Diatom communities associated with elevated and variable groundwater salinity in Karoo farm reservoirs, South Africa

The geology of the Karoo Basin of South Africa largely determines groundwater hydrochemistry. In this semi-arid region groundwater resources are utilized for agriculture and often stored above ground in reservoirs. The present study documents the dominant diatom species (>1% relative abundance) found in two reservoirs where electrical conductivity value ranged from 2777 µS/cm to 8400 µS/cm. Both these borehole-reservoir sites are located in the Aberdeen district of the Eastern Cape Province. The most commonly occurring diatom species from Breipaal (EC 8400 µS/cm; >10% relative abundance) were Nitzschia elegantula, Halamphora sp. 0 and Navicymbula pusilla whereas for Hopsrivier (EC 2777 µS/cm) they were Denticula kuetzingii and species belonging to the Family Chaetocerotaceae. Entomoneis paludosa was also recorded at Hopsrivier with an abundance total of 6.3% from all the samples. According to current biogeographical information, this is the second time the species from Chaetocerotaceae have been recorded in the interior of South Africa. Three Fragilaria species were observed which could not be identified. The distinct diatom communities observed at the two reservoirs were associated with marked differences in hydrochemistry, particularly dissolved salt concentrations and related ionic composition.