Relations between planktivorous fish and zooplankton in two very shallow lakes of the Pampa Plain

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1 Verh. Internat. Verein. Limnol Stuttgart, July 2002 Relations between planktivorous fish and zooplankton in two very shallow lakes of the Pampa Plain A. M. Rennella and R. Quirós Introduction It is widely accepted that predation plays a critical role in determining zooplankton community structure in ponds and lakes. The relationship between fish planktivory and zooplankton species composition has been discussed since the early work of HRBACEK et al. (1961) and BROOKS & DODSON (1965). Size-selective predation by fish may produce changes in zooplankton community structure. Phenomena frequently attributed to heavy planktivory include the small individual size of cladocerans and the reduced representation of the vulnerable Daphnia sp. (BROOKS & DODSON 1965). A strong relationship between fish assemblage and zooplankton was proposed from results from a large set of Argentinean lakes (QUIRÓS 1995). Fish assemblage was stated as the principal factor affecting phytoplankton and macrozooplankton biomass, once nutrient concentration has been taken into account. Lakes with planktivores, but without piscivores, had the lowest macrozooplankton biomass (QUIRÓS 1998). The main purpose of this study was to compare the zooplankton community structure between two hypereutrophic shallow lakes of the Pampa Plain. Both lakes are naturally subject to different intensities of planktivorous fish predation through time, which allowed the assessment of the possible effects of planktivore abundance on the zooplankton community structure. Study area The Pampa Plain is situated in the Subtropical Region (33 39 S, W). Climate is warm temperate and precipitation is about 900 mm year 1. The Pampa Plain lakes have very high levels of nutrients (QUIRÓS & DRAGO 1999). Phytoplankton abundance is usually high, with chlorophyll levels often exceeding 50 mg m 3 during summer (QUIRÓS et al. 1988) (Table 1). Because of their shallowness, these lakes are strongly influenced by wind and are therefore polymictic. The present study was carried out in Gómez and Carpincho Lakes, which are located in the upper Salado River basin at the border of an intensively used agricultural region (Fig. 1). Macrophytes are poorly developed and restricted to littoral zones. The pelagic zone of both lakes has essentially three trophic levels (primary producer, herbivore, planktivore). Herbivorous zooplankton are dominated by Ceriodaphnia dubia, Moina micrura, Leydigia quadrangularis, Alona intermedia, Daphnia silvestrii and Daphnia spinulata (RINGUELET et al. 1965). The pampean silverside (Odontesthes bonariensis, Atherinidae), a visual planktivore as juvenile, and facultative visual planktivore as young adult, dominates the pelagic fish assemblage in both lakes. Material and methods Both zooplankton and fish sampling were conducted monthly from December 1998 to May Two Table 1. Some limnological characteristic of the study lakes. Mean values for summer Sites Carpincho Gómez North Gómez East Total phosphorus (mg m 3 ) Secchi disk (cm) Chlorophyll a (mg m 3 ) Macrozooplankton biomass (dw, µg L 1 ) Microzooplankton biomass (dw, µg L 1 ) /02/ $ E. Schweizerbart sche Verlagsbuchhandlung, D Stuttgart

2 2 Verh. Internat. Verein. Limnol. 28 (2002) Fig. 1. Study area, sampling sites for Gómez Lake and Carpincho Lake are shown as white squares. sites were sampled in Gómez Lake (Gómez north, GN and Gómez east, GE) and one site in Carpincho Lake (Fig. 1). All sampling sites were in the pelagic zone. Zooplankton samples were collected with vertical net tows (15-cm diameter conical net, 69-µm mesh size) from 0.4 m above the lake bottom to the surface. Samples were immediately preserved in a 4% sucrose formalin solution (HANEY & HALL 1973). Three 5-mL subsamples (representing up to 10% of the total sample) were counted and at least 100 specimens of cladocerans, copepod cyclopoida and copepod calanoida were measured. Biomass, as dry weight, was calculated using length weight regressions (BOTTRELL et al. 1976). Fish samples were collected using gillnets composed of two 100-m panels ranging from 30- to 170-mm stretched mesh. Floating sets were made at night, and were usually set for a period of 2 3 h. Catch per unit effort (CPUE) of silversides ranging from 130 to 180 mm long was used as a surrogate of the planktivore predaceous pressure on the zooplankton community. Neither cyclopoids nor calanoids were affected by silversides (RENNELLA unpublished data) so they were not taken into account in this study. Results The abundance of small silversides in Carpincho Lake was high in December 1998 and declined through summer and autumn 1999 to the lowest values in winter Silverside abundance increased again in autumn 2000, reaching its highest values in April (Fig. 2). For the Gómez Lake sites, the CPUEs of silversides were low in summer, autumn and winter Small silverside abundance increased in GN in spring 2000 whereas in GE they did not increase until late summer (Fig. 2). In summary, high planktivore densities were observed in Carpincho Lake during summer 1999 and in GN during summer In summer 1999, cladocerans biomass was higher in Gómez sites than in Carpincho Lake (Table 2). During this season, Daphnia sp. was CPUE small silversides Fig. 2. Seasonal variation of small silverside abundance for Carpincho Lake (black triangles), Gómez North (GN, grey circles) and Gómez East (GE, open squares) from December 1998 to May 2000.

