Ecosystem Changes in the Northern South China Sea - Earth Science: New Methods and Studies - page 162

Chu, 1949). The average P concentration was also low. The SSP and P av were 0.33 and
0.56 μmol l −1 , respectively, which is closed to the low limit of suitable P concentra-
tion for diatom growth (0.48 μmol l −1 , Zhao et al., 2000). However, Si concentrations
(SSSi and Si av were 12.38 and 14.82 μmol l −1 , respectively) outclassed the low limit
of suitable for diatom growth (4.40 μmol l −1 , Harvey, 1957). Therefore, Si pool was
sufficient. N:P ratios were less than half the Redfield ratio of 16 (SSN:SSP and N av :P av
were 5.5 and 7.5, respectively).
In contrast, for the second phase after 1997, the average DIN concentration has
been clearly increasing (SSDIN and DIN av were 5.75 and 7.78 μmol l −1 respectively,
Table 3), and exceeded the low limit of suitable N concentration for diatom growth
(5.71 μmol l −1 , Chu, 1949). However, the average P concentration has decreased (SSP
and P av , were 0.25 and 0.48 μmol l −1 , respectively, Table 3). Consequently, there was
a rapid increase in DIN:P ratios (SSDIN:SSP and DIN av :P av were 23.0 and 16.2, re-
spectively, Table 3), which were close to the Redfi eld ratio (16, Richardson, 1997;
Hutchins et al., 1998). Therefore, it is favorable to phytoplankton growth in the second
phase. The Si concentration also decreased (SSSi and Si av were 9.19 and 12.26) in this
phase, resulting in rapid decrease in the ratio of SSSi:SSDIN (Si av :DIN av ), from 6.3
(2.5) to 1.6 (1.6) (Table 3). The Si concentration decrease may be due to the dam of the
river courses which signifi cantly reduced the silica delivery to the SCS.
Based on the studies on the kinetics of nutrient uptake, the thresholds of SiO 3 -Si,
DIN and PO 4 -P for phytoplankton growth have been estimated to be 2.0, 1.0, and
0.1 μmol l −1 , respectively (Justic et al., 1995). In the study area, the values of all
the nutrient parameters were over these threshold concentrations, except for those in
the fi rst phase, when the mean of SSDIN was close to the threshold of N (Table 3).
According to chemical stoichiometry, in the fi rst phase both SSN:SSP and DIN av :P av
was lower than 10, and SSSi:SSDIN, Si av :DIN av , and Si 200 :DIN 200 were all over 1, in-
dicating potential N limitation. In the second phase SSDIN:SSP and DIN 200 :P 200 were
higher than 22, and DIN av :P av was equal to the Redfi eld ratio. All Si:DIN ratios ranged
from 1.3-1.6, and Si:P ranged 25.5-53.8 (Table 3), which indicated that the potential
of P limitation increased and N limitation decreased. Furthermore, Si was always suf-
fi cient during the observation period, even in the second phase when its concentration
decreased (Table 3).
Response of Ecological Environment to ENSO Events
When El Nino occurs, the warm pool of the western Pacific Ocean moves eastward,
whereas it moves westward during La Nina (Takeuchi, 1987; White et al., 1985; Zhang
and Huang, 1993). The nSCS is located to the west of the warm pool however the
response of the nSCS has not been well documented. In the present study, pronounced
responses to ENSO were found. During the observation period, nine El Nino events
(1976, 1982-1983, 1986-1987, 1991, 1993, 1994, 1997, 2002, and 2004) and four La
Nina events (1981, 1988, 1995, and 1998-1999) occurred (Levimson, 2005; Mcphaden,
2004; Qin, 2003; Wang and Gong, 1999). In general, whenever El Nino/La Nina oc-
curred, SST and Tav was low/high in the nSCS (Figure 6, Table 4).
In the area southwest of Dongsha Islands, near Station 4 (19-20°N, 116-117°E,
Figure 1), pronounced responses to ENSO were observed. In general, whenever an El