Mierob Eeol (1989) 17:63-76

MICROBIAL ECOLOGY @Springer-VerlagNew York Inc. 1989

Population Dynamics of Bacteria in Arctic Sea Ice Ralph E. H. Smith,* Pierre Clement, and Glenn F. CotaJDepartment of Fisheries and Oceans, BedfordInstitute of Oceanography,Box 1006, Dartmouth, Nova Scotia, Canada B2Y 4A2

Abstract.

The dynamics of bacterial populations in annual sea ice were measured throughout the vernal bloom of ice algae near Resolute in the Canadian Arctic. The maximum concentration of bacteria was 6.0.101~ cells, m-2 (about 2.0- 10 lo cells. 1-1) and average cell volume was 0.473 tim 3 in the lower 4 cm of the ice sheet. On average, 37% of the bacteria were epiphytic and were most commonly attached (70%) to the dominant alga, Nitzschia frigida (58% of total algal numbers). Bacterial population dynamics appeared exponential, and specific growth rates were higher in the early season (0.058 day-l), when algal biomass was increasing, than in the later season (0.0247 day-l), when algal biomass was declining. The proportion ofepiphytes and the average number ofepiphytes per alga increased significantly (p < 0.05) through the course of the algal bloom. The net Production of bacteria was 67.1 m g C . m -2 throughout the algal bloom Period, of which 45.5 m g C . m -2 occurred during the phase of declining algal biomass. Net algal production was 1942 m g C . m -2. Sea ice bacteria (both arctic and antarctic) are more abundant than expected on the basis .of relationships between bacterioplankton and chlorophyll concentrations m temperate waters, but ice bacteria biomass and net production are nonetheless small compared with the ice algal blooms that presumably support them.

Introduction tieterotrophic microorganisms account for a significant proportion of the carbon and nutrient flux through the plankton communities of many temperate and tropical marine waters [10, 34]. The efficiency with which primary pro,du.ction may be channeled to consumers at higher trophic levels through the microbial loop' [1] is still controversial but seems certain to be lower than through the more classical diatom-copepod food chain. It has often been suggested that the growth of heterotrophic bacteria is strongly inhibited by the low

* Present address: BiologyDepartment, Universityof Waterloo, Waterloo, Ontario, Canada N2L 3Gl. Present address: Graduate Program in Ecology,Universityof Tennessee, Knoxville,Tennessee 37996.

64

R.E.H. Smith et al.

temperatures encountered seasonally in t e m p e r a t e waters and routinely in polar or profundal waters [7, 24]. T e m p e r a t u r e - m e d i a t e d inhibition o f the heterotrophic microbial loop relative to autotrophic p r o d u c t i o n has been p r o p o s e d to contribute to the high apparent productivity o f cold water shelf ecosystems [26]. In contrast, recent work in a temperate coastal e m b a y m e n t [22] found n o evidence for the suppression o f heterotrophic microbial activity during the spring bloom. Studies o f the activity o f bacteria and other heterotrophic microorganisms in chronically cold waters are still few, however, some suggest that a highly active microbial loop operates in polar waters [15, 21 ]. An important, and chronically cold, c o m m u n i t y o f microorganisms develops in annual sea ice in both arctic and antarctic waters [16, 30]. T h e c o m m u n i t i e s contribute substantially to marine p r i m a r y p r o d u c t i o n in seasonally ice-covered areas [ 14, 18] but also include an active heterotrophic microbial flora. Recent work in the Antarctic has shown that heterotrophic ice bacteria actively grow and assimilate dissolved organic substrates [13, 20, 21 ]. T h e r e is also evidence that ice bacteria are grazed where they occur on and in the undersurface o f the ice [21] although rates are still difficult to quantify. By comparison, little is known about the size o f ice bacteria populations in the Arctic or their rates o f growth and nutrient assimilation, although there is evidence for the assimilation o f dissolved organic substrates by ice bacteria from the Beaufort Sea [12, 17]. Apart f r o m the i n f o r m a t i o n they can yield on the ability o f microbes to adapt to low temperatures, ice bacteria afford an o p p o r t u n i t y to study algal-bacterial interactions in an unusually concentrated yet natural setting. In the spatially restricted zones o f peak microbial abundance in sea ice, chlorophyll concentrations m a y exceed 3 mg.l -~ [28, 32] and bacterial cell concentrations can surpass 1.4.109 cells. 1-t [21 ]. A m a j o r practical advantage is that the populations are not subject to advection and mixing effects, so that net poulation dynamics can be observed through time-series sampling o f the ice. Antarctic ice bacteria populations have thus been shown to grow exponentially during m u c h o f the spring b l o o m o f ice algae, while their association with the cooccurring ice algae suggests a c o m m e n s a l or even mutualistic relationship [ 13, 31]. N o similar observations have been m a d e in the Arctic, where species composition o f the ice algae and the structure and m e c h a n i s m s o f ice growth [ 16] generally differ from those in the Antarctic. T h e object o f the present study was to quantify the population d y n a m i c s o f ice bacteria during the spring b l o o m o f ice algae in the Canadian high Arctic, to determine their associations with the d o m i n a n t species o f ice algae, and to estimate the magnitude o f net bacterial p r o d u c t i o n vs primary production. Materials and Methods

