McMurdo LTER: Inorganic geochemical studies with ...
Denton, G.H., J.G. Bockheim, S.C. Wilson, and M. Stuiver. 1989. Late Wisconsin and early Holocene glacial history, inner Ross embayment, Antarctica. Quaternary Research, 31(2), 151-182. Doran, P.T., R.A. Wharton, Jr., and W.B. Lyons. 1994. Paleolimnology of the McMurdo Dry Valleys, Antarctica. Journal of Paleolimnology, 10(2),85-114. Green, W.J., M. Angle, and K. Chave. 1988. The geochemistry of antarctic streams and their role in the evolution of four lakes in the McMurdo Dry Valleys. Geochimica et Cosmochimica Acta, 52(5), 1265-1274. Lawrence, M.J.F., and C.H. Hendy. 1989. Carbonate deposition and the Ross Sea ice advance, Fryxell basin, Taylor Valley, Antarctica. New Zealand Journal of Geology and Geophysics, 32(2),267-277.
Nesje, A. 1992. A piston corer for lacustrine and marine sediments. Arctic and Alpine Research, 24(3), 257-259. Rau, G., T. Takahashi, and D. Des Marais. 1989. 13 C depletion in antarctic marine plankton: Parallels to past oceans? Nature, 341(6242), 516-518. Stuiver, M., G.H. Denton, T.J. Hughes, and J.L. Fastook. 1981. History of the marine ice sheet in West Antarctica during the last glaciation, a working hypothesis. In G.H. Denton and T.H. Hughes (Eds.), The last great ice sheets. New York: Wiley-Interscience. Wharton, R.A., Jr., W.B. Lyons, and D.J. Des Marais. 1993. Stable isotopic biogeochemistry of carbon and nitrogen in a perennially icecovered antarctic lake. Chemical Geology, 107, 159-172.
McMurdo LTER: Inorganic geochemical studies with special reference to calcium carbonate dynamics K. WELCH and W.B. LYONS, Department of Geology, University ofAlabama, Tuscaloosa, Alabama 35487-0338 J.C. PRISCU and R. EDWARDS, Department of Biology, Montana State University, Bozeman, Montana 59712 D.M. MCKNIGHT and H. HOUSE, Water Resources Division, U.S. Geological Survey, Boulder, Colorado 80303 R.A. WHARTON, JR., Biological Sciences Center, Desert Research Institute, Reno, Nevada 89506
uring the first field season (1993-1994) of the McMurdo D Dry Valleys Long-Term Ecological Research (LTER) program, we collected, processed, and analyzed samples for geochemical and biogeochemical analysis. The following analyses were conducted on the majority of these samples: pH, dissolved inorganic carbon (CO 2 ), sodium (Na), potassium (K), magnesium (Mg 2 -), calcium (Ca2 ), chloride (Cl-), sulfate (SO421, nitrate (NO3 -), nitrite (NO2 j, ammonium (NH4), phosphate (PO4 3-), dissolved organic carbon (DOC), and stable isotope ratios (8 180 and ÔD) of the water. Each of the three major lakes in Taylor Valley (Bonney, Fryxell, and Hoare) were sampled at least three times during the field season. Lakes were systematically sampled at regular depths, and biological as well as chemical samples were obtained (Priscu, Antarctic Journal, in this issue). Although previous work on the geochemistry of these lakes has been published since the early 1960s, little comparison work has been undertaken. Chloride profiles from the third "limno run" of the season (21 December 1993, 23 December 1993, 29 December 1993, and 7 January 1994) are shown for both the east and west lobes of Lake Bonney as well as Lakes Fryxell and Hoare in figure 1. This comparative approach emphasizes the radically different chemical compositions of lakes, with Lake Hoare being the freshest and Lake Bonney (both lobes) being the most saline. One of the most intriguing questions about the McMurdo Dry Valleys is, how did these lakes, essentially evolving in a similar climatic region, draining similar geologic materials, evolve into such different lake chemistries? Figure 2 is a plot of the Ca:Cl ratios in the lakes. Included in this figure are data for Lake Vanda in Wright Valley from Green and Canfield (1984). The Ca 2 profiles are "normalized" to Cl-to eliminate any variation due to changes in total dis-
LTER LIMNO RUN 3 CI (meq/L) 110 100 1000
10
a)
