동중국해 북부해역의 식물플랑크톤 군집구조 및 분포특성

Abstract

Construction of the Three Gorges Dam has changed both water mass characteristics in the northern East China Sea(ECS) and the fluctuations of diluted water outflow in the Chang Jiang; this study aimed to investigate whether these alterations to the marine environment in these places are affecting phytoplankton community structure. Changes to the marine environment were determined by investigating long-term variations in (water temperature, salinity, Chlorophyll-a, nutrients) in the northern ECS from 1995 to 2017. We compared the marine environmental factors and phytoplankton communities at one station in the summer of 1998 to the results of this survey (2015-2017; same station). Notably, the distribution characteristics of picophytoplankton, which is becoming more important to the offshore zone, were compared. The water temperature in the northern ECS decreased by 0.62℃ over 23 years, which is opposite to the trend of global warming, and the salinity decreased by, which was the result of a decrease in freshwater inflow from the Chang Jiang in the summer. In addition, the effect of the Yellow Sea bottom cold water(YSBCW) became strong whereas that of the high-temperature and high-salt Kuroshio water(KW) became weak. The remarkable changes in these two water masses, which are opposite in terms of water temperature and salinity, seemed to have greatly influenced the water temperature and salinity in the northern ECS. In the northern ECS, nitrate and nitrite levels increased from 1995 to 2017 whereas phosphate and silicate levels decreased. However, levels of all nutrient salts have decreased since 2010; this may be due to the decreased influence of the Changjiang diluted water(CDW)and Kuroshio water, which are the main sources of nutrient salt inflow, and decreased levels of nutrients due to the frequent occurrence of red tide in the Chang Jiang estuary since 2000. In conclusion, the marine environment in the northern ECS has changed due to natural phenomena such as changes in water masses and frequent red tide occurrences at the estuary of Chang Jiang, thus resulting in changes to the phytoplankton community. Diatoms and dinoflagellates were the dominant species in the phytoplankton community during the spring and summer seasons of the early and mid-2000s. On the other hand, nanoflagellates of less than 10 μm showed the dominant rate of 15.8–53.8 % in all seasons except in the winter and were the most dominant species; these results are different from those of previous community structure studies. Comparing the marine environment at the same station during the summer of 1998 and that in this study showed that the overall levels of nutrient salts were lower in this study than in the summer of 1998; in particular, phosphate concentration was very low (0.1 µM or less). In 1998, nitrogen level was the limiting factor, but in this survey, it was replaced by phosphorus. Thus, the 1998 community centered on diatoms and the community in this survey centered on nanoflagellates of 10 μm or less. In terms of chlorophyll-a‘s according to size, picochlorophyll Chl-a showed a high contribution rate of 60% or higher in the spring and summer; the highest contribution rate was observed in the surface mixed layer during the summer when the phosphate concentration was low. In addition, the contribution rate of picochlorophyll Chl-a was higher at the eastern station than at the western station, where nutrient salt levels were high. Therefore, we expect the phytoplankton community to undergo miniaturization in the northern ECS, where nutrient salt levels are decreasing, resulting in a decrease in primary productivity. The food web will also change from a simple food web centered on diatoms with high primary productivity to a complex microbial food web based on picophytoplankton with low primary productivity. In addition, the rate of carbon transfer to higher consumer levels will be lowered, thus leading to reduced marine productivity. To investigate the temporal-spatial distribution of picophytoplankton in relation to different water masses in the northern ECS, picophytoplankton abundance were investigated using flow cytometry with environmental factors in 2016–2017. The results from the analysis of flow cytometer data showed that Synechococcus appeared across all seasons, exhibiting its minimum abundance in winter and maximum abundance in summer. Furthermore, high abundance was detected in the surface mixed layer during spring and summer when vertical stratification occurs; in particular, Synechococcus exhibited maximum abundance in thermocline layer, indicating a close correlation to water temperature and thermocline formation. In addition, the abundance of Synechococcus indicated a decrease in the western seas in 2017 compared to 2016 under the strong influence of the Changjiang Diluted Water(CDW). This was determined by the significant influence of the CDW on the abundance of Synechococcus during summer in the northern waters of the ECS. In contrast, Prochlorococcus did not appear during winter and spring, and its distribution was limited during summer and autumn in the eastern seas under the influence of the Kuroshio current. The largest range of Prochlorococcus distribution was confirmed during autumn without the influence of the CDW. Thus, the distribution pattern of each picophytoplankton genus was found to be changing in accordance to the extension and reduction of sea current in different seasons and periods of time. This is anticipated to be a useful biological marker in understanding the distribution of sea currents and their influence in the northern ECS.