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of water temperature change were higher in the nSCS than in the Yellow Sea and the
Bohai Sea (Lin et al., 2001, 2005).
The increase in DIN in the nSCS during 1989-2004 was consistent with the in-
crease in DIN along the coast of the nSCS, such as Qinzhou Bay (Wei et al., 2002,
2003) and Daya Bay (Qiu et al., 2005). It also shows the same trend with the rise of
DIN observed throughout the global marginal seas (Seitzinger et al., 2002). Along
with the rapid economic development in China, DIN concentration in the Pearl River
estuary and shelf of the China Sea has been dramatically increased, due to the increas-
ing urbanization near the coastal areas, which resulted in more municipal sewage,
agricultural fertilizer, mariculture waste, and so forth inputs (SOAC, 2001b). Through
analysis based on DIN and PN models, combining with spatially explicit global da-
tabases, Seitzinger et al. (2002) showed that DIN input rates increased from approxi-
mately 21 Tg N y
−1
in 1990 to 47Tg N y
−1
by 2050. The largest increases are predicted
for Southern and Eastern Asia, associated with predicted large increases in population,
will increase fertilizer use, and increased industrialization. The DIN from the Pearl
River discharge increased by three times in 2002 as compared to 1986 (He et al., 2004,
Table 1), and NO
3
-N input from the Pearl in River Estuary was 1.7 times in 1999 of
that in 1987 (Guan et al., 2003; Wang and Peng, 1996, Table 1). The SSDIN should
be infl uenced by the increase in DIN from the river discharges. In addition, signifi cant
inputs of DIN into the nSCS have also occurred through atmospheric dry and wet
deposition (Zhang et al., 1999) and the upwelling of the deep waters (Zhao et al.,
2005). The mitigation of N limitation in the upper layer since 1998 was clearly related
to these DIN inputs (Figure 3). The increase in the annual rate of DIN was higher at
the 200 m layer (DIN
200
) than the water column average (DIN
av
) and at the sea surface
layer (SSDIN). This may be due to possible strengthening of the deep water upwell-
ing. However, during 1989-1997, the DIN
av
was low, the multi-annual mean was 4.2
μmol l
−1
and the lowest value of DIN
av
was only 1.7 μmol l
−1
in 1989. Since 1998, the
multi-annual mean of the water column average DIN concentration has reached 7.7
μmol l
−1
and the maximal value of the annual mean even reached up to 10.8 μmol l
−1
in
2001 (Figure 2b). And therefore, the previous status of N limitation in the nSCS was
signifi cantly alleviated. Moreover, the increases in the annual rates of DIN and NO
3
-N
were much higher in the nSCS than in the Yellow Sea (Lin et al., 2005).
The increase in the N:P ratio was due to an increase in DIN and a decrease in P
concentration. Before 1997, the N:P ratios (SSN:SSP, DIN
av
:P
av
, and DIN
200
:P
200
) were
lower than 10. Since 1998, these ratios have rapidly increased to 28, 18, and 16 in
2004, respectively (Figure 4). In 2004, the average values of DIN
av
:P
av
and DIN
200
:P
200
were close to the Redfi eld ratio (16:1), and therefore favorable to phytoplankton
growth (Hu et al., 1989; Jiang and Yao, 1999; Richardson, 1997). The high value of
SSN:SSP (28) in 2004 was probably due to dry and wet deposition. The peak value of
SSN:SSP appeared in 2002 (up to 86, Figure 3a), due mainly to high rainfall as high
Pearl River discharge, and this high ratio corresponded to the lowest surface salinity
(Figure 3a). Moreover, the fact that the annual rates of DIN and DIN:P were higher in
the shallow water area (<200 m) than in the deep water area (>200 m) (Table 2) sug-
gested the infl uence of the anthropogenic activities on the ecosystems of the shallow
or coastal waters. These results agree well with Xu et al. (2008) who also showed that
