Wave Control on Reef Morphology and Coral Distribution: Molokai, Hawaii

Abstract

A multidisciplinary project lead by the U.S. Geological Survey is currently studying the fringing reef off southern Molokai, Hawaii, in an effort to characterize the biological structure and geologic variability of coral reef systems. Wave modeling and field observations were utilized to help understand the physical controls on reef morphology and the distribution of coral species. The morphology of the reef crest, which extends roughly 50 km from east to west and up to 1500 m offshore, appears to be primarily controlled by the amount of wave energy impinging on the coastline. Extratropical cyclones and inter-anticyclonic systems crossing the North Pacific during the winter months generate the wave energy regime that appears to dominate the reef. This North Pacific swell generates near-bed orbital velocities greater than 3.0 m/sec and shear stresses greater than 2.5 N/m 2 that inhibit substantial coral development in shallow water (<10 m). The reef is sheltered from these waves by the island of Molokai; however, refraction around the east and west ends of the island cause the reef crest to pinch out roughly 5 km from each end of the island. Where the reef crest merges into the shoreline, more robust high-energy corals and low coral cover typify the fore reef. In contrast, more delicate branching corals characterize the central portion of the reef, which is sheltered from the large North Pacific swell, and up to 80% live coral cover.

EL Nino Influence on Holocene Reef Accretion in Hawai'i

“…Cores from Hikauhi, a few kilometers away, all show rapid and continual accretion over the late Holocene. This striking difference in modern reef accretion has been convincingly explained by Storlazzi et al (2002). Using a PACIFIC SCIENCE· April 2004 combination of wave modeling and field observations, they found high correlations between changes in reef morphology and total coral cover versus benthic shear stress from North Pacific swell.…”

“…Sites of substantial Holocene accretion are limited to areas that receive some degree of sheltering from large waves and have substrate at depths suitable to support coral growth (Dollar and Tribble 1993, Grigg 1998, Storlazzi et al 2002. New observations of reef accretion from several locations, however, show that accretion before the mid-Holocene occurred in some areas where today it is precluded by the wave regime.…”

“…Long-period (12 to 20 sec) North Pacific swell on the other hand has been extensively reported to exert a substantial control on reef distribution (Grigg 1983, Storlazzi et al 2002. Occurring during the winter season, these swells approach from a directional range of west-northwest through northeast with typical deepwater wave heights of 1.5 to 5 m, although waves as large as ca.…”

“…As a result, hurricane-induced