The Underwater Photic Environment of Two Subarctic Icelandic Lakes P.H

Home , Underwater environment

ARCTIC VOL. 46. NO. 1 (MARCH 1993) P. 17-26 The Underwater Photic Environment of Two Subarctic Icelandic Lakes P.H. HEINERMANN’ and M.A. ALI2

(Received 16 July 1991; accepted in revised form 15 June 1992)

ABSTRACT. The Underwater light fields of two Icelandic lakes of volcanic origin and differing trophic status, Thingvallavam (oligotrophic) and Myvatn (eutrophic), were investigated. Gilvin and turbidity depth profiles were also measured. Diurnal variation in photosynthetically active radiation (PAR) reached almost3 orders of magnitude. Downwardirradiance spectra were variable near the surface, but with increases in depth transmission peaks at 510,560 and 570 nanometres (nm)became apparent in Thingvallavatn, Myvatn-East Basin and Myvatn-South Basin respectively. Upward irradiance transmission maxima shiftedfrom 480 to 500 nm with depth in Thingvallavatn, while in Myvatn they remainednear 570 nm. An irradiance trough at 520 nm was noted in both the upward and downwardspectra of Thingvallavatn.The importance of phytoplankton (chlorophyll) and gilvin in mcdiiing the underwater light climateof Myvatn is clearly demonstrated. The meandownwelliig and upwellii imuiiance curves for Thingvallavam coincide very well with the spectral sensitivities of resident adult arctic charr. This provides support for the sensitivity hypothesis. Key words: subarctic, limnology, imdiance, underwater light, arctic charr, spectral sensitivitjr, Iceland

RIhUM6. L’environnement photique sous-marinde deux lacs islandais,le Thingvallavatn (oligotmphe)et le Myvatn (eutrophe), d’origine volcanique et prksentant un ktattrophique diffkrent, ont kk? ktudiks. La quantitk de matibre organique dissoute (gilvin) et la turbidit6ont kt6 mesurhs en fonction de la profondeur dans chaque lac.La radiation disponible pour la photosynthbse(PAR) pouvait varier quotidiennementjusqu’h trois ordres de grandeur. Alors que les spectres de l’irradiance descendante variaient prbssurface, de la ils augmentaient avec la profondeur aux de pics transmission de 510, 560 et 570 nanombtres (nm)respectivement dansle Thingvallavatn et le bassin duest Myvatnet le bassin sud du Myvam.Les maxima dans la transmission de l’irradiance ascendante ontkt6 dCalks avec la profondeur de 480 B 500 nm dans le Thingvallavatn tandis que dansle Myvatn ils sont demeurks p&s de 570 nm. Une zone d’absorptionB 520 nm a kgalement 6te observ6.e dansles spectres descendants et ascendantsdu Thingvallavatn. L’importance du phytoplancton (chlorophylle) deet la matikre organique dissoute (gilvin)dans la modificationde l’environnement photique sous-marindu Myvatn a kt6 nettement dkmontr6.e. Les spectres moyens de l’irradiance descendante et ascendante co’incident trbs ktroitemnt avec les sensibdies spectrales chez les ombles-chevaliers adultes indigbnes. Ceci appuie l’hypothbse de la sensibilit6 chez ces poissons. Mots clks: subarctique, limnologie, irradiance, lumibre sous-marine, omble-chevalier, sensibilite spectrale, Islande

INTRODUCTION lakes, Thingvallavatn and Myvatn. These volcanic lakes are a valuable addition to a continuing survey of lakes at different The underwater photic environments of fresh waters have beenlatitudes and of differing trophic states (Heinermann, 1986; shown to be quite variable, such that the quantity and quality Heinermann and Ali, 1988; Heinermannet al., 1990). As well, of light transmissionmay change both temporally and spatially a detailed investigationof the underwater light in these lakes (Smith et al., 1973; Kirk, 1977a; Eloranta, 1978; Roemer and is lacking, even though most other aspects of the ecosystems Hoagland, 1979; Dubinsky and Berman, 1979; Bowlinget al., of Thingvallavatn and Myvatn have been studied (Antonsson, 1986; Heinermannand Ali, 1988; Chambers and Prepas, 1988). 1977; Jhasson, 1979;Lindegaard, 1980; Snorrason, 1982; In spite of these fluctuations a crude optical classification for Lastein, 1983; J6nsson, 1987; J6nasson and Lindegaard, 1988; inland water hasbeen presented by Kirk (1980). It is based uponJonsson et al., 1988; Gardarssonet al., 1988). These data were the relative contributions of gilvin (dissolved organic matter), collected in conjunction with a concurrent study by the authors tripton (inorganic particulates), algae and the water itself to theon the visual abilitia of the resident arcticchan in Thingvallavatn. absorption of underwater quanta. Fresh waters may then have As such, they forma critical framework within which the visual their light fields categorized as being influenced primarilyby task and response of these charr can be investigated. one or more of these factors. Scattering, though not included The primary objective ofthis study was therefore to charac- in this classification, may also be very important in affecting terize the underwater light fieldstwo of neo-volcanic subarctic the amount and at times the spectral distribution of the light lakes, Thingvallavatn and Myvatn. With this information and passing through the water column (Kirk, 1983, 1985, 1989). the existing spectral sensitivity data for the vision of resident Thesemodifications to the underwater light field brought arctic charr, we will determine whether they are well adapted about by absorptive and scattering processes have biological to their photic environment. This investigation is part of the importance for the organisms present. Algal productivity and continuing Thingvallavatn and Myvatn projects that began in species composition may be altered (Talling, 1971; Kirk, 1976;1971 and focus upon the functioning and dynamic ecologyof Eloranta,1978; Kirk, 1979; Jeffrey, 1980; Kirk, 1981), North Atlantic ridge lakes. zeitgebers(triggering stimuli) may beprovided for diel zooplankton migrations (Siebeck, 1960; Ringelberg, 1964) and SITE DESCRIPTION visual tasks of other aquatic animals, such as fish, may also be affected(Barlow, 1974; McFarland and Mum, 1975; Thingvallavatn, the largest lake in Iceland, is located in the Lythgoe, 1979; Levine et al., 1980; Dabrowski and Jewson, southwest (64”6’N, 21”8’W) at100.7 m abovesea level 1984; Guthrie, 1986; Lythgoe, 1987; Munk et al., 1989). (Fig. 1A). It has a surface area of83.7 km2, a maximum depth With this latter aspect in mind, we set out to characterize theof 114 m and a mean depth of 34.1m (Rist, 1975). The primary underwater photic environments of two of Iceland’s largest inflow (70-90%) is subterranean, entering the lake near the

