The Relationships of the Nutrients in the Changjiang River Estuary and the Flow of the Changjiang River Water

Abstract

In the Changjiang estuary, the high concentration area of phosphate (PO₄–P), SiO₃–Si and NO₃–N gradually reduced with the reduction of the area of the Changjiang diluted water from summer, autumn to winter. The seasonal distributions of the nutrients were mainly controlled by the river flow and were also related to the growth and decline of phytoplankton . The conservation of SiO₃–Si and NO₃–N in the estuary in the flood season was poorer than that in the dry season . The behaviour of PO₄–P shows that in addition to biological removal , the buffering of PO₄–P is possible in the estuary. The Changjiang’s annual nutrient transports to the sea were estimated. The investigation shows that the seasonal variations of the nutrient transport closely correlate with that of the flow of the Changjiang River and is confirmed by the investigations in 1997–1998 (Shen et al. in Ambio 32:65–69, 2003; Shen in Acta Geogr Sinica 61:741–751, 2006) and 2004 (Shen et al. in Environ Monitor Assess 184:6491–6505, 2012).

It is well known that the 6300-km-long Changjiang River is the largest river in China and the third largest in the world. The average Changjiang River runoff into the East China Sea is 9282 × 10⁸ m³ a⁻¹ or 29,000 m³ s⁻¹. Flood season (May to October) runoff is 71.7% of annual runoff . After entering the sea, the Changjiang River diluted water is mixed continuously with Taiwan Warm Current Water from the south and Yellow Sea water from the north. The convergence of various water masses in the area near the Changjiang River estuary and the great quantity of nutrient material transported annually by river water to the estuary makes the area a most important fishing ground in China and known as one of the best in the world. This study is mainly based on the results of eleven investigations in the Changjiang River estuary from August 1985 to July 1986 (station locations in Fig. 1). The seasonal variations of the phosphate (PO₄–P), silicate (SiO₃–Si ) and nitrate (NO₃–N) , and their relation to the Changjiang River flow , and effects on the transportation, distributions, variations and the behaviour of the nutrients in the estuary, nutrients transport , etc., are discussed.

Fig. 1

Sampling stations

1 The Seasonal Variations of the Nutrients in the Changjiang River Mouth

The seasonal variations of the average nutrient concentrations in the Changjiang River mouth freshwater and the average flow of the Changjiang River at Datong hydrological station are indicated in Fig. 2. The concentrations of PO₄–P and NO₃–N were high (average 0.62 and 63.7 µmol L⁻¹, respectively) in the flood season and low (average 0.48 and 51.1 µmol L⁻¹, respectively) in the dry season . The highest concentrations were in August for PO₄–P (average 0.88 µmol L⁻¹) and in May for NO₃–N (average 81.6 µmol L⁻¹), and the lowest were in March (average 0.34 and 43.7 µmol L⁻¹, respectively). The relationship between the seasonal variation of SiO₃–Si concentration and the flow of the Changjiang River water was not regular. The highest concentration of SiO₃–Si was 191.5 µmol L⁻¹ in November, and the lowest was 58.5 µmol L⁻¹ in April.

Fig. 2

Seasonal variations of the nutrients and flow in the Changjiang River water: filled diamond—SiO₃–Si ; filled triangle—NO₃–N; filled circle—PO₄–P ; filled square—Q

2 The Relationships Between the Seasonal Variations of the Nutrient Concentrations in the Changjiang River Estuary Sea Water and the Flow of the River Water

The relationships among the seasonal variations of the average concentrations of the nutrients in the surface sea water (the area west of 123º30′ E), the flow of the river water and phytoplankton are indicated in Fig. 3, which shows that during the annual phytoplankton peak in the flood season (August to September) PO₄–P , SiO₃–Si and NO₃–N concentrations all decreased. After September, although the flow was higher, there was a rapid decrease of phytoplankton and marked increase in the concentrations of SiO₃–Si and NO₃–N to the annual maximum of 30.9 µmol L⁻¹ for SiO₃–Si and 13.3 µmol L⁻¹ for NO₃–N in November. The concentration of PO₄–P rose only after October and reached a peak in November. In late autumn and winter (dry season ), phytoplankton was at low ebb; the concentrations decreased a little for PO₄–P and markedly for SiO₃–Si and NO₃–N. The annual lowest values of SiO₃–Si and NO₃–N appeared in April. Then, all nutrient concentrations increased with the increase of the river water flow . In May to June, due to phytoplankton bloom , all of them decreased. The seasonal variations of PO₄–P , SiO₃–Si and NO₃–N show that in addition to control by the Changjiang River water flow , their concentrations were also related to the growth and decline of phytoplankton .

