Geoscience Reference
In-Depth Information
decreased the P concentration. Hong et al. (1983) reported that diatoms can take up 30
times more P than they need and store it for use when P is defi cient. In the most years
except for 1994 and 2000, the concentrations of P in the upper layer (<75 m) were
even lower than the P concentration required for diatom growth (<0.1 μmol l
−1
, Zou et
al., 1983). The decreasing trends in Si concentration have probably been infl uenced by
the decrease in Si concentration in the Pearl River runoff since the 1970s (Lei et al.,
2003), but still adequate for diatom growth. At 200 m, the interannual fl uctuations of
both P and Si concentrations were high (Figures 8 and 9) and probably attributed to the
interannual changes in upwelling of deep water. In 1996 and 1999 the depth of the 20°
isopleth was shallow (average value from multi-stations was 93 m), high concentra-
tions of P and Si occurred (Pav = 0.61 and 0.62 μmol l
−1
, P
200
= 1.154 and 1.31 μmol
l
−1
, Si
av
= 25.18, and 19.55 μmol l
−1
, and Si
200
= 42.12 and 44.28 μmol l
−1
, respectively,
in the 2 years). In contrast, during 2004 when the depth of the 20° isopleths was deep
((the average value from multi-stations was 105 m), concentrations of P (P
av
= 0.37
μmol l
−1
, and P
200
= 1.13 μmol l
−1
), and Si (Si
av
= 8.9 μmol l
−1
, and Si
200
= 20.5 μmol
l
−1
) were obviously low.
Table 3.
Comparison of key nutrient concentration (μmol l
−
1
) and the ratio between the two phases
(before 1997, the first phase and after 1997, the second phase).
Nutrient and ratio
mean in the mean fi rst phase
mean in the second phase
SSDIN
1.83
5.75
DIN
av
4.22
7.78
DIN
200
12.77
20.33
SSP
0.33
0.25
P
a
υ
0.56
0.48
P
200
0.49
0.48
SSSi
12.38
9.19
Si
av
14.82
12.26
Si
200
27.54
25.85
SSDIN:SSP
5.5
23.0
DIN
a
υ
:P
a
υ
7.5
16.2
DIN
200
:P
200
26.1
42.4
SSSi:SSDIN
6.3
1.6
Si
a
υ
:DIN
a
υ
2.5
1.6
Si
200
:DIN
200
2.2
1.3
SSSi:SSP
37.5
36.8
Si
a
υ
:P
a
υ
26.5
25.5
Si
200
:P
200
56.2
53.8
