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ARCTIC VOL. 43, NO. 2 (JUNE 1990) P. 129-136

The Underwater Photic Environmentof a Small Arctic Lake P.H. HEINERMANN,' L. JOHNSON2and M.A. ALII

(Received 27 February 1989; accepted in rmised form 17 October 1989)

ABSTRACT. Theunderwater light fieldinasmallarcticlakeon Victoria Island,,was examined. Downward radiancewas found to be bimodal, with transmission peaksat 480 and 640 nanometres (nm,or 10-9m). Upward radiancewas similar nearthe surface, with peaksat 480 and 620 nm, but becameunimodal with depth andshifted to580 nm near the bottom. Diurnal variation inthe underwater downward and upward irradiance of PAR (photosynthetically active radiation) was approximatelytwo orders of magnitude. The spectral qualityof light transmission also changed over this24 hour period. Unimodal transmissionof red light occurred inthe early morning(1:OO and 5:OO) and late evening(22:00), while bimodal transmissionof blue-green and red light was observedduring the day(900-1730). Kd(z,), the vertical attenuation coefficient downwardfor irradiance at themidpoint of the euphotic zone, was relatively insensitiveto changes in solar elevation. Diurnal variation the in reflectance of PAR differed from that predicted by previous simulation models, whilethe inverse relationship between reflectanceand the absorption coefficient was in agreement with these same models. Gilvin, humic material-dissolvediron complexes, algal fucoxanthin, chlorophylla and triptonall contribute to theattenuation of light and areresponsible forthe unique underwaterspectral transmission in Keyhole Lake. Key words: arctic, , underwater light, diurnal variation, spectral quality, irradiance, radiance

RESUME. L'environnement photique sous-marin d'un petit lacarctique sur l'Isle Victoria, Territoires du Nord-Ouest a ete examine.La radiance descendante aeu unspectre bimodal avec deux depics transmission B 480 et a 640 nanometres (nm ou lt9m).La radiance ascendante a et6 semblable pres de la surface avec les mCmes deux pics(480 et 620 nm) mais avecune augmentationde la profondeur la transmission estdevenue unimodale et a et6 deplacepres du fond vers580 nm. La variation quotadienne de l'irradiance sous-marine descendante et ascendantede la PAR (la radiation disponsible pour la photosynthese) a et6a peu pres deux ordresde grandeur. La qualite spectralede la transmission lumineuse a egalement change pendant cette periodede 24 heures. Une transmissionunimodale de la lumiere rouge eua lieu tbtle matin (1:OO et 500) et tardle soir (2200) tandis qu'une transmission bimodalede la lumiere bleue-verteet rougea et6 observedurant le jour (900-1730).Kd(z,), le coefficient verticald'attenuation de l'irradiance descendante aumilieu de la zone photique,n'a pas vraiement change avecdes changements de l'6llevation solaire. La variation diurne de la reflexionde la PAR s'estdiffer6 de celle predit par desmodeles de simulation precedents tandisque le rapport inverse entre la reflexion et la coefficient d'absorption a et6en accord avec ces mCmes mod6les.l.a matiereorganique dissoute, les composesde matiere organique- fer dissout, la fucoxanthinedes algues, le chorophyllea et letripton contribuent tous1 l'attenuation de la lumiere et sont responsablepour l'unique transmission spectrale sous-marine dans le lac Keyhole. Mot cles: arctique, limnologie, lumiere sous-marine, variation diurne, qualit6 spectrale, irradiance, radiance

