MEPS 234:55-69 (2002)  -  doi:10.3354/meps234055

Changes in the phytoplankton community and microbial food web of Blanes Bay (Catalan Sea, NW Mediterranean) under prolonged grazing pressure by doliolids (Tunicata), cladocerans or copepods (Crustacea)

Alexis Katechakis1,*, Herwig Stibor1, Ulrich Sommer2, Thomas Hansen2

1Zoologisches Institut, Ludwig-Maximilians-Universität, Abteilung Aquatische Ökologie, Karlstraße 23-25, 80333 München, Germany
2Institut für Meereskunde, Christian-Albrechts-Universität, Abteilung Experimentelle Ökologie, Düsternbrooker Weg 20, 24105 Kiel, Germany

ABSTRACT: We report how different zooplankton groups (doliolids, cladocerans and copepods) are able to influence the coastal pelagic food web, including the microbial food web, in waters of the NW Mediterranean. We studied the effect of grazing and of grazing-induced nutrient recycling mediated by different types of zooplankton grazing on a natural phytoplankton community. Experiments were conducted in semicontinuous 2-stage chemostats. The 1st stage vessels contained seawater from Blanes Bay, Spain (NW Mediterranean) including its natural phytoplankton community; the 2nd stage vessels contained the same seawater and copepods, cladocerans or doliolids. At daily intervals we transferred part of the medium from the 2nd to the 1st stage flasks, which contained ungrazed algae and excreted nutrients. In this way, the zooplankton could influence phytoplankton dynamics both by selective grazing and by differential excretion of limiting nutrients. In the 2nd stage flasks grazing changed the algal community composition. Doliolids and cladocerans promoted the growth of large algae and copepods shifted the size spectrum towards small sizes. This effect was transferred to the 1st stage flasks. Doliolids, cladocerans and copepods also affected the microbial food web in different ways. Size-selective grazing led to differences in the nanoplankton concentrations. These in turn affected bacterial concentrations in a trophic cascade. The potential to modify a given algal population increased with increasing selectivity of the grazer.

KEY WORDS: Doliolids · Cladocerans · Copepods · Grazing · Marine pelagic food web · Microbial food web · Trophic cascade

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