On the Influence of the Freshwater Jellyfish Craspedacusta Sowerbii on the Zooplankton Community

Verh. Internat. Verein. Limnol. 27 1–4 Stuttgart, December 2000

On the influence of the freshwater jellyfish Craspedacusta sowerbii on the zooplankton community

Thomas Jankowski and Hans-Toni Ratte

Introduction ico–chemical water parameters (PO4, NO3, TP, TN and Si), phyto- and zooplankton. Oxygen, tempera- Craspedacusta sowerbii was described for the first ture, pH and conductivity were measured by elec- time in 1880 (LANKESTER 1880). Since then, the trodes. Zooplankton and phytoplankton specimens medusae have been found in many places – some- were counted and measured by an inverted micro- times at considerable densities (DEJDAR 1934, STA- scope. Three zooplankton subsamples (each 1 L) per DEL 1961, DUMONT 1994). Occasionally, an increase enclosure and sampling day were completely in the jellyfish population is accompanied by a counted. Each cladoceran species was enumerated by decrease in some zooplankton species (DUNHAM size, number and developmental stage of the eggs 1941, ACKER & MUSCAT 1976, STRAUß 1996). How- ever, hardly anything is known about the quantita- (for the first 50 specimens encountered). The eggs tive impact of Craspedacusta on the zooplankton were divided into three stages of development (NEILL community (DODSON & COOPER 1983, DEVRIES 1981, LIEDER 1996). Copepods were divided into 1992, DUMONT 1994). In 1995 we studied a large nauplii, copepodids and adults (male and female). population of Craspedacusta (1,000 ind./m²) in a For every treatment, arithmetic means and stan- small shallow hypertrophic lake near Aachen (Ger- dard deviations were calculated from the replicates. many). Here, with the increase in jellyfish numbers, Chlorophyll-a data were converted into algae-C con- a decrease of bosminids and cyclopoid copepods to tent (JØRGENSEN 1979) to analyse the food availabil- very low densities was observed. At the same time, ity for the crustaceans. Zooplankton abundance was the lake was dominated by whitefish (Rutilus rutilus). converted into biomass (µg dry weight/L) according In order to investigate whether Craspedacusta was a major reason for the zooplankton decline, we con- to BOTTRELL et al. (1976) for copepods, DUMONT et ducted an enclosure experiment in 1996. al. (1975) for cladocerans, and RUTTNER-KOLISKO (1977) for rotifers. The divergence of the communities was demonstrated and tested by means of the Materials and methods similarity indices of Stander and Steinhaus and the U-test (P ≤ 0,05) using the software, Community During summer 1996, a 23-day enclosure experi- Analysis (HOMMEN et al. 1994) ment was performed (19 June–12 July). The six enclosures (100-µm polyethylene/polyamide foil, 2 m depth, 1 m diameter; described by BROCKMANN et Results al. (1974)) were filled with 800-µm filtered lake water to prevent the presence of medusae and fish At the start of the experiment, all measured (controls), whereas three replicates were enriched parameters showed a close correspondence in all with jellyfish (800 ind./enclosure; nearly 1,000 ind./ enclosures. During the experimental period, m²). On each sampling day, three 1-L subsamples (0, 1 and 2 m) were taken by a Ruttner water sampler temperatures ranged between 16 and 19 °C. and mixed. From the mixed sample subsamples were There were no differences between the six taken for the determination of chlorophyll-a, phys- enclosures.

Control Medusae-Encl. Zooplankton 200 200

150 150 At the start, the zooplankton community [µgTG L ] -1 showed the typical structure of the lake com- 100 100

-1 munity (Fig. 2), dominated by various rotifer ] [µgTG L [µgTG 50 50 species (Asplanchna sp., Keratella cochlearis,

0 0 Pompholyx sp., Filinia longiseta, Polyarthra sp. 0 5 10 15 20 25 0 5 10 15 20 25 and Synchaeta sp.) at low cladoceran (mostly day day Bosmina longirostris) and copepod densities. The communities proved to be similar in the Fig. 1. Change in chlorophyll a (µg Chla/L) in the enclosures (left: controls; right: medusae enclo- different treatments (similarity L-data near 1, sures). Each point is the mean ± SD of three repli- Fig. 6). During the course of the experiment, cates. the zooplankton community developed differ- ently depending on the treatment (Fig. 2). In the controls, the rotifers decreased and the Phytoplankton copepods increased during the first days (Fig. The chlorophyll (Chl) data (Fig. 1) indicated 3). At the end of the experiment, the most equal phytoplankton densities at the beginning important copepod was Mesocyclops leuckarti, a of the experiment. In both treatments, the phy- small cyclopoid with a winter diapause in the toplankton was dominated by Chlamydomonas CIV and CV instar (EINSLE 1993). Bosmina throughout the experiment. In the controls a increased from day 15 onwards and dominated constant decrease in phytoplankton down to 30 the zooplankton community at the end of the µg Chl/L was observed. In contrast, in the experiment (Fig. 4). From day 15 onwards, medusae enclosures, after an initial decrease, small populations of Ceriodaphnia cf. quadran- the phytoplankton practically recovered to gula and Daphnia cucullata became established. starting densities (120 µg Chl/L). Algae-C content ranged from 6 mg C/L at the beginning to As opposed to this, the medusae enclosures 1 mg C/L (controls) and 5.5 mg C/L (medusae were dominated by rotifers for the entire exper- enclosures) at the end of the experiment. There imental period (Asplanchna, Pompholyx and was no food limitation during the experiments Keratella cochlearis; Fig. 2). Nauplii decreased to for Bosmina longirostris (LIEDER 1996). very low densities during the first 10 days. At

