Abstract: Picophytoplankton (0.2-2.0 μm in size) is the smallest group of autotrophic plankton, being abundant and widespread in the world ocean and playing an important role in the organic matter cycling in ocean. By the method of epifluorescence microscopy (EFM), the abundance and its spatial and diurnal variations of the picophytoplanktons in Qingdao offshore and its adjacent waters in winter were investigated. The results showed that in the study area in winter, phycoerythrin-rich (PE) Synechococcus cells were dominant, followed by Picoeukaryote (Euk) cells, while the abundance of phycocyanin-rich (PC) Synechococcus cells was low and no Prochlorococcus (Pro) cells were observed. The abundance of Synechococcus (Syn) and Euk varied from 8.97×10³ to 1.95×10⁵ cells·ml^-1(averaged 4.67×10⁴ cells·ml^-1) and from 1.95×10² to 1.01×10⁴ cells·ml^-1(averaged 2.39×10³ cells·ml^-1), respectively. There was a high-value of Syn abundance in Jiaonan offshore and a low-value in Jimo and southeast Laoshan off-shores, while a high-value of Euk abundance in Rizhao offshore and a low-value in Laoshan offshore. No significant difference was observed in the vertical distribution of Syn and Euk abundance among four water layers (P>0.05) at a continuous station located in the center of Jiaozhou Bay, the abundance had an obvious diurnal fluctuation. Pearson correlation analysis indicated that Syn was positively correlated with water temperature and electrical conductivity (P<0.01) but negatively correlated with dissolved oxygen concentration (P<0.01), and Euk was negatively correlated with water salinity and dissolved oxygen concentration. In the study area in winter, picophytoplankton contributed about 20％ to the total phytoplanktonic biomass.

Key words: mineralization characteristics, organic matter, labile organic matter, organic fertilizer, nitrogen, phosphorus and potassium, carbon management index (CMI)