Corals need clear water which are low in nutrients. They cannot not tolerate freshwater or nutrients carried in the water run-off from the mainland. That is why the most diverse and abundant corals grow offshore where the environmental conditions are more suitable. The clarity of water on the reef is determined by a combination of the amount of sediment and the amount of phytoplankton in the water. Sediment becomes suspended due to increased water motion caused by tide changes, high winds and storms.
Phytoplankton are the microscopic plants that drift around in the water.
They are more numerous in areas where the nutrient levels of the water are higher particularly around coastal reefs which receive nutrient rich runoff from the land. Whales are normally encountered during the winter months when they migrate up to the reef from Antarctic waters to mate and give birth. One of the most spectacular visitors during this period is the Humpback. They are seen in the shallow coastal waters of the Great Barrier Reef ranging from Harvey Bay to Port Douglas. Whale watching is conducted by a number of tourist operators through these areas.