Limnology of two lake systems of Katmai National Park and Preserve, Alaska: Part II. Light penetration and Secchi depth

Seven large lakes in the Naknek River drainage and four in the Alagnak Rive r drainage within the Katmai National Park and Preserve, Alaska, were surve yed once a summer during the period 1990-92 to determine baseline limnologi cal conditions. All of the lakes are oligotrophic based on Secchi depth (SD ) transparency and light penetration. Overall, SD transparency varied from 4.4 m to 17 m, the vertical light extinction coefficient (K-d) ranged from 0.411 m(-1) to 0.070 m(-1) and the depth of 1% light penetration (I-1%) var ied from 11 m to 67 m. However, because of greater light scattering, the pe rcent of photosynthetic radiation (PAR) at SD was nearly twice as much in B attle Lake (30.4%) and Naknek Lake (32.8%), compared with the other nine la kes (mean 16%). Consequently, the ratio of I-1% to SD was about 4 in these two lakes compared to a mean value of 2.6 for the other lakes. However, Bat tle Lake is a `deep blue' calcium sulfate lake with little phytoplankton, w hereas Naknek Lake contains some inorganic glacial flour and volcanic ash, as well as planktonic algae, but where sampled exhibits minimal turbidity. Biomass of planktonic algae (indexed by total chlorophyll concentration) ex plained most of the variation in SD (r(2)=0.66), K-d (r(2)=0.75), and I-1% (r(2)=0.85). In contrast, neither color nor turbidity were significant pred ictors of any optical variable. Considering all 11 lakes, there was a signi ficant linear relationship between SD and both K-d (r(2)=0.80) and I-1% (r( 2)=0.72); however, most of the unaccounted for variation was attributed to Battle Lake and Naknek Lake. Although changes in water transparency are oft en linked to changes in algal biomass (chlorophyll), simple measures of SD transparency alone may not be appropriate for assessing whole-scale watersh ed or regional changes toward oligotrophication or eutrophication in lakes of the remote and pristine Katmai National Park and Preserve.