Control Medusae-Encl Rotatoria Cladocera Copepoda Rotatoria Cladocera Copepoda 100% 100% 80% 80%

60% 60%

40% 40%

20% 20%

0% 0% day 1 4 9 15 18 23 149151823 day

Fig. 2. Change in Zooplankton community composition (percent of total dry weight) in controls (left) and medusae enclosures (right). T. Jankowski & H.-T. Ratte, Influence of C. sowerbii on zooplankton 3

Control Medusae-Encl. Control Medusae-Encl. 0,45 0,45 300 300 0,40 0,40 250 250 n = 148

[µgTG L 0,35 0,35 ] 200 200 -1 n = 114 0,30 0,30 150 150 n = 399 n =423

-1 0,25 0,25 ] length [mm] length [mm]

[µgTG L [µgTG 100 100 n = 447 0,20 n = 122 0,20 n = 450 n = 391 50 50 n = 148 n =148 n =401 n = 144 0,15 0,15

0 0 1 4 6 9 1 4 6 9 12 15 18 23 12 15 18 23 0 5 10 15 20 25 0 5 10 15 20 25 day day day day Fig. 5. Bosmina longirostris body length in the con- Fig. 3. Population dynamics of cyclopoid copepods trols (left) and medusae enclosures (right). N, num- (dry weight: µg/L) in controls (left) and medusae ber of measured individuals; point, 95% of data; enclosures (right). Each point is the mean ± SD of error bar, 80% of data; box, 50% of data; broken three replicates. line, median; solid line, mean.

Control Medusae-Encl. Steinhaus Index Stander's Index 1000 1000 1,0 1,1 0,9 W-data 1,0 B-data 800 800 0,8 L-data 0,9

[µgTG L 0,8 ] 0,7 -1 0,7 600 600 0,6 0,6 0,5

400 400 -1 0,5

] 0,4 [µgTG L [µgTG 0,4 0,3 200 200 0,3 0,2 W-data B-data 0,2 0 0 0,1 L-data 0,1 0,0 0,0 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 day day day day

Fig. 4. Population dynamics of Bosmina logirostris (dry weight: µg/L) in controls (left) and medusae Fig. 6. Zooplankton similarities according to the enclosures (right). Each point is the mean ± SD of similarity indices of Stander and Steinhaus. W-data: three replicates. mean within similarity; B-data: mean between similarity (control vs. Medusae enclosures); L-data = B/ W. the end of the experiment a short peak of copepodids and adult cyclopoids were observed presence of medusae. In contrast, in the controls (Fig. 3). Bosmina densities were nearly equal all these crustaceans built up large populations the time (Fig. 4). Additionally, differences in approaching full carrying capacity. Here the zoop- Bosmina length were found (Fig. 5). From day 9 lankton increase was accompanied by a phytoplank- (28 June) onwards, control Bosmina were ton decrease, probably produced by the grazing pressure. The largest differences in zooplankton longer than those in medusae enclosures. At the between the treatments occurred on day 15 (similar- end of the experiment, control Bosmina had a ity L-data near 0.2, Fig. 6). After day 15, the com- length of 0.325 mm as compared to only 0.250 munities became more similar, because more than mm in those from medusae treatments. 600 medusae/enclosure died during the experimental period and thus the predation pressure decreased. Conclusions Additionally, the predation rates of Craspedacusta were quantified by laboratory experiments (JAN- Depending on the presence or absence of medusae, KOWSKI 1998). The medusae were able to kill marked differences in zooplankton structure and between 13% (copepods) and 39% (Bosmina) of the chlorophyll-a content were observed. Chlorophyll standing crop, indicating that in the small shallow and algae-C content indicated that food limitation lake the predation of Craspedacusta could be as high could not be responsible for the observed differences. as that of the fish (STRAUß personal communica- Therefore, the reason for low Bosmina and copepod tion). The results suggest that Craspedacusta could be densities in medusae enclosures appears to be the an important predator in small lakes during summer. 4 Food webs – predator/prey interactions

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