Modifying the functional diversity in the zooplankton assemblage of an oligotrophic lake differentially affects pelagic community structure and biomass

Experimental ecology represents a fundamental tool to predict how a system reacts to the loss of functional groups in response to contingent disturbance.Zooplankton, the heart of pelagic food webs, is characterized by short life cycles; therefore it can serve as a model system to test ecological theories.In this study we performed a removal experiment to test the hypothesis that the absence of calanoid or cyclopoid copepods could have a different effect on plankton dynamics due to their distinct functional roles in freshwater ecosystems.We selectively removed either Calanoida or Cyclopoida from the original zooplankton assemblage of the oligotrophic lake Brunnsee (Bavaria, Germany), then inspecting temporal changes in plankton community structure and biomass. Experimental communities were incubated in plastic enclosures, fed with cultured phytoplankton and analysed at four time steps over four weeks.Our manipulations led to significant variations in environmental parameters (i.e. chlorophyll-a and microalgae concentration), in zooplankton community composition (dominance, diversity and equitability) and in normalized biomass spectra of zooplankton. The main driver of such changes was the small cladoceran Ceriodaphnia reticulata, the most abundant species at the final sampling in all the treatments. Calanoida facilitated the development of C. reticulata; Cyclopoida played a keystone role, indirectly regulating the abundance of this cladoceran through predation on Calanoida.Our results offer a wider picture of the pelagic food web in oligotrophic freshwater environments and strongly support the view that ecological studies should rely on the measurement of both specific and functional diversity.

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