Journal of Plankton Research Vol.22 no.9 pp.1757–1778, 2000
Long-term changes of Copepoda community (1957–1996) in a
subtropical Chinese lake stocked densely with planktivorous filterfeeding silver and bighead carp
P.Xie2 and Y.Yang1
Donghu Experimental Station of Lake Ecosystems, The State Key Laboratory for
Freshwater Ecology and Biotechnology of China, Institute of Hydrobiology,
Chinese Academy of Sciences, Wuhan, 430072, P.R. China and 1Science Center of
Shantou University, Shantou, 515063, P.R. China
2To
whom correspondence should be addressed
Abstract. In contrast to the relatively well documented impact of particulate-feeding fish on
zooplankton communities, little attention has been devoted to the impact of filter-feeding fish.
Filter-feeding silver and bighead carp are the most intensively cultured fish species in Asia and
comprise much of the production of Chinese aquaculture. However, little information is known
about the impact of either fish on the zooplankton community. Long-term changes in the Copepoda
community (1957–1996) were studied at two sampling stations of a subtropical Chinese lake (Lake
Donghu) dominated by silver and bighead carp. For both calanoids and cyclopoids, the littoral
station (I) was much more resource profitable than the pelagic station (II). There has been a
tremendous increase in the annual fish catch over the past 30 years due to the increased stocking
with fingerlings of the two carp species. There was a notably higher fish density at Station I than at
Station II. Cyclopoid abundance was notably higher at Station I than at Station II during the 1950s
to the 1980s, while the reverse became true in the 1990s. This is probably because when fish
abundance increased to an extremely high level, the impact of fish predation on the cyclopoids
became more important than that of food resources at the littoral station. At both stations,
cyclopoid abundance was relatively low in spite of the presence of abundant prey. Similarly,
calanoid density did not differ significantly between the two stations in the 1950s and 1960s, but
was significantly lower at Station I than at Station II during the 1980s and 1990s. Such changes are
attributed to the gradient of fish predation between the stations and an increasing predation
pressure by the fish. The increased fish predation also correlated with a shift in summer-dominant
calanoids from larger species to smaller ones. In conclusion, the predaceous cyclopoids are affected
by fish predation to a much lesser extent than the herbivorous calanoids, and therefore increased
predation by filter-feeding fish results in a definite increase in the cyclopoid/calanoid ratio.
Predation by filter-feeding fish has been a driving force in shaping the copepod community
structure of Lake Donghu during the past decades.
Introduction
In contrast to the relatively well documented impact of particulate-feeding fish
on zooplankton communities, little attention has been paid to the impact of filterfeeding fish [see review by (Lazzaro, 1987)]. So far, there have been only a few
field studies documenting the responses of the zooplankton community to filterfeeding fish (Drenner et al., 1982, 1984).
The planktivorous filter-feeding silver carp (Hypophthalmichthys molitrix) and
bighead carp (Aristichthys nobilis) are the most intensively cultured fish species
in Asia and comprise much of the production of Chinese aquaculture (Tang, 1970;
Liang et al., 1981). The silver carp has been introduced to 34 countries (Li et al.,
1990) and has also been suggested as an important biological control agent for
algal blooms in eutrophic waters (Kajak et al., 1975; Sirenko et al., 1976; Smith,
© Oxford University Press 2000
1757
P.Xie and Y.Yang
1985, 1988, 1989; Starling, 1993; Xie, 1996, 1999). However, little information is
known about the impact of either fish on the zooplankton community.
Copepods play an important role in ecosystems by virtue of their place in food
webs, i.e. they are an excellent food for many predators, mainly zooplanktivorous
fish (Balcer et al., 1984), and are themselves predators of Rotifera, Cladocera and
Protozoa [e.g. (Dziuban, 1937; Anderson, 1970; Brandl and Fernando, 1974, 1975;
Yang and Brandl, 1996)]. Of the free-living copepods, cyclopoid and calanoid
species are by far the best known and most important components of freshwater
plankton. Generally, most calanoids are herbivorous and feed chiefly by filtration
on algae, flagellates and bacteria, while cyclopoids are usually omnivorous with
their mouth parts modified for grasping and chewing algae as well as animal prey,
sometimes attacking other copepods or even their own nauplii or copepodites
(Dussart and Defaye, 1995). However, there are few datasets on the change of
the Copepoda community covering a time span of several decades. Ecological
studies on the planktonic Copepoda community of Chinese lakes are especially
rare, probably due to the difficulty of identifying these animals.
The main objectives of the present study were to describe the long-term
changes in the Copepoda community (from 1957 to 1996) in the shallow eutrophic
Lake Donghu where there has been a tremendous increase in annual fish production (consisting mainly of filter-feeding silver and bighead carp) through
increased stocking, to discuss the possible mechanisms underlying the changes in
both dominant copepods and the ratio of Cyclopoida to Calanoida, and to evaluate the relative importance of food resources and fish predation in driving the
copepod community in such a lake ecosystem with extremely abundant filterfeeding fish.
Method
Description of study sites
Lake Donghu (East Lake) (30°33N, 114 °23E) is a subtropical shallow lake with
an average depth of 2.5 m. It is located near the middle reaches of the Yangtze
River, about 5 km from the river, and has a surface area of 32 km2. There was
free passage between the river and the lake through the Qingshan Canal, but
since the completion of the flood control works on the canal in the early 1960s,
flow between the two water bodies has been brought under control. In the latter
half of the 1960s, the lake was divided, by artificial dykes, into several parts—
Guozheng Hu, Tanglin Hu, Hou Hu, Miao Hu and Niuchao Hu. As only minimal
interconnections remain, these parts of the lake are effectively isolated from each
other. The Guozheng Hu (with a surface area of 10.0 km2) is our study area.