Site and Sampling Methods The study site was in the Canadian Arctic, approximately 4 km south of Cornwallis Island in Barrow Strait (74~ 94~ Ice and snow thickness varied from 170 to 190 cm and 2 to 25 cm, respectively, with localized drifts providing deeper snow cover. The water column was approximately 100 m deep. Cota et al. describe the sampling area in more detail [9]. Samples were taken at weekly, or more frequent, intervals from early April through early June

DYnamics of Ice Bacteria

65

19.85, corresponding to the period of growth and decline of the ice algal bloom. Cores were taken using a SIPRE corer (a 7.62 cm diameter cylinder equipped with flights and teeth [28]) and then processed in one of two ways. To sample the entire core area, the bottom 3-4 cm of each core was cut off, melted in a 20-30~ water bath, and aliquots were taken for enumeration o f bacterial and algal cells and for determination of chlorophyll concentration. Samples were removed from the bath as soon as the ice was melted, and were not found to warm above 5"C. Previous sampling [9] and visual inspection showed that most of the algal population was concentrated in the lower 4 cm of the ice, and especially in the highly porous skeletal layer [23]. A subsampling method was Used more commonly in the latter half o f the observation period when ice was softer and intact cores were harder to retrieve. In the subsampling method, a 2.54 cm diameter steel punch was Used to remove subcores (3 cm deep) from areas of the fragments o f bottom ice that appeared intact (i.e., that possessed an intact skeletal layer of normal appearance). The subcores were then melted and sampled for bacteria and chlorophyll. Limited comparisons (two dates) between the !eehniques did not reveal consistent differences. Temperature of the surface seawater, and by reference of the sampled layer of ice, remained in the range - 1.7 to - 1.8~ throughout the Observation period. Sampling was from areas with snow cover of 2-4 cm in depth. It was usually possible to discern that the snow cover was stable and long established, and thus to ensure that the sampled populations !algal as well as bacterial) were of a reasonably uniform and known light history. Snow melt began in the first week of June, at which time light penetration and upper water column chemistry changed rapidly.

