then the submerged aquatic plants will note be able to receive light for photosynthesis. then those submerged aquatic plants will not have food/nutrients to survive, hence die. aquatic animals will not receive sufficient oxygen from those submerged aquatic plants, hence die also.
decomposes will breakdown all dead organisms to release carbon dioxide, and also deplete oxygen concentration during decomposition process.
eventually, habitat is rendered not livable. only the algae continue to proliferate uncontrollably at the water body surface.