Ans. Algae thrive in the upper 60 to 90 metres (200 to 300 feet) of ocean water because they require warmth, sunlight, and nutrients to develop and reproduce. The epipelagic zone, the uppermost layer of water, is rich in oxygen, is exposed to sunshine, and is warmer than water at lower levels. When algae and other ocean animals die, they sink to the ocean's bottom, where they degrade and release the chemicals that they were composed of. These nutrients can deplete the oxygen in the water under certain circumstances.The density of water and how it travels is determined by temperature and salt concentration (currents). Coldwater is denser (heavier) than warm water and sinks to the bottom (downwelling). Other water flows across to take its place. Water at the surface is eventually replaced by water that has risen, or upwelled, from the ocean's depths to the surface. Upwellings transport nutrient-rich water to the surface. Algal blooms may be triggered by this increase in nutrients.