‘Department of Biology, University of Ottawa, 30 Marie Curie, Ottawa, Ontario, Canada KIN 6N5 V6partement de Sciences biologiques, Universite de MontrM, Case postale 6128, Succursale A, MonW, Qubbec, Canada H3C 3J7; offprint requests to: M.A. Ali @The Arctic Institute of North America 18 / P.H. HEINERMANN and M.A. ALI

FIG.1. Bathymetric maps of (A) Thingvallavatn and (B) Myvatn. Sampling locations for Thingvallavatn were T (irradiance profiles) and Mj6anes (diurnal variation); those for Mwatn were SB (south basin) and EB (east basin). ,In (A)., 20, . 60 and 100 m contours are shown; in (B) 2, 3 and 4 m contours are presented. Inset shows theiriocation in Iceland.

northern and eastern shoreline through porous rocks in the surface areaof 8.2 km2, 3800 years old,mean depth of 1.05 m catchment area (Arnason, 1976). As a result, water chemistry, and a maximum depth of 2.4 m) and the south basin (S-basin, levels and temperatures are quite stable both seasonally and surface areaof 29.1 km2, 2000 years old, mean depthof 2.3 m yearly. The formation and continued modificationof the lake and a maximum depth of 4.2 m). The S-basin may also be basin have been due to the combined effects of glacial and divided into the central S-basin (22 kmz) and the east basin tectonic activity (Saemundsson, 1976). Thingvallavatnmay be (E-basin,7 kmz) (Adalsteinsson,1979a; Olafsson, 1979a). classified as a dimictic oligotrophic lake (a nutrient-poor lake Water inflow to the lake occurs primarily from cold and warm that mixes completely twice a year). Accordingly, phytoplanktonsprings (constant flow and temperature) along the eastern shore- gross production averaged 70 gC*m-2.yr-1 over the period line (E-basin). These springs strongly influence the major ion 1975-82 (Lastein, 1983; Kairesaloet al., 1989). The very large composition and distribution in Myvatn (input loading rates from pelagiczone contains somewhata simple zooplankton the springs are 1.4, 1.5 and 340 gm-2*yr-1for N, P and Si community (Antonsson, 1977), while the littoral zone is home respectively)(Olafsson, 1979b). The formation and present toadiverse invertebrate community (Lindegaard, 1980; shape of the lake basin are primarily the result of postglacial Snorrason, 1982). Arctic charr(Salvelinus alpinus) and thethree volcanic activity (Th6rarinsson, 1979). Myvatn is a shallow, spine stickleback(Gasterosteus aculeatus) are the most abundant strongly eutrophic polymictic lake (a nutrient-rich lake that fish species found, while a few brown trout(Salmo tnctta) are frequently mixes completely).(Jhasson, 1979). Phytoplankton also present. Four very distinct morphs of arctic charr are gross production is 118 gC-m-2-yr-1 in the S-basin (1971-76), present: small benthivore, large benthivore, planktivore and 75 gC.m-2.yr-1 in the N-basin (1971-76), 50 gC*m-2*yr-1 in piscivore (Sandlundet al., 1987; Jonssonet al., 1988; Slnilason theE-basin (1972-73) and 5 gC*m-*-yr-'near the E-basin et al., 1989; Snorrason et al., 1989). springs (J6nasson and Adalsteinsson, 1979). The zooplankton Myvatn is the fourth largest lake in Iceland and is found in community is again not very diverse (Adalsteinsson, 1979a). the northeast (65'35 ' N, 17'00 ' W) at 277.8 m above sea level The zoobenthos production is quite high but the number of (Fig. 1B). It iscomposed of two basins: the north basin (N-basin, species is limited (Lindegaard, 1979; Lindegaard and Jbnasson, UNDERWATERPHOTIC ENVIRONMENT 1 19

