nitrogen, phosphorus, and organic carbon cycling in an arctic lake
TRANSCRIPT
Nitrogen, Phosphorus, and Organic Carbon Cycling in an Arctic lake'
S. C. Whalen and 1. C. Cornwell' institute of Marine Science, University of Alaska, Fairbanks, AK 9970 1 , USA
Whalen, S. C., and 8 . C. Cornwell. 1985. Nitrogen, phosphorus, and organic carbon cycling in an arctic lake. Can. 8 . Fish. Aquat. Sci. 42: 797-808.
Budgets for nitrogen, phssphorus, and organic carbon in Toolik Lake, Alaska, were assembled from data collected during 1977-81. The annual total organiccarbon (TOC), total nitrogen (TN), and total phosphorus (TP) loads to the lake were 8557,290, and 4.64 r n m ~ l ~ m - ~ . Inlet streams were the major source sf nutrients to the lake, as direct precipitation provided only 1,2, and 5%, respectively, of the annual TOC, TN, and TP loads to the Bake. Up to 30% of the annual N and P inputs to the lake from riverine sources occurred during the first 10 d of stream f8ow following breakup when cold water temperatures and snow-covered ice limited primary production. Due to the short water renewal time (0.5 yr), efficiency of nutrient retention was poor and 90,82, and 70% of the annual TOC, TN, and TP inputs to the lake were discharged at the outlet stream. Regeneration within the water column supplied40-66% and 68-78% of the N and P necessary for measured primary production. Yearly accumulation rates for C, N, and P in the sediment were about 220,21.0, and 1.75 mmol-m-5 Phosphorus remineralized within the sediment was completely retained due to adsorption onto Fe oxide minerals in the oxidizing surface layer. Annual rates of release of C and N to the overlying water column were 110 and 11.5-22.2 mm~ l - rn -~ . Mass balance considerations showed no serious errors in estimates of any terms of the annual sediment and water column 61, P, and organic C budgets.
Les bilans de I'azote, du phosphore et du carbone organique dans le IacToolik (Alaska) ont ete determines A ['aide des dsnnees recueillies de 1977 a 1981. Les charges annuelles de carbon organique total (COT), d'azote total (AT) et de phosphore total (PT) s'devaient respectivement a 8557, 298 et 4 , M mmoI-m-5 Les tributaires etaient les principales sources de biodements car les precipitations n'ont fourni que 1,2 et 5 % respectivement des charges annuelles lacustres de COT, d'AT et de PT. Jusqu'a 30 % des apports annuels de N et de P prsvenant de cours d'eau ont eu lieu pendant les 10 premiers jsurs apres la debicle des glaces, guand la basse temperature de l'eau et la couverture de neige sur la glace limitaient la production primaire. A cause du court temps de renouvellement des eaux (8,s an), I'efficacite de retention des bioelements etait [aible; 98,82 et 78 % des charges annuelles lacustres de COT, dfATet de PTont kt6 evacuees par I'emissaire. A I'interieur de la cslsnne d'eau, le regeneration a fourni 40-66 % et 68-78 % du N et du P necessaires A la production primaire quantifiee. Les taux annuels d'accumulation de C, du N et du P dans le sediment s'elevaient respectivement a environ 220, 21,Q et 1,75 mm~ l - rn -~ . Le phssphore remineralise dans le sediment a kte compl&tement retenu par adsorption aux oxydes mineraux ferrugineaux presents dans la couche superficielle oxydante. Les taux annuels de degagement du C et du N dans la colonne d'eau surjacente s'elevaient respectivement a 110 et 11,s-22'2 m r n ~ l - m - ~ . Un examen de I'eqhtilibre global n'a pas revel6 d'erreurs graves dans les estimations des facteurs lies aux bilans annuels de N, de Pet de C organique dans les sediments et la colonne d'eau.
Received lu/y 18, 1984 Accepted December 5! 1984 (J7878)
rom a chemical standpoint, early limnological investiga- tions in the arctic (reviewed by Hobbie 1973) generally consisted of sporadic observations on single lakes or small regions. More recently, detailed annual nutrient
cycles have been reported for culturally eutrophied Meretta Lake (Schindler et al. 1974a) and Char Lake (Schindler et al. 1974b; de March 19751, both in Canada, and for shallow thaw ponds near B m o w , Alaska (Prentki et al. 1980). Nonetheless,
'~gastitute of Marine Science Contribution No. 57 1 . "resent address: Department sf Oceanography, Texas A $L M
University, College Station, TX 77843, USA.
Recu /e 18 jus'88et 1984 Accepte le 5 decembre 1984
compared with other biogeographic provinces of the world, there is a dearth s f published information concerning annual elemental cycles in arctic lacustrine systems. We have e x m - ined various components of the N, B, and organic C cycles in an Alaskan arctic lake from 1977 through 1981. However, due to logistics problems it was not possible to assess all features of each elemental cycle during every field season. Therefore, the purpose of this paper is to assemble the available information to give reasonable estimates of annual inputs and outputs of N, B, and organic C to a nutrient-poor arctic lake and to understand further the function of an arctic freshwater ecosystem by assessing the role of phytoplankton in modifying nutrient cycles.
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Study Site
Except where noted, the following is from our unpublished data.
The study site, Toolik Lake (68"38'N; 149"38'W), is a deep arctic tundra lake located at an elevation of 720 m about 20 km north of Alaska's Brooks Range. The TFoolik watershed covers 65 h2 and the lake surface is 1.5 kmqor a ratio of catchment to lake surface of 43: 1. Tussock tundra dominates the vegetation and ground cover is nearly 380%. Bemafrost with a maximum thaw depth of about 0.5 m (K. Kielland, Inst. Arctic Biol., Univ. Alaska, pers. comm.) underlies the entire area. Annually, the temperature averages - 10°C with approximately 28 em of precipitation, equally divided between rain and snow (Brown and Berg 1980).