Analytic and Microscopical Methods Aliquots of melted sea ice were preserved with 2% glutaraldehyde and stored at 4~ pending bacterial and algal cell counts. Parallel aliquots were filtered on glass fiber filters (Whatman GF/ F) and immediately frozen pending analysis for chlorophyll a. Both types o f sample were generally taken within 2 hours of first retrieving the ice core. Samples for chlorophyll a were extracted for 16-24 hours in 90% acetone at - 2 0 ~ without grinding. The fluorescence of the extract before and after acidification (with HC1) was measured on a flUorometer previously calibrated with pure chlorophyll a [35]. Samples were assayed within 3 Weeks of collection. Samples for enumeration of bacteria were filtered onto 0.2 #m pore size polycarbonate filters using 0.45 ~tm pore size cellulose acetate backing filters to obtain a uniform distribution of cells. The Polycarbonate filters were previously stained with Irgalan Black (Monsanto Ltd., Montreal, Que.) to enhance contrast. The fluorescent dye DAPI (4',6-diamidino-2-phenylindole-2 HC1, Sigma) was added at a concentration of 100 mg.ml ' to wet the entire face o f the filter, and allowed to stand for 5 rain before removal by filtration. The filter was mounted in immersion oil on a microscope slide before examination by epifluorescent microscopy. Bacteria were enumerated at a final magnification of 1,200 • (I 00 x objective). Filtered seawater blanks were routinely examined to control for contamination of equipment and reagents. To measure bacterial cell sizes, photomicrographs were projected to a final magnification of about 20,000 x, together with a simultaneous photomicrograph of a stage micrometer to ensure accuracy of the image size calculations. The measurement of epifluorescent images is a common method of assessing cell sizes of bacterioplankton (and antarctic ice bacteria, [21]) but may overestimate the cell dimensions [27]. Only unattached cells were measured, but there was no subjectively evident size difference between unattached and epiphytic bacteria. Epiphytic ice bacteria in Antarctic bottom ice are larger than unattached cells [13] so our estimates of epiphytic and total bacterial biomass may be conservative. Cell volumes were calculated from geometric approximations, and converted to carbon assuming 220 fgC.#m 3 [6, 21]. Algae were counted on the same filters used for enumeration of bacteria but at 200 x final ~agnification. Empty (dead) algae lacked DAPI fluorescence, but were quite rare and are not included in the counts reported here. Their relative abundance was estimated from less frequent examinations using both phase contrast and chlorophyll-derived fluorescence in live material within

66

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Sampling Date Fig. 1. The dynamics of (A) chlorophyll (mg.m 2) and bacterial cell (B) concentrations (cells" m -2) in annual sea ice in 1985, near Resolute, Canadian Arctic. Values shown are logarithms to the base 10. The lines were fitted by linear least squares (Table 1),

1 hour of collection. Fewer than 1% of the algal cells failed to show the fluorescence indicative of chlorophyll-containing, intact, photosynthetic organelles. Aliquots of melted core sections were filtered through glass fiber filters (Whatman GF/F, precombusted) and analyzed for carbon and nitrogen content using an elemental analyzer. The values of particulate organic carbon (POC) allowed calculation of the total (algal plus bacterial) microbial carbon production during the ice algal bloom, and comparison against bacterial production calculated from cell volumes.

Dynamics of Ice Bacteria

67

Table1,

The fitted parameters of the relationship between sampling date (Day) and bacterial cell concentrations ~N, cells.m -2) in sea ice ofthe Canadian Arctic: Iog,o(N) = a + u.Day where g has units of day-~ ffOpulation Intarval

a

~ 9 SE

r2

n

Epiphytic

Early 1 , 1 1 9 0,0795 ~ 0.0075 0.94 8 Late 8.905 0.0217 +- 0.0070 0.34 17 Unattached E a r l y 4.589 0.0536 ~+0.0073 0.88 8 Late 7.305 0.0277 • 0.0042 0.73 17 Total Early 4 , 2 0 7 0.0581 +_ 0.0049 0.95 8 ~ Late 8.008 0.0247 -+__0.0050 0.61 17 [mervals_Corresp~rLdto the divisions shown in Figure 1B, C, D; SE ~ta~dard error

