PICOPLANKTON DYNAMICS IN DAVIES REEF LAGOON, THE GREAT-BARRIER-REEF, AUSTRALIA

The diel variations in abundance and frequency of dividing cells (FDC) of coccoid cyanobacteria in a coral reef lagoon were investigated in June, September, and December 1989, and April 1990. Cyanobacteria and picoplanktonic eukaryotes (<3 mum) were sampled monthly from January to December 1990. The average abundances of cyanobacteria and eukaryotes ranged between 1.17-10.06 x 10(4) cells ml-1 and 0.16-2.41 x 10(4) cells ml-1, respectively, with abundances of both being higher in summer (November-April) than in winter (May-October). The ratio of cyanobacteria to eukaryotes fluctuated from 1.93 to 8.67, independent of the seasonal variation in their abundances. The instantaneous growth rate of cyanobacteria, which was estimated from the daytime abundance increment, ranged between 0.430 and 3.144 day-1. The estimated daily specific growth rate of cyanobacteria by the FDC method ranged between 0.231 and 0.966 day-1. In April, despite the high specific growth rate and low flushing conditions, cyanobacterial abundance showed a cyclic diel pattern, suggesting a strong grazing impact on their population.