dore, john e., and john c. priscu phytoplankton...

1331

Limnol. Oceanogr., 46(6), 2001, 1331–1346q 2001, by the American Society of Limnology and Oceanography, Inc.

Phytoplankton phosphorus deficiency and alkaline phosphatase activity in the McMurdoDry Valley lakes, Antarctica

John E. DoreSOEST, Department of Oceanography, University of Hawaii, Honolulu, Hawaii 96822

John C. PriscuDepartment of Land Resources and Environmental Sciences, Montana State University, Bozeman, Montana 59717

Abstract

We assessed the nutrient (N, P) deficiency of phytoplankton from the perennially ice-covered lakes in TaylorValley, Antarctica, with 14C-based bioassays. Phytoplankton in the near-surface waters from three of the four lakesdisplayed elevated photosynthetic carbon assimilation in the presence of added P relative to controls. Carbon as-similation in samples from deep chlorophyll layers was also enhanced by P addition in two of the lakes. No effectof N addition without simultaneous P addition was noted in any lake. Lake Bonney near-surface phytoplanktonconsistently showed a strong response to P and N 1 P additions, suggesting P deficiency. The near-surface phy-toplankton of Lake Hoare responded to N 1 P but not N or P addition alone, suggesting a more general nutrientdeficiency. Further investigation of phytoplankton phosphorus deficiency was carried out in the east lobe of LakeBonney and in Lake Hoare. Water column dissolved and sestonic elemental ratios and planktonic alkaline phos-phatase activity measurements corroborated the results of the bioassays. Despite the proximity of the lakes and theirsimilarity in ice cover and watershed, a high degree of phytoplankton phosphorus deficiency was evident in LakeBonney but not in Lake Hoare. We hypothesize that the difference in the degree of phytoplankton P deficiencybetween these lakes may not be due to differences in external P fluxes, but rather to differences in the internalcycling of phosphorus stemming from large differences in water column chemistry.

The McMurdo Dry Valleys region of Antarctica is aunique environment in which the biological communitieshave had to adapt to some of the most extreme conditionsavailable on Earth. The largest ice-free region on the con-tinent, it is among the coldest and driest deserts in the world,having a mean annual temperature of #2208C and #10 cmyr21 precipitation (Heywood 1984). Within this desert are anumber of lakes, many of which maintain a body of liquidwater beneath a permanent ice cover. This ice cover protectsthe underlying water column from experiencing the extremelow temperatures and large temperature variations that arecommon in the environment above the lake ice (Spigel andPriscu 1998). These lakes offer habitats for specializedaquatic communities that have adapted to low light levels,lack of water column mixing, and intensely stratified chem-ical environments ranging from extreme oxygen supersatu-ration to anoxia (Simmons et al. 1993; Priscu et al. 1999).

Life in the dry valley lakes is primarily microbial, with aconspicuous absence of metazoans (Takacs and Priscu 1998).The plankton of the dry valley lakes is composed primarily

AcknowledgmentsWe thank E. Adams, R. Edwards, C. Fritsen, D. Gordon, A.

Lundberg-Martell, C. Takacs, and C. Wolf for assistance in the field.Many of the nutrient and particulate matter samples were analyzedat McMurdo Station by Antarctic Support Associates. The U.S.Navy provided helicopter transportation during even the most ex-treme winter weather. We also thank D. Karl and E. Laws for gen-erously providing laboratory facilities at the University of Hawaii.This research was supported by the NSF Office of Polar Programsgrants OPP 92-11773, OPP 94-19413, OPP 98-15512, OPP 98-15998, and OPP 00-96250 awarded to J.C.P. SOEST contribution#5803.

of microalgae, bacteria, and protozoans (James et al. 1998;Priscu et al. 1999). Among the phytoplankton, flagellatedcryptophytes dominate shallow populations, and chryso-phytes and chlorophytes make up deep chlorophyll maxima;cyanobacteria may also be present (Lizotte and Priscu 1998).Because of the thick ice cover, light levels in the lake watercolumns are generally less than 3% of incident irradiationand are seldom sufficient for saturation of phytoplanktonphotosynthesis (i.e., primary production is primarily light-limited; Neale and Priscu 1998). The layer in which net pro-ductivity is nonzero, or the trophogenic zone, deepens withincreasing summer light. In the east lobe of Lake Bonney,for example, the trophogenic zone extends to a maximumdepth of about 20 m (Priscu et al. 1999). Because of theconsistently low light levels and high water column stability,antarctic lake phytoplankton are extremely shade-adapted(Neale and Priscu 1998). They also have an abundance oflight-harvesting carotenoid pigments that are efficient at ab-sorbing the blue-green wavelengths of light that penetratethe ice (Lizotte and Priscu 1998). As within any aquaticcommunity, the phytoplankton in antarctic lakes must havean adequate supply of nutrient elements for balanced growth,whether introduced exogenously or through the biochemicalrecycling of organic matter. Because of losses, such as theescape of lift-off cyanobacterial mats and associated sedi-ments through the ice cover (Parker et al. 1982), recyclingcan never be 100% efficient, so new supplies of nutrientsmust be brought into the system from its surroundings. Aninadequate supply of nitrogen, phosphorus, or other requirednutrient during the summer months may limit the photosyn-thetic capacity of the phytoplankton and, hence, limit theirability to utilize the available light energy (Lizotte and Pris-

1332 Dore and Priscu

cu 1994; Neale and Priscu 1998). Nitrogen deficiency hasbeen implicated in some antarctic lakes (Vincent and Vincent1982; Priscu et al. 1989); however, the phytoplankton ofothers appear to suffer from potential phosphorus deficiency(Parker and Simmons 1985; Priscu 1995).

Because of the protective ice cover of the lakes and thestrong salinity gradients within some of them, convectivemixing and turbulence are virtually absent (Spigel and Priscu1996). Because of this extreme water column stability, ver-tical fluxes of dissolved nutrients from the deep pools areessentially controlled by molecular diffusion, providing onlya slow refueling of nutrient supplies (Priscu 1995; Spigeland Priscu 1998). New nutrients from outside the lakes mayenter with seasonal meltwater streams for only a few weeksper year, and much of this limited freshwater input appearsto spread across the lake in a thin layer just below the icecover (Spigel and Priscu 1998). Material transport to thelakes is also achieved through aeolian transport of terrestri-ally derived sediment onto the ice cover. Although the icecover prevents the lakes from trapping as much windblownsediment as would ice-free lakes, some sediment does gettrapped and can slowly migrate downward through the ice(Priscu et al. 1998). Such migration is facilitated by cracksand liquid water channels in the ice; thus, this form of ma-terial transport to the water column is also enhanced by theelevated temperatures during the summer season (Fritsen etal. 1998). Through absorption of solar radiation, the sandand gravel deposited on the ice surface also may form theirown localized meltwater pockets, speeding its gravitationalsettling through the ice cover (Psenner et al. 1999). However,nutrients introduced in this fashion can be used by microbialcommunities in meltwater pockets within the ice cover itself(Paerl and Priscu 1998; Priscu et al. 1998); these commu-nities may act as a ‘‘biofilter,’’ removing readily leachablenutrients from deposited material into less available partic-ulate organics before the gravel-associated material can passthrough the ice and into the lakes.

