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.
