Fourteen steelhead trout (Salmo gairdneri) were captured in the Matamek and Moisie rivers on the North Shore of the Gulf of St. Lawrence, near Sept-Îles, Québec, in the late summer and early fall of 1979. Steelhead trout are not indigenous to this area. The relatively large numbers of fish taken and their presence in these two rivers on the North Shore may indicate an expanding distribution and a potential threat of colonization in this area of pristine Atlantic salmon (Salmo salar) habitat.Key words: steelhead trout, Salmo gairdneri; geographic range, exotic fish

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This study examines the seasonal variation in the underwater spectral distribution of light in a mesotrophic (Lake Cromwell) and an oligo-mesotrophic (Lake Croche) temperate lake. Gilvin is primarily responsible for the strong selective attenuation of blue light in both lakes. As a result of differing gilvin concentrations light transmission maxima of downwelling and upwelling spectra are near 615 nm in Lake Cromwell and 599nm in Lake Croche. With increases in depth both upwelling and downwelling radiance fluxes decrease, are shifted to longer wavelengths and become more monochromatic. The greatest penetration of light occurs in the summer and spring after which a gradual decrease occurs through fall to a minimum value in winter. Under the winter cover the AP,, of downwelling light shifts 10 to 20 nm towards shorter wavelengths. Seasonal changes in downwelling irradiance are related to solar altitude, concentration of suspended particles, phytoplankton populations, amount of gdvin, mixing and winter cover. The brownish colouration of these lakes is explained by reflectance of spectrally impure orangish-red light.

ABSTRACT. Theunderwater light field inasmallarcticlakeon Victoria Island,Northwest Territories, was examined. Downward radiance was found to be bimodal, with transmission peaks at 480 and 640 nanometres (nm, or 10-9m). Upward radiance was similar near the surface, with peaks at 480 and 620 nm, but became unimodal with depth and shifted to 580 nm near the bottom. Diurnal variation in the underwater downward and upward irradiance of PAR (photosynthetically active radiation) was approximately two orders of magnitude. The spectral quality of light transmission also changed over this 24 hour period. Unimodal transmission of red light occurred in the early morning (1:OO and 5:OO) and late evening (22:00), while bimodal transmission of blue-green and red light was observed during the day (900-1730). Kd(z,), the vertical attenuation coefficient for downward irradiance at the midpoint of the euphotic zone, was relatively insensitive to changes in solar elevation. Diurnal variation in the reflectance of PAR differed from that predicted by previous simulation models, while the inverse relationship between reflectance and the absorption coefficient was in agreement with these same models. Gilvin, humic material-dissolved iron complexes, algal fucoxanthin, chlorophyll a and tripton all contribute to the attenuation of light and are responsible for the unique underwater spectral transmission in Keyhole Lake. Key words: arctic, limnology, underwater light, diurnal variation, spectral quality, irradiance, radiance RESUME. L'environnement photique sous-marin d'un petit lac arctique sur l'Isle Victoria, Territoires du Nord-Ouest a ete examine. La radiance descendante a eu un spectre bimodal avec deux pics de 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 avec une augmentation de la profondeur la transmission est devenue unimodale et a et6 deplace pres du fond vers 580 nm. La variation quotadienne de l'irradiance sous-marine descendante et ascendante de la PAR (la radiation disponsible pour la photosynthese) a et6 a peu pres deux ordres de grandeur. La qualite spectrale de la transmission lumineuse a egalement change pendant cette periode de 24 heures. Une transmission unimodale de la lumiere rouge a eu lieu tbt le matin (1:OO et 500) et tard le soir (2200) tandis qu'une transmission bimodale de la lumiere bleue-verte et rouge a et6 observe durant le jour (900-1730). Kd(z,), le coefficient vertical d'attenuation de l'irradiance descendante au milieu de la zone photique, n'a pas vraiement change avec des changements de l'6llevation solaire. La variation diurne de la reflexion de la PAR s'est differ6 de celle predit par des modeles de simulation precedents tandis que le rapport inverse entre la reflexion et la coefficient d'absorption a et6 en accord avec ces mCmes mod6les.l.a matiere organique dissoute, les composes de matiere organique -fer dissout, la fucoxanthine des algues, le chorophyllea et le tripton contribuent tous...