In the present study, zooplankton sampling was conducted at two stations in
the Guozheng Hu area (Figure 1). Station I is a littoral station located in the
middle of the Shuigo Hu Bay near the west end of the lake. The banks of the bay
are only 500 m apart. Water depth at this station usually fluctuates from 1.8 to
3.0 m. Station II is in the central part of the Guozheng Hu area, about 1500 m off
the southern shore. Water depth at this station usually fluctuates in the range 3.5
to 4.7 m.
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Copepod community changes in a Chinese lake
Fig. 1. Map of Lake Donghu.
In Lake Donghu, the maximum water temperature usually occurs in July and
August, slightly exceeding 30ºC; the minimum occurs in February and is approximately 5ºC. There is little difference in the annual mean water temperature
between the two stations. The difference in temperature between the surface and
the bottom layers of the water column is less than 2ºC (Shei et al., 1993).
The dissolved oxygen content (DO) is often high. Nowhere is there an oxygenfree layer that might be insufficient for aquatic animals. The dissolved oxygen
content is usually slightly lower at Station I than at Station II. For example, in
1995, DO in the surface layer varied between 5.2 and 9.5 (mean 7.3) mg l–1 at
Station I, and between 6 and 10.4 (7.8) mg l–1 at Station II; DO in the bottom
layer varied between 4.4 and 8.4 (mean 6.5) mg l–1 at Station I, and between 5.3
and 8.4 (mean 7.0) mg l–1 at Station II (Xie et al., 1996); the DO difference
between the surface and bottom layers was 0.3–2.5 mg l–1 at Station I and 0–2.2
mg l–1 at Station II.
In Lake Donghu, a total of 83 macrophyte species was recorded during
1962–1964, and the average biomass of the whole lake reached 1068 g (wet
weight) m–2 in autumn (Chen and Ho, 1975). In the 1970s, large-scale reduction
of macrophytes took place in most parts of the lake, which was mainly due to the
1759
P.Xie and Y.Yang
over-stocking of grass carp (Chen, 1980). From the 1980s, macrophytes in the
Guozheng Hu area disappeared almost completely, and their role in the primary
production of the lake ecosystem is negligible (Ni, 1996). No macrophytes have
been observed at the two sampling stations since the 1970s.
In Lake Donghu, the dominant phytoplankton were Pyrrophyta and Bacillariophyta in the 1950s, and Chlorophyta and Cyanophyta in the 1960s (Jao and Zhang,
1980). In the Guozheng Hu area, from the 1970s, large-sized cyanobacteria
colonies (mainly Microcystis, Anabaena and Oscillatoria) occurred in such abundance as to cause a striking ‘water bloom’ which continued until 1985. After 1985,
the ‘water bloom’ suddenly disappeared from this basin and the dominant phytoplankton switched to Bacillariophyta (mainly Cyclotella), Cryptophyta (mainly
Cryptomonas) and Cyanophyta (mainly small-sized Oscillatoria and Merismopedia) (Shei et al., 1993). Using a series of enclosure experiments, Xie (Xie, 1996)
showed that the increase in gazing pressure by two planktivorous filter-feeding
fish, silver carp (Hypophthalmichthys molitrix) and bighead carp (Aristichthys
nobilis), was the key factor in eliminating the ‘water bloom’ from the lake.
Sampling methods of zooplankton
Zooplankton samples were taken using a 5 l modified Patalas’ bottle sampler. At
each station, two to eight samples were taken each month before 1986, and only
one sample each month thereafter. Each sample was a mixture of several subsamples collected between the surface and the bottom at 0.5 m intervals. Two
types of samples were taken on each sampling date. The first was taken by placing
1 litre mixed lake water (preserved with Lugol’s iodine and formaldehyde) into
a glass column; after sedimenting for more than 24 h, the supernatant fluid was
carefully removed and the residue collected. Protozoans and rotifers were examined using this concentrated 1 litre sample. The second type of sample was taken
by straining 30–50 l of the lake water through a 60 µm mesh plankton net. Cladocerans and copepods were examined using the concentrated 30–50 l sample.
Data sources of zooplankton density, phytoplankton production,
chlorophyll a, bacterial density and fish yield
Zooplankton density data were mainly collected from past studies (published or
unpublished). Protozoan density data were available for 1956–1957 (Shen,
unpublished), 1962 (Shen and Gu, 1965), 1973–1975 and 1979–1985 (Huang,
1990), and 1989–1990 (Shei et al.., 1993). Rotifer density data were available for
1956–1957 (Wang, unpublished), 1962–1963 (Wang et al., 1965), 1973–1975 and
1979–1985 (Huang, 1990), 1986–1988 (Huang, unpublished), 1989–1990 (Shei et
al.., 1993), 1991 (Zhuge and Huang, 1993), 1995–1996 (Shao, unpublished).
Cladoceran density data were available for 1956–1957 (Chiang, unpublished),
1962–1963 (Chiang, 1965; Chiang, unpublished), 1973–1975 and 1979–1985
(Huang, 1990), and 1986–1996 (present study). Copepod density data were available for 1956–1957 (Chen, unpublished), 1962–1963 (Chen, 1965), 1973–1975 and
1979–1985 (Chen, 1985; Huang, 1990), 1986–1990 and 1994–1996 (present study).
1760
Copepod community changes in a Chinese lake
Data on the annual average of the daily gross production of phytoplankton
were available for 1963–1964 and 1973–1975 (Zheng et al. unpublished),
1979–1985 (Wang, 1990), 1988–1992, 1994 and 1996–1997 (Rong, unpublished).
The gross production of phytoplankton was measured monthly by the light- and
dark-bottle techniques (oxygen method) at each station. A coefficient of 0.3 was
used to convert the phytoplankton production from oxygen to carbon units.
