Bioremediation of South Africa’s hypertrophic Hartbeespoort Dam: evaluating its effects by comparative analysis of a decade of MERIS satellite data in six control reservoirs

Abstract

Medium Resolution Imaging Spectrometer (MERIS) satellite data on chlorophyll a (Chl-a, μg L⁻¹) and cyanobacteria (Cyan, %) content in Hartbeespoort Dam (HBPD) and 6 “control” reservoirs between 2002 and 2012 were compared to assess potential effects of a bioremediation project (HMaM) implemented on HBPD in 2008. Planktonic autotrophs showed clear but inconsistent seasonal periodicity in all 7 reservoirs. Neither average (±95% confidence interval) values of Chl-a (172.1 ± 33.3 vs. 189.0 ± 40.3 μg L⁻¹) nor Cyan (45.1% ± 8.6 vs. 45.0% ± 9.0) differed significantly (p ≤ 0.05) in HBPD before and after HMaM, but differed (higher and/or lower) in 3 control systems. Across 3 timespans (overall, before 2008, and after 2008), Mann-Kendall tests for Chl-a and Cyan revealed mixed trends between and within reservoirs with similarly mixed trends in monthly anomaly values (ChlAnom and CyanAnom) that offset confounding effects of interannual variability. Chl-a and ChlAnom, however, declined consistently in all reservoirs after 2008, significantly so for Chl-a in HBPD and 2 controls, and for ChlAnom in HBPD and 5 controls. Although Cyan and CyanAnom trends varied, both were significantly negative in HBPD after 2008. Directionally concordant anomaly trends were provided by parallel 50th quantile regression analysis. Comparable declines in Chl-a and ChlAnom across all reservoirs preclude causal interpretation of the declines in HBPD as direct consequential effects of HMaM intervention alone, giving whole-lake support to published empirical evidence contesting the suitability of biomanipulation for eutrophic reservoir management in the region.

Keywords:

• biomanipulation
• biomass removal
• bioremediation
• eutrophication management
• remote sensing
• warm temperate reservoirs
• water security

Acknowledgements

This study was enabled by an NRF Research Incentive Award to RCH and constitutes an output of the Water Research Commission Project K5/2458 led by MWM. Particular thanks to Associate Editor Dr Michelle Palmer for helpful statistical pointers and generous extension of revision deadlines to accommodate unavoidable circumstantial delays. Dr Craig Morris (same affiliation as first author) kindly provided the 50th quantile regression data reported here. We acknowledge inputs provided by anonymous referees to improve the paper.

Postscript by RCH (12 January 2018) – A local press statement (Kormorant; 5 May 2017) reported that amidst allegations of corruption, nepotism, and infighting, DWS had abruptly pulled the plug on the (perceptively) successful HMaM programme, resulting in explosive growth of water hyacinth (Eichornia) to then cover more than 30% of HBPD’s surface. The closure of all DWS website links to HMaM during 2017 suggests an official embargo on this bioremediation venture, now disclosed (DWS 2017: Case Study 6, page 22) to have cost ZAR 500 million by 2015. [DWS] Department of Water and Sanitation. 2017. Water Quality Management Policies and Strategies for South Africa. Report No. 3.2. Integrated Water Quality Management (IWQM) Strategy. Water Resource Planning Systems Series, DWS Report No.: 000/00/21715/16. Pretoria, South Africa. xxiii + 116 pp.