20
Ii
40 Figure 1. Depth profile of Cl- in Lake Bonney, Lake Hoare, and Lake Fryxell.
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237
io
Ca/Cl (mM) 0.001 0.01 0.1 1 El 10 20
50 60 70 Figure 2. Depth profile of the Ca2 to Cl- ratios in the lakes.
10
solved solids alone. The Ca:Cl profiles from Lakes Hoare and Vanda are essentially constant with depth, whereas Ca2 depletion, relative to Cl-, occurs for both lobes of Lake Bonney and Lake Fryxell (figure 2). The table summarizes the Ca:Cl and Ca:HCO 3 ratios for snow, other southern Victoria Land streams, and some of our (i.e., LTER) recent data from streams contributing to Lake Hoare. When all the information is combined, a set of phenomena becomes apparent. Precipitation in regions where little to no ice-free area exists has Ca:C1 approximately twice as great as the sea water ratio, whereas, even in areas of high elevation above the valley floors, the ratio for precipitation is approximately 6 to 10 times greater than that of sea water (table). This indicates that in the ice-free areas in Antarctica, a nonmarine, terrestrial dust or salt source of Ca 2 relative to Cl- exists (e.g., Welch et al., in preparation). As the glaciers melt and become stream discharge, however, the Ca:Cl ratio increases to very high values (table), with the shorter streams (i.e., the Lake Hoare streams) having the highest Ca:Cl ratios. These data indicate that calcium minerals, undoubtedly calcium carbonate (CaCO 3), are rapidly being dissolved from soils as liquid water becomes available in the austral summer. Because the regolith in the McMurdo Dry Valleys has abundant CaCO3 present (Keys and Williams 1981), this is certainly not a surprising observation. After the streams enter the lakes, the ratios change dramatically again, as the Ca:Cl is greatly decreased. Note that the surface value of Lake Fryxell does look much like a stream value. On the other hand, the surface values are an order of magnitude less than the streams, suggesting rapid Ca2 removal. The precipitation of carbonate minerals in antarctic lacustrine environments has been reported by several authors (Wharton, Parker, and Simmons 1983; I T
1
1OQO. i__
Stream and precipitation data from the McMurdo Dry Valleys (molar ratios) aIuiiIiILLY 1.JU and Chave inii; tsira et al. ini). me interaction and interplay between the dynamics of CaCO3 formation and dissolution and the production and oxidation 4.4 Sea water 0.019 of org anic carbon are undoubtedly - Welch et al. (in preparation) Snow (non dry valley) 0.04-0.08 responsible for the dramatic differences Welch et al. (in preparation) Snow (dry valley area) 0.12-0.25 I observed in the Ca2 and HCO3- chemis0.40 Green and Canfield (1984) Onxy River 0.56 tries in the dry valley lakes, with the over0.29 DeMora, Whitehead, and Gregory (1991) AIph River 0.88 printing of biological processes on the Lyons and Welch (unpublished data) r,diit.tin nnd d p triii'tinn nf C(iL. Streams Entering Lake Hoare S.,-Simmons varying from lake to lake. This is the case 0.45 9 December 1991 1.3 because the rates of biological processes 0.45 10 January1994 0.59 vary from lake to lake. For example, the 1.0 26 January1994 1.2 pelagic primary production maxima in Vestal these lakes differs by at least one order of 2 December 1993 1.4 0.53 1.4 0.61 11 January1994 magnitude in midsummer, with Lake 26 January1994 3.5 0.51 Fryxell and the west lobe of Lake Bonney having the highest rates (Vincent 1988; Wharton 2 December 1993 2.0 0.49 Priscu and Edwards unpublished data). 4.1 0.48 11 January1994 This biological influence is reflected not just in the variations in ICO2, but also in McKay 2 December 1993 3.0 0.51 the 813C composition of the XCO2 as well 3.4 0.54 11 January1994 (Wharton et al. 1993). In addition, rela-
ANTARCTIC JOURNAL - REVIEW 1994 238
tively high rates of sulfate reduction and methane production occur close to the sediment-water interface in Lake Fryxell (Howes and Smith 1990). All these data indicate that Lake Fryxell, and perhaps the west lobe of Lake Bonney, will have CaCO3 dynamics dominated by biological activity, whereas the other two lakes, especially Lake Hoare, may not. Our aim is, in part, to compare and contrast the affects of high biological vs. low biological activities on the dynamics (i.e., production and dissolution) of CaCO3 in these lake systems. This research was supported by National Science Foundation grant OPP 92-11773. Special thanks are given to L. Mastro, A. Butt, G. Dana, and P. Doran for help with sample collection.
