comparisons between lakes and seas during the arctic winter

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Comparisons between Lakes and Seas during the Arctic WinterAuthor(s): Harold WelchSource: Arctic and Alpine Research, Vol. 23, No. 1 (Feb., 1991), pp. 11-23Published by: INSTAAR, University of ColoradoStable URL: http://www.jstor.org/stable/1551432 .

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Arctic and Alpine Research, Vol. 23, No. 1, 1991, pp. 11-23

COMPARISONS BETWEEN LAKES AND SEAS DURING THE ARCTIC WINTER*

HAROLD WELCH Canada Department of Fisheries and Oceans, Freshwater Institute 501 University Crescent, Winnipeg, Manitoba R3T 2N6, Canada

ABSTRACT

Lakes and seas are oases of life in winter, when the land is frozen. Differences between lakes and seas result because the sea has a much higher salinity and much larger size. Thus the frozen sea has a lower temperature, a macroscopic porous ice structure at the water interface, higher current velocities, and discontinuous ice cover compared with frozen lakes. Both systems exhibit complex differential movements of water masses with differing solute content resulting from freezeout distillation. The biological differences resulting from the physical differences include the absence of ice communities, shorter food chains, lower diversity, lower benthic and higher fish biomass, and lack of mammals and birds in lakes compared with seas in winter.

INTRODUCTION

Seen in winter, the ice-covered seas and lakes of the Arc- tic appear as barren, wind-swept areas of snow and ice with no hint of life. Yet they are constant-temperature baths containing a great many species that remain active year-round; indeed, except for a few specialized land mammals and birds, ice-covered lakes and seas constitute the only living oases in a vast frozen landscape devoid of metabolically active creatures. There are, however, major differences between a frozen sea and a frozen lake, resulting from two physical factors - salinity and size. The purpose of this paper is to explore the physical and biological differences between the two environments. These are summarized in Table 1.

It should be remembered that relatively little is known about arctic aquatic environments. Small arctic lakes have been fairly well studied, but large lakes have not. The In-

ternational Biological Programme Char Lake Project at Resolute (Rigler, 1972) came close to describing all the components of an arctic aquatic system, and Char Lake has only a few species because it is so far north (75?N). Limnological studies at Saqvaqjuac, northwest Hudson Bay (63?N) (Welch, 1989) have provided considerable winter data. Extensive work has also been done on ponds at Barrow, Alaska (Hobbie, 1973), but because these freeze to the bottom in winter they are not useful for this review. More recent research on Toolik Lake, Alaska (68?37'N) has included little winter work. The arctic marine biology that has been done has largely been on species distribution, zooplankton, algal photosynthesis, and marine mammal and bird ecology. I have used examples from the literature as appropriate, but have made no attempt to include an exhaustive review of the pertinent literature.

PHYSICAL PROCESSES

FREEZEUP CONDITIONS The relatively high concentration of dissolved salts in

the ocean compared with arctic lakes results in differences

*A version of this paper was presented at the Circumpolar Eco- systems in Winter Symposium, 16.21 February 1990, Churchill, Manitoba, Canada.

in ice structure, freezing point, and water circulation, which in turn control biological processes. Lake water typically has a total dissolved solids content of 0.01 to 0.2 g L-', low enough that the freezing point of 0?C is practically unaffected. Maximum density is at 3.94?C. When lakes in the temperate zone freeze, the ice floats over a thin layer of cold water beneath which the main body of

?1991 Regents of the University of Colorado H. WELCH / 11



the lake is usually much warmer, up to 4?C (Figure 1); the return of summer heat stored in the sediments will also raise the mean water temperature of a temperate lake overwinter. But arctic lakes, unprotected by trees and exposed to higher mean wind speeds, usually freeze during or just after stormy weather which circulates and cools the water column well below 3.94?C, precluding the establishment of a cold low-density surface layer until the entire lake may be at or very close to 0?C (Figure 1) (Welch et al., 1987; Schindler et al., 1974).

The salt content of seawater depresses the freezing point to approximately -1.8?C, and decreases the temperature of maximum density down to the freezing point. Although density gradients limit the depth to which the water must be cooled before freezing can occur, under full seawater (nonestuarine) conditions the upper few tens of meters are close to the freezing point (Figure 1), and in shallow seas the entire water column is not much above that temperature after ice cover is established (e.g. Bergmann, 1989).

ICE STRUCTURE The initial ice sheet in both salt and freshwater includes

numerous small, randomly oriented crystals. Because an

Properties

Physical Salinity Freezing point

Winter temperature Ice structure

Light

Currents

Movement of water with different salt contents

Biological Winter oxygen and photosynthesis

Species diversity

Biomass

Trophic structure

Mammals and birds Antifreeze properties

ice crystal grows fastest perpendicular to its c-axis, the growth rate of each crystal down into the water column varies. Slower-growing crystals are quickly wedged out, so that by the time the ice sheet is about 10 cm thick, the freezing ice bottom exposes only crystals with horizon- tal c-axes (Weeks and Ackley, 1982; Maykut, 1985).

