APPLIED AND ENVIRONMENTAL MICROBIOLOGY, Oct. 1981, p. 565-5730099-2240/81/100565-09$02.00/0

Vol. 42, No.4

Nitrate Accumulation in Aerobic Hypolimnia: RelativeImportance of Benthic and Planktonic Nitrifiers in an

Oligotrophic LakeWARWICK F. VINCENT* AND MALCOLM T. DOWNES

Freshwater Section, Ecology Division, Department of Scientific and Industrial Research,Taupo, New Zealand

Received 18 February 1981/Accepted 26 June 1981

Both nitrate and nitrous oxide accumulate in the hypolimnion of the oligo-trophic Lake Taupo, New Zealand, throughout stratification. The two forms ofoxidized nitrogen increase in concentration with increasing depth toward thesediments, where the dissolved concentrations of reduced nitrogen are two ordersof magnitude higher than concentrations in the overlying water. Nitrificationrates were measured by dark [14C]C02 assays with and without the inhibitornitrapyrin. The fastest rates were recorded for planktonic nitrifiers in the epilim-nion and benthic species in the surficial 2.5 mm of the sediments. Nitrifyingbacteria were least active in the deep hypolimnion. Deepwater accumulation ofNO3 in Lake Taupo must therefore be a product ofbenthic rather than planktonicnitrification.

Accumulation of nitrate beneath the thermo-cline is a general feature of aerobic water col-umns at all latitudes (Table 1). Nitrate levels inthe oxygenated bottom waters of many lakesmeasurably increase soon after the onset ofstratification, typically rising to 1 to 5 mmol -

m-3 above surface concentrations. In those moreenriched hypolimnia which deoxygenate later inthe stratification cycle, nitrate production mayaccount for a significant proportion of total ox-ygen consumed (e.g., 25% in Lake Grasmere,English Lake District [14]) and may therebyaccelerate the onset of anoxic conditions. Inaddition, oxidation of ammonia to nitrate earlyin the season provides the substrate for laterdenitrification and ultimate loss of nitrogen asN2 gas. In oligotrophic waters which remainaerobic, the accumulated N03-N is eventuallymade available to surface euphotic populationsof algae during periods of circulation; nitratetransfer by mixing may affect both the timingand magnitude of primary production maximain such lakes (e.g., Lake Tahoe in California andNevada, [30] and Lake Tanganyika in East Af-rica [6]).Although nitrate production beneath the ther-

mocline is a common phenomenon that mayexert a controlling influence on a wide range oflake processes, the primary sites of lacustrinenitrifier activity have not been clearly deline-ated. Paerl et al. (23) observed an accumulationof nitrate in flasks of water that had been col-lected from the hypolimnion of Lake Tahoe and

incubated with 15-mmol - m-3 enrichments ofammonium for 4 weeks. They concluded thatsinking detritus was the main source of reducednitrogen in Lake Tahoe and that oxidation tonitrate was effected by bacteria suspendedthrough the deepwater column. In Lake Men-dota, Wisconsin, Brezonik (P. L. Brezonik, Ph.D.thesis, University of Wisconsin, Madison, 1968)measured substantial rates of nitrification atmid-depths in late spring and early summer andemphasized that this N03-N production in-creased the importance of denitrification as anitrogen sink. Sediments from a range of otherWisconsin lakes only demonstrated nitrificationwhen they were suspended in water and stirredto increase oxygen diffusion; for these waterbodies, significant ammonia oxidation in the sed-iments was only likely to occur in the littoralregion, and then only at the sediment-waterinterface (4). In contrast with these reports, ni-trifiers in some aquatic environments have beenreported to be very active (e.g., aerated riversediments [8, 26]). This has led Belser (1) toconclude in a recent review that "in aquaticsystems nitrification appears to be associatedwith the sediments rather than the overlyingwater that is transporting the ammonium."

