice algal communities.

5. Conclusions and future outlook

The seasonal sea ice cover is a distinct feature of the BalticSea, although its extent may be mainly limited to the northern-most parts of the Gulf of Bothnia at the end of this century ifclimate changes as predicted. Such a drastic change in the seaice warrants more intense investigations into the role sea iceplays currently in the Baltic Sea system. We know severalpieces in the puzzle, but the complete picture of the role ofsea ice in the Baltic Sea system in regards to ecosystem func-tioning and biogeochemistry is lacking.

Within the framework of possible effects of climate change,an assessment of the changes to the future Baltic Sea when seaice absent is hard to make. However, there is no doubt in thepresent climate predictions that significant large-scale changesin the Baltic Sea region will occur (Houghton et al., 2001).These include changes in the water balance of the whole Balticcatchment area, and substantial increase of mean temperatureswith the predicted changes being most pronounced during thecold season. Average winter temperatures in northern Europemay increase by several degrees by the year 2100 (Meier,

2002). In conjunction with such increases the ice-coveredarea in the Baltic Sea would decrease and during mild wintersonly the northernmost and easternmost parts of the Gulf ofBothnia and the Gulf of Finland would freeze. For example,such changes would be a major threat to all southern popula-tions of the Baltic ringed seal. In the future the only remainingwinter sea ice habitat for this species will be found in theBothnian Bay (Meier et al., 2004). Rainfall is also predictedto increase in autumn and winter, which together with frost-free soil could lead to added dissolved organic and inorganicnutrient load from the catchments to the coastal waters.

The complex structure of sea ice cover need further consid-erations with regard both to dynamical (e.g. ice thickness dis-tribution) and thermodynamic aspects of the Baltic Seaphysics (Omstedt et al., 2004). Because of the importance ofmeteoric ice formation, its sensitivity to a warming climateneeds to be investigated more thoroughly. Recent observations(Haas, 2004) indicate that the total ice volume in the Balticmight be larger than previously thought. This has obviousconsequences for the studies of ice-associated biota and inparticular large-scale estimations of seasonal ice-associatedproductivity, which has not been attempted to date.

Since the atmosphere is a major source of inorganic nutrientsand pollutants in the Baltic Sea, the seasonal sea ice cover accu-mulates significant amounts and at the time of ice/snow melt thesea ice is the major source of nutrients and trace elements to thesurface Baltic Sea. A pertinent question to be addressed is towhat extent the ice and snow fields are passive or active in thetransformation of accumulated substances before release atice melt. Release of nutrients from the melting ice and its poten-tial consequences for the productivity of the Baltic Sea duringthe subsequent spring bloom period need also be studied.

Better estimates on contribution of ice to the overall pro-ductivity of Baltic Sea are also needed. The necessary datafor estimating the contribution of ice production in relationto the overall productivity of the Baltic Sea system are simplynot available. It is speculated that sea ice cover in the Balticmay have critical role in the overall nutrient and carbon cy-cling of the Baltic Sea. However, the necessary spatial andtemporal data to address such ideas are missing. There is a per-tinent need for sustained temporal biogeochemical pro-grammes to enable such measurements to be made, althoughclearly these need to be embedded within multidisciplinaryprogrammes where measurements of the physical propertiesof the ice and underlying water masses are also made.

Specifically, we lack information on the heterotrophic com-ponents of the ice-related food webs, and we also know verylittle on the overwintering of cyanobacteria, which are so typ-ical to the Baltic. Rate measurements of bacterial activitycompared with primary production rates are lacking, and vitalif we are to understand the organic matter and nutrient dynam-ics within the sea ice and underlying waters in the Baltic Sea.On the rather scant information available at present the role ofnutrients entrapped in the ice during formation and subsequentregeneration of this nutrient pool seem to be more important inthe Gulf of Bothnia than in the Gulf of Finland. Clearly suchstudies need to be extended.


To date most of the studies of Baltic Sea ice have been re-stricted to land-fast ice that can be easily accessed from shorefacilities. Comprehensive biogeochemical studies of the packice are rare, and should be the focus of more intensive studiessince most of the Baltic Sea ice volume resides in the heavilydeformed pack ice areas. It is proposed that the low-salinityBaltic Sea ice, and the processes associated with it could po-tentially be used as a model for Arctic sea ice (or regions ofthe Arctic such as the White Sea) in a climate change inducedwarming, especially where high river discharge reducescoastal salinities to a significant degree.

Acknowledgements

Financial support for many of the activities over the yearsthat have led up to this review was provided by the Walterand Andree de Nottbeck Foundation, Koneen Saatio Founda-tion, Jenny and Antti Wihuri Foundation and the Academyof Finland (Snow and Ice Graduate School and BIREME-programme). Tvarminne Zoological Station (University ofHelsinki) and Bothnian Bay Research Station (University ofOulu) are acknowledged for access to their facilities and sup-port by the stations staff. We also like thank our colleagues,too numerous to all be mentioned here, who have shared theirideas and enthusiasm over the years.

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Sea ice in the Baltic Sea – A review