L . Zou et al. J. Exp. Mar. Biol. Ecol. 249 2000 111 –121
117
Fig. 3. Relationship between EV and TQ.
23 21
CR was observed at station A, that is, 21.20 mg m l
; while significant but positive
23 21
CRs occurred at stations C and D, i.e., 3.80 mg m l
. Results of the effects of light on Chl-a concentrations are shown in Fig. 3.
Concentrations of Chl-a were reduced when light radiation decreased.
4. Discussion
It has been reported that photosynthesis was phosphorus-limited in coastal waters of China Zhang, 1988; Zhang, 1994, whereas nitrogen limitation probably takes place in
Europe and in North American coastal waters McCarthy et al., 1977; Willey and Cahoon, 1991; Mallin et al., 1993; Zhang, 1994. Our studies indicate that phyto-
plankton is most likely P limited in the coastal Yellow Sea, which appears to be a result
32
of both low absolute PO concentration and a high N P ratio about 80:1. Wet
4
deposition carries both phosphate and other limiting elements e.g. Fe to the Yellow Sea. Given that the N P ratio in rainwater is 83 1, addition of rain to the surface
seawater will further increase the N P ratio. The incubation with phosphorus shows that concentrations of Chl-a at station A, C,
and D were increased to various degrees. The P limitation situation is alleviated followed by a phytoplankton biomass increase when P was added. Ammonium appears
to be the more important limited nutrient compared to nitrate, and Chl-a in ammonium incubations were twice as high as that in nitrate incubations. The experimental data in
this study are in agreement with previous studies that ammonium is preferred by phytoplankton over nitrate Kanda et al., 1990.
Martin and Fitzwater 1988 and Behrenfeld et al. 1996 have indicated that Fe could stimulate primary production in areas with high nutrient concentration Martin and
Fitzwater, 1988; Liu, 1992; Behrenfeld et al., 1996 and low Chl-a concentration. Chl-a, amino acid and protein content in phytoplankton increased and reached its maximum
when the Fe concentration is at 5 mM Liu, 1992; Chen et al., 1997. A 2 mM FeIII
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. Zou et al. J. Exp. Mar. Biol. Ecol. 249 2000 111 –121
solution in our incubation in situ led to a large increase in Chl-a concentration, which indicated again the special effect of Fe on the uptake of phytoplankton.
Chl-a concentrations in rainwater groups were 6.2 lower than that for the control in station A. This is most likely due to DIP dilution by the rainwater in the incubation
water sample 0.30 mM, although the DIN concentration was increased in the rainwater incubation experiment. Moreover, the N P ratio in the rainwater was about 83 1, which
is much larger than the 22 1 at station A. In contrast, DIP concentration at station C, diluted by rainwater, appeared positive. Chl-a concentration with rainwater incubation
23
was 0.19 mg m higher than the batch control. What could be the reason? First, the
change of DIP concentration was only 0.01 mM from 0.21 to 0.20 mM, which was quite limited compared to the absolute concentration of the batch control. Second, DIN
concentrations were significantly increased from 3.75 to 4.54 mM by rainwater, so that phytoplankton species of station C may have a different composition and respond
differently to nutrient level changes. At station D, both DIN and DIP significantly increased from rainwater, which increased the DIN concentration by 15 and the DIP
concentration by 17. Such a change in nutrient levels resulted in an increase of Chl-a twice as high as the batch control.
