The body of water experiences eutrophication.
The process of eutrophication takes place primarily in ecosystems with gradual changing waters, mainly in deep lakes. In the depths of the lake, where deposition of dead algae takes place, the aerobic bacteria, which feeds on them proliferate that in turn consumes more amount of oxygen.
Though in the absence of enough circulation of water that is usually found in the case of deep lakes, the bottom of the lake is poorly oxygenated and the bacteria eventually deplete the oxygen found in the deep layers of water. Thus, they can no longer degrade all the dead organic matter and gets accumulated in the sediments. The lake is now considered to be aging.