Long-term ecological interactions between nutrient and phytoplankton community in the Changjiang estuary

Both nitrate (NO3−) and soluble reactive phosphate (PO43−) concentration in the freshwater end-member at the mouth of the Changjiang River have increased dramatically since the 1960s. Within the same period in the sea area, with surface salinity>30, NO3− concentration has shown an obvious increase, PO43− has not changed greatly and dissolved reactive silica (SiO32−) has deceased dramatically. An examination of the elemental ratio of NO3− to PO43− at the mouth of the Changjiang River did not show a systematic trend from the 1960s to 2000s largely because both nutrients increased simultaneously. In comparison, the elemental ratio of dissolved inorganic nitrogen (DIN) to PO43− in surface seawater, with salinity>22, has shown a clearly increasing trend. Furthermore, an overall historical change of the SiO32−:PO43− ratio has undergone a reverse trend in this area. Based on the changes of SiO32−:PO43− and DIN:PO43− ratios, we can conclude that an overall historical change of SiO32−:DIN ratio has decreased in this area from the 1950–1960s to 2000s. The argument that phytoplankton productivity in the Changjiang estuary has been enhanced by increasing nutrient input from the riverine transport was supported by these results. A comparative study analyzing the shift of phytoplankton composition from the mid-1980s to 2000s was also made. The results indicated that the average yearly percentage of diatom species in the Changjiang estuary has decreased from 84.6% during 1985–1986 to 69.8% during 2004–2005. Furthermore, the average yearly percentage of diatom abundance in the Changjiang estuary decreased from 99.5% during to 75.5% over the same time period, while the abundance of dinoflagellates has increased dramatically, from 0.7% to 25.4%.