Final answer:
Most ocean life is concentrated near the surface due to the availability of sunlight for photosynthesis, warmer temperatures, and higher levels of dissolved oxygen and nutrients. Deeper ocean layers have much higher pressures and colder temperatures, which create challenging conditions for life.
Step-by-step explanation:
The reason most life in the ocean is concentrated near the surface is mainly due to the availability of sunlight, which is vital for photosynthesis. This process is carried out by phytoplankton, forming the base of the marine food web and capturing about half the energy that flows through the Earth's biosphere. In addition to light, solar radiation warms the surface water, influencing the rates of growth for organisms and the dissolved oxygen levels, which are higher near the surface. Dissolved oxygen is crucial for aerobic respiration in marine life.
Furthermore, surface waters generally have a higher concentration of dissolved nutrients than deeper waters, as they are often carried into the ocean from land through runoff, streams, and rivers. However, it's worth noting that decomposers on the ocean floor can release nutrients back into the water as they break down dead organisms, therefore the very bottom may also be nutrient-rich.
As for temperature, it tends to be more stable near the surface compared to the extreme cold found at great depths. Pressure is another limiting factor for deep-sea life; the farther from the surface, the higher the pressure, which can affect the structure and function of biomolecules. Yet, life does persist in these conditions around features like hydrothermal vents, through unique adaptations and processes such as chemosynthesis.