the McMurdo Dry Valleys. Geochimica et Cosmochimica Acta, 52, 1265-1274.
Green, W.J., and D.E. Canfield. 1984. Geochemistry of the Onyx River and its role in the chemical evolution of Lake Vanda. Geochimica et CosmochimicaActa, 48, 2457-2467.
Howes, B.L., and R.L. Smith. 1990. Sulfur cycling in a permanently ice-covered amictic antarctic lake, Lake Fryxell. Antarctic Journal of the U.S., 25(5), 230-232.
Keys, J.R., and K. Williams. 1981. Origin of crystalline, cold desert salts in the McMurdo region, Antarctica. Geochimica et Cosmochimica Acta, 45, 2299-2309.
Lawrence, M.J.F., and C.H. Hendy. 1989. Carbonate deposition and Ross Sea ice advance, Fryxell Basin, Taylor Valley, Antarctica. New Zealand Journal
of Geology and Geophysics, 32, 267-277.
Priscu, J.C. 1994. McMurdo LTER: Phytoplankton nutrient deficiency in lakes of the Taylor Valley, Antarctica. Antarctic Journal of the
U.S., 29(5). Vincent, W.F. 1988. Microbial ecosystems
References Bird, M.I., A.R. Chivas, C.J. Randell, and H.R. Burton. 1991. Sedimentological and stable-isotope evolution of lakes in the Vestfold Hills,
Antarctica. Palaeogeography, Palaeoclimatology, and Palaeoecology, 84, 109-130. DeMora, S.J., R.F. Whitehead, and M. Gregory. 1991. Aqueous geochemistry of major constituents in the Mph River and its tributaries in Walcott Bay, Victoria Land, Antarctica. Antarctic Science, 3, 73-86. Green, W.J., M.P. Angle, and K.E. Chave. 1988. The geochemistry of antarctic streams and their role in the evolution of four lakes in
of Antarctica. Cambridge: Cambridge University Press. Welch, K.A., P.A. Mayewski, J.E. Dibb, M.S. Twickler, and S.I. Whitlow. In preparation. Marine and polar continental air mass influence in glaciochemical records from the Dry Valley region of Antarctica. Atmospheric Environment.
Wharton, R.A., Jr., W.B. Lyons, and D.J. Des Marais. 1993. Stable isotope biogeochemistry of carbon and nitrogen in a perennially icecovered antarctic lake. Chemical Geology, 107, 159-172. Wharton, R.A., Jr., B.C. Parker, and G.M. Simmons, Jr. 1983. Distribution, species composition and morphology of algal mats in antarctic dry valley lakes. Phycologia, 22, 355-365.
