Ecophysiological influence of light and mixing on Anabaena circinalis (Nostocales, Cyanobacteria)

The physiology (growth rate, biochemical composition and flotation rate) of the toxic cyanobacterium Anabaena circinalis was studied in relation to irradiance and mixing regime in two small-scale (250 ml or 11) batch culture experiments and one medium scale (1 m high, 4.4 1), semi-continuous microcosm experiment. In batch cultures, A. circinalis had a relatively high irradiance requirement as indicated by its compensation irradiance, E-c (13 +/- 2 mu mol m(-2) s(-1)). The growth efficiency (alpha(g)) of A. circinalis, estimated as the initial slope of the growth-irradiance curve, was in the mid to high range depending on the model applied (Monod: 0.33 +/- 0.19 m(2) mol(-1), exponential: 0.43 +/- 0.15 m(2) mol(-1)). The flotation rate of the A. circinalis population was 0.69 +/- 0.11 m d(-1) at 100 mu mol m(-2) s(-1) and decreased at higher irradiance to be negative at irradiances over 135 mu mol m(-2) s(-1). In a microcosm experiment, different mixing intervals were tested (10 min (MIXED) and 48 h (CALM)) across three irradiance treatments. The irradiance treatments differed in both their degree of vertical light attenuation and mean integrated irradiance. The mixing regime had no effect on growth rate in the high irradiance ( HE) and medium irradiance (ME) treatments (mean integrated irradiances 60 and 15 mu mol m(-2) s(-1), respectively). However, an increase in A. circinalis flotation rate in the lowest irradiance treatment (LE; mean integrated irradiance 7 mu mol m(-2) s(-1)) resulted in a 44% higher growth rate in the CALM microcosm through access to greater irradiance. This suggests that short stable periods (e.g. diurnal strati. cation) in highly light limited treatments will assist the growth of A. circinalis due to its ability to gain ready access to light in the euphotic zone.