The main inlet stream to the lake, Inlet 1, drains 75% of the watershed including a series of 82 small lakes above Toolik. These have a total surface area of 1 -0 h" and seven that have been surveyed show an average maximum depth of 4.1 m (C. Luecke, Dep. Zool., Univ. Washington, pers. comm.). A secondary inlet (Inlet 2) drains 7% of the watershed, while several ephemeral streams on the lake's west side drain the remaining 18%. A single outlet is located on the north side of the lake. Flow commences in the major inlets and outlet during mid-May and terminates in mid-September.
Toolik Ldce consists of five basins separated by rocky shoals. Maximum and mean depths are 25 and 7 m, respectively, and the volume is 30.6 x 10%'. The lake is ice-covered from late September until late June. Thermal stratification is evident for a 5- to 6-wk period during July and August, with the maximum depth of the thermocline about 8-10 m. Maximum surface and bottom water temperatures are 16 and 7°C. Lake water is a calcium bicarbonate type with a total alkalinity of about 0.4 mequiv-LO'. The photic zone chlorophyll a concentration averages 1.3 p,g-L-', while I4C primary production is about 10 g C..m-"yr-'. A map detailing the bathymetq and location of inlet and outlet streams for Toolik Lake is given in Comwell (1985).
Streams, Lake Water, and Precipitation
Lake water was collected from 8, 1 ,3 ,5 ,8 , 12, and 16 m at a permanently established central lake station using a plastic Van Dom sampler or a submersible pump. Temperature detemina- tions were made by immersing a handheld thermometer in the pump outflow or by lowering a Yellow Springs Instrument Co. model 57 themister probe in the lake. Dissolved oxygen determinations were made immediately by the Winkler tech- nique (American Public Health Association 197 1) on Van Dom- collected samples ody. Inlet and outlet streams were sampled within 50 m of Toolik Lake by directly submersing rinsed 4-L polyethylene bottles into the main flow. Concurrent with stream water collection, midwater flow measurements were made with a Gurley meter at 3-m intervals in a cross-stream transect. Discharge was calculated as the product of current velocity and the cross-sectional area. Frequency of sampling was approxi- mately 10 d for lake profiles, daily during initial stream flow, and approximately weekly thereafter or following episodic s tom events for Inlet 1 and Outlet. Inlet 2 was sampled whenever flow was sufficient for accurate gauging. Precipita- tion samples were collected in polyethylene pans for chemical
analyses but no volume determinations were made. The duration of sampling (lake, streams, precipitation) was from mid-May or June through August, 1977-8 1.
Water collected for nutrient analysis was quickly returned to a field laboratory and filtered through ashed (6 h at 450°C) 47-mm Gelrnan A/E glass fiber filters. Filters were stored frozen and later analyzed for particulate phosphoms (PP), particulate nitrogen (PN), and particulate organic carbon (POC) according to the methods of Menzel and Cowin (1965), Sol6rzano and S h q (1980), and Menzel and Vaccaro (1964). In some cases, PN and POC determinations were made with a Perkin-Elmer 24OC Elemental Analyzer. Filtrates (frozen samples) were analyzed for nitrate (NO3- -t- NO2--N) and ammonium accord- ing to Strickland and Parsons (19721, while total dissolved phosphoms (TDP) determinations followed Menzel and Corwin ( 1965). Dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) determinations followed the methods of Sol6rzano and Sharp (1 980) and Stainton (1 973), respectively.
Phytoplankton "c-labelled HC03- and "N-labelled DIN (dissolved inorganic nitrogen = NO3- f NH4') uptake were measured at six depths through the photic zone at approximately 10-d intervals from mid-May or June though August in 1980 and 1983 according to methods described by Whalen and Alexander (1984). The entire data were time- and depth-inte- grated to calculate areal 14C and I5N primary production for the 1980 and 198 1 growing seasons.
Sediments
Losses of PP, BN, and POC from the trophogenic zone were determined at the central lake station. Two acrylic sediment traps 16.2 cm in diameter and 47 crn tall were placed l 00 m apart at a depth of 16 m (2 m above the lake bottom) for 3- to 9-d intervals from June through August 1981. Trap design and method of use follow Kirchner (1975). A 10-cm layer of NaCl solution marked with a dye was placed in the bottom sf each trap to establish a density gradient and to indicate disturbance. Upon collection, undisturbed samples were homogenized by continu- ous stirring and subsamples were withdrawn, filtered (ashed 47-mrn Gelman AIE glass fiber filters), and the filters analyzed as previously described for POC, PN, and PP.
Seven cores for solid phase chemistry were collected in 8980 using a KB corer, while five cores were collected in 1979 by subcoring an Ekman dredge. Both techniques yielded undis- turbed cores. Total P (TP) was determined on LiB02 digests (Medlin et al. 1969) using a colorimetric technique (Strickland and Parsons 1972). Total N (TN) and total C were determined using a Perkin-Elmer 240C Elemental Analyzer. Carbonate C was determined using the syringe gas stripping method of Stainton (1973). Total organic C (TOC) was calculated as the difference between total and carbonate C. Rates of nutrient acumulation in the sediment were determined by 210~b dating of the cores according to Comwell (1985).
Results and Discussion
Hydrology
Stream sampling commenced on the first day of spring flow only in 1980. Because our sampling season terminated at the end of August, it never encompassed the entire water year. Based on the data of Craig and McCart (1975) for the annual freeze-thaw regime of tundra streams in the Alaskan Arctic Foothill and Coastal Plain Provinces, we estimate that 2 wk of
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0 MY June duly Aug.
FIG. 1 . Temporal variations in water discharge rates at Inlet 1 and Outlet, Toolik Lake.
ungauged low-volume discharge occurred in the inlet and outlet streams prior to freeze-up in mid-September.