INTRODUCTION been less studied (Talling, 1971; Spence et d., 1971; Kirk, 1977b, 1979; Bowling et al., 1986), while detailed investiga- As sunlight penetrates the surface of natural waters, its at- tions of the underwater light in arctic lakes are tenuation with depth is brought about by a combination of virtually absent. Some arcticstudies have during the course scattering and absorption processes. The factors primarily of their limnological surveys measured relative transparency responsible for this attenuation are the water itself, dissolved using a secchi disc (McLaren,1964; Oliver, 19641, determined organic matter (gilvin or "Gelbstoff"), suspended non-living extinction coefficients forbroad regions of the visible spec- particulate matter (tripton), the photosynthetic biota (phyto- trum (Hobbie, 1962; Schindler et al., 19741, or have presented and macrophytes where present), bacterioplankton limited downward photosynthetically active radiation spec- and zooplankton (Furch et aZ., 1985; Hulburt, 1945; Kirk, 1976, tra (Kalff, 1967; Schindleret al., 1974). 1985; Robarts and Zohary, 1984; Ruttner and Sauberer, 1938; The objective of this study is toprovide a detailed descrip- Tilzer, 1980; Westlake, 1980). Thetransmission of light tion of the underwater light field of a small arctic lake underwater and its spectral composition have been found (Keyhole Lake) and, in addition, to investigate diurnal to vary with depth, location and season (Dubinsky and variation in the intensity and spectral quality of this unique Berman, 1979; Kirk, 1977a; Roemerand Hoagland, 1979). photic environment. This work is part of a continuing Maximum transmission of light underwater can vary across ecosystemstudy (Hunter, 1970; Johnson, 1976,1983; the entire visible spectrum. , the closest freshwa- Vanriel, unpubl.data) relating to primaryproduction, ter equivalent to distilled water, has an average maximum trophic level energy dynamics and arctic charr production in transmission near 420 nm (Tyler and Smith, 1967). Lakes this lake. having a relatively high biomass tend to transmit better in the green part of the spectrum (Yentsch, SITE DESCRIPTION 1960; Jewson, 1977; Schanz, 1986). As the concentration of gilvin and tripton increases in fresh waters, transmission Keyhole Lake is locatedat 69'22'45" N latitude and 106'14' maxima of underwater light shift to the yellow and red parts W longitude on Victoria Island, Northwest Territories. It is of the spectrum (Kirk, 1976,1985; Visser, 1984). found in a shallow valley at an elevation of 27.4 m above Studies of the underwater photic environment in marine level. The only input is from melting of the active surface systems are numerous (Jerlov, 1951; Herjerslev, 1975; McFar- layer, which provides a continuous filtered summer water and Munz, 1975; Morel, 1982).Freshwater systems have supply.

'Universite de Montreal, Departement de Sciences biologiques,C.P. 6128, Succ. 'A,Montreal, Quebec, H3C 3J7 2Freshwater Institute, Departmentof Fisheries and , 501 University Crescent, Winnipeg, Manitoba, CanadaR3T 2N6 OThe Arctic Instituteof North America 130 / P.H. HEINERMANN et al.

