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The notable decrease in phytoplankton biomass of Lake Ziway in 2005, which became evident when comparison of data generated in different years (See Table VI) was made, indicates that the lake has experienced deterioration in its water column conditions as related to phytoplankton growth over the last 15 years.
Why the authors are so certain that the decrease of phytoplankton biomass means the deterioration of the lake condition ?
The data show that the lake trophic status has decreased from hypertrophic conditions (chl-a > 150 μg/l) to eutrophic conditions (mean value 36 μg/l).
Consequantly, it would be interesting to discuss the possible causes of this change and the possible consequence to the lake ecosystem.
But, in my opinion, it is wrong to say that the decrease from very (!) high to moderately high phytoplankton biomass unambiguously means the deterioration of water quality (because in most cases the decrease of phytoplankton biomass clearly shows the decrease of lake trophic status and improvement of water quality) |