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Food selectivity of the herbivore Daphnia magna

Food selectivity of the herbivore Daphnia magna -...

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PRIMARY RESEARCH PAPER Food selectivity of the herbivore Daphnia magna (Cladocera) and its impact on competition outcome between two freshwater green algae Xu Wang Yin Peng Fei Liu Sha Sha Zhu Xiao Xia Chen Received: 25 March 2010 / Revised: 15 July 2010 / Accepted: 27 July 2010 / Published online: 3 September 2010 Ó Springer Science+Business Media B.V. 2010 Abstract Freshwater green algae, Chlorella , have heavy cell walls and their size usually exceeds the lower limits of limb size of herbivorous Daphnia (Cladocera). According to the optimal foraging theory, we speculated that Daphnia would graze more exposed and relatively large Clamydomonas rather than Chlorella , and this process would lead to small-sized Chlorella becoming a superior competi- tor in the presence of Daphnia . We used Daphnia magna , Clamydomonas sajao and Chlorella pyre- noidosa to test this hypothesis. Our grazing experi- ments showed that Daphnia preferred C. sajao to C. pyrenoidosa, regardless of the concentration and relative abundance of these two algae. The decrease in relative abundance of high-quality Clamydomonas in Clamydomonas–Chlorella assemblages did not diminish the grazing efficiency of Daphnia on this algal species, but increased selectivity of low-quality Chlorella . However, when the concentration of Clamydomonas was extremely high, the grazing of Daphnia on Clamydomonas decreased. In competi- tion experiments, we observed that the presence of Clamydomonas restrained the growth potential of Chlorella ; however, the introduction of herbivorous Daphnia into the competing environment weakened this influence and to some extent enhanced the growth ability of Chlorella . Moreover, we also observed that the intensity of herbivory, imposed by different densities of Daphnia , had an obvious influence on the competition outcome between Clamydomonas and Chlorella . At the highest inten- sity of herbivory (10 Daphnia ), C . sajao was eliminated from the culture medium whereas C . pyrenoidosa could persist, but at low cell density. Keywords Filtration rate ± Population growth rate ± Optimal foraging ± Clamydomonas sajao ± Chlorella pyrenoidosa Introduction In freshwater ecosystems, competition and herbivory are important processes in determining the commu- nity structure of phytoplankton (Wetzel, 2001 ; Lampert & Sommer, 2007 ). Since Tilman ( 1977 , 1981 , 1982 ) developed resource competition theory, the majority of studies associated with competition between phytoplankton species have focused on algae competing for limiting resources (e.g. N, P and Si) (Smith & Kalff, 1983 ; Sommer, 1986 ; Hu & Zhang, Electronic supplementary material The online version of this article (doi: 10.1007/s10750-010-0399-0 ) contains supplementary material, which is available to authorized users. Handling editor: Piet Spaak X. W. Yin ( & ) ± P. F. Liu ± S. S. Zhu ± X. X. Chen Liaoning Provincial Key Laboratory for Hydrobiology, College of Life Science and Technology, Dalian Ocean University, Dalian 116023, People’s Republic of China e-mail: [email protected] 123 Hydrobiologia (2010) 655:15–23 DOI 10.1007/s10750-010-0399-0
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1993 ; Hyenstrand et al., 2000 ). Other studies evalu- ated the effects of abiotic factors (e.g. light) and
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