Rates of water discharge at Inlet 1 and Outlet varied over an order of magnitude during the 1980 thaw season (Fig. 1). High rates of discharge at Inlet 1 were observed for a week following breakup and within several hours of each major s tom event due to the rapid saturation of tundra with a shallow active layer. Variations in water discharge rates at Inlet 2 closely tracked those at Inlet 1 (data not shown). Changes in the water discharge rate at Inlet 1 were rapidly reflected at Outlet. Thus, variations in water flow both to and from Toolik Lake were closely tied to spring runoff and the occurrence and magnitude of precipitation during the thaw season. The annual pattern of flow described here was also observed in 1978 and 1981 (only other years for which we have stream flow data), but sampling did not include the entire spring runoff.
In 1980, Inlets B and 2 provided '7 1 and 9% of the total stream discharge to Toolik Lake, while diffuse sources accounted for the remaining 20% (Table 1). Volumes of flow given here are slightly lower than the true annual values because we do not include any estimate of the low-volume September discharge. Unpublished National Weather Service (NWS) data collected 1 5 h south of Toolik Lake from September 19'79 through August 1988 (no data available for April and July 1980) show a total precipitation of 23 cm. Linear interpolation between
TABLE 1. Summary of stream flows to and from Tool& Lake during 1980. Undefined inflow represents input from ephemeral and ungauged streams and is calculated as the difference between measured outflow and inflows assuming lake stage remained constant and evaporation and input from precipitation were negligible.
Inflow volume Water level (millions of m3) change (m)
Inlet 1 13-7 9.19 Inlet 2 1.7 1.14 Undefined inflow 4.8 2.68 Total inflow 19.4 13.02 Outlet 19.4 13.02 Water renewal time (yr) 0.5 Sampling season 13 May - 31 August
80 DOC ---- nYC--8--be-----_- -----
Moy June July Aug. 1980
FIG. 2. Temporal variations in concentrations of soluble and particulate C, N, and P at Inlet 1, Toolk Lake. All values as pmol .~ - ' . TDP, total dissolved P; PP, particulate P; BIN, dissolved inorganic N; PN, particulate N; POC, particulate organic C; DON, dissolved organic N; DOC, dissolved organic C.
months to estimate values for the missing data increases the total to 30 cm. The estimated sleam input to the lake during 1980 was 13 m equivalent of water (Table 1). Thus, direct precipita- tion supplied to Toolik Lake a volume of water roughly equivalent to 2% of the stream input.
Estimates of evaporative water loss from arctic lakes are sparse. Dingman et al. (1988) indicated annual rates of evaporation on the order of 9.6- 1 1.2 cm in the Bmow area. Miller et al. (1988) and Brown et al. (1968) reported pan evaporation values of 21 and 16 cm, June though August, for the same area. Assuming a total evaporation of 15 cm, an amount equivalent to 1 % of the 1980 total stream inflow was lost as evaporation from the lake surface. Thus, direct precipitation and lake water evaporation were minor components of the annual hydrologic regime of Toolik Lake.
Stream, Lake, and Precipitation Chemistry
During 1980 all chemical characteristics at Inlet 1 generally showed elevated levels at the initiation of stream flow in mid-May (Fig. 2). Concentrations dropped rapidly until early June and remained more or less constant or decreased slightly with time thereafter. The data for BP and TBP conform best to this pattern, while those for DIN deviate in that increasing and sporadically high concentrations were found in July and August. Increased NO3- levels were responsible for the mid- to late-season DIN increase, as N H ~ + concentrations at Inlet 1 were relatively invariant at about 0.2 pmol-L-' from June through August. The increased soil active layer depth, greater potential for soil nitrification due to warming of the tundra, and subsequent leaching may have led to the observed mid- to late-season increase in stream water NO3-. Gersper et al. (1980) summarized several unpublished studies of nitrification in tundra soils including one that implicated temperature as a major limiting factor.
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By comparison sf Fig. 1 and 2, we suggest that the rate of stream discharge had little influence on concentrations of chemical constituents. Most solutes and particulates remained relatively unaffected by temporal variations in flow rates, simply decreasing in concentration as the thaw season pro- gressed. Correlation coefficients between water discharge rates at Inlet B during 1980 and concentrations of nutrients measured therein were never statistically significant (P > 0.01 ; df = 15- 17; r = -0.09-0.41). On the other hand, highly significant (P<0.01; df- 15-18; r = 0.60-0.83) correlations were found between increasing time from the initiation of flow and decreasing stream water concentrations of DOC, POC, TDP, PP, m N , and PN.
The temporal patterns outlined here with respect to stream water concentrations of N and P held for 1978 and 198 1 as well, except that early season levels were not as high because runoff was already in progress when sampling was initiated. When sampled, Inlet 2 and several smaller inlets with discontinuous ffow were higher in particulates and lower in concentrations of dissolved C , N, and P.
The inflowing water to Toolik Lake was high in dissolved organic material (Fig. 2). Discharge-weighted mean concentra- tions of DOC and POC at Inlet 1 in 1980 were 625 and 29 p,mole~- ' . Corresponding figures for DON, PN, and DIN were 19.6,2.1, and 0.50 pmol .~-8 . Thus, DOC was more than 20 times as abundant as FQC, while DON levels exceeded DIN + PN by a factor of about 7. Discharge-weighted mean concentrations of TDP and PP at Inlet 1 in 1980 were 0.3 1 and 0.12 p,mol*L-' . Sohble reactive phosphorus (SRP) concentra- tions were generally not determined, but when measured represented 23-68% of the TDP. Thus, a large fraction of the TDP was in the organic phase, but probably not as great a fraction as for total dissolved nitrogen (TDN = DIN 4- DON). The data of Schindler et al. (1974b) for Char Lake inlet water show reduced levels of TDP, PN, PP, and DON relative to Toolik Inlet 1 water, roughly equivalent concentrations of POC, and higher levels of DIN. The dominance of the dissolved organic and particulate fractions in the Toolik inflow un- doubtedly reflects the nearly complete plant coverage of the drainage as opposed to only 5-7% coverage in the Char Lake watershed (Schindler et al. 19743).
