Bacterial biomass and production in pack ice of Antarctic marginal ice edge zones

Deep.SeaResearch,Vol.37, No. 8, pp. 1311-1330,1990. Printedin GreatBritain.

0198-0149/90$3.00+ 0.00 (~ 1990PergamonPressplc

Bacterial biomass a n d p r o d u c t i o n in pack ice o f Antarctic m a r g i n a l ice edge zones STEVENT. KOTrMEIER* and CORNELIUSW. SULLIVAN* (Received 26 October 1989; in revisedform 9 March 1990; accepted 9 April 1990) Abstract--Bacterial biomass and production in pack ice is little known even though the pack accounts for the majority of the 20 million square kilometer Antarctic sea ice habitat. On three cruises in marginal ice edge zones, spring 1983 ( A M E R I E Z I), autumn 1986 ( A M E R I E Z II), and late winter 1985 (Wintercruise I), considerable bacterial biomass and production was found throughout ice floes up to 2.22 m thick. We hypothesize that bacteria accumulate in pack ice as a result of both physical and biological processes. During the formation and growth of ice, physical processes act to concentrate and accumulate bacteria within the ice matrix. This is followed by in situ growth along physiochemical gradients found in several sea ice microhabitats. Bacterial biomass and production in ice were equal to that present in several meters of underlying seawater during all seasons. Among microhabitats, highest bacterial production and most rapid rates of growth (>1 d-l) were found in saline ponds on the surface of floes and porewater in the interior of floes. Bacterial carbon production ranged from 2% of primary production in surface brash to 45221% of primary production in surface ponds and porewater. Bacterial growth and microalgal photosynthetic metabolism in pack ice appear to be coupled in a fashion similar to that described for fast ice. The presence of substantial numbers of active, feeding protozoans and metazoans in pack ice suggests, albeit indirectly, that bacterial production supports microheterotrophs of the microbial loop, which in turn may support organisms at higher trophic levels. Bacterial growth in pack ice may be important to the potential for primary production. Thus ice bacteria may provide remineralized inorganic nutrients necessary for continued microalgal growth in localized microhabitats within the ice or they may compete with algae for nutrients. Upon release from melting ice, actively growing bacteria also contribute to microbial biomass in seawater. From these seasonal studies, we conclude that bacterial production in pack ice contributes substantially to the trophodynamics of marginal ice edge zones during all seasons.

INTRODUCTION

BACTERIALproduction in sea ice has generally been overlooked in estimates of production for the Southern Ocean. Microalgae are thought to dominate the production of sea ice microbial communities (SIMCO), which are composed of psychrophilic microalgae, bacteria and protozoans (BUNT and WOOD, 1963; HORNEt, 1976; W n l T ~ , 1977; ACKLEY et al., 1979; SULLIVANand PALmSANO, 1984; PALMISANOand SULLIVAN, 1985a,b; KOTrMEIERand SULLIVAN, 1988). Although a microbial loop has been proposed to function within land-fast sea ice (SULLIVANand PALMIS~O, 1984; GltOSSIet al., 1985; KOTrMEmRet

*Marine Biology Research Section, Department of Biological Sciences, University of Southern California, Los Angeles, CA 900894)371, U.S.A. 1311

1312

S.T. KOTrMEIER and C. W. SULLIVAN

al., 1987), the relative importance of bacterial production and direct evidence for components of the microbial loop in pack ice have been lacking. Sea ice bacteria have been studied primarily in land-fast ice around the coastline of Antarctica (hzux~, et al., 1966; Burrr, 1971; SOLLrVANand PAL~tlSANO, 1981; McCoNVILLE and WErnEI~EE, 1983; GROSSI et al., 1984; SULLlVAN and PALMm^NO, 1984; KOrrM~mRet al., 1987). Although bacteria are distributed throughout fast ice of McMurdo Sound, they are most concentrated in the bottom 20 cm of the ice, associated with high concentrations of microalgae (SvLLrVAN and PAL~nSANO, 1984; GROSS~ et al., 1984; GARI~SON et al., 1986). Several morphological types of bacteria are present, often occurring as dividing or paired cells (SULLIVAN and PALmSANO, 1981, 1984). When bacteria of fast ice are incubated under simulated in situ conditions that maintain the physical structure of their habitat (i.e. small cubes of ice perfused with seawater containing 3H-thymidine), they incorporate the radiolabel into acid-insoluble material characteristic of DNA, suggesting that they are capable of in situ growth (SULLWAr~et al., 1985; Korr~Emx et al., 1987). Sea ice bacteria are generally more abundant and of 5-10 times greater volume than bacterioplankton and are found frequently in long chains, typical of bacteria from organic-rich environments (SuLLrVANand PAL~SANO, 1984; G~OSS~et al., 1984; SOLLWAN, 1985; GAm~SON et al., 1986; KOTI~mE~ et al., 1987). Physical and metabolic coupling apparently exists between bacterial growth and microalgal photosynthetic metabolism in fast ice (SuLLrVAN and PALmSAr~O, 1984; GROSSl et al., 1984; KorrM~ER et al., 1987). We have estimated bacterial production in fast ice to be as high as 9% of primary production with growth rates up to 0.2 d -1 during the austral spring bloom of ice microalgae (KorrMEmR et al., 1987). In contrast to bacteria of fast ice, much less is known about the bacteria inhabiting pack ice, even though pack ice accounts for an estimated 99% of Antarctic sea ice (CLARXEand ACXLE~', 1984). Similar to the bacteria of fast ice, bacteria of pack ice are more abundant and larger than bacterioplankton and are associated with high concentrations of microalgae (MARl~Aet al., 1982; M~LLERet al., 1985; SULLIVAN,1985; GAm~SOr~et al., 1986). Net incorporation of thymidine has been measured recently for bacteria inhabiting pack ice during late winter west of the Antarctic Peninsula (Korr~F.IER and StrLLrVA~r, 1987), suggesting that these bacteria are also capable of growth in situ. The first two AMERIEZ (Antarctic Marine Ecosystem Research at the Ice Edge Zone) cruises during spring and autumn and Wintercruise I of the R.V. Polar D u k e provided us with opportunities for seasonal studies of bacterial abundance and metabolic activity in pack ice of marginal ice edge zones. Here we describe the distribution of biomass, rate of growth, and possible seasonal importance of carbon production by bacteria associated with pack ice during austral spring, autumn and winter. METHODS