In this study, we set out to evaluate the potential phos-phorus deficiency of phytoplankton in the major lakes ofTaylor Valley, Antarctica. In the assessment of phytoplank-ton nutrient status, individual methods can provide mislead-ing results if considered alone; the best approach is to con-duct several independent yet complementary assayssimultaneously (Vincent 1981). Investigators have used anumber of indicators for nutrient deficiency in studies of lakeecosystems, including the response of phytoplankton in nu-trient addition bioassays, the elemental compositions of thedissolved nutrient supplies and of the seston, and the activityof alkaline phosphatases (Healy and Hendzel 1980; Morrisand Lewis 1988; Hecky et al. 1993). The McMurdo DryValley lakes represent extreme aquatic environments inwhich to examine the relative roles of nitrogen and phos-phorus in nutrient deficiency and have a number of uniqueaspects relevant to the study of nutrient limitation. For ex-ample, the limited stream flow and lack of terrestrial vege-tation in the watershed provide an opportunity to study ses-tonic elemental ratios in the near absence of potentiallyconfounding terrestrial detritus (Hecky et al. 1993). Also,because the lakes are in a hydrologic balance between lim-ited seasonal stream inflow and ablation at the ice surface

with no outflows (Chinn 1993), they have extremely lowrates of flushing. These lakes therefore represent end mem-bers in which the results of short-term biological assays ofnutrient deficiency should converge on the predictions basedon elemental ratios of seston (Hecky et al. 1993). In addition,the virtual absence of external inputs of nutrients during theearly part of the season (prior to the initiation of stream flow)allows for examination of nutrient deficiency in a relativelysimple, practically closed system. We therefore proceeded to(1) conduct an initial nutrient assessment using 14C-based Nand P deficiency bioassays on these lakes and then (2) gen-erate complementary data on phosphorus deficiency (dis-solved inorganic and organic N : P ratios, sestonic C : N : Pratios, alkaline phosphatase activity assays) from two lakesdisplaying different degrees of P deficiency in the 14C bio-assays.

Materials and methods

All field work was conducted in Taylor Valley, McMurdoDry Valleys, Antarctica. Four lakes, each permanently cov-ered with an ice cover approximately 4 m thick, were ex-amined: Lake Bonney (east and west lobes), Lake Fryxell,and Lake Hoare. The two lobes of Lake Bonney are con-nected by a shallow constriction only and are considered asdistinct basins. The Taylor Valley lakes represent a centralfocus of the ongoing McMurdo Dry Valleys Long-Term Eco-logical Research (LTER) project, which uses these systemsas sentinals of global environmental change (see http://huey.colorado.edu/LTER/). The exact geographic locationand physical setting of each of the lakes have been welldescribed elsewhere (Priscu 1998).

14C-based nutrient bioassays were performed during latespring and summer (November–December) 1994 on samplesfrom immediately beneath the ice and from the deep chlo-rophyll layers within each lake. Associated profiles of chlo-rophyll a (Chl a), bacterial numbers and nutrients were col-lected at this time. Additional field work focusing onphosphorus was conducted during late winter and earlyspring (September–October) 1995 in the east lobe of LakeBonney and Lake Hoare only. Particulate carbon, nitrogen,phosphorus, and Chl a were measured, and a variety of al-kaline phosphatase assays were carried out. Dissolved inor-ganic and organic pools of nitrogen and phosphorus werealso assessed. Dissolved organic carbon was not measuredat this time, but results from November 1996 are presentedhere for comparison to other nutrient data. Lake water sam-ples were collected with 2- or 5-liter Niskin samplers on acable lowered by winch through a hole melted in the icecover. All depths are reported with respect to the water levelin the sampling hole (piezometric depth), which was about30–50 cm below the surface of the ice. Temperature andconductivity were measured with a SEABIRD Model 25CTD sonde; salinity was computed as outlined by Spigel andPriscu (1996) and is reported in practical salinity units(PSU). Photosynthetically active radiation was measuredwithin the water column and at the ice surface during No-vember–December 1996 using Licor spherical and flat platesensors, respectively. Experiments and sample preparation

1333Phosphorus deficiency in Antarctic lakes

were carried out in the field laboratories at Lake Bonney andLake Hoare. Analyses were conducted at McMurdo Station(Crary Laboratory) or at the University of Hawaii (UH).

14C bioassays—Water samples (20 liters) were collectedfrom selected depths in each lake and transported by heli-copter within 30 min of collection to the Lake Bonney fieldlaboratory. Samples were kept dark and near the temperatureof collection during transport. From each sample, a 5.6-litersubsample was drawn and amended with 14C-bicarbonate toa final activity of 4.1 kBq ml21. This subsample was thenused to prepare eight 700-ml aliquots in 1,000-ml acid-washed high-density polyethylene (HDPE) bottles. Nutrientsupplements were added to duplicate bottles to yield the fol-lowing enrichments: N 5 20 mM NH , P 5 2 mM PO , N1 32

4 4

1 P 5 20 mM NH 1 2 mM PO , control 5 no nutrient1 324 4

additions. The bottles were incubated in an environmentalchamber at the temperature of collection and at saturatingirradiance (70 mmol photons m22 s21; Lizotte and Priscu1992). Subsamples (60 ml) were withdrawn at 24-h intervalsover 4–6 d and filtered through Whatman GF/F filters. Thefilters were placed in 20-ml glass scintillation vials and acid-ified (0.5 ml of 3 N HCl) to remove inorganic carbon. Thefilters were dried at 508C and counted using standard liquidscintillation spectrometry. Nutrient addition response factors(cf. Elser et al. 1990) were calculated as the ratios of theslopes of the least-squares fits of the natural log of the ra-dioactivity (disintegrations per minute) on the filter againsttime of nutrient-enriched samples versus control samples.The statistical significance of these results was determinedby comparing the treatment slopes to the control slope witha one-way analysis of variance followed by a post hoc Dun-nett’s test (Zar 1984).

Dissolved nutrients—Samples for dissolved inorganic ni-trogen (DIN) analyses were vacuum-filtered (,0.5 atm)through combusted Whatman GF/F filters into acid-washedHDPE bottles and immediately frozen at 2208C. These sam-ples were returned to Crary Laboratory, where they werethawed at room temperature and analyzed for NH , NO ,1 2

4 2

and NO using an autoanalyzer (Lachat Instruments). No23

special modifications to standard autoanalytical methods(Strickland and Parsons 1972) were required, except thatsamples with salinities exceeding 35 PSU were diluted withdeionized water to final salinities at or below this value be-fore analysis. Samples for dissolved organic carbon (DOC)were similarly filtered into combusted acid-washed amberglass bottles, preserved with phosphoric acid, and returnedunfrozen to Crary Laboratory, where they were analyzed us-ing an OI Model 700 carbon analyzer (Oceanography Inter-national Corp.).

Samples for soluble reactive phosphorus (SRP), total dis-solved phosphorus (TDP), and total dissolved nitrogen(TDN) were filtered through acid-washed, combusted GF/Ffilters and were frozen in HDPE bottles. These samples werereturned to UH and kept frozen until the day of analysis.SRP was determined with a manual colorimetric method(Strickland and Parsons 1972); highly saline samples fromLake Bonney were diluted with deionized water to about 50PSU prior to analysis. TDP was measured as SRP after UV

oxidation (1,200 W, 2.5 h) of the sample (Armstrong et al.1966). Before the oxidation step, hypersaline samples werediluted with deionized water, and fresh samples were amend-ed with an NaCl solution such that all samples fell withinthe range of 10–50 PSU. Oxidation was enhanced by adding75 ml of 30% H2O2 to 20 ml of sample prior to UV exposure.Dissolved organic phosphorus (DOP) was calculated by sub-tracting SRP from TDP. TDN was measured using high-tem-perature combustion with chemiluminescent detection(Walsh 1989). Hypersaline TDN samples were diluted withdeionized water to 50 PSU before analysis. Dissolved or-ganic nitrogen (DON) was calculated by subtracting DINfrom TDN.

