Data Al
Resume:
Chinese Journal of Oceanology and Limnology
Vol. 28 No. 4, P. 887-898, 2010
DOI: 10.1007/s00343-010-9059-5
Long-term ecological interactions between nutrient and
phytoplankton community in the Changjiang estuary*
JIANG Tao,, YU Zhiming SONG Xiuxian,
CAO Xihua, YUAN Yongquan Key Laboratory of Marine Ecology and Environmental Sciences, Institute of Oceanology, Chinese Academy of Sciences,
Qingdao 266071, China
Graduate University of the Chinese Academy of Sciences, Beijing 100049, China
Received Apr. 2, 2009; revision accepted May 11, 2009
Chinese Society for Oceanology and Limnology, Science Press, and Springer-Verlag Berlin Heidelberg 2010
Both nitrate (NO3) and soluble reactive phosphate (PO3 ) concentration in the freshwater
Abstract 4
end-member at the mouth of the Changjiang River have increased dramatically since the 1960s. Within
the same period in the sea area, with surface salinity>30, NO3 concentration has shown an obvious
increase, PO3 has not changed greatly and dissolved reactive silica (SiO2 ) has deceased dramatically. An
4 3
examination of the elemental ratio of NO3 to PO3 at the mouth of the Changjiang River did not show a
4
systematic trend from the 1960s to 2000s largely because both nutrients increased simultaneously. In
comparison, the elemental ratio of dissolved inorganic nitrogen (DIN) to PO3 in surface seawater, with
4
salinity>22, has shown a clearly increasing trend. Furthermore, an overall historical change of the
SiO2 :PO3 ratio has undergone a reverse trend in this area. Based on the changes of SiO2 :PO3 and
3 4 3 4
DIN:PO3 ratios, we can conclude that an overall historical change of SiO2 :DIN ratio has decreased in
4 3
this area from the 1950 1960s to 2000s. The argument that phytoplankton productivity in the Changjiang
estuary has been enhanced by increasing nutrient input from the riverine transport was supported by these
results. A comparative study analyzing the shift of phytoplankton composition from the mid-1980s to
2000s was also made. The results indicated that the average yearly percentage of diatom species in the
Changjiang estuary has decreased from 84.6% during 1985 1986 to 69.8% during 2004 2005.
Furthermore, the average yearly percentage of diatom abundance in the Changjiang estuary decreased
from 99.5% during to 75.5% over the same time period, while the abundance of dinoflagellates has
increased dramatically, from 0.7% to 25.4%.
Keyword: eutrophication; nutrients; phytoplankton community; Changjiang estuary
1 INTRODUCTION N and P limitation and increasing Si limitation for
phytoplankton growth may favor a shift from
Increasing anthropogenic inputs of nutrients can diatoms to non-siliceous phytoplankters (Justi et al.,
cause eutrophication, which is manifested as a 1995).
dramatic increase in phytoplankton production and The Changjiang River ranks the fourth largest in
biomass. For example, in the western-most tidal inlet the world, with annual mean water discharge of up to
9.04 1011 m3 (Chen et al., 2006). The Changjiang
of the Wadden Sea, phytoplankton biomass and
production almost doubled at the end of the 1970s (Yangtze) River drainage provides for 24% of
and has remained high ever since (Philippart et al., national arable land, 35% of national crop
2000). N and P concentrations have increased over production, 32% of national gross output of
time in rivers under anthropogenic perturbation,
mainly due to fertilizer application and detergent use. * Supported by the National Natural Science Foundation of China for
Thus in the rivers and coastal ecosystems strongly Creative Research Groups (No. 40821004), the High Technology
Research and Development Program of China (863 Program) (No.
affected by them, the stimulated phytoplankton
2008AA09Z107), and the National Basic Research Program of China
production exhausts the cushion of excess Si, which
(973 Program) (No. 2010CB428706))
results in Si limitation of diatom growth. Decreasing ** Corresponding author: ***@****.**.**
CHIN. J. OCEANOL. LIMNOL., 28(4), 2010 Vol.28
888
agriculture and 34.5% of national gross output of were used during 1985 1986 and are shown
industry (Liu et al., 2003). Under immense pressure elsewhere (Shen et al., 1992). Water samples
from rapid agricultural and industrial development, collected at the mouth of the Changjiang River (Fig.1)
the Changjiang River drainage has suffered from and from the sea area with surface salinity>30 were
severe environmental deterioration, such as designated to represent freshwater and seawater
eutrophication in lakes and in the Changjiang estuary. end-members, respectively. To analyze nutrient
The Changjiang River carries a large amount of limitation, we divided the sea area into two parts by
nutrients into the Changjiang estuary. For example, demarcating salinity 22, inside which, phytoplankton
dissolved inorganic nitrogen (DIN) and soluble production is mainly controlled by light penetration.
