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Contribution of metazoan plankton to internal phosphorus cycling in
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Lake Taihu
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Hongkai Pan1, 2, Feizhou Chen1, Zhengwen Liu1, * & Guijun Yang1
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Nanjing Institute of Geography & Limnology, CAS, Nanjing 210008, P. R. China
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Graduate School, Chinese Academy of Science，Beijing 100039, P. R. China
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Author for correspondence. E-mail: zliu@niglas.ac.cn
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Key words: Lake Taihu, metazoan plankton, P release
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This paper has not been submitted elsewhere in identical or similar form, nor will it be
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during the first three months after its submission to Hydrobiologia
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Abstract
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The contribution of metazoan plankton to internal phosphorus (P) cycling was
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investigated in a eutrophic, shallow lake (Lake Taihu, China) in spring 2004 by
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determining biomass and biomass specific P release rates. Sampling was performed in
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Meiliang and Wuli Bays. Zooplankton in eutrophic Meiliang Bay was dominated by
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cladocerans, particularly Daphnia spp. and copepods, while the dominant groups in
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hypereutrophic Wuli Bay included rotifers, copepods, and cladocerans. Release rates
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of PO4-P ranged from 0.21 - 0.57 mg P gDW-1 h-1 in Meiliang Bay and from 0.20 -
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0.76 mg P gDW-1 h-1 in Wuli Bay. In most cases, P release rates were higher in Wuli
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Bay than Meiliang Bay. Phosphorus flux estimates from zooplankton excretion varied
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from 3.34 - 48.63 mg P m-2 d-1 in Meiliang Bay and 4.59 - 66.95 mg P m-2 d-1 in Wuli
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Bay. Since P released by zooplankton in this study was in a form available to
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phytoplankton, zooplankton may have a large impact on the ecosystem of Lake Taihu.
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Introduction
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Lake Taihu is a large, shallow, eutrophic lake in eastern China. Although the lake is
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well mixed and turbid due to sediment resuspension, a clear water phase may be
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observed in spring and early summer due to low phytoplankton abundance (Liu,
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unpubl. data). Water transparency decreases later in summer due to Microcystis
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blooms，while large zooplankton, particularly Daphnia spp., decrease in density. High
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densities of large zooplankton, particularly cladocerans, are key to regulating
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phytoplankton biomass and species composition (Carpenter et al., 1987; Reynolds,
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1994). However, zooplankton also influence phytoplankton via nutrient excretion
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(Lehman, 1980a; Elser et al., 1988; Sterner & Hessen, 1994). Phosphorus often limits
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biological production in lakes, and zooplankton excretion is an important P source
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(Peters & Rigler 1973; Lehman, 1980b; Urabe et al., 1995; Vadstein et al., 1995).
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This paper evaluates the role of P release by zooplankton to nutrient cycling in
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Lake Taihu in spring. This was achieved by experimental determination of excretion
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rates multiplied by zooplankton biomass in the lake.
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Materials and methods
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Zooplankton was collected with a tube sampler about every two weeks at two stations
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in Lake Taihu, one in eutrophic Meiliang Bay and another in hypereutrophic Wuli Bay
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(Fig. 1). Organisms were filtered with a 63-m sieve into plastic vials and preserved
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with 5% formalin solution. The samples were identified according to Wang (1961),
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Zhuge (1997), Shen (1979) and Chiang & Du (1979). The animals were counted, and
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the lengths of 20 - 30 individuals measured using a microscope. Zooplankton biomass
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(dry weight) was estimated according to length-weight relationships from the
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literature (Dumont et al., 1975; Huang & Hu, 1986).
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On each experiment day, zooplankton were collected with horizontal hauls using
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a 63-m mesh net. Animals were transferred into a 20-l tank containing filtered lake
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water and transported to the laboratory, where zooplankton were concentrated with a
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63-m sieve. After rinsing three times with distilled water, animals were transferred
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into three experimental flasks containing 500 ml filtered (0.45-m) lake water.
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Temperature was maintained at or near in situ lake temperature. Three reference flasks
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without animals were run during each experimental series as controls. After
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incubation (1h), a 200-ml water sample was taken by a pipette with the tip opening
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covered with a fine-mesh net to prevent particle introduction, such as feces. Water
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samples were analyzed for PO4-P according to Murphy & Riley (1962).
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At the end of the experiments, animals in each flask were filtered onto GF/C
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filters and dried at 60C for 24 h. Dry biomass was determined using a microbalance.
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Phosphorus (PO4-P) release rates were calculated as the difference between
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experimental and control flasks and expressed per gram dry biomass per hour.
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Zooplankton community P flux was estimated by multiplying release rates by
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zooplankton biomass (expressed as gDW m-2) on each day.
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Results
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Metazoan plankton composition during the study period was similar in the two bays,
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but there were differences in dominant species. In Meiliang Bay, the dominant species
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included cladocerans Daphnia hyalina, D. longispina, D. pulex, Moina spp., Bosmina
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spp. and copepods Sinocalanus dorrii, Cyclops vicinus vicinus, C. strenuous,
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Thermocyclops taihokuensis. However, in Wuli Bay, the dominant zooplankton
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species were rotifers Brachionus calyciflorus, B. angularis, Asplanchna priodonta,
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Keratella tropic, Polyarthra spp., Filinia longiseta, copepods Cyclops vicinus vicinus,
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Cyclops strenuous, Thermocyclops taihokuensis, Mesocyclops thermocyclopoides and
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cladocerans Moina spp.
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Biomass (dry weight) of metazoan plankton in Meiliang Bay was variable (Fig.
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2). Cladocerans, dominated by Daphnia spp. from March to April and Bosmina spp.
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and Moina spp. in late May, were the largest component of zooplankton biomass,
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followed by copepods, while the biomass of rotifers was negligible.
