Rates of primary production and growth for
Rates of primary production and growth for phytoplankton in Lake Bonney
0 C\J X 0 C
a
80
Primary Production + Under—ice Irradiance o Chlorophyll a
60
15 a
40
-c 10 a-
20
-c c0
THOMAS R. SHARP and JOHN C. PRIscu Department of Biological Sciences Montana State University Bozeman, Montana 59715
The permanently ice-covered lakes in the McMurdo Dry Valleys of southern Victoria Land offer a rare opportunity to study natural phytoplankton assemblages in a nonturbulent environment. In a turbulent environment, phytoplankton are subject fluctuating light, because of vertical displacement in the water column. Although measurements of primary production determined by bottle methods at stationary depths may not adequately mimic conditions in a turbulent environment, these methods should give a more accurate estimate of primary production in a nonturbulent environment. Another factor that makes permanently ice-covered lakes amenable to primary production and growth studies, is the virtual absence of planktonic grazers. In most pelagic ecosystems, grazing results in significant loss of phytoplankton biomass. Here, we present a detailed seasonal study of phytoplankton primary production and growth in a dry-valley lake. The lack of mixing and grazing activity allowed us to examine the seasonal dependence of phytoplankton photosynthesis on irradiance and to compute phytoplankton growth rates over the season. Primary production of phytoplankton from the center of the east lobe of Lake Bonney was measured by the carbon-14 method at approximately weekly intervals from 30 October to 11 January during the 1990-1991 season. The methods and depths are similar to those described previously (Priscu et al. 1990) except that samples were incubated for 24 hours. Primary production measured in a single 24-hour incubation was shown to be equal to three 8-hour incubations. Excretion of photosynthetically assimilated carbon-14 was measured by acidifying and drying down the filtrate, from the sample. The amount of organic carbon-14 was determined via liquid scintillation spectroscopy. Earlier research (Parker et al. 1977) reported that up to 90 percent of the carbon-14 assimilated during their incubation was excreted as dissolved organic carbon. Because this rate seems unusually high, we measured dissolved organic carbon excretion on samples collected from the photosynthetic maximum (4.5 to 5 meters) during both the 1989 and 1990 austral summers. We found that, using vacuum filtration (300 millibar), between 16 and 25 percent of the photoassimilated carbon14 was excreted. The proportion of carbon-14 excretion was reduced to about 9 percent when samples were filtered by gravity, presumably because of a reduction in cell breakage with gravity filtration. This latter rate is within the range normally reported for antarctic freshwater phytoplankton (Heywood 1984). Rates of integrated (4.5 to 20 meters) primary production ranged from 5.6 to 22.6 milligrams of carbon per square meter per day (figure). Integrated primary production was positively correlated (r2 = 0.73) with irradiance immediately under the ice.
1991 REVIEW
20
>\
0 0
O 10 30 50 70 Days from 30 October 1990 Seasonal patterns in primary production (milligrams of carbon per meter squared per day), chlorophyll a (milligrams per meter squared), and irradiance (moles quanta per meter squared per day). Seasonal primary production (30 October 1990 to 11 January 1991) was 1.05 grams of carbon per square meter. Chlorophyll a concentrations, a measure of phytoplankton biomass, increased over our sampling season (figure). The decline in integrated (4.5 to 20 meters) chlorophyll which occurred during December is probably not caused by a loss of biomass but rather by dilution of the surface population by glacial meltwater. A growth rate was derived by linear regression of log transformed values. The growth rate estimated by this method was low, 0.012 per day. This is equivalent to a doubling time of 56 days. Our results show that the underice community was present at the start of our sampling season. Either phytoplankton growth occurred before the start of our field season or some portion of the phytoplankton populations remains viable through the winter dark period. In addition, the phytoplankton may remain photosynthetically active through March when adequate light still exists to drive photosynthesis. Our next field season (1991-1992) will begin in early September in an attempt to ascertain the standing stocks and activities of these phytoplankton populations at the onset of daylight. This early season study will give a better estimate of annual primary productivity in dry valley lakes of the McMurdo Sound region. We thank Michael Lizotte, Patrick Neale, Robert Spigel, Ian Forne, and Ian Sheppard for their assistance in the field. This work was supported by the National Science Foundation grant DPP 88-20591 to John C. Priscu.
References
Heywood, R.B. 1984. Inland Waters. In R.M. Laws (Ed.), Antarctic ecology. London: Academic Press. Parker, B.C., R.C. Hoehn, R.A. Paterson, J.A. Craft, L.S. Lane, R.W. Stavros, H.G. Sugg, J.T. Whitehurst, R.D. Fortner, and B.L. Weand. 1977 Changes in dissolved organic matter, photosynthetic production and microbial community composition in Lake Bonney, Southern Victoria Land, Antarctica. In G.A. Llano (Ed.), Adaptations within antarctic ecosystems. Washington, D.C.: Smithsonian Institution. Priscu, J.C., T.R. Sharp, M.P. Lizotte, and P.J. Neale. 1990. Photoadaptation by phytoplankton in permanently ice-covered antarctic lakes: Response to a nonturbulent environment. Antarctic Journal of the U.S., 25(5), 221-222.
