The lakes in the dry valleys of Victoria Land have diverse assemblages of phytoplankton and bacterioplankton beneath the permanent ice caps (Vincent 1981; Priscu et al. 1987); we are unaware of reports on microbial communities living within the ice cap itself. During previous studies on planktonic microorganisms in Lake Bonney, we observed a discolored layer of ice occurring at approximately mid-depth within the ice cap (Priscu unpublished data). This study presents results from an investigation of the components in that discolored ice layer, including chlorophyll-a, gravel weight, nutrient concentration, primary productivity, and bacterial activity.
Ice cores were analyzed from Lake Bonney, a meromictic lake with a 3.5- to 4-meter (m) permanent ice cap. Samples were collected in the center of the east lobe (station E30) and in the center of the west lobe (station W20) during the austral summer of 1992-1993 (see Spigel, Fourne, and Sheppard 1991 for precise site locations). The east lobe station was sampled three times from 17 November through 17 December, with approximately 7 days separating each sample run; the west lobe was sampled twice from 20 December through 1 January. Replicate coring holes were within 1 meter of the original hole. Cores were obtained using a SIPRE hand-coring device [diameter 10 centimeters (cm)]. Cores were sectioned into 14-16-cm lengths before being placed in acid washed, screw-cap glass jars to melt. Specific core depths mentioned hereafter indicate the depth at the core end nearest to the ice surface; for example, a core named "260 cm" would encompass 14 cm of ice from depths of 260 to 274 cm. Sample jars were kept dark and melted slowly so that the ice/water sample never exceeded 5C. Immediately after complete ice melt, gravel content, chlorophyll-a, nitrate (NO3, nitrite (NO2, ammonium (NH4), Soluble Reactive Phosphate (SRP), primary productivity, and bacterial activity were also collected for later examination. Except for nutrient analyses, all work was conducted in a cold (10 to 15C) and dark environment.
Chlorophyll-a was detected only below 200 cm in the ice cap of the east lobe (figure, block B). The shallowest core containing measurable chlorophyll-a [0.03 milligram per cubic meter (mg/m3)] came from 190 cm and the highest levels were found in cores from 200 to 240 cm from the ice surface. Primary productivity rates (PPR), measured with the 14-carbon method, showed no trend with depth, nor did bacterial activity, measured by monitoring the rate of 3H-thymidine incorporation into bacterial cells (TDR) (table). Linear regressions showed no significant relationship between chlorophyll-a levels and either NO3 or NH4 (r2<0.50).
Gravel was present in shallow cores (130 to 180 cm) where no chlorophyll-a existed, as well as in the deeper cores (figure, block A). A linear regression of data from the third east lobe run, which sampled only from 200 to 310 cm, revealed a positive relationship between gravel weight and chlorophyll-a concentration (r2=0.93). Neither of the other two east lobe cores showed linear gravel vs. chlorophyll-a relationships, nor did a regression of all east lobe core runs product a significant gravel to chlorophyll-a relationship (r2<0.50).
East lobe NO3 was markedly higher above 200 cm, although it was measurable through the entire ice column (3.5 m) at levels less than 1 millimole per cubic meter (mmol/m3) (figure, block C). NO2 showed no clear trend with depth and was always less than 0.04 mmol/m3 (figure, block D). NH4 was also first detected at 200 cm and was present in most samples below that depth to the bottom of the ice cap (figure, block E). East lobe SRP (figure, block F) was detected at an average of 0.18 millimole per m3 in the shallower cores and only rarely appeared below 200 cm. A t-test between overall NO2 and NH4 above and below 200 cm (t=5.26, P<0.001) in the east lobe indicated that these nutrients were in significantly greater amounts below 200 cm.
No linear relationship was evident between gravel or any of the nutrients and chlorophyll-a in the west lobe (r2<0.50). PPR measurements in this lobe showed a slight increase with decreasing depth, whereas TDR showed no clear trend with depth (table). West lobe nitrate became detectable at 200 cm (figure, block C) as in the east lobe, but both the gravel and chlorophyll-a peaks were slightly above this depth, at 179-180 cm. The west lobe contained nitrite and SRP throughout most of the ice column at concentrations generally less than 0.5 mmol/m3 (figure, blocks D and F). NH4 was present in all west lobe samples but showed no clear trend with depth (figure, block E). A t-test comparing combined nitrogenous nutrients (NO3 plus NO2 plus NH4) above and below 200 cm in the west lobe showed no significant differences (P>0.05).
All data indicate significant biological activity near the middle of the ice cap in association with the gravel layer. The coexistence of gravel and microorganisms presumably relates to the fact that liquid water exists within the region of the gravel. We have noticed distinct liquid water layers associated with gravel when coring the ice. It seems that the liquid water pockets within the lake ice are corollaries to the brine channels within sea ice, which harbor a marine microbial assemblage. We currently have thermistors anchored within the ice cap of Lake Bonney to determine the vertical and temporal extent of potential regions of liquid water within the ice column.
Microscopic examination of preserved samples from the ice cores revealed a preponderance of filamentous cyanobacteria morphologically similar to the genera Oscillatoria and Phormidium. Our studies within the liquid water column of Lake Bonney have shown Oscillatoria to be a minor component of the photosynthetic planktonic community. The lake ice microalgae may be derived from the plankton within the lake or may originate from surrounding terrestrial communities which are dominated by filamentous cyanobacteria of the genus Phormidium. Precise identification of these species using both morphological and molecular characteristics is required before the question of origin is resolved. We plan to continue our work on lake-ice microorganisms in an attempt to answer the question regarding the seed population and to determine their role in total ecosystem dynamics.
This work was supported in part by National Science Foundation grants OPP 88-20591 and OPP 91-17907 to John C. Priscu. Kate T. Wing was supported by a National Science Foundation REU grant to John C. Priscu.
References
Priscu. J.C., L.R. Priscu, W.F. Vincent, and C. Howard-Williams. 1987. Photosynthate partitioning by microplankton in permanently ice-covered antarctic lakes. Limnology and Oceanography, 32(1), 260-270.
Spigel, R.H., I. Fourne, I. Sheppard, and J.C. Priscu. 1991. Differences in temperature and conductivity between the east and west lobes of Lake Bonney: Evidence for circulation within and between lobes. Antarctic Journal of the U.S., 26(5), 221-222.
Vincent, W.F. 1981. Production strategies in antarctic inland waters: Eco-physiology in a permanently ice-covered lake. Ecology, 62(5), 1215-1224.