Chlorophyll a data were available for 1990 (Shei et al., 1993) and 1991 (Xie,
unpublished). Data on bacterial density (measured by the acridine-orange direct
count method) were available for 1991–1992 (Lin, 1993).
The data on the annual fish yield in Lake Donghu was provided by the Donghu
Lake Fish Farm.
Feeding rate of Cyclops vicinus and Mesocyclops notius on various
zooplankton
Laboratory experiments were conducted to estimate the feeding rate (FR) of
adult Cyclops vicinus and Mesocyclops notius on various zooplankton of Lake
Donghu. Both predator and prey were collected from Lake Donghu. The predator was collected with a 120 µm mesh plankton net and only adult females without
egg sacs were used in the experiment. Small-sized prey were collected with a 64
µm mesh plankton net. All laboratory experiments were carried out in 500 ml
plastic vessels covered with aluminum foil at room temperature (9–13ºC for the
experiment with C.vicinus, 20–25ºC for the experiment with M.notius). Each
experiment had two treatments, one with predators (usually 20 predators in each
vessel) and the other without predators (control), and each treatment consisted
of three replicates. The feeding experiment with C.vicinus lasted for 24 h and that
of M.notius, 7 days, under a light intensity of 1 µ mol m–2 s–1. Feeding rates were
calculated from differences in prey numbers between the vessels with predators
and the control vessels
FR=(Nc-Ne)/(T*Np)
where FR is the feeding rate (prey/predator/day), Nc is the mean prey number in
control vessels (prey/vessel), Ne is the number of live prey in the vessels with
predators at the end of incubation, Np is the number of predators
(predator/vessel) and T is the duration of the experiment (days).
Results
Changes in fish production
In the 1950s and 1960s, the annual fish yield of Lake Donghu fluctuated very
widely (39–177.8 kg ha–1) and was low on average (92 kg ha–1). During 1972–1978,
the Institute of Hydrobiology, CAS, in co-operation with the Donghu Lake Fish
Farm, took a series of measures to increase the fish yield of the lake: increasing
the stocking density and proportion of large-sized fingerlings, reconstructing the
fish screens, controlling the predatory fish (Chu et al., 1976), and adopting the
1761
P.Xie and Y.Yang
Fig. 2. Changes in annual fish yield and gross primary production. Values are monthly means.
method of bulk harvesting. Of the total stocked fingerlings during the periods of
1973–1978 and 1983–1997, silver carp constituted 46.5%, bighead carp 40.3%,
grass carp 5.4% and the others, 7.8%. The annual fish yield increased steadily
from 124.5 kg ha–1 in 1971 to 1067.5 kg ha–1 in 1997 (Figure 2). From the 1970s
onward, more than 85% of the total fish yield comprised stocked species (Liu,
1984); especially in recent years, over 90% was usually from the two planktivorous species, silver carp and bighead carp (Huang and Xie, 1996).
The remarkable increase in fish catch is not only attributed to the improvement
of fishery management mentioned above, but also to a notable increase in
primary productivity (Figure 2). The increase in the primary productivity of Lake
Donghu comes from the progress of eutrophication during the past decades (Jao
and Zhang, 1980). It was found that the annual average of the daily gross primary
production (g C m–2 day–1) was significantly higher at the littoral Station I (mean:
0.17, range: 0.66–2.70) than at the pelagic Station II (mean: 1.29, range 0.56–1.90)
(by a paired t-test with years as pairs, P = 0.000002, n = 21).
Changes in the density of cyclopoid and calanoid copepods
The seasonal changes in the density of cyclopoid and calanoid copepods
(copepodites + adults) at Stations I and II are shown in Figure 3. The maximum
1762
Copepod community changes in a Chinese lake
Fig. 3. Seasonal changes in the densities of Cyclopoida and Calanoida at Stations I and II.
densities of cyclopoids at Station I and Station II were 215.3 No. l–1 (number per
litre) (in August 1981) and 114.3 No. l–1 (in February 1981), respectively, and the
maximum densities of calanoids at Station I and Station II were 25.3 No. l–1 (in
May 1986) and 19.2 No. l–1 (in April 1980), respectively. After the mid-1990s,
calanoid abundance showed a remarkable decline at both stations, while
cyclopoid abundance showed an obvious decline only at Station I.
The maximum annual mean density of cyclopoids was 30.9 No. l–1 (in 1980) and
21.3 No. l–1 (in 1980) at Stations I and II, respectively (Figure 4). The average of
the annual mean density of Cyclopoida was significantly higher at Station I (13.45
No. l–1) than at Station II (9.51 No. l–1) during the 1950s–1980s (t-test, P = 0.030,
one tail, paired data, n = 10). However, it was significantly lower at Station I (6.74
No. l–1) than at Station II (11.69 No. l–1) in the 1990s (t-test, P = 0.003, one tail,
paired data, n = 7).
The maximum annual mean density of Calanoida was 6.01 No. l–1 (in 1982) at
1763
P.Xie and Y.Yang
Fig. 4. Changes in the annual average densities of Calanoida and Cyclopoida, the proportion of
Cyclopoida in total Copepoda, and the fish yield.
Station I and 7.47 No. l–1 (in 1982) at Station II, respectively (Figure 4). Calanoid
abundance was relatively low in the 1950s–1960s, peaked in the early 1980s, and
declined remarkably afterwards. The annual mean density of calanoid copepods
was not significantly different between the two stations during the 1950s–1960s
(t-test, P = 0.355, one tail, paired data, n = 3). However, it was significantly lower
at Station I than at Station II during the 1980s–1990s (t-test, P = 0.022, one tail,
paired data, n = 14).