In freshwater, crystals with the c-axis oriented horizon- tally continue to grow in diameter as the ice sheet thickens. The growing crystal lattice excludes dissolved substances. This process of distillation by freezeout is about 9807o efficient for dissolved oxygen (Welch and Bergmann, 1985a) and probably other gases, and about 95 to 99%o for major ions (Welch and Legault, 1986); thus melted arctic lake ice can be as pure as distilled water. Nonetheless, some impurities do collect at the growing crystal interfaces, and during thaw, solar radiation and downward percolating meltwater combine to open tiny cracks which tend to follow crystal edges top to bottom in the ablating ice sheet. This "candling" process isolates vertical shards of ice whose jagged upper surface relief becomes more pronounced and coarser as melt progresses, corresponding to increasing diameter with depth of the original ice crystals. At all stages of freezeup and melt, the underside of lake ice is smooth as glass, although

TABLE 1

Comparisons between arctic lakes and seas in winter

Lakes

0.01 to 0.2 g L-1 O?C

0? to -3?C Crystals large; ice-water interface smooth; solute content extremely low; ice sheet con- tinuous; snow ice common; frazil ice uncommon. Controlled primarily by overlying snow; transmission through ice dependent upon ice type (Fig. 4). Thermally driven, very slow (-0.01 cm s-').

Dilute meltwater flows out of lakes in spring, leaving lakes more concentrated than their inflows.

Respiration rates 30 to 100 g 02 * m-2 * yr-1, constant throughout the year; considerable photosynthesis before ice-out; no ice community. Low; strong latitudinal gradient decreasing northward; insects important. Low; 1 to 2 g dry wt . m-2 benthos and zooplankton; fish high, 1 to 4 g * m2. Four trophic levels; benthic detritivores important; lake trout (rarely Arctic char) are the top carnivores. Absent overwinter. Absent.

Seas

32 to 34 g L-1 --1.8?C; Tf = -0.055 Sw, where Tf is temperature of freezing and Sw the salinity in per mill. --1.8?C Crystals small; ice-water interface a complex skeletal layer; freeze-out at 65 to 75% in first- year ice; ice sheet ridged, broken, polynyas present; frazil ice common. Controlled primarily by overlying snow; first- year sea ice fairly transparent (Fig. 4).

Currents driven by solute freezeout are slow, - 1 to 6 cm s-1; tidal and geostrophic currents are rapid, up to m s-1. Dense brines collect in basins; export of low- salinity water from the arctic basin affects the vertical circulation of subarctic seas.

Respiration rates 100 to 200 g O2 * m-2 * yr-1; highest in late summer, decreasing overwinter; ice community productive in late winter.

High; moderate latitudinal gradient; no insects; mammals important. High; on the order of 0.5 kg * m-2 dry wt benthos; fish low, -0.1 g dry wt m-2. Four to five trophic levels; polar bear the top carnivore.

Present to varying degrees over winter. Present.

12 / ARCTIC AND ALPINE RESEARCH



when the melting ice is candled, cracks up to several milli- meters wide craze the bottom surface (Welch et al., 1987).

In addition to this clear congelation (also called "black") ice, another kind of lake ice forms. If snow loading becomes heavy enough relative to the ice thickness to depress the ice surface below the water level, lake water rises through cracks (caused by contraction as the ice sheet contracts and expands with changing air temperature) and saturates the overlying snow cover, which may subse- quently freeze to form so-called white, snow, or slush ice overlying the earlier black ice (Adams, 1981; Welch et al., 1987). In general the proportion of white ice decreases with increasing latitude because snow loading is less and ice growth more rapid. At 63?N there is usually a few centimeters of snow ice over the black ice, whereas at 75?N extensive areas of snow ice are unusual (pers. obs.).

In salt water the growing ice crystals also freeze out solutes, but because the solute concentration is so high, crystal growth is more complex than in freshwater (Maykut, 1985). The result is that the lower surface of growing ice crystals is composed of parallel knife-like blades 0.5 to 1.0 mm wide (Figure 2), called the skeletal

VARIABLE SNOW COVER WATER LEVEL

It

layer. Microturbulence at the growing crystal interfaces confers advantage to certain c-axis orientations relative to water currents, such that the exposed knife edges of each crystal tend to be oriented in parallel (Weeks and Gow, 1978). This can be seen in a cast of the under-ice surface in Barrow Strait (75?N) (Figure 3). Note also that each set of parallel blades in Figure 3 is on the order of 1 to 2 cm in diameter, corresponding to the probable size of individual ice crystals.