3
reactive phosphate (PO 4 ) fluxes reached 174.6 104 The values of the ratios between different nutrients
4
and 2.3 10 tons in 1998, respectively, due to the were analyzed using a Box and Whisker plot
large flood that year (Shen et al., 2008). technique, which provided the following information:
Previous studies (Wang, 2006; Zhou et al., 2008) 1. the median and average; 2. the 25 and 75 percentile
indicate increasing eutrophication of the Changjiang values.
estuary from the 1960s. Maximum Chl-a
concentration has increased from 6.6 mg/m3 in
August 1984 to 24.2 mg/m3 in August 2002. This has
been ascribed to the gradual increase in riverine
nutrient input from the Changjiang River (Chai et al.,
2006; Zhou et al., 2008). However, analyses of the
long-term interaction between phytoplankton
production/composition and nutrients in this area are
limited, mainly due to the lack of systematic and
long-term investigations.
This study focused on three major objectives: (1)
to examine the long-term changes in nutrient
concentrations and structure in the Changjiang
estuary; (2) to evaluate changes in phytoplankton
abundance in response to nutrient concentrations;
and (3) to analyze the shift in phytoplankton Fig.1 Sampling stations in the Changjiang estuary
composition in relation to nutrient structure. : 33 stations from 2003 to 2006; : 18 stations for 1959; dashed-line rectangle
inside the Changjiang Mouth: freshwater end member; TWC: Taiwan Warm
2 DATA SOURCES AND METHODS Current; YWMW: Yellow Sea Mixing Water
Nutrient concentrations and phytoplankton
abundance have been monitored periodically in the 3 RESULTS AND DISCUSSION
Changjiang estuary since 1958. In this study data
3.1 Historical changes in nutrients concentration
collected during the following periods was used:
1958 1959 (Office of Integrated Oceanographic An obvious increase in riverine nutrient (nitrate
Survey of China, 1961), 1963 1964 (Gu et al., 1981), and phosphate) concentration has been observed
1985 1986 (Shen et al., 1993; Guo et al., 1992), since the 1960s (Fig.2). The nitrate (NO3) concentration
2003 2007 (this study) and other published increased by a factor of more than four, from
references (described individually as appropriate). 20.5 mol/L in the 1960s to 65.1 mol/L in the 1980s,
Samples were taken on a monthly or seasonal basis and to 89.4 mol/L in the 2000s; and that of soluble
3
and analyzed using comparable analytical methods. reactive phosphate (PO 4 ) rose about two-fold, from
Detailed information of analytical methods for 0.5 mol/L in the 1960s to 0.6 mol/L in the 1980s,
nutrients has been described elsewhere (Gu et al., to 1.1 mol/L in the 2000s. Yan et al. (2003) found
1981; Shen et al., 1992; Chai et al., 2006). P and Si significant linear relationships between the annual
concentration were not measured during the periods average river nitrate concentration in the Changjiang
1958 1959 and 1963 1964, respectively. River and a number of different variables including:
The study area and sampling stations during fertilizer N input, population, manure N and total N
1958 1959 and 2003 2006 are shown in Fig.1. A input, while synthetic N fertilizer input was
similar study area and similar set of sampling stations assumed to be the major contributor. Application of
No.4 JIANG et al.: Interactions between nutrient concentration and structure and the phytoplankton community 889
Fig.2 Long term variation of NO3, PO 4 and SiO2 at the mouth of the Changjiang River
3
3
The solid line represents the average concentration of different nutrients at different stages. Additional data sources: Admond et al. (1985); Shen et al. (1987);
Hu et al. (2002); Lin et al. (1996)
synthetic N fertilizer began in the early 1950s and the 2000s (data before 1980 were unavailable). This
climbed dramatically after 1980 (Yan et al., 2003), phenomenon seemed to be contrary to the findings of
2
Li et al. (2007), which showed that SiO 3 decreased,
coinciding with a dramatic increase of riverine NO3
at the mouth of the Changjiang River from the 1960s sharply from the 1950s to 1980s. This was thought to
3
to 1980s. Riverine PO 4 concentration increased mainly be mainly due to the increasing numbers of dams in
from the 1980s to 2000s. This was most likely due to the Changjiang drainage over this time period as
