Interaction of temperature and light on the growth rates of two planktonic Oscillatoria species under a short photoperiod regime

The growth rates under a 3:21 light:dark cycle of Oscillatoria redekei van Goor and O. agardhii Gom. were measured over a range of temperatures (5-23°C), and irradiances (5-200 µE m-2s-1). The temperature quotient of light-saturating growth, Q10, was close to 2·0 only over limited temperature ranges, 14-20°C for O. agardhii and 11-20°C for O. redekei. The temperature quotient of growth rate was greater than 2·0 at lower temperatures which coincided with increases in algal carbohydrate at the start of the light period. Carbohydrate increases during the light period were proportional to growth rate for both species. At subsaturating irradiances the growth rates of the two species were independent of temperature. A comparison of laboratory growth rates indicated that O. agardhii grew faster than O. redekei only at temperatures greater than 11°C when irradiances were greater than 100 µE m-2s-1. This is in broad agreement with the succession of the two species in Lough Neagh where O. redekei begins growth in the early spring, then gives way to O. agardhii in the summer.