In t ems of relative chemical composition, water at Outlet was similar to that at Inlet 1 during 1980 (cf. Fig. 2 and 3). In addition, the general trend toward a rapid decline in concentra- tion of most chemical parameters to June was clearly evident. The decrease in nutrient levels with increasing time from the initiation of spring flow was also observed in the other study yeas. Similarities between influent and effluent waters in tems of chemical character and seasonal trends resulted from the short water renewal time of the lake, 8.5 yr (Table I).
The range and mean concentrations of chemical variables for Toolik Lake differed somewhat from other arctic systems (Table 2). Bmow ponds had higher levels of all forms of C, N, and P than Toolik Lake. Sediment-water interactions in the shallow (mean depth 0.4 m; Miller et al. 1980) ponds un- doubtedly serve to increase water column concentrations of chemical constituents. Concentrations of DIN, TDP, and all particulate species were somewhat higher in Tsolik than in Char Lake. Levels of DON were probably markedly higher in Toolik than in Char Lake, as TDN average 5.4 ymol .~- ' in the latter (Schindler et al. 1974b).
Dissolved oxygen profiles for Toolik Lake indicate that the bottom water remained oxic both during the long arctic winter
0.4 a 0
0.2 a- e,
0.0 May June duly Aug.
FIG. 3. 'Temporal variations in concentrations of soluble and particulate C, N, and P at Outlet, 'Toolik Lake. All values as yrnol-l-l.
TABLE 2. Comparison of ranges and mean values (yrnol-L- I ) of several chemical characteristics of 'Toolik Lake in 1980 with other arctic environments.
Pameter Toolik Lake Char Lakea Barrow pondsb
Dissolved 267-783 - 375- 1290' organic C 576 g7.5 -
Particulate 8-57 - 17-121 organic C 18 14 5 1
N03--N 0-3.29" 0-2.14 -
0.46 - 1.44
NH~'-N 0-0.28 - 0.68-6.07 0.17 <O. 14 2.32
Dissolved 11-30 - 57-100 organic N 17 60
Particulate N 0.8-6.0 0.5-1.9 2.2-1 1.4 2.1 1 . 1 4.3
Total 0.09-0.59 -
dissolved P 0.11 0.06 0.45
Particulate P 0.03-0.18 0.03-0.16 -
0.12 0.86 0.33
"From Schindler et al. (1974b). %om Rentki et al. (1980). "Estimated from Premaki et al. (1980, p. 152, fig. 4-30). d ~ e a n during open water perid = 0.10 pmol-L- ' (see text for
details).
and the brief summer period of themal stratification (Fig. 4). As a result, the lack of a chemocline, even at the August peak of thermal stratification (Fig. 51, is the most conspicuous feature of the annual chemical regime in Toolik Lake. Early season data for PN and NO3- provide exceptions to this general lack of vertical structure in water column nutrient concentrations. For a 2-wk period in late May, PN values just beneath the ice were about 2 times higher than in the remainder of the water column,
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0 Dissolved Oxygen % m g C' 1 8 2 4 6 8 80 82 14 I6
0 2 4 6 8 10 I2 14 I6 &A Temperature ( " C )
FIG. 4. Typical midsummer and midwinter profiles for dissolved oxygen md temperature, Toolik Lake.
while NO3- concentrations >3 Fmol .~- l were observed at depth. Analysis of chlorophyll a and productivity data indicates that the former anomaly was associated with an under-ice algal bloom, while the latter probably represented overwinter nitri- fication. Klingensmith and Alexander (1983) found active nitrification in 'Psolik Lake sediment, and our unpublished experiments using a 15N isotope dilution technique demonstrate the potential for nitrification in the bottom water during the winter. Schindler et al. (1974b) also reported a general lack of vertical structure for N and P in Char Lake as well as elevated under-ice levels of NO3-.
The chemical composition of rain and snow in the Toolik area in 1980 and 1981 was extremely variable as indicated by the large standard deviations about sample means (Table 3). This has also been noted in other investigations of precipitation chemistry (see review by Chapin and Uttomark 1973). None- theless, our data are similar to those of Schindler et al. (1974b), Barsdate and Alexander (1975), de March (1975), and Prentki et d. (1980) for other arctic environments. These investigators reported mean concentrations of N h 9 and NO3- ranging from 5.1 to 18.2 and from 1.8 to 6.9pmol.L-' in rainfall with corresponding means for snowfall ranging from 0.6 to 4.2 and from 1.9 to 4.1 pmole~-l. Mean TDP concentrations in arctic rain and snow ranged from 0.31 to 0.53 and from 0.08 to 0.31 ~mol -L - ' , respectively. Concentrations of PP in arctic rain and snow ranged from 0.43 to 1.61 and from 8.85 to 0.35 pmol .~- l .
Annual Elemental Input via Streams and Precipitation
Concentration ( , u r n o i . ~ ' )
FIG. 5. Typical late summer chemical profile, Toolik Lake.
nutrient concentration therein, summed over the sample season and normalized to 1 m2 lake surface area. Input from "undefined inflow" was calculated as the product of the volume of H20 necessary to complete the water balance times the discharge- weighted mean nutrient concentration of Inlets 1 plus 2. The efficacy of this approach was tested by monitoring the (pre- sumed) conservative cation Na9; agreement to about 3% was found between the Na9 loads o f t he outflow and summed inflows. Except where noted, nutrient loads to the lake via direct precipitation were calculated from data in Table 3 and the unpublished 1979-80 NWS precipitation data cited earlier. Brentki et al. (1980) suggested that nutrient input to Bmow tundra ponds via dfy fallout was negligible. We have made that assumption here.