Bacteria associated with SIMCO were studied during the first two AMERIEZ cruises: AMERIEZ I cruise (November-December 1983) in the Scotia and Weddell Seas and AMERIEZ II cruise (February-April 1986) in the Weddell Sea and Wintercruise I (August-September 1985) in the Bellingshausen Sea (Fig. 1 and Table 1). Distribution of SIMCO biomass and rates of primary and bacterial production were examined in a variety of microhabitats associated with sea ice including the internal consolidated ice of floes, brash ice and ponds on the surface of floes, porewater from the

1313

Bacterial biomass and production in pack ice

8°W 70°

60°

30°

40 °

55° ~ ° 60°

,

i ~s ..OoOo..... ,...I......

~:.-.'::::.-:.... ~

65°

~ i ~

7o~ -~ Fig. 1. Locations of study areas, cruise tracks and stations in marginal ice edge zones. (A): AMERIEZ I in the western Scotia and Weddell Seas (November-December 1983), B: AMERIEZ II in the western Weddell Sea (February-April 1986), and C: Wintereruise I in the Bellingshausen Sea (August-September 1985), (B) AMERIEZ I, (C) AMERIEZ II, and (D) Wintereruise I (reprinted from KoTr~E,r.~ and SULUVAN,1987).

interior of floes, blooms of microalgae exposed on the periphery of floes, and seawater underlying the pack ice (Fig. 2). Cores of ice were taken using a CRREL (Cold Regions Research and Environment Laboratory) auger (7.6 cm dia.) and cut horizontally into sections in the field. Sections of ice were allowed to melt in 1 liter volumes of 0.45/~m poresize filtered seawater of approximately 34%0 and controlled to maintain sample temperatures between -1.8 and +2°C, which minimized the effects of osmotic and thermal shock to SIMCO (PALmSANOet al., 1985). Salinity of final ice meltwater samples ranged from 20 to 26%0 determined by refractometer (Bausch and Lomb). Water samples were pumped manually from saline ponds found on or near the surface of floes, from the interior of floes (porewater), and beneath the pack ice by a clean all-plastic bilge pump equipped with Tygon tubing and used exclusively for sampling..The pump was flushed with several volumes of surface seawater between samples and rinsed with deionized water between stations. Samples of brash ice floating at the sea surface were hand sampled from an inflatable boat using an ethanol-cleaned Nalgene polyearbonate flask. Bacterial number and biomass were determined by direct counts of DAPI (4',6-diamidino-2-phenylindole 2 HC1, Sigma) stained specimens as described by GRossz et al. (1984). Bacterial biovolumes were estimated from cell dimensions (ZzM~mI~AN, 1977) and biomass derived from biovolume using a conversion factor of 220 fg C p m - 3 ( B l ~ A r and DUNDAS, 1984). Bacterial production in various samples of sea ice and water incubated in a sea ice/seawater bath at -1.8°C was determined by incorporation of 3H-thymidine following the method of FUHI~MANand AZ^M (1982) as modified for ice samples by KorruEi,~ and SULLIVAN(1987). Several potential problems have been reported concerning the interpretation of the incorporation of 3H-thymidine as a measure of bacterial production. These include uptake by eukaryotic algae (~VraN, 1986), catabolism by microrganism, and incorporation into macromolecular products other than DNA (KARL, 1982; HOLLm^UGn,

1314

S.T. KoyrMEmrt and C. W. SULLXVAN

1988; CARMAN et al., 1988) a n d e s t i m a t e s o f t h e specific a c t i v i t y w i t h i n s o l u b l e p o o l s o f n u c l e o t i d e s in bacteria. W e h a v e s t u d i e d s o m e o f t h e s e p r o b l e m s d u r i n g i n v e s t i g a t i o n s o f b a c t e r i a l p r o d u c t i o n in l a n d - f a s t s e a ice. T h e s e s t u d i e s h a v e d e m o n s t r a t e d t h a t at t h e c o n c e n t r a t i o n o f t h y m i d i n e u s e d in o u r s t u d i e s e u k a r y o t i c a l g a e w e r e n o t f o u n d b y a u t o r a d i o g r a p h y to t a k e u p t h y m i d i n e , y e t f r e e b a c t e r i a a n d b a c t e r i a o n t h e s u r f a c e o f t h e m i c r o a l g a e w e r e (SULLIVAN et al., 1985). C a t a b o l i s m a n d i n c o r p o r a t i o n i n t o m a c r o m o l e c u lar p r o d u c t s was i n v e s t i g a t e d b y u s i n g specific e n z y m e s t o t r a c e t h e fidelity o f t h e i n c o r p o r a t i o n o f 3 H - t h y m i d i n e i n t o D N A . T h e r e s u l t s s h o w e d t h a t 8 6 % o f t h e l a b e l was i n c o r p o r a t e d into D N A ( u n p u b l i s h e d d a t a ) . W e h a v e n o t e x a m i n e d t h e a c t i v i t y o f the s o l u b l e p o o l s o f i c e - a s s o c i a t e d b a c t e r i a b e c a u s e o f t e c h n i c a l difficulties. T h e k i n e t i c s o f b a c t e r i a l cell p r o d u c t i o n using t h e 3 H - t h y m i d i n e m e t h o d a n d n e t a c c u m u l a t i o n o f b a c t e r i a l cells in l a n d - f a s t ice w e r e in a g r e e m e n t (KoTrMEmR et al., 1987). E s t i m a t e s o f b a c t e r i a l cell and carbon production and growth rates were calculated from incorporation of 3 H - t h y m i d i n e as d e s c r i b e d b y KOa'rMEmR et al. (1987). M i c r o a l g a l b i o m a s s w a s e s t i m a t e d b y f l u o r o m e t r i c a n a l y s i s o f c h l o r o p h y l l a c o n t a i n e d in m e l t w a t e r s a m p l e s o f s e a ice a n d w a t e r f o l l o w i n g t h e m e t h o d o f STRICKLAND a n d PARSONS

Table I. Ice core sampling locations in the Weddell-Scotia and Bellinghausen Seas

Core cruise

Sampling date

Thickness of sea ice (m)

Sample location S. lat.

