Photosynthetic response of phytoplankton to changing light intensity in a southcentral Alaskan lake

Abstract

Little fresh-water work has been done (compared with published ma­rine studies) using photosynthesis-irradiance (P-I) curves to determine photosynthetic response of natural assemblages of phytoplankton to light above and below thermal stratification structures. Limnological data including physical, chemical, and algal taxonomy and biomass were collected through the summers of 1985 and 1986 at Wasilla Lake, Alaska (approximately 61oN., 148oW.). Algal photosynthesis­irradiance relationships were also determined through the summer of 1986 by means of P-I curves. P-I curve light-limited initial slope (a) was ≈2.5 times higher in phytoplankton populations at 6 m than those in the wind-mixed zone when populations were separated by thermal stratification. Wasilla Lake’s trophic status was estimated to fall between mesotrophic and eutrophic classifications. Phytoplankton in Wasilla Lake were found to require approximately 4-5 days to best adapt to a changing light environment.