R~sults During our observation period in 1985, the ice algae d e v e l o p e d a large standing crop, in excess o f 100 m g . m -2 o f chlorophyll a at its peak. Figure 1A shows the dynamics o f chlorophyll a standing crop, illustrating two clearly different Phases o f the bloom. T h e initial phase was a roughly exponential increase, SUstained for the first 3 weeks o f our observation period, followed b y a declining phase. There m a y actually have been a substantial decrease and recovery between the two phases, constituting a m a j o r oscillation in standing crop, b u t the data do not clearly resolve events between the increasing and decreasing phases. The declining phase was, in its later stages, a c c o m p a n i e d b y shedding o f algal mats from the ice into the water column. Sampling u n d e r various depths of Snow cover (from zero to 20 cm) revealed that the thin snow c o v e r areas (24 cm) that are described here supported the largest algal populations I29]. Chlorophyll concentrations reached a peak o f 120 rag. m -2 u n d e r 2--4 cm snow COver but no m o r e than 60 nag. m -~ u n d e r other depths o f snow. Bacterial laOl~ulations also showed at least two different phases in their dynamics. Total bacterial cell n u m b e r s appeared to increase exponentially during bold the increasing and decreasing algal phase (Fig. IB), b u t at a faster rate dur/ng the increasing algal phase. The same was true o f b o t h epiphyfic and unattached bacteria considered separately (Fig. 1C, D), T h e unattached cells, like the chlorophyll a standing crop but not the epiphytes, appeared to decrease at the end o f the early phase. Unlike the chlorophyll concentrations, however, the Unattached bacteria subsequently increased again during the latter half of the observation period (Fig. 1C), /~acterial cell volumes were m e a s u r e d on seven dates spanning the observation period, with a grand average o f 0.473 # m 3 . e e l F ~ and a 95% confidence range o f 0.07. The range o f volumes (each the m e a n o f 50 or m o r e cells for each date) ~r f r o m 0.422 to 0.499 ~m3,cell -~, except for a value of 0,299 urq3.cell_~ on the last sampling day. At least to that point, there was n o e~ider~ce

68

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Fig. 2. The dynamics of(A) algal species composition and (B) the distribution of epiphytic bacteria among algal species in sea ice in 1985, near Resolute, Canadian Arctic. The values shown are cumulative proportions of total numbers. The lines were drawn by eye to the observed points indicated by the plot symbols.

Jun. 2

Sampling Date of a systematic change of cell volume with time, and a constant value was assumed in subsequent calculations of bacterial production. The bacterial population dynamics (Fig. 1B, C, D) were fitted to an exponential growth model, which explained significant (P < 0.05) variation in each case (Table 1). Variation about the fitted line was much greater in the later phase than in the earlier phase of the bloom, although growth rates were significantly larger than zero in all cases. There was some pattern in the residual variation, notably for epiphytes in the later season, and it may be that the simple exponential growth model oversimplifies events to a significant degree. There may have been one or more episodes of major loss from the bacterial populations, with subsequent replacement through increased growth. If so, the fitted growth rates are likely to underestimate the true net growth rates and must be viewed as minimal. Growth rates decreased significantly (P < 0.05, t test) between early and later phases for both epiphytic and unattached populations, but the two populations did not differ significantly in growth rate within either phase (Table 1). Epiphyte growth rates were almost (0.1 > P > 0.05) significantly larger than those for

DYnamics of Ice Bacteria

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June 2

Sampling Date Unattached in the early phase, however, and further analysis showed that epiPhytes did become relatively more abundant during the bloom. Two species dominated the algal community and were the primary substrates z G runow , a p ennate dmtom that forms 1for bacteria " 1attachment. Nitzschiafrig'da " arge branching colonies, comprised on average 58% of total algal numbers and maintained its dominance throughout the bloom and the decline (Fig. 2A). Nitzschia grunowii Hasle, a pennate diatom forming large ribbon-like colonies, Was also a major constituent throughout the observation period (Fig. 2A), Veraging 28% o f total numbers. Other species comprised individually less than 70 of total numbers, and were lumped into the single category of 'others.' Although cell sizes were not quantified, both of the dominant species appeared to be average in size and so their share of algal biomass is probably similar to their share of numbers. The relative species abundances (Fig. 2A) had average standard errors of 15% of the mean for both of the dominant species, so within the limits of precision there did not appear to be major shifts in dominance despite the large increase and decrease in overall algal standing crop. ,,~.Eph~"Ph y t es were distributed r o u ghl y"m p ro p ortton " to algalspecles abundance trig. 2B), but on average displayed a significant preference for N. frigida. The Proportion ofepiphytes found upon N. frigida was significantly larger than the proportion of the alga/population comprised by N. frigida (paired t test, P < 0.01) but the same was not true for N. grunowii or other algae. The average number of bacteria per cell (Fig. 3) was significantly larger (paired t test, P < 0.05) for N. frigida (12.9) than for N. grunowii (5.6) or for other algal species (8, 9). The number ofepiphytes per algal cell (Fig. 3) increased significantly through the observation period. Linear regression of the logarithm ofepiphytes per algal

70

R.E.H. Smith et al.