W. long.

Wintercruise 122 31 Aug. 1985 131 1 Sep. 1985 166 4 Sep. 1985 203 6 Sep. 1985 209 6 Sep. 1985 228 8 Sep. 1985 282 10 Sep. 1985 297 11 Sep. 1985

1.34 0.96 1.08 0.47 0.21 1.79 1.31 0.54

65*08' 65018' 66"12' 66018' 66"12' 65035 ' 65o26' 65*04'

64°05 ' 65"01 ' 67"41 ' 68*25' 67*46' 64*45' 64*47' 65o41 '

AMERIEZ I B 15 Nov. C 15 Nov. D 16 Nov. F 16 Nov. G 16 Nov. H 16 Nov. J 22 Nov. K 22 Nov.

1983 1983 1983 1983 1983 1983 1983 1983

1.06 1.21 2.22 1.16 0.94 1.78 1.12 0.71

62°18,0 ' 62018,0 ' 61059.6 ' 61059.6 ' 61"59.6 ' 61o59.6' ,61051.0' 61°51.0'

36*59.3 ' 36059.3 ' 36o27.3' 36*27.3' 36*27.3' 36O27.3' 38*08.7 ' 38*08.7 '

AMERIEZ II A 5 Mar. B 6 Mar. D 10 Mar. E 12 Mar. F 14 Mar. G 16 Mar. I 19 Mar. J 20 Mar.

1985 1986 1986 1986 1986 1986 1986 1986

0.68 1.25 1.36 1.18 1.39 0.93 2.09 1.59

65*58.0 ' 65*57.8 ' 65032.8 ' 65035.5 , 65049.5 , 65032.6 , 65*24. i ' 65016.3 ,

50012.0 ' 50012.9' 48007.2 ' 48014.2' 48044.3 ' 49O25.3' 49o08.2' 49047.4 '

~

"t .............

TUIES

CONOELATIONICE

ICE

PORE 8PACEIWATER

SNOW

Fig. 2. Conceptualized drawing of microhabitats in which bacteria are distributed within an ice floe. Views: top, full profile, cross-section. (1) Pressure ridge, (2) snow cover and slush layer, (3) hummock, (4) surface pond, (5) consolidated ice (composed of frazil and congelation ice crystals), (6) porewater layer, (7) submerged mantle of consolidated ice, (8) labyrinthine channels in deteriorating ice floe, and (9) platelet ice layer (composed of frazil ice).

IJ I m i i l J i

r~

L~

0

e~ "a

0

¢0

1316

S.T. KOTTMEIERand C. W. SULLIVAN

(1972). Conversion to carbon was made using a carbon : Chl a ratio of 38 (SULLIVANet al., 1985). Concurrent with the determination of bacterial production, primary production was determined in ice meltwater and seawater samples by incubation with NaH14CO3 (ICN). Replicate bottles (two light bottles and one bottle darkened with wraps of electrical tape) were incubated for 24 h in a Plexiglas deck incubator in circulating surface seawater at temperatures of - 1.0 to - 1.9°C. In situ irradiance in a variety of sea ice microhabitats was measured using a QSI-140 scalar irradiance meter (Biospherical Instruments Inc.). Surface downwelling irradiance, I0, varied from 10 to 45 Ein m - 2 d - 1 (SooHoo et al., 1987a,b). Light bottles were screened to simulate the irradiance in situ where the sample was taken: 2% I0 for bottom sections of ice, 12% I0 for surface brash, porewater and upper sections of ice, and 25-100% I0 for water from surface ponds and bands of exposed ice from the perimeter of floes. Following incubation, the contents of each bottle were filtered onto Whatman GF/C filters and reduced on an LKB model 1210 scintillation counter with carbon fixation determined as described previously (KcrrrMEmR and SULLIVAN,1987). Estimates of rates of algal growth were calculated from carbon fixation and a C: Chl a ratio of 38 (SULLIVANe t al., 1985), using equation (5) of EPPLEY (1972). RESULTS B a c t e r i a a n d m i c r o a l g a e w e r e v e r t i c a l l y d i s t r i b u t e d t h r o u g h o u t t h e c o n s o l i d a t e d ice o f floes, w h i c h a v e r a g e d f r o m 0 . 8 6 t o 1.47 m i n t h i c k n e s s ( T a b l e 2). T h e r e w e r e t h r e e g e n e r a l

Table 2. Characteristic features of consolidated ice cores collected during spring (November 1983) and autumn (March 1986) /n the Weddell Sea and during winter (August-September 1985) in the Beilingshausen Sea and mean (-S.E.). Number of samples indicated in parentheses Characteristic

Spring

Autumn

Ice thickness (m) Range

1.47 + 0.09 0.87-2.22 (n = 22)

1.34 + 0.14 0.68-2.09 (n = 9)

0.86 + 0.13 0.20-1.79 (n = 15)

Snow cover (m) Range

0.19 + 0.03 0-0.50 (n = 19)

0.16 + 0.04 0.05--0.40 (n = 9)

0.18 _ 0.04 0-0.50 (n -- 14)

Freeboard (m)

0.20 + 0.04

0.78 + 0.32

ND

(n = 3)

(n = 6)

Depth of porewater (m)

0.17 + 0.05

0.21 + 0.03

(n = 6)

(n = 9)

Salinity of meltwater in sections of consolidated ice (%.)