Although the estimates of input via direct precipitation are probably the weakest component of the annual external nutrient loading budget for Toolik Lake, nutrient input from this source was a minor component of the annual budget for the lake; direct precipitation provided only about 1 ,2 , and 5%, respectively, of the annual TOC, TN, and TP loads to the lake (Table 4). These values may in fact be overestimates, as it was assumed that the total snowfall to the ice surface from September 1979 through May 1980 remained intact until the spring thaw. In contrast with Toolik Lake, atmospheric input was of greater importance to the annual nutrient budget for Char Lake, supplying 1 1 - 12% of the annual C, N, and P loads (de March 1975). Bulk precipitation contributed about 20% to the annual N and P loads to the subarctic (68"N) Lake Stugs~on, Sweden (Jansson 1979), and comprised about 50% of the annual N and P inputs to Lake Rawson (Schindler et al. 1976) and Lake 227 (Schindler et al. 1973) at 49% in the Canadian Precambrian Shield.
The 15 September 1979 to 3 1 August 1980 TOC, TN, and TP loads to Toolik Lake were 8557,298, and 4.6 mmol.m-' (Table 4). Extrapolation of TOC, TN, and TP loading rates from the last week in August 1980 to estimate the nutrient input to the lake during the first 2 wk of September (last 2 wk of river flow) increased the annual loading rates by 1 .$-4.7%. We concluded,
The stream input of nutrients to the lake was calculated as the therefore, that the error introduced io the estimation of the 1980 product of stream water discharge (Inlets 1 and 2) times the Tool& Lake nutrient load through incomplete sampling of
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TABLE 3. Mean, standard deviation, and number of observations (n) for some chemical components of precipitation, Toolik Lake. All values as Fmol*~- ' .
Year F o m N03--N Dissolved Total Particulate
NH4'-N organic N dissolved P P
1980 Rain 1.6821.94) (13) 0.7720.71 (13) - - - Snow 2.52 (1) 0.28 (1) 4.3k2.9 (4) 0.1420.06 0.50k0.32 (5)
1981 Rain 1.29k 1.74 (9) 3.91 k2.44 (8) - 0.3326.23 (8)' -
Snow 1.55k2.03 (3) 2-24)? 1.63 (3) - 0.06k0.85 (3) 8.27k0.09 (3)
'Value represents total P.
TABLE 4. Annual loading rates for N, P, and organic @ in Tool& Lake. Strem inputs and outputs were measured fmm 13 May though 31 August 1980 and do not include approximately 2 wk of Bow volume flow in September. Estimates of precipitation input are from 15 September 1979 though 1% September 1980. All values as mrn~l-an-~ lake surface area.
Total stream Direct Total Stream Net % input
Parmeter inflow precip. input outflow (in-out) re tained
Dissolved organic C Particulate organic @ Total organic C Dissolved inorganic N Dissolved organic N Particulate N Total N Total dissolved P Particulate P Total P Na9
"Estimate derived from concentration in bulk precipitation at Char Lake (de March 1975). %istimate assumes equal concentration of DON in rain and snow. "Estimate uses concentration measured in 1981 rainfall and 1980 snow pack.
stream inputs was negligible. Although early season flow was not sampled, the years 1978 and 198 1 appear to have been lower water yeas than 1988 and measured nutrient loads to the lake were about 50% of 1980 values. Because of the high ratio of catchment to lake surface (Ad:& = 43: I), the nearly complete plant cover in the drainage, and year to year variability in annual water and nutrient budgets, the 1980 nutrient loading data for Toolik Lake were higher than for other undisturbed, oligotro- phic systems. For example, the combined data from Schindler et al. (1 974b) and de March (1975) for Char Lake (&:Ao = 8: 1) show annual loading rates of 208, 23, and 0.7 ~nnaol.m-~ for TOC, TN, and TP. In a 4-yr study, annual loading of TBC, TN, and TP to Rawson Lake &$Aa, = 6: 1) averaged 3290,90, and 2.1 mmol~rn-~ (Schindler et al. 1976), while the 5-yr mean annual TN and TP loads to Lake Stugsjon (Ad:& = 6: 1) were 21 and 0.3 r n m ~ l . m - ~ QJansson 1979). Clear Lake, Ontario (&:Ao = 1.4: 11, received 1.4 mmol T ~ m m - ~ in a 1-yr investigation (Schindler and Nighswmder 1970).
The percent retention of the annual nutrient influx to Toolik Lake (Table 4) was low compared with that of other nutrient- poor systems. Schindler et al. (1976) and de March (1975) found approximately 50-80% retention of the annual TN and TB loads to Rawssn Lake and Char Lake, respectively, while Schindler and Nighswander (1970) reposted 80% retention of the annual TP influx to Clear Lake. Over 5 yr, the percent retention of the spring TN and TP inputs to Lake Stugsjon averaged 20 md 14%, while corresponding values for summer
loading were 30 and 50% (Jmsson 1979). In the latter investigation, the low efficiency of nutrient retention in the spring is similar to the annual efficiency of retention in Toolik Lake (Table 4) and was attributed to the fact that the lake volume was renewed 2.6 times during spring runoff (Jansson 1979). Likewise, the short water renewal time in Toolik Lake (0.5 yr) was probably responsible for the poor nutrient retention efficiency. In contrast, water renewal times ranged from about 5 to 16 yr for the Canadian lakes discussed.
