Kelp forest structure.

Physical Conditions Light Both seaweeds and reef-building corals grow only in surface waters, where there is sufficient light to support photosynthesis. The depth of light penetration sufficient to support kelp and coral varies with local conditions from a few meters to nearly 100 m.  Temperature Temperature limits the distribution of both kelp and coral. Most kelp are limited to temperate shores, to those regions where temperatures may fall below 10°C in winter and rise to a bit above 20°C in the summer. Reef-building corals are restricted to warm waters, to those regions where the minimum temperature does not fall below about 18°C to 20°C and average temperatures usually vary from about 23°C to 25°C. Reef-building corals are also sensitive to high temperatures, however, and temperatures above about 29°C are usually lethal.