As the blades grow downward they continually trap tiny pockets of brine which make thin young sea ice quite salty (about 10 to 15%oo; Cox and Weeks, 1974). As the ice ages, thickens, and cools, the salt content is reduced by brine migration and gravity drainage to about 6%o by early spring (Cox and Weeks, 1974; Nakawo and Sinha, 1981). Much of this remaining brine drains quite abruptly in April before air temperatures reach 0?C, sinking from the ice sheet in plumes of dense cold brine of about 65 to 70%/o salinity and often freezing the surrounding seawater into stalactites up to a meter in length (Lewis and Milne, 1977; pers. obs.). Thus the salinity of first-year sea ice is only about 2%o or less in summer

c- axis

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TEMPERATURE, ?C FIGURE 1. Typical temperature profiles in winter. Sea-17 April 1988, northwest coast of Hudson Bay; Char Lake-October 1969 from Schindler et al. (1974); Lake 305-December 1970 from Schindler (1971). Also illustrated is the mechanism of snow ice formation, caused when accumulating snow depresses the ice below the water level, allowing water to flow upward through cracks to saturate the snow, further depressing the ice. If it finally freezes, the resultant white ice is different from the black congelation ice below.

- 5- 1I 0.5- I mm

FIGURE 2. Schematic illustration of the platelet structure on the bottom of a growing salt water ice crystal (from Maykut, 1985, reproduced by permission of the author).

H. WELCH / 13

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FIGURE 3. Urethane foam cast of the undersurface of first-year sea ice, 8 May 1986, Barrow Strait (74? 55'N). The foam reaction is exothermic and has partly melted the platelet layer, but the crystal orientations are distinct. Bars are 1 mm apart.

(Cox and Weeks, 1974), and when melted is used as a source of drinking water throughout the Arctic. When sea ice survives the summer it becomes multiyear ice and is characterized by visible annual layers, of which all but the current year band is relatively fresh.

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LIGHT Ice types have important biological effects because they

control light penetration into the water body. The percent transmission of visible sunlight through 10 cm of lake water, or clear (black) lake ice, is 97 to 99%; it is 95% for 10 cm of candled ice, 44% for 10 cm of white ice, and only 4% for the top 10 cm of arctic snowpack (Figure 4). Thus snow cover is of overriding importance to photosynthesis before the beginning of melt. Sea ice is less transparent than black freshwater ice, with a uniform cloudy appearance. Ten centimeters of first-year sea ice transmits about 90% of the light absorbed at the surface (Figure 4).

CURRENTS In arctic lakes, the currents found beneath winter ice

are slow (unless rivers are present). Fall freezeup gives water near 0?C overlying slightly warmer sediments which contain heat stored during the summer open water period (Welch and Bergmann, 1985b). Water in contact with the bottom warms a few hundredths of a degree, increases very slightly in density, and runs downslope, in turn caus-

0.1 10 20 30 40 50 60

THICKNESS, cm FIGURE 4. Percent light transmittance as a function of substrate thickness. 100% equals photosynthetically active radiation at the substrate surface, before reflectance. (Adapted from Welch et al., 1987; sea ice data unpubl., from Barrow Strait, 1-10 May 1985).

FIGURE 5. Currents beneath ice in small arctic lakes during winter (from Welch and Bergmann, 1985b).




ing shoreward currents just below the ice surface and set- ting the whole lake body in motion (Figure 5). Current speeds are on the order of tens of meters per day (Welch and Bergmann, 1985b).

In the sea, sinking brine excluded from the growing ice sheet drives a slow thermohaline circulation, with dense water sinking and running downslope, replaced by a countercurrent beneath the ice moving shoreward (New- bury, 1983). Speeds of such currents are slow, on the order of one to a few centimeters per second as measured in the western Beaufort Sea (Matthews, 1980). This circulation pattern is thus similar to that in lakes, but is driven primarily by freezeout of solutes rather than heat input from the sediments.

Of more immediate biological importance are the tidal and geostrophic currents that result from the sheer physical size of the sea as compared with lakes. Such current speeds are usually much higher, ranging up to meters per second, and in certain constricted areas they create ice-free areas called polynyas, where open water persists year round (Stirling and Cleator, 1981). These range in size from tidal races as small as hundreds of square meters, to the North Water in north Baffin Bay, where even in mid-winter many square kilometers are essentially ice free. Polynyas enlarge rapidly in spring, constituting centers of expanding open water. Currents and tides also keep much of the offshore (nonfast) ice in motion and broken, with important ramifications for marine birds and mammals. Currents and winds, particularly those associated with fall and early winter storms, also crush pans of sea ice together, resulting in keels of jumbled ice projecting downwards into the water to 30 m depth (McLaren, 1988).

K ..... OUTFLOW . INLET

A. FREEZE-UP

C. EARLY ICE

C. EARLY MELT

ICE

B. MAXIMUM ICE

D . . LATE MELT.. .

D. LATE MELT

FIGURE 6. Movement of water through arctic lakes (arrow heads indicate relative flow volumes). A. At freezeup the lake is well mixed. B. During winter, freezeout concentrates solutes; outflow and inflows are frozen. C. During early snow melt, flow volumes are high, but solute concentrations are low. The colder, more dilute meltwater flows over the deeper water. Ice remains on the lake, preventing it from being mixed by the wind. During late melt, flow volumes decrease as inflow solute concentrations increase. The lake remains more concentrated than its inflows and the turnover time of lake water is longer than calculated from outflow divided by inflow (after Schindler et al., 1974; Bergmann and Welch, 1985).

