Fish and other aquatic life depend on oxygen gas dissolved in the water. which event is most likely to harm aquatic life by reducing the oxygen concentration of the water?

The mass flow of currents, or waters, are vital in understanding that how the movement of heat energy takes place between the water bodies of the Earth, atmosphere, and the landmasses.  

About 70 percent of the planet is covered with ocean and holds ninety-seven percent of its water, thus, forming the ocean an important element in the transfer and storage of heat energy through the planet.  

The motion of this heat via the global and local ocean currents influences the stabilization of global climatic patterns, the regulation of local weather conditions and temperature extremes, delivery of nutrients, and cycling of gases in the aquatic ecosystems.  

In the process, an influx of warm water, which elevates the temperature of the lake by 5 degree Celsius is most probably to affect the aquatic life by minimizing the concentration of oxygen in the water.