In this study, we first measured the rate ofaccumulation of nitrate-nitrogen and the nitri-fication intermediate, nitrous oxide (N20) in thewell-oxygenated hypolimnion of the largest oli-gotrophic lake in New Zealand. We then exam-ined, by short-term radioisotopic assay, the ver-

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TABLE 1. Nitrate accumulation in aerobic hypolimniaaN03-N (mmolm-3) Rate of net N03-N accu-

Lake mulation (mmol m-2 ReferenceSurface Bottom Differ- day-')bence

Char Lake, Arctic (winter <0.1 1.4 1.4 0.2 (20-27 m) 27stratification)

Lake Titicaca, Peru and Bolivia 7.9 12.1 4.2 26Lake Victoria, East Africa 0.4 5.0 4.6 1.4 (40-60 m) 31Lake Tanganyika, East Africa 0.1 11.3 11.2 7Lake Ohrid, Yugoslavia 0.1 1.4 1.3 30Castle Lake, California 0.3 5.7 5.4 2.0 (15-30 m) 21Lake Baikal, USSR 4.9 8.7 3.8 22Lake Michigan, Wisconsin 7.1 15.7 8.6 2.4 (50-100 m) 24Wastwater, English Lake 23.2 24.4 1.2 W. F. Vincent, unpub-

District lished dataLake Tahoe, California and <0.1 1.6 1.6 1.6 (200-450 m) 25Nevada

Lake Waikaremoana, New 0.1 5.8 5.7 1.5 (30-250 m) W. F. Vincent, unpub-Zealand lished data

a Surface- and bottomwater levels during mid- to late stratification.b Rates of accumulation are derived from discrete depth values trapezoidally integrated over the region

indicated in parentheses.

tical distribution of nitrifying bacteria to estab-lish the relative contribution of benthic andplanktonic species to the observed deepwateraccumulation of oxidized nitrogen.

MATERIALS AND METHODSStudy site. Lake Taupo is a moderately large (612

km2), deep (maximum depth, 163 m; average depth, 97m), oligotrophic lake located on the central volcanicplateau in North Island, New Zealand (see Fig. 1).Thermal stratification begins in November or Decem-ber and persists until June or July of the followingyear. Complete isothermy (ca. 10.5°C) and homoge-neous nutrient profiles are normally achieved at thedeepest station in July or August. The euphotic zoneextends from 0 to 60 m, and maximum algal photosyn-thesis is at 10 to 15 m (determined by in situ 14Cincubations). The hypolimnion remains aerobicthroughout the year, with the maximum dissolvedoxygen depletion of 4 g * m-3 in the bottommostwaters at the end of stratification (17). (Further lim-nological description of Lake Taupo is presented else-where (34). Sampling was at six sites: A, B, and C,where the water approximated the average depth ofthe lake; D, over the deepest water; and E and F, twoshallow-water (30 m) sites in Taupo Bay (Fig. 1).

Nutrient analysis. All water samples were col-lected at discrete depths by a Van Dorn sampler. Forchemical analysis, the water was filtered through acid-washed GF/C glass fiber filters and stored frozen. Allanalyses were performed on a Technicon Auto-Analyzer II. Nitrate was analyzed by the method ofDownes (9). Ammonium was assayed by the salicylate-isocyanurate method of Crooke and Simpson (7), andsoluble reactive phosphate was assayed by a modifiedmolybdenum blue method (11).

Nitrous oxide measurement. Samples for nitrousoxide analysis were removed from the outlet hose ofthe sampler with a 50-ml plastic syringe containing 0.2

FIG. 1. Sampling sites on Lake Taupo, New Zea-land.

ml of 5% HgCl2 solution to inhibit microbial activity.The samples were capped and, within 5 h of collection,extracted by multiple-phase equilibration for N20measurement (21). The sample volume in the syringewas reduced to 25 ml, and then 25 ml of helium wasintroduced. The gas-liquid combination was vigorouslyshaken to equilibrate the two phases. The gas phasewas then passed through a short column of anhydroneto remove water vapor into a 1-ml calibrated loop ofa six-port Carle 5518 gas sampling valve for injectioninto the gas column. The Perkin-Elmer Sigma 4 elec-tron-capture gas chromatograph was fitted with a 2-m(3-mm outer diameter) stainless steel column of Chro-