DIFFERENTIAL MOVEMENT OF LOW SALINITY WATER The freezeout phenomenon, a sort of distillation caused

by ice formation, also allows for the differential movement of water masses with varying salt contents. In lakes, despite the low dissolved solute content, there is differential export of dilute surface water in spring time (Berg- mann and Welch, 1985). Very dilute and cold ice meltwater floats just below the melting ice sheet. Surface inflows are also dilute and cold and tend to float above the warmer, saltier stable lake body, which has a relatively high solute content as a result of freezeout overwinter. Dilute surface water leaves the lake via the outflow during the spring melt (Figure 6), when most of the runoff occurs, and only after iceout does the lake mix vertically and the outflow solute content rise. One result is that headwater arctic lakes are about 60% higher in solute content than their inflows (Welch and Legault, 1986). Another result is that inflowing water does not completely mix with the lake, altering nutrient inputs and increasing the actual water turnover time compared with the theoretical turnover time (Bergmann and Welch, 1985).

In the sea, the differential movement of low-salinity water and ice is a much more important phenomenon than in lakes. It occurs on a small scale in shallow near- shore basins where sinking brine can accumulate (New- bury, 1983), where salinities may temporarily reach levels

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0 10 20 30 40 50 60 70 80

Salinity %o

FIGURE 7. Salinity profile for Hawk Inlet, a small (0.5 x 0.1 km) water body separated from Saqvaqjuac Inlet, northwest coast of Hudson Bay, by a 10 m deep sill at its entrance).

H. WELCH / 15

I - I I I I I I I



several times that of the parent seawater (Schell, 1975; Aagaard, 1978). If the basin is sufficiently protected and deep, the stratification may become permanent, as in the example of Hawk Inlet on the northwest coast of Hud- son Bay (Figure 7). When such basins are uplifted by glacial rebound they become lakes with high-salinity deep water which is gradually washed out depending upon the size and morphometry of the basin. Such meromixis may last many centuries, as in Garrow and Sophia lakes in the Canadian High Arctic (Stewart and Platford, 1986).

The very large scale differential movement of low- salinity ice and water caused by freezeout distillation is extremely important to the vertical circulation of the high- latitude ocean, as outlined by Aagaard and Carmack (1989). The export of ice and relatively dilute water from the Arctic Ocean via the East Greenland Current to the Greenland, Iceland, and Labrador seas determines the mixing efficiency of their convective gyres. If the freshwater input is too high, as occurred during the last deglaciation, the circumpolar seas become more strongly

stratified and deep water convection is slowed or stopped. This in turn has major consequences for climate and global circulation (see Broecker and Denton, 1989).

However, the efficient deep convection of the northern seas is also dependent upon some freshwater input. This is because compressibility is temperature dependent. In winter, a sea with a slight salinity gradient cools and the lower-salinity surface layer eventually convects downward. Because it is colder, it compresses more than the surrounding water and continues to sink, thereby circu- lating the deep water. Without this initial slight salinity gradient, continual cooling is required to convect the mixed layer downward, a slower process which is less efficient at ventilating the deep sea (summarized from Aagaard and Carmack, 1989). The system thus appears to be delicately balanced. This mechanism likely caused large-scale halocline disruptions in the past, and could do so again if climate change alters runoff and ice cover (Aagaard and Carmack, 1989; Mysak and Manak, 1989).

BIOLOGICAL PROCESSES

WINTER OXYGEN AND PHOTOSYNTHESIS Because polar lakes have a finite oxygen supply sealed

in by surface ice and basin permafrost in winter, they act as respirometers whose metabolic rates can be measured accurately (Welch, 1974; Welch and Bergmann, 1985a). Furthermore, the combination of snow cover and polar night eliminates photosynthesis for much of the winter,

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FIGURE 8. Mass of oxygen per square meter in Char Lake (75?N) and P&N Lake (63?40'N) overwinter. The slope of the regres- sion is the metabolic rate of the lake. The deviation above the line in late winter is caused by oxygen released from resuming photosynthesis.

at least November through March at all latitudes, so that whole-lake respiration can be measured independently of the effects of photosynthesis. Since the total respiration of a system must be equal to its total photosynthesis (minus storage in sediments and export via the outflow, which are small components of most lake energy budgets), lake respiration measurements are convenient estimators of lake production. Practically all the respiration of an arctic lake occurs on the bottom, and at a constant rate throughout the year, so that the oxygen depletion rate in winter is constant when expressed per unit

100

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0.1 0 5 10 15 20 25 30 35

Snow Depth, cm FIGURE 9. Ice algal chlorophyll concentration as a function of overlying snow depth, Resolute, 30 April 1986 (from Welch and Bergmann, 1989).


