Terrestrial biology
Terrestrial biology Dissolved organic material in desert lakes in the dry valleys
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D.M. MCKNIGHT, G.R. AIKEN, E.D. ANDREWS, E.C. BOWLES, and R.L. SMITH U.S. Geological Survey, WRD Arvada, Colorado 80002
J.M. DUFF and L.G. MILLER U.S. Geological Survey, WRD Menlo Park, California 94025
6
CE 8 CFL
10
12 Dissolved organic material (DOM) is often a major pool of organic material in freshwater ecosystems and is predominantly comprised of refractory organic acids. In most freshwaters, DOM is derived from the soils and plants of the watershed and from algae and other microorganisms growing in the lake or stream (Steinburg and Munster 1985; McKnight and Feder in press). In the McMurdo Dry Valleys, in contrast, the watersheds are extremely barren, which greatly limits the contribution from the watershed to the DOM of the permanently ice-covered lakes found in these valleys. Therefore, these lakes are unique environments for studying the biogeochemical processes involving DOM derived from phytoplankton and bacterial productivity within lakes. The approach taken in this study was to characterize chemically the two major fractions of the DOM (fulvic acids and hydrophilic acids) obtained using large-scale preparative chromatography (Aiken 1985), and to determine the major pathways of carbon cycling, and dissolved organic carbon (DOC) production, in several dry valley lake ecosystems. Lake Fryxell, in the Taylor Valley, was chosen for study because it is one of the more productive dry valley lakes (Vincent 1981). Despite the low-light intensities (10-3 microeinstems per square meter per second) caused by the 4.5-meterthick ice cover, abundant algal populations develop in the oxic zone (above 9.5 meters) during the austral summer. The extreme stability of the water column, resulting partially from the ice cover, may contribute to the development of plates of microbial dominance within the lake. Figure 1 shows the data for in vivo fluoresence, a measure of chlorophyll concentration and, indirectly, phytoplankton abundance, on three dates during December 1987. In December, the phytoplankton was dominated by filamentous blue-green algae; and the chlorophyll peak at 7.0-meter depth was composed chiefly of Oscillatoria sp. and the peak at 8.5-meter depth was composed chiefly of Phormidium angustissima. The increases in in vivo fluorescence at 8.5 meters, which occurred between 6 December and 23 152
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18
0 1 2 3 4 5 6 7 8
IN—VIVO FLUORESCENCE (IVF UNITS)
Figure 1. Depth profile of in vivo fluorescence measured In Lake Fryxell, McMurdo Dry Valleys, on three dates during December, 1987. (m denotes meter.)
December, may indicate rapid growth of P. angustissitna during this period. The difference between the values at 8.5 meters on 23 and 24 December may reflect a small inaccuracy in the depth of sampling (± 0.1 meter). The depth profile for DOC in the lakes (figure 2) is quite distinct from the depth profile for in vivo fluorescence. DOC was measured in samples filtered through 0.45-micrometer silver membrane filters within several days of sample collection using an OceanographicInternational* carbon analyzer operated at the Ecklund Biological Laboratory at McMurdo Station. The DOC concentrations increased with depth throughout the oxic and anoxic zones to a maximum concentration of 25 milligrams of carbon per liter at 18.0 meters. Sediment interstitial water was obtained by centrifugation of bottom sediment sampled with an Eckman dredge, and the organic material concentration in these samples was even greater, 135 milligrams
* The use of trade names in this report is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey. ANTARCTIC JOURNAL
of carbon per liter. The glacial meltwater inflows entering the lake were sampled in late December and had DOC concentrations ranging from 0.3 to 1.0 milligrams carbon per liter, substantially less than the most dilute water in the lake immediately below the ice cover. These DOC concentrations were also lower than DOC concentrations reported previously for glacial meltwater streams in the dry valleys, 1.6 to 5.8 milligrams carbon per liter (Downes, Howard-Williams, and Vincent 1986). Because several of the same streams were sampled, the different values may reflect the differences in sampling time during the austral summer or different analytical methods for DOC analysis. The DOC depth profile is generally similar to the depth profiles for specific conductance and several major cations such as sodium and calcium, as shown in figures 2 and 3. Dissolved calcium and sodium were determined in samples filtered through 0.4 micrometer Nucleopore filters (and diluted if necessary) using a Jarrel-Ash 975 Inductively Coupled Plasma Spectrometer. The profiles of these solutes, which follow the trend of increasing with depth, probably result from: • upward chemical diffusion of ions from the saline bottom water, • inflow of solutes in dilute glacial meltwater streams, and • various chemical reactions (Green and Canfield 1984). The resulting density gradient also contributes to the stability of the water column. The similarity in the DOC and cation depth profiles indicates that one source of DOM in Lake Fryxell is degradation of particulate organic material derived from algae and bacteria in the sediments or in the deeper zones of
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DISSOLVED ORGANIC CARBON (mg/L) 0 4 8 12 16 20 24 28
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Na (meq/L) 60 90 120 150
0 2 4 6
CL oil
10 12 14 16 18
0 2 4 6 8 10 12 14 SPECIFIC CONDUCTANCE (mS)
Figure 3. Depth profiles of specific conductance (In millisemins) and dissolved sodium (in milliequivalents per liter). (m denotes meters.)
the anoxic bottomwaters. The trend of increasing DOC with depth may result from the upward diffusion of the more refractory components of the DOM from the deeper zones. This hypthesis will be tested by analyzing for changes with depth in the chemical characteristics of the DOM, especially the major DOM pools represented by fulvic and hydrophilic acids.