Fig. 3

Relationships among the seasonal variations of the nutrients in the Changjiang River estuary sea water and the flow of the Changjiang River water and phytoplankton : filled diamond—SiO₃–Si ; filled triangle—NO₃-N; filled circle—PO₄–P ; filled square—Q; multiplication sign—phytoplankton

The seasonal variations of the nutrient concentrations in the sea area were similar in many ways to those in the Changjiang River mouth. The maximum concentrations of PO₄–P all appeared in August, and the seasonal variations were small. The monthly average concentrations of PO₄–P at the seaward side of the river mouth were between 0.29 and 0.68 µmol L⁻¹ and a little higher in winter than in summer, which may be related to oxidation and decomposition of organisms. The change of SiO₃–Si concentration here was very similar to that in the river freshwater, the maximum in November, second in October, and the minimum in April. The relationship between them was as follows:

$$\begin{aligned} {\text{SiO}}_{3} {-}{\text{Si}}\;({\text{in seawater}}) & = 0.200\;{\text{SiO}}_{ 3} {-}{\text{Si}} \\ & \quad - 2.454\;({\text{in river water}})\;\left( {r = 0.877,n = 10} \right) \\ \end{aligned}$$

The concentration of NO₃–N in the sea area was also higher in the flood season (average 10.4 µmol L⁻¹) than in the dry season (average 7.6 µmol L⁻¹), and there was a delay in the seasonal variation as compared with that in the river water area. The concentration of NO₃–N in the river water area decreased to minimum from October to next March, rapidly rose again until May and then decreased again. NO₃–N concentration in the sea area decreased from November to next April (minimum in April), rapidly rose again until June and then decreased again.

3 The Effects of the Changjiang River Water Flow on the Distributions and Changes of the Nutrients in the Sea Area

From August , November to January (from the flood to dry season ), the average Changjiang River runoffs into the sea were 3.38 × 10⁴, 1.95 × 10⁴ and 0.96 × 10⁴ m³ a⁻¹, respectively. The Changjiang River diluted water area gradually reduced from August to January, and the 30 isohalines contracted towards the river mouth. The research shows positive linear correlationships between the Changjiang River diluted water area and the Changjiang River runoff into the sea (Zhang et al. 1987). With the reduction of the area of the diluted water, the higher concentration areas of the nutrients were reduced and all the isograms gradually moved towards the river mouth. For SiO₃–Si and NO₃–N (Figs. 4 and 5), the areas covered by the 20 µmol L⁻¹ isogram were smaller and smaller from summer to winter, and in winter the waters of 20 µmol L⁻¹ concentration for SiO₃–Si were limited to west of 122º30′ E and for NO₃–N near the river mouth. This shows that the seasonal distributions and changes of the nutrients are mainly controlled by the flow of the Changjiang River.

Fig. 4

Distribution of SiO₃–Si (µmol L⁻¹) in surface sea water: solid line—August; dashed line—November; dotted line—January

Fig. 5

Distribution of NO₃–N (µmol L⁻¹) in surface sea water: solid line—August; dashed line—November; dotted line—January

4 The Effects of the Changjiang River Flow on the Behaviours of the Nutrients in the Estuary

Correlation statistics show that there were monthly negative linear correlationships between SiO₃–Si or NO₃–N and salinity and that the correlationship between PO₄–P and salinity was not close. The correlation coefficients and statistical ranges are listed in Table 1, showing that the correlationships between SiO₃–Si or NO₃–N and salinity , respectively, in the flood season were poorer than that in the dry season . This was due to the phytoplankton bloom (average 7648 × 10⁴ cells m⁻³ in August). In the Changjiang River flood season, nutrients were largely taken up by phytoplankton in surface seawater and the decomposition of organisms released nutrients in the lower seawater (mainly in the waters of salinity higher than 30). It is evident that the change of SiO₃–Si with salinity was similar to that of NO₃–N with salinity and that there was positive linear correlationship between SiO₃–Si and NO₃–N (in August) indicated as:

Table 1 Correlationships of the nutrients and salinity

$${\text{NO}}_{3} {-} {\text{N }} = \, 0.582\,{\text{SiO}}_{3} {-} {\text{Si}} + 0.913\;(r = 0.936, \, n = 51)$$

It clearly shows that part of them were removed through biological processes .