Particulate bioelements, Chl a, and bacteria—Particulateorganic carbon (POC) and particulate nitrogen (PN) sampleswere collected on combusted GF/F filters and stored frozenfor later acidification and analysis at Crary Laboratory witha Carlo-Erba model 1500 CHN analyzer. Samples for par-ticulate phosphorus (PP) were collected on acid-washed,combusted GF/F filters and returned frozen to UH. PP waslater analyzed as SRP after high-temperature ashing withsubsequent acid extraction (Karl et al. 1991). Particulate Chla samples were collected on GF/F filters and frozen fortransport to Crary Laboratory. The filters were then extractedin 90% acetone and analyzed by fluorometry (Holm-Hansenet al. 1965). For bacterial enumeration, formalin-preservedwater samples (10 ml) were stained with acridine orange,filtered onto black 0.2-mm polycarbonate filters, and countedby epifluorescence microscopy (Takacs and Priscu 1998).Non-autofluorescing rods and cocci were counted on eachfilter at 31,000 magnification.

Alkaline phosphatase activity—The enzymatic activity ofalkaline phosphatase (APase) in lake water samples was es-timated using the compound 4-methylumbilliferyl phosphate(MUP) as a substrate analog during timed, temperature-con-trolled incubations, with subsequent analysis of the hydro-lysis product 4-methylumbilliferone (MUF) by fluorometry(Pettersson and Jansson 1978). Incubations were carried outin acid-washed 15-ml polystyrene centrifuge tubes. In eachtube, 0.5 ml of buffered (Tris, pH 8.3) MUP was added toa 4.5-ml water sample and incubated in the dark. The con-centration of the MUP addition was varied to adjust the finalMUP to the desired concentration (from 0 to 100 mM, de-pending on the particular experiment). Denatured controlswere prepared by boiling the samples for 5 min and coolingback to incubation temperature prior to the addition of MUP.Selected samples were enriched with 5 mM KH2PO4 prior toMUP addition. At time zero, and at selected time pointsthereafter, the enzymatic reaction was stopped by freezingof the sample in liquid nitrogen (Chrost and Velimirov1991). After rapid freezing, the samples were maintainedbetween 220 and 2408C until returned to Crary Laboratoryfor analysis. A 208C water bath was used to rapidly thawthe samples, which were then immediately analyzed with aPerkin-Elmer LS-50B luminescence spectrometer. An APasesubstrate–response curve was generated using an iterativenonlinear regression technique (Marquardt-Levenberg algo-


Table 1. Results and background data from Nov–Dec 94 nutrient bioassay experiments. SRP concentrations below the detection limitof 0.02 mM are treated as 0.02 mM for ratio calculations. DIN is nitrate 1 nitrite 1 ammonium, each with a 0.03-mM detection limit usedfor ratio calculations. Nutrient bioassay response factors represent the ratios of treatment to control responses, as outlined in Materials andmethods.

Lake Depth (m)Chl a

(mg L21)

Bacteria(cells L21

3 109)

Nutrient concentration(mM)

DIN SRP DIN : SRP

Response factor

1N 1P 1(N 1 P)

Bonney (west lobe) 513

2.85.6

7.72.2

8.832.0

0.020.02

4401600

1.050.98

1.38**1.22*

1.32**1.20*

Bonney (east lobe) 51318

1.10.70.4

5.32.53.8

9.226.3

110.5

0.020.020.03

46013153683

0.951.061.01

1.42**0.991.16

1.30**1.191.15

Hoare 514

9.51.8

2.81.1

1.03.0

0.020.02

50150

1.080.89

1.050.79

1.47**0.80

Fryxell 59

6.88.0

0.9915.25

0.093.32

0.110.18

0.818.4

0.731.04

1.78**1.38*

2.02**1.31*

Statistical results of bioassays (two-tailed Dunnett’s test) were different from control at * a 5 0.05 and ** a 5 0.01.

rithm). Because of logistic constraints, only one time-courseAPase incubation from Lake Hoare was performed.

Results

14C bioassays of dry valley lakes—Phosphorus stimulatedcarbon assimilation significantly in all samples except thosefrom Lake Hoare and from the deep chlorophyll layers inthe east lobe of Lake Bonney (Table 1). The addition ofnitrogen alone never stimulated carbon assimilation relativeto controls in samples at the a 5 0.05 level. Simultaneousaddition of nitrogen and phosphorus had nearly the sameeffect as the addition of phosphorus alone in all but the 5-m sample from Lake Hoare. The results from this near-sur-face sample from Lake Hoare indicated a synergistic stim-ulatory effect of N 1 P addition but no effect of N or Pwhen added singly. Nutrient concentrations during thespring–summer 1994 season varied considerably amonglakes, with elevated inorganic nitrogen levels generally oc-curring in deeper water, whereas SRP levels were relativelylow at all depths (Table 1). Bulk nutrient concentrations werenot related to phytoplankton biomass (Chl a) among thelakes. DIN : SRP ratios in all samples except the near-surfacewater of Lake Fryxell exceeded, often considerably, the 16 :1 Redfield ratio that is typically required for balanced algalgrowth (Redfield et al. 1963), implying a deficiency of Prelative to N for phytoplankton growth.

Physical limnology and microbial biomass—Lake Hoareand the east lobe of Lake Bonney differed markedly in tem-perature and salinity (Fig. 1). Salinity in the east lobe ofLake Bonney was nearly zero immediately under the ice,then rose with depth through a steep chemocline to a hy-persaline 140 PSU in the waters below 25 m. Temperaturein this lake displayed a maximum of about 68C at 15 m, withvalues around 08C immediately under the ice and 228C nearthe bottom. Lake Hoare was nearly fresh throughout and hadonly a slight salinity gradient down to 14 m (Fig. 1). Tem-perature in Lake Hoare fell in a narrow range (0.2–0.88C)with a maximum at about 8 m. Light penetration through

the ice was low in both lakes, but higher in the east lobe ofLake Bonney (;3%) than in Lake Hoare (;0.3%). The wa-ter column concentrations of Chl a were substantially higherand the bacterial abundances somewhat lower in Lake Hoarethan in the east lobe of Lake Bonney (Fig. 1).

Nutrient pools and elemental ratios—Vertical profiles ofDOC, TDN, and DIN during the winter–spring 1995 seasonrevealed high concentrations below the chemocline and lowconcentrations through most of the trophogenic zone in theeast lobe of Lake Bonney (Fig. 2). In Lake Hoare, thesepools were of similar magnitude in the trophogenic zone butdid not change with depth except for a small increase at thelake bottom. POC and PN were similar within the tropho-genic zones of the two lakes but stayed mostly constant withdepth in Lake Hoare, while rising through large maxima justbelow the chemocline (about 22 m) in the east lobe of LakeBonney (Fig. 2). The SRP profile obtained from the east lobeof Lake Bonney revealed a depleted layer to 18 m, a sharpgradient (coincident with the gradient in salinity) to a max-imum at 24 m, and a more gradual decline to the bottom(Fig. 2). This profile is consistent with data presented earlier(Priscu et al. 1999), but the trophogenic zone SRP concen-trations we measured (0.02–0.04 mM) are on the low end ofthose measured previously. This already extremely low levelshould be considered an upper limit to the actual free ortho-phosphate concentration for three reasons: (1) the SRP assaymay measure some acid-labile organics as well as ortho-phosphate (Strickland and Parsons 1972), (2) potential ar-senate interference was not examined separately (Karl andTien 1997), and (3) metallophosphate complexes and other‘‘bound’’ forms of phosphate that may not be truly availableto the plankton in situ may be released by the acid molybdateassay (Cembella et al. 1984). The DOP profile resembledthat of SRP, except that DOP in the trophogenic zone wasnot completely depleted, having a rather constant concentra-tion averaging 0.23 mM in the upper 18 m. The trophogeniczone PP was also extremely low (0.03–0.04 mM) but rosebelow the chemocline to a maximum of 0.38 mM at 24 m.Total phosphorus (TP 5 SRP 1 DOP 1 PP) was composed


Fig. 1. Temperature (8C), photosynthetically active radiation (PAR, % incident PAR at the icesurface; 400–700 nm), salinity (PSU), Chl a (mg L21), and bacterial numbers (109 cells L21) in thewater columns of the east lobe of (a, c) Lake Bonney and (b, d) Lake Hoare.

