Abstract:To explore the seasonal variations in the phytoplankton community structure and their relationship with environmental factors in the offshore area of Laoshan, we designed 15 monitoring stations and made six investigative voyages in the spring, summer, and autumn during 2016~2017 while monitoring other environmental factors simultaneously. We detected 69 species belonging to 31 genera and two phyla of phytoplankton, most of which belong to temperate coastal species. Diatoma was the dominant phylum comprising 25 genera and 60 species, and dinoflagellates comprised 6 genera and 9 species. The seasonal variations in the dominant species were obvious. The most dominant species were Coscinodiscus oculus-iridis, Coscinodiscus asteromphalus, and Ceratium fusus. The total number of phytoplankton ranged from 732.10×104 to 1142.19×104 cell/m3, with an average of 937.15×104 cell/m3. The number of phytoplankton cells changed significantly during the year. The highest peak in 2016 appeared in autumn and the second highest peak in summer; the highest peak in 2017 appeared in summer and the second highest peak in autumn. The spatial distribution showed a decreasing trend from north to south, and the high-value areas appeared in Wanggezhuang Bay, Nanyao River, and Shilaoren Beach. The diversity and abundance of phytoplankton in the surveyed area are at a high level, and the phytoplankton community structure in this area is relatively stable, indicated by the analysis of the phytoplankton community structure index. The result of cluster analysis showed that the interannual variations in phytoplankton cell abundance are unobvious in spring, and the similarity is approximately 70% both in 2016 and 2017. However, the interannual variations in phytoplankton cell abundance are relatively obvious in summer and autumn, and the similarity is only approximately 40%, which is mainly related to environmental factors. The correlation analysis between phytoplankton community structure and environmental factors showed that the correlation coefficient between cell abundance average and COD is the highest (0.536) and the two factors that were most related to environmental factors were pH and COD, COD and DIN, and DO and COD. Therefore, the key limiting factors for phytoplankton cell abundance in the offshore area of Laoshan are COD, pH, DIN, and DO.
