Geoscience Reference
In-Depth Information
and 0.005 y
−1
, respectively, Figure 5, Table 2). The SSS decreased by 0.653, while the
S
av
and S
200
increased by 0.206 and 0.151. The R
xt
values of time series of SSS, S
av
, and
S
200
were −0.41(P ≤ 0.05), 0.38 (P ≤ 0.05), and 0.44 (p ≤ 0.02), respectively (Table 2).
Concentration of Dissolved Oxygen (DO)
A significant (p ≤ 0.01) decrease in DO concentrations was observed (Figure 7, Table
2), and the annual rate was between −0.91 and −1.93 μmol l
−1
y
−1
. The regional mean
of DO of the SS, the water column average in the upper 200 m and at 200 m signifi-
cantly decreased by 26.5, 55.8 and 47.9 μmol l
−1
. The R
xt
values of time series for DO
ranged −0.48 to −0.81 (p ≤ 0.01, Table 2).
Concentrations of P, Si, and N
Both P and Si concentrations exhibited decreasing trends (Figures 8 and 9, Table 2).
Their annual rates were −0.008 to −0.017 (for P) and −0.071 to −0.387 (for Si) μmol
l
−1
y
−1
, respectively.
The concentrations of NO
3
-N, NO
2
-N, NH
4
-N, and DIN (DIN = NO
3
+ NO
2
+NH
4
)
in the nSCS exhibited obvious increasing trends during 1989-2004 (Figure 2, Table
2). The annual rates of the increases in DIN and NO3-N were 0.38-0.81 and 0.34-0.71
μmol l
−1
y
−1
, respectively. The regional means of DIN values of the SS, water column
average and 200 m layer increased by 8.06, 6.10, and 13.01 μmol l
−1
, respectively.
The DIN concentrations in the 200 m layer were signifi cantly higher than those of the
average for the water column and at the SS. Its annual rate was 1.61 times greater than
that of the SS. And its multi-year average value during observation period was 3.9
times higher than that of the SS layer. The R
xt
value for the time series of NO
3
-N and
DIN were between 0.55-0.74. The variation trends of the NO
3
-N and DIN time series
were highly signifi cant (p ≤ 0.01) (Table 2), except for the time series of NO
3
-N
200
,
which was signifi cant (p ≤ 0.03). It was also found that the annual rate of DIN was
higher in the shallow (<200 m) than in the deep water (>200 m) areas (Table 2). The
spatial-temporal distributions of the DIN
av
(mean concentrations in the water column)
indicated that since 1998 the high DIN
av
concentrations have exhibited a pronounced
large area increasing, that is after 1998 in the most study area, DIN
av
concentration
>5.71 μmol l
−1
(Figure 3, the shadow shows the area), and exceeded the low limit of
suitable N concentration for diatom growth (5.71 μmol l
−1
, Chu, 1949). So it is favor-
able for phytoplankton growth in the most study area.
Nutrient Ratios
During 1989-2004, the nutrient ratios (DIN:P and Si:DIN) changed significant-
ly. For example, the DIN: P ratios increased by 1.04-2.75 y
−1
, and it reached 28.1
(SSDIN:SSP) and 18.0 (DIN
av
:P
av
) in 2004 (Figure 4, Table 2); the Si:N ratios de-
creased at an annual rate of −0.10~−0.44 y
−1
, and it dropped to 1.0 (SSSi:SSDIN) and
1.4 (Siav:DIN
av
) in 2004 (Table 2). The variation trends of all time series of N:P and
Si:N were significant (p ≤ 0.01). The annual rates of increase in DIN:P and decrease
in Si:DIN were higher in the shallow coastal waters (<200 m) than in deep continental
shelf waters (>200 m) (Table 2).