METHODS Irradiance and radiance series were recorded at 0 m (just Water samples weretaken with a Van Dorn type sampler below the water surface) and at 1 m intervals to the bottom. at a depth of 0.5 m at four locations in Keyhole Lakeon 28 and Measurement of a complete vertical profile required 10-15 29 July 1985. The parameters that were ana- min. The effectof boat shadow was eliminated bylowering lyzed are found in l. Analytical methods used were the measuring device on the sunny side of the boat byhand to the desired depth. A measurement of in-air downward those of Stainton et al. (1977) with the following modifica- tions: alkalinity - Gran titration; chloride, sulphate and quantum flux was takenat the beginning of each downwel- organic acids - ion chromatography; total dissolved nitro- ling and upwelling series to ensure that little variation in gen and total dissolved phosphorus -ultraviolet photocom- incident radiation was occurring over the measurement in- terval. All recordings werestarted only under cloudless skies bustion and chlorophyll u - the solvent used was methanol (P. Vanriel, pers. comm. 1986). was measured on and taken within 1 h of the sun's maximum altitude. Diurnal variation in the underwater light flux and spectral quality site with a Hach Ratio turbidity meter (model no. 18900-00). Estimates of the dissolved colour or gilvin concentration as was investigated over a 24 h period at 1 m depth andhourly g,, (Kirk, 1976)were calculated as 2.303 A/0.1 m-l, where A intervals on 24-25 July 1985. Withthe downwelling (E,) and wasthe measured absorbance of a membrane-filtered upwelling (EU)irradiance data obtained at each interval, the (Whatman GF/C glass fibre and Millipore GS 0.22 ymfilters) downward (K,)and upward Mu) vertical attenuation coeffi- water samplein a 10 cm cuvette. cients, as well as irradiance reflectance values (EU€,*), were Underwater downward and upwardirradiances for PAR calculated. Solar elevationswere computedusing the method were measured on21,22 and 30 July 1985 at the deepest point described byKirk (1983). An estimationof the absorption (a) in the pelagic zone (6.5 m). The instrument used to record and total scattering coefficients (b)from the diurnal measure- irradiance was a LI-COR LI-185quantum meter and a LI-192s ments followed the procedure outlined by Kirk (1981a). underwater quantum sensor. The spectral quality of the underwater light was measured concurrently as radiance RESULTS with an underwaterspectroradiometer. A completedescrip- The water quality analysis of Keyhole Lake water is pre- tion of this wide-angle radiance meter is found in Heiner- sented in Table 1. Nutrient element levels in Keyhole Lake mann and Ali (1988). were lowand conductivity was moderate. During sampling the water column was completely mixed and isothermal at 11°C. Chlorophyll a concentrations were also very low. pH values indicate that the lake was basic in . Turbidity TABLE 1.Water quality analysis of Keyhole Lakewater on 28 and 29 was moderate andgilvin concentrations were low. July 1985 Since the incident flux and the spectral distribution of the Parameter Mean (S.E.)* downwelling and upwelling light inKeyhole Lake were similar on 21,22 and 30 July 1985, only the data for 30 July are Alkalinity" peql-l (7.0) 1330 presented. A description of the spectral distribution of the Calciuma 13 500 (40) pgl" photic environment on this date will be followed by an Chloride" 22 500 (300) pgl" examination of the diurnal changes in the underwater light Chorophyll aa 0.6 (0.02) pgl-l Conductivity at 25°C a 214.5 (1.0) pScrn-' field. Dissolved inorganic carbona 1600 (71) pgl" In Figures la and lb, bimodal spectra are evident in the Dissolved iron" 40 (0) pgl" upper 1 and 2 m of the downward and upward radiances Dissolved organic carbon' 395 (12) pgl" respectively. Transmission maxima are found near 480 nm Dissolved oxygenb 9.28 (0.50) mgl-Iat 8.94 (0.01) "C Gilvin (gwY 0.60 (0.07) m-l (blue-green light) and 640 nm (red light). As depth increases Magnesiuma 9380 (20) ygl" the spectra become unimodal, witha loss of the short wave- Manganese" <20 pgl-1 length component and a shift to long wavelength transmis- Organic acids" 32 (11.7) peql-I sion. In Figure lb, blue-green light transmission appears pH" (8.04-8.12) pH" range slightly greater than red light penetrance nearthe surface. In Potassium" 1180 (40)pgl" Primary productivityb 2.3 (0.86) mgCm-3day1 addition, the long wavelength peak is located near620 nm. Secchi disc" 2.9 (0.6) m Near the bottom unimodal spectra are again found, with Sodium" 12 280 (350) pg1-I maximum transmission occurring near 580 nm. Measured Soluble reactivephosphorus" 1 (0) pgl" upward radiance curves extended over a smaller range of Soluble reactive silicon" 148 (3)pgl" wavelengths (less wide) than downwelling ones. Sulphate" 6250 (250) pgl" Suspended carbon" 1162.5 (362) pgl-l Figures 2a and 2b present the diurnal variation in the Suspended nitrogen" 157.8 (45.6) pgl-1 downward and upwardirradiances of PAR on 24 and 25 July Suspended phosphorus" 7.75 (0.48) pgl" 1985. Even though at this latitude on these dates there are Total dissolved nitrogen" 440 (35) pg1-I about 22 h 12 minof daylight, the sun's altitude does change Total dissolved phosphorusa 15 (1) pgl-l Turbidity' 5.09 (0.04) NTU substantially (maximum altitude was 33.3" for 24 July) and Water " 11.o (0) "C results in a variation inthe underwater light flux of more than three orders of magnitude. Increases and decreases inirradi- "Data fromVan Riel (unpubl.). bData fromHunter (1970). ance are monotonic except for noticeable changes in the 'Data from this study. measurements at 13:30 and 17:OO Summer Time 'Number of samples = 4, except for secchi diskand gilvin, where n = 2. (20:30 and 24:OO Greenwich Mean Time),where the sun was UNDERWATERPHOTIC ENVIRONMENT / 131

A l- A TE 0.003 -a $ lo4 a E E T -0 rUI E, io3 1' Q Air E Y - K g! 0.002 10' I 2 Y -0 8 E .-mE x 0.001 ii :K $ 10" p"E 0.000 0 0 5 10 15 20 25 500 700 n 400 600 Time (hr) Wavelength (nm)