1979). Three fish species are found in Myvatn: arctic charr, downwelling and upwelling irradiance to document variation in brown trout and the three spine stickleback (Adalsteinsson, incident radiation during the sampling interval. In-air scans of 1979b; Kristjansson and Adalsteinsson, 1984). spectral quality were also conducted before and after each downwelling and upwelling series. A scan from 350 to 750 nm took MATERIALS AND METHODS 35 seconds. Electroretimgram (ERG)determined light-adaptedspectral sen- Water samples from the pelagic zone were taken with a Vansitivity data for all four adult arctic charr morphsin Thingvallavatn Dorn type sampler at the water surface and everym 10 to 70 m were obtained with permission from Thorarensen (1987).Details depth in Thingvallavatn (Station T) and at m1 intervals in the of the electrophysiological methods used may be found in his E- and S-basins of Myvatn (Stations EB andSB, Fig. 1). Three unpublished thesis and in Thorarensen and Einarsson (1985). replicate profile samplings were done at each station. Water temperatureprofiles were also measured during sampling. RESULTS Turbidity (NTU) was measured in the laboratory within of 4 h sampling at Thingvallavatn and within 24 h at Myvatn (Hach Water and Optical Quulity Ratio Turbidity meter, Modelno. 18900-00). Estimates of the dissolved humic substances or gilvin concentrationas g440 (Kirk, Vertical attenuation, absorption and scattering coefficients, 1976)were calculated as 2.303A/0.1 m-l, where A is the gilvin, turbidity and chlorophyll a values were all at least two measured absorbanceof a membrane-filtered (WhatmanGF/C timesgreater in Myvatn than they were in Thingvallavatn glass fibre and Millipore GS 0.22 pm filters) water sample in (Table 1). On the otherhand, Secchi disk and euphotic zone depth a 10 cm cuvette (Varian DMS90 W Visible Spectrophotometer). were much larger in Thingvallavatn, reflecting its much greater Transparency was measured using a standard Secchi disk at transparency. Another difference is evident when the valuesof Myvatn. Myvatn's east and south basins are compared. This time, all Underwater depth series of downward and upward irradiances parameters were greater in the S-basin, with the exceptions of for PAR were measured from0 m (just below the water surface) Secchi disk and euphotic zone depth. to 10 m in 1 m intervals and then to 30 m in 5 m intervals in 1. Water quality and optical characteristics of Thingvallavatn recorded TABLE Thingvallavatn on 10 August 1989.In Myvatn they were and Myvatn at 0.5 m intervals to2 m in the E-basin andto 3 m in the S-basin on 2 August 1989. The instrument used to record irradiance of Myvam PAR was a LI-COR LI-185B quantum meter and a LI-192SB ThingvallavatnParameter S-basin E-basin underwater quantum sensor. The meter was lowered with an outrigger projecting 75 cm out into the water over the side1989August of 10 Date 19892 August the boat facing the sun, effectively eliminating any boat shadow. Water tempxatu~ Measurementswere only conducted under cloudless skies (surface to (Myvatn)or when the sun was not obscured by clouds bottom, "C) 10.5-6.4 10.5 8.5 (Thingvallavatn) within 3 h of solar noon. Gilvin, g,, (m") 0.068 (0.004, 24)a 0.278 (0.007,12) 0.152 (0.003, 9) Diurnal variation in the downward irradiance of PAR was Turbidity 0.59 (0.05, 24) 6.8 (0.1, 12) 2.8 (0.11, 9) recorded at 1.5 m depth at Mj6anes on 20 May, 22 June, 1 August and 23 September 1984. Solar elevations were computed secchi disk (m) 12 -16b 0.5 1 using the method described by Kirk (1983). Chlorophyll a From the depth series of PAR, mean upward and downward OLgl-9 0.31 (0.05, 18.674)b (5.21, 6)" 9.25 (1.25, 4)" verticalattenuation coefficients for PAR (Ku,Kd) were 0.28 0.69 0.63 0.69 a (at zm,m")d 0.28 calculated by linear regression (Kirk, 1977b). Absolute values for KdCz,,the downward vertical attenuation coefficient at depth b (at z,, m:l)e 0.06 9.00 4.41 (z), were determined for individual depth strataso that changes1.72 MaxK,(,, (m-l)f 0.43 1.25 in broadband Kd values that accompany spectral changes with - 0.14 1.47 0.71 depthwould be-accounted for. From these minimum (Min Kd (m-')g &(,)), mean (&) and maximum (Max Kd(z)) valueswere Mill&(z) (m-ly 0.03 1.24 0.33 tabulated for each lake. Reflectance of PAR was calculated as K,(m-')' 0.12 1.37 0.61 E, Ed-l at any depth (Kirk, 1977b). Euphotic depth, z,, the depth where incident radiation was reduced to 1% , was deter- Euphotic zone m) 30 (zw9 m) 3.1 >3.0 mined diredly from downward PAR measurements. An estimate of the absorption (a) and scattering (b) coefficients at z, (the BWater column mean (standard error, number of samples). bFrom Snorrason, 1982. depthwhere incident radiation was reduced to 10%) from Worn J6nasson and Adalsteinsson, 1979. upwellingand downwelling values of PAR followed the d, eAbsorption (a)and scattering (b)coefficients at the midpoint of the euphotic procedure outlined by Kirk (1981). zone. f, hMaximum and Wum vertical attgnuation coefficients for PAR. Thespectral distribution ofupwelling and downwelling & 'Mean downward (aand upward (K,) vertical attenuation coefficients of ifiadiance (350-750 nm) was measured at the same depths as PAR PAR for the entire water column. usingLI-CORa underwater spectroradiometer (Model LI-l8OOUW/12). The instrument was held rigidly and lowered With the exceptions of turbidity at 10 m and gilvin at 30 m, on the sunny side of the boat about0.5 m from the boat's edge. there is a general increase in theseparamem with depth (Fig. 2). A recording of in-air downward quantum flux (LI-COR LI-185) Gilvinvalues begin to plateau after 50 m depth.Turbidity wastaken simultaneously with underwater measurements of measurements at 0,20 and 30 m were significantly lower than 20 / P.H.HEINEFWANN and M.A. ALI at 10 m (p< .05, t-tests). This higher value at 10 m coincides is in contrast with that found by Kirk (1977a). Values ranged with a mild thermocline at this depth. The amount of gilvin at from 0.041 (0m) to 0.103 (25m) in Thingvallavatn,0.05 (0 m) 30 m was not found beto statistically different from that at either to 0.08 (2.5m) in the east basin of Myvatn and 0.09 (0 m) to 20 or 40 m (p> .05,t-tests). 0.145 (2.5m) in the south basin of Myvatn. 8

0.09, T 1" 7

-+ EastBasin * South Basin .-E5 rr

2! I I i O.O1 1 0 1 2 3 I I" Depth (m) 0 10 20 30 40 50 60 70 Depth (m) mo. 2. Turbidity and gilvin depth profiles in Thingvallavatn. Mean values and standard emrs at each depth are shown.