The percentage of cyclopoids in the total copepod density showed a general
increase at both stations during the 1950s–1990s (Figure 4). It varied between 63.2
and 98.2% (average, 84.0%) at Station I and 40.0 to 94.5% (average, 78.7%) at
1764
Copepod community changes in a Chinese lake
Fig. 5. Changes in the annual average densities of dominant Calanoida and Cyclopoida at Stations I
and II.
Station II. The average percentage was significantly higher at Station I than at
Station II (t-test, P = 0.003, one tail, paired data, n = 17).
Changes of dominant copepods
In 1962, the dominant calanoids at both stations were Neutrodiaptomus incongruens and Neodiaptomus yangtsekiangensis (Figure 5). Neutrodiaptomus incongruens dominated in the spring with a maximum density of 22.2 ind. l–1 at Station
I (in April) and of 16.0 ind. l–1 at Station II (in April), respectively. Neodiaptomus yangtsekiangensis (mixed with a few N.schmackeri) dominated in the
summer with a maximum density of 25.1 ind. l–1 at Station I (in August) and 8.3
ind. l–1 at Station II (in August and September). Sinocalanus dorri (female length
1.44–1.73 mm) was relatively abundant in the winter, and its maximum density
1765
P.Xie and Y.Yang
was 1.4 ind. l–1 at Station I (in December) and 1.55 ind. l–1 at Station II (in
January). The dominant cyclopoids at both stations were Cyclops vicinus in the
spring and Mesocyclops notius in the summer. The other species were rare and
constituted only a minor part of the copepods in quantity.
During 1980–1983, in addition to N.incongruens, Schmackeria forbesi and
N.schmackeri became the dominant calanoids while N.yangtsekiangensis was
almost absent (Figure 5). The maximum density of N.incongruens varied between
5.3 and 21.1 ind. l–1 at Station I and between 8.7–17.8 ind. l–1 at Station II.
Schmackeria forbesi occurred almost throughout the year, but usually peaked in
the summer or autumn, and its maximum density varied between 4.9 and 15.1
ind. l–1 at Station I and between 7.0 and 11.0 ind. l–1 at Station II. Neodiaptomus
schmackeri usually peaked in the summer, and its maximum density varied
between 0.2 and 9.5 ind. l–1 at Station I and between 0.14 and 11.6 ind. l–1 at
Station II. Sinocalanus dorrii was also dominant at Station II, and its maximum
density varied between 1.5 and 3.3 ind. l–1 at Station I and between 1.0and 9.9
ind. l–1 at Station II. The dominant cyclopoids were still C.vicinus (from the
autumn to the spring) and M.notius (in the summer) (Figure 5). The maximum
density of M.notius was as high as 215.3 ind. l–1 at Station I (in August 1981) and
42.8 ind. l–1 at Station II (in August 1981).
In 1995, S.forbesi was the only dominant calanoid species left (Figure 5), but
its density remained at a low level (<2.35 ind. l–1 at Station I, and <3.24 ind. l–1 at
Station II). The density of S.dorrii was <1.2 ind. l–1 at Station I and <0.2 ind. l–1
at Station II. No other calanoid species was found in the quantitative samples.
The first of the dominant cyclopoids was still C.vicinus (Figure 5), and its
maximum density was 30.0 ind. l–1 at Station I (in January) and 36.4 ind. l–1 (in
April) at Station II. Mesocyclops notius (maximum 9.1 ind. l–1 in July at Station
I, 10.7 ind. l–1 in June at Station II) and Thermocyclops taihokuensis (maximum
4.6 ind. l–1 in November at Station I, 3.0 ind. l–1 in October at Station II) dominated in the warmer months.
Change in species richness and biodiversity index of copepods
Community structure of the planktonic copepods changed significantly from the
1960s to the 1990s, i.e. the species number of copepods declined from 14 to seven
species at Station I, and from seven to five species at Station II; the Margalef
diversity index also declined remarkably (Table I). Similar changes were also
observed for rotifers and cladocerans (Appendices I and II). Some copepod
species, e.g. Macrocyclops albidus, Eucyclops speratus, E.serrulatus serrulatus,
Acanthocyclops viridis and Microcyclops varicans, that were present at the littoral
station (I) in the 1960s, disappeared in the 1990s.
Feeding rate of Cyclops vicinus and Mesocyclops notius on various zooplankton
The highest feeding rate (FR) of C.vicinus was on nauplii (5.75), followed by
those on Brachionus calyciflorus (1.65) and Rotaria neptunia (1.63). The FR of
C.vicinus on Daphnia galeata was the lowest (0.05) (Table II). During the
1766
Copepod community changes in a Chinese lake
Table I. Changes in the species composition of copepods in Lake Donghu
Stations
Calanoida
Centropagidae
Sinocalanus dorrii (Brehm)
Pseudodiaptomidae
Schmackeria forbesi Poppe et Richard
Diaptominae
Neutrodiaptomus incongruens (Poppe)
Neodiaptomus schmackeri (Poppe et Richard)
N.yangtsekiangensis Mashiko
Eodiaptomus sinensis (Burckhardt)
Cyclopoida
Cyclopidae
Macrocyclops albidus (Jurine)
Eucyclops serrulatus serrulatus (Fischer)
E.speratus (Lilljeborg)
Cyclops vicinus vicinus Uljanin
Acanthocyclops viridis (Jurine)
Microcyclops varicans (Sars)
Mesocyclops notius Kiefer
Thermocyclops taihokuensis Harada
T.hyalinus (Rehberg)
Total number of species
Margalef diversity index
1960s
I
II
+
+
+
+
+
+
1980s
I + II
1990s
I
II
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
14
3.55
+
+
7
2.00
+
+
+
+
+
+
+
+
10
+
+
+
+
7
1.45
5
0.87
experiment, approximately 50% of the prey provided were eaten by C.vicinus,
and C.vicinus were able to capture all the prey provided.