0.0003 b E air l- rUI E 5 0.0002 E Y E; E, + 4rn

E8 m 0.0001 :a z 5 0 5 10 15 20 25 = 0.0000 1 Time (hr) 400 500 600 700 RG.2.Diurnal variation in the (a) downward(b) upward and irradianceof PAR Wavelength (nm) in KeyholeLake on 24 and 25July 1985. Solarelevations at measurement times are: 01:00 (-5.8"), 0200 (-3.9'), 0300 (-0.9O),04:00 (3.1"), 05:OO (7.69, 0600 FIG. 1. Downward (a) and upward(b) radiance at eachdepth in KeyholeLake (12.6"),0700(17.7°),08:00 (22.6"),09:00(26.9"),1000 (30.3"), 11:OO (32.5"),13:30 on 30 July 1985. Note the bimodal spectral distributionin the upper2 m and (31.6"),1430(28.7"), 15:N (24.8"),16:30(20.2"), 1730 (15.2"),1830(lO.lo), 1930 1 m of downward and upward radiancerespectively. (5.3'), 22:OO (-3.9"), 2300 (-5.8"), 24:OO (-6.4")and 25:OO (-5.8"). partially obscured by a thin cloud cover. The greatest inci- midpoint of the euphotic zone, 4 m) and the average K, have dent downwardflux was foundat 15:30. Theupward irradi- been computed for each time interval and presented with ance displayed a similar pattern of change and was about onerespect to solar elevation (p) in Figures 4 and 5a. Figure 4 order of magnitude less intense than the downward flux. shows Kd(zm)to be relatively insensitive to changes insolar Diurnal changes in the spectral quality of the underwater altitude, whereas the average K, appears to be somewhat photic environment are shownin Figures 3a and 3b. Meas- larger at low and high solar elevations (Fig. 5a).During the urements of the downward and upward radiance at 1 m 24 h measurementperiod the average K, was quite variable depth are displayed at approximately4 h intervals. In Figure and didnot change in any recognizable pattern with the sun's 3a, a slightly greater transmission of red light is apparent in varying position (Fig.5b). the early morning (1:OO and 5:OO) and late evening (22:00), Reflectance values for PAR just belowthe surface (0 m) and while blue-green penetrance increases during the day (9:OO- at zm (4 m)are shown aas function of solar elevation in Figures 17:30). This resulted ina bimodal distribution, with maxima 6a and 6b respectively.In Figure 6a there is a small increase found at 500 and 640 nm. Similar trends were found in the in reflectancewhen the solar altitude increases. However, at upward radiance; however, the blue-green peaks were much zm,large increases were seen in reflectance up to 5" above the less important at 9:OO and 1730. horizon, while further increases in the sun's height resulted With the downward irradiance data a downwelling at- in little directed changein reflectance (Fig. 6b). data The agree tenuation coefficient for PAR (K,) was calculated. Themean well with a third order polynomial regression equation value of K, for the entire water column on24 and 25 July 1985 (R=0.87). Very similartrends were seen at 3 and 5 m, while was 0.657 (S.E. 0.042). Inaddition, Kd(z,), the vertical attenu- similar but more variable patterns were found at 1 and 2 m. ation coefficient fordownward irradiance at the depth where The estimated absorption and scattering coefficients are irradiance is 10% of that found just beneath the surface (the plotted as a function of solar elevation in Figures 7 and 8 132 / P.H. HEINERMANN et al.

n 7 - 0.9 E 0.003 a v- ru) -E 0.8 g 0.002

v2 P) 0 0.7 C n .-m v- '0 m 0.001 E -n a v U E 0.6 L h( m If v 3 s I 2 0" 0.000-r 0.5 400 500 600 700 Wavelength (nm) Y = 0.599 - 0.0015~+ 3.504e - 4x2 - 9.1 19e-6x3 R = 0.24 0.4