Turbidity values in Myvatn were significantly higher at all depths in the S-basin than they were in the E-basin (Fig. 3a, p < .01,t-tests). No significant differences (p > .05)were found with depth but a slight increase wasseen at 3 m. These somewhat uniform turbidity levels,as well as isothennic conditions (Table l), reflect a complete mixingof the water column at the time of sampling. In the E-basin turbidity was significantly higher at 2 m than at the surface (p < .05). In Figure 3b gilvin values at all depths were also consistentlypter in the S-basin(p < .05, t-tests). Increases in gilvin with depth were seen in both basins -I+ EastBasin such that in the S-basin measurements2 andat 3 m were greater C than at 0 and 1 m @<.05) and in the E-basin values at 1 and -s 0.24 2 m were greater than those at the surface (p < .05). Myvatn has highermean absorption coefficients than Thingvallavatn at all wavelengths measured (Fig.4). In Myvatn the mean gilvin concentrations (Table 1; Fig. 4) in the S- and

E-basins were 4.1 and 2.2 times that found in Thingvallavatn. I 1 0.1 I I I Small absorption peaks were seen near 700 nm in all spectra. 0 1 2 3 4 Depth (m) Underwater Light mo. 3. Turbidity (a) and Gilvin (b) depth profiles in the east and south basins The curves for downward (Ed) and upward (E,) irradiance for of Myvam. PAR in Thingvallavatn and Myvatn are approximately linear, in agreement with the exponential manner by which irradiance Thedownward irradiance of PAR in Thingvallavatn near diminishes with depth in natural waters (Fig.5). Some departure Mj6anesexhibits significant diurnal and seasonal variation from this linearity occurs in the upper 2 m of Thingvallavatn (Fig. 6). Changing day lengths (Fig.6) and solar altitudes further (E,-Thing) and in the upper metre of the east basin of Myvatn illustratethe variation that can occur even at this latitude. (E,-MyEB). On average upwelling PAR was about28 times less Maximum solar heights on 20 May, 22 June, 1 August and intense than downwelling PAR in Thingvallavatn. In Myvatn, 23 September 1984were45.9",49.2",43.9"and25.7"respec- the same comparison yielded values of 15 and 8 times for the tively. Day lengthand maximum solar height on2 and 10 August eastand south basins respectively. The upwelling and 1989 were 17 h50 min, 43.5 " and 16 h 57min, 41.3 " respec- downwelling irradiance' values measured at a given depth (Fig.5) tively. Increases and decreases in irradiance were monotonic were used to calculate the reflectance PAR of for that depth.No except for noticeable changes in measurements07:OO at and 09:OO asymptotic state for reflectance was reached in either lake, which (GMT) on 20 May and 10:00 on 23 September, where the sun UNDERWATER PHOTIC ENVIRONMENT / 21 was partially obscured by cloud cover. The greatest incident are shifted to 500 nm at all other depths. A similar absorption downward flux occurred at either 1190 or 12:OO and varied by troughwas located at 520 nm. Itsmagnitude delta E,, almost three orders of magnitude over the course ofa day. The (510:520 nm) also decreases exponentially with depth. When the in-water PAR curves are much narrower than that for the solar size of the trough at each depth was compared with the same irradiance at the top of the atmosphere. This reflects losses parameter in the downwelling spectra, the two were quite well resulting from scattering and absorption of the direct and diffusecorrelated (r = 0.97). This provides further evidence for the close radiation within the atmosphere. relationship between downward and upward irradiance. -2. -1 Irradiance of PAR (prnokm s ) 10 100 1000 10000 0

* MeanAbCo-SB t * MeanAbCo-Th

t 10

YE 5 Q

+ Ed-Thing -c- Eu-Thing

20 + Ed-MyEB + Eu-MyEB * Ed-MySB + 350 450 550 650 750

Wavelength (nm) 30

FIG. 4. Mean absorption spectra of dissolved colour (gilvin) in filtered water FIG.5. Attenuation of downward (Ed) and upward&)irradiance of PARin samplesfrom the entire water columns of Thingvallavatnand Myvam. Thingvdavam (Thing) and Myvam (MyEB-east basin, MySB-south basin). MeanAbCo-EB,MeanAbCo-SB and MeanAbCo-Th are mean absorption coeffiicierns for the east and south basins of Myvam and Thingvallavatn respectively. n = 8 €or Thingvallavatn, 3 for the east basin and 4 for the south basii. Standard errors were small and are not shown for clarity. 1200 1

