DYNAMICS OF PICOCYANOBACTERIA IN THE SETO INLAND SEA (JAPAN) DURING SUMMER

Temporal and vertical changes of picocyanobacteria (PC) were monitored daily together with environmental variables during the summer stratification period (16 July to 9 August 1992) in the Seto Inland Sea, Japan. Coupled with wind mixing, N- and P-nutrients were introduced to the surface layer twice (19 July and after 30 July). A PC bloom (5 x 10(4) ml-1) occurred following the first mixing event, and a rapid decrease of this bloom seemed to be attributed to high abundance (> 2 x 10(3) ml-1) of heterotrophic nanoflagellates (HNAN). In contrast, the second mixing event did not induce a PC bloom in spite of the high nutrient concentration, low abundance of HNAN (< 2 x 10(3) ml-1) and optimum temperature for growth. Field experiments on PC growth in the < 2 mum seawater fraction conducted during the second mixing period revealed that addition of metal buffer solution enhanced the PC growth rates significantly. Although trace metal concentrations were not monitored and clean techniques were not used in the experiments, we infer that growth stimulation was not an experimental artifact, but due to Fe-deficiency of the PC populations. This is based on results obtained in our previous studies concerning trace metal contamination. It is suggested that Fe-deficiency was responsible for the lack of a PC bloom during the second mixing period.