The temporal pattern of nutrient loading may have important ramifications in terns of nutrient availability for primary producers. Of the total 1980 influx of TP to the lake via Inlet 1, 30% entered during the initial 10 d of flow, while only 10% of the total water discharge to the lake from this source was received during the same period. Hobbie et al, (1983) demon- strated with Rhodamine dye that much of the under-ice flow from Inlet 1 through Toolik Lake followed the eastern shore and appeared at Outlet in 5 d. Within the first 15 d of flow, only 5% of the annual water discharge at Outlet was realized, but 12% of the 1980 TP efflux from the lake occurred. This effect was not as pronounced for TO&: or TN, but nonetheEess underscores the extreme impomnee of sampling the initial flow to Toolik Lake when calculating annual nutrient budgets, particularly for P. Furthemore, the data illustrate a situation where maximum utility of limiting nutrients (N, P) to lacustrine primary production is obviated by reception of a dispropoflionately large fraction of the annual input during a period when low water
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temperature and rapid attenuation sf incident radiation by ice and snow cover additionally restrict productivity.
Elemental Export from the Terrestrial Watershed
The water retention time of the ponds upstream fmm Toolik Lake was undoubtedly short, as flow at Inlet 1 increased dramatically within hours of major storms. It is therefore likely that nutrient retention in these ponds was negligible, and we calculated that nutrient export rates from the Inlet 1 watershed during 8980were 184,6.3, and0.09 m m ~ l . r n - ~ f o r ~ , N, andP. The TP ex ort rate is consistent with the 197 1-73 mean of 0.07 mmo1.m-fyrl we calculated for the Char Lake watershed from the data of Schindler et al. (1974b) and de March (1 973 , while the data for Toolik are higher than the 21 and 2.3 rnmolom-' calculated for the mean annual export of POC and PN from the Char Lake watershed. These differences are again attributable to the 5-7% plant cover in the Char Lake area as opposed to the nearly complete cover at Tool&, which lends itself to increased export of organic material. The TOC export value for the Toolik watershed is nearly identical to the mean value of 187 mmol* m-2ayr-' given by Mulholland and Watts (1982) for seven large Alaskan watersheds, most sf which were subarctic.
The 1980 nutrient load to the lake via Inlet 1 was 4.42 h o l TP and 305 h o l TN, while the total input to the Inlet 1 watershed via precipitation was 11.8 kmol TP and 308 kmol TN. This indicates that the tundra was approximately 60% efficient at retaining the annual TP input via precipitation, in agreement with the 50% efficiency calculated by Chapin et al. (1978) for Alaskan coastal tundra. However, in the Toolik tundra only about 1% of the TN input from precipitation was retained. The actual efficiency of N retention by the Toolik tundra is unknown due to the unquantified input from N fixation. Barsdate and Alexander (1975) suggested that 75% of the annual input of N to the arctic tundra is from N fixation and the efficiency of retention is 80%. In a 3-yr study, Alexander et al. (1 978) reported a mean annual input of 8.23 mmol ~Srn-' to the Bmow tundra by N fixation. This could theoretically add 480 h o l N to the Inlet 1 watershed annually, more than doubling the input from precipitation. Our data for efficiency of nutrient retention by the watershed assume uniform winter snowpack across the watershed (i.e. no aeolian import or export) based on NWS precipitation totals and are therefore approximations only.
Sedimentation and Diagenesis
The amount of particulate organic matter reaching the profundal sediment in Toolik Lake from June through August 198 1 closely tracked seasonal phytoplankton production dyna- mics (Fig. 6). Coefficients of variation between duplicate sediment traps averaged 9 + 8, 1 1 t 10, and 8 + 6% for C, N, and B ( n = 11- 12), indicating good agreement between traps with respect to quantity of material captured. Rates of removal of particulate C, N, and P from the water column ranged from 1.67 to 7.87,0.19 to 8.89, and 8.007 to 0.029 mmolm-'ads', respectively. Total flux to the sediment ( + s ~ ) , June through August, was 342 + 16,40 + 2, and 1.36 -f 0.04mmol-m-' for C, N, and P. Thus, the sedimenting tripton had the following time-weighted atomic ratios ( + s ~ ) : C:N, 8.5 -+ 0.6; C:P, 251 * 15; N:P, 30 + 2. If the atomic ratio of algal biomass is assumed to be about 104C:16N:lP (Vallentyne 1974), the sedimenting material was enriched in C relative to N and P and high in M relative to P with respect to algal stoichiometry. This
Particulate P
0.00 J he ne July A ug.
8981 FIG. 6. Phytoplankton primary production and flux s f particulate 6 , N , and P to sediment traps, 'Foolik Lake, 1981.
indicates preferential removal of N and particularly P from material leaving the trophogenic zone.
Radiocarbon and 1 5 ~ primary production (?so) from June through August 1981 totalled 735 2 15 and 67 2 1 raam~l.rn-~. Assuming a 16M: 1P atomic ratio for algal biomass (PN:PP ratio in lake averaged 17.5; Table 2), the corresponding value for P was about 4.2 mmol-m-5 The total allochthonous inputs of PN and PP to the lake in the same time period were 17 and 0.43 mmol-m-"no data for PO@). If this material is assumed to be refractory and totally sedimented or alternatively is assumed to be totally mineralized within the water column. limits can be set on the fraction of N and P primary production from June through August removed by the profundal sediment during the same period. Thus, we calculated that 34-60 and 22-32% of the N and P primary production was sedimented with 40-66% of the N and 68-78% of the P being recycled in the water column. A maximum of 47% of the algal C production also reached the sediment. In comparison, Kimmel and Goldman (1977) found that 27 and 19% of the POC and PN produced by phytoplankton in meso-oligotrophic Castle Lake, California, reached the sediment surface. These values are considerably lower than those calculated for Toolik Lake, particularly for N. However, the water turnover rate for Castle Lake in the summer was only 0.001 -d-I, suggesting that recycling within the water column provided essentially all of the DIN for phytoplankton production. Epilimnetic recycling provided 35-7096 of the N and 55-8596 of the P needed for measured primary production in two eutrophic Swiss lakes (Bloesch et al. 1977). Suqrising- ly, these values are nearly identical to those for oligotrophic Tmlik Lake. The short water renewal time may reduce the need for extensive water column recycling of nutrients essential for phytoplankton production in Toolik Lake.