the rapid development of industry and urbanization reservoirs were considered to trap silica-rich particles
in the Changjiang drainage from the 1980s, including diatom frustules, and thus retain silica
particularly in the lower drainage area (Shen et al., (Turner et al., 1998; Flemer et al., 2006). Although
2006), since phosphate was sourced mainly from the number of large dams has continued to increase
2
quickly, the present study has indicated that SiO 3
industrial and domestic sewage. Concentrations of
2
dissolved reactive silica (SiO 3 ) increased slightly concentration did not decrease as might be expected
2
in the last 20 years. This indicates that SiO 3 concentration
from 106.1 mol/L in the 1980s to 134.8 mol/L in
CHIN. J. OCEANOL. LIMNOL., 28(4), 2010 Vol.28
890
may also be influenced by other factors, such as increased obviously from 2.7 mol/L in 1960s and
anthropogenic perturbation (e.g., deforestation, 2.5 mol/L in 1980s, to 6.2 mol/L in 2000s (Fig.3).
extensive leaching from agricultural activity) and even Although this area is mainly controlled by oceanic
natural climate change (Zhang et al., 1994; Conley, waters, freshwater fronts could influence the
2
2002). Similarly, SiO 3 concentration in the Mississippi distribution of DIN as the study area was quite close
River has declined by 50% from 1950 to 1970s, and to the mouth of the Changjiang River. In addition, the
then increased in 1980s. Turner et al. (1991) proposed increasing NO3 concentration in this sea area was
2
that the increase of SiO 3 was due to the decrease of perhaps partly due to the increasing nitrogen
nitrate during the same period. This is unlikely for the concentration in precipitation which has been
3
Changjiang River since DIN and PO 4 concentrations considered an important N source for coastal areas
have both increased within this time period. (Shen et al., 2006; Fisher et al., 1991). Unfortunately,
The NO3 concentration of the surface seawater
no systematic data concerning nitrogen concentration
from the outer shelf with surface salinity>30 from precipitation were available for either the
Fig.3 Long term variation of NO3 PO34 and SiO2 in the surface seawater in the outer shelf with salinity>30, 3
The solid line represents the average concentration of different nutrients at different stages
No.4 JIANG et al.: Interactions between nutrient concentration and structure and the phytoplankton community 891
Changjiang estuary or the Changjiang drainage. 3.2 Historical changes in phytoplankton production
Inorganic N contents in precipitation in the Donghu
The argument that phytoplankton productivity in
region were found to have increased by a factor of
the Changjiang estuary has been enhanced by
two, from 31.9 mol/L during 1962 1963 (Liu et al.,
increasing nutrient input from the Changjiang River
1983), to 41.8 mol/L during 1979 1981 (Zhang et
was supported by our results (Fig.4). Previous studies
al., 1984), to 64.8 mol/L during 1997 1998 (Shen et
(Wang, 2006; Zhou et al., 2008) have suggested that
al., 2006). The average concentration of DIN (NH4+
Chl-a concentration has increased since mid-1980s,
NO3) was observed to be 51.9 mol/L from 2000 to
before which no data were available. These authors
2002 in the East China Sea (Zhang et al., 2005),
did not study phytoplankton abundance due to lack of
slightly lower than that in Donghu region during
long term datasets. The present study indicated a
1997 1998. It is highly likely that increasing
decrease in the year-round average phytoplankton
nitrogen deposition was a contributing factor to the
density from 4.7 104 cell/L in 1958 1959 to
increased nitrate-N contents of the sea area
2.6 104 cell/L in 1985 1986, then increased greatly
influenced by the offshore seawaters. The other
to 9.9 104 cell/L in 2004 2005. Average phytoplankton
nutrient source which should be considered is the
abundance during 1985 1986 was the lowest value of
upwelling of bottom waters (Pei, 2008). However, no
these three periods; this can be probably ascribed to
obvious changes in the DIN concentration for bottom
lower freshwater discharge from the Changjiang
waters with salinity>30 were observed from the
River. A relationship between DIN flux (which
1960s to 2000s (data not given).