3.53 + 0.25

3.24 + 0.30

(n = 46)

(n =

Salinity of porewater (%.)

24.4 + 1.06 (n = 17)

32.3 + 0.62 (n = 8)

*From KOTTMEI~and SULLIVAN(1987). ND, not determined.

Winter*

ND ND

55) ND

1317

Bacterial biomass and production in pack ice

0

BACTERIA (10II cells m"3 ) 0 10 0 10 0

~

10

20

i

,~0

0

B

,

• 0

oC ~ ~

J

D

F i

i

'11

•

.

•

.

G

200

H

• '

'

J

K

Fig. 3. Variability in vertical distribution of bacteria through eight floes collected during spring 1983 (AMERIEZ I) in northwest Weddell Sea ice edge zone. Horizontal bars represent the range of replicate counts. The length of core F was 222 cm. Ice core locations and characteristics are shown in Table 1.

patterns by which bacteria and microalgae were distributed though vertical profiles of ice microhabitats (Figs 3 and 4); Chl a concentration, an indicator of microalgal biomass, was distributed similarly to bacterial distribution (Figs 5 and 6). The peak in biomass of

0

4

0

loo

4

BACTERIA (10I1 ceils m"3 ) 0 4 0 4

=,"

0

~J

4

I

ha

C

A

_z

B ___~

~

L.

' - ~

[

.___l~I _]

200

i

i

i ~

i

10 20 "~0O

,

E

I

I-Ti H ~

F

J

I

1

-1

I i

I

I

j I J

,

Fig. 4. Variability in distribution of bacteria through nine floes collected during autumn 1986 ( A M E R I E Z I I ) in the western Weddell Sea. The length of core I was 209 cm. Note change in scale of bacterial cell concentration and biomass for this core and in Fig. 8. Ice core locations and characteristics are shown in "]'able 1.

1318

S.T. KorrMEmR and C. W. SULLIVAN

Ch[ G • Pheo (rag m"3) 2 & 6 0 2 /. 6 0 2 /, 6 0 2 4 6 8

0

u~

0

c ' ~ '.2z2! - ~ - I ID

B

F

-y "" 100

I I

I

G

2OO

H

i

J

a

K

Fig. 5. Variability in distribution of total photosynthetic pigments (the sum of chlorophyll a and pha¢opigments) through eight floes from spring 1983 ( A M E R I E Z I). Ice core locations are shown in Table 1.

bacteria and algae was observed to occur either at the surface, in the interior or at the bottom of the ice floe. Bacterial biomass in consolidated ice was 55% and 21% of microalgal biomass during spring and autumn, respectively, in the Weddell Sea (Figs 7, 8 and Table 3).

0

0

10

CHI. o (O) AND PHE0 (e} (mgm "3) 10 0 10 0 10 0

10

20

5O I00 150 0

:

:

A :

Z

B

D

LI::1:

E

F

/

G

Fig. 6.

H ~2o~ ,

1

,

,

J

Variability in distribution of chlorophyll a (open histograms) and phaeopigments (shaded histograms) through nine floes from autumn 1986 ( A M E R I E Z II).

1319

Bacterial biomass and production in pack ice

BACTERIAL BIOMASS (mg 200 0 200 0 200

0

• ,~" i ~

so I -

t-

m

'

•

•

"

:

|

m-3) 2O0

4OO

J

11)0 150 B • 0

:

;

I

C~ ;

!

:

F

O

100 150 G

2O0 Fig. 7.

H

J

Variability in distribution of bacterial biomass through eight floes from spring 1983 ( A M E R I E Z I).

Bacteria were abundant in several ice microhabitats. Highest concentrations of bacterial cells and biomass were found in saline ponds on the surface of floes, followed by brash ice at the surface, and then porewater in the interior of floes (Table 3). Chlorophyll a was distributed in a pattern similar to that of bacteria in all ice microhabitats (Table 4). Bacterial biomass varied from 10 to 143% of microalgal biomass in saline surface ponds and porewater, respectively (Table 3).

50

BACTERIAL BIOMASS ( m"3) SO 50 ~g C 50

0

0

50 ._j

i

i

100

IO0 ~150 1,1 n-

• Bi '

A

8o

z

~20o~o ! . . . .

E

;

;

.F

I'

T

E50 ¸ m ttl El

100

]

1so 200

Fig. 8.

J ,

6

,

,

,H~.~

,

, I

J

Variability in distribution of bacterial biomass through nine floes from autumn 1986 (AMERIEZ II).

ND

55.1

ND

Bacterial growth rate (d -1)

Bacterial biomass: microalgal biomass~t (%)

Bacterial production: primary production:~ (%)

ND

ND

0.530-+0.054 (n = 75)

1.07-+0.206

48.7-+9.36 (n = 105)

31.4

2.72+0.233

0.866+0.065 (n = 137)

Seawater

22.1

21.2

0.115-+0.020

4.03-+0.460

4.79-+0.540 (n = 12 sections of 2 cores

63.5+3.12

38.6-+5.70

6.08+0.86 (n = 56 sections of 9 cores)

Ice

2.35

17.7

0.031

3.72

6.24

59.6

118

19.8 (n = 1)

Slush

44.9

10.5

1.37-+1.23

1094-+1059

127-+81.9

93.6-+13.4

208-+35.6

22.9+6.12 (n = 2)

Surface pond

Autumn

221

42.9

1.01+0.589

46.9-+45.0

75.1-+71.6

57.7-+6.93

11.5-+4.07

2.21-+0.979 (n = 5)

Porewater

ND

ND

0.156+_0.014

0.226-+0.016

10.28-+0.733 (n = 138)

21.8

2.59-+0.140

1.19+0.063 (n -- 115)

Seawater

*Data from KorruEmlt and SULLIVAN(1987). t Production in meltwater for ice samples. ~tData from Table 4. ND, not determined.