primarily of DOP in the trophogenic zone, but included sig-nificant proportions of SRP and PP within and below thechemocline (Fig. 2). The concentration of SRP in LakeHoare was very low (0.01–0.20 mM) throughout the watercolumn, except for a sharp increase to 2.53 mM just abovethe lake bottom (Fig. 2). This enrichment was associatedwith anoxic bottom water, not with a salinity gradient as was

seen in Lake Bonney. DOP showed no such near-bottomenrichment, remaining nearly constant with depth at an av-erage concentration of 0.21 mM. PP was low (0.24 mM)throughout the water column, with the highest values justbelow the ice cover at 5–6 m. PP in the anoxic bottom water(30 m depth) was only 0.10 mM. The TP enrichment ob-served in the bottom water was therefore almost entirely


Fig. 2. Carbon, nitrogen, and phosphorus pools of the water columns of (a, c) the east lobe ofLake Bonney and (b, d) Lake Hoare: particulate organic carbon (POC), particulate phosphorus (PP),soluble reactive phosphorus (SRP), total phosphorus (TP), and dissolved organic phosphorus (DOP)concentrations (mM); dissolved inorganic nitrogen (DIN) and total dissolved nitrogen (TDN) (1024

M units); particulate nitrogen (PN, 1027 M units); dissolved organic carbon (DOC, mM).

composed of dissolved acid-labile phosphorus compounds(i.e., SRP).

Molar carbon-to-phosphorus and nitrogen-to-phosphorusratios showed a tremendous degree of variability with depthbetween operationally defined nutrient pools and betweenlakes (Fig. 3). The POC : PP and POC : PN ratios in the tro-phogenic zone of Lake Bonney were at or above their re-spective 106 : 1 and 106 : 16 Redfield ratios, suggesting gen-

eral planktonic nutrient deficiency. In this same layer, PN :PP was at or slightly above the 16 : 1 Redfield ratio, possiblyindicating a sestonic deficiency of P relative to N. At thesame time, POC : PP and PN : PP ratios below the tropho-genic zone were lower than Redfield, indicating an excessof P over N and C in this deep particulate matter. DIN : SRP,DIN : TDP and TDN : TDP ratios throughout the water col-umn of the east lobe of Lake Bonney were high (36–1,042),


Fig. 3. Elemental ratios in the particulate and dissolved pools of the water columns of (a, c)the east lobe of Lake Bonney and (b, d) Lake Hoare. All ratios (mol : mol) are presented on loga-rithmic scales. Upper panels present C : P (top axis) and C : N (bottom axis) ratios. Lower panelspresent N : P ratios. Particulate and dissolved pools are as given in Fig. 2. The vertical dotted linein each panel indicates the Redfield C : N : P stoichiometry of 106 : 16 : 1.

indicating an extreme deficiency of phosphorus relative tonitrogen in the dissolved nutrient pools throughout the lake.Dissolved inorganic and total nutrient pools in Lake Hoarewere also depleted in P relative to N, but to a lesser degree,except for the anoxic bottom water, which approached theRedfield 16 : 1 N : P ratio in both dissolved organic and in-organic pools (Fig. 3). However, the DIN : TDP ratio wasless than 16 : 1 in the trophogenic zone of Lake Hoare. Theparticulate pool in Lake Hoare had an N : P ratio well below16 : 1 and a C : P ratio near 106 : 1, suggesting sestonic Ndeficiency (relative to P), except in the anoxic bottom water,

where the particulate C : N : P approached the Redfield 106 :16 : 1 ratio.

APase activity in Lake Hoare and Lake Bonney—The hy-drolysis of the fluorescent MUP substrate analog by alkalinephosphatases in 5-m water samples from the east lobe ofLake Bonney followed a linear time course from time zeroto 10–20 h incubation time (Fig. 4), with hydrolysis ratedecreasing thereafter. Based on this observation, all subse-quent APase activity measurements were determined fromincubations of ,20 h. The addition of 5 mM phosphate to


Fig. 4. Time course of hydrolysis of MUP substrate during incubation for the measurement ofalkaline phosphatase activity at 5 m in the east lobe of Lake Bonney. Incubations were performedafter addition of 1 mM MUP, with and without the previous addition of 5 mM phosphate. Controlsamples were boiled and cooled before addition of MUP. Incubation temperature was 48C. Errorbars are 61 SD of triplicate incubations.

APase incubations resulted in significantly reduced rates ofMUP hydrolysis. Boiled control samples showed virtuallyno APase activity out to 50 h incubation time. A large frac-tion of the total MUP hydrolyzed was hydrolyzed during thefirst few seconds after substrate addition, as evidenced bythe high time-zero measurements of MUF produced, but lin-ear MUF production continued thereafter (Fig. 4). Subse-quent APase activity rates have been determined after time-zero correction.

The substrate–response curve for alkaline phosphatase ac-tivity at 5 m in the east lobe of Lake Bonney was welldescribed by standard Michaelis–Menten enzyme kinetics.

V 5 (VmaxS)/(Km 1 S)

V is the reaction velocity at a given substrate concentrationS, Vmax is the maximum potential reaction velocity at thetemperature of incubation, and Km is the substrate concen-tration at which V is half of Vmax (Fig. 5). The results shownare for incubation temperatures of 58C, but the temperatureoptimum for APase activity was about 358C (Fig. 6). Mostof the rate measurements in this study were made at 4–58C

with 1 mM MUP substrate to more closely simulate in situconditions; however, it should be noted that because of thesuboptimal temperatures and subsaturating substrate concen-trations used, maximum potential APase activity (cf. Healyand Hendzel 1979) may actually have been considerablyhigher. A single time-course experiment from 5 m in LakeHoare made at saturating substrate concentration (100 mMMUP) yielded an order of magnitude less APase activitythan was measured in Lake Bonney (Fig. 5). To more di-rectly compare the results from the two lakes, normalizationsof the APase activities to Chl a and particulate phosphoruswere performed. It should be noted that the Chl a valuesused for normalization were collected from the same samplesused for APase analyses (September–October 1995); theseChl a values were considerably lower than the ones pre-sented in Fig. 1 (November 1994) because they were col-lected early in the season while biomass was accumulating.Kinetic parameters from Lake Bonney (determined by non-linear regression for both the raw data and the normalizeddata), when compared to the single substrate-saturated ratemeasurement from Lake Hoare, revealed that the large dif-


Fig. 5. Substrate response of APase activity at 5 m in the east lobe of Lake Bonney. APaseactivity is estimated as the hydrolysis rate of added MUP substrate during incubation at 58C. Solidcurve is fit to the data by iterative nonlinear regression. The MUP hydrolysis rate from a singletime-course incubation of 5-m water from Lake Hoare at 58C is included for comparison.

ference in APase activity observed between the lakes wasnot an artifact produced by a difference in biomass (Table2).

The depth distribution of APase activity in the trophogen-ic zone of the east lobe of Lake Bonney showed the highestrates near the ice cover at 5 m decreasing to near-zero ratesat 18 m (Fig. 7). The addition of 5 mM phosphate to thesesamples suppressed APase activity at all depths, but the de-gree of suppression was greatest in the 5-m sample. Sizefractionation of a 5-m sample from the east lobe of LakeBonney revealed very little APase activity in the free dis-solved (,0.2 mm) pool (Fig. 8). Most of the activity oc-curred in the 0.2–2.0-mm size class. APase activity in allfractions was suppressed by the addition of 5 mM phosphate.

