Laboratory experiments on the effects of light and temperature on the uptake of nutrients by Lake Rotongaio phytoplankton

Abstract

Laboratory experiments were carried out to investigate the effect of sustained light and temperature on the uptake of PO4 ³ ‐P, NH/‐N, and NO3 ^‐‐N and on the biomass of the phytoplankton of Lake Rotongaio. Two sets of experiments compared differences between the seasonally contrasting dominant phytoplankton species, which consist of a filamentous nitrogen‐fixing cyanophyte in summer and diatoms in winter. The extent to which nutrients became exhausted, cell N/P ratios changed, and nitrogen‐fixation was enhanced, depended on the amount of light supplied and strongly indicated that light is the limiting factor for phytoplankton growth under normal lake conditions. Re‐illumination after extended periods of darkness suggested that sedimentation below the euphotic zone could be tolerated by the phytoplankton, and that growth could be quickly reestablished upon re‐entry to better light conditions. Temperature could have some effect on metabolic rate but is probably of minor importance compared to light availability for the species selection and overall nutrient dynamics of the lake.

Key words:

• phytoplankton
• nutrients
• light
• temperature
• Lake Rotongaio