New Zealand Journal of Marine and Freshwater Research abstracts
Trophic interactions promote dominance by cyanobacteria (Anabaena spp.)
in the pelagic zone of Lower Karori Reservoir, Wellington, New Zealand
K. F. Smith*
P. J. Lester
School of Biological Sciences
Victoria University of Wellington
P.O. Box 600
Wellington, New Zealand
*Present address: Cawthron Institute, Private Bag 2, Nelson, New Zealand. email:
Kirsty.Smith@cawthron.org.nz
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.
Keywords cyanobacteria;
Anabaena; phytoplankton; zooplankton;
thermal stratification; nitrogen; phosphorus;
Perca fluviatilis
New Zealand Journal of Marine and Freshwater Research, 2007, Vol. 41:
143–155
0028–8330/07/4102–0143 © The Royal Society
of New Zealand 2007
M06006; Online publication date 27 April 2007. Received 17 January
2006; accepted 13 September 2006
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