7 1000 -E- May 20.1984 The in-air downward irradiance measurements (Fig. 7a) at + June 22,1984 Thingvallavatn reveala number of.small irradiance troughsoccur- € -& Sept. 23. 1984 ring at430,490,520,690 and 720 nm. The underwater spectra E recorded in Thingvallavatn in the first2 m are somewhat variable 2 800 because of some wave action during the measurement (Fig. 7a). a a One can see very clearly that wavelengths longer than 600 nm n r are rapidly attenuated in the water column (most removed by 600 4 m). This may be attributed to absorption by the water itself u .-m (Smith and Baker, 1981; Kirk, 1983). There is less attenuation 0 near the blue end of the spectrum such that more bluethan red 4 400 light remains at10 m. With increasesin depth the band of trans- E mission narrows, such that most of the flux by10 m was found g between 390 and 600 nm. All of the underwater recordings are b 200 unimodal andthe wavelength of maximumtransmission cl eventually centres around 510 nm. The irradiance trough at 520 nm seen in the in-air spectrum is reflected in all the underwatercurves down to a depth of 20 m. 0 4 6 10 12 14 16 2422 Its magnitude delta Edcz,,(510:520 nm) decreases exponentially 0 2 8 1820 with depth. Time (hr) Upward irradiance (Fig. 7b) spectra show trends similar to E,, as far as the loss of red and blue light is concerned. The FIG. 6. Diurnal variation in downward irradiance measured at 1.5 m depth in width of the transmission zone is, however, narrower than E, Thiidavatn (Mj6anes) on various summer dates.Note the presenceof clouds at 07:OO and 09:00 on 20 May and 1000 on 23 September 1984. The did in the upper 2 m, but by 10 m it is very similar toE, (390-600 variation of the solar irradiance in the PAR band, which is incident to the top nm). Transmission maxima of the unimodal upward irradiance of the atmosphere at Thingvallavam, is included for comparison. The mrded spectra are located at480 nm down to 2 m depth and then they value is divided by 2 to facilitate comparison. Local time = GMT. 22 / P.H. HEINERMANN and M.A. ALI

maxima occurring at or near 570, 650 and 700 nm. Irradiance troughs can be seen near 430, 620 and 680 nm. There is also a a greater attenuation of blue than red light, and as a result the spectra are narrower than in the east basin. The upwardirradiance curves (Fig. 9b) showtroughs and transmission peaks that are similar but more evident than in the east Q Air basin. Peaks are again seen near 570, 650 and 700 nm, while troughs are found near 430, 630 and 680 nm. The comparison between the spectral distribution of mean downward and upward irradiance measured in Thingvallavatn

* i0m with spectral sensitivities of adult resident arctic charr morphs * 20m highlights several important observations (Fig. 10). Maximum + 30m light transmission occurs near10 5 nm for downward irradiance '0 and 500nm for upward irradiance. All arctic charr morphs show L Q somewhat similar spectral sensitivity curves. Three of the morphs 3 -e have their maximum sensitivity near 500 nm, while the large 350 450 550 650 750 650 550 450 350 benthivore is most sensitive at 510nm. The spectral sensitivity 0 of all the arctic charr morphs coincides very well withspectral the quality of light available in the upper 30 m of Thingvallavatn.

Q Q air 0 c 0.06 .-Q 0.04 L -L 0.02 E !Q 0.00 -350 450 -350 750 650 550 3 Wavelength (nm) 450 550 650 750 650350 550 450 FIG. 7. The spectral distribution of downward (a) and upward (b) irradiance at various depths in Thingvallavatn on 10 August 1989. 0

The spectral distribution of in-air downward irradiance at the east basinof Myvatn demonstrates the same series of absorption peaks that were observed in Figure 7a. However, theseirradiance troughs are not seen in any subsequent underwater measurements (Fig. 8a). Moderate wave action caused the downwelling curves be near the surface (0-1 m) to quite variable. This subsides -LE between 1 and 2 m, so that they appear unimodal, quite broad 0 (350-730 nm) and have a maximum transmission near 560nm. 5 0.3 v There also is a slightly better transmission ofthan red blue light. Q + 1.5m The upwelling irradiance spectraare much less variable (Fig. 8b). 0.2 They are somewhat trimodal, demonstrating small irradiance .-tu troughsthat occur near 430, 630 and 670-680 nm. The U 2 wavelength of maximum transmission is located at or near 570 -L 0.1 nm. Upwelling curves appear to be somewhat narrower than downwelling ones ( - 50 nm). A better transmission of red E compared to blue light is again evident. crr 0.0 350 450 550 650 5502 450 350 750 Downward irradiance spectra for the south basin of Myvatn, 3 measured at 0 and 0.5 m, are again influenced by wave action, Wavelength (nm) but the characteristic shape of the downwelling light is evident FIG. 8. The spectral distribution of downward (a) and upward (b) irradiance at by 1m (Fig. 9a). The curves are somewhat trim&, with different depths in the east basin of Myvatn on 2 August 1989. UNDERWATER PHOTIC ENVIRONMENT / 23

h ?- DISCUSSION E Water quality and optical parameters (Table 1) measured in -c 71a Thingvallavatn all confm the oligotrophic nature of this deep, 6 stable, transparent volcanic/glacial lake. Myvatn, on the other hand; is shownto be a somewhat productive, moderately turbid, 5 shallow volcanic/glacial lake.As a result of its unique array of islands and subterranean inflowing springs, the E-basin maybe + 0.5m 4 considered very much less eutrophic, while the productivity and 4- 1.5m 3 chlorophyll a concentrations of the S-basin evidence the general 0 2.5m eutrophic nature ofthis lake. The productivity of Myvatn is also 2 integrally linked to the regular complete mixing of its water column. Sediments are frequently brought into suspension by 1 prevailing winds, enabling nutrients for phytoplankton and other organismstobe much more available (Jhasson and 0 I""""_"" "" , I----.-"" E- Adalsteinsson, 1979). 350 450 350 550 650 750 Thingvallavatn is similar in water and optical quality charac- 5 Wavelength (nm) n teristics to two clear, oligotrophic Tasmanian sub-Antarctic and south-temperatelakes (Lakes Prion antPerry respectively). Gilvin (g440), turbidity, Secchi disk and (&) values reported for Lake Prion were: 0.115 m-l, 0.39 NTU, 11.5 m and 0.14 m-l respectively,while those for the glacial Lake Perry were: 0.058 m-l, 0.37 NTU, 15.0 m and 0.21 m-' (Bowling et al., 1986). Gilvinvalues for Thingvallavatn are verylow and comparable to those of two Canadian arctic lakes, Little Nauyuk - (gU = 0.046) and Ciavia Lake (gU = 0.023; Heinermann et al., Om 0.5m 1990), while much less than some north-temperate shield lakes lm (Lakes Cromwell and Croche,g,, = 5.56 and 2.5; Heinermann 1 5m 2m and Ali, 1988). Though Thingvallavatn may be one of the most 2.5m transparent lakes in Iceland (S. Slnilason, pep. comm. 1989), 3m - its mean downward extinction coefficient (K,)of 0.14 m-l is still more than double that of Lake Tahoe(0.06 m-l) and Crater Lake (0.04 m-l), bothfound in the western United States. Increased gilvin concentrations have shifted its wavelength of maximum downward transmission near 510 nm, compared to 350 450 350 550 650 750 3 475 and 460 for the latter two lakes, while its euphotic zone Wavelength (nm) (30 m) is roughly half that of Lake Tahoe (60 m) and Crater Lake (80 m; Smith et al., 1973). FIG. 9. The spectral distribution of downward(a) and upward (b) irradiance at men the mean absorption spectra of gilvin were measured, each depth in the south basin of Myvatn on 2 August 1989. a small absorptionpeak near 760 nm was-observed in all curves

