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 sunlmer stratification period (16 July to 9 August 1992) in the Seto Inland Sea, Japan. Coupled with wind mixing, Nand P-nutrients were introduced to the surface layer twice (19 July and after 30 July). A PC bloom (5 x 104 ml-l) occurred following the first mixing event, and a rapid decrease of this bloom seemed to be attnbuted to high abundance (>2 X 103 ml-'1 of heterotrophic nanoflagellates (HNAN). In contrast, the second w i n g event did not induce a PC bloom in spite of the high nutrient concentration, low abundance of HNAN (< 2 X 103 ml-l) and optimum temperature for growth. Field experiments on PC growth In the < 2 pm 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 experin~ental artifact, b'ut due to Fe-deficiency of the PC populations. This is based on results obtained in our previous studies concerning trace metal contaminatlon. It is suggested that Fe-deficiency was responsible for the lack of a PC bloom during the second rnixlng penod.