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References Aiken, G.R. 1985. Isolation and characterization techniques for aquatic humic substances. In G.R. Aiken, D.M. McKnight, R.L. Wershaw, and P. MacCarthy (Eds.), Humic substances in soil, sediment, and water: Geochemistry, isolation, and characterization. New York: John Wiley.
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Downes, M.T., C. Howard-Williams, and W.F. Vincent. 1986. Sources of organic nitrogen, phosphorous and carbon in antarctic streams. Hydrobiologia, 134, 215-225. Green, W.J., and D.E. Canfield. 1984. Geochemistry of the Onyx River (Wright Valley, Antarctica) and its role in the chemical evolution of Lake Vanda. Geochimica et Cosmochimica Acta, 48, 2457-2467. McKnight, D.M., and G.L. Feder. In press. Ecological aspects of humic substances in the environment. In P. MacCarthy, R.L. Malcolm, R.
12 14 16 18
Swift, and M. Hayes, (Eds.), Humic substances: Environmental interactions. London: John Wiley and Sons.
0 2 4 6 8 10 Ca (meq,'L)
Figure 2. Depth profiles of dissolved organic carbon (in milligrams of carbon per liter) and dissolved calcium (in milliequivalents per liter). (m denotes meter.) 1988 REVIEW
Vincent, W.F. 1981. Production strategies in Antarctic inland waters: Phytoplankton eco-physiology in a permanently ice-covered lake. Ecology, 62(5), 1215-1224. Steinberg, C., and U. Meunster. 1985. Geochestistry and ecological role of humic substances in lakewater. In G.R. Aiken, D. M. McKnight, R.L. Wershaw, and P. MacCarthy (Eds.), Humic substances in soil, sediment, and water: Geochemistry, isolation, and characterization. New York: John Wiley. 153
0 2 4
D.M. MCKNIGHT, G.R. AIKEN, E.D. ANDREWS, E.C. BOWLES, and R.L. SMITH U.S. Geological Survey, WRD Arvada, Colorado 80002
J.M. DUFF and L.G. MILLER U.S. Geological Survey, WRD Menlo Park, California 94025
6
CE 8 CFL
10
12 Dissolved organic material (DOM) is often a major pool of organic material in freshwater ecosystems and is predominantly comprised of refractory organic acids. In most freshwaters, DOM is derived from the soils and plants of the watershed and from algae and other microorganisms growing in the lake or stream (Steinburg and Munster 1985; McKnight and Feder in press). In the McMurdo Dry Valleys, in contrast, the watersheds are extremely barren, which greatly limits the contribution from the watershed to the DOM of the permanently ice-covered lakes found in these valleys. Therefore, these lakes are unique environments for studying the biogeochemical processes involving DOM derived from phytoplankton and bacterial productivity within lakes. The approach taken in this study was to characterize chemically the two major fractions of the DOM (fulvic acids and hydrophilic acids) obtained using large-scale preparative chromatography (Aiken 1985), and to determine the major pathways of carbon cycling, and dissolved organic carbon (DOC) production, in several dry valley lake ecosystems. Lake Fryxell, in the Taylor Valley, was chosen for study because it is one of the more productive dry valley lakes (Vincent 1981). Despite the low-light intensities (10-3 microeinstems per square meter per second) caused by the 4.5-meterthick ice cover, abundant algal populations develop in the oxic zone (above 9.5 meters) during the austral summer. The extreme stability of the water column, resulting partially from the ice cover, may contribute to the development of plates of microbial dominance within the lake. Figure 1 shows the data for in vivo fluoresence, a measure of chlorophyll concentration and, indirectly, phytoplankton abundance, on three dates during December 1987. In December, the phytoplankton was dominated by filamentous blue-green algae; and the chlorophyll peak at 7.0-meter depth was composed chiefly of Oscillatoria sp. and the peak at 8.5-meter depth was composed chiefly of Phormidium angustissima. The increases in in vivo fluorescence at 8.5 meters, which occurred between 6 December and 23 152
14 16
18
0 1 2 3 4 5 6 7 8
IN—VIVO FLUORESCENCE (IVF UNITS)
Figure 1. Depth profile of in vivo fluorescence measured In Lake Fryxell, McMurdo Dry Valleys, on three dates during December, 1987. (m denotes meter.)