2 (Fig. 4). Since all the samples were filtered, whole algae could not be responsible for these minor peaks. Still, the filtering 18 processmay have ruptured cells and released their pigment 1.6 contents. As a result chlorophyll and its breakdown products r- E 1.4 could still have an influence upon the absorption spectra of these F water samples (French et al., 1972). $ 1.2 E Even though the attenuation of downwelling and upwelling i irradiance for PAR in Thingvallavatn and in the east basin of - Myvatn is mostly exponential (Fig. 5), some departure fromthis 8 0.8 C does occur in the upper parts of the water columns. This zone 5e 06 of greater light attenuation may be related to particulates and " 0.4 dissolved substances present. Unfortunately,no gilvin or turbidity samples were taken at these depths. This biphasic characteristic 0.2- of the downwelling irradianceis common to clearer waterscK;lrk, 0- A400 450 500 550 600 650 700 7 1983). In more turbid water bodies, exemplified by the south 3 Wavelength (nm) basin of Myvatn, the curve remains quite linear (Ed-MySB). The hirzher attenuation coefficients presented in Table 1 for Mvvatn 24 / P.H. HEINEMANN and M.A. ALI

Markeddifferences in the attenuationof PAR were seen itselfacting in conjunction with chlorophyll concentrations between the two basins of Myvatn. Thesemay be attributed to present.These effects are somewhat limited becauseof the the diluting influence of springs entering the 'east basin. These shallow nature of this lake. springs contribute significantly to a reduction in primary produc- The trimodal nature of the upwelling curves (Figs. 8b, 9b) in tivitywithin the east basin and asubsequent increase in both basins can be directly linked to the chlorophylla and algae transparency when compared with the south basin. present. The predominant alga found in the east and south basins The in-air downward irradiance spectra at Thingvallavatn andatthe time ofmeasurement was Anabaena jlos-uquae, a Myvatn (Figs. 7a, 8a) both show a number of small irradiance blue-greenalga containing chlorophyll a. In vivo absorption troughs at 430, 490, 520, 690 and 720 nm. These appear to measurements of another blue-greenSynechococcus leopoliensis coincide with Fraunhofer lines G (Fe and Ca, 430.8 nm), G' have demonstrated absorption maxima of440,625 and 680 nm (H, 434.0 nm), F (H, 486.1 nm), b, (Fe and Mg, 516.8 and (Doucha and Kubin, 1976). These coincide reasonably well with 516.7 nm), b2 (Mg, 517.2 nm), b, (Mg, 518.4 nm), D2 (Na, absorption troughs inthis study and with chlorophylla absorption 589.0 nm), D, (Na, 589.6 nm) and B (0, 687.0 nm; Weast, maxima in diethyl ether at 430, 615 and 662 nm (Kost, 1988). 1974). The trough at 720 nm is not represented by a FraunhoferChlorophyll a has been shown in living cells to have its broad line but may be the result of absorption by water vapour in the absorption in the red shifted to a maximum of 676 nm (Kirk, atmosphere (United States Air Force, 1960). The in-air spectra 1983; Davies-Colley et al., 1986). weresimilar to those previously recorded in arctic areas This comprehensive analysis ofthe underwater photic environ- (Henriksen et al., 1989) but quite different in terms of spectral ments of Thingvallavatn and Myvatn provides useful information distribution from those measured in subarctic regions (Nordtug that may be integrated with the existing and future ecological and Mela, 1988). Their measurements at As, Norway (59'40') studies in these ridge lakes. As well,it formsthe basis for further show considerably less light in the 500-700 nm range. comparisons with data gathered by remote sensing techniques The underwater downward irradiance spectral scans nearthe in this region. surface in Thingvallavatn and Myvatn (Figs. 7a, 8a) displayed Information regarding the visual abilities of the arctic charr some variability due to wave action. It has been clearly shown in Myvatn is not available but some data do exist for those in that waves may function as lenses with multiple focal lengths Thingvallavatn.The light-adapted ERGdetermined spectral (Schenck, 1957; McFarland and Loew, 1983) and contribute to wide intensity fluctuations in the near surface environment. sensitivity curves of arctic cham in Thingvallavatn (Fig.10) show The increased blue (blue-green) light penetration (Fig. 7) is a maximum sensitivity near 500 nm, which may be the result linked tothe relatively small amount ofgilvin present in of an interaction between blue and green cone receptors in the Thingvallavatn (Table 1). The importance of gilvin in the removal eye(Thorarensen and Einarsson, 1985; Thorarensen, 1987). of blue light from the downwelling stream has been observed They also found their dark-adapted spectral sensitivity measure- by many workers (Shapiro, 1957; Kirk, 1976; Verduin, 1982; ments to be similar in form and maxima. This was attributed Kishiho et al., 1984; Watras and Baker, 1988; Heinermann and to the absorption of a visual pigment (rhodopsin) with a maxi- Ali, 1988; Heinermann et al., 1990). mum absorption (b)of 509 nm (Mum and Beatty, 1965; The unusual reflection of the in-air 520 nm irradiance trough Mum and McFarland, 1965). In these studies no concurrent insubsequent underwater spectra (Fig. 7a) deserves some measurements of the underwater light fields were made. Other comment. There is a clear exponential decrease of its size with studieson fish have also demonstrated this close matching depth. This indicates that no amplification of the trough size between has ERG-recorded spectral sensitivities and visual pigment occurred. Onthe contrary, the processesof absorption and absorption maxima (Douglas, 1983; Pankhurst and Montgomery, scattering actto erode the size of the trough. The upward&& 1989). Douglas (1983) found in the rainbow trout thatthe light- fluorescenceof organic matter mayalso contribute to the adapted spectral sensitivity curve provided a good match with reduction of this trough (Bristowet al., 1981). This factormay 'hypothetical blue (A- = 440 nm) and green (& = 535 nm) be minimal in importance because of thesmall amount of organic cone pigments. He did not observe any interaction between these matter found in Thingvallavatn. Assuch, after verifying that this two receptors, nor did he measure underwater light. was not a result of an instrumenterror, this trough maybe related Few studies have attempted a direct comparison betweenthe to Fraunhofer lines b,, b2 and b,, found near 520 nm. quality of the underwater light and spectralthe sensitivity or visual Acomparison ofdownwelling and upwelling curves pigment absorptionof resident fish (Heinermann and Ali, 1985; (Figs. 7a,b) shows the upwelling onesto be somewhat narrower Crescitelli et al., 1985;Pankhurst and Montgomery, 1989; in the upper 2 m. One would expect the upwelling curvesbe to Heinermann and Ali, 1989).In the first three of these works and this study a good correspondence was found between the under- not as wide as the downwelling ones because they represent a part of the downward light from a greater depth (i.e., narrowerwater light and the visual sensitivity of the fish. Our findings spectraldistribution) that has been redirected upwards by provide support for the sensitivity hypothesis. It states that fish scattering (Kirk, 1983). This upward directed light is also morevisual pigments have their A- at wavelengths that are most diffuse, resulting in less variability in the spectral distribution abundantin theunderwater environment (Munz, 1965) to curves (Figs. 7b, 8b, 9b). optimizequantal catch and, consequently, maximize visual The downward irradiance recordings in Myvatn were all quite sensitivity. According to this proposal, objects in deep water, broad, with a maximum transmission occurringat 560 nm in the objects viewed at a distance and objects darker than the back- east basin and 570 nm in the south basin (Figs. 8a, 9a). East ground space light would be most effectively detected by a basin spectra wereunimodal, while thosein the south basin were pigment whose spectral absorbance matches the colour of the trimodal. Gilvin plays an important role in the reduction of background. The advantage of this kind of adaptation for the wavelengthsless than 500 nm, whilethe absorption of arctic charr in Thingvallavatn may be directly linked to their wavelengths longer than 600 nm can be attributed to the water feeding ecology. UNDERWATER PHUTIC ENVIRONMENT / 25