McMurdo LTER: Phytoplankton nutrient deficiency in lakes of the Taylor Valley, Antarctica JOHN C. PRISCu, Department of Biology, Montana State University, Bozeman, Montana 59717
revious reports on nutrient deficiencies in antarctic lakes p have been based on indirect evidence such as nitrogento-phosphorus ratios in the water column (Vincent 1981; Priscu et al. 1989), nutrient ratios in streams entering the lakes (Canfield and Green 1985), and direct measurement of nitrogen uptake using nitrogen-15 labeled compounds (Priscu 1989, pp. 173-182; Priscu et al. 1989). With the inception of studies focusing on photosynthesis (Priscu et al. 1990), nitrogen transformations (e.g., Priscu, Ward, and Downes 1993), and long-term ecological research (Wharton, Antarctic Journal, in this issue) in the lakes of the dry valley region of McMurdo Sound, knowledge of nutrient regulation of primary productivity in these systems is imperative. This article presents results from experimental nutrient (nitrogen and phosphorus) bioassays conducted on Lakes Bonney (east and west lobes), Hoare, Fryxell, and Vanda. Experiments were conducted on phytoplankton populations at 5, 13, and 18 meters and 5 and 13 meters in the east and west lobes of Lake Bonney, respectively, and at 5 meters in Lakes Hoare, Fryxell, and Vanda. The depths selected for Lake Bonney were from phytoplankton biomass and productivity maxima; those for the other lakes represent the phytoplankton
populations immediately beneath the permanent ice covers. All experiments were conducted at the Lake Bonney field camp during November and December 1993. A 4-liter sample was enriched with carbon-14 bicarbonate (0.1 to 0.2 microcuries per milliliter (mL) final concentration), and 500 mL was decanted into each of eight acid-washed high-density polyethylene bottles. Two bottles each were then enriched with 20 micromolar (!IM) ammonium, 2 tM phosphorus, and 20 iM ammonium plus 2 tM phosphorus; 2 nonamended bottles served as controls. All bottles were placed in an environmental chamber that simulated light and temperature conditions from which the samples were collected. Subsamples (80 mL) were removed from each bottle at 24-hour intervals (for 144 hours) and filtered through Whatman GF/F filters. The filters were acidified with 0.5 mL of 3 normal hydrochloric acid and dried at 50°C to remove unincorporated isotope. Radioactivity on the filters (which represents photosynthetic activity) was determined by standard liquid scintillation spectrometry at McMurdo Station. Nutrient chemistry was measured using methods described by Sharp and Priscu (1990). Photosynthesis in all phytoplankton populations sampled from Lake Bonney was stimulated strongly, relative to the non-
ANTARCTIC JOURNAL - REVIEW 1994
239
Nesje, A. 1992. A piston corer for lacustrine and marine sediments. Arctic and Alpine Research, 24(3), 257-259. Rau, G., T. Takahashi, and D. Des Marais. 1989. 13 C depletion in antarctic marine plankton: Parallels to past oceans? Nature, 341(6242), 516-518. Stuiver, M., G.H. Denton, T.J. Hughes, and J.L. Fastook. 1981. History of the marine ice sheet in West Antarctica during the last glaciation, a working hypothesis. In G.H. Denton and T.H. Hughes (Eds.), The last great ice sheets. New York: Wiley-Interscience. Wharton, R.A., Jr., W.B. Lyons, and D.J. Des Marais. 1993. Stable isotopic biogeochemistry of carbon and nitrogen in a perennially icecovered antarctic lake. Chemical Geology, 107, 159-172.