Table 2. Net production of bacteria and particulate organic carbon (POC) in Arctic sea ice near Resolute, N.W.T.

Population Epiphytic bacteria Unattached bacteria Total bacteria POC

Interval

Production mgC.m -z

Early Late Early Late Early Late Early Late

7.967 18.055 14.305 27.418 21.634 45.515 3,226.0 -284.0

Production C.I. rate mgC.m -2 mgC-m-2-d1.503 1!.648 3.896 8.314 3.649 18.427 13.22 37.61

0.419 0.821 0.753 1.246 1.139 2.169 111.31 - 18.95

Intervals as in Table 1 and Figure I; C.I. denotes the 95% confidence range cell vs day yielded a highly significant fit (P < 0.002) for all algae together a n d for the d o m i n a n t and other species analyzed separately, despite the fact that at least one species (N. grunowii) did not reveal simple, m o n o t o n i c d y n a m i c s o f e p i p h y t e n u m b e r s . T h e relationship was best for iV. frigida (R ~ = 0.56) and poorest for N. grunowii (R 2 = 0.29). T h e pattern, or lack o f it, in residual variation suggested that the exponential m o d e l was a good description for N. frigida but not for N. grunowii, which displayed a decreasing frequency o f epiphytes at the end o f the o b s e r v a t i o n period (Fig. 3). T h e n u m b e r o f e p i p h y t e s per cell on the two d o m i n a n t algal species (Fig. 3) had no significant linear correlation with the relative a b u n d a n c e s (Fig. 2A) o f the algae (linear least squares, P > 0.2), n o r did scatter plots suggest any nonlinear relationship between the two variables. The net p r o d u c t i o n o f bacteria a n d algae was calculated for b o t h phases o f the o b s e r v a t i o n period (Table 2). Bacterial p r o d u c t i o n was calculated f r o m the fitted exponential growth m o d e l (Table 1) o v e r the a p p r o p r i a t e intervals. Production rate (Table 2) was an a r i t h m e t i c average (carbon p r o d u c e d d i v i d e d by t i m e interval); the exponential growth m o d e l indicates that bacterial p r o d u c t i o n rate actually increased constantly through the o b s e r v a t i o n period. T h e total p r o d u c t i o n o f P O C was calculated f r o m the m e a s u r e d changes in P O C standing crops o v e r the a p p r o p r i a t e intervals. P O C in the ice prior to the increase o f algal chlorophyll a concentrations was less than 10% o f the p e a k values attained, suggesting that algal b i o m a s s a c c o u n t e d for m o s t o f the P O C increase. In the early season, net P O C p r o d u c t i o n was a b o u t 100 times larger t h a n bacterial net carbon production, i m p l y i n g a large d o m i n a n c e o f algal p r o d u c t i o n o v e r bacterial (Table 2). In the later season, net P O C p r o d u c t i o n was negative but bacterial p r o d u c t i o n increased, implying a net loss o f algal c a r b o n o f a b o u t 21 m g C - m - Z . d J a n d a bacterial p r o d u c t i o n rate o f 2.2 m g C . m - 2 - d -~ (Table 2). O v e r the entire o b s e r v a t i o n period, bacterial net p r o d u c t i o n was a b o u t 2.2% o f algal net production. Epipbytic bacteria c o n t r i b u t e d a b o u t 40% o f the bacterial production. T h e d y n a m i c s o f ice algal chlorophyll differed f r o m those o f the bacteria (Fig. 1) but there was nonetheless a significant overall correlation between bacterial n u m b e r s a n d chlorophyll a concentrations in the ice (Table 3, Fig. 4). T h e

DYnamics of Ice Bacteria

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Fi~. 4. The relationship between bacterial cell concentrations and chlorophyll concentrations in annual sea ice in 1985 near Resolute, Canadian Arctic (dosed squares), and in congelation (crosses) and platelet (open squares) ice in Antarctica. The antarctic data were read from Figure 4 of Grossi et al. [14] and Figure 2 of Kottmeier et al. and Sullivan [20]. The values shown are logarithms to the base 10. The lines were fitted by linear least squares to the separate arctic and antarctic data sets (cf Table 3).