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TABLE 5 . Sediment concentrations and '"~b-derived sedimentation rates of P, M, and organic C in Toolik Lake sediment. Concentrations expressed per gram total sediment and per gram total AZ in the sediment. All concentrations and burial fluxes based on 12 cores.
Total organic @ Total N Total P
Concentration m o l - g - ' m o l n ( g ~ 1 ) ~ ' m o 1 . g - m o l . ( g Al)-' rnrn~l .~ - ' mmol . (g All- ' Eange 4.08-9.55 88- 188 0.44-0.74 8.4-26.9 0.057-0.194 0.43-1.26 % + SD 5.68&1.19 122935 0.5490.10 12.7+3.2 0.078k0.038 0.94k0.29
Flux mol-m-"yr-' mrnol-~n-~-yr-' mmol - m-"yr- ' Range Whole I&ea M o l e l&eb Sediment trapsc
- -
"Estimate based on flux rates weighted by fraction of lake bottom in depth intervals. b~stirnate based on mean wsntrient:Al ratio and whole I&e Al flux rate. 'Estimate based on extrapolation to annual rate from data collected 1 June to 31 August 1981 (see text for details).
These June through August 1981 sediment trap data were extrapolated to an annual basis giving loss rates from the trophogewic zone of 387, 41-50, and 1.4-1.7rnmol-m-~ for POC, PN, and PP. In making these estimates the following assumptions were made: (a) the proportion sf the annual 14C and I5N primary production that occurred early (15 May - 1 June) and late (1-15 September) in the growing season was constant from 1979 to 1981 (we have early and late season productivity data for 1979 and 1980 only), and of the early and late season primary production, 47% sf the C, 34-60% of the N, and 22-32% of the P left the trophogenic zone; (b) the conversion of inorganic N and P to algal biomass occurred in an N:P ratio (atoms) of 16 early and late in the growing season; and (c) the measured stream input of PN and PP to the lake (no data for POC) during 1981 was 70% of the annual total stream input (see Annual Elemental Input via Streams and Precipitation) and the total unmeasured PN and PP input from streams left the trophogenic zone.
Suficial sediment concentrations of TOC, TN, and TP averaged 5.68,0.54. and 0.078 mmol-g-' (Table 5). The molar TOC:TN ratio (X 5 SD) of surficial sediment, 10.5 * 1.5, was significantly higher than that of freshly sedimenting material (Student's &-test; df = 9; P < 0.01), indicating a greater rate sf N than C mobilization therein. Ratios (x 5 SD) of TBC:TP and TN:TP in suriicial sediment, 80 k 23 and 7.8 + 2.4, were significantly lower than those for sediment trap material (Student's &test; df = 9; P < 0.01 both cases), indicating greater retention of P than C or N in sediment. This resulted from the strong adsorption of inorganic P onto Fe oxide minerals in oxidizing surficial sediment, which has been shown (Morti- mer 1971; Williams et al. 1971, 1976; Prentki et al. 1980) to prevent diffusion of remineralized P into the water column. Comwell ( I 983) found a mean ( a e = 3 1) inorganic Fe:P ratio of 44 in Toolik Lake surfficial sediment. This value greatly exceeds the minimum of 7 suggested by Williams et al. (1976) for effective sediment P retention. In addition, postdepositional migration may have increased the P concentration in the surficial sediment, thereby decreasing the C:P and N:P ratios. This mechanism has been shown by Caignan and Flett (198 1) to enrich sediment horizons with P in freshwater systems. Comwell (1983) gave evidence that this occurred in Toolik Lake.
The utility of the 'l0Pb sedimentation technique for measur-
ing nutrient accumulation rates in lake sediment has been demonstrated by Kipphut (1978) and Evans and Wigler (1980; 1983). However, use of this method for calculating nutrient accumulation rates in Toolik Lake is complicated by the postdepositional migration of Mn, Fe, and P. This diagenetic enrichment of surficial sediment with Mn and Fe oxides dilutes the '60riginal" sediment particles with these elements (Comwell 1983). It was therefore necessary to normalize TOC, TN, and TP concentrations in Toolik Lake sediment to a sediment component relatively unaffected by diagenesis. We chose total Al as the basis for expressing sediment nutrient concentrations, while sedimentation rates are expressed as Al flux (Comwell 1985). Estimation of whole lake TOC, TN, and TP accumula- tion rates were made in two ways. The first consisted of weighting the mean accurnulatlon rates in five depth intervals with the proportion of lake bottom encompassed by each interval (see Cornwell 1985). The second approach involved multiplication of the whole lake Al input rate (1.8g %ale n ~ - ~ ~ y r - ' ) by the mean nutrient:Al ratio for the whole lake. The two methods gave essentially the same results (Table 5).
The "'Pb-derived rates of deposition of TOC and TN within surficial sediment were 57 and 92-5 1 % of the rates estimated from sediment trap data (Table 5). The difference results from mineralization of organic material at the sediment-water interface. Comparison of sediment trap and " '~b data for P again suggests efficient retention within the sediment.
Sediment accumulation rates of N and P were 40 and 126% of the net inputs to Toolik Lake from the 1980 mass balance (cf. Tables 4 and 5). Nearly identical differences (43 and 129%) were found between measured and estimated (mass balance) N and P accumulation rates for Char Lake (de March 1978). He concluded that the measured and estimated accumulation rates were in reasonable agreement considering the high year to year variability reported for loading rates in long-term lake nutrient budget studies and the fact that the accumulation rates were historic averages. We extend the same reasoning to the Toolik Lake data.