The experimental data show that phytoplankton growth e.g. Chl-a can be stimulated by single nutrient additions to various extents. Supposing that only nitrate, nitrite,
phosphate and silicate are considered, then the effect of rainwater in changing the Chl-a concentration introduced by rainwater can be estimated by
2 1
Chl-a ? [NO ]
1 Chl-a ? [NH ]
1 Chl-a ? [P] 1 Chl-a
NO 3
rain NH
4 rain
P rain
Si
3 4
23 2
23 1
? [Si] 5 0.016 mg m
per mM NO ? 4.81 mmol 1 0.032 mg m per mM NH
rain 3
4 3
23
? 7.00 mmol 1 0.040 mg m per mM P ? 0.14 mmol 1 0.003 mg m per mM Si
23 21
? 0.5 mmol 5 0.31 mg m l
3 where Chl-a
represents the Chl-a change stimulated by 1 mM of nitrate; and
NO
3
2 2
similarly for Chl-a , Chl-a , and Chl-a . [NO ]
represents NO con-
NH P
Si 3
rain 3
4
1
centration in rainwater; similarly for [NH ] , [P]
, and [Si] . Chl-a
?
4 rain
rain rain
NO
3
2
[NO ] represents the change in Chl-a concentration affected by nitrate in rainwater
3 rain
and similarly for Chl-a ?[NH ]
, Chl-a ?[P] and Chl-a ?[Si]
.
NH 4 rain
P rain
Si rain
4
23 21
Compared with the effect of natural rainwater incubation 2.13 mg Chl-a m l
, the estimated changes of Chl-a affected by supposed rainwater is only one sixth of that
from natural rainwater. It can be concluded that trace elements and organic materials in rainwater, which are under estimated in this study, could stimulate phytoplankton
growth.
Furthermore, compare the effect of rainwater EV, experimental value and quality of surface water TQ, total quality shown in Fig. 3, and suppose that
EV5 [Chl-a 2 Chl-a ] Chl-a
4
R C
S
TQ 5 DIN ? DIP ? Chl-a 5
S
where Chl-a , Chl-a , and Chl-a represent Chl-a concentrations in rainwater, the
R C
S
L . Zou et al. J. Exp. Mar. Biol. Ecol. 249 2000 111 –121
119
control and at the start. We concluded that there is a negative relationship between EV and TQ, which means that under the same rainwater environment, the lower nutrient
condition station station D, has a greater increase in Chl-a; while at the higher nutrient condition station station A, there is less change in Chl-a compared to the control.
Moreover, EV and TQ have a somewhat negative exponent relationship.
Solar radiation is an important factor for primary production Nielsen et al., 1979; Ning et al., 1991. From the data in this experiment, it may be seen that the Chl-a
concentrations appear to decrease with the lowering of the radiation. This can be brought into a broad concept that an increase in suspended matter concentration would
dramatically reduce photosynthesis, as shown in Fig. 4. A non-linear relationship between radiation and Chl-a concentration has been identified. Chl-a concentrations
23
decreased from 1.00 to 0.73 mg m with a reduction in light intensity of 100 to 80.
When light intensity was reduced to 0 i.e. black bottle incubation, Chl-a con- centrations were reduced to 42.
The Yellow Sea receives a significant amount of nutrients via atmospheric deposition. Atmospheric wet deposition may represent 65 and 70 of total input for DIN and DIP
nutrients, respectively Zhang, 1994; Zhang and Liu, 1994. Nutrient-enriched rains could dramatically enhance marine bioproduction McCarthy et al., 1977; Zhang, 1994.
The production enhanced by rainwater is ‘new production’, which is especially important for oligotrophic waters, such as the central Yellow Sea.
Rainfall data are available from local authorities. Annual rainfall in the east coastal Yellow Sea averages 600–700 mm, of which 60–80 takes place in the summer season
when the water column is stratified with DIN and DIP concentrations of 55.5 and 1.90 mM in rain Zhang, 1994. This suggests that approximately 505 mg N and 2.8 mg P are
put into the upper layer per square meter. Based on the Redfield ratio C:N:P5106:16:1,
22
a value of 115 mg C m of carbon fixation new production would be expected
annually from the input of nutrients by rain.
Fig. 4. Effects of irradiance on biomass.
120 L
. Zou et al. J. Exp. Mar. Biol. Ecol. 249 2000 111 –121
5. Conclusion