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BENTHIC AND PLANKTONIC NITRIFIERS 567

mosorb 102 and was operated at a column temperatureof 55°C, with a carrier-gas (95% Ar, 5% CH4) flow of24 ml * min-'. The 63Ni detector was operated at375°C, with a standing current of 3.5 x 10-9 amp. Themachine was calibrated against commercial nitrousoxide standards. Air samples from the mid-lake sam-pling site were similarly analyzed. The concentrationof N20 in each water sample was calculated from twoconsecutive equilibrations by the formulae of Mc-Auliffe (21).Sediment analysis. Sediment samples were ob-

tained with an Ekman dredge. Lake Taupo sedimentsare cohesive and well compacted; each sample ob-tained with an Eckman dredge remained an intactblock during subsequent profiling. Redox profiles weremeasured immediately upon collection with a polishedplatinum electrode (calibrated with quinhydrone) anda calomel reference electrode. The platinum electrodehad a horizontal blade (6.5 x 1 mm) and was polishedwith rhyolitic tephra (pumice) immediately beforeeach profile. The readings were measured with anOrion pH/Millivoltmeter and are expressed relative tothe standard hydrogen electrode (Eh, 0).

Layers of sediment for profile analysis were sepa-rated in the field immediately after the measurementsof redox potential. Interstitial water was extractedfrom subsamples by high-vacuum filtration throughGF/C filters. The filtrate was diluted to 1:10 withdeionized water and then chemically analyzed as de-scribed above. For measuring nitrifier activity, furthersubsamples were brought into dilute suspension (gen-erally 10 mg [dry weight] of sediment per ml) inbottom lake water.

Nitrification assays. Rates of nitrification wereestimated by dark '4C-HCO3 incubations with andwithout the specific nitrifier inhibitor, nitrapyrin (2,27). The lake water and sediment suspensions werepreincubated with nitrapyrin (5-mg * liter-' final con-centration) dissolved in ethanol (7.8-,umol*liter-1 finalethanol concentration) or with ethanol only (control)for 1 h. '4C-HCO3 was then added at a final activityof 0.25 pCi ml-', and the incubation continued for 4h. Throughout these incubations, the samples weremaintained in a dark, temperature-controlled room at15°C. At the end of each experiment, subsamples werefiltered through either 0.22-pum Millipore membranes(50 ml of lake water filtered) or GF/C glass fiber filters(0.3 ml of sediments filtered). These were air dried andthen counted by liquid scintillation spectrometry. Inthe earlier sediment experiments, duplicate sampleswere combusted in a furnace, and the ['4C]CO2 wastrapped in /3-phenethylamine, which was then countedby liquid scintillation. These samples had counts 20%higher on the average than the filtered sediment sam-ples counted directly; however, variability was muchhigher. All direct filtered sediment counts were there-fore multiplied by a factor of 1.2 to correct for self-absorption.

Bacterial profiles. The vertical distribution ofbacteria was determined with acridine orange by epi-fluorescent counts (19). Bacterial activity was esti-mated from rates of deoxyribonucleic acid synthesismeasured with tritiated thymidine (13, 31). Samples(10 ml) of lake water or dilute sediment suspensionwere incubated with [3H]methyl thymidine (10-pmol

* m-3 final concentration) for 20 min. The sampleswere then processed with trichloroacetic acid, HCl,and ethyl acetate, and the counts were converted topmol of C * m-3 * h-' (13).

Extent of mixing. The chlorophyll fluorescenceindex cellular fluorescence capacity (33) was used asa biological tracer of vertical mixing. Cellular fluores-cence capacity ranges from 0 to 1.0 and is an indicatorof phytoplankton cells able to photosynthesize.

RESULTSBetween November 1974 and June 1975, ni-

trate accumulated in the hypolimnion at allthree average-depth sites (sites A, B, C; Fig. 2).The net rate of accumulation was more rapidover the first 3 months of stratification (0.28mmol * m-1 * day-) than it was over the sub-sequent 4 months (0.1 mmol * m-2 - day-1).Differences among sites were most pronouncedin the early stages of winter circulation (July1975); sites A and B mixed 1 month earlier thandid site C, which is less exposed to the prevailingsouth-westerly winds. With the onset of strati-fication in November 1976, nitrate once againaccumulated at a relatively constant rate of 0.24mmol * m-2 * day-'. There were no consistentdifferences among sites.