The highest feeding rate of M.notius was also on nauplii (4.07), followed by
those on Aspanchna (2.92) and B.calyciflorus (0.99). The FR of M.notius on
Lecane was the lowest (0.15) (Table II). During the experiment, approximately
43% of the prey provided were eaten by M.notius.
The feeding rate of C.vicinus was significantly higher on small-sized D.galeata
than on large-sized ones (95% LSD, P < 0.05). A similar phenomenon also
occurred for the feeding of C.vicinus on Moina micrura (95% LSD, P < 0.05) and
for the feeding of M.notius on M.micrura (95% LSD, P < 0.05) (Table III). This
indicates that both C.vicinus and M.notius prefer small prey to large ones.
Discussion
Although it is generally believed that diversity is increased when predators selectively remove individuals of the most abundant species, thus facilitating the coexistence of potential competitors by reducing the uptake of limited resources
[e.g. (Paine, 1966; Glasser, 1978, 1979)], extremely high predation pressure
(Figure 2) by the filter-feeding fish resulted in a remarkable decline in both
species richness and biodiversity index of planktonic copepods (Table I) in Lake
Donghu.
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P.Xie and Y.Yang
Table II. Mean feeding rates (number of prey eaten/predator/day) by adult Cyclops vicinus and
Mesocyclops notius on the mixture of small-sized (120–330 µm) zooplankton prey. Each treatment had
three replicates
Prey
C.vicinus feeding exp.*
M.notius feeding exp.**
—————————————————— —————————————————
FR ± S.D.
No. eaten /no. provided
FR ± S.D.
No. eaten/no. provided
Copepodites
Nauplii
Daphnia galeata
Brachionus diversicornis
B.urcealaris
B.calyciflorus
B.angularis
Polyarthra dolichoptera
Aspanchna
Rotaria neptunia
Lecane
Vorticella
Total
0.53 ± 0.04
5.75 ± 0.73
0.05 ± 0.01
0.08 ± 0.02
0.20 ± 0.03
1.65 ± 0.08
0.85 ± 0.28
0.18 ± 0.02
0.23 ± 0.07
1.63 ± 0.19
0.47 ± 0.06
0.20 ± 0.03
11.75
33/109
346/673
3/9
5/9
11/32
99/120
48/96
8/16
14/24
99/265
28/39
12/27
710/1419
0.96 ± 0.11
4.07 ± 0.32
51/227
216/598
0.25 ± 0.04
13/39
0.99 ± 0.03
53/84
2.92 ± 0.17
155/196
0.15 ± 0.02
8/16
9.34
496/1160
*Prey density 429–511 ind. 500 ml–1, predator density 20 ind. 500 ml–1, water temperature 9–13°C.
**Prey density 355–403 ind. 500 ml–1, predator density 20 ind. 500 ml–1, water temperature 22–25°C.
Table III. Mean feeding rate (no. of prey eaten/predator/day) by adult Cyclops vicinus and
Mesocyclops notius on Daphnia galeata and Moina micrura. Each treatment had three replicates. Only
one prey type was added in each experimental vessel
Prey type
Experiment A*
D.galeata (120–330 um)
D.galeata (330–650 um)
Experiment B**
M.micrura (200–357 um)
M.micrura (357–500 um)
FR of C.vicinus*
FR of M.notius**
0.220
0.093
0.480
0.287
0.35
0.54
*Prey density 20 ind. 500 ml–1, predator density 20 ind. 500 ml–1, water temperature 9–13°C.
**Prey density 18–23 ind. 500 ml–1, predator density 18–23 ind. 500 ml–1, water temperature 22–25°C.
Most calanoids are herbivorous and feed chiefly by filtration on algae, flagellates and bacteria (Dussart and Defaye, 1995), although some species are facultative detritivores (De Mott, 1986) or even distinctly carnivorous (Chow-Fraser
and Wong, 1986; Williamson and Butler, 1986). For the calanoid copepods of
Lake Donghu, the littoral station (I) was much more resource profitable than the
pelagic station (II) in terms of primary production of phytoplankton, chlorophyll
a amount and bacterial abundance (Table IV). However, the annual mean density
of calanoid copepods did not differ significantly between the two stations in the
1950s and 1960s, and was significantly lower at the resource-profitable littoral
station than at the pelagic station during the 1980s and 1990s.
The most likely explanation is the gradient of fish predation. Tatsukawa et al.
conducted an acoustic survey to estimate the fish population of Lake Donghu and
found that the distribution of fish was not uniform (Tatsukawa et al., 1989). In the
shallow area (Area A–I where Station I is situated), the average fish density
during June–October was 0.149 ind. m–2 in 1985 and 0.208 ind. m–2 in 1987.
1768
Copepod community changes in a Chinese lake
Table IV. Comparison of the average annual mean abundance of cyclopoid and calanoid copepods
(excluding nauplii), the average daily gross primary production of phytoplankton (GP), the annual
mean chlorophyll a concentration, and the annual mean abundance of bacteria between Stations I and
II of Lake Donghu
Station I
1950s–1960s
1980s
1990s
Station II
1950s–1960s
1980s
1990s
Cyclopoida
(no. l–1)
Calanoida
(no. l–1)
Total
(no. l–1)
GP
Chl a
(mg C l–1 day–1) (µg l–1)
Bacteria
109 no. l–1
14.39
13.05
6.74
3.51
3.53
0.45
17.90
16.58
7.19
0.75
1.85
1.98
60.0–77.7
1.69–2.08
6.21
10.93
11.69
3.37
4.00
1.10
9.58
14.92
12.79
0.66
1.34
1.64
34.3–47.4
0.99–1.23
However, in the deep area (Area H–Q where Station II is located), it was only
0.072 ind. m–2 in 1985 and 0.090 ind. m–2 in 1987. This indicates that fish predation on calanoid copepods might be much stronger at Station I than at Station II.
