Effects of light history on primary productivity in a phytoplankton community dominated by the toxic cyanobacterium Cylindrospermopsis raciborskii

1. The effects of instantaneous irradiance and short-term light history on primary production were determined for samples from a subtropical water reservoir dominated by the toxic cyanobacterium Cylindrospermopsis raciborskii. C-14-bicarbonate uptake incubations were conducted on water samples from the reservoir, for irradiance (photosynthetically active radiation) ranging from 0 to 1654 mu mol quanta m(-2) s(-1). Prior to the C-14 incubations, cells were pre-treated at irradiance levels ranging from 0 to 1006 mu mol quanta m(-2) s(-1). 2. The average irradiance experienced by cells during the 2-2.5 h pre-treatment incubations affected the productivity-irradiance (P-I) parameters: exposure to high light in pre-treatment conditions caused a substantial decrease in maximum rate of primary production P-max and the photoinhibition parameter beta when compared to cells pre-treated in the dark. 3. While the data collected in this study were not sufficient to develop a full dynamic model of C. raciborskii productivity, P-max and beta were modelled as a function of pre-treatment irradiance, and these models were applied to predict the rate of primary production as a function of both instantaneous and historical irradiance. The results indicated that while cells with a history of exposure to high irradiance will be the most productive in high irradiance, production rates will be highest overall for dark-acclimated cells in moderate irradiance. 4. Our results may explain why optically-deep mixing favours C. raciborskii. If the mixing depth z(m) exceeds the euphotic depth z(eu), cells will be dark-acclimated, which will increase their rate of production when they are circulated through the euphotic zone. These results also predict that production rates will be higher during morning hours than for the same irradiance in the afternoon, which is consistent with other phytoplankton studies. 5. Since the rate of production of C. raciborskii-dominated systems cannot be described by a single P-I curve, accurate estimates of production rates will require measurements over the daily light cycle.