The Underwater Photic Environment of Two Subarctic Icelandic Lakes

Abstract

The underwater light fields of two Icelandic lakes of volcanic origin and differing trophic status, Thingvallavatn (oligotrophic) and Myvatn (eutrophic), were investigated. Gilvin and turbidity depth profiles were also measured. Diurnal variation in photosynthetically active radiation (PAR) reached almost 3 orders or magnitude. Downward irradiance 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 shifted from 480 to 500 nm with depth in Thingvallavatn, while in Myvatn they remained near 570 nm. An irradiance trough at 520 nm was noted in both the upward and downward spectra of Thingvallavatn. The importance of phytoplankton (chlorophyll) and gilvin in modifying the underwater light climate of Myvatn is clearly demonstrated. The mean downwelling and upwelling irradiance curves for Thingvallavatn 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