,z- 0.0003 b 0.3 -i.,.,.,',.r',',.,.l E 10 -5 0 5 10 15 20 25 35 30 7 v) -0- 01:oo r * Solar Elevation (degrees) E 05:OO 0.0002 " 09:oo RG.4.Variation of the vertical attenuation coefficient downward for irradiance -0 * 13:30 at themidpoint in the euphotic zone with solar elevation. * YE, 17:30 P) " 22:oo 0 C (Barsdate and Prentki, 1972), while in terms of solu- m 2 0.0001 able reactive phosphorus, K+,SO,--, pH, conductivity, chloro- K phyll a and dissolved oxygen, it is similar to Char Lake, Cornwallis Island (Schindler et al., 1974). z One obvious characteristic of the downward light trans- z mission in Keyhole Lake isthe loss of blue light with depth = 0.0000 1 400 500 600 700 (Figs. la,b). This selectiveattenuation of blue light with depth has been observed by many workers (Verduin, 1982; Davies- Wavelength (nm) Colley, 1983; Kishinoet al., 1984; Watras and Baker, 1988).It FIG. 3. Diurnal variation in the spectral distribution of(a) downward and (b) has often been attributed to absorption by gilvin found upward radiance at 1 m for various times on 24 July 1985. Times listed are Alberta Summer Time (AST), whichis equivalent to Greenwich Mean Time within the water column (Kirk, 1976). Gilvinconcentrations (GMT) minus 7 h. Note the loss of the bimodal spectral distribution in the in Keyhole are relatively low compared to other reported evening and early morning. See legend of Figure 2 for solar elevations. values (Kirk, 1983), but their contribution to this reduction may be substantial. No gilvin values from arctic waters have respectively. The absorption coefficient, even though being been reported, but recent measurements(Heinermann, quite variable, appears to decrease as the sun's angle from the unpubl. data) from other arctic lakes (Nauyuk Lake, g,, = horizon increases (Fig. 7). Theaverage absorption coefficient 0.345 m", Little Nauyuk Lake, g,, = 0.046 m", Gavia Lake,g,, at zmfor the 24 h period was 0.31 (S.E. 0.01). On the other hand, = 0.023 m-I, Northwest Territories) show Keyhole Lake to the total scattering coefficient increases to the point where the have the highest gilvin concentrations measured so far. Trip- sun meets the horizon and then it remains relatively stable ton or inanimate suspensoids may alsocontribute to this loss with further increases in j3 (Fig. 8). The average scattering of shorter wavelengthsthrough absorptionby insolublehumic coefficient at zmfor the samplinginterval was 1.22 (S.E. 0.10). material adsorbed on the surface of these particles (Hart, 1982; Kirk, 1985).A small reduction in blue light transmission DISCUSSION may also be attributed to absorption by chrysophytes and Water quality parameters in Keyhole Lakewere somewhat diatoms in the water. These groups have a broad absorption variable but typically low and representative of a nutrient- band from 400 to 540 nm due to chlorophylls a and c, as well poor water body. Based on chlorophyll a and primary pro- as fucoxanthin, which they possess (Owens, 1986) ductivity values, it may be classifiedas anoligotrophic lake. A shift in the peak transmission of the upward radiance to Greater values of Na+, Mg++and Mn++were present, while shorter wavelengths with depth was observed on 30 July. smaller measures of silicon and primary productivity were This is similar to shifts in upwardradiance found in temper- recorded than have been reported from other arctic lakes ate dystrophic waters (Heinermann and Ali, 1988). Upwel- (Livingstone, 1963; Hobbie, 1973).In terms of soluable reac- ling spectra were found to be narrower than downwelling tive phosphorus, total dissolved phosphorus, dissolved or- ones because they represent a part of the downward light ganic carbon, suspendedphosphorus, temperature and from a greater depth (Le., narrower spectral distribution) that chlorophyll a, Keyhole Lakeis similar to the Barrow in has been redirected upwards by scattering (Kirk, 1985). UNDERWATER PHOTIC ENVIRONMENT / 133

0.9 - (a) 0.08 - (a) Y = 0.6781 - 0.0024~- 1.7e - 4x2 + 7.386e-6x3 R = 0.51 ' 0.8 - Y = 0.0287 (OJ"'39x) R = 0.56 Q

A 13 13 13 El A 0.06- 7 'E 0.7- E v s m El L" 13 8 Q Q) 0.6- 0.04- 0) c Q)0 Q 1313 ' E E -0 Q 13 130 $! 0.5- 8 U 0.02 - ""1 O.~!.I.I.I.I.~.I.I.I.I -10 -5 0 5 10 35302520 15 -10 -5 0 5 10 1535 3025 20 Solar Elevation (degrees)Elevation Solar (degrees)Elevation Solar

Y = 0.0325 + 0.0032~- 1.7628 - 4x2 + 2.837e-6x3 R = 0.87 Y = 0,4762 + 0.0042~+ 9.373e - 5x2 - 6.2588-6x3 R = 0.37 El 0.8 13 0.06 4 BB c1 Q

Q

-10 -5 0 5 10 1535 3025 20 -10 -5 0 5 10 15 353025 20 Solar Elevation (degrees) Solar Elevation (degrees) FIG.6.Solar elevationeffects on (a)the reflectance just beneath the water surface FIG.5. Diurnal changes in the average Kd (a) and average Ku (b) with solar (0 m) and (b) the reflectance atzm. altitude.