McMurdo LTER: Inorganic geochemical studies with special reference to calcium carbonate dynamics K. WELCH and W.B. LYONS, Department of Geology, University ofAlabama, Tuscaloosa, Alabama 35487-0338 J.C. PRISCU and R. EDWARDS, Department of Biology, Montana State University, Bozeman, Montana 59712 D.M. MCKNIGHT and H. HOUSE, Water Resources Division, U.S. Geological Survey, Boulder, Colorado 80303 R.A. WHARTON, JR., Biological Sciences Center, Desert Research Institute, Reno, Nevada 89506
uring the first field season (1993-1994) of the McMurdo D Dry Valleys Long-Term Ecological Research (LTER) program, we collected, processed, and analyzed samples for geochemical and biogeochemical analysis. The following analyses were conducted on the majority of these samples: pH, dissolved inorganic carbon (CO 2 ), sodium (Na), potassium (K), magnesium (Mg 2 -), calcium (Ca2 ), chloride (Cl-), sulfate (SO421, nitrate (NO3 -), nitrite (NO2 j, ammonium (NH4), phosphate (PO4 3-), dissolved organic carbon (DOC), and stable isotope ratios (8 180 and ÔD) of the water. Each of the three major lakes in Taylor Valley (Bonney, Fryxell, and Hoare) were sampled at least three times during the field season. Lakes were systematically sampled at regular depths, and biological as well as chemical samples were obtained (Priscu, Antarctic Journal, in this issue). Although previous work on the geochemistry of these lakes has been published since the early 1960s, little comparison work has been undertaken. Chloride profiles from the third "limno run" of the season (21 December 1993, 23 December 1993, 29 December 1993, and 7 January 1994) are shown for both the east and west lobes of Lake Bonney as well as Lakes Fryxell and Hoare in figure 1. This comparative approach emphasizes the radically different chemical compositions of lakes, with Lake Hoare being the freshest and Lake Bonney (both lobes) being the most saline. One of the most intriguing questions about the McMurdo Dry Valleys is, how did these lakes, essentially evolving in a similar climatic region, draining similar geologic materials, evolve into such different lake chemistries? Figure 2 is a plot of the Ca:Cl ratios in the lakes. Included in this figure are data for Lake Vanda in Wright Valley from Green and Canfield (1984). The Ca 2 profiles are "normalized" to Cl-to eliminate any variation due to changes in total dis-
LTER LIMNO RUN 3 CI (meq/L) 110 100 1000
10
a)
20
Ii
40 Figure 1. Depth profile of Cl- in Lake Bonney, Lake Hoare, and Lake Fryxell.
ANTARCTIC JOURNAL - REVIEW 1994
237
io
Ca/Cl (mM) 0.001 0.01 0.1 1 El 10 20
50 60 70 Figure 2. Depth profile of the Ca2 to Cl- ratios in the lakes.
10
solved solids alone. The Ca:Cl profiles from Lakes Hoare and Vanda are essentially constant with depth, whereas Ca2 depletion, relative to Cl-, occurs for both lobes of Lake Bonney and Lake Fryxell (figure 2). The table summarizes the Ca:Cl and Ca:HCO 3 ratios for snow, other southern Victoria Land streams, and some of our (i.e., LTER) recent data from streams contributing to Lake Hoare. When all the information is combined, a set of phenomena becomes apparent. Precipitation in regions where little to no ice-free area exists has Ca:C1 approximately twice as great as the sea water ratio, whereas, even in areas of high elevation above the valley floors, the ratio for precipitation is approximately 6 to 10 times greater than that of sea water (table). This indicates that in the ice-free areas in Antarctica, a nonmarine, terrestrial dust or salt source of Ca 2 relative to Cl- exists (e.g., Welch et al., in preparation). As the glaciers melt and become stream discharge, however, the Ca:Cl ratio increases to very high values (table), with the shorter streams (i.e., the Lake Hoare streams) having the highest Ca:Cl ratios. These data indicate that calcium minerals, undoubtedly