3 A. M. Rennella & R. Quirós, Comparative study of zooplankton community structure 3 the most abundant cladoceran in Gómez sites, while in Carpincho, cladocerans were dominated by smaller Moina sp. and Ceriodaphnia sp. (Fig. 3). During winter, cladoceran biomass was low in all sites (Table 2) and the cladoceran structure was dominated by small-bodied species (Bosmina sp., Alona sp., Leydigia sp. and Ceriodaphnia sp.) (Fig. 3). Daphnia sp. populations reached a maximum in all sites in late spring 1999, with cladocerans achieving the highest biomass values during this period. However, cladoceran biomass declined in early summer (Table 2). Although Daphnia sp. dominated the cladoceran structure in GE and Carpincho until autumn, Moina sp. replaced Daphnia sp. populations in GN in mid summer (Fig. 3). No differences in mean cladoceran biomass were detected in GN and GE between summer 1999 and 2000, whereas in Carpincho Lake cladoceran biomass was higher in summer 2000 than in 1999 (Table 2). Changes in cladoceran species composition were reflected in the cladoceran size structure. For all sites, the lowest cladoceran size was observed during winter when cladocerans were dominated by small-bodied species (Fig. 4) However, the highest cladoceran size was observed during the Daphnia sp. population maximum in late spring (Figs. 3 and 4). Moreover, in Carpincho Lake, cladocerans mean individual size fluctuated between 0.62 and 0.66 mm in summer 1999, when Moina sp. and Ceriodaphnia sp. were the most abundant. However, cladoceran sizes of between 1.03 and 1.11 mm were observed in summer 2000 when the community was dominated by Daphnia sp. (Figs. 3 and 4). The same trend was observed in Fig. 3. Seasonal variation of cladoceran community composition for (a) Carpincho Lake, (b) Gómez North Lake, (c) Gómez East Lake from December 1998 to May GN. In summer 1999, the community was dominated by Daphnia sp. and the mean size was higher than in summer 2000, when Moina sp. was the most abundant cladoceran (Fig. 4). Discussion A clear seasonal pattern in cladoceran structure Table 2. Means and ranges of cladocerans biomass as dw (µg L 1 ) for study lakes during summer (January, February, March), autumn (April, May, June) winter (July, August, September) and spring (October, November and December). Sites Carpincho Gómez North Gómez East Summer (50 440) 1160 ( ) 1600 ( ) Autumn ( ) 680 ( ) 900 ( ) Winter ( ) 170 (54 370) 290 (57 578) Spring ( ) 1440 ( ) 870 ( ) Summer ( ) 1200 ( ) 1630 ( )

