The organisms are as essential a part of the environment as the water, substratum, etc., and it would not be proper to consider the environmental conditions with the organisms excluded. Conditions in a coral reef are, however, so dominated by the characteristics of the living corals and calcareous algae themselves that it is more convenient to defer detailed discussion of these conditions until the reef inhabitants have been described. It is sufficient at this stage to mention certain external conditions which limit the existence of reef corals. To begin with, the vertical range is limited. Pacific reefs extend up slightly into the intertidal zone, but Atlantic reefs appear to be limited to below extreme low watermark. The depth to which reefs extend varies, probably being controlled largely by the transparency of the water, and rarely exceeds 40-60 meters.
Coral reefs are restricted to the warmer seas, not occurring where the water temperature drops much below 21°C. in winter. In Bermuda, the northern limit of Atlantic reefs, the winter minimum is about 18°C., whereas off Miami, the northern limit on the American mainland, the winter minimum is probably about 20°C. At Miami Beach, where conditions are probably more extreme, the average winter minimum is 18.7°C. and the recorded minimum 16.7°C.