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

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