4 4 Verh. Internat. Verein. Limnol. 28 (2002) Cladocera mean size (mm) Fig. 4. Seasonal variation of Cladocera mean size for Carpincho Lake (black triangles), Gómez North Lake (GN, grey circles) and Gómez East Lake (GE, open squares) from December 1998 to May was observed in the study lakes. While Daphnia sp. and Moina sp. dominated during summer, Alona sp., Leydigia sp., Ceriodaphnia sp. and Bosmina sp. were the most abundant cladocerans during winter. Cladoceran species composition during summer was strongly influenced by planktivore abundance. For sites where small silverside abundance was high (Carpincho during summer 1999 and GN during summer 2000), cladocerans were dominated by Ceriodaphnia sp. and Moina sp. Moreover, when planktivorous fish were scarce, Daphnia sp. was the most important cladoceran during the entire summer season. Fish zooplankton interactions in pampean lakes are in agreement with the size efficiency hypothesis (BROOKS & DODSON 1965). For sites where planktivore predation pressure was higher, the cladoceran size was small, and large Daphnia sp. were usually replaced by small Moina sp. and Ceriodaphnia sp. In accord with trophic cascade theories (CAR- PENTER et al. 1985), an inverse relationship between cladoceran biomass and planktivore abundance was observed for Carpincho Lake. In fact, cladoceran biomass was higher in Carpincho Lake in summer 2000 than in summer 1999, when the highest planktivorous fish abundance was observed. However, this inverse relationship was not observed for Gómez Lake. No differences in zooplankton biomass were observed between summer 1999 and 2000, although there were clear differences in planktivorous fish abundance. After increasing in late spring, a sharp decline in cladoceran abundance was observed in all sites, independently of the sampled planktivorous fish abundance. The gill nets used in this study strongly select against small fish, so 0+ year class was usually not caught. However, juvenile planktivorous fish abundance usually rises in early summer (QUIRÓS personal observation). Therefore, it has to be expected that the early summer decline of daphnids would be, in part, a consequence of predation by 0+ silversides. The effects of planktivorous fish on phytoplankton are complex (CROWDER et al. 1988). Fish can influence algae directly through nutrient regeneration and indirectly through impacts on zooplankton grazing (SHAPIRO & WRIGHT 1984). The results of QUIRÓS (1998) suggest that changes in fish assemblage may cascade through the food web. On the other hand, grazing is strongly affected by zooplankton size structure and composition (PACE 1984). In this study, it was clearly shown that planktivorous fish can affect both mean cladoceran size and cladoceran composition for pampean lakes. Therefore, it would be expected that fish effects on cladocerans may influence the phytoplankton abundance at times in the very shallow pampean lakes. Acknowledgements R. QUIRÓS acknowledges research support from the Consejo Nacional de Investigaciones Cientificas y Tecnologicas. We thank M. BOVERI, J. J. ROSSO, C. PETRACCHI, H. T. VON BERNARD and D. BLACO BELLO for their assistance. References BOTTRELL, H. H., DUNCAN, A., GLIWICZ, Z. M., GRYGIEREK, E., HERZIG, A., HILLBRICHT-ILKOWSKA, A., KURASAWA, H., LARSSON, P. & WEGLENSKA, T., 1976: A review of some problems in zooplankton studies. Norw. J. Zool. 24: BROOKS, J. L. & DODSON, S. I., 1965: Predation, body size and the composition of plankton. Science 150: CARPENTER, S. R., KITCHELL J. F. & HODGSON, J. R., 1985: Cascading trophic interactions and lake productivity. Bio- Science 35: CROWDER, L. B., RAPPORTEUR, R. W., DRENNER, C. W., KER-

5 A. M. Rennella & R. Quirós, Comparative study of zooplankton community structure 5 FOOT, C., MCQUEEN, D. J., MILLS, E. L., SOMMER, U., SPEN- CER, C. N. & VANNI, M. J., 1988: Food web interactions in lakes. In: CARPENTER S. R. (ed.): Complex Interactions in Lake Communities: Springer-Verlag New York. HANEY, J. F. & HALL, D. J., 1973: Sugar-coated Daphnia: a preservation technique for Cladocera. Limnol. Oceanogr. 18: HRBACEK, J., DVORAKOVA, M., KORINEK, V. & PROCHAZKOVA, L., 1961: Demonstration of the effect of fish stock on the species composition of zooplankton and the intensity of metabolism of the whole plankton association. Int. Ver. Theor. Angew. Limnol. Verh. 14: PACE, M. L., 1984: Zooplankton community structure, but not biomass influences the phosphorus chlorophyll relationship. Can. J. Fish. Aquat. Sci. 41: QUIRÓS, R., 1995: Fish effects on pelagic-trophic relationships in a comparative lake study. Lake Reservoir Manage. 11: QUIRÓS, R., 1998: Fish effects on trophic relationships in the pelagic zone of lakes. Hydrobiologia 361: QUIRÓS, R. & DRAGO, E., 1999: The environmental state of Argentinean lakes: An overview. Lake Reservoir Res. Manage. 4: QUIRÓS, R., BAIGUN, C. R. M., CUCH, S., DELFINO, R., DE NICHILO, A., GUERRERO, C., MARINONE, M. C., MENU MARQUE, S. & SCAPINI, M. C., 1988: Evaluación del rendimiento pesquero potencial de la republica Argentina: I. Datos 1. Instituto Nacional de Investigación y Desarrollo Pesquero. Informes Técnicos del Departamento de Aguas Continentales No 7: 55 pp. RINGUELET, R. A., MORENO, I. & FELDMAN, E., 1965: El zooplancton de las lagunas de la pampa deprimida y otras aguas superficiales de la llanura bonaerense (Argentina). Physis 74: SHAPIRO, J. & WRIGHT, D. I., 1984: Lake restoration by biomanipulation: Round Lake, Minnesota, the first two years. Freshwater Biol. 14: Authors address: A. M. RENNELLA, R. QUIRÓS, Sistema de Producción Acuática. Facultad de Agronomía. Universidad de Buenos Aires. Av. San Martín 4453, (1417) Buenos Aires, Argentina. rennella@mail.agro.uba.ar