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In Wuli Bay, however, rotifers contributed a significant portion to zooplankton
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biomass. Although cladocerans also were an important contributor to zooplankton
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biomass, the proportion was lower than in Meiliang Bay, and copepod biomass was
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similar in the two bays.
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Mean P release rates ranged from 0.21 to 0.57 mg P gDW-1 h-1 in Meiliang Bay
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and from 0.20 to 0.76 mg P gDW-1 h-1 in Wuli Bay (Fig. 3). In most cases, P release
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rates were higher in Wuli Bay than in Meiliang Bay.
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Based on biomass (data from Fig. 2 were converted to gDW m-2) and release
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rates, zooplankton contribution to internal P cycling was estimated (Fig. 4).
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Phosphorus flux varied from 3.34 - 48.63 mg P m-2 d-1 in Meiliang Bay and 4.59 -
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66.95 mg P m-2 d-1 in Wuli Bay. Maximal values for P flux occurred at highest
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zooplankton biomasses.
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Discussion
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Zooplankton are important in phytoplankton succession during spring and summer via
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grazing activity (Sommer et al., 1986). However, nutrient regeneration by
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zooplankton also is an important factor. Earlier studies revealed that most soluble P
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released by zooplankton is in the form of PO4-P and, therefore, is available to
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phytoplankton (Lehman, 1980a). During the study period, large zooplankton were
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abundant in Lake Taihu. The highest biomass of Daphnia spp. was more than 2.0
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mgDW l-1. Zooplankton P release rates were variable but fall within ranges reported in
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other studies. In an experimental study, Lehman (1980b) reported a range of 0.58 -
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0.83 mg P gDW-1 h-1 for zooplankton P release rates. Urabe et al. (1995) showed that
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zooplankton P release rates in Lake Biwa ranged from 0.01 - 0.11 mg P gDW-1 h-1. In
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a tropical reservoir, Pinto-Coelho & Greco (1999) reported zooplankton P excretion
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rates from 0.49 - 1.05 mg P gDW-1 h-1.
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Our results showed differences in P release rates between the two bays. In most
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cases, P release rates were higher in Wuli than Meiliang Bay, which may be attributed
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to differences in zooplankton community structure. Wuli Bay is close to the urban
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area of Wuxi (population > 100 million) and receives (treated and untreated) sewage.
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There were more rotifers in Wuli than Meiliang Bay, while large zooplankters, mainly
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Daphnia spp., were more numerous in Meiliang Bay. Nutrient release rates are related
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inversely to zooplankton size (Pérz-Martínez & Gulati, 1999; Ruan, 1999). Given
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equal biomass, small zooplankters, such as rotifers, should play a more important role
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in P cycling than large-sized zooplankton, such as daphnids (Ejsmont-Karabin, 1983;
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Ejsmont-Karabin et al., 2004). Difference in zooplankton composition likely is a main
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factor explaining differences in P release rates between Meiliang and Wuli Bay.
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Contribution of zooplankton to internal P cycling was estimated by multiplying
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biomass specific excretion rates by integrated biomass on each sampling date. The
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lowest estimate was 3.34 mg P m-2 d-1 in Meiliang Bay when zooplankton biomass
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was lowest, while the highest value of 66.95 mg P m-2 d-1 was observed when biomass
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was highest in Wuli Bay. A recent study showed that mean PO4-P release from
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Meiliang Bay sediments was below 3.0 mg P m-2 d-1 (Fan, unpub. data). In 1998, the
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external load of total P to Lake Taihu was ~10.0 mg P m-2 d-1 (Qin et al., in press),
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which is the same order of magnitude as PO4-P flux from zooplankton excretion.
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Considering that P released by zooplankton in this study was in a form available to
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phytoplankton, zooplankton may have a large impact on the Lake Taihu ecosystem.
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Acknowledgements
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Funds for this study were provided by NSFC (No: 40371103), NIGLAS (No:
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CXNIGLAS-402-03) and CAS (No: KZCX1-SW-12). The authors are grateful to
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Peisheng Huang, Xiaolan Song and Shouxuan Wu for their assistance in sampling and
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chemical analyses, and Mark McCarthy for English correction and valuable
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suggestions.
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Figure captions:
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Figure 1. Sampling stations in Meiliang Bay and Wuli Bay, Lake Taihu.
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Figure 2. Biomass (dry weight) composition of metazoan plankton of Meiliang Bay
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and Wuli Bay, Lake Taihu during the study period.
Figure 3. Phosphorous release rates of zooplankton in Meiliang Bay and Wuli Bay,
Lake Taihu during the study period. Vertical bar shows standard deviation.
Figure 4. Phosphorus flux by metazoan plankton in Meiliang Bay and Wuli Bay, Lake
Taihu during the study period.
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Figure 1
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Lake Taihu
Wuli Bay
N
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Meiliang Bay
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15
10 km
Sampling Station
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1
2
Figure 2
3.0
2.5
Wuli Bay
2.0
1.5
-1
Biomass (mg DW l )
1.0
0.5
Rotifers
Copepods
Moina
Bosmina
Daphnia
0
3.0
2.5
Meiliang Bay
2.0
1.5
1.0
0.5
0
3
March
April
May
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1
2
Figure 3
P release rate (mg gDW -1 l -1 )
1.2
Meiliang Bay
1.0
Wuli Bay
0.8
*
0.6
*
*
*
0.4
0.2
0.0
3.9
3
3.18
3.26
4.6
4.15
4.22
5.16
5.25
Date
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1
Figure 4
-2 -1
P flux (mg m d )
80
70
Meiliang Bay
60
Wuli Bay
50
40
30
20
10
0
3.9
2
3
4
3.18
3.26
4.6
4.15
4.22
5.16
5.25
Date
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