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Experimental investigations on selective feeding behaviour in freshwater crustacean zooplankton

Experimental investigations on selective feeding behaviour in freshwater crustacean zooplankton

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KUNZMANN, Alessandra Janina, 2020. Experimental investigations on selective feeding behaviour in freshwater crustacean zooplankton [Dissertation]. Konstanz: University of Konstanz

@phdthesis{Kunzmann2020Exper-51930, title={Experimental investigations on selective feeding behaviour in freshwater crustacean zooplankton}, year={2020}, author={Kunzmann, Alessandra Janina}, address={Konstanz}, school={Universität Konstanz} }

In freshwater habitats, food webs are generally characterized by the body size ratios of occurring organisms. Members of the zooplankton community are common components of pelagic ecosystems playing a key role for energy transfer, redistribution of nutrients, and growth regulation within natural plankton associations. For example, water fleas (Cladocera) and copepods (Copepoda) are major contributors to the structure and composition of lower plankton size classes. In Lake Constance, both groups are represented by a diverse set of taxa whose population dynamics have been recorded along with the changing trophic conditions of the lake since the beginning of the past century. Although several studies already addressed the general impact of cladocerans and copepods on their respective predators and prey organisms, there is little to no information on how the feeding behaviour of individual taxa might influence the planktonic communities recently occurring in Lake Constance under oligotrophic conditions.<br /><br />In my PhD thesis, I aimed to identify and compare the functional role of particular zooplankters within their food web. In order to do this, I analysed the food preferences of selected cladoceran and copepod taxa, particularly with regard to contrasting selection patterns, e.g. related to prey size or prey type, and observed short-term population dynamics in a plankton community under predation by planktivorous fish.<br /><br />First, I investigated the impact of the calanoid copepod Eudiaptomus gracilis on its natural prey spectrum in Lake Constance by the end of the 1990’s when the lake was still mesotrophic. In this work, I described short-term effects of copepod feeding pressure on primary prey as well as potential cascading effects on a secondary level (heterotrophic flagellates and bacteria). Data of autumnal in-situ experiments demonstrated a strong preference of this copepod for ciliate prey whereas phytoplankton and rotifers were less strongly selected for. Additionally, the second group experienced a disruptive selective feeding pressure, i.e. small and large cell sizes were discriminated against and the mean body size variance in phytoplankton increased over time. Those observations imply the omnivorous, highly selective mode of life of a copepod species which was previously considered as herbivorous and shed new light on the importance of E. gracilis in the pelagic zone of Lake Constance.<br /><br />Building on the results of our first study, I strived to find out if and how the feeding habits of E. gracilis would change during the course of the year. By conducting six field experiments over an annual cycle, I analysed the interrelation of copepod clearance rates on individual prey groups and temperature and assessed indicator species in order to emphasise seasonal fluctuations in the prey spectrum of this copepod species. Altogether, we observed that even under oligotrophic conditions, ciliates were among the preferred diet of E. gracilis; additionally, rotifers experienced high clearance rates during summer/early autumn. In winter, predation pressure was low whereas in spring, increased selection of E. gracilis for ciliates favoured phytoplankton growth. Therefore, we were able to demonstrate the dietary preferences and thus the impact of this copepod species on the planktonic food web to be coupled with alternating seasons.<br /><br />In the course of those experiments, four additional zooplankters (the copepods Cyclops sp. and Mesocyclops leuckarti plus the cladocerans Daphnia x obscura and D. cucullata) were examined along with E. gracilis. These fixe taxa represent the dominating zooplankters of Lake Constance. Preliminary results indicate diverging selective patterns between individual species, even appearing within the group of copepods. In general, the annual mean revealed the cyclopoid copepods to predominantly select for both ciliates and rotifers whereas E. gracilis efficiently cleared larger algal particles from the water column, in addition to ciliate prey of all sizes. None of the two Daphnia taxa exhibited size-selective feeding behaviour. However, both of them significantly controlled phytoplankton and ciliate biomasses in their respective treatments. Those findings highlight the fact that the functional role of copepods and cladocerans might not only significantly differ between higher taxonomic groups but even on species level.<br /><br />Finally, I conducted a joint mesocosm experiment in collaboration with other PhD students aiming to assess how the influence of a natural zooplankton community on phytoplankton growth might be affected by either the presence or absence of predators (i.e. fish). We found that the growth of small Daphnia species was not affected by fish predation whereas large taxa were equally frequently preyed on by the two introduced fish species. Under lacking fish predation, biovolumes of large daphniids significantly increased towards the end of the experimental period in relation to smaller species whose biovolumes drastically declined. Furthermore, phytoplankton was strongly released from the feeding pressure of zooplankton under fish presence. Our results show that when interpreting group-specific taxonomic data, the genus Daphnia should not only be considered as a uniform entity since more accurate results can be achieved by means of a size-specific subcategorisation.<br /><br />In summary, I was able to outline several factors decisively influencing the selective feeding behaviour and thus the functional role of different zooplankters within pelagic environments. Moreover, I could demonstrate that a higher taxonomic and functional resolution of the investigated organisms can yield an even more distinct picture of their dietary preferences. Those insights can help us to comprehend the structures of planktonic food webs more thoroughly and precisely as well as to better understand the complex ecology of zooplankton. Furthermore, they offer a conceptual and comparable basis for similar research approaches dealing with experimental investigations on predator-prey interactions in Lake Constance. terms-of-use 2020-11-26T12:23:51Z 2020-11-26T12:23:51Z Kunzmann, Alessandra Janina eng Experimental investigations on selective feeding behaviour in freshwater crustacean zooplankton Kunzmann, Alessandra Janina 2020

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