Probably, it is easier for calanoid copepods to survive fish predation in the deeper
zones of a lake.
In Lake Donghu, despite the fact that the fish yield of planktivorous fish has
increased steadily, the population density of calanoid copepods has also increased
since 1956 and peaked in 1980–1982 (Figure 4). The increase in calanoid density
in the lake may benefit from a considerable increase in phytoplankton productivity (Figure 2). Fish predation on calanoids might have been minor before the
early 1980s. Afterwards, calanoid density began to decline drastically although
phytoplankton productivity remained high (Figure 2). Thus, it is unlikely that the
low calanoid density after 1983 was caused by food shortage. It appears that as
the fish yield increased further after the early 1980s, fish predation began to
suppress most zooplankton prey, especially the larger cladocerans and copepods,
while the smaller rotifers and protozoans were little affected by fish predation as
their density changed little or even increased slightly during the 1980s (Figure 6).
In Lake Donghu, the fish farm produces millions of sizeable fingerlings of the
silver and bighead carp each year for stocking the lake in January or February of
the next year. The growth in body length and weight of the carp takes place from
April to late October with a peak in July–September (Liu et al., 1982). Fishing
effort in Lake Donghu is concentrated between October and December, and the
major portion of the annual catch is landed in this period.
Food consumption of fish in Lake Donghu in 1990 has been roughly estimated.
At the beginning of the year, about 185 t fingerlings (98 t silver carp, 87 t bighead
carp) were stocked into the Guozheng Hu and Tanglin Hu areas. The annual fish
yield (over 90% from silver and bighead carp) was 1240 t (1140 t in Guozheng
Hu and 100 t in Tanglin Hu). The crop of the two species reached 6.7 times the
original weight of the fingerlings at stocking. Since the surface area of the
Guozheng Hu area is ~10 km2, the fish yield was ~1140 kg ha–1 or 45.6 g m–3,
taking the average depth of the Guozheng Hu area as 2.5 m. Assuming that the
exploitation rate by the fishermen was 50% (Tatsukawa et al., 1989 ), the fish
1769
P.Xie and Y.Yang
Fig. 6. Changes in the annual average densities of total planktonic Protozoa, Rotifera, Copepoda
(including nauplii) and Cladocera at Stations I and II.
1770
Copepod community changes in a Chinese lake
biomass during the fishing period in the Guozheng Hu area might have been
~91.2 g m–3 in 1990. Taking the annual fish yield, the exploitation rate and the
quantity of stocked fingerlings into account, the estimated fish biomass in the
Guozheng Hu area might have been ~52.4–91.2 g m–3 throughout the year of
1990. This was, obviously, several times higher than that of the total plankton
during the year (Shei et al., 1993). On the other hand, the annual average daily
ration of the two carps is estimated to be ~9.4% (for silver carp) and 8.0% (for
bighead carp) of their body weights (Chen et al., 1989) and thus, daily food
consumption by fish might be 4.6–7.9 g wet wt m–3. The annual average biomass
of zooplankton (phytoplankton) was 2.0 (11.5) g wet wt m–3 at the littoral Station
I and 1.5 (6.9) g wet wt m–3 at the pelagic Station II. Therefore, grazing pressure
by fish on the plankton community should be rather strong in the Guozheng Hu
area.
Cyclopoid copepods prefer a diverse diet and are usually considered omnivorous. They are known to prey on protozoans, rotifers, cladocerans, calanoid copepods and copepod nauplii (Brandl and Fernando, 1975; Williamson, 1980;
Stemberger, 1985; Wiackowski et al., 1994). The present study indicates that
cycolpoids prefer small prey to large ones, which agrees with the results of many
authors [e.g. (Dziuban, 1937; Anderson, 1970; Smyly, 1970; Brandl and Fernando,
1974, 1975; Yang and Brandl, 1996)]. However, cyclopoid copepods are also food
items for many planktivorous fish. It is usually rather complex to evaluate the
effect of an increase of fish stock on the population dynamics of the predacious
cyclopoids as fish suppress not only cyclopoids but also its zooplankton prey.
Compared with the daily feeding rate of adult cyclopoids (Tables II and III),
their prey densities (Table V) were extraordinarily abundant. Thus, the lower
abundance of cyclopoid copepods in Lake Donghu could not be attributed to
food limitation.
During the 1950s–1980s, the cyclopoid abundance was notably higher at the
resource profitable littoral station (I) than at the pelagic station (II) (Table I).
However, when the fish abundance increased to an extremely high level in the
1990s, the reverse was true. It appears that at the littoral station in the 1990s, the
impact of fish predation on the cyclopoids became more important than that of
food resources.
The ratio of cyclopoid to calanoid copepods is related to various biological
interactions of the aquatic community such as fish predation, invertebrate predation and nutrient levels (Hurlbert and Mulla, 1981). There has been little information on the impact of filter-feeding fish on the ratio of cyclopoid to calanoid
copepods. The present study shows that in Lake Donghu, the proportion of
Cyclopoida in the total Copepoda increased at Station I (II) from 74.8% (59.7%)
in the 1950s and 1960s to 78.5% (74.6%) in the 1980s, and further, to 93.4%
(91.0%) in the 1990s. This proportion was significantly higher at the fish-abundant littoral station (I) than at the pelagic station (II). Meanwhile, the annual fish
yield (mainly composed of filter-feeding silver and bighead carp) increased from
98.3 kg ha–1 in the 1950s–1960s to 662.1 kg ha–1 in the 1980s, and further, to 1003.2
kg ha–1 (extremely high levels in natural lakes!) in the 1990s.
