THE EVOLUTION OF A TROPICAL PARADISE.

Hawaii is one of the few places in the world where the majority of the stages of reef evolution can be seen.  Recently created lava flow on the big islands become home to new reefs and developing communities. Fringing reef forms  near shore can be seen throughout Hawaii. Barrier reefs are seen off  the coast of Oahu and Kauai and coral atolls are built on top of sunken volcanic islands in parts of the north-western Hawaiian Islands (Figure 1).

Figure 1: A. The Hawaiian Archipelago. B. The Island of Oahu.

After my last entry you will remember that the accretion of coral reefs during the Holocene is sea level dependent, after the peak of the Wisconsin glaciation some 21ka the glacier melted, sea level rose and approached the critical depth (30m) of modern reefs....

.... BUT wave exposure is also a major determinant that controls the development of coral reefs (Grigg 1998). In many parts of the world windward reefs may be favoured over leeward reefs because of the greater exposure to ‘moderate’ wave action which in turn optimises misting, and nutrient availability.

Crigg (1998) showed that of the coast of the island of Oahu (Figure 1) areas exposed to between 0-30% wave exposure have an average long term growth of corals of 2mm/y. Albeit small this is still positive.  However along wave exposed coasts (waves greater then 7.5m) Holocene reef formation is only a thin layer sitting on top of a Pleistocene limestone foundation.  

Modern coral communities in wave exposed environments undergo constant turnover associated with morality, recruitment or re-growth of fragmented colonies and are rarely thicker than a single living colony. High Hawaiian islands are highly susceptible to wave action and it is this that has been associated with its lack of mature barrier reefs. The only reef off  the coast of Oahu that is close to become a barrier reef is off Kaneohe bay. This side of Oahu faces east and is therefore exposed to only moderate trade winds. This lack of destruction from large waves and hurricane swells has allowed the reef to establish itself.   

But barrier reefs are present in wave exposed coastline in other parts of the world (e.g. tropical pacific islands) so another factor must play an important role. One proposed theory is the low species diversity of coral in the high Hawaiian islands.  Acropora the fastest growing species of coral is present in the middle of the Hawaiian archipelago but absent from the high islands. On the island of Oahu in protected and deep water environments the dominant coral species is P.compressa. Whilst in more exposed habitats  P.lobata is the dominant reef builder. Another theory is that Hawaii’s waters are on the cool side for substantial coral growth. 

In short it isn’t only the change in Holocene sea levels which influence the Hawaiian corals and their formation, community structure appears to be controlled by wave energy and depth, alongside species diversity. 

Crigg, R.W. 1998. DOI: 10.1007/s003380050127