The unique feature of the underwaterlight environment is absorption by carotenoids between 440 and 470 nm and the presence of bimodal spectral distributions. This is a chorophyll u absorbance maxima near 450 and 650 nm. Kirk relatively rare occurrence and may be attributed to a couple and Tyler (1986) and Bowling and Tyler (1986) have also of factors. First, dissolved organic matter in the water column found bimodal spectra in a tropical Australian and a Tas- may form complexes with dissolved iron (Shapiro, 1964, manian lake respectively. They tooattributed these distribu- 1967; Mantoura et ul., 1978). Thesehumic material-metal ion tions to chorophyll u absorption maxima. The absorption compounds absorb strongly between 500 and 560 nm (C.G. spectra of algal species have been shown to significantly Trick, pers. comm. 1989).The dissolved iron measured in this affect the underwater spectral quality when the chorophyll u study (40 pgl")is high enough so that when complexed with concentrations are greater than 10 mgm-3 (Talling,1960; Kirk, gilvin itwould selectively produce the transmission troughs 1977b). Belowthis level, Bowlinget al. (1986) concluded that seen in downward and upward radiance spectra. With in- phytoplankton is unlikely to be a significant factor. Since creases in depth and removal of short-wavelength light by chorophyll a levels in Keyhole Lake are very low, effectson gilvin the spectra become unimodal, with transmission light transmission would be minimal. This statement as- maxima in the red end of the visible spectrum. A second, but sumes that the vertical distribution of algal populations in the less likely, possibility theis effect of algal populations on the lake is uniform. This, however, was not verified. According transmission of underwater light. Dubinsky and Berman to previous studies at Keyhole Lake (Hunter, 1970; Kling, (1979) attributed the bimodal distribution of the downwel- unpubl. data), phytoplankton biomass was near its maxi- ling spectral irradiance in Lake Kinneret to a Peridinium mum for the year (29.13 m@) during oursampling period, (Dinophyceae)bloom. Transmission peaks were found at 580 and the most abundant species for 1985, in order of impor- and 680 nm. These were stated to be the result of strong tance, in terms of biomass were Dinobryon cylindricurn (Chryso- 134 / P.H. HEINERMANN et al.

0.5 Both downwelling and upwelling radiance spectra showed Y = 0.3315 - 0.0056~+ 2.69e - 4x2 - 4.416e-6x3 R - 0.55 a loss of the blue-green transmission maximum in the early Q morning and late evening.At these times the sun’s position was relatively close tothe horizon; as a result of this longer atmospheric path, shorter wavelength (blue and blue-green) h l- light is more readily scattered and, thus, removed from the E direct solar beam. Near sunset or sunrise, therefore, direct 0.4 E Q light becomes red-shifted (Nordtug and Mels, 1988; Kirk, .-al 1983). This may account forthe loss of the blue-green trans- =0 3 mission maximum. On the other hand, at these low solar 0 elevations indirect light (skylight), being richer in shorter E wavelengths, makes up a larger proportion of the total irra- -0 c.n diance (Nordtug and Mels, 1988). Without actual data con- 5 0.3 cerning these parameters no simple relation can be drawn nu) between solar altitude and spectral distribution of total irra- a diance. The mean K, for the entire water column and secchi disc Q readings for Keyhole Lakeare similar to late July values for Q Q Lake Schrader, Alaska ( Kd=0.59and secchi disc=4 m, Hobbie, 1962) and to coefficients of some European north temperate 0.2 -10 - 5 025 205 15 10 30 35 lakes (e.g., Lake Esthwaite, Kd=0.59and secchi disc=3.7 m, Macan, 1970; LakeHampen, K,=0.60 and secchi disc=3.7 m, Solar Elevation (degrees) -Jensen and Ssndergaard, 1981; Lake Vechten, Kd=0.60 and secchi disc=3.5m, Steenbergerand Verdouw, 1982). Lake FIG. 7. Variation in the absorption coefficientwith changes in solar elevation. Schrader is an oligotrophic lake (summer chlorophyll a=0.98 mgm-3, while Lakes Esthwaite, Hampen and Vechten are mesotrophic (summer chlorophyll a > 2 mgm3).In terms of algal biomass, diatoms and blue-green algae predominate in Lakes Esthwaite and Hampen, green algae are most impor- tant in Lake Vechten and Chrysophyceae are the dominant group in Lake Schrader. Therefore only Lake Schrader is somewhat similar to the type of algal community found in Keyhole Lake. Q Q The relative insensitivity of Kd(zm)to changes in solar angle / in the moderately scattering waters of Keyhole Lake(average ba-’=3.97)is in agreement with the characteristics of a “quasi-

