New Zealand Journal of Marine and Freshwater Research abstracts
Cyanobacterial blooms appear to be driven by top-down rather than bottom-up
effects in the 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 The lower reservoir of the Karori Wildlife Sanctuary (Wellington)
has experienced cyanobacterial blooms for the last 8 years. In this study,
we sought to test the role of nutrient resources (bottom-up) versus cascading
effects of zooplanktivorous perch (Perca fluviatili) (top-down) in controlling
cyanobacteria in this lake. A 2 ´ 2 factorial field experiment was performed
within plastic enclosures positioned in the lake, using four treatments: (1)
the addition of juvenile perch; (2) the addition of nitrogen and phosphorus;
(3) the addition of juvenile perch and of nitrogen plus phosphorus; and (4) a
control treatment of no perch or nutrient addition. Four species of Anabaena were
found with the dominant species being A. lemmermannii. Cyanobacterial
abundance was significantly higher in treatments with perch. The addition of
nutrients had no significant effect on cyanobacterial densities, but there
was a weak fish ´ nutrient interaction on cyanobacteria abundance. There was
no overall difference in total zooplankton abundance in any of the treatments.
However, zooplankton species diversity was increased and large crustaceans were
at higher densities in treatments without perch. Community composition of both
phytoplankton and zooplankton species was altered by the addition of juvenile
perch and nutrients. Our results suggest that an eradication of perch may cause
a reduction in cyanobacteria abundance with the plankton being dominated by both
larger phytoplankton and zooplankton species.
Keywords Cyanobacteria; Anabaena; top-down; bottom-up; Perca
fluviatilis; nitrogen; phosphorus; Karori Wildlife Sanctuary
New Zealand Journal of Marine and Freshwater Research, 2006, Vol. 40:
53–63
0028–8330/06/4001–0053 © The Royal Society
of New Zealand 2006
M05002; Online publication date 31 January 2006Received 19 January
2005; accepted 25 October 2005
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