Both the large and small benthivores feed heavily under low . 1979b. Size andfood of arctic charSalvelinus alpinus and stickleback light situations (at dusk and at night) on a pulmonate gastropod Gnsterosteus anrleatus in Lake Myvam. Oikos 32:228-231. Lymmea peregra,while the small benthivore frequently includes ANTONSSON, U. 1977.Zooplankton of lake Thigvallavam 1972-75. Postgraduate thesis, Universityof Iceland, Reykjavik, Iceland.(In Icelandic.) chironomid larvae and pupae in its diet (Malmquistet al., 1985; ARNASON, B. 1976. Groundwater systems in Iceland traced by deuterium. Sandund et al., 1987). These prey are often seen against the Societas Scientiarum Islandica 42: 1-236. bottom itself and frequently would appear darkerthan it. In these BARLOW, G.W. 1974. Contrasts in social behaviour between central American cases having a spectral sensitivity matched tothe ambient light cichlid fishes and coral-reef surgeon fishes. American Zoologist 14:9-34. would be a very efficient adaptation. The planktivore feeds largelyBOWLING, LC, STEANE,M.S., and TYLER, P.A. 1986.The spectral distribution and attenuation of &water irmhce in Tasmanianinland waters. atdusk on zooplankton (Cyclops abyssorurn and Daphnia Freshwater Biology 16:313-335. longispinu) and some chironomid pupae. The zooplankton is BRISTOW, M., NIELSEN, D., BUNDY, D., and FURTEK, R. 1981. Use of relatively transparent, except for dark eyespots and their digestive water Raman emission to correct airborne laser fluorosensordata for effects system, so visual detection from a distance would be difficult. of water optical attenuation. Applied Optics 20:2889-2906. Their jerky movement in conjunction withdark eye spot could CHAMBERS, P.C., and PREPAS, E.E. 1988. Underwater spectral attenuation the and its effect on the maximum depth of angiosperm colonization. Canadian render them visible. For this morph movement detection may Journal of Fisheries and Aquatic Sciences 45:lOlO-1017. be more important than sensitivity matching. Piscivorous charr CRESCITELLI, F., McFALLNGAI, M., and HORWZ, J. 1985. The visual feed almost exclusively onthree spined sticklebacks. These prey pigment sensitivity hypothesis: Further evidence fromfishes of varying habitats. may often be seen at a distance and at times, depending upon Joumal of Comparative Physiology A 157:323-333. theangle of view, wouldappear darker than theambient DABROWSKI, K.R., andJEWSON, D.H. 1984. The influence of light environment on depth ofvisual feeding by larvae and fry of Coregonus polh spacelight. Again, matching its sensitivity theto background light (Thompson) in Lough Neagh. Journal of Fish Biology 25:173-181. would be a more effective strategy. It appears that for at least DAVIESCOLLEY, R.J., PRJDMORE, R.D., and HEm, J.E. 1986. Optid three of the four arctic charr morphs, having a spectral sensitivity properties of somefreshwater phytoplanktonic algae. Hydrobiologia matched to theunderwater light field would be a distinct 133:165-178. advantage in their feeding strategies. DOUGLAS, R.H. 1983. Spectral sensitivity of rainbow trout(Salnw guihn?. Revue Canadienne de Biologie Exphimentale 42:117-122. DOUCHA, J., and KIJBIN, S. 1976. Measurement of in vivo absorption se CONCLUSIONS of microscopic algae using bleached cells as a reference sample. Archivfur Hydrobiologie 49:199-213. The underwater light fields of Thingvallavatn and Myvatnare DUBINSKY, Z., and BERMAN, T. 1979. Seasonal changes in the spectral quite different in terms of both the quantity and quality of light composition of downwelling irradiance in Lake Kinneret (Israel). Limnology present. The amount of chlorophylla, gilvin and turbidity, which and Oceanography 24:655-663. ELORANTA, P. 1978. Light penetration in different types of lakes in central are to some degree a reflexion trophicof