Assessment of Lake Victoria's trophic status using satellite-derived Secchi Disk depth

Abstract

As a result of the accelerated population growth in Lake Victoria’s catchment area accompanied by the large scale urbanization in the area, the lake has increasingly faced water quality issues. Trophic status index is a major indicator of water quality which can effectively be determined from secchi disk depth, a water quality parameter. Secchi disk depth is a method of determining water clarity, traditionally performed and monitored through field work activities. This method is very cumbersome, expensive and limited in time and space. To overcome these limitations, satellite data is used because it offers a synoptic view of the study area. This study therefore explores the use of satellite imagery to monitor secchi disk depth in Lake Victoria. It aims at determining the best secchi disk depth retrieval algorithm from MODIS imagery from which the lake’s secchi disk depth and trophic status can be determined. Five secchi disk depth algorithms were explored namely: single band model, band ratio model, multi band model, Mueller algorithm and Lee et. al algorithm. The output of these algorithms was compared to the in situ data collected on 27th July 2015. Of the 22 points collected on that day, 11 were used because the rest were obscured and greatly affected by cloud cover thereby rendering them unusable. From the statistical analysis performed, the multi band model was deemed to be the best performing algorithm with a R2 of 0.709, RMSD of 0.295m, RPD of 29.278%, a bias of 0.593m and CPRMSE of 1.152m. The identified algorithm was applied to MODIS Aqua images from 2013 to 2017 from which the lake’s trophic status was determined using Carlson’s Trophic Status Index. Both monthly and yearly image averages were developed and used for further analysis. The outputs showed that the water in middle of the lake was more transparent as compared to that closer to the shore. This was more so for water close to the highly industrious areas along the lake’s shore line. It was also observed that the lake’s secchi disk depth has been declining over the years and this was attributed to anthropogenic activities in the lake’s catchment area. The trophic status values from Lake Victoria indicated that it exhibits mesotrophic, eutrophic and hypertrophic characteristics. This corresponded with the results from the model with the areas having shallow secchi disk depths being hypertrophic, those with intermediate secchi disk depths being eutrophic and those with the greatest secchi disk depth being mesotrophic.