positively correlates with water discharge) and
3
PO 4 concentration in surface seawater with a
phytoplankton production (Chl-a) has been observed
salinity>30 did not change markedly from the 1960s
in some major river systems (Lohrenz et al., 2008 and
to the 2000s (Fig.3). This was mainly due to the
citations therein; Dagg et al., 2004). As the river
buffering effect of suspended matter and sediment
discharge was quite similar before August in 1959
2
(Shen et al., 2008). SiO 3 concentration showed a
and in 2005 (Fig.5), comparison between these two
dramatic decrease from 26.1 mol/L in the 1960s to
periods was the focus of this study. The annual
4.9 mol/L in the 1980s and 15.3 mol/L in the
duration of high phytoplankton abundance seems to
2000s. This can presumably be ascribed to increasing
have increased slightly in 1985 1986 and in
phytoplankton production and Chl-a concentration in
2004 2005. In spring (from March to May) of 1959
the Changjiang estuary (Wang, 2006; Zhou et al.,
and 1986, the average phytoplankton density was
2008). Increased diatom growth will have led to a net
quite low, while it reached 1.9 105 cell/L in some
loss of silicon, as well as nitrogen and phosphate,
samples in May 2005 (about three orders higher than
from the water column. However, nitrogen and
found at the same time of year for the two former
phosphate may be quickly recycled or replenished by
sampling occasions (Fig.4). Recent studies have also
precipitation and buffering from suspended
indicated that since 2001, large-scale HAB outbreaks
particulate materials while diatom Si seems to have
have frequently occurred in the Changjiang estuary
been permanently buried in the sediments. Silicon
and in coastal waters of Zhejiang Province during
depletion due to biological uptake for diatom
spring and summer (Chen et al., 2003; Zhou et al.,
production has also been observed in other areas,
2006; 2008).
such as the Lake Michigan (Schelske et al., 1971) and
Although average phytoplankton abundance over
the Amazon River estuary (Milliman et al., 1975).
the entire study area increased by only a factor of two
Moreover, the occurrence of dust storms,
from 1958 1959 to 2004 2005, in the sea area with
transporting silicon, from the Gobi Desert region in
surface salinity>30, it showed a significant increase
Northwest of China has steadily declined since the 3
of two orders of magnitude (Fig.4). PO 4 concentration
1970s (Chen et al., 2008). This has been
in the surface seawater with salinity>30 did not
accompanied by decreasing silicon deposition due to 2
change greatly during this period. Furthermore, SiO 3
wet or dry precipitation in the East China Sea (Zhang
concentration showed an obvious decrease. The main
et al., 2005). It is noteworthy that the abnormally low
reason for the significant increase in phytoplankton
2
SiO 3 concentration during the 1980s when compared
density was thus assumed to be increasing N
with that during the 2000s can be very likely ascribed
concentration in this area. Since nitrogen was not
to biological consumption in the lower and middle
measured during cruises in 1958 1959, data from
salty area during the summer time as described by
1963 to 1964 were used to test the suggestion. The
Shen et al. (1993).
