Trophic Interactions Promoting Dominance by Cyanobacteria in a Freshwater Pelagic Food Web

Cyanobacterial blooms have occurred in the Lower Karori Reservoir (Wellington) for approximately the last eight years. The dominant species in the past have been identified as Anabaena lemmermannii and A. circinalis. The reservoir was built in 1874 as part of the first water supply scheme for Wellington City and has been closed to the public since 1906. The catchment was created into a "predator-free" native wildlife sanctuary in 1999. The first severe bloom in the Lower Karori Reservoir occurred during the summer of 2000/01. The objectives of this study were: - To establish a baseline knowledge of the food web within the Lower Karori Reservoir. - To follow the chemical and biological dynamics of the Lower Karori Reservoir for an extended period of time to observe seasonal changes associated with the cyanobacterial blooms. - To experimentally manipulate the trophic levels of the food web to determine which factors (i.e. resources or consumers) were most significant in promoting the growth of cyanobacteria. I sampled temperature, dissolved oxygen, nutrient levels, and phytoplankton and zooplankton communities from 7 October 2003 to 7 July 2004. I also monitored the diet of the zooplanktivorous fish (perch, Perca fluviatilis) present in the lake. The dominant cyanobacteria species was found to be A. lemmermannii. Three other cyanobacteria species were present during the study at low abundances, including A. planktonica which had not been previously recorded in the reservoir. The bloom of cyanobacteria was associated with thermal stratification. Nutrient concentrations were at moderate levels, characteristic of mesotrophic lakes. The perch population consisted mostly of smallsized individuals that feed predominately on large species of zooplankton that were at low abundances within the lake. During March 2004, I conducted a food web manipulation experiment within the Lower Karori Reservoir. I sought to test the role of nutrient resources (bottom-up) versus cascading effects of zooplanktivorous perch (Perca fluviatilis) (top-down) in controlling cyanobacteria in this lake. Experimental treatments with perch had higher levels of cyanobacterial densities, lower zooplankton species diversity and zooplankton species were generally smaller-sized than in treatments without perch. Nutrient treatments were not favourable for cyanobacteria as the addition of nitrogen probably meant that nitrogen fixing Anabaena species lost competitive advantage over other phytoplankton taxa. It is not known by which mechanism perch promoted cyanobacterial growth since this was not directly measured. Plankton community composition was altered by all treatments. This study indicates that blooms of Anabaena species in the Lower Karori Reservoir are the result of a number of complex interactions within the lake food web. Low nitrogen levels favour dominance by the nitrogen fixing cyanobacteria, which bloom during thermal stratification as they are able to maintain their position in the water column due to buoyancy control. Predation pressure by perch is likely keeping large filter-feeding zooplankton at low levels and thus reducing grazing pressure on phytoplankton. Phosphorus excretion by perch also probably favours cyanobacteria. An eradication of perch within the Lower Karori Reservoir would probably contribute to prevent cyanobacteria predominance. The removal of perch from the reservoir should be reasonably simple due its small size and contained area. Other methods which may help reduce cyanobacterial blooms within the reservoir include artificial mixing to prevent stratification. However, these methods can be expensive to install and run.