Similar rates of nitrate increase were recordedin the bottom waters (90 to 150 m) of the deepWaitahanui Basin site (site D) over the periodof January to June 1979 (0.26 mmol *day-'), but much faster rates were recordedthere during early stratification (October andNovember) in 1980 (1.34 mmol . m-2 - day-1).

Planktonic nitrifier activity was first examinedduring mid-stratification on 13 March 1979 (Ta-ble 2). By this date, nitrate-nitrogen concentra-

U N D J F M A M J J A S O N D J1974 1975 1976

FIG. 2. Nitrate accumulation beneath the euphoticzone, 1974 to 1976. Each point is the mean integralvalue for sites A, B, and C ± 1 standard deviation.

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568 VINCENT AND DOWNES

tions had risen to a deepwater maximum of 1.23mmol * m-3, with decreasing concentrations athigher depths. Ammonium-N levels were lessthan 0.1 mmol . m-3 at all depths. Dark CO2fixation rates were highest in water from the 0-

TABLE 2. Physicochemical profiles and dark CO2fixation at site D with and without the nitrification

inhibitor nitrapyrinDark CO2 fixation

Depth Temp No3-N. mol-m-3.h-1)a(m) (OC) m3) Control Plus Differ-

inhibitor ence

0 20 ND b 4.63 3.69 0.9430 16 ND 3.78 2.32 1.4660 11.5 0.32 1.28 0.99 0.2990 10.5 0.75 1.09 0.93 0.16120 10.5 0.97 1.12 1.14 NSC148 10.5 1.23 1.13 1.20 NSa Each CO2 fixation rate is the mean for duplicate

bottles.bND, <0.02 Mmol NO3 m3.'NS, No significant difference in dark CO2 fixation

with and without inhibitor.

Nitrous oxide500.

301

E

I

uia

60!

90

120

140

temp. II 0'

- NO3-

I I

NH4I

I~~~~~-ti

1~~~I

and 30-m levels but varied very little between60 and 148 m (2 m above the sediments). Thenitrification inhibitor nitrapyrin decreased fixa-tion at the 0- and 30-m levels by 30 to 40% buthad a much lesser effect at the 60- and 90-mlevels. No significant effect of nitrapyrin on darkcarbon uptake was recorded in 120-m or 148-msamples.

This experiment was repeated during latestratification on 13 May 1980, when NO3 concen-trations rose with increasing depth below 60 mto a maximum of 2.09 mmol . m-3 at 140 m (Fig.3). Ammonium levels were more or less constantdown the profile at 0.2 to 0.3 mmol m-3.Concentrations of dissolved nitrous oxide werearound saturation (10.7 ,imol . m-3 at 140C) inthe surface water of Lake Taupo, but were su-persaturated below 50 m and increased withincreasing depth to a maximum above the sedi-ments (156% saturation). Cellular fluorescencecapacity profiles demonstrated that partial mix-ing had not occurred below 60 m; there were nophytoplanktons capable ofphotosynthesis belowthis depth (Fig. 4).

Epifluorescent counts, total bacterial deoxy-

!% saturation100 150

N20 july

/, N20 may

1 2NO3 or NH4 m mol.m3

I I I I

0 5 10 15Temperature C

FIG. 3. Physicochemical profiles at site D during late stratification (13 May 1980) or winter mixing (N20only, 28 July 1980).

