Microbial communities in the permanent ice cap of Lake Bonney ...
Microbial communities in the permanent ice cap of Lake Bonney, Antarctica: Relationships among chlorophyll-a, gravel, and nutrients KATE T. WING, Smith College, Northampton, Massachusetts 01063 JOHN C. PRISCU, Department of Biological Sciences, Montana State University, Bozeman, Montana 59717
he lakes in the dry valleys of Victoria Land have diverse T 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in Lake Bonney, we observed a discolored layer of ice occurring at approximately mid-depth within the ice cap (Priscu unpublished data).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, screwcap 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 5°C. 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 measured. Preserved samples of microalgae and bacteria were also collected for later examination. Except for nutrient analyses, all work was conducted in a cold (10 to 15°C) 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 regres-
sions showed no significant relationship between chlorophyll-a levels and either NO 3 or NH4 (r2
he lakes in the dry valleys of Victoria Land have diverse T 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, screwcap 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 5°C. 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 measured. Preserved samples of microalgae and bacteria were also collected for later examination. Except for nutrient analyses, all work was conducted in a cold (10 to 15°C) 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 regres-
sions showed no significant relationship between chlorophyll-a levels and either NO 3 or NH4 (r2
