In Situ Quantum Efficiency of Lake Superior Phytoplankton

Abstract

In situ quantum efficiencies were measured in Lake Superior over a 4-day period in 1978 and on one occasion in 1980. In 1980, experimental artifacts caused by exposing deep phytoplankton to elevated irradiances were minimized by the use of SCUBA divers. The trends of quantum efficiency with depth agreed well with theory. In the nutrient-limited upper portion of the euphotic zone, quantum efficiencies increased with depth, whereas in the light-limited lower portion of the euphotic zone, quantum efficiencies were relatively constant. Maximum quantum efficiencies calculated with downwelling irradiances ranged from 0.041 to 0.069 moles C fixed • Einst abs −1 with a mean maximum quantum efficiency of 0.0538 ± 0.0025 moles C fixed • Einst abs −1. Maximum quantum efficiencies in morning experiments ranged from 0.041 to 0.053 moles C fixed • Einst abs −1 and were slightly less than noon values, 0.057 to 0.067 moles C fixed • Einst abs −1. Correction for scalar irradiance would reduce all quantum efficiencies by 25%.