Question: why is it that hydrothermal vents have a greater density of organisms surrounding it than the seabed around it?

Verity Nye answered on 15 Jun 2011:

Nice question. It’s all to do with food availability. Bacteria use the hydrogen sulfide from the vent fluid to produce energy in a process called chemosynthesis. This forms the basis of the food web at hydrothermal vents. As long as the vent keeps pumping out fluid there is a plentiful supply of hydrogen sulfide for the bacteria to feed on. Primary production is high and this sustains the “oases” of life found at vents. Some animals at vents have special adaptations that enable them to use the energy produced by the bacteria, whereas others feed on prey and dead animals.
In contrast the surrounding deep sea is a relatively food-poor environment. The animals rely on pulses of organic matter (bits of poo, vegetation, dead animals etc) reaching the seafloor so there isn’t a consistent supply of food. What does reach the deep sea doesn’t fall within a concentrated area. This is why scientists used to think of the deep sea as a food-limited environment. That view changed when hydrothermal vents were discovered.