title: Daphnia versus copepod impact on summer phytoplankton: functional compensation at both trophic levels.
abstract: Here we report on a mesocom study performed to compare the top-down impact of microphagous and macrophagous zooplankton on phytoplankton. We exposed a species-rich, summer phytoplankton assemblage from the mesotrophic Lake Schöhsee (Germany) to logarithmically scaled abundance gradients of the microphagous cladoceran Daphnia hyalinaxgaleata and of a macrophagous copepod assemblage. Total phytoplankton biomass, chlorophyll a and primary production showed only a weak or even insignificant response to zooplankton density in both gradients. In contrast to the weak responses of bulk parameters, both zooplankton groups exerted a strong and contrasting influence on the phytoplankton species composition. The copepods suppressed large phytoplankton, while nanoplanktonic algae increased with increasing copepod density. Daphnia suppressed small algae, while larger species compensated in terms of biomass for the losses. Autotrophic picoplankton declined with zooplankton density in both gradients. Gelatinous, colonial algae were fostered by both zooplankton functional groups, while medium-sized (ca. 3,000 microm3), non-gelatinous algae were suppressed by both. The impact of a functionally mixed zooplankton assemblage became evident when Daphnia began to invade and grow in copepod mesocosms after ca. 10 days. Contrary to the impact of a single functional group, the combined impact of both zooplankton groups led to a substantial decline in total phytoplankton biomass.
authors.affiliation: Institut für Meereskunde, Düsternbrooker Weg 20, 24105, Kiel, Germany. usommer@ifm.uni-kiel.de
authors.forename: Ulrich
authors.initials: U
authors.lastname: Sommer
authors.forename: Frank
authors.initials: F
authors.lastname: Sommer
authors.forename: Barbara
authors.initials: B
authors.lastname: Santer
authors.forename: Eckart
authors.initials: E
authors.lastname: Zöllner
authors.forename: Klaus
authors.initials: K
authors.lastname: Jürgens
authors.forename: Colleen
authors.initials: C
authors.lastname: Jamieson
authors.forename: Maarten
authors.initials: M
authors.lastname: Boersma
authors.forename: Klaus
authors.initials: K
authors.lastname: Gocke
mesh_terms: D000818:Animals; D018533:Biomass; D002250:Carbon Radioisotopes / Q000378:metabolism; D002734:Chlorophyll / Q000378:metabolism; D000077194:Chlorophyll A; D033342:Copepoda / Q000254:growth & development* / Q000502:physiology; D003621:Daphnia / Q000254:growth & development* / Q000502:physiology; D005247:Feeding Behavior / Q000502:physiology*; D020387:Food Chain; D005618:Fresh Water; D005858:Germany; D010839:Phytoplankton / Q000254:growth & development*; D011156:Population Density; D011157:Population Dynamics; D012621:Seasons; D015048:Zooplankton / Q000254:growth & development* / Q000502:physiology
chemical_list: D002250:Carbon Radioisotopes; D002734:Chlorophyll; D000077194:Chlorophyll A