In Lake Donghu, the dominant Calanoida of the 1960s was Neutrodiaptomus
1771
P.Xie and Y.Yang
Table V. The average (1957–1996) of the annual mean densities of Protozoa, Rotifera, nauplii,
Cladocera and Cyclopoida (copepodites + adults) at Stations I and II of Lake Donghu
No. of years
Station I
Station II
Protozoa
15
Rotifera
21
Nauplii
11
Cladocera
24
Cyclopoida (copepodites + adults)
17
16544
(1469–30474)
2573
(687–5566)
60.9
(8.8–171.3)
22.9
(5.7–61.0)
10.7
(4.4–30.9)
7692
(800–13493)
1779
(181–3979)
57.9
(6.0–139.1)
21.9
(6.7–47.2)
10.4
(2.9–21.3)
incongruens (maximum length, 1.68 mm) in the spring and Neodiaptomus
yangtsekingensis (maximum length, 1.63 mm) in the summer (Chen, 1965). These
two species disappeared completely from both stations in the 1990s. A smaller
calanoid, Schmackeria forbesi (maximum length, 1.40 mm), showed a smaller
increase in density in the 1990s than in the 1960s at both stations. Two cyclopoids,
Cyclops vicinus (maximum length, 2.63 mm, dominant in the winter and spring)
and Mesocyclops notius (maximum length, 1.2 mm, dominant in the summer)
were dominant throughout the study period. Thermocyclops taihokuensis
(maximum length, 1.53 mm) has also become a summer dominant cyclopoid since
the 1980s.
There have been some studies to show that the cyclopoid/calanoid ratio
increases when predation by particulate-feeding zooplanktivorous fish increases.
Lynch reported that bluegill sunfish (a particulate feeder) predation caused a
remarkable increase in the ratio of Cyclopoida/Calanoida as it suppressed populations of the calanoid Diaptomus siciloides (maximum length, 1.5 mm) in some
enclosures but enhanced the population of a carnivorous cyclopoid species,
Cyclops vernalis (maximum length, 1.3 mm) (Lynch, 1979). Hurlbert and Mulla
reported that predation by mosquito fish (a particulate feeder) reduced Diaptomus pallidus but had little impact on Cyclops vernalis (C.vernalis is markedly
smaller than D.pallidus) (Hurlbert and Mulla, 1981). The cyclopoid/calanoid ratio
was higher in fish than in control ponds, it was higher in the ‘normal’ fish ponds
than in the ‘low fish’ fish ponds, and in all fish ponds it was higher in summer,
when Gambusia predation was more intense, than in winter. Wells reported that
the remarkable increase in the Alewife (Alosa pseudoharengus, a particulate
feeder) population from 1954 to 1966 in Lake Michigan has resulted in a sharp
density decline of the large-sized Limnocalanus macrurus (average length
2.45–2.50 mm), Epischura lacustris (1.33–1.70 mm), Diaptomus sicilis (1.25 mm)
and Mesocyclops edex (1.15 mm) but a density increase of the relatively smallsized Cyclops bicuspidatus (0.91–0.92 mm), C.vernalis (0.88 mm) and Diaptomus
ashlandi (0.84–0.93 mm) (Wells, 1970). The average proportion of Cyclopoida in
the total Copepoda increased from 60.8% in 1954 to 73.1% in 1966. Hutchinson
reported that in a small glacial lake, intensive stocking of Alewife resulted in the
1772
Copepod community changes in a Chinese lake
elimination of most large cyclopoids, i.e. dominance of larger species such as
Epischura lacustris, Diaptomus minutus and Mesocyclops edax were replaced by
a small-sized species, Tropocyclops prasinus, and a different large-sized species,
Cyclops vernalis (Hutchinson, 1971). Hillbricht-Ilkowska and Weglenska
reported that in a small Polish lake (Lake Warniak), the copepods were dominated by a calanoid, Eudiaptomus graciloides (average adult size 1.02–1.29 mm)
and three cyclopoids (average adult size 0.68–0.78 mm), Mesocyclops sp., Acanthocyclops sp. and Cyclops sp., and the increased fish-stocking (mainly the
common carp Cyprinus carpio, a particulate feeder) during 1967–1969 resulted in
a remarkable increase in the proportion of cyclopoids in the total copepods from
90% in 1967 to 97.5% in 1969 (Hillbricht-Ilkowska and Weglenska, 1973).
In conclusion, although there are notable differences in both fish density and
the duration of the experiments in the above studies, the field data suggest that
the predaceous cyclopoids were affected by fish (both particulate-feeding and
filter-feeding) predation to a much lesser extent than were the herbivorous
calanoids, and in some cases, cyclopoids were enhanced. That is, increased predation by planktivorous fish results in a definite increase in the cyclopoid/calanoid
ratio. Generally, planktivorous fish occasion a shift of copepod from larger
species to smaller ones, as in these studies; the calanoids suppressed or eliminated
by fish predation are usually conspicuously larger than the less affected
cyclopoids. Such an effect is especially notable in summer when fish predation is
more intense. In some cases, such as in Lake Donghu, increased fish predation
leads to a shift of summer-dominant calanoids from larger species to smaller ones.
Acknowledgements
This work was supported by the State Key Basic Research and Development
Plan. (G2000046800) and by a key project of CAS titled ‘Formation and Sustainability of Ecosystem Productivities’ (Grant Nos. KZ95T-04, KZ-951-A1-301). We
would like to express our deep thanks to all the researchers (listed in the text)
from the Institute of Hydrobiology, CAS, for providing their unpublished data.
Prof. Stanley I.Dodson in University of Wisconsin, USA, gave many useful
suggestions on the manuscript. We also wish to give our thanks to Dr
D.H.Cushing for his kind help.