state, all make important Finland. Holarctic Ecology 1~362-366. contributions to these modifications. As a result the deep, oligo-FRENCH, C.S., BROWN, J.S., and LAWRENCE, M.C. 1972. Four universal trophic Thingvallavatn has a distinctive blue to bluish-green forms of chlorophyll a. Plant Physiology 49:421429. appearance, while the shallow, eutrophic Myvatn appears green GARDARSSON, A., GISLASON, G.M., and EINARSSON, A. 1988. Long to greenish-yellow Differences in light transmission betweenthe term changes in the lake Myvam ecosystem. Aqua Fennica 18:125-135. . GUTHRIE, D.M. 1986. Role of vision in fish behaviour.In: Pitcher, T.J., ed. east and south basins of Myvatn appear to be strongly influencedThe behaviour of teleost fishes. London: Croom Helm. 75-113. bysprings entering the east basin. The importance of HEINERMANN, P.H. 1986.The photic environment and scotopicvisual phytoplankton (chlorophyll) in modifying the underwater light pigments: A historical review and experimental approach using Senwtilus climate of Myvatn is clearly demonstrated.In addition, the mean ammacularus, C2tostomus commersoni and Emglossum maxillingua. M. Sc. thesis, Universitk de MonW, Montreal, Quebec. downwelling and upwelling irradiance spectra in Thingvallavatn HEINERMANN, P.H., and ALI, M.A. 1985. Correlation between the photic coincide very well with the light-adapted spectral sensitivities of environment and porphyropsin in thecutlips minnow, Exoglosm miUingua. the resident arctic charr. This has important consequences for Naturwissenschaften 72:488-489. the feeding ecology ofthese fish. Thesedata present a quantitative HEINERMANN,P.H.,andALI,M.A.1988. Seamalchangesintheunderwater andqualitative description of the underwaterlight fields of light climate of two Canadian shield lakes. Hydrobiologia 169:107-121. HEINERMANN,P.H., and ALI,M.A. 1989. The photic environment and Thingvallavatn and Myvatn and indicate that Thingvallavatn's scotopic visual pigments of thecreek chub,Sonotilus atromudatw and white charr have adaptedto maximize photon capture in support of the sucker, Catostomus conunersoni. Journal of ComparativePhysiology sensitivity hypothesis. A 164~707-716. HEINERMANN, P.H., JOHNSON, L., and ALI, M.A. 1990. The underwater photic environment of a small arctic lake. Arctic 43:129-136. ACKNOWLEDGEMENTS HENRIKSEN, K., STAMNES, K., and @STENSEN,P. 1989. Measurements of solar u.v., visible and near i.r. irradiance at 78 "N. Atmospheric Envhment We thank Jean Franqois St. Pierre and Marcel Babin, of Environ- 23~1573-1579. ment Canada, and Dr. Ellis Luew for the loan of the two underwater JEFFREY, S.W. 1980. Algal pigment systems. In: Falkowski, P.G.,ed. primary spectroradiometersused in this study. We are especially thankful to Smi productivity in the sea. New York: Plenum Press. 33-58. SWnfor his contribution to the organization,logistics and execution JONSSON, B., SKlkASON, S., SNORRASON, S.S., SANDLUND, O.T., of the sampling program at both Thingvallavatn and Myvatn. We are MALMQUIST, H.J., J6NASSON, P.M., GYDEMO, R., and LINDEM, T. grateful to S. Snorrasson and H.Malmquist for theirvaluable discussions 1988. Life history variation of polymorphic arctic charr (Salvelinus alpinus) and field assistance at Thingvallavatn. Our appreciation alsois extended in Thingvallavam, Iceland.Canadian Journal of Fisheries and Aquatic Sciences to Dr. G. Renninger and Dr. A. Mayumdar for their critical revisions 45~1537-1547. J6NSSON, G.S. 1987. The depthdistributionand biomass of epihthic periphyton of this manuscript. Financial support for this research was provided by in lake Thingvallavatn, Iceland. Archiv fir Hydrobiologie 108:531-547. NSERC to M.A. Ali. J6NASSON, P.M. 1979.The lake Myvatn ecosystem, Iceland. Oikos 32:289-305. 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