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area of unfrozen bottom (Figure 8) (the actual decrease in oxygen concentration is not constant because freezeout concentrates dissolved gases in the remaining water, re- quiring a correction when making the calculation).

The range in arctic lake respiration is 30 to 100 g 02 * m2 * yr-1, in a gradient decreasing from south to north

(Welch and Bergmann, 1985a). In lakes up to a few hun- dred hectares in size, the photosynthesis that is required to drive this respiration may be up to 80% benthic rather than contributed by phytoplankton (Welch and Kalff, 1974; Welch, unpubl.). Benthic photosynthesis must be relatively less important in very large, deep lakes although no data are available.

Under ice, as lake water acquires heat from the bottom and slowly sinks downslope it gives up oxygen to the sediments, resulting in the accumulation of low-oxygen- content water in deep basins. There, further respiration may give anaerobic conditions near the bottom even though the main lake body is well oxygenated. While div- ing in Char Lake, I have also noted that anaerobic conditions develop in natural depressions in the moss, creating and maintaining moss-free areas of 1 to 100 m2. The shallower the lake, the lower the oxygen level by spring time. For this reason, arctic lakes less than 1.5 to 1.8 m mean depth (depending on the productivity and hence respiration rate) winter kill and have no permanent fish populations, even though they do not freeze to the bottom (I prefer the term pond for water bodies which do freeze to the bottom).

When sufficient light penetrates the lake in spring, photosynthesis by both phytoplankters and phytobenthos begins anew and is controlled primarily by latitude and secondarily by snow and ice cover (the smooth undersurface of lake ice precludes the attachment of ice-associated organisms). Beginning about 1 April at 63?N, and 1 May at 75?N, sufficient photosynthesis occurs beneath lake ice to change the oxygen budget; 3 to 6 wk later, by the beginning of snowmelt, the lakes produce more gaseous oxygen than they consume (Figure 8). In low-snow years, phytoplankton blooms may appear before snowmelt, and as much as 75% of total annual phytoplankton production can occur before ice out (Welch et al., 1989).

Arctic seas behave differently. The respiration rate of the sea bottom is not constant, as in lakes, but parallels the changes in phytoplankton production, being low in winter and increasing sharply in July and August (Welch and Kalff, 1974). It appears that the arctic sea bottom, particularly the abundant filter and deposit feeders, responds to increased input of phytoplankton, and pos- sibly of sedimenting ice algae, by increasing activity and metabolic rate. When phytoplankton production ceases in fall, filter feeders rapidly clear the water and the metabolic rate gradually drops. Lake bottoms, on the other hand, metabolize a great deal of stored detritus and have few filter feeders, which stabilizes respiration rates across the seasons (Welch and Kalff, 1975).

The knife edges at the bottom of freezing sea ice (Figure 2) provide habitat for numerous species of microalgae and small invertebrates, which together form the sea-ice

microbial community, or sympagic (from the Greek, meaning "with the ice") community. Some of these are present throughout the winter (Horner, 1985) but as the sunlight reappears and intensifies in February/March, diatom colonies grow on the ice crystals, and where the overlying snow is thin they double every 1 to 2 wk until the bottom centimeter of sea ice becomes deep golden brown. Even though ice algae are adapted to grow under very low light, the opacity of the arctic snowpack con- trols algal growth, so that ice algal abundance becomes an inverse logarithmic function of overlying snow depth (Figure 9). The algae use micronutrients from the water column and a low nitrate supply can also limit ice algal growth (Maestrini et al., 1986; Cota et al., 1987). Where nitrate is adequate, ice algal biomass during the exponen- tial growth phase is predictable from cumulative surface light and snow depth. Maximum biomass can reach 0.5 tonne dry matter per hectare beneath low snow cover (Welch and Bergmann, 1989).

This abundance of plant matter beneath the ice pro- vides an energy source utilized by marine zooplankton months before the more bountiful summer phytoplankton bloom occurs (Conover et al., 1986; Runge and Ingram, 1988). Nematodes and other small invertebrates, in- cluding many larval planktonic and benthic species, also grow amongst the ice algae in densities of many thousands per square meter, constituting a sort of upside down benthic meiofaunal food web (Grainger et al., 1985; Grainger and Hsiao, 1990).

The most obvious sympagic fauna are amphipods. Over shallow water, several species dominate at a given locale, swimming and floating against the algal-rich ice surface in numbers up to thousands per square meter (Pike and Welch, 1990) (Figure 10). They originate from the sea floor each winter and their abundance is inversely related to underlying water depth (Pike and Welch, 1990). Over deep (> 100 m) water, and in the Polar Basin where multiyear ice dominates, Gammarus wilkitzkii and the mysid Mysis polaris are associated with ice year round (Gullikson and T0nne, 1989). They are frequently join- ed by more pelagic species such as Apherusa glacialis and juvenile Parathemisto, which are associated seasonally with the ice community. Some sympagic amphipods graze ice algae directly (Carey and Boudrias, 1987; Lewis, 1987), while others appear to be omnivorous.

