Assessment of Lake Victoria’s Trophic Status Using Satellite-Derived Secchi Disk Depth
Trophic status (TS) is a water quality indicator effectively determined from Secchi Disk Depth (SDD). SDD is an in situ method of determining water clarity which is cumbersome, expensive and limited in time and space. This prompted the exploration of satellite imagery which provides a means around these challenges. Whereas algorithms for SDD retrieval from satellite imagery have been developed, their performance has not been tested on Lake Victoria. Therefore, this study aimed at determining the best SDD retrieval algorithm for MODIS satellite imagery from which the lake’s SDD and TS were determined. To this effect, five algorithms were explored and their output was compared to the in situ data collected on 27 July 2015. The multi band model performed best with a R2 of 0.709, RMSD of 0.295 m, RPD of 29.278% and bias of 0.593 m. This algorithm was applied to MODIS Aqua monthly and yearly aggregates from 2013 to 2017 and the trophic status was determined using Carlson’s Trophic Status Index (CTSI). The outputs showed that the water closer to the lake’s shore had a shallower SDD as compared to that in the middle of the lake and the CTSI values indicated that the lake presents as mesotrophic with persistent eutrophic hotspots at the shoreline. It was concluded that Lake Victoria was exhibiting water quality issues and recommended that they are dealt with to prevent its further degradation.
KeywordsSecchi disk depth Trophic status MODIS Lake Victoria Remote sensing
I acknowledge financial support through an IEEE GRSS/AARSE TRAVEL FELLOWSHIP.
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