Eutrophication and cyanobacterial blooms in South African inland waters: 10Â years of MERIS observations

The medium resolution imaging spectrometer full resolution (MERIS FR) archive (2002 to 2012) over South Africa has been processed with the maximum peak height (MPH) algorithm for the 50 largest standing water bodies in South Africa. Time series of chlorophyll a (chl-a), cyanobacteria and surface scum area coverage were used to establish the status, seasonality and trends for each of the water bodies. The majority (62%) of the 50 water bodies were hypertrophic (mean chl-a > 30 mg m− 3), while 23 water bodies had intermediate to extensive cyanobacteria coverage. Cyanobacterial surface scum events posing a high health risk occurred in 26 of the water bodies in varying extents. Analysis of significant trends showed that water bodies both worsened and improved with regard to eutrophication, cyanobacteria and surface scum coverage between 2005 and 2012. Validation of the MPH algorithm using an independent in situ dataset demonstrated that gross trophic status through chl-a can be accurately determined in both eukaryote and cyanobacteria-dominant waters. Chl-a estimation in oligo/mesotrophic waters remains challenging due to a wide range of potential sources of error. This study is the first of its kind providing quantitative chl-a and phytoplankton species information in lakes from a time series of satellite remotely sensed data on a sub-continental scale, demonstrating how global analyses of changes in lake biogeochemistry might be performed in the future. It demonstrates the pivotal role that satellite remote sensing can play in supplementing in situ monitoring efforts, particularly in the developing world. The study concludes that eutrophication and cyanobacterial blooms are widespread in South African water bodies and remain issues of critical concern for water quality.