Correlation reflects the more than order-of-magnitude increase of both algae and bacteria during the growth season, and also helps to compare the results against those obtained elsewhere. The relationship between mean bacterioPlankton abundance and chlorophyll a concentrations in a variety of freshwater and marine systems (Table 3) significantly underestimates the abundance of ice bacteria; only 9 of our 24 observations at Resolute fall within the upper 95% confidence limit of the literature relationship (P < 0.01). Ice bacteria in McMurdo Sound, Antarctica (Table 3, Fig. 4), are also significantly more abundant than the literature relationship predicts. Although more abundant than predicted, ice bacteria were still present in low concentrations relative to chlorophyll according to the data summarized here (Table 3). The average ratio of bacterial cells (. 109) to mg of chlorophyll a Was 2.6 for the arctic samples and 20-511 for antarctic samples of platelet and COngelation ice, respectively. The average ratio for bacterioplankton (calCUlated from [4]) was 610 by comparison, thus, it is not that ice bacteria were nU.sually abundant compared with bacterioplankton at higher temperatures. Uther, the literature relationship appears to predict too drastic a decrease in the rati~ of bacteria to chlorophyll at the high chlorophyll concentrations to be found in the ice community. The relationship between bacterial numbers and chlorophyll a concentration

72

R . E . H . Smith et al.

Table 3. The fitted parameters of the relationship between chlorophyll concentration (Chla, m g - m -2) and bacterial cell concentration (N, numbers.m-2): log~o(N)= a + b.log~o(Chla) Population

n

a +_ SE

b + SE

r2

24 24 24

8.02 -2_ 0.43 9.15 + 0.24 9.15 _+ 0.27

1.58 +_ 0.24 1.07 _+ 0.13 1.19 _+ 0.15

0.64 0.73 0.72

29

10.73 +_ 0.05

0.19 _ 0.04

0.42

53

10.76 +_ 0.05

0.27 +_ 0.03

0.57

39

8.87 +_ 0.06

0.78 _+ 0.05

0.88

Resolute Epiphytic Unattached Total M c M u r d o Sound Total All sea ice Total Literature Total

Data for Antarctic ice bacteria ( M c M u r d o Sound [20]) were combined with our o w n for Arctic ice bacteria (Resolute) to define the c o m m o n relationship (all sea ice). The literature relationship was derived by Bird and Kalff [4] for the m e a n bacterioplankton concentration in a variety o f marine and freshwater systems. SE = standard error

appeared quite different for the arctic and antarctic samples (Fig. 4, Table 3). Although the residual variation was large for both data sets, the slope and the R 2 were significantly larger and the intercept was significantly smaller for the arctic than the antarctic data. The approximate geometric mean regression slope (the linear least squares slope divided by the correlation coefficient) was also smaller for the antarctic data (0.45) than the arctic (1.65), thus suggesting that the functional relationship truly differed between the two sites. Though the antarctic data were collected from two different ice communities-the platelet and the congelation ice layers--the former were too few for separate analysis. The arctic and antarctic samples together formed a significant relationship (Table 3), but comparisons against the literature relationship for bacterioplankton are premature. The latter relationship was based on mean rather than individual concentrations for different aquatic systems, and we presently lack a comparable data base for sea ice populations.

Discussion

Our results indicate sustained exponential increase of bacteria in annual sea ice during the spring bloom of ice algae, and thus provide the basis for the first published estimates of net production of ice bacteria in the Arctic. Bacterial populations increased in size even when algal populations were static or declining (Fig. 1) and growth of the ice sheet had ceased, evidence that simple physical entrapment from the water column cannot account for the bacterial production. Measurements of bacterial population dynamics and rates of assimilation of thymidine and other organic substrates in antarctic communities [13, 20, 21] also show that bacteria can grow actively in bottom ice.

