Phytoplankton periodicity of the Waitaki lakes, New Zealand

Abstract

The Waitaki River system in the South Island of New Zealand includes three large glacially-formed headwater lakes, Tekapo, Pukaki and Ohau, which drain into the manmade Lake Benmore. Phytoplankton periodicity was followed from December 1975 to January 1980 as part of a study investigating possible changes in these lakes as a consequence of hydroelectric development. The phytoplankton was highly dominated by diatoms, e.g., Diatoma elongatum, Cyclotella stelligera, Asterionella formosa, and Synedra acus, but in lakes Ohau and Benmore populations of green algae occasionally developed. In all four lakes seasonal phytoplankton periodicity was observed with maximum biomass in spring and summer. In Lake Tekapo, the first lake in the chain, maximum biomass did not exceed 300 mg m-3, but in the very turbid Lake Pukaki the maximum summer biomass ranged between 300 and 800 mg m-3. In Lake Ohau, the least turbid lake, maximum biomass was around 1 000 mg m-3. In the newly created Lake Benmore periodicity was less evident and summer maxima reached over 1 500 mg m-3. The phytoplankton periodicity in these lakes is greatly influenced by seasonal patterns of turbidity from inflowing glacial silt.