MEPS 279:305-309 (2004)  -  doi:10.3354/meps279305

Cascading effect of three-spined stickleback Gasterosteus aculeatus on community composition, size, biomass and diversity of phytoplankton in shallow, eutrophic brackish lagoons

Tue Stenholm Jakobsen1, Pia Borch Hansen1,2, Erik Jeppesen1,2,*, Martin Søndergaard1

1Department of Freshwater Ecology, National Environmental Research Institute, Vejlsøvej 25, PO Box 314, 8600 Silkeborg, Denmark 2Department of Plant Biology, Biological Institute, Aarhus University, Nordlandsvej 68, 8240 Risskov, Denmark
*Corresponding author. Email:

ABSTRACT: We conducted a 4 mo mesocosm experiment to elucidate the cascading impact of the zooplanktivorous three-spined stickleback Gasterosteus aculeatus on phytoplankton in slightly brackish lakes. The mesocosms were nutrient-enriched with low salinity (2 psu) and contrasting densities of sticklebacks (0 to 10 fish m-2). Total phytoplankton biovolume increased by 2 orders of magnitude when stickleback density increased from 0 to 3 to 6 to 10 fish m-2, most likely reflecting a parallel change in zooplankton biomass and size distribution. Phytoplankton size distribution was affected in that large forms dominated at high fish density, which most likely reflects a parallel reduction in size of zooplankton. A few taxa (especially Cryptophyceae) dominated at fish densities above 3 to 6 fish m-2, whereas higher diversity (characterised by taxa of Bacillariophyceae and Chlorophyceae) was recorded at fish densities below 3 to 6 m-2. However, genera richness showed no significant relationship with fish density. The threshold fish density for a transition from low to high phytoplankton biovolume at these high nutrient levels was 3 to 6 fish m-2. This level lies well below stickleback densities found in many eutrophic, low-salinity lagoons, implying that the intensity of the cascading top-down effect exerted by planktivorous fish on the phytoplankton abundance and community structure is of importance.

KEY WORDS: Sticklebacks · Phytoplankton diversity · Top-down control · Trophic cascade · Saline · Brackish lakes

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