CHIN. J. OCEANOL. LIMNOL., 28(4), 2010 Vol.28
892
Fig.4 Long term variation in phytoplankton density in the Changjiang estuary (note that the scale of right axis (phytoplankton
density) is expressed in logarithmic form)
a. average density for the whole study area; b. average density in the sea area with surface salinity >30; c. maximum density for the whole study area. The solid line
represents the average concentration of different nutrients at different stages
flux of DIN from the Changjiang River was quite
stable before 1980 (Li et al., 2007) and thus it is
likely that there would be no major difference
between 1958 1959 and 1963 1964. Analyses
indicated that from August to October in 1963 (data
not shown; Gu, 1981), for all stations with surface
3
salinity>30, DIN:PO 4 ratios were below 10 and DIN
concentration was below 1.0 mol/L, indicating a
considerable N limitation in that period. An marked
increase of DIN occurred from 1963 1964 to
2003 2006 (Fig.3), which was accompanied by
3
increasing DIN:PO 4 ratios (data not shown; a similar
Fig.5 Water discharge at the Datong hydrological station of pattern was shown for the sea area with surface
the Changjiang River
No.4 JIANG et al.: Interactions between nutrient concentration and structure and the phytoplankton community 893
3
Historical increase from quite low riverine PO 4
salinity>22, Fig.6). Similarly, significant quantities
3
of Excess Nitrate relative to PO 4 were found in concentration can change estuaries from being
3
sources to sinks for PO 4 . Because of this buffering
waters with salinities as high as 30.5 in the East
3
mechanism, the PO 4 concentration in the sea area
China Sea in the early spring of 1993 (Wong et al.,
1998). The transport of nitrate into the euphotic zone beyond the TMZ would be less influenced by the
2
riverine input than for other nutrients (eg SiO 3 and
was also suggested to be a major factor regulating the
standing stock and production of phytoplankton in DIN). This was reflected by the relatively minor
3
historical change in PO 4 in the sea area with surface
southern California coastal waters (Eppley et al.,
1979). salinity>22 (data not shown), as well as for that with
Maxima in phytoplankton abundance generally surface salinity>30 (Fig.3). Over the same time
2
period it could thus be presumed that the SiO 3
occurred during the warmer seasons of the year.
Maxima occurred in summer for the periods concentration decreased in the Changjiang estuary.
1958 1959 and 1985 1986 (Fig.4c). Notably a In contrast, an obvious increase in DIN was observed
spring bloom with 7.0 105 cell/L (one third of the in this area from 1959 to 2006.
maximum phytoplankton abundance) occurred in Overall, the rapid increase of nitrogen
May 2005. However, this was not observed in the concentration in the Changjiang River or the estuary,
spring of the former two periods. The maxima in which has accelerated the production of
phytoplankton abundance reached values of phytoplankton, should be the focus of attention.
1.0 106 cell/L and 1.1 106 cell/L, in July and Unfortunately, no nitrogen pollution control has been
September 1959, respectively, which were half the carried out in China until now. With economic
value recorded (2.2 106 cell/L) in September 2005. advances and population increase, the nitrogen
Monthly maxima in phytoplankton abundance concentration in the Changjiang River can be
occurred over a wide range of salinities, mainly from expected to continue to rise (Zhang et al., 1995).
12 to 33 (data not shown). Boynton et al. (1982) Consequently, the Changjiang estuary could become
summarized seasonal patterns of primary production an ecological disaster, unless nutrient (especially
for a number of estuaries and found that maxima in nitrogen) loading from cities and agricultural activity
phytoplankton abundance were most closely related are reduced (Chen et al., 2003).
to nitrogen availability. However, they were unable
3.3 Historical changes in nutrient structure and
to make generalizations regarding the spatial patterns
phytoplankton communities
of estuarine production because of the diverse nature
of the systems examined. Further research on the
Nitrate delivered by freshwater discharges from
distribution of maxima in phytoplankton abundance
the Changjiang River have been observed to greatly
would be worthwhile but further discussion on this 3
exceed PO 4, relative to the nutrient requirement of
issue is beyond the scope of the present study.
phytoplankton (N:P 16:1, Redfield et al., 1963)
Lohrenz et al. (1997) proposed that increased
(Fig.7). An examination of the elemental ratio of
phytoplankton production was most closely 3
NO3 to PO 4 at the mouth of the Changjiang River
associated to an increase in N. This appears to be the
did not show a systematic trend from the 1960s to
same in the Changjiang estuary, and was also
2000s mainly because they increased simultaneously
demonstrated for the sea area with salinity>30, as
(Fig.2, 7). In comparison, changes in the elemental
discussed above. Based on the mass balance of 3
ratio of DIN to PO 4 in the surface seawater with
phosphorus in the turbidity maximum zone (TMZ) in
salinity>22 showed clear long-term trends. In the
the Changjiang estuary, Shen et al. (2008) suggested 3
cold season (from October to March), DIN:PO 4
that phosphorus was strongly limited by the buffering
ratios were variable and lower, rising gradually from
of suspended matter and sediment. According to
1963 to 2006. On the other hand, a dramatic increase
Shen et al. (2008), the majority of particulate 3
in the DIN:PO 4 ratio was observed in the warm
adsorbed inorganic phosphorus, over one-third of
3 season (from April to September) over the same time
total particulate phosphorus and one-fifth of PO 4
period (Fig.6). The overall historical change in the
were found in the TMZ. This suggests that the TMZ
2 3
SiO 3 :PO 4 ratio showed a reverse trend in surface
in the Changjiang estuary plays a role as a sink for
seawater with salinity>22 (Fig.8). It decreased
both particulate and dissolved phosphorus. Moreover,
3
dramatically from 1959 to 1985 1986 and then
Prastka et al. (1998) pointed out that riverine PO 4
3
during 2003 2006, it exhibited major fluctuations.