Q/ inherent” or “apparent” optical property of an aquatic me- dium, asdiscussed by Bakerand Smith (1979) and simulated by Kirk (1984). Theaverage Kd, on theother hand, displayed increases at low (13%)and high (22%) solar elevations. The former increase has been described by Kirk (1984); however, the increase in K, when the sun was high in the sky remains somewhat problematic. The large variability shown by the average K, with solar elevation changes does not allow any reasonable explanation. ,.,.,.,.,.I. I.I. I 10 15 20 25 30 35-5 30 0 25 205 15 10 The observations that reflectance just below the surface and at zm increase with solar elevation are in opposition to Solar Elevation (degrees) calculations presented by Gordon et al. (1975), Kirk (1981b) and Kirk (1984). They found that reflectance values should FIG.8. Diurnal variation in the scattering coefficient with changes in solar decrease as f3 gets larger. These simulations may, however, altitude. not apply well to the relatively gilvin-poor,moderately tur- phyceae), Diatorna elongaturn, Asterionellaformosa and Eunotia bid of Keyhole Lake. zasumensis (Bacillariophyceae).Species such as Dinobryon are An inverse relationship between reflectance and the ab- quite motile and may migrate upwards from the lake bottom sorption coefficient became apparent as solar elevation in the mornings, thus being concentrated near the water changed. As f3 increases, reflectance just below the surface surface at midday. Pigments such as xanthophylls found in increases (Fig. 6a),while the absorption coefficient decreases these algae and, inparticular, fucoxanthin absorb strongly in (Fig. 7). This inverse proportionality between R(0) and a has the 500-560 nm region (Kirk, 1983; Wetzel, 1983; Davies- been shown by previous modelling studies (Gordon et al., Colley et al., 1986). These two factors may account for the 1975; Kirk, 1984).The average absorption coefficient at zmre- bimodal spectral distribution of underwater light in Keyhole ported here is lower than values of a at zmdescribed for some Lake. Australian inland waters (Kirk, 1981a). UNDERWATER PHOTIC ENVIRONMENT / 135