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Received on January 2, 2000; accepted on April 7, 2000
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Appendix I. Changes in the species composition of Rotifera in Lake Donghu
Species
Epiphanidae
Proalides tentaculatus De Beauchamp
Liliferotrocha subtalis (Rodewald)
Rhinoglena frontalis Ehrenberg
Philodinedae
Rotaria neptunia (Ehrenberg)
Philodina megalotrocha Ehrenberg
Colurellidae
Lepadella patella (Muller)
L. triptera Ehrenberg
Brachionidae
Colurella obtusa (Gosse)
C.uncinata (O.F.M.)
Trichotria truncata (Whitelegge)
Brachionus angularis (Gosse)
B.calyciflorus Pallas
B.forficula Wierzejski
B.budapestiensis Daday
B.quadridentatus Hermann
B.urcealaris (O.F.M.)
B.diversicornis (Daday)
B.leydigi Cohn
B.caudatus Barrois & Daday
Platyias militaris (Daday)
Mytilina mucronata (O.F.M.)
M.ventralis (Ehrenberg)
M.trigona (Gosse)
Euchlanis triquetra Ehrenberg
E.pellucida Harring
E.piriformis Gosse
E.dilatata Ehrenberg
Anuraeopsis fissa (Gosse)
Keratella cochlearis (Gosse)
K.quadrata (O.F.M.)
K.valga (Ehrenberg)
Notholca labis Gosse
Epiphanes senta (O.F.Muller)
E.brachionus (Ehrenberg)
Lecanidae
Lecane ungulata (Gosse)
L.luna (O.F.Muller)
L.sibina Harring
L.curviconis (Murray)
L cornuta (O.F.M.)
L.closterocerca (Schmarda)
L.crenata Harring
L.lunaris (Ehrenberg)
L.bulla (Gosse)
L.furcata (Murry)
Asplanchnidae
Asplanchna brightwellii (Gosse)
A.priodonta Gosse
A.intermedia (Hudson)
Notommatidae
Notommata sp.?
Eosphora najas Ehrenberg
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1962.04–1963.05
1991
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Copepod community changes in a Chinese lake
Appendix I. continued
Cephalodella catellina (O.F.Muller)
C.exigna (Gosse)
C.gibba (Ehrenberg)
Cephalodella sp.
Gastropodidae
Gastropus hyptopus (Ehrenberg)
Ascomorpha ovalis (Bergendahl)
A.ecaudis (Perty)
A.saltans Bartsch
Proalidae
Proales sordida Gosse
Trichocercidae
Trichocerca bidens (Lucks)
T.stylata (Gosse)
T.porcellus (Gosse)
T.insignis (Herrick)
T.rousseleti (Voigt)
T.brachyura (Gosse)
T.dixon-nuttalli Jennings
T.cylindrica (Imhof.)
T.capucina Wierzejski & Zacharias
T longiseta (Schrank)
T.rattus (O.F.M.)
T.pusilla Lauterborn
T.elongata (Gosse)
T.bicristata (Gosse)
T.similis (Wierzejski)
T.gracilis (Tessin)
Synchaetidae
Polyarthra dolichoptera Idelson
P.vulgaris Carlin
Synchaeta oblonga Ehrenberg
S.pectinata Ehrenberg
S.stylata Wierzejski
Ploesoma truncatum (Levander)
Testudinellidae
Testudinella patina (Hermann)
T.paratridentata Wang
Pompholyx sulcata Hudson
P.complanata Gosse
Hexarthra mira (Hudson)
Filinia longiseta (Ehrenberg)
F.passa (O.F.Muller)
F.maior (Colditz)
F.terminalis (Plate)
F.minuta (Smirnov)
F.cornuta (Weisse)
Tetramastix opoliensis Zacharias
Conochilidae
Conochilus unicornis Rousselet
Collothecidae
Collotheca pelagia (Rousselet)
C.mutabilis (Hudson)
C.ornata (Ehrenberg)
C.balatonic Varga
Collotheca sp.?
Average density (No./l)
Total number of species
Margalef diversity index
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
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+
+
+
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+
+
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+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
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+
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+
+
+
+
+
+
+
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+
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687.5
68
10.3
331.9
56
9.47
+
+
+
+
2612
38
4.70
+
+
+
2771
38
4.67
1777
P.Xie and Y.Yang
Appendix II. Changes in the species composition of Cladocera in Lake Donghu
Species
Leptodoridae
Leptodora kindti
Sididae
Sida crystallina
Diaphanosoma brachyurum
D.leuchtenbergianum
D.sarsi
Latonopsis australis
Daphniidae
Daphnia carinata ssp.
D.hyalina
D.pulex
Scapholeberis mucronata
Simocephalus vetulus
S.vetuloides
S.serrulatus
Ceriodaphnia quadrangula
C.cornuta
C.pulchella
Moina micrura
M.affinis
Bosminidae
Bosmina longirostris
B.coregoni
Bosminiopsis deitersi
Macrothricidae
Ilyocrypyus sordidus
I.spinifer
Chydoridae
Camptocercus rectirostris
Alona quadrangularis
A.costata
A.guttata
A.rectangula
A.karua
Rhynchotalona falcata
Graptoleberis tesudinaria
Ledigia acanthocercoides
L.quadrangularis
Pleuroxus hamulatus
P.laevis
Dunhevedia crassa
Chydorus sphaericus
C.ovalis
C.globosus
C.barroisi
Average density (no. l–1)
Total number of species
Margalef diversity index
1778
1956–1963
———————————
Station I + Station II
1980–1988
————————————
Station I + Station II
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+
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+
+
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+
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+
+
+
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+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
19.74
33
10.73
+
+
+
+
+
27.36
27
7.86