ND

Bacterial carbon productiont (nag C m -3 d-1)

78.7-+14.2

Average cell biomass (fg C cell-t)

ND

77.2 = 20.5

Bacterial biomass (rag C m -a)

Bacterial ceU productiont (101°cellsm-ad -l)

8.82-+2.19 (n = 43 sections of 8 cores)

Ice

Bacteria (1011 cells m -3)

Characteristic

Spring

8.58

ND

ND

5.74-+2.37

22.6+9.32 (n = 5)

25.4

ND

ND

Ice

5.20

ND

ND

0.314 + 0.212

1.48 + 1.00 (n = 4)

21.2

ND

ND

Seawater

Winter*

Table 3. Bacterialcharacteristics (mean -+ S.E.) of various sea ice microhabitats and underlying seawater during spring 1983 and autumn 1986 in the Weddell-Scotia Sea and during winter 1985 in the Bellingshausen Sea. Number of samples indicated in parentheses

7~

7~ ),

t-

e~

bO

ND

Phaeopigments (mg m -3)

ND

Microalgal growth rate (d-')

0.073 + 0.006 (n = 119)

0.229+0.028 (n = 126)¶

16.3+2.40

ND

0.430+0.063 (n = 172)t

Seawater

0.193 + 0.034

18.2+4.06 (n = 12)

182+26.6

0.22+0.05

4.12+0.260 (n = 58 sections of 9 cores)

Ice

0.213

158

665

0.040

17.5 (n = 1)

Slush

0.752 + 0.110

2435+1399

1980+ 1160

4.49+2.50

52.1+21.7 (n = 2)

Surface pond

Autumn

*Data from KO~TMEIERand SULLIVAN(1987). t D a t a from NELSON et aL (1989). ~Sum of chlorophyll a and phaeopigments. §Chlorophyll a x 38 ( S U L L I V A N et al., 1985). IIProduction in meltwater for ice samples. ¶Samples incubated in screened bottles under surface irradiance (data from W. Smith). ND, not determined.

ND

Primary productionll ( m g C m - 3 d -I)

( m g C m -I)

biomass§

140+41.9

3.69+1.09 (n = 43 sections of 8 cores)

Chlorophyll a~ (rag m -a)

Microalgal

Ice

Spring

0.365 + 0.120

21.2+15.2 (n = 5)

26.8+ 11.3

0.060+0.030

0.77+0.29 (n = 9)

Porewater

0.0026 +_0.0002 (n = 117)

2.05+0.170 ~n = 118)

2.97+0.096 (n = 213)

0.027+0.001

0.076+0.003 (n = 224)

Seawater

0.007 + 0.003

66.9+21.0 (n = 5)¶

479+44.2

2.08+0.300

12.6+1.15 (n = 55 sections of 15 cores)

Ice

Seawater

0.030+0.007

6.04+3.18 (n = 4)

14.7-+ 13.4

0.090 + 0.063

0.388 + 0.305 (n = 4)

Winter*

M icroalgal characteristics (mean + S.E. ) of various sea ice microhabitats and underlying seawater during spring 1983 and autumn 1986 in the Weddell Sea and during winter 1985 in the Bellingshausen Sea. Number of samples indicated in parentheses

Characteristic

Table 4.

L,O t,o

W

O t~

r~

O

1322

S.T. KOrrMEmR and C. W. SULLIVAN

Net bacterial cell and biomass production, estimated from incorporation of thymidine, were found for all ice microhabitats (Table 3). The highest bacterial production was found in surface ponds and the lowest in consolidated ice of floes. Likewise, the most rapid bacterial growth was also found in surface ponds, followed by porewater, consolidated ice, and then brash. Estimates of bacterial production in consolidated ice probably exceed bacterial activity in situ, since sections of ice were allowed to melt into filtered seawater prior to incubation with 3H-thymidine. These estimates therefore should be considered as "potential" production for consolidated ice due to physical disruption of ice structure and changes in the microenvironment of the bacteria. However, previous studies in which ice "cubes" were perfused with seawater containing 3H-thymidine gave similar values for specific growth rates of bacteria (KoTrMEIERe t al., 1987). Distribution of primary production in pack ice microhabitats paralleled bacterial production (Table 4). Bacterial carbon production ranged from 2% of primary production in brash ice on the surface of floes to 221% of primary production in porewater of the interior of floes (Table 3). Spearman rank correlations (TATE and CLELLAND,1957) were calculated for several microbial characteristics of various pack ice microhabitats in autumn 1986. There was a significant correlation (P < 0.05) between total bacterial number and Chl a. Significant correlations (P < 0.05) were also found between bacterial biomass and production (cell and biomass) and Chl a and primary production (Table 5). Ice microhabitats were concentrated sources of actively growing bacteria compared to seawater. Bacterial concentrations in ice were 2-19 times greater than in seawater and ice bacteria, which often occurred as long chains of cells, were up to 4 times larger in biovolume than bacterioplankton in seawater (Table 6). As a result, ice microhabitats had 4-80 times more bacterial biomass per m 3 than seawater. Although bacterial cell production and growth in some microhabitats were less than that found in seawater, bacterial biomass production was 16--4840 times that found in comparable volumes of seawater. This results from the greater size of bacteria in sea ice compared with bacterioplankton (Table 3). In addition to bacteria and microalgae, the ice contained a variety of heterotrophic

Table 5.

