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Marine Ecology Progress Series

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MEPS 337:15-26 (2007)  -  doi:10.3354/meps337015

Disruption of the microbial food web and inhibition of metazooplankton development in the presence of iron- and DOM-stimulated Baltic Sea cyanobacteria

Betina Kozlowsky-Suzuki1,2,6,*, Miina Karjalainen3, Marja Koski4, Per Carlsson1,2, Willem Stolte1, Maija Balode5, Edna Granéli1

1Department of Biology and Environmental Science, University of Kalmar, 392-82 Kalmar, Sweden
2Marine Biology, Campus Helsingborg, Lund University, Box 882, 251-08 Helsingborg, Sweden
3Finnish Institute of Marine Research, Eric Palménin aukio 1, 00561 Helsinki, Finland
4Danish Institute for Fisheries Research, Kavalergården 6, 2920 Charlottenlund, Denmark
5Institute of Aquatic Ecology, University of Latvia, 19 Raina Boulevard, 1586 Riga, Latvia
6Present address: Departamento de Ecologia e Recursos Marinhos, Universidade Federal do Estado do Rio de Janeiro (UNIRIO), Av. Pasteur 458, Urca, 22290-040 Rio de Janeiro, Brazil

ABSTRACT: Summer N2-fixing cyanobacterial blooms are a common feature in the Baltic Sea, and the occurrence of Nodularia spumigena toxic blooms is of particular concern. Cyanobacterial blooms can be favoured by certain conditions including high concentrations of dissolved organic matter, which may increase the availability of iron critical for N2 fixation. Cyanobacteria may negatively affect grazers because many species produce toxins and generally lack fatty acids essential for zooplankton reproduction. In this study we investigated the effect of riverine high-molecular weight dissolved organic matter (DOM-)/iron-stimulated cyanobacteria on the development of proto- and metazooplankton, and evaluated the role of DOM in stimulating the zooplankton part of the microbial food web. A plankton community was incubated in cylinders with either nitrate (NO3) or DOM alone or combined with iron (Fe) or zooplankton >100 µm (G). The development of proto- and metazooplankton was followed for 10 d. Trophic relationships between metazooplankton taxa and their potential food items were assessed by ordination analysis and by feeding and reproduction bottle incubations with the calanoid copepod Acartia bifilosa. Contrary to our expectations, DOM did not stimulate the microbial food web, and proto- and metazooplankton developed similarly in all treatments until the middle of the experiment. However, by the end of the experiment, the biomass of proto- and metazooplankton as well as the biomass of diatoms and dinoflagellates was greatly depressed in all DOM and NO3Fe treatments. In these treatments, cyanobacterial and bacterial biomasses were highest leading up to phosphate depletion. Plankton development seemed to be bottom-up controlled and to be affected by extracellular compound(s) produced by the dominant cyanobacteria, possibly triggered by phosphate limitation. Diatoms, dinoflagellates, protozoans and metazooplankton were instead stimulated in the NO3 and NO3G treatments, where cyanobacterial biomass was low. Accordingly, A. bifilosa reproduction and survival were sustained in NO3 bottles. Deleterious effects of cyanobacteria on metazooplankton were diminished in NO3 and NO3G tanks where other food resources were available. Overall, the results suggest that increases in the input of DOM to the Baltic Sea can potentially stimulate cyanobacterial blooms that may disrupt the microbial food web and inhibit metazooplankton development.

KEY WORDS: Riverine high-molecular weight dissolved organic matter (DOM) · Iron · Microbial food web · Acartia bifilosa · Zooplankton · Cyanobacteria · Extracellular compounds

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