The annual rates of accumulation sf TOC, TN, and TP in Toolik Lake sediment (Table 5) were 2.6,3.4, and 3 -0 times the rates reported by de Mach (1978) for Char Lake. This may reflect the increased potential for dlochthonous input to Tmlik Lake due to the nearly complete plant cover of the watershed.
The microbial decomposition of organic matter in lake
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O R G A N I C C A R B O N
118 I DISSOLVED INORGANIC
I TOTAL
FIG. 7 . Organic carbon cycle, Tmlik Lake. All fluxes normalized to lake surface uea. Flux rates to and from water column via stream flew and primary production derived from data collected in 1980, while rate of particulate loss from water column to sediment estimated from 1981 sediment trap data. Rates of permanent sedimentation and return of @ fmm sediment determined from cores collected in 1979 and 1980. Organic @ input via snow and rain estimated from National Weather Service data for precipitation in Tool& area md bulk precipitation chemistry for Char Lake area (de Much 1975). See text for methodological details.
sediment results in progressively lower concentrations of TOC and TN with increasing depth of burial. Three complete vertical sediment profiles in Toolik Lake showed that (% + so) 50 2 7% of the TOC and 55 + 3% of the TN originally deposited was remineralized within the top l O cm of sediment. Vertical profiles of the loss on ignition (500°C) of organic matter from six cores showed that (% 2 so) 57 + 15% of originally deposited material had been remineralized. This corroborates well the above data for percent remineralization of sedimented C and N. Furthermore, the Toolik Lake data are comparable with those for Castle Lake where Kimmel and Goldman (1977) found that 49 and 68% of C and N reaching the sediment was remineralized
1979-88 overwinter accumulatisn of DIN in the water column (stream flow nil and phytoplankton activity minimal). This gave an N remineralization rate of 22.2 m m ~ l . r n - ~ y r - ~ . Although this rate is about 2 times the long-term average remineralization rate given above, it again reflects a single yea 's data as opposed to a historic average. There is no available information concerning rates of C and N remineralization in sediment of arctic lakes. However, the rate of N remineralization calculated for Toolik sediment is considerably lower than the rates cited by Fenchel and Blackbum (1979) for several temperate lakes (range: 529-2 197 mmol-rn-'y-I).
following initial deposition. The above data yield whole lake C and N remineralization Annual N, B, and Organic C Cycles
rates of 1 10 and -1 1.5 r n m ~ l n m - ~ - ~ r - ' for Toolik Lake. In Flow diagrams for organic C, TN, and TP in Toolik Lake addition, the rate of N remineralization and release to the water were constructed from our data (Fig. 7 , 8 , and 9). Input-output column was also obtained by extrapolating to an annual rate the budgets for the water column balance to within 11, 8, and 7%
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N I T R O G E N
0.7 DISSOLVED lNORGAMlC
STREAM INFLOW STREAM OUTFLOW
41 - 50 PARTICULATE
11.5, 22.2 DISSOLVED INORGANIC
I SEDIMENT
FIG. 8. Tool& Lake nitrogen cycle. As for Fig. 7, except precipitation chemistry determined on locally collected samples in 1980 and 1981 and estimates of DIN flux from sediment to water column are caleullated from sediment N profiles and extrapolation from 1979-80 overwinter BIN accumulation in water column.
for N, P, and organic C. This indicates that no serious errors exist in our estimation of the dominant input-output tems, namely the riverine influx and efflux for N, P, and organic C as well as the burial tern for P. Sediment mass balances agree to within 5-42, 1-30, and 8% for N, P, and organic C. We accept a greater error for the sediment mass balances because flux rates to the sediment are for a single year, while burial tems and rates of return to the overlying water are time-averaged (exception: for N we also provide a 1-yr estimate sf flux from sediment).
We give here an overview of the N, P, and organic C cycles in Toolik Lake. Of particular importance, we provide the first published information concerning the role s f phytoplankton in the nutrient budget for an arctic lake. The total data represent a major contribution to the scant regional limnology of the North American arctic and will prove valuable for management purposes as public access t s the Alaskan arctic increases. Nutrient input via precipitation and rates of nutrient release from the sediment are the weakest components of our budgets. Also, we provide little infomation on year to year fluctuations in the magnitudes of various tems in the N, P, and organic C budgets.
We recommend a long-term investigation sf important comp- nents of these elemental cycles in SFoolik Lake with particular emphasis on determination of nutrient fluxes to the lake water via release from the sediment and direct precipitation.
We thank V. Alexander, G. K. Kipphut, S. F. Sugai, H . E. Welch, and an anonymous reviewer for their valuable comments on early drafts of this manuscript. D. L. Witt provided the logistic support necessary to undertake this study at a remote field location. M. C. Miller generously supplied water temperature data for the summer of 1978. Financial support for this work came from State of Alaska (J.C.C.) and Noyes Foundation (S.C.W.) fellowships as well as from NSF grants DPP79-008 15 and DPP77-23475 to. V. Alexander and R. J. Barsdate.
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ALEXANDER, V. , M. BILLINCT~N, AND B. M. SCHELL. 1978. Nitrogen fixation in arctic and alpine tundra, p. 540-558. In L. L. Tieszen [cd.] Vegetation and production ecology of an Alaskan arctic tundra. Springer-Verlag, New York, NY.
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P H O S P H O R U S
RAIN & SNOW
DISSOLVED
009 I PARTICULATE lo,'5
STREAM INFLOW STREAM OUTFLOW
PAWTBCULAT PARTICULATE
1.4 - 1.3 PARTICULATE
4
SEDIMENT
1.69 - 1.81 TOTAL
BURIAL
FIG. 9. Toolik Lake phosphoms cycle. As for Fig. 7, except precipitation chemistry determined on locally coBBeeted samples in 1980 and 1981.
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Can. J . Fish. Aquar. Sci., Vol. 42, 1985
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