%O I

IL 0 I I

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BENTHIC AND PLANKTONIC NITRIFIERS 569

12Nitrif ier activi ty (jimol Cmr3hfl) or 10 bacteria .m,3

E

wa

Total bacterial activity pmolC.m3. h1

0 0.1 0.2CFC

I I I 00.3 0.4 0.5 0.6

FIG. 4. Bacterial distribution and photochemical (cellular fluorescence) capacity of the algae at site D, 13May 1980.

ribonucleic acid synthesis, and nitrifier activityhad similar profiles, with maxima in the surfaceepilimnion and values decreasing with depth(Fig. 4). Bacteriol deoxyribonucleic acid synthe-sis and nitrifier activity both decreased by afactor of 4 between 0 and 60 m. Nitrapyrin-sensitive C02 fixation in the lake water sampleswas highly correlated with both total bacterialnumbers (r2, 0.9832; P < 0.001) and total bacte-rial activity (r3, 0.8811; P < 0.01).On this date of sampling, water was also col-

lected at the sediment-water interface. Thissample, containing some suspended sediment,was tested by all three bacterial assays. Bothbacterial numbers and total activity were overan order ofmagnitude higher than those ofwaterfrom shallower depths (Fig. 4). Nitrifier activitywas two orders of magnitude higher than that ofthe 120-m level on the same day.The water colunm was further sampled during

mid-winter mixing on 28 July 1980. On this date,cellular fluorescence capacity was more or lessconstant at 0.5 to 0.6 at all depths to 140 m,

thereby demonstrating complete circulation ofrecently euphotic algae to the bottom of thelake. Nitrate concentrations were also reasona-bly constant throughout the profile, but close tothe detection limits (0.03 mmol * m-3 at 0 m;0.02 mmol . m-3 at 140 m). Ammonium levelswere higher than those in late stratification (ca.0.28 mmol . m-3 at all depths), and nitrous oxidewas detected in concentrations above saturationthroughout the water column (Fig. 3). Nitra-pyrin tests were performed on water from depthsof 0, 30, 60, 90, 120, and 140 m at site D. Therewere no significant differences between dark C02fixation with and without the nitrification inhib-itor at any depth.Sediment samples were collected from the

bottom of Lake Taupo at the end of wintermixing in 1980, immediately before the onset ofhypolimnetic nitrate accumulation. Nitrate lev-els were an order of magnitude higher in theinterstitial water of surficial sediments than theywere in the overlying water column (Fig. 5), andNO3 concentration decreased with depth down

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570 VINCENT AND DOWNES

0

N03 mmol.m35

-C1-0,

0 50NH4 mmol.m3

FIG. 5. Interstitial nitrate (shaded) and ammo-

nium (unshaded) in the surficial sediments and over-

lying water at site D, 14 October 1980.

the sediment profile. Ammonium levels were

two orders of magnitude higher in the sedimentsthan they were in the overlying water but, unlikenitrate, concentrations increased with sedimentdepth to a maximum in the region of 2.5 to 15mm.

Surficial sediments (O to 2.5 mm) were sam-pled from four sites to measure nitrification ac-

tivity. At all four sites, Eh rapidly decreasedwith depth in the sediments (Table 3); at only 5mm below the surface, Eh was at or below thelower threshold reported for nitrification (210mV; [2]). Nitrifier activity at the offshore sites(D and E) was highly variable per gram ofsediment (dry weight), but much less variablewhen expressed on an areal (m-2) basis. Theinshore Two Mile Bay sample (site F) was aboutfour times higher in activity than offshore sitesD and E, both per gram of sediment and persquare meter.

Further sediment samples were collected fromsite D to measure the vertical distribution ofheterotrophic bacteria and nitrifier activity.Both total bacteria and nitrifying bacteria fol-lowed the redox profile, with decreasing activityobserved at increasing depth (Fig. 6). This de-crease was particularly sharp for nitrification,which dropped by 95% between 2.5 and 7.5 mm.Nitrifier activity is therefore restricted to thefew surficial millimeters in Lake Taupo sedi-ments. On this date (14 October 1980), lakewater samples 1 m above the sediments were

concurrently tested for the presence of nitrifiers.There was no effect of nitrapyrin on dark CO2fixation by these samples.