Spearman rank correlation matrix o f microbial parameters measured in 21 samples o f ice and water from all sea ice microhabitats during autumn (1986) in the Weddell Sea

TBN TBN TBB BCP BBP BGR CHL PHE MPP MGR

TBB

BCP

BBP

BGR

CHL

PHE

MPP

MGR

0.95**

0.36 0.41

0.41 0.52* 0.90**

-0.51"* -0.46* 0.46* 0.34

0.53* 0.61"* 0.63** 0.62** -0.02

0.08 0.04 0.33 0.32 0.24 0.13

0.40 0.48* 0.60** 0.58** 0.14 0.83** 0.13

0.10 0.17 0.17 0.22 0.26 0.02 0.31 0.46*

TBN, total bacterial number; TBB, total bacterial biomass; BCP, bacterial cell production; BBP, bacterial biomass production; BGR, bacterial growth rate; CHL, chlorophyll a; PHE, phaeopigments; MPP, microalgal primary production; MGR, microalgal growth rate. *0.01 < P < 0.05, **0.001 < P < 0.01.

1323

Bacterial biomass and production in pack ice Table 6. Enrichment of bacterial characteristics in sea ice. Estimated by determining the ratio of bacterial characteristics in various sea ice microhabitats to underlying seawater during spring 1983 and autumn 1986 in the Weddell Sea and during winter 1985 in the Bellingshausen Sea. Derived from mean data presented in Table 2 Autumn Spring Ice

Ice

Slush

Bacteria

10.2

5.11

16.6

19.2

1.86

ND

Bacterial biomass

28.4

45.6

80.3

4.44

ND

Average cell biomass

2.80

3.98

2.73

2.38

1.20

Bacterial cell production

ND

0.466

0.607

Bacterial carbon production

ND

Bacterial growth rate

ND

Characteristic

14.9

17.8 0.737

16.5 0.199

Surface pond

Porewater

4.16 12.4 4840 8.78

7.31

Winter Ice

15.3

208

18.3

6.47

ND

ND, not determined.

eukaryotes, including potential bacterial grazers such as amoebae, naked flagellates, choanoflageUates, dinoflagellates, ciliates and a few micrometazoans (GARRISON, 1989). These eukaryotic microheterotrophs were distributed throughout consolidated ice (Fig. 9) and contributed 4-6% of the microalgal plus bacterial biomass (Table 7).

MICROALGAL (D) AND BACTERIAL. (O) BIOMASS (mgC. rn'3~} 10 20 30 0 100 200 300 400 O 417 (

"=' a 10Q

0

-o

•

j-[. ~

I I I I I I I 10 20 30 0 20 40 60 MICROHETEROTROPH (O) BIOMASS [mgC. m -3)

loo

80

Fig. 9. Variability in distribution of microalgal, bacterial, and microheterotrophic biomass through two floes from spring 1983 ( A M E R I E Z I), Note (D) refers to histogram.

1324

S.T. KOTTMEIERand C. W. SULLIVAN Table 7. Microbial and micrometazoan biomass (mg C m -2) in two cores of consolidated ice collected from different floes during spring 1983 in the Weddell Sea Core J*

Core K*

Microalgal bioma3st (diatoms, autotrophic dinoflagellates and other flagellates)

24.1

200

Bacterial biomass

25.6

217

Microheterotrophic biomass~: (flagellates, ciliates, amoebae and micrometazoans)

3.09

Total biornass Bacterial: microalgal biomass

(%)

52.8 106

17.2

434 108

Microheterotrophic: microalgal biomass

12.8

8.60

Microheterotrophic: bacterial biomass

12.1

7.93

Microheterotrophic: microalgal + bacterial biomass

6.22

4.12

t Chlorophyll a x 38 (SULLIVANet al., 1985). .~Data courtesy of D. Garrison and K. Buck, University of California, Santa Cruz. * See Fig. 1 area WC and Table I for ample location. Core J is I. 12 m and core K 1.14 m in length.

DISCUSSION

Sea ice microhabitats within marginal ice edge zones are concentrated sources of bacterial biomass and production compared to underlying seawater. Although viable bacteria are abundant and capable of growth in every ice microhabitat examined, consolidated ice comprises the major microhabitat (by volume) for bacteria and serves as the substrate within which other microhabitats develop during the processes of ice deformation and deterioration induced by melting. Bacteria are distributed throughout consolidated ice and exhibit a variety of abundance vs depth profiles, related to a combination of the physical, chemical and biological characteristics of the ice. The mechanisms by which bacteria colonize and become distributed in pack ice appear to be partially related to the crystalline layers of frazil and congelation ice comprising consolidated ice. An analysis of variance indicated significantly greater numbers of bacteria associated with layers of frazil ice than congelation ice in four cores taken during A M E R I E Z '83 (F 1.16 = 6.12, P < 0.05) (RonI.F and SOKAL, 1969; SOgAL and ROHLF, 1981; GARRISONand Buct:, 1986). Small (approx. i mm dia.) frazil ice crystals forming rapidly in turbulent water (MARTIN, 1981; WEEKS and ACKLEY, 1982) may physically concentrate bacteria, similar to microalgae and foraminifera (BUNT, 1968; BUST and LEE,

