112 L
. Zou et al. J. Exp. Mar. Biol. Ecol. 249 2000 111 –121
1990. Studies in N-limited estuarine, coastal and oceanic waters show that the marine primary production is stimulated by N compounds of wet and dry forms from
atmospheric discharge Thayer, 1974; Pael et al., 1990, 1999; Willey and Cahoon, 1991; Willey and Paerl, 1993; Paerl and Fogel, 1994; Peierls and Paerl, 1997. Moreover,
rainfall produces a greater stimulation than either ammonium or nitrate when the amount of N is considered Pael et al., 1990; Paerl and Fogel, 1994. This may due to the other
elements in rainwater, which include P, Si, Al and Fe Martin et al., 1989; Duce and Tindale, 1991; Duce et al., 1991. Since Fe is a component of the enzymes responsible
for nitrate reduction to ammonium and N
fixation Stewart, 1974, the cooperation
2
between Fe and N in enhancing marine primary production has been affirmed Harrison et al., 1987; Martin et al., 1991; Ditullio et al., 1993; Pael et al., 1999.
The Yellow Sea is located in the northwestern Pacific Ocean, and has an oligotrophic character, which is somewhat similar to the central Pacific Ocean. Since input from
rivers and upwellings are small, atmospheric deposition plays an important role for nutrients. Previous studies indicate that 65 dissolved inorganic nitrogen DIN and
70 dissolved inorganic phosphorus DIP can be delivered to the surface of the Yellow Sea via the atmosphere Zhang and Liu, 1994. Rain-stimulated primary production is
considered as a category of ‘new production’ due to the nutrient input to the system by atmospheric deposition. It is estimated that nitrate in precipitation contributes about
4.3–9.2 of the nitrate requirement for the annual new production in the Yellow Sea. Three times higher production would be expected if dry nitrate deposition, and wet and
dry ammonium deposition are included Chung et al., 1998.
On an average, episodic deposition of nutrient elements accounts for only a small fraction of the concentration in seawater. However, individual rain events are directly
deposited on the sea surface and result in temporal eutrophication of surface waters. In this manner, chlorophyll a Chl-a and phytoplankton biomass in the surface water may
be greatly increased over short periods Owens et al., 1992; Mallin et al., 1993; Pearl, 1995, which may result in deleterious blooms Zhang, 1994; Paerl, 1997.
Very limited data are available in the literature on the relative production between phytoplankton and atmospheric deposition. However, in this study we provide data from
in situ observations on the effects of wet deposition on phytoplankton growth in the Yellow Sea during the summer, when stratification is dominant in the water column.
2. Materials and methods
2.1. Rainwater sampling Rainwater used in the experiment was collected on board ship at station B Fig. 1
when a strong typhoon passed through between August 18 and 20, 1997. The plastic rain collector was rinsed in 10 HCl and then rinsed with double distilled water DDW
before use. The collector was placed on the upper deck and left open to rain. The rainwater was immediately collected after the rain ceased, filtered through precleaned
0.45-mm pore-size filters and kept frozen until analyzed. The nutrient concentration of
L . Zou et al. J. Exp. Mar. Biol. Ecol. 249 2000 111 –121
113
Fig. 1. Location of sample stations.
rainwater was determined by traditional colorimetric methods for seawater samples Parsons et al., 1987.
2.2. In situ incubation experiment Sample location sites are shown in Fig. 1. Sub-surface water samples were collected
with a 5-l plastic sampler at A, C and D, and passed through a 180-mm mesh to remove large particles, and then transferred into 0.50-l incubation bottles. Triplicate samples
were prepared for each nutrient rain incubation group.
2.3. Incubation with rainwater A 50-ml volume of rainwater was added to 450 ml of sub-surface water. Then the
bottles were floated on the sea surface. The effect of salinity change was examined by incubation using 50 ml DDW and 450 ml seawater. The incubation experiment was
carried out for 24 h.
2.4. Nutrients effects NaNO , NH Cl, KH PO , NaSiF and FeCl from stock solutions were added to the
3 4
2 4
6 3
500-ml water samples to reach nutrient concentrations of 20, 10, 2, 10 and 2 mM, respectively, for each sample. NaNO , NH Cl, KH PO , and NaSiF stock solutions
3 4
2 4
6
114 L
. Zou et al. J. Exp. Mar. Biol. Ecol. 249 2000 111 –121 Table 1
Methods of chemical analysis for nutrient species, including precision and detection limits mM Species
Methods Detection
Precision limits
Nitrate Cd–Cu reduction
0.05 1.2
Nitrite Azo dye complex
0.02 0.5
Ammonium Indophenol blue complex
0.15 3.7
Orthophosphate Phosphomolybdate complex
0.03 0.6
Silicate Silicomolybdic complex
0.05 0.5
were prepared in the laboratory and stored at 48C. The unchelated FeCl solution was
3
freshly mixed with Milli-Q water in situ prior to experimentation. All containers and tubing for storing and dispensing were made of polyethylene or natural rubber, soaked
with 20 HCl for 24 h, washed by distilled water and then Milli-Q water. The following incubations were carried out for 24 h in the same manner as in the rain experiment.
In all experiments, control samples were prepared. After incubation, samples were filtered immediately through 0.45-mm pore-size filters and frozen until analysis.
2.5. Nutrient analysis All experimental containers e.g. incubation bottles were rinsed in 10 HCl for 24 h
and then rinsed thoroughly with DDW before use. Nitrate, nitrite, ammonium, orthophosphate and dissolved silicate were analyzed by classic colorimetric methods
Parsons et al., 1987. Detection and precision of the analyses are shown in Table 1.
2.6. Chlorophyll a analysis Samples for Chl-a were filtered using Whatman GF F filters. Particles on the filter
were extracted with 90 acetone at 48C in the dark for 24 h. Chl-a concentrations were determined by a fluorometric method Parsons et al., 1987.
3. Results