calcium carbonate (CaCO 3), are rapidly being dissolved from soils as liquid water becomes available in the austral summer. Because the regolith in the McMurdo Dry Valleys has abundant CaCO3 present (Keys and Williams 1981), this is certainly not a surprising observation. After the streams enter the lakes, the ratios change dramatically again, as the Ca:Cl is greatly decreased. Note that the surface value of Lake Fryxell does look much like a stream value. On the other hand, the surface values are an order of magnitude less than the streams, suggesting rapid Ca2 removal. The precipitation of carbonate minerals in antarctic lacustrine environments has been reported by several authors (Wharton, Parker, and Simmons 1983; I T
1
1OQO. i__
Stream and precipitation data from the McMurdo Dry Valleys (molar ratios) aIuiiIiILLY 1.JU and Chave inii; tsira et al. ini). me interaction and interplay between the dynamics of CaCO3 formation and dissolution and the production and oxidation 4.4 Sea water 0.019 of org anic carbon are undoubtedly - Welch et al. (in preparation) Snow (non dry valley) 0.04-0.08 responsible for the dramatic differences Welch et al. (in preparation) Snow (dry valley area) 0.12-0.25 I observed in the Ca2 and HCO3- chemis0.40 Green and Canfield (1984) Onxy River 0.56 tries in the dry valley lakes, with the over0.29 DeMora, Whitehead, and Gregory (1991) AIph River 0.88 printing of biological processes on the Lyons and Welch (unpublished data) r,diit.tin nnd d p triii'tinn nf C(iL. Streams Entering Lake Hoare S.,-Simmons varying from lake to lake. This is the case 0.45 9 December 1991 1.3 because the rates of biological processes 0.45 10 January1994 0.59 vary from lake to lake. For example, the 1.0 26 January1994 1.2 pelagic primary production maxima in Vestal these lakes differs by at least one order of 2 December 1993 1.4 0.53 1.4 0.61 11 January1994 magnitude in midsummer, with Lake 26 January1994 3.5 0.51 Fryxell and the west lobe of Lake Bonney having the highest rates (Vincent 1988; Wharton 2 December 1993 2.0 0.49 Priscu and Edwards unpublished data). 4.1 0.48 11 January1994 This biological influence is reflected not just in the variations in ICO2, but also in McKay 2 December 1993 3.0 0.51 the 813C composition of the XCO2 as well 3.4 0.54 11 January1994 (Wharton et al. 1993). In addition, rela-
ANTARCTIC JOURNAL - REVIEW 1994 238
tively high rates of sulfate reduction and methane production occur close to the sediment-water interface in Lake Fryxell (Howes and Smith 1990). All these data indicate that Lake Fryxell, and perhaps the west lobe of Lake Bonney, will have CaCO3 dynamics dominated by biological activity, whereas the other two lakes, especially Lake Hoare, may not. Our aim is, in part, to compare and contrast the affects of high biological vs. low biological activities on the dynamics (i.e., production and dissolution) of CaCO3 in these lake systems. This research was supported by National Science Foundation grant OPP 92-11773. Special thanks are given to L. Mastro, A. Butt, G. Dana, and P. Doran for help with sample collection.
the McMurdo Dry Valleys. Geochimica et Cosmochimica Acta, 52, 1265-1274.
Green, W.J., and D.E. Canfield. 1984. Geochemistry of the Onyx River and its role in the chemical evolution of Lake Vanda. Geochimica et CosmochimicaActa, 48, 2457-2467.
Howes, B.L., and R.L. Smith. 1990. Sulfur cycling in a permanently ice-covered amictic antarctic lake, Lake Fryxell. Antarctic Journal of the U.S., 25(5), 230-232.
Keys, J.R., and K. Williams. 1981. Origin of crystalline, cold desert salts in the McMurdo region, Antarctica. Geochimica et Cosmochimica Acta, 45, 2299-2309.
Lawrence, M.J.F., and C.H. Hendy. 1989. Carbonate deposition and Ross Sea ice advance, Fryxell Basin, Taylor Valley, Antarctica. New Zealand Journal
of Geology and Geophysics, 32, 267-277.