225
0 C\J X 0 C
a
80
Primary Production + Under—ice Irradiance o Chlorophyll a
60
15 a
40
-c 10 a-
20
-c c0
THOMAS R. SHARP and JOHN C. PRIscu Department of Biological Sciences Montana State University Bozeman, Montana 59715
The permanently ice-covered lakes in the McMurdo Dry Valleys of southern Victoria Land offer a rare opportunity to study natural phytoplankton assemblages in a nonturbulent environment. In a turbulent environment, phytoplankton are subject fluctuating light, because of vertical displacement in the water column. Although measurements of primary production determined by bottle methods at stationary depths may not adequately mimic conditions in a turbulent environment, these methods should give a more accurate estimate of primary production in a nonturbulent environment. Another factor that makes permanently ice-covered lakes amenable to primary production and growth studies, is the virtual absence of planktonic grazers. In most pelagic ecosystems, grazing results in significant loss of phytoplankton biomass. Here, we present a detailed seasonal study of phytoplankton primary production and growth in a dry-valley lake. The lack of mixing and grazing activity allowed us to examine the seasonal dependence of phytoplankton photosynthesis on irradiance and to compute phytoplankton growth rates over the season. Primary production of phytoplankton from the center of the east lobe of Lake Bonney was measured by the carbon-14 method at approximately weekly intervals from 30 October to 11 January during the 1990-1991 season. The methods and depths are similar to those described previously (Priscu et al. 1990) except that samples were incubated for 24 hours. Primary production measured in a single 24-hour incubation was shown to be equal to three 8-hour incubations. Excretion of photosynthetically assimilated carbon-14 was measured by acidifying and drying down the filtrate, from the sample. The amount of organic carbon-14 was determined via liquid scintillation spectroscopy. Earlier research (Parker et al. 1977) reported that up to 90 percent of the carbon-14 assimilated during their incubation was excreted as dissolved organic carbon. Because this rate seems unusually high, we measured dissolved organic carbon excretion on samples collected from the photosynthetic maximum (4.5 to 5 meters) during both the 1989 and 1990 austral summers. We found that, using vacuum filtration (300 millibar), between 16 and 25 percent of the photoassimilated carbon14 was excreted. The proportion of carbon-14 excretion was reduced to about 9 percent when samples were filtered by gravity, presumably because of a reduction in cell breakage with gravity filtration. This latter rate is within the range normally reported for antarctic freshwater phytoplankton (Heywood 1984). Rates of integrated (4.5 to 20 meters) primary production ranged from 5.6 to 22.6 milligrams of carbon per square meter per day (figure). Integrated primary production was positively correlated (r2 = 0.73) with irradiance immediately under the ice.
1991 REVIEW
20
>\
0 0
O 10 30 50 70 Days from 30 October 1990 Seasonal patterns in primary production (milligrams of carbon per meter squared per day), chlorophyll a (milligrams per meter squared), and irradiance (moles quanta per meter squared per day). Seasonal primary production (30 October 1990 to 11 January 1991) was 1.05 grams of carbon per square meter. Chlorophyll a concentrations, a measure of phytoplankton biomass, increased over our sampling season (figure). The decline in integrated (4.5 to 20 meters) chlorophyll which occurred during December is probably not caused by a loss of biomass but rather by dilution of the surface population by glacial meltwater. A growth rate was derived by linear regression of log transformed values. The growth rate estimated by this method was low, 0.012 per day. This is equivalent to a doubling time of 56 days. Our results show that the underice community was present at the start of our sampling season. Either phytoplankton growth occurred before the start of our field season or some portion of the phytoplankton populations remains viable through the winter dark period. In addition, the phytoplankton may remain photosynthetically active through March when adequate light still exists to drive photosynthesis. Our next field season (1991-1992) will begin in early September in an attempt to ascertain the standing stocks and activities of these phytoplankton populations at the onset of daylight. This early season study will give a better estimate of annual primary productivity in dry valley lakes of the McMurdo Sound region. We thank Michael Lizotte, Patrick Neale, Robert Spigel, Ian Forne, and Ian Sheppard for their assistance in the field. This work was supported by the National Science Foundation grant DPP 88-20591 to John C. Priscu.
References
Heywood, R.B. 1984. Inland Waters. In R.M. Laws (Ed.), Antarctic ecology. London: Academic Press. Parker, B.C., R.C. Hoehn, R.A. Paterson, J.A. Craft, L.S. Lane, R.W. Stavros, H.G. Sugg, J.T. Whitehurst, R.D. Fortner, and B.L. Weand. 1977 Changes in dissolved organic matter, photosynthetic production and microbial community composition in Lake Bonney, Southern Victoria Land, Antarctica. In G.A. Llano (Ed.), Adaptations within antarctic ecosystems. Washington, D.C.: Smithsonian Institution. Priscu, J.C., T.R. Sharp, M.P. Lizotte, and P.J. Neale. 1990. Photoadaptation by phytoplankton in permanently ice-covered antarctic lakes: Response to a nonturbulent environment. Antarctic Journal of the U.S., 25(5), 221-222.
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