Remote sensing of Harmful Algal Blooms (HABs) in water bodies of Vhembe district area, Limpopo province, South Africa

Abstract

Satellite remote sensing techniques have been proved to be the best methods for quantifying chlorophyll-a levels by estimating algal concentrations in water bodies. Harmful algal blooms (HABs) are posing a significant threat to the many water bodies in South Africa. This study aims at developing remote sensing solution to estimate chlorophyll concentrations in water bodies of Vhembe district municipality using recently launched Landsat 8 OLI. It is the first study to provide quantitative water quality information for the Vhembe region’s water bodies from a time series of satellite remotely sensed data and in-situ laboratory data. The objectives of this study was to evaluate spatial and temporal distributions of algae in water bodies and fish-ponds, to assess water quality parameters, namely: chlorophyll-a and turbidity and to compare data obtained from satellite remote sensors with in situ data. The 30 meters spatial resolution multispectral Landsat 8 OLI for 2016, 2017 and 2018 were used to derive chlorophyll-a estimate from an existing model at three water bodies. The chlorophyll-a concentrations measured during in-situ were employed to validate the Landsat derived chlorophyll-a estimates. The results from this study shows that Landsat derived chlorophyll-a estimates are correlating with field measurements. In all the reservoir, it was detected that there is low content of harmful algal blooms and thus the water bodies are in good condition since the chlorophyll-a concentrations were very low (ranging from 0 to 0.6 mg.m-3). In conclusion, it can be stated that Landsat 8 OLI sensor has the potential to map inland water bodies dominated with algal blooms at certain extent. It can further be stated that Landsat 8 OLI is suitable for monitoring the growth of HABs in aquatic ecosystem and is cost effective. This study also evaluated the potential of Banana peels powder and K2SO4 to inhibits the growth of algae (batch experiment). The water samples were collected at Tshifulanani and Lwamondo fish ponds where there are floating algae. The samples were collected seasonally and analysed for pH, water temperature, Total Dissolved Solids, Electrical conductivity, Dissolved Oxygen, turbidity, chlorophyll-a and absorbance. From the laboratory experiments, there was a variation in the values of absorbance (0.936A-1.234A), PH (7.1-8.3), EC (63.1- 87.9 μs/cm), TDS (52.6-69.7mg/L), water temperature (25.5-29.3°C) and Dissolved oxygen (5.3-7.1mg/L). The concentration of chlorophyll-a for Tshifulanani and Lwamondo fish ponds ranges were (2.14-15.96 mg/m-3) and (0.65-15.66 mg/m-3) respectively. A batch experiment was conducted to determine the potential of banana peels powder on inhibition of algal blooms in water samples by measuring absorbance at 750nm. It can be concluded in this study that banana peels have a potential to inhibits the growth of algae in fish ponds. The Absorbance has shown a rapid v decrease from 0.936A to Zero from day 1 to day 7 respectively. The inhibition of cyanobacteria by banana peels is followed using Potassium sulphate in treating the algal blooms in water samples. Both banana peels and potassium Sulphate has shown a positive response in treatment of algae on the batch experiment. The results of this study revealed that high concentration of physico-chemical parameters promote the growth of cyanobacteria in fish ponds but does not have negative effects on the fish except the oxygen competition with algal blooms. The statistical analysis in correlating the chl-a field measurements and remotely sensed data showed a positive outcome where K values were very high from 70% to 89%. These results show high level of agreement of correlation values of field chlorophyll-a concentration and satellite remotely sensed variables.