Discussion

Phytoplankton P deficiency as revealed by 14C bioas-says—The results of the 14C bioassays imply that photosyn-thetic carbon assimilation by phytoplankton during the 1994

season was potentially phosphorus limited in the trophogeniclayers of all four lakes examined, except perhaps in LakeHoare, where phytoplankton responded to N 1 P additionbut not to N or P added individually (Table 1). In addition,deep chlorophyll populations in Lake Fryxell and the westlobe of Lake Bonney exhibited signs of phosphorus defi-ciency (Table 1). In none of the lakes was significant nitro-gen deficiency exhibited at any depth, except for the syn-ergistic response to N 1 P addition seen at 5 m in LakeHoare. With the exception of the 5-m sample from LakeFryxell, all samples used for bioassays had DIN : SRP ratiosabove the balanced 16 : 1 Redfield ratio. The P-deficient re-sponse from 5 m in Lake Fryxell, in spite of apparently lowDIN : SRP, may be explained by the difficulty in preciselyquantifying SRP and DIN concentrations at levels near andbelow their respective detection limits.

Elemental ratios and implications for phytoplankton nu-trient deficiencies—The trophogenic zones of both the eastlobe of Lake Bonney and Lake Hoare were characterized by


Fig. 6. Temperature response of alkaline phosphatase activity at 5 m in the east lobe of LakeBonney, as estimated by the rate of hydrolysis of a 1-mM MUP addition in parallel incubations.Vertical error bars are 61 SD of triplicate incubations at a given temperature; horizontal error barsare 61 SD of three temperature measurements (beginning, middle, and end of incubation).

Table 2. Alkaline phosphatase kinetic parameters from two dry valley lakes.

LakeDepth

(m) Vmax Units of Vmax K (mM substrate)

Bonney (east lobe) 5 51.4 6 2.8122.6 6 6.4

36.8 6 2.0

nmol L21 h21

nmol (mg Chl a)21 h21

nmol (mg PP)21 h21

2.4 6 0.53.8 6 0.82.4 6 0.5

Hoare† 5 3.038.230.43

nmol L21 h21

nmol (mg Chl a)21 h21

nmol (mg PP)21 h21

ND‡

NDND

* Parameters (mean 6 standard error) determined by nonlinear regression of 15 rate measurements.† Single rate measurement at saturating substrate concentration (100 mM).‡ ND, not determined.

very low SRP concentrations (Fig. 2). High DIN : SRP ratiosin Lake Bonney and low DIN : SRP ratios in Lake Hoarehave been cited as one line of evidence for P deficiency inthe former and N deficiency in the latter (Priscu 1995). Thepresent results (Fig. 3) are consistent with the previous dataset from the east lobe of Lake Bonney, but the DIN : SRP

ratios for Lake Hoare measured here (9.2–163 in the upper14 m) are considerably higher than earlier results (0.3–0.5;Priscu 1995). This is apparently a result of higher SRP val-ues in the prior study, possibly because of analytical diffi-culties in the accurate measurement of phosphate at very lowlevels. A recent investigation, using a steady-state radio-


Fig. 7. Depth profile of alkaline phosphatase activity in trophogenic zone of the east lobe ofLake Bonney. Both unamended samples and those treated with 5 mM phosphate are shown at 5,10, 13, and 18 m. APase activity is estimated as the rate of hydrolysis of a 1-mM MUP substrateaddition during incubation at 58C.

bioassay technique to measure phosphate in a diverse set ofNorth American lakes, found that the standard SRP analysisoverestimated true phosphate concentrations by two to threeorders of magnitude (Hudson et al. 2000). Notwithstandingthis potential artifact, the measured DIN : SRP ratios in thetrophogenic zone of Lake Bonney were about an order ofmagnitude higher than those in Lake Hoare, implying loweravailability of P relative to N for phytoplankton growth inthe former lake.

When considering nutrient availability, one should not fo-cus solely on the inorganic nutrients, as many organic formsare potentially available through direct assimilation, andmore importantly, after exoenzymatic hydrolysis (Cembellaet al. 1984). The TDN : TDP ratio in aquatic environmentsmay therefore be more indicative of the nutrient status ofthe ecosystem than the DIN : SRP ratio (Smith et al. 1986).Furthermore, based on their concentrations in glacial melt-water, organic nutrients are suspected to be important to themaintenance of antarctic lake ecosystems (Downes et al.1986). Our results revealed that TDN : TDP in the lakes stud-ied was similar to DIN : SRP, except in the trophogenic zone

of Lake Bonney (Fig. 3). From 5 to 20 m, the TDN : TDPratio in the east lobe of Lake Bonney was substantially lowerthan the DIN : SRP ratio, implying that the dissolved organicnutrients were depleted in nitrogen relative to phosphorus.However, the lack of a vertical gradient in DOP (Fig. 2) andother evidence based on APase activity (see below) suggestthat this organic material, although hydrolyzable by UV-ox-idation, is resistant to enzymatic hydrolysis and thus is prob-ably not readily available for phytoplankton growth. There-fore, if labile DOP inputs are important as a phosphorussource, they are likely used rapidly after entering the lakeand do not accumulate from one season to the next. Anotherpotential index for discriminating between N and P limita-tion is the DIN : TDP ratio, which may be preferable to theDIN : SRP and TDN : TDP ratios because of the relativelyhigh bioavailability of DOP relative to DON in aquatic eco-systems (Morris and Lewis 1988). In the east lobe of LakeBonney, DIN : TDP ratios are substantially higher than Red-field, and differ little from the TDN : TDP ratios (Fig. 3). InLake Hoare, however, DIN : TDP in the trophogenic zone islow, indicating N deficiency, in contrast to the P-deficient


Fig. 8. Size fractionation of alkaline phosphatase activity at 5 m in the east lobe of Lake Bonney.Samples were size-fractionated by differential filtration prior to any additions. APase activity isestimated as the rate of hydrolysis of a 1-mM MUP substrate addition during incubation at 48C.Both unamended samples and those treated with 5 mM phosphate are shown. There was no mea-surable activity in the ,0.2-mm fraction with 5 mM phosphate added.

TDN : TDP results. Taken together, the various elemental ra-tios of the dissolved pools strongly support P deficiency rel-ative to N in the east lobe of Lake Bonney but are somewhatambiguous regarding N versus P deficiency in Lake Hoare.

Particulate phosphorus was low in the trophogenic zonesof both lakes studied (Fig. 2), though a slight increase towardthe under-ice surface was noted in Lake Hoare. The deepwater in the east lobe of Lake Bonney contained higher lev-els of PP, but the lack of carbon assimilation (Priscu et al.1999) and the virtual absence of bacterial thymidine incor-poration (Takacs and Priscu 1998) in this layer suggests thata majority of this particulate material is nonliving. This deepPP pool may therefore represent organic detritus, mineralphases, or both. Particulate elemental ratios were quite dif-ferent between the two lakes (Fig. 3). In the trophogeniczones, where much of the particulate matter is composed ofphytoplankton (Priscu 1995), we saw slightly high PN : PPratios (compared to Redfield) in the east lobe of Lake Bon-ney and low PN : PP ratios in Lake Hoare. This suggests thatphytoplankton in Lake Bonney are deficient in phosphorus

relative to nitrogen and that the opposite holds true for LakeHoare phytoplankton. Lake Hoare particulate matter was N-deficient throughout the water column, except in the anoxicbottom water, where all nutrient pools appeared to be neara balanced Redfield 106 : 16 : 1 C : N : P ratio. The particulatematter below the chemocline in Lake Bonney was enrichedin C and P relative to N, suggesting that it was composedprimarily of mineral or organic detrital matter, because theprimary potential source of living organic matter (phyto-plankton) is depleted in P relative to N. We suspect that atleast some of this material represents either fine aeolian min-eral particles (which are P-rich; J. E. Dore unpubl. data) thatsank through the less dense surface waters and became sus-pended in the denser hypersaline bottom waters, or mineralprecipitates that formed in situ (carbonates, phosphates). Acombination of these scenarios is also possible.