levels are an important regulator of PO 4 in estuaries.
CHIN. J. OCEANOL. LIMNOL., 28(4), 2010 Vol.28
894
Fig.6 Long term variation of the DIN: PO3 ratio in the surface seawater in the outer shelf with salinity>22 (Dashed line
4
represents DIN: PO3 =16)
4
The median and the average of each sample is marked with a horizontal line and a blank square, respectively
3
Fig.7 Long term variation in the NO3:PO 4 ratio at the mouth of the Changjiang River
2 3
Fig.8 Long term variation in the SiO 3 :PO 4 ratio in the surface seawater in the outer shelf with salinity>22 (Dashed line
represents SiO2 : PO3 =16)
3 4
2
Historical changes in the DIN:SiO 3 ratio could not specifically in 1958 1959 or 1963 1964. However,
2 2 3 3
be completed since DIN and SiO 3 were not measured based on changes in the SiO 3 :PO 4 and DIN:PO 4
No.4 JIANG et al.: Interactions between nutrient concentration and structure and the phytoplankton community 895
2
ratios, an overall historical decrease in the SiO 3 :DIN the periods of 1985 1986 and 2003 2006. The
ratio in this area from the 1950 1960s to 2000s can results indicated that a marked difference between
2
be concluded. Similarly, the SiO 3 :DIN ratio at the two sea areas demarcated by salinity levels of 22
Datong hydrological station along the Changjiang (Table 1). The inner shelf (sea area with surface
River has decreased since the 1960s (Wang, 2006). salinity
with values of 79.1% and 95.8% for the periods of
Nutrient stoichiometry is generally used to assess
1985 1986 and 2003 2006, respectively. In contrast,
phytoplankton limitation in coastal and marine water.
the probable phosphorus limitation was negligible.
Nutrient limitation can potentially occur for
Similarly, the probable nitrogen and silicon
phosphorus with Si:P>22 and N:P>22, for N with
limitation were also negligible for both periods apart
N:P 1 and for Si with Si:P22
Frequency of single nutrient limitation (% occurrence)
1985 1986 n=172-****-**** n=165-****-**** n=311-****-**** n=240
Phosphorus limitation
Stoichiometric (DIN:P>2 2; Si:P>22) 79.1 95.8 27.3 50.0
Probable (P22; Si:P>22) 0 0.6 2.9 7.1
Nitrogen limitation
Stoichiometric (DIN:P1) 0 0.6 1.6 6.7
Probable (N1) 0 0 0 0.4
Silicon limitation
Stoichiometric (Si:P22) disputed until now. Zhou et al. (2008) observed that
exhibited quite different nutrient limitation compared silicon concentration in the Changjiang estuary and
with the inner shelf (Table 1). For the outer shelf adjacent waters had remained high and thus unlikely
phosphorus limitations were 27.3% and 50.0% became a limiting factor for phytoplankton growth in
during 1985 1986 and 2003 2006, respectively, the near future. However, Wang (2006) observed an
much lower than for the inner shelf, while the obvious decrease in the Si: N ratio from 3.8 in 1959
probable phosphorus limitations were 2.9% and to 0.85 in 2002, which indicated potential silicon
7.1%, respectively, which were negligible for the limitation currently. The results of the present study
inner shelf. Meanwhile, silicon (probable silicon) supported that the occurrence of silicon limitation
limitation appeared to be higher than for the inner over the last 20 years. However, due to lack of data,
2
whether SiO 3 limitation has increased from the
shelf. It was noteworthy that clear silicon (probable
silicon) limitation of 26.0% (15.1%) existed for the 1980s to 2000s could not be conclusively verified.
outer shelf during 1985 1986, which declined to Silicon limitation for this latter period appears to
7.9% (3.8%) during 2003 2006. In contrast, both have been periodical, for example, no limitation was
phosphorus (probable phosphorus) and nitrogen observed in August 1988 (Tian et al., 1993a).