DYnamics of Ice Bacteria

73

The photosynthetic rates and patterns of intracellular photosynthate allocation of the ice algal populations varied with the strength of tidal mixing in the underlying water column through the latter part of the observation period [28]. Photosynthetic rates did not decrease systematically as the algal populations declined during the end of the growth season, however, suggesting rather !hat algal population loss rates increased. Likely loss mechanisms include grazing in and on the bottom ice community [5, 19] and loss of cells to the water Column by physical means [8]. Both loss mechanisms will influence the bacteria, but the grazing effect may be especially significant to the epiphytes rather than Unattached cells if the dominant grazers prefer larger food particles. Some of the apparent loss of algal biomass may be due also to the difficulties of accurately Sampling the softer ice typical of the late season [33], but we did attempt to COntrol for this effect by subsampling our cores (see Materials and Methods). Thegrowth and production rates inferred from population dynamics are minimal insofar as they ignore losses throughout the season, but appear likely to be especially conservative for the latter part of the observation period. The diminished specific growth rates (Table 1) in the later season may therefore not indicate poorer growing conditions but greater loss rates. Bacterial population dynamics in bottom ice communities of Antarctica indicate specific growth rates of 0.04-0.08 d- L[ 13, 21 ] for populations growing Under thin snow cover (< 10 cm) in the presence of substantial algal populations. Our early season value of 0.058 d -z (Table 1) agrees remarkably well with the antarctic range. Net growth of antarctic ice bacteria in situ agreed reasonably Well with thymidine incorporation [21], suggesting only small loss rates in situ, but the factor for converting thymidine incorporation to cell production was Uncertain over a wide range. More recently, it has been reported that up to 90% of ice algal production at the same antarctic site may be exported or COnsumed during the growth season [ 14], suggesting major losses from bacterial POpulations as well. Further work is necessary to determine the extent to which bacterial biomass is cycled in the ice or lost to the water column. The growth rate of psychrophilic and psychrotrophic marine bacteria [24] is often strongly temperature-dependent in the range - 1.8 to + 5.0~ [7, 24]. Our results, together with those for antarctic populations, indicate slow growth rates for ice bacteria compared with many estimates for bacterioplankton at higher temperatures, but the influence of temperature alone is confounded by methOdological differences. The frequency of dividing cells method indicated high growth rates, about 0.5-2.0 d -z, for bacterioplankton in the Southern Ocean at temperatures between 2.5 and 5~ [15]. Over a similar temperature range, a Sustained net increase of bacteria in Lake Erken indicated a relatively low growth rate of 0.07 d -z or less [2]. Other estimates for bacterioplankton growth rates in cold waters ( - 1 . 8 to +2.00C), mostly by the thymidine method, are largely in the range 0.1-0.5 d -z [21]. There is some overall temperature-de13endence of growth rate in natural populations, but it does not appear to be readily quantifiable with extant data. Even with consistent methodology, temPerature-independent variation of growth rate can be large in cold-water popUlations [ 15]. To define the growth-temperature relationship for naturally adapted POPulations, as opposed to selected clones in culture, would appear to require further studies directed specifically at that goal. Bacteria growing in congelation and platelet ice in Antarctica accumulated

