Seasonal relative abundance and diversity of microalgae in freshwater eutrophic systems of Namibia
Authors: Garus-oas Carol Hermionee , Julies Elsabe 
Published: May 2020

BACKGROUND: Microalgae are very important primary producers in aquatic ecosystems. Their presence directly correlate with instead of to trophic levels and at times toxicity. They are commonly used as bio indicators in determining water quality and anthropogenic stresses caused by pollution. Changes in microalgae blooms’ population, composition and diversity is influenced by trends in environmental factors, physico-chemical water quality parameters and varying physiological advantages amongst microalgae. Namibian freshwater man-made dams are periodically frequented by microalgae blooms that pose devastating effects on water quality, making water treatment processes troublesome and expensive. Currently, there is no published research on identifying and assessing the seasonal relative genera abundance and diversity (richness and evenness) of microalgae in Namibian freshwater eutrophic systems. The present study provides an insight into the composition and diversity of microalgae in these systems against physico-chemical water quality parameters. This is an inception study that prompts follow-up or future studies in the Namibian freshwater phycology and water quality assessments. OBJECTIVE: The objective of this study was to determine whether there is a difference in the seasonal relative abundance and diversity (richness and evenness) of microalgae in three fresh water eutrophic systems (Goreangab, Von Bach and Swakoppoort Dams) sharing the Swakop catchment in central Namibia. METHODS: Sampling was carried out during the cold dry seasons (14 June 2013 and 4 June 2014) and the warm wet seasons (14 November 2013 and 28 February 2014). The microalgae collected were morphologically identified to genus level using taxonomic keys and enumerated in terms of cell numbers by the use of a haemocytometer. Physico-chemical water quality parameters such as turbidity, pH, temperature, and sdissolved oxygen were measured concurrently using a portable Eutech multi probe. The concentrations of macronutrients (i.e. phosphates, nitrates and ammonia) in samples were determined thereafter using ion chromatography to assess the correlation coefficient against the relative genus abundance. RESULTS: Results revealed that seasonal relative abundance was not normally distributed and not statistically significant for all the dams. Microcystis was the most abundant genus in terms of cell numbers for Swakoppoort and Goreangab Dams during both seasons, while Aulacoseira was the most abundant genus in terms of cell numbers in Von Bach Dam. The Shannon-Weiner diversity index showed that Von Bach Dam was the most diverse, followed by Goreangab Dam and Swakoppoort Dam. Species evenness (evenness index) was the highest for Von Bach Dam and species richness (Margaleff’s index) was the highest for Swakoppoort Dam. CONCLUSIONS: There is no difference in seasonal relative abundance and diversity (richness and evenness) of microalgae sampled in the three Namibian freshwater eutrophic systems (Goreangab, Von Bach and Swakoppoort Dams) throughout the duration of this study. Although, Microcystis was the most abundant genus in cell numbers in two of the systems (Swakoppoort and Goreangab Dams) during both the wet and dry seasons, there were no toxicology tests carried out to determine the mycrocystin concentrations that would be used to compare with the 1998 World Health Organization (WHO) Guideline Value of 1 μg L–1 for microcystin-LR in drinking-water. Therefore, it is recommended that a follow up study be conducted to determine the microcystins concentrations along with other cyanotoxins and bioactive compounds during these blooms. Further recommendations include biweekly assessments of microalgae diversity and abundance over a period of one year; analysis of other macro-and micronutrients, trace elements and chlorophyll a; topographical and mechanical assessments, especially during water transfers which may affect microalgae compositions as well as employing the use of a diversified set of phytoplankton nets with various pore sizes.