Beneath a 40 cm snow cover, less than 0.1%o of surface light penetrates to the water; even under low snow cover, only 1 to 2% of the surface light reaches the ice algal layer (unpubl.), and of this as much as 90% is absorbed by the ice algae at the peak of the ice algal bloom (Welch and Bergmann, 1989). This leaves very little light for the water column. As a result, phytoplankton production in the sea is negligible until both the snow and ice algae disappear during melt, with chlorophyll concentrations of 0.01 to 0.1 mg m-3 in the water column below permanent ice cover from March through June (M. Bergmann, Freshwater Institute, unpubl.). We can speculate that large polynyas such as the North Water, and the Roes Welcome Sound polynya in northwestern

H. WELCH / 17



FIGURE 10. Gammarus setosus (Crustacea: Amphipoda) floating and feeding against the undersurface of sea ice, Barrow Strait, May 1985. Note the pattern of the skeletal ice layer, emphasized by the attached algae.

Hudson Bay may have much higher phytoplankton production and biomass in May and June, when light input into the Arctic is maximum (see Figure 5 in Schindler et al., 1974), but there are no data as yet. It is also likely that benthic photosynthesis beneath ice in shallow water increases sharply after snowmelt.

Another biological effect of freezeout and subsequent salinity distribution occurs in summer. Melting sea ice dilutes the surface water and stabilizes the water column for most of the open water season at high latitudes. This density stratification allows the development of intense phytoplankton blooms in the upper 10 to 20 m of water shortly after iceout when light penetration into the water suddenly increases, but the phytoplankton quickly deplete micronutrients and settle through the stratified water column (Figure 11). The blooms are therefore short-lived if stratification persists and no upwelling mechanism is present to bring deeper nutrient-rich water to the surface (Alexander and Niebauer, 1989). Where such upwelling does exist, as caused for example by coastal currents, wind-driven circulation at ice edges, and density differences at the faces of bergs and ice fields, phytoplankton blooms may persist until terminated by decreasing light in the fall.

DIVERSITY While many differences in ecosystem structure between

arctic lakes and seas pertain to the entire year rather than just the ice-covered period, it may be useful to summarize them briefly, emphasizing those that result from winter conditions.

Both arctic lakes and seas hold a surprisingly high diversity of microalgae. Saqvaqjuac lakes (63?N) have over 200 species of phytoplankton and well over 100 species of phytobenthos (Kling, pers. comm., 1989), and even Char Lake at 75?N has about 150 species of phytoplankton and at least 98 species of phytobenthos (Kling, unpubl.; Rigler, 1972). This compares favorably with the 211 species of marine phytoplankton collected at Frobisher Bay (Hsiao, 1987), of which 196 are also

20 AUGUST 1988

CHL AND N03 (MG/M3)

1 3 5 7 9 11

0

20

40

60 I

UJ 80 w 0

80

100

120

1025 1026

DENSITY (KG/M3)

1027

FIGURE 11. Vertical profiles of density, phytoplankton chlorophyll, and inorganic nitrate in Barrow Strait off Resolute, 74?55'N, 20 August 1988. The Strait had been ice free about 1 mo. Phytoplankton have removed NO3 from surface waters and settled, so that peak biomass occurs below the depth of maximum nutrient depletion.




found in the ice community, nearly all diatoms (Hsiao, 1980).

Macrofauna exhibit a strong latitudinal gradient in lakes, with decreasing diversity poleward. Macrozoo- plankton are typically represented by 6 or 7 species in lakes at 60 to 65?N, although very large lakes on major river systems such as Garry Lake on the Back River may hold 12 to 15 species (K. Patalas, Freshwater Institute, unpubl.). At latitude 75?N the copepod Limnocalanus macrurus is the only common macrozooplankter in lakes (Rigler et al., 1974). Insects, particularly the midges (Diptera: Chironomidae), dominate lake benthos at high latitudes, with 30 or more species at 63?N (Welch, unpubl.) and only 6 to 8 at 75?N (Rigler, 1972). Fish species are typically 4 to 6 in low arctic lakes, but only the Arc- tic char (Salvelinus alpinus) and occasionally the sculpin Myoxocephalus quadricornis occur in lakes on the islands north of Parry Channel at 75?N.

This contrasts with the sea, with its far higher macro- faunal diversity and generally lower gradient of decreasing diversity with increasing latitude. For example, in the northern half of Baffin Bay are found at least 23 species of benthic macrophytes, 330 species of macroinverte- brates, and 35 fish species (compiled from Dunbar and Moore [1980] and other sources). However, sampling at any one location would probably not yield all these species, and the diversity undoubtedly decreases westward in the passages amongst the central arctic islands, where polar water moves eastward unmixed with North Atlan- tic water. Arctic marine benthos is dominated by coelenterates, polychaetes, bivalves, and crustacea with other phyla contributing to the high diversity. Insects are absent. Arctic lakes have many insects but no coelenterates or polychaetes, and bivalves are sparse and small in size.

