Final answer:
The area of rapid change in salinity with slight change in depth is called a halocline, which is crucial for marine life adapted to specific salinity levels.
Step-by-step explanation:
The area of rapid change in salinity with a slight change in depth in the ocean is called a halocline. Haloclines often occur where there is freshwater influx, such as at the mouth of rivers and estuaries, and can also occur in the open ocean.
The existence of a halocline can significantly affect marine life, as different species are adapted to particular salinity levels. For example, estuaries, where rivers meet the ocean, provide nourishment and shelter for various species thanks to the mix of fresh and saltwater creating a diverse range of salinity conditions. In the open ocean, the oceanic zone often exhibits haloclines, especially in areas where there is less mixing of water layers.