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about 2.0-2.8 mgC.m -2 during the spring bloom of ice algae [13, 21], a net production quite similar to our results at Resolute (Table 2). The accumulated biomass of antarctic ice bacteria was only about 1% of algal biomass, and bacterial production was less than 1% of primary production [21]. We estimate somewhat higher values for bacterial biomass (up to 3% of algal) and production (2.2% of algal) at our arctic site. We also estimate larger cell sizes (average 104 fgC.cell-') than previous values from the Antarctic (23-40 fgC-cel1-1 [13, 21]), due to our larger cell volume estimates. The apparent size difference may result from our sampling only the bottom few centimeters of the ice, where bacteria can be larger [31]. Our bacterial production values are still small compared with results for a variety of planktonic communities at higher temperatures [ 10, 21], and indicate that ice bacteria are not likely to be major consumers of primary production in annual arctic sea ice with substantial algal populations. Stimulation of bacterial growth by association with diatom algae has been demonstrated and shown to reflect the use of algal exudates by the bacteria [3, 25]. Epiphytes grew faster than unattached bacteria in antarctic bottom ice, and preferentially colonized one of the two dominant diatoms, Amphiprora sp [13]. Bacterial growth also varied with algal growth among areas of differing snow cover [l 3, 21], additional evidence for stimulation of bacteria by the algae. Similarly, we observed an increasing relative abundance of epiphytes through the observation period (Fig. 3) even though growth rates were not quite significantly different between epiphytes and unattached cells (Table 1). We also observed species-specificity in algal-bacterial association, with the dominant diatom Nitzschia frigida the preferred host. There was no evidence that the association was detrimental to N. frigida. The relative degree of colonization of another important diatom, N. grunowii, increased in parallel to or even faster than that of IV. frigida through much of the observation period but declined significantly (linear least squares, P < 0.05) over the last 10 days (Fig. 3). It is not unusual to observe an increasing degree of colonization of diatoms by bacteria through the course of a bloom but it is unusual to observe a significant reversal of such a trend. Occurring in the absence of any major systematic changes in the relative abundance of the two algal species, the changes in degree of colonization are difficult to explain without invoking changes in the attractiveness and/or resistance of the algae to the epiphytes. There is evidence that diatoms can produce antibacterial compounds to inhibit the growth of bacteria [3] and N. grunowii may either have produced such inhibitors or have reduced its production of utilizable exudates. A highly significant (R 2 = 88%) relationship between bacteria and chlorophyll concentrations has been demonstrated for freshwater and marine plankton [4]. It was conjectured that the relationship might overestimate bacterial abundance in highly productive marine systems, since it overestimated it in one case of an estuarine spring phytoplankton bloom. We cannot expect the same levels of explained variance or even, necessarily, a similar functional response to the literature relationship because we are analyzing individual observations rather than means (Table 3, Fig. 4). Our results and those from the Antarctic do show, however, that the model tends to underestimate in the productive (up to 5 g.m -3 of chlorophyll a and 30 gC. m -3.d -1 primary productivity [20, 28]) but cold environment of the bottom-ice community. The model was grossly extrapolated for comparison against our arctic data (mean chlorophyll concen-

DYnamics of Ice Bacteria

75

tration of 69.39 mg. m -2, ~ 2,100 ~tg.1-t, vs a mean chlorophyll concentration of 2.37 rag.m-2 and maximum of 120 #g.1 -~ for Bird & Kalff's main data set), but the antarctic data were not so far removed from the model's main data base (Fig. 4). If expressed in terms of bacterial biomass, the model would presumably fare even worse because both the arctic and antarctic ice bacteria are much larger than the average for bacterioplankton [4]. The greater than predicted abundance of ice bacteria in the antarctic data set (Fig. 4) may reflect, at least in part, the growth and accumulation of bacteria Prior to the spring increase of insolation and the ice algal bloom [21], abetted by the scavenging action of ice platelets in the water column [ 11 ]. The comParatively small slope parameter for the antarctic data (Table 3) is consistent With this idea. The arctic data, however, seem quite clearly to show that the model underestimates even when the populations are overwhelmingly established by biological production in the course of an intense algal bloom. Further, the hypothesis [4] that specific productivity of bacteria may increase with Chlorophyll and bacterial cell concentrations would not be consistent with the comparatively low growth rates and net productivity of the ice bacteria. This may reflect the influence of low temperatures, suggesting that future refinements of the general bacteria-chlorophyll model might do well to consider temperature as an additional variable.

Acknowledgments. This work was financed by the

Department of Fisheries and Oceans, Canada, Withinvaluable support from the Polar Continental Shelf Project, Resolute. We thank our colleagues from the Bedford Institute of Oceanography and the Freshwater Institute for their help.

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