Surface sediment (O to 10 mm) from site Dwas tested for denitrifier activity on 3 December1980. The sediment was suspended in a smallamount of lake water, and 50 ml of this fluid wastransferred to 125-ml Hypovials. These weresealed with neoprene stoppers and then incu-bated aerobically with or without 10% acetyleneon a shaker for 4 days. At the end of the incu-bation, nitrous oxide levels in the headspace of

the control (no acetylene) sample had risen to500 nmol* liter-1. A nitrous oxide level of only0.7 nmol * liter-' was recorded in the headspaceof the sample incubated with acetylene. Acety-lene is a potent inhibitor of bacterial oxidationof ammonia (16). It also acts on denitrifiers, butby blocking nitrous oxide consumption, not bysynthesis (35). Therefore, the denitrifier contri-bution to net nitrous oxide production from thissediment appears negligible compared with thecontribution of nitrification.

DISCUSSIONBoth nitrate and nitrous oxide increase in

concentration in the hypolimnion of Lake Taupothroughout the 6 to 8 months of stratificationeach year. Net rates of nitrate accumulation areslow (generally in the range of 0.1 to 0.3 mmolm -2 *day-) in contrast to other deep oligo-

trophic lakes (1 to 2 mmol . m-2 * day-'; Table1). This might reflect the weak thermocline de-velopment in Lake Taupo and relatively highrates of transfer of dissolved substances to theeuphotic zone by turbulent diffusion.

Elkins et al. (11) concluded that nitrificationrepresents a dominant source of N20 in bothmarine and freshwaters. They estimated that 1mol of N20 is formed for every 700 mol of NH4oxidized, producing a molar ratio of ca. 1.43 x10-3. In the hypolimnion of Lake Taupo, N20and NO3 accumulate at approximately this ratio(1.89 x 10-3), which suggests a common originby nitrifiers for both inorganic species.Both N20 and NO3 increase toward the sedi-

ments, where interstitial nitrate and ammoniumconcentrations are far in excess of values in theoverlying water. No gradient in NH4 levels wasdetected down the water column, suggesting thatammonium-N is oxidized before it leaves thesediments. No planktonic nitrifier activity wasdetected at any time of the year in the region ofmaximum nitrate and nitrous oxide accumula-tion in the bottom waters of the lake. However,sediment nitrifiers were very active at all sites

TABLE 3. Surficial redox potential and nitrifieractivity in four sediment samples collected October-

November 1980Redox potential Nitrifier activity

Water (mV) at depth of:Site depth nmol of l,mol(m) Imlo uo(1mm 5mm C-g-' C-m2-

h-' h-'D 145 420 130 55.3 44.8D 140 380 110 89.2 39.0E 30 310 200 62.5 36.8F 30 450 180 237.3 185.4

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BENTHIC AND PLANKTONIC NITRIFIERS 571

sampled. Benthic nitrification rates in LakeTaupo fall in the middle of the wide range re-

ported in the literature (Table 4; Lake Tauporates were multiplied by 8.3 to convert frommoles of carbon fixed to moles of nitrogen nitri-fied [2]). Even the minimum rate recorded (0.31mmol of N m-2 h-1) could easily account forthe maximum rate of net nitrate accumulationin Lake Taupo (0.06 mmol of N * m-2 h-') or

indeed for rates of NO3 accumulation reported

Nitrifier activit10 20

E h

D3>tF act

y

generally (0.04 to 0.08 mmol of N m-2 *h-. ;Table 1).

Despite the relatively aerobic sediments ofLake Taupo, benthic nitrifier activity is re-

stricted to the few surficial millimeters, whereelectrode potentials are well above 200 mV. Thisconfirms the findings of Chen et al. (4), whoconcluded that benthic nitrification was onlylikely to occur near the sediment-water inter-face. The distribution of total bacterial activity

n mol C. gsed1 h1

30 40 50

0 0.5 1.0 1.5

Total bacterial activity nmol C. gsed½1

I I I I I I

0 100 200 300redox potential

400 500mV

FIG. 6. Sediment profiles for nitrifier and total bacterial activities, and redox potential, 14 October 1980.