Alkaline phosphatase activity and phytoplankton P defi-ciency—The time course of APase activity revealed someimportant features (Fig. 4). First, it can be seen that the ob-


served APase activity as measured by the hydrolysis of MUPto MUF was probably not an artifact of variable backgroundfluorescence or nonenzymatic hydrolysis, as evidenced bythe near-zero fluorescence of boiled controls. Second, therewas a distinct suppression of APase activity produced by theaddition of phosphate, to be expected under P-deficient en-vironmental conditions (Cembella et al. 1984). Finally, anintriguing result of this experiment was the high time-zerohydrolysis evident in the live treatments. The reaction in allsamples was stopped by immediate freezing in liquid nitro-gen after addition of the MUP substrate, with an elapsedtime of no more than 2 min; nevertheless, this large time-zero fluorescence was evident in all experiments. Becausethere is presumably a suite of enzymes responsible for theobserved activity, an enzyme with particularly rapid kineticsmay have caused the time-zero fluorescence. Alternatively,it is possible that some background fluorescence from dis-solved compounds in the water samples may have contrib-uted to the time-zero fluorescence but that these compoundswere denatured by boiling in the control samples. However,the time-zero fluorescence of 0.2-mm filtered samples wasless than half that of parallel unfiltered samples (data notshown), suggesting that the time-zero fluorescence was, atleast in part, due to particulate enzyme activity, not dissolvedfluorescent compounds. A third potential explanation for thehigh time-zero response involves enzymatic ‘‘burst’’ kinetics(see for example Copeland 2000).

Consider the following multistep enzymatic reaction,

k k k11 2 3

E 1 S ⇔ ES ⇒ P 1 EQ ⇒ E 1 Q,k21

where E is the enzyme, S the substrate, P and Q are products,and the kn are the rate constants for the reactions indicated.If the final step is rate-limiting, then in the steady-state, theenzyme will exist almost entirely as EQ, and the overallrelease of both P and Q will be governed by the rate constantk3. Consider the case when in the initial condition S is verysmall but there is a substantial amount of enzyme. Then theenzyme exists primarily as E, and upon addition of a pulseof S, P and EQ are produced rapidly, but the production ofQ and release of E cannot keep up. In this transient phase,P is rapidly produced without production of Q; as steady-state is reached, the production rates of P and Q becomeequal.

In the case of these alkaline phosphatase experiments, Prepresents the hydrolysis product MUF, and Q represents thephosphate released. S is the MUP substrate added (see re-action scheme for APase outlined in Jansson et al. 1988).Initially, available phosphate monoesters are nearly absentfrom the system, but a substantial amount of uncomplexedalkaline phosphatase is on hand. When the MUP is added,MUF is produced in a burst (governed by k11 and k2), butthe phosphate removed from the MUP remains complexedto the APase, releasing free APase and phosphate at a slowerrate (governed by k3). What we presumably see in the time-zero samples is the burst of MUF, but the actual rate ofrelease of phosphate is seen in the steady state kinetics fol-lowing time zero.

The burst kinetics phenomenon, if confirmed, has impor-

tant analytical and ecological implications. Analytically, itimplies that time-zero corrections must be applied followingtime zero, or gross overestimates of potential steady-staterates of phosphate liberation may result. Ecologically, it im-plies that in situ substrate concentrations are vanishinglylow, yet high levels of enzyme are maintained. The main-tenance of high levels of APase may represent an adaptationto an environment in which substrate inputs are rapid andstochastic. Such inputs could be expected from release ofdissolved organic matter from gravel-associated ice capcommunities (Priscu et al. 1998) as these gravel packets fallinto the water column during the formation of summer melt-water channels in the ice cover or from the delivery of theorganic-rich initial melt of nearby glacial faces (Downes etal. 1986).

The substrate response of APase activity in the east lobeof Lake Bonney was described by a standard Michaelis–Menten function (Fig. 5). The apparent half-saturation con-stant Km of 2.4–3.8 mM (Table 2) was on the low end of thereported range for APase in aquatic environments (about 1–100 mM; Jansson et al. 1988). The low Km implies a highaffinity of Lake Bonney APase for available phosphatemonoesters. The maximum reaction velocity of 51.4 nmolL21 h21 indicates the potential for turnover of the SRP poolin less than 1 h following a sufficiently large input of phos-phomonoester substrate. The single data point from LakeHoare has been included on this plot, demonstrating themuch reduced APase activity there compared with the eastlobe of Lake Bonney. This comparison reveals more severephosphorus deficiency in the east lobe of Lake Bonney thanin Lake Hoare, consistent with the unambiguous stimulationof photosynthesis by P addition observed among near-sur-face phytoplankton in Lake Bonney but not in Lake Hoare(Table 1; see also Priscu 1995).

Maximum APase activity has been evaluated experimen-tally as a function of the degree of P deficiency in a varietyof algal cultures; slight P deficiency is indicated by maxi-mum potential hydrolysis rates of .2 nmol (mg PP)21 h21

or .3 nmol (mg Chl a)21 h21, whereas severe P deficiencyis indicated by maximum potential hydrolysis rates of .10nmol (mg PP)21 h21 or .5 nmol (mg Chl a)21 h21 (Healyand Hendzel 1979). Maximum APase activities from the eastlobe of Lake Bonney, whether normalized to PP (36.8 nmol[mg PP]21 h21) or to Chl a (122.6 nmol [mg Chl a]21 h21),were clearly indicative of extreme P deficiency (Table 2).The degree of P deficiency implied by the maximum APaserate in Lake Hoare was somewhat ambiguous; when nor-malized to PP (0.43 nmol [mg PP]21 h21), no P deficiencywas suggested, but when normalized to Chl a (8.23 nmol[mg Chl a]21 h21), severe P deficiency was implied (Table2). This apparent contradiction may be due to a substantialfraction of the APase activity (in both lakes) being associ-ated with bacterial and free dissolved pools, rather than thealgal pool alone. Microscopic analysis of phytoplankton inthe dry valley lakes shows that most species are larger than2.0 mm (J.C. Priscu unpubl. data). When APase activity inLake Bonney was measured on whole water from 5 m deepand on ,2.0- and ,0.2-mm filtrates to evaluate the relativeproportions of algal, bacterial, and dissolved APase, the re-sults showed that the 0.2–2.0-mm (bacterial) fraction held


most (68%) of the total APase activity, and the ,0.2-mm(dissolved) fraction also held some (8%) of the activity (Fig.8). Normalization to PP rather than to Chl a may thereforebe more appropriate here if APase activity is considered asan overall community P deficiency indicator.

The temperature response of APase activity in the eastlobe of Lake Bonney showed a maximum potential rate atabout 358C (Fig. 6), typical of alkaline phosphatases in otheraquatic environments (Healy and Hendzel 1979). AlthoughAPases with temperature optima as low as 58C have previ-ously been described from antarctic microbial sources (Ba-nerjee et al. 2000), no special adaptation to the relativelycold water temperature of the east lobe of Lake Bonney (Fig.1) was evident in our study. The relatively rapid rise in ac-tivity between 0 and 108C suggests that in situ rates may bestrongly influenced by even minor changes in temperature.Although potential rates of APase activity were higher at 5m than at 13 m (Fig. 7), it is still conceivable that true insitu APase activities reached on substrate input may actuallybe higher in the lower trophogenic zone than immediatelybelow the ice because of the approximately 38C higher tem-perature at 13 m. Also, it is clear that even should adequatesubstrate at times become available, APase activity as awhole will be suboptimal owing to the low overall temper-ature of the lake environment.

