A large no.of fish are suddenly found suddenly floating dead on lake.There is no evidence of toxic dumping but you can find an abundance of phytoplankton.Suggest a reason why were the dish found dead?
The fish asphyxiated due to a lack of oxygen.
This is an example of eutrophication, where agricultural run off containing fertilizer stimulates algal growth in the waterways. Now during the day, the algae photosynthesize, and are net producers of dioxygen. During night-time, however, photosynthesis stops (why?), and the algae are net consumers of dioxygen gas.
If the algal bloom is large enough, the algae might consume all the dissolved oxygen in a lake or body of still water, so that during the night, there is insufficient dissolved oxygen to support fish and other animals, and all the fish are turned off.