CHIN. J. OCEANOL. LIMNOL., 28(4), 2010 Vol.28
896
The historical change in nutrient structure has dentatum and Skeletonema costatum, with abundance
of 2.5 105 cell/L, 1.6 105 cell/L and 1.6 105 cell/L,
profound implications for coastal phytoplankton
communities and therefore, for coastal food webs respectively (Luan, 2007). Large-scale harmful algal
(Justi et al., 1995). A decreased percentage of blooms (HABs) caused by dinoflagellates, such as
diatom species in the Changjiang estuary and its Prorocentrum dentatum, K. mikimotoi, Alexandrium
adjacent coastal waters has been observed in catenella, Scrippsiella trochoidea, have been
previous studies (Wang, 2002a; Zhou et al., 2008). A reported to occur frequently in the Changjiang
comparative study showed that the average yearly estuary and adjacent coastal waters during spring in
percentage of diatom species in the Changjiang recent years (Wang, 2002b; Zhou et al., 2008).
estuary decreased from 84.6% during 1985 1986 to Reflecting the trends observed in the historical data,
69.8% during 2004 2005 (Table 2). Re-analysis of the replacement of diatoms by dinoflagellates in the
previous data suggested that the percentage of diatom Changjiang estuary has been the main feature of
species was consistently more than 80% before 1990, phytoplankton community changes in recent years.
after which an obvious decrease occurred (Zhou et al., The shift from diatoms to dinoflagellates was also
2008). The main response of phytoplankton observed in Bohai, China, which was ascribed to the
communities to the change in nutrient structure was increase in the N/P ratio and decrease in the Si/N
thus a decrease in the percentage diatom abundance, (Wei et al., 2004). In Jiaozhou Bay, although silicon
accompanied by an increasing abundance of limitation has increased, a species shift from diatom
dinoflagellates. The average yearly percentage of to non-diatom communities was not been observed.
diatom abundance in the Changjiang estuary However, a change from large diatoms to smaller
decreased from 99.5% during 1985 1986 to 75.5% cells has been recorded (Shen, 2001). Phytoplankton
during 2004 2005 (Table 2), while that of species respond in different ways to nutrient
dinoflagellate abundance increased dramatically enrichment, most probably depending on their
from 0.7% to 25.4% (Table 2). This was most specific life-history characteristics, such as growth
apparent during autumn (November) and spring curves and storage capacities (Philippart et al., 2000).
(May), particularly for 2004 2005. As shown in It seems that long-term changes in phytoplankton
Table 2, the percentage of diatom abundance composition are different in different environments.
decreased from 98.9% in May 1986 to 32.3% in May Unfortunately, one issue has not been resolved in
2005 while that of dinoflagellate abundance this paper. Although considerable silicon limitation
increased from 1.0% to 67.7%. existed during 1985 1986, the dominant species
were diatoms (especially S. costatum). Due to the
Table 2 Diatom abundance as a percentage of overall
lack of data, the existence of long-term Si limitation
phytoplankton abundance, and diatom species as a
during the 1980s cannot be determined. Further
percentage of total phytoplankton species during
studies are required to investigate this shift in
1985 1986 and 2004 2005
phytoplankton species composition coupled with the
changing nutrient structure.
Percent diatom Percent Percent diatom
Time dinoflagellate species 4 CONCLUSIONS
1985.11 98.8 1.0 87.0
3
Clear increases in NO and PO 4 concentration
1986.1 99.7 0.3 83.7 3
were observed at the mouth of the Changjiang River
1986.5 98.9 1.0 85.0
since the 1960s. The NO 3 concentration increased
1986.8 99.6 0.4 82.7
3
more than four fold while that of PO 4 increased
Sub-average 99.5 0.7 84.6
around two-fold from the 1960s to 2000s. Meanwhile,
2004.8a 95.2 4.0 64.2 2
concentration of SiO 3 also increased slightly from
2004.11 a 68.4 28.9 70.0
106.1 mol/L in the 1980s to 134.8 mol/L in the
a
2005.1 95.9 0.9 79.8
2000s. The NO3 concentration in the surface seawater
2005.5 a 32.3 67.7 65.2 in the outer shelf with salinity>30 showed a clear
Sub-average 75.5 25.4 69.8 increase from 2.7 mol/L in the 1960s and
2.5 mol/L in the 1980s, to 6.2 mol/L in the 2000s.
