Trophic interactions promote dominance by cyanobacteria (Anabaena spp.) in the pelagic zone of lower Karori reservoir, Wellington, New Zealand

Abstract

Cyanobacterial blooms have occurred in the Lower Karori Reservoir (Wellington, New Zealand) since 2000, resulting in reduced water quality and potential health risks. This study examined the factors that contribute to the cyanobacterial blooms in the reservoir (October 2003 to July 2004). In particular, it examined how thermal stratification influences the chemical dynamics in the reservoir, and the potential cascading effect of zooplanktivorous perch (Perca fluviatilis) in promoting high phytoplankton biomass. Thermal stratification, coupled with increased water column stability occurred from October to March. Oxygen levels were reduced in the hypolimnion during stratification. Nutrient concentrations declined during stratification and became elevated after the collapse of the bloom in February 2004. Concentrations of Anabaena lemmermannii increased with the onset of thermal stratification. Zooplankton abundance peaked during stratification and consisted mainly of rotifers. The perch caught during this study were typically small‐sized individuals, which had consumed mostly large‐sized zooplankton. The development of cyanobacterial blooms in the Lower Karori Reservoir is probably owing to a combination of factors including low nitrogen concentrations that favour nitrogen‐fixing cyanobacteria and the ability of cyanobacteria to adjust their buoyancy and maintain their position in the water column during thermal stratification. Predation by perch probably maintains large zooplankton species at low densities and consequently reduces grazing pressure on phytoplankton populations.