The scattering coefficient remained relatively stable while REFERENCES the sun was above the horizon, but a large decrease occurred as it dipped below the horizon. This decrease may result from BAKER, K.S., and SMITH, R.C. 1979. Quasi-inherent characteristics of the the lack of a direct solar beam entering the water column. The diffuse attenuationcoefficient forirradiance. Proceedings of the Society of Photogrammetry and InstrumentEngineering 208:60-63. average scattering coefficient at zm in this study is similar to BARSDATE, R.J., and PRENTKI,R.T. 1972. Nutrient dynamics in tundra that reported for Corin Dam, a mesotrophic (chlorophyll a = ponds. In: Tundra Symposium. Seattle: Universityof Washington 2.9 mgm-3), slightly turbid (1.7 NTU)Australian lake. Press. 192-199. In terms of the quality of its downward spectral transmis- BOWLING, L.C., and TYLER, P.A. 1986. The underwater light-field of lakes sion, Keyhole Lakeappears to differ from other arctic lakes. with marked physicochemical and biotic diversity in the water column. Journal of Plankton Research 869-77. Char Lake, on Cornwallis Island, transmits best in the blue- BOWLING, LC., STEANE, MS., andTYLER, P.A. 1986.The spectraldistribu- green (Schindler et d., 19741, while Lake Peters and Lake tion and attenuationof underwater irradiancein Tasmanianinland waters. Schrader, in Alaska, have their greatest transmission in the Freshwater Biology 16313-335. green part of the spectrum (Hobbie, 1962, 1973). Red light CHAMBERS, PC., andPREPAS, E.E. 1988. Underwater spectral attenuation and its effect on themaximum depth of angiospermcolonization. Canadian penetration is very poor in these lakes and no bimodal Journal of Fisheries and Aquatic Sciences451010-1017. spectral curves have been recorded. DAVIES-COLLEY, R.J. 1983. Optical properties andreflectance spectra of 3 When compared with other temperate and north temper- shallow lakes obtained from a spectrophotometric study. ate Canadian, American and European lakes, the underwater Journal of Marine and Freshwater Research17445-459. downwelling light spectra in Keyhole Lake are also quite -, PRIDMORE, R.D.,and HEWITT, J.E. 1986. Optical properties of some freshwater phytoplanktonicalgae. Hydrobiologia 133165-178. different (see Chambers and Prepas, 1988; Watras and Baker, DUBINSKY, Z., and BERMAN, T. 1979. Seasonal changes in the spectral 1988). Its spectral quality only somewhat resembles that composition of downwelling irradiancein Lake Kinneret (). Limnol- found in two eutrophic Albertan lakes (Lakes Cooking and ogy and Oceanography24655-663. Joseph; Chambers and Prepas, 1988). Therefore,the under- FURCH, B., CORREA, A.F.F., NUNES DE MELLO, J.A.S., and OTTO, K.R. 1985. Lichtklimadaten in drei aquatischen Okosystemen verschiedener water photic environment of Keyhole Lakeis unique among physikalisch-chemischer Beschaffenkeit.I. Abschwachung, Ruckstreuung arctic, north temperate and temperate lakes. und Vergleich zwischen Einstrahlung, Ruckstrahlung und spharischen gemessener Quantenstromdichte (PAR). Amazoniana9411-430. CONCLUSIONS GORDON, H.R., BROWN,O.B.,and JACOBS,M.M. 1975.Computed relation- ships between the inherent and apparentoptical properties of a homo- Keyhole Lake isa slightly basic, monomictic arctictundra geneous . Applied Optics14:417-427. lake. It is not very productive and nutrient poor. Although HART, B.T.1982. Uptake of trace metalsby and suspended particu- continuous light is present at this latitude during the sum- lates: a review. Hydrobiologia 91299-313. HEINERMANN, P.H.,and ALI, M.A.1988. Seasonal changes thein underwa- mer, significant diel changes in the spectral quality and ter lightclimate of two Canadian shield lakes. Hydrobiologia169:107-121. intensity of the underwaterlight field do occur. The spectral HOBBIE, J.E. 1962. Limnological cyclesand primary productivityof two lakes distribution of the downward and, some to degree, the upward in the Alaskan arctic. Ph.D. thesis,Indiana University, Bloomington, Indi- radiance is bimodal and may be primarily attributed to ana. -. 1973. Arctic limnology:a review. In: Britton,M.E., ed. Alaskan arctic absorption by humic material-dissolvediron complexesfound tundra. Arctic Institute of North America Technical Paper25127-168. in thewater column. A second, and less important, contribu- HIZIJERSLEV, N. 1975. A spectral light absorptionmeter formeasurements in tor may be absorption by accessory pigments (namely xan- the sea.Limnology and Oceanography201024-1034. thophylls) and photosynthetic pigments such as chlorophyll HULBURT, E.O. 1945. Optics of distilled and natural water. Journalof the a found in phytoplankton species present at the time. The Optical Societyof America 35698-705. HUNTER, J.G.1970. Production of arctic cham(Salvelinus alpinus Linnaeus) in relative insensitivity of Kd(z,) to changes in solar elevation a small arctic lake, Keyhole Lake, Victoria Island,Northwest Territories. supports the idea that this parameter may be viewed as a Fisheries ResearchBoard of Canada Technical Report231.190 p. "quasi-inherent" optical property. Diurnal variation in the JERLOV, N.G. 1951. Optical studies of ocean water. Report of the Swedish reflectance of PAR differed from that predicted by previous Deep Sea Expedition 3:l-59. simulation models, while the inverse relationship between JEWSON, D.H.1977. Light penetration in relationto phytoplankton contentof the euphotic zoneof Lough Neagh, N.Ireland. Oikos 2837483. reflectance and the absorption coefficient was in agreement JOHNSON, L. 1976. Ecology of arctic populations of , Salvelinus with these same models. These data provide a quantitative namaycush, lake cham, S. alpinus and associated species unexploitedin lakes and qualitative description of depth and diurnalvariations in of the Canadian NorthwestTerritories. Journal of the Fisheries Research the unique arctic underwater photic environment of Keyhole Board of Canada 332459-2488. -.1983. Homeostatic characteristicsof single species stocks in arctic Lake. lakes. Canadian Journalof Fisheries and Aquatic Sciences40:987-1024. KALFF, J. 1967. Phytoplankton dynamics in an arctic lake. Journal of the ACKNOWLEDGEMENTS Fisheries Research Boardof Canada 241861-1871. KIRK, J.T.O. 1976. Yellow substance (Gelbstoff) and its contribution to the We would like thankto P. Vanriel for helpful comments and accessattenuation of photosynthetically active radiation in some inland and to someof his unpublished data, J.P. Parent and V. Bolliet for their coastal south-eastern Australian waters. Australian Journalof Marine and help during sampling, and H. Bourassa for her assistance in the dataFreshwater Research27:61-71. analysis.Our appreciationis also extended toE.R. Loew for the loan -. 1977a. Attenuation of light in natural water. Australian Journalof of hisunderwater spectroradiometer. Financial support for this Marine and Freshwater Research28497-508. research has come from the Department of Fisheries and Oceans, -.1977b. Use of a quantameter to measure attenuation and underwater reflectanceofphotosyntheticallyactiveradiationinsomeinlandandcoastal Canada, to L. Johnson,from the National Science and Engineering south-eastern Australian waters. Australian Journalof Marine and Fresh- Research Council, the Fonds pour la Formation de Chercheurs et water Research 28:9-21. 1'Aide A la Recherche and the Department of Indian and Northern -. 1979. Spectral distribution of photosynthetically active radiationin Affairs to M.A. Ali and from the Joseph-Arthur Paulhus Foundation some southeastern Australian waters. Australian Journalof Marine and to P.H. Heinermann. Freshwater ResearchX81-91. 136 / P.H. HEINERMANN et al.

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