BIOMASS Plant biomass is low in most arctic lakes, although the

luxurious beds of benthic mosses present in some high arctic lakes is a notable exception; in Char Lake there is about 1 kg dry wt * m-2 of moss between about 4 and 10 m depth (Rigler, 1972). The biomass of benthos and zooplankton is also low in arctic lakes. In Char Lake there is about 1.2 g dry weight of benthos and zooplankton combined (Rigler, 1972). The equivalent in the Saqvaq- juac lakes is slightly higher although biomass measurements have not been made for all groups. Fish biomass, on the other hand, is relatively high, being 0.92 g wet wt * m-2 in Char Lake (MacCallum and Regier, 1984) and averaging 3.7 g wet wt * m-2 in four Saqvaqjuac lakes (Welch, unpubl.).

There are almost no comparable data for arctic seas. Near Resolute the biomass of the three most common kelp species averages 300 g dry wt * m-2 at 5 m depth, decreasing to zero at 30 m (Welch et al., submitted). In Barrow Strait (74?N) the average biomass of the bivalve Mya truncata alone is on the order of 20 g dry wt * m"2, reaching 300 g m-2 at 15 m depth, and the total for all other bivalves combined is probably equivalent (Welch et al., submitted). With the addition of all other groups,

the biomass of the marine benthos is at least an order of magnitude higher than in lakes. This is partly because many of the marine species are relatively long-lived. For example we have found M. truncata up to 55 yr old and Macoma calcarea up to 45 yr old, and other common bivalves such as Serripes groenlandicus average 10 to 20 yr old, whereas arctic chironomids have 1- to 3-yr life cycles (Welch, 1976; and unpubl.), freshwater gammarids 2-yr life cycles, and caddis flies (Insecta: Trichoptera) 3- to 5-yr (unpubl.). Fish biomass is far less in arctic seas, being on the order of 0.1 g wet wt * m-2 in Resolute Bay (unpubl.), although some spectacular aggregations of Arctic cod, numbering millions of individuals, occur sporadically throughout the Arctic (Craig et al., 1982; Welch et al., 1990).

TROPHIC LEVELS Arctic aquatic food chains are unusually long, espe-

cially considering their relatively low productivity per unit area. Lake food webs usually include four well-defined trophic levels, with plants at the base (Figure 12). Her- bivores include zooplankton, benthic insect larvae, and benthic crustacea. Arctic char are the most common and obvious first-level carnivores in lakes throughout the Mid to High Arctic, with other fish species such as lake whitefish (Coregonus clupeaformis) and suckers (Catostomus spp.) replacing char at lower latitudes. South of Parry Channel, lake trout (Salvelinus namaycush) oc- cupy the third trophic level while young, becoming piscivorous when about 45 cm in length, so that in many lakes where there are only landlocked char and lake trout, the invertebrate-char-large trout food chain is well defined, as in Figure 12. In lakes with only arctic char, or with only lake trout, a few large individuals become exclusively piscivorous, feeding on their own kind and creating a fourth trophic level.

Arctic seas have even longer food webs which include five fairly distinct trophic levels (Figure 13). The Arctic cod (Boreogadus saida) is pivotal, feeding primarily on herbivorous crustacea and in turn providing food for ringed seal (Phoca hispida, harp seal (Phoca groenlan- dica), beluga (Delphinapterus leucas), narwhal (Monodon monoceros), and several seabird species. The fifth trophic level is clearly defined by polar bear (Ursus maritimus), which feeds almost exclusively on ringed seal, plus occasional other marine mammals.

OTHER BIOLOGICAL CHARACTERISTICS The most visible difference between ice-covered seas

and lakes is the presence of mammals, and to a much lesser extent birds, year-round in the sea. Distinction must be made between those species that do not migrate and are capable of inhabiting unbroken fast ice, and those that depend upon open water for access to either air for breathing or water for feeding, and migrate to find such conditions during the polar winter. The migrants include all the whales, all the birds, and the seals except for ringed seal and some bearded seals (Erignathus barbatus).

H. WELCH / 19



Winter migration may not be far, as in the case of birds and mammals that live in or near polynyas year round. The broken ice condition beyond the shore-fast ice zone also allows animals like narwhal and black guillemots (Cepphus grylle) to overwinter in what is essentially an ice-covered environment.

Walrus (Odobenus rosmarus) routinely break and use breathing holes in new ice 10 to 15 cm thick but must move seaward as freezing progresses (Vibe, 1950). Bowhead whales (Balaena mysticetus) also break ice 10 to 20 cm thick with the dorsal hump in the vicinity of their blowhole, a feature postulated to be an adaptation for making and breathing through such holes (George et al., 1989). Bowhead migrating into the Beaufort Sea off Point Barrow, Alaska make hundreds of such breathing holes, reusing some, and breaking young ice in frozen leads in preference to using patches of open water. Natives there say they are able to break ice up to 60 cm thick (George et al., 1989).