Priscu, J.C. 1994. McMurdo LTER: Phytoplankton nutrient deficiency in lakes of the Taylor Valley, Antarctica. Antarctic Journal of the
U.S., 29(5). Vincent, W.F. 1988. Microbial ecosystems
References Bird, M.I., A.R. Chivas, C.J. Randell, and H.R. Burton. 1991. Sedimentological and stable-isotope evolution of lakes in the Vestfold Hills,
Antarctica. Palaeogeography, Palaeoclimatology, and Palaeoecology, 84, 109-130. DeMora, S.J., R.F. Whitehead, and M. Gregory. 1991. Aqueous geochemistry of major constituents in the Mph River and its tributaries in Walcott Bay, Victoria Land, Antarctica. Antarctic Science, 3, 73-86. Green, W.J., M.P. Angle, and K.E. Chave. 1988. The geochemistry of antarctic streams and their role in the evolution of four lakes in
of Antarctica. Cambridge: Cambridge University Press. Welch, K.A., P.A. Mayewski, J.E. Dibb, M.S. Twickler, and S.I. Whitlow. In preparation. Marine and polar continental air mass influence in glaciochemical records from the Dry Valley region of Antarctica. Atmospheric Environment.
Wharton, R.A., Jr., W.B. Lyons, and D.J. Des Marais. 1993. Stable isotope biogeochemistry of carbon and nitrogen in a perennially icecovered antarctic lake. Chemical Geology, 107, 159-172. Wharton, R.A., Jr., B.C. Parker, and G.M. Simmons, Jr. 1983. Distribution, species composition and morphology of algal mats in antarctic dry valley lakes. Phycologia, 22, 355-365.
McMurdo LTER: Phytoplankton nutrient deficiency in lakes of the Taylor Valley, Antarctica JOHN C. PRISCu, Department of Biology, Montana State University, Bozeman, Montana 59717
revious reports on nutrient deficiencies in antarctic lakes p have been based on indirect evidence such as nitrogento-phosphorus ratios in the water column (Vincent 1981; Priscu et al. 1989), nutrient ratios in streams entering the lakes (Canfield and Green 1985), and direct measurement of nitrogen uptake using nitrogen-15 labeled compounds (Priscu 1989, pp. 173-182; Priscu et al. 1989). With the inception of studies focusing on photosynthesis (Priscu et al. 1990), nitrogen transformations (e.g., Priscu, Ward, and Downes 1993), and long-term ecological research (Wharton, Antarctic Journal, in this issue) in the lakes of the dry valley region of McMurdo Sound, knowledge of nutrient regulation of primary productivity in these systems is imperative. This article presents results from experimental nutrient (nitrogen and phosphorus) bioassays conducted on Lakes Bonney (east and west lobes), Hoare, Fryxell, and Vanda. Experiments were conducted on phytoplankton populations at 5, 13, and 18 meters and 5 and 13 meters in the east and west lobes of Lake Bonney, respectively, and at 5 meters in Lakes Hoare, Fryxell, and Vanda. The depths selected for Lake Bonney were from phytoplankton biomass and productivity maxima; those for the other lakes represent the phytoplankton
populations immediately beneath the permanent ice covers. All experiments were conducted at the Lake Bonney field camp during November and December 1993. A 4-liter sample was enriched with carbon-14 bicarbonate (0.1 to 0.2 microcuries per milliliter (mL) final concentration), and 500 mL was decanted into each of eight acid-washed high-density polyethylene bottles. Two bottles each were then enriched with 20 micromolar (!IM) ammonium, 2 tM phosphorus, and 20 iM ammonium plus 2 tM phosphorus; 2 nonamended bottles served as controls. All bottles were placed in an environmental chamber that simulated light and temperature conditions from which the samples were collected. Subsamples (80 mL) were removed from each bottle at 24-hour intervals (for 144 hours) and filtered through Whatman GF/F filters. The filters were acidified with 0.5 mL of 3 normal hydrochloric acid and dried at 50°C to remove unincorporated isotope. Radioactivity on the filters (which represents photosynthetic activity) was determined by standard liquid scintillation spectrometry at McMurdo Station. Nutrient chemistry was measured using methods described by Sharp and Priscu (1990). Photosynthesis in all phytoplankton populations sampled from Lake Bonney was stimulated strongly, relative to the non-
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