We will focus on photosynthetic communities which derive
their energy, either directly or indirectly, from solar radiation.
The biological effect of photosynthesis extends far beyond the sunlit surface waters or the mixed layer.
Remember from our previous work, that solar radiation typically penetrates
less than 200 meters into the sea, therefore, these photosynthetic communities
depend on processes in the mixed (surface) layer of the ocean, which forms less than 4% of the water
volume of the ocean.
However, the remains of organisms living in the surface waters sink into the deep-sea, as particles of organic matter, called marine snow, thereby supporting life in
the mid-water environment, on the abyssal plain and in deep-sea sediments.