The ringed seal is the arctic mammal most adapted to permanent ice cover. Although it is also capable of living in broken pack ice such as in north Baffin Bay (Finley et al., 1983), its preferred habitat is shore-fast ice. Each seal maintains several permanent breathing holes in ice up to 2 m or more thick. Pupping occurs in March in birth layers excavated in snow drifts, typically downwind of pressure ridges (a feature mostly confined to the sea) where drifts are deep and the ice relatively thin (Smith and Stirling, 1975; Smith, 1987; Hammill and Smith, 1989).

Although lake ice is harder and more difficult to erode than sea ice, several races and subspecies of ringed seal have colonized large lakes throughout the Northern Hemisphere, of which the Lake Saimaa seal in northern Finland is an example. The Saimaa seal gives birth in lairs beneath snow drifts against the shore, since large drifts associated with pressure ridges are absent. Fish are eaten exclusively (Becker, 1984). The harbor seal (Phoca vitulina) is also found in Canadian arctic lakes but ap- parently does not maintain breathing holes, either migrating to the sea or retreating to open river rapids in winter (Mansfield, 1967).

FHYTOPLANKTON BENTHIC MICROALGAE

/ /

The abundance of ringed seal in the sea has allowed the development of a fifth trophic level, occupied by the polar bear. The primary habitat of this rapidly-evolving mammal is sea ice, where it takes its ringed seal prey, itself a permanent inhabitant of fast ice. There is no ecological equivalent of the polar bear in fresh water.

Both arctic lakes and seas share one common feature, the year-round activity of its inhabitants. While some her- bivores may reduce their activity in winter, and the general pace picks up in the summer ocean, most animal species appear to carry on as usual. Most fish species are active all year, for example Greenland cod (Gadus ogac), Arctic cod, and Greenland halibut (Reinhardtius hip- poglossoides) in salt water, lake trout, whitefish, and bur- bot (Lota lota) in fresh. An exception is the arctic char; both anadromous and landlocked forms cease feeding from about October through May in lakes (unpubl.).

The salt content of the sea combined with winter has caused one important physiological difference between freshwater and marine fish. Freshwater freezes at zero and even with occasional supercooling as in turbulent rivers in the fall, the water temperature does not reach the freezing point of fish blood, which for temperate teleosts is -0.6 to -0.8?C (Holmes and Donaldson, 1969). Such fish can tolerate supercooling to -2?C but if they come in contact with ice crystals they quickly freeze (Scholander et al., 1957). Polar marine fish have evolv- ed a unique class of antifreeze proteins which confers pro- tection against freezing without significantly raising the osmotic pressure of blood plasma. They are only found in fish that risk exposure to subzero water temperatures and ice crystals (Fletcher, 1986) and are absent from arctic freshwater fish, even the charrs (Shears et al., 1989).

Invertebrates have not been thoroughly investigated for antifreeze properties. The mussel Mytilus edulis is found in arctic waters such as Hudson Bay and contains antifreeze proteins (Theede et al., 1976). The common ice- associated amphipod G. wilkitzkii is often found living

N I B

I PHYTOPLANKTON I BENTHIC ALGAE I ICE ALGAE I

\

YOUNG CHAR & LAKE TROUT I CHAR | J LAKE TROUT < 45 cm

LARGE PISCIVOROUS LAKE TROUT

FIGURE 12. Simplified trophic diagram for lakes in the Saqvaq- juac area, northwest coast of Hudson Bay, 64?N.

I POLAR BEAR

FIGURE 13. Simplified trophic diagram for Lancaster Sound (74?N), showing the development of five trophic levels. A great many ecosystem components are missing from this figure.


I ZOOPLANKTON || CHIRONOMID LARVAE || CADDIS LARVAE || AMPHIPODS I



in cold brine channels in sea ice (Cross, 1982), and it the increased salinity rather than by using antifreeze pro- lowers its freezing point by osmotically conforming to teins (Aarset and Aunaas, 1987).

CONCLUSIONS

This review emphasizes the winter differences between the two environments that are virtually constant-temperature oases of life year round in the Arctic. These differences result from the larger size and higher salinity of the ocean. The reader will also note that for very large lakes, and most marine phenomena, little is known about winter behavior, particularly biological. While this enor- mous gap in our knowledge of arctic ecology is to some extent caused by the "summer expedition" mentality of southern scientists, it will be filled only when sufficient resources are provided to encourage investigators to per-

form long-term research annually during the polar night. In the meantime, two thirds of the annual biological cycle throughout most of the Arctic remains largely unknown.

ACKNOWLEDGEMENTS

I thank K. Martin-Bergmann, M. Bergmann, T. Siferd, and M. Curtis for critical reviews of the manuscript, and the Churchill Northern Studies Center for the opportunity to present this work. I am indebted to arctic scientists everywhere for the hard- won data used herein.

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comparisons between lakes and seas during the arctic winter

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