a
Data from Luan (2007)
3
PO 4 concentration did not change greatly during this
2
The dominant species in the study area in May period, while SiO 3 deceased dramatically. An
3
2004 were Karenia mikimotoi, Prorocentrum examination of the elemental ratio of NO to PO 4 at
3
No.4 JIANG et al.: Interactions between nutrient concentration and structure and the phytoplankton community 897
(Changjiang) Estuary and the adjacent East China Sea,
the Changjiang Mouth showed no systematic trend
China. Hydrobiologia, 563: 313-328.
from 1963 to 2007 mainly because both nutrients
Conley D J. 2002. Terrestrial ecosystems and the global
increased simultaneously. The elemental ratio of
biogeochemical silica cycle. Global Biogeochemical
3
dissolved inorganic nitrogen (DIN) to PO 4 in the Cycles, 16(4): 1 121. doi:10.1029/2002GB001894.
surface seawater with salinity>22 also showed a Dagg M, Benner R, Lohrenz S, Lawrence D. 2004.
clearly increasing trend. In contrast, overall the Transformation of dissolved and particulate materials on
2 3 continental shelves influenced by large rivers: plume
historical change in the SiO 3 :PO 4 ratio showed a
processes. Continental Shelf Research, 24(7-8): 833-858.
2 3
reverse trend. Based on the changes in the SiO 3 :PO 4
Eppley R W, Renger E H, Harrison W G. 1979. Nitrate and
3
and DIN:PO 4 ratios an overall historical decrease in phytoplankton production in southern California coastal
2
the SiO 3 :IN ratio from the 1950 1960s to 2000s can waters. Limnology and Oceanology, 24(3): 483-494.
be concluded. The argument that phytoplankton Fisher D C, Oppenhelmer M. 1991. Atmospheric nitrogen
deposition and Chesapeake Bay estuary. Ambio., 20:
productivity in the Changjiang estuary is enhanced
102-108.
by increasing nutrient input from riverine transport is
Flemer D A, Champ M A. 2006. What is the future fate of
supported by these results. Comparative analysis of
estuaries given nutrient over-enrichment. freshwater
the phytoplankton composition from the mid-1980s diversion and low flows? Marine Pollution Bulletin, 52:
to 2000s showed that the average yearly percentage 247-258.
diatom species in the Changjiang estuary decreased Gu H K, Xiong X X, Liu M X, Li Y. 1981. Marine geochemistry
of nitrogen near estuary of Yangtze River- nitrate in sea
from 84.6% during 1985 1986 to 69.8% during
water near estuary. Journal of Shandong College of
2004 2005. Furthermore, the average yearly
Oceanology, 11(4): 37-46. (in Chinese with English
percentage diatom abundance in the Changjiang abstract)
estuary decreased from 99.5% during the former Guo Y J, Yang Z Y. 1992. Quantitative variation and
period to 75.5% during the latter period while that of ecological analysis of phytoplankton in the estuarine area
of the Changjiang River. Studia Marina Sinica, 33:
dinoflagellate abundance increased dramatically
167-188. (in Chinese with English abstract)
from 0.7% to 25.4%.
Hu F X, Hu H, Gu G C. 2002. Studies on fronts in the
Changjiang Estuary. East China Normal University Press,
5 ACKNOWLEDGMENTS
Shanghai, China. 93p. (in Chinese)
The authors gratefully acknowledge Dr. WANG H Justi D, Rabalais N N, Turner R E, Dortch Q. 1995. Changes
in nutrient structure of river-dominated coastal waters:
L and Miss SUN S for their help with data collection,
stoichiometric nutrient balance and its consequences.
and other participants who contributed to field
Estuarine, Coastal and Shelf Science, 40: 339-356.
sampling and sample analysis in the laboratory. Li M T, Xu K Q, Watanabe M, Chen Z Y. 2007. Long-term
variations in dissolved silicate, nitrogen, and phosphorus
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