Sources, sinks, and internal cycling of P in antarcticlakes—The four antarctic lakes studied here all lie withinthe same valley and have similar ice covers and watersheds(Fountain et al. 1999; Lyons et al. 2000). Nevertheless, theirphytoplankton communities exhibit different degrees ofphosphorus deficiency. Because the lakes have no significantoutflows (Chinn 1993), these differences must be related tothe supply of exogenous phosphorus to the lakes and theinternal cycling of phosphorus within the lakes. New phos-phorus may enter the lakes via intermittent stream flow dur-ing the summer or possibly via release from the permanentice cover. Such exogenous nutrient inputs are likely to beimportant on an annual basis. However, the field work inthis study was conducted when air temperatures rarely ex-ceeded 2208C, and flowing water was absent and had beenso for several months. Primary photosynthetic production inthese effectively closed systems must therefore have beensupported during this period by endogenous phosphorus sup-plies. Because of the strong stability of the lake water col-umns, this phosphorus is presumably supplied via upwarddiffusion across the lakes’ chemoclines (Priscu 1995). An-other possible yet undemonstrated mechanism for bringingphosphorus from the chemocline into the trophogenic layeris via the vertical phototactic migrations of microorganisms.Many of the phytoplankton species in these lakes are flag-ellated and may move vertically in order to achieve optimallight-harvesting potential and possibly nutrient supply (Pris-cu and Neale 1995; Lizotte and Priscu 1998). This processwas not examined in our study, yet remains an importantconsideration for future research.

If the primary source of phosphorus supporting phyto-plankton growth prior to the onset of stream flow is upwarddiffusion across the chemocline, then we expect that the de-gree to which phosphorus (vs. nitrogen) limits primary pro-

ductivity during this period will be determined by the N : Pratio of the upward diffusive flux of inorganic and bioavail-able organic nutrients. Priscu (1995) showed that the re-sponse of phytoplankton carbon assimilation to N and P en-richments in these lakes was consistent with the DIN : SRPflux ratios from nutrient gradients immediately below thephytoplankton populations sampled. In the present study, wefind a more consistent response of carbon fixation to phos-phorus amendment in Lake Bonney than in Lake Hoare, aswell as much higher APase activity in the east lobe of LakeBonney than in Lake Hoare. These results are in accord withour measured diffusive flux ratios across the chemoclines ofDIN : SRP (12.3 in Lake Hoare vs. 284 in Lake Bonney),DIN : TDP (12.3 in Lake Hoare vs. 166 in Lake Bonney) andTDN : TDP (6.1 in Lake Hoare vs. 302 in Lake Bonney).

It is possible, therefore, that because of the extremely highDIN : SRP and TDN : TDP ratios of the Lake Bonney deep-water nutrient source, the lake ecosystem is driven towardphosphorus deficiency by factors producing these deepwaternutrients. However, regeneration of organic matter withmoderate N : P sinking out of the trophogenic zone cannotexplain the observed N : P values of the dissolved nutrientsin the deep water. The dense deepwater brine of Lake Bon-ney is thought to have originated from several evaporativeperiods over the last 10,000 yr and represents a resourcelegacy of past events (Chinn 1993; Lyons et al. 2000). At-mospheric dinitrogen fixation appears today to be an insig-nificant process in Lake Bonney (Priscu 1995), but it is pos-sible that past periods of higher dinitrogen fixing activitycould have contributed to the high N : P ratios in the deepwater. A more likely scenario is that enhanced precipitationof phosphate minerals during evaporative periods selectivelyremoved phosphorus from the water column. Conversely,Lake Hoare is a younger lake and has not experienced suchevaporative periods; thus, it remains relatively fresh from topto bottom (Fig. 1; Fountain et al. 1999; Lyons et al. 2000).Dissolved inorganic and organic nutrients in the anoxic bot-tom water of Lake Hoare are balanced in phosphorus relativeto nitrogen. However, this bottom water is of limited arealextent; hence, the significance of its overall contribution tophytoplankton nutrient supply in the lake is unclear.

We have presented several lines of evidence that the LakeBonney ecosystem is extremely P-deficient. However, thedifferent approaches taken to assessing nutrient status haveyielded somewhat ambiguous results in Lake Hoare that sug-gest N and P codeficiency. In the east lobe of Lake Bonney,the stage appears to have been set for phosphorus deficiencythrough the legacy left by past geological and geochemicalmodifications of the lake. The relative roles of stream-borneand diffusive phosphorus supplies, the mechanisms by whichmicrobial populations of this lake have adapted to their P-deficient environment, and the relative roles of phytoplank-ton and bacteria in phosphorus cycling are important areasfor future research.

References

ARMSTRONG, F. A. J., P. M. WILLIAMS, AND J. D. H. STRICKLAND.1966. Photo-oxidation of organic matter in sea water by ultra-


violet radiation, analytical and other applications. Nature 211:481–483.

BANERJEE, M., B. A. WHITTON, AND D. D. WYNN-WILLIAMS. 2000.Phosphatase activities of endolithic communities in rocks ofthe antarctic dry valleys. Microb. Ecol. 39: 80–91.

CEMBELLA, A. D., N. J. ANTIA, AND P. J. HARRISON. 1984. Theutilization of inorganic and organic phosphorous compoundsas nutrients by eukaryotic microalgae: A multidisciplinary per-spective: Part 1. CRC Crit. Rev. Microbiol. 10: 317–391.

CHINN, T. J. 1993. Physical hydrology of the dry valley lakes, p.1–52. In W. J. Green and E. I. Friedmann [eds.], Physical andbiogeochemical processes in antarctic lakes. Antarctic Re-search Series, v. 59. American Geophysical Union.

CHROST, R. J., AND B. VELIMIROV. 1991. Measurement of enzymekinetics in water samples: Effect of freezing and soluble sta-bilizer. Mar. Ecol. Prog. Ser. 70: 93–100.

COPELAND, R. A. 2000. Enzymes: A practical introduction to struc-ture, mechanism, and data analysis, 2nd ed. Wiley.

DOWNES, M. T., C. HOWARD-WILLIAMS, AND W. F. VINCENT. 1986.Sources of organic nitrogen, phosphorus and carbon in antarc-tic streams. Hydrobiologia 134: 215–225.

ELSER, J. J., E. R. MARZOLF, AND C. R. GOLDMAN. 1990. Phos-phorus and nitrogen limitation of phytoplankton growth in thefreshwaters of North America: A review and critique of ex-perimental enrichments. Can. J. Fish. Aquat. Sci. 47: 1468–1477.

FOUNTAIN, A. G., AND OTHERS. 1999. Physical controls on the Tay-lor Valley ecosystem, Antarctica. Bioscience 49: 961–971.

FRITSEN, C. H., E. E. ADAMS, C. P. MCKAY, AND J. C. PRISCU.1998. Permanent ice covers of the McMurdo Dry Valleyslakes, Antarctica: Liquid water contents, p. 269–280. In J. C.Priscu [ed.], Ecosystem dynamics in a polar desert: The Mc-Murdo Dry Valleys, Antarctica. Antarctic Research Series, v.72. American Geophysical Union.

HEALY, F. P., AND L. L. HENDZEL. 1979. Fluorometric measurementof alkaline phosphatase activity in algae. Freshw. Biol. 9: 429–439.

, AND . 1980. Physiological indicators of nutrient de-ficiency in lake phytoplankton. Can. J. Fish. Aquat. Sci. 37:442–453.

HECKY, R. E., P. CAMPBELL, AND L. L. HENDZEL. 1993. The stoi-chiometry of carbon, nitrogen, and phosphorus in particulatematter of lakes and oceans. Limnol. Oceanogr. 38: 709–724.

HEYWOOD, R. B. 1984. Antarctic inland waters, p. 279–344. In R.M. Laws [ed.], Antarctic ecology, v. 1. Academic.

HOLM-HANSEN, O., C. J. LORENZEN, R. W. HOLMES, AND J. D. H.STRICKLAND. 1965. Fluorometric determination of chlorophyll.J. Cons. Perm. Int. Explor. Mer 30: 169–204.

HUDSON, J. J., W. D. TAYLOR, AND D. W. SCHINDLER. 2000. Phos-phate concentrations in lakes. Nature 406: 54–56.

