Dynamics of phytoplankton and phytobenthos in Lake Loskop (South Africa) and downstream irrigation canals
The relationships between water quality and the phytoplankton community within Lake Loskop and irrigation canals downstream were studied over a period of one year from April 2009 to March 2010. The phytoplankton assemblage in Lake Loskop during this sampling period was dominated by the phytoplankton Ceratium hirundinella, with the highest biovolume of 12.1 mm 3 l- 1 recorded in late summer during January 2010. From the data generated the algae assemblage showed a clear trend in the two canals during the study period and also among sampling stations. The filamentous macroalgae Cladophora glomerata dominated the phytobenthos of the two irrigation canals during the whole sampling period. However, much higher biovolumes (8.5; 6.3 mm 3 l- 1) of Cladophora glomerata and total phosphates were observed in the long and short irrigation canals during lake overturn in the months of March and September, while much lower average biovolumes (2.4; 1.5 mm 3 l- 1) were recorded during the summer months. The dominance of the water column phytoplankton assemblage in the two irrigation canals by Ceratium hirundinella, Fragilaria crotonesis, Closterium stellenboschense and Closterium polystictum during autumn and spring was related to the observed lake overturn. Withdrawal of irrigation water from the upperhypolimnia during these two time periods transported phytoplankton species usually occurring in the epilimnion zone of Lake Loskop into the irrigation canals. This phenomonen resulted in these species becoming dominant during autumn in the water column of the two irrigation canals downstream of Lake Loskop. The phytoplankton assemblage data generated from this study can be used for management and control of nuisance macroalgae like Cladophora glomerata in irrigation canals. © 2011 E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, Germany.