TABLE 4. Taupo nitrification rates compared with other sediments and waters

Site Ratea Reference

SedimentLimfjord.... 0.004-0.008 13Danish offshore waters.... 0.01-0.06 16Unenriched stream... 0.09-0.21 4Estuary .... .... 0.16-1.43 34Lake Taupo sediments.. 0.31-1.54 This studyNorth Sea.... 2.4-4.0 2Enriched stream .. ... 10.2 28

WaterLake Vanda .. ... 0.25-5.8 36Lake Taupo ... 1.5-12 This studyLake Mendota.. 2-44 3Blelham Tarn (enriched enclosure) 134 6Scheldt Estuary ............. 170-490 29Scheldt Estuary (potential) 50-1700 29

a For sediment sites, mmol of N-m-2.h-'; for water sites, ,umol of N.m-3h-'.

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572 VINCENT AND DOWNES

along the redox profile in the sediments followsthe general pattern found in most lacustrinesediments (18) of decreasing rates of hetero-trophic metabolism at decreasing Eh.Although planktonic nitrifying bacteria can-

not account for the accumulation of hypolim-netic nitrate, they may be important in theepilimnion of Lake Taupo, where both micro-heterotrophic activity and algal production areat a maximum. Planktonic nitrification rates inTaupo are slow in comparison with some othermarine- and freshwaters (Table 4), but whenintegrated down the water column, they appearto be as high per unit area as sediment rates-e.g., 0.59 mmol of N * m-2 * h-1 on 13 March1979 (O to 148 m), 87% of which occurred in thetop 60 m. Nitrifying bacteria are necessarilydependent upon the rate of supply of reducednitrogen as the substrate for chemoautotrophy.Belser (1) concludes that nitrification is closelyregulated in both disturbed and undisturbedhabitats by the activity of ammonifiers. In LakeTaupo, ammonifying bacteria are likely to bemost active in the surface trophogenic zone andin the surfilcial sediments where availability oforganic N for the ammonifiers is maximal. Ni-trifying bacteria are similarly distributed, butthey are least active in the deep hypolimnion,where the end product of their metabolism, ni-trate-nitrogen, is most abundant.

ACKNOWLEDGMENTSWe thank Karen Law for technical assistance, E. White

and C. Howard-Williams for helpful criticisms, J. A. Gibb andA. Pritchard for editorial comments, and Jan Simmiss fortyping each draft.

LITERATURE CITED1. Belser, L. W. 1979. Population ecology of nitrifying bac-

teria. Annu. Rev. Microbiol. 33:309-333.2. Billen, G. 1976. Evaluation of nitrifying activity in sedi-

ments by dark "4C-bicarbonate incorporation. WaterRes. 10:51-57.

3. Chatarpaul, L., J. B. Robinson, and N. K. Kaushik.1980. Effects of tubuficid worms on denitrification andnitrification in stream sediment. Can. J. Fish. Aquat.Sci. 37:656-663.

4. Chen, R. L., D. R. Keeney, and J. G. Konrad. 1972.Nitrification in sediments of selected Wisconsin lakes.J. Environ. Qual. 1:151-154.

5. Christofi, N., T. Preston, and W. D. P. Stewart. 1981.Endogenous nitrate production in an experimental en-closure during summer stratification. Water Res. 15:343-349.

6. Coulter, G. W. 1977. Approaches to estimating fish bio-mass and potential yield in Lake Tanganyika. J. FishBiol. 11:393-408.

7. Crooke, W. M., and W. E. Simpson. 1971. Determina-tion of ammonium of Kjeldahl digests of crops by anautomated procedure. J. Sci. Food Agric. 22:9-10.

8. Curtis, E. J. C., K. Durrant, and M. M. L. Harman.1975. Nitrification in rivers in the Trent Basin. WaterRes. 9:255-268.

9. Downes, M. T. 1978. An improved hydrazine reductionmethod for the automated determination of low nitrate

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levels in freshwater. Water Res. 12:673-675.10. Downes, M. T. 1978. An automated determination of low

reactive phosphorus concentrations in natural waters inthe presence of arsenic, silicon, and mercuric chloride.Water Res. 12:743-745.

11. Elkins, J. W., S. C. Wofsy, M. B. McElroy, C. E. Kolb,and W. A. Kaplan. 1978. Aquatic sources and sinksfor nitrous oxide. Nature (London) 275:602-606.

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