Bacterial biomassand productionin pack ice

1325

1970; ANDERSEN,1977;ACKLEY,1982; GARRISONet al., 1983; CLARKEand ACKLEY,1984; SULLIVAN,1985; DIECKIdANNet al., 1986). Frazil ice crystals may be nucleated by bacteria and/or buoyant crystals may non-selectively scavenge bacteria from seawater onto their surfaces as they move toward the sea surface (SULLIVAN,1985). By contrast, large (approx. 1 cm dia.) columnar-grained crystals of congelation ice form slowly during the removal of heat from an existing ice sheet and the physical accumulation of bacteria is not as likely (SULLIVAN,1985). After consolidated ice forms, additional microhabitats develop within it and may be colonized subsequently by bacteria from within t h e ice or the surrounding seawater. Bacteria are found in snow, slush and ponds at the surface of floes in a layer termed plankton ice, snow ice or infiltration ice (MEGURO, 1962; BURKHOLDERand MANDELLI, 1965; WHITAKER, 1977; PALMISANO and SULLIVAN,1985a,b) and may colonize this microhabitat from the melting of snow and consolidated ice, washing of seawater onto the ice, and possibly through aerosols. They also are found in porewater which perfuses the interior of floes close to sea level and may colonize this microhabitat by incomplete rejection of brine from chambers in floes (ACKLEYet al., 1979) and horizontal pumping of seawater by wave action (KOTTMEIERand SULLIVAN,1987). Bacteria are capable of growth at the low temperatures and variable salinities characteristic of pack ice following their colonization of the ice (Table 2). A necessary prerequisite for bacterial growth, however, is an aqueous environment within the ice; brine concentrations and therefore water activity known to be strictly a function of temperature (POUNDER, 1965; KO'ITMEIERand SULLIVAN,1987). Bacteria frozen into ice crystals cannot grow but may survive (SULLIVAN,1985). Growth of bacteria in situ is indicated by net increase of cells over time (assuming minimal losses to bactivory and melting), synthesis of DNA (indicated by incorporation of 3H-thymidine), and observation in the scanning electron microscope of dividing cells (SULLIVAN, 1985). Since we did not sample floes repetitively on these cruises, we were unable to determine a net change in bacterial concentration over time as we have observed for fast ice (KoTI~EIER et al., 1987). We did observe, however, net incorporation of thymidine for all ice microhabitats, which indicated synthesis of DNA, a prerequisite for growth of bacteria. Net incorporation of thymidine by bacteria in fresh pancake ice during autumn (KoTrMEIER and SULLIVAN, unpublished observations) suggests that bacteria recently incorporated into ice are capable of growth. The qualitative observation of dividing cells by epifluorescence microscopy in all ice microhabitats also supported growth of bacteria/n situ. Growth rates of >1 d -1 for bacteria of melt ponds and porewater in autumn pack ice exceed greatly the rates of up to 0.2 d-1 we estimated for bacteria inhabiting consolidated pack ice, summer congelation and p!atelet ice of McMurdo Sound (KOTTMEmRet al., 1987). The distribution and rate of growth of bacteria in pack ice apparently are coupled to the distribution and rate of growth of microalgae. In all cases the distribution of bacterial biomass paralleled the distribution of Chl a. We found the highest rates of thymidine incorporation in surface ponds of floes where blooms of microalgae were continually exposed to high irradiance. Significant positive correlations between bacterial number, biomass and production, and Chl a and primary production, suggest indirectly that bacterial growth and microalgal photosynthetic metabolism in pack ice are coupled in a fashion similar to that reported for fast ice (KoTTMEIERet al., 1987). Growth of bacteria in pack ice is probably stimulated by blooms of microalgae, which in turn are triggered and sustained primarily by high surface irradiance (GROSSl et al., 1984). Ice microalgae may

1326

S.T. KOTrMEIERand C. W. SULLIVAN

provide bacteria with dissolved organic matter (DOM) as a source of carbon and energy, while bacteria may provide microalgae with inorganic nutrients and possibly vitamins (GRossl et al., 1984; KOrI~EmR et al., 1987). These reciprocal interactions between algae and bacteria may sustain the observed structure of pack ice associated microbial communities for long periods, since these communities appear to be very well developed in the multi-year floes we observed in the western Weddell Sea in autumn. We also observed uncoupling of bacterial and primary production seasonally and in different microhabitats of pack ice. Bacterial production in porewater exceeded primary production during autumn (Table 3) and in some ice floes studied during winter in the Bellingshausen Sea (KorI~EmR and SULLIVAN,1987). Low ambient irradiance contributed to lower rates of primary production in these porewater and pack ice microhabitats. Significant attenuation and spectral shift in downwelling irradiance also occur throughout the year in some ice microhabitats due to the combined effects of snow cover, ice thickness and biomass of microalgae present within the ice (SooHoo et al., 1987a,b). Snow cover alone, which averaged 0.16--0.19 m (range from 0 to 0.50 m) in the floes sampled (Table 2), can reduce downweUing irradiance to less than 1% of surface irradiance (SULLIVANet al., 1985). Although bacterial production temporarily exceeds primary production under these conditions, obviously it cannot exceed primary production on an annual basis without import of DOM. Sustained bacterial production depends upon continuing microalgal photosynthetic metabolism for supply of DOM to the various ice microenvironments described here. The quantitativ e importance of bacterial production in pack ice of marginal ice edge zones is difficult to assess. The ice edge region is very dynamic on time scales ranging from hours to months (COMISOand ZWALLY,1984; CoMISOand SULLIVAN,1986; SULLIVANet al., 1988). It is also a heterogeneous, physical environment composed of several distinct regions: open water (seaward of the pack ice edge and in polynas), young sea ice (grease, pancake, and thin <10 cm new ice), annual ice ( - 1 - 2 m thick) and multi-year ice (>2.5 m thick). Individual ice floes that we sampled are likely to have different histories. We do not know precisely in what water mass they were formed, how far they have moved since their formation, nor do we know the impact of such factors as melting, freezing, wave action and ice deformation processes generally. As a result, each floe might be considered an "island" of bacterial production with an unknown previous history. Recent work has suggested a relatively high spatial heterogeneity in the distribution of ice algae on a scale of one to a few meters in all ages of ice (GARRISONand BUCK, 1989b). Since the distribution of bacteria is correlated with the distribution of Chl a in pack ice, high spatial variability may exist for bacteria as well. There is also considerable seasonal variability in bacterial production of pack ice, since some microhabitats, such as the productive saline surface ponds, are not present throughout the year. Pack ice is clearly a site of concentrated bacterial biomass and production throughout the year. Total bacterial biomass in pack ice is equivalent to that found in 4-80 m of underlying seawater (Table 6). This is due in part to high densities of large bacteria, several times larger than bacterioplankton of the surrounding seawater, from organically rich ice microenvironments (SULLIVANand PLAMISANO,1984; SULLIVAN,1985). In contrast to our reported bacterial concentrations in total ice meltwater, in situ concentrations within the liquid brine of tubes, channels and chambers of pack ice may be as high as 1013 cells m -3. These concentrations are comparable to those found in the bottom 20 cm of land-fast ice during summer (KoTTMEI~.Ret al., 1987). If one considers only the bacteria inhabiting

