Dynamics of phytoplankton and phytobenthos in Lake Loskop (South Africa) and downstream irrigation canals

Abstract

The relationships between water quality and the phytoplankton community within Lake Loskop and irrigation channels 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 mm3 l-1 recorded in late summer during January 2010. From the data generated the algae assemblage showed a clear trend in the two channels during the study period and also among sampling stations. The filamentous macroalgae Cladophora glomerata dominated the phytobenthos of the two irrigation channels during the whole sampling period. However, a much higher biovolumes (8.5; 6.3 mm3 l-1) of Cladophora glomerata and total phosphates were observed in the long and short irrigation channels during lake overturn in the months of March and September, while much lower average biovolumes ( 2.4; 1.5 mm3 l-1) were recorded during the summer months. The dominance of the water column phytoplankton assemblage in the two irrigation channels by Ceratium hirundinella, Fragillaria crotonesis, Closteruim stellenboschense and Closteruim polystictum during autumn and spring was in relationship with the observed lake overturn. Withdrawal of irrigation water from the upper-hypolimnia during this two time periods did contained and transport phytoplankton species in the irrigation channels usually occurring in the epilimnon zone of Lake Loskop. This phenomina resulted in these species to become dominant during autum in the water column of the two irrigation channels downstream of Lake Loskop. The phytoplankton assemblage data generated from this study can be use for management and control of nuisance macroalgae like Cladophora glomerata in irrigation channels.