The correlationship between PO₄–P and salinity was poor in the flood or dry season . The investigations show that the concentration of PO₄–P in surface water outside the river mouth was higher than that within the river mouth in many months, and that its distribution was even in some months. In April (Fig. 6), the concentration of PO₄–P was 0.38 ± 0.058 µmol L⁻¹ all over the surveyed area where salinity was between 0 and 33.7. The behaviour of PO₄–P in the estuary shows that in addition to biological removal , PO₄–P removal can also possibly be caused by the buffering action of suspensions and sediments of the estuary. The author thinks this is the main reason that in the long term PO₄–P content is at the same level in the sea area of the Changjiang River estuary .

Fig. 6

Relationship between PO₄–P and salinity (April)

5 Nutrient Transport to the Sea from the Changjiang River

The monthly average concentrations of the nutrients in the river water area of the Changjiang River mouth can be used to calculate the nutrient transport from the Changjiang River. Their seasonal variations and the relationships between them and the Changjiang River runoff to the sea are indicated in Fig. 7. Related statistics show that the seasonal variations of the nutrient transports F (kg s⁻¹) of PO₄–P , SiO₃–Si , NO₃–N, NO₂–N, NH₄–N and DIN (dissolved total inorganic nitrogen ) and the water flow Q (m³ s⁻¹) of the Changjiang River were very closely correlated as indicated in the equations:

Fig. 7

Relationships between the nutrient transports and the flow Q of the Changjiang River: filled diamond—SiO₃–Si ; filled triangle—NO₃–N; filled circle—PO₄–P ; filled square—Q

$$\begin{aligned} {\text{F}}({\text{PO}}_{4}{-}{\text{P}}) & = 0.110{ \exp }\;\left( {0.0000497Q} \right)\;(r = 0.880,n = 10) \\ {\text{F}}({\text{SiO}}_{3} {-} {\text{Si}}) & = 23.070{ \exp }\;\left( {0.0000366Q} \right)\;(r = 0.735,n = 10) \\ {\text{F}}({\text{NO}}_{ 3} {-} {\text{N}}) & = 4.899{ \exp }\;\left( {0.0000518Q} \right)\;(r = 0.929,n = 10) \\ {\text{F}}({\text{NO}}_{2} {-} {\text{N}}) & = 0.0342{ \exp }\;\left( {0.0000446Q} \right)\;(r = 0.850,n = 10) \\ {\text{E}}({\text{NH}}_{4} {-} {\text{N}}) & = 29.212{ \exp }\;\left( { - 0.0000759Q} \right)\;(r = - 0.692,n = 10) \\ {\text{F}}({\text{DIN}}) & = 16.270{ \exp }\;\left( {0.0000208Q} \right)\;(r = 0.754,n = 10) \\ \end{aligned}$$

That clearly shows that the nutrient transports are mainly controlled by the Changjiang River flow . Monthly calculations summing up the Changjiang’s annual nutrient transports to the sea listed in Table 2 show them to be 1.4 × 10⁴ tons for PO₄–P , 204.4 × 10⁴ tons for SiO₃–Si , 63.6 × 10⁴ tons for NO₃–N, 0.38 × 10⁴ tons for NO₂-N, 24.90 × 10⁴ tons for NH₄–N and 88.9 × 10⁴ tons for DIN , which are about 10 times that from the Huanghe River (Shen et al. 1988).

Table 2 Nutrient transports

The above results of close correlations between nutrient transport from the Changjiang River mouth and the Changjiang River runoff have already been confirmed by the investigations in 1997–1998 (Shen et al. 2003; Shen 2006) and 2004 (Shen et al. 2012).