Bacterial biomass and production in pack ice

1327

consolidated ice, then approximately 15-28 times more bacterial biomass is present in the ice than an equivalent volume of underlying seawater. This conservative estimate does not reflect the contribution of bacterial biomass from other ice microhabitats, which have 4--80 times more bacterial biomass than comparable volumes of seawater (Table 3). Although bacterial carbon in pack ice is thought by some to be physically unavailable to consumers because it is "frozen" into the ice matrix, our observations suggest that this concentrated biomass is available to microheterotrophs in several ice microhabitats throughout the year. The presence of microheterotrophs with significant biomass and numbers in pack ice microhabitats suggests indirectly that accumulation of bacterial biomass may be partially regulated by bactivory. Microheterotrophs observed in pack ice are a diverse group of protozoans composed of choanoflageUates, dinoflagellates, ciliates, tintinnids, foraminifera and amoebae (FENCHEL and LEE, 1972; GARRISONet al., 1984, 1986; DIECrMANNet al., 1986; KoTrMEIERet al., 1987). Although little is known about rates of microheterotrophic production in pack ice (GARRISON et al., 1986), evidence for microheterotrophs as bactivores has been inferred from microscopic observations (Buck and GARRISON, submitted). Indirect evidence suggests that bactivory may influence bacterial cell production in fast ice (KoTmEmR et al., 1987). Microheterotrophs are important links between bacterial production of the microbial loop and the classic food chain of pack ice (GARRISONet al., 1986; KOTmEmR et al., 1987). Some ice bacteria may be large enough (chains 10-30/am long) to serve as food for juvenile krill since choanoflagellates, forming colonies up to 18/am diameter, have been shown to serve as sources of food for krill (MEYER and EL-SAYED, 1983; QUErIN and Ross, 1985; MARCHANTand NASH, 1986; TANOUEand HARA, 1986). Peripheral surfaces of ice floes are readily accessible to larger pelagic consumers such as amphipods and krill (HAMNERet al., 1983; KOrrMEmRand SULLIVAN,1987; AARSEa'r, 1987; STRETCH et al., 1988). Although vertical brine channels are usually lacking in floes consisting primarily of frazil ice (CLARKEand ACKLEY, 1984), microheterotrophs and small metazoans may be able to colonize consolidated ice and porewater, depending upon the crystalline structure and porosity of the ice. Deteriorating floes have labyrinthine channels (several cm in diameter), with the appearance of "Swiss cheese" (GARRISONet al., 1986; SULLIVANand AINLEY,1987), which provide an extremely large surface area to volume ratio and favorable habitat for feeding by larger fauna. Subsequent rafting and refreezing of floes may account for observation of fauna, such as krill, occurring within ice floes throughout the year (AINLEYand SULLIVAN,1984; KOrrMEIER and SULLIVAN,1987; SULLIVANand AINLEY, 1987). Bacterial production in pack ice contributes to overall trophodynamics of marginal ice edge zones throughout the year. By production of high concentrations of particulate carbon, bacteria of pack ice apparently support a significant biomass of microheterotrophs that participate in a microbial loop associated with the ice and may in turn support higher trophic levels of classic food chains (GARRISON et al., 1986; KOXa~EmR et al., 1987). Actively growing bacteria released from melting ice also contribut.e to microbial blooms in the surrounding seawater. Bacterial production accessible to consumers in several microhabitats of pack ice and seawater may partially explain why the ice edge zone is a focus for organisms of cryo-pelagic food webs (AINLEVet al., 1986). Acknowledgements--The authors wish to thank their AMERIEZ colleagueswho assisted with sampling and data

collection; in particular we acknowledge field assistance from J. SooHoo, M. P. Lizotte and D. Robinson. W,

1328

S.T. KOTrMEIERand C. W. SULLIVAN

Smith provided information on phytoplankton chlorophyll a and primary production of the water column. We acknowledge support from the National Science Foundation grant DPP-84-44783 to CWS.

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Antarctic nutrient cycles and food webs (Proceedings of the fourth SCAR symposium on Antarctic biology), W. R. SIEOrraED, P. R. CONDYand R. M. LAws, editors, Springer-Verlag, Berlin, pp. 372-377. RIVKIN R. B. (1986) Incorporation of tritiated thymidine by eucaryotic microalgae. Journal of Phycology, 22, 193-198. RoitLr F. J. and R. R. Sor~L (1969) Statistical tables. W. H. Freeman, San Francisco. SOKALR. R. and F. J. ROHLF(1981) Biometry, 2nd edn, W. H. Freeman, San Francisco. SooHoo J. B., M. P. LIZOTTE,D. H. ROmNSONand C. W. SULLIVAN(1987a) A M E R I E Z 1986: Photoadaptation of phytoplankton and light limitation of primary production in the ice edge zone of the Weddell Sea. Antarctic Journal Review of the United States, 22, 185--187. SooHoo J. B., A. C. PALVaSANO,S. T. Kcyrr~EmR, M. P. LlzffrrE, S. L. SooHoo and C. W. SULLIVAN(1987b) Spectral light absorption and quantum yield of photosynthesis in sea ice microalgae and a bloom of phaeaocystis pouchetii from McMurdo Sound, Antarctica. Marine Ecology Progress Series, 39, 175-189. STRETCHJ. J., P. P. HAMNER,W. M. t-IAMNER,W. C. MmHEL, J. COOKand C. W.. SULLIV,~ (1988) Foraging

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