December, may indicate rapid growth of P. angustissitna during this period. The difference between the values at 8.5 meters on 23 and 24 December may reflect a small inaccuracy in the depth of sampling (± 0.1 meter). The depth profile for DOC in the lakes (figure 2) is quite distinct from the depth profile for in vivo fluorescence. DOC was measured in samples filtered through 0.45-micrometer silver membrane filters within several days of sample collection using an OceanographicInternational* carbon analyzer operated at the Ecklund Biological Laboratory at McMurdo Station. The DOC concentrations increased with depth throughout the oxic and anoxic zones to a maximum concentration of 25 milligrams of carbon per liter at 18.0 meters. Sediment interstitial water was obtained by centrifugation of bottom sediment sampled with an Eckman dredge, and the organic material concentration in these samples was even greater, 135 milligrams
* The use of trade names in this report is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey. ANTARCTIC JOURNAL
of carbon per liter. The glacial meltwater inflows entering the lake were sampled in late December and had DOC concentrations ranging from 0.3 to 1.0 milligrams carbon per liter, substantially less than the most dilute water in the lake immediately below the ice cover. These DOC concentrations were also lower than DOC concentrations reported previously for glacial meltwater streams in the dry valleys, 1.6 to 5.8 milligrams carbon per liter (Downes, Howard-Williams, and Vincent 1986). Because several of the same streams were sampled, the different values may reflect the differences in sampling time during the austral summer or different analytical methods for DOC analysis. The DOC depth profile is generally similar to the depth profiles for specific conductance and several major cations such as sodium and calcium, as shown in figures 2 and 3. Dissolved calcium and sodium were determined in samples filtered through 0.4 micrometer Nucleopore filters (and diluted if necessary) using a Jarrel-Ash 975 Inductively Coupled Plasma Spectrometer. The profiles of these solutes, which follow the trend of increasing with depth, probably result from: • upward chemical diffusion of ions from the saline bottom water, • inflow of solutes in dilute glacial meltwater streams, and • various chemical reactions (Green and Canfield 1984). The resulting density gradient also contributes to the stability of the water column. The similarity in the DOC and cation depth profiles indicates that one source of DOM in Lake Fryxell is degradation of particulate organic material derived from algae and bacteria in the sediments or in the deeper zones of
0
DISSOLVED ORGANIC CARBON (mg/L) 0 4 8 12 16 20 24 28
0 30
Na (meq/L) 60 90 120 150
0 2 4 6
CL oil
10 12 14 16 18
0 2 4 6 8 10 12 14 SPECIFIC CONDUCTANCE (mS)
Figure 3. Depth profiles of specific conductance (In millisemins) and dissolved sodium (in milliequivalents per liter). (m denotes meters.)
the anoxic bottomwaters. The trend of increasing DOC with depth may result from the upward diffusion of the more refractory components of the DOM from the deeper zones. This hypthesis will be tested by analyzing for changes with depth in the chemical characteristics of the DOM, especially the major DOM pools represented by fulvic and hydrophilic acids.
2 4 6
References Aiken, G.R. 1985. Isolation and characterization techniques for aquatic humic substances. In G.R. Aiken, D.M. McKnight, R.L. Wershaw, and P. MacCarthy (Eds.), Humic substances in soil, sediment, and water: Geochemistry, isolation, and characterization. New York: John Wiley.
10
Downes, M.T., C. Howard-Williams, and W.F. Vincent. 1986. Sources of organic nitrogen, phosphorous and carbon in antarctic streams. Hydrobiologia, 134, 215-225. Green, W.J., and D.E. Canfield. 1984. Geochemistry of the Onyx River (Wright Valley, Antarctica) and its role in the chemical evolution of Lake Vanda. Geochimica et Cosmochimica Acta, 48, 2457-2467. McKnight, D.M., and G.L. Feder. In press. Ecological aspects of humic substances in the environment. In P. MacCarthy, R.L. Malcolm, R.
12 14 16 18
Swift, and M. Hayes, (Eds.), Humic substances: Environmental interactions. London: John Wiley and Sons.
0 2 4 6 8 10 Ca (meq,'L)
Figure 2. Depth profiles of dissolved organic carbon (in milligrams of carbon per liter) and dissolved calcium (in milliequivalents per liter). (m denotes meter.) 1988 REVIEW
Vincent, W.F. 1981. Production strategies in Antarctic inland waters: Phytoplankton eco-physiology in a permanently ice-covered lake. Ecology, 62(5), 1215-1224. Steinberg, C., and U. Meunster. 1985. Geochestistry and ecological role of humic substances in lakewater. In G.R. Aiken, D. M. McKnight, R.L. Wershaw, and P. MacCarthy (Eds.), Humic substances in soil, sediment, and water: Geochemistry, isolation, and characterization. New York: John Wiley. 153