JAMES, M. R., J. A. HALL, AND J. LAYBOURN-PARRY. 1998. Pro-tozooplankton and microzooplankton ecology in lakes of theDry Valleys, southern Victoria Land, p. 255–267. In J. C. Pris-cu [ed.], Ecosystem dynamics in a polar desert: The McMurdoDry Valleys, Antarctica. Antarctic Research Series, v. 72.American Geophysical Union.

JANSSON, M., H. OLSSON, AND K. PETTERSSON. 1988. Phosphatases:origin, characteristics and function in lakes. Hydrobiologia170: 157–175.

KARL, D. M., AND G. TIEN. 1997. Temporal variability in dissolvedphosphorus concentrations in the subtropical North PacificOcean. Mar. Chem. 56: 77–96.

, J. E. DORE, D. V. HEBEL, AND C. WINN. 1991. Proceduresfor particulate carbon, nitrogen, phosphorus and total massanalyses used in the US-JGOFS Hawaii Ocean Time-seriesprogram, p. 71–77. In D. Spencer and D. Hurd [eds.], Marine

particles: Analysis and characterization. Geophysical Mono-graph 63. American Geophysical Union.

LIZOTTE, M. P., AND J. C. PRISCU. 1992. Photosynthesis–irradiancerelationships in phytoplankton from the physically stable watercolumn of a perennially ice-covered lake (Lake Bonney, Ant-arctica). J. Phycol. 28: 179–185.

, AND . 1994. Natural fluorescence and quantumyields in vertically stationary phytoplankton from perenniallyice-covered lakes. Limnol. Oceanogr. 39: 1399–1410.

, AND . 1998. Pigment analysis of the distribution,succession, and fate of phytoplankton in the McMurdo DryValley lakes of Antarctica, p. 229–239. In J. C. Priscu [ed.],Ecosystem dynamics in a polar desert: The McMurdo Dry Val-leys, Antarctica. Antarctic Research Series, v. 72. AmericanGeophysical Union.

LYONS, W. B., A. FOUNTAIN, P. DORAN, J. C. PRISCU, AND K. NEU-MANN. 2000. The importance of landscape position and legacy:The evolution of the Taylor Valley Lake District, Antarctica.Freshw. Biol. 43: 355–367.

MORRIS, D. P., AND W. M. LEWIS, JR. 1988. Phytoplankton nutrientlimitation in Colorado mountain lakes. Freshw. Biol. 20: 315–327.

NEALE, P. J., AND J. C. PRISCU. 1998. Fluorescence quenching inphytoplankton of the McMurdo Dry Valley lakes (Antarctica):Implications for the structure and function of the photosyn-thetic apparatus, p. 241–253. In J. C. Priscu [ed.], Ecosystemdynamics in a polar desert: The McMurdo Dry Valleys, Ant-arctica. Antarctic Research Series, v. 72. American Geophysi-cal Union.

PAERL, H. W., AND J. C. PRISCU. 1998. Microbial phototrophic,heterotrophic and diazotrophic activities associated with aggre-gates in the permanent ice cover of Lake Bonney, Antarctica.Microb. Ecol. 36: 221–230.

PARKER, B. C., AND G. M. SIMMONS, JR. 1985. Paucity of nutrientcycling and absence of food chains in the unique lakes ofsouthern Victoria Land, p. 238–244. In W. R. Siegfried, P. R.Condy, and R. M. Laws [eds.], Antarctic nutrient cycles andfood webs. Springer-Verlag.

, , R. A. WHARTON, JR., K. G. SEABURG, AND F. G.LOVE. 1982. Removal of organic and inorganic matter fromantarctic lakes by aerial escape of bluegreen algal mats. J. Phy-col. 18: 72–78.

PETTERSSON, K., AND M. JANSSON. 1978. Determination of phos-phatase activity in lake water—a study of methods. Verh. Int.Ver. Limnol. 20: 1226–1230.

PRISCU, J. C. 1995. Phytoplankton nutrient deficiency in lakes ofthe McMurdo Dry Valleys, Antarctica. Freshw. Biol. 34: 215–227.

[ED.]. 1998. Ecosystem dynamics in a polar desert: The Mc-Murdo Dry Valleys, Antarctica. Antarctic Research Series, v.72. American Geophysical Union.

, AND P. J. NEALE. 1995. Phototactic response of phytoplank-ton forming discrete layers within the water column of LakeBonney, Antarctica. Antarct. J. U.S. 30: 301–303.

, W. F. VINCENT, AND C. HOWARD-WILLIAMS. 1989. Inor-ganic nitrogen uptake and regeneration in perennially ice-cov-ered Lakes Fryxell and Vanda, Antarctica. J. Plankton Res. 11:335–351.

, AND OTHERS. 1998. Perennial antarctic lake ice: An oasisfor life in a polar desert. Science 280: 2095–2098.

, C. F. WOLF, C. D. TAKACS, C. H. FRITSEN, J. LAYBOURN-PARRY, E. C. ROBERTS, AND W. B. LYONS. 1999. Carbon trans-formations in the water column of a perennially ice-coveredantarctic lake. Bioscience 49: 997–1008.

PSENNER, R., B. SATTLER, A. WILLIE, C. H. FRITSEN, J. C. PRISCU,M. FELIP, AND J. CATALAN. 1999. Lake ice microbial com-


munities in alpine and antarctic lakes, p. 17–31. In P. Schinnerand R. Margesin [eds.], Adaptations of organisms to cold en-vironments. Springer-Verlag.

REDFIELD, A. C., B. H. KETCHUM, AND F. A. RICHARDS. 1963. Theinfluence of organisms on the composition of seawater, p. 26–77. In M. N. Hill [ed.], The sea, ideas and observations onprogress in the study of the seas, v. 2. Wiley Interscience.

SIMMONS, G. M., JR., J. R. VESTAL, AND R. A. WHARTON, JR. 1993.Environmental regulators of microbial activity in continentalantarctic lakes, p. 491–541. In E. I. Friedmann [ed.], Antarcticmicrobiology. Wiley-Liss.

SMITH, S. V., W. J. KIMMERER, AND T. W. WALSH. 1986. Verticalflux and biogeochemical turnover regulate nutrient limitationof net organic production in the North Pacific gyre. Limnol.Oceanogr. 31: 161–167.

SPIGEL, R. H., AND J. C. PRISCU. 1996. Evolution of temperatureand salt structure of Lake Bonney, a chemically stratified ant-arctic lake. Hydrobiologia 321: 177–190.

, AND . 1998. Physical limnology of the McMurdoDry Valleys lakes, p. 153–187. In J. C. Priscu [ed.], Ecosystemdynamics in a polar desert: The McMurdo Dry Valleys, Ant-

arctica. Antarctic Research Series, v. 72. American Geophysi-cal Union.

STRICKLAND, J. D. H., AND T. R. PARSONS. 1972. A practical hand-book of seawater analysis. Fisheries Research Board of Cana-da.

TAKACS, C. D., AND J. C. PRISCU. 1998. Bacterioplankton dynamicsin the McMurdo Dry Valley lakes, Antarctica: Production andbiomass loss over four seasons. Microb. Ecol. 36: 239–250.

VINCENT, W. F. 1981. Rapid physiological assays for nutrient de-mand by the plankton. II. Phosphorus. J. Plankton Res. 3: 699–710.

, AND C. L. VINCENT. 1982. Factors controlling phytoplank-ton production in Lake Vanda (778S). Can. J. Fish. Aquat. Sci.39: 1602–1609.

WALSH, T. W. 1989. Total dissolved nitrogen in seawater: A newhigh-temperature combustion method and a comparison withphoto-oxidation. Mar. Chem. 26: 295–311.

ZAR, J. H. 1984. Biostatistical analysis, 2nd ed. Prentice-Hall.

Received: 23 August 2000Accepted: 25 April 2001

Amended: 4 June 2001

dore, john e., and john c. priscu phytoplankton...

Documents