Imagine the busiest cities on the planet and their population sizes and ask yourself - how are so many people able to reside within such a small area? The best answer would be infrastructure. This is the also the answer to why coral reefs, which only make up 0.1 % of the earth’s surface, are able to house 25 % of all marine biodiversity. As corals grow, their calcium carbonate skeleton creates a complex rigid framework, which other organisms can utilize for their own purposes. Environmental conditions, such as wave exposure, temperature and light, control the coral communities and create different types of coral reefs.
Over the past four years, while surveying for the XL Catlin Seaview Survey, we have come across many different types of coral reef cities including Acropora meadows on the sheltered Great Barrier Reef, boiler reefs in Bermuda and the boulder star coral reefs in the sheltered Belize Barrier Reef. More diverse complex coral communities create busier underwater cities rich in marine life. This is one of the main goals with this project, relating variations in coral reef structural complexity present under different environments to the marine residents that rely on it.
Here in Hawaii, reefs are more isolated than the reefs we have previously surveyed, and are dominated by fewer coral species, namely Porites, Montipora and Pocillopora. As these reefs are strongly affected by oceanic currents and wave exposure, changes in the distribution of these coral communities, as a result of these dominant drivers, can be reflected in the make-up of their residing marine organisms. Over the past couple of days, the team has been surveying reefs along the western coast of Hawaii Island, and we were impressed by the high coral cover of the few species present. We also saw dramatic shifts in the coral reef communities as we scooted along our 2 km transect following the reef contour at 10 m depth. Reefs dominant in Pocillopora species, made up of independent colonies of small compact branches, would give way to Porites fields carpeting the reef with their fingerlike projections. Coral colonies could also be seen growing over large terrigenous rocks, where these rocks appeared to help enhance the complexity of the reef structure and creates surfaces and spaces for corals and other organisms to grow. This is contrasting to what we have seen before, where corals were the main engineers of infrastructure.
As we continue to survey the different islands, the team is excited to observe how these reefs and their complexity may change from one island to the next as their environments also change. Considering the isolated position of Hawaiian reefs, and the presence of fewer species of corals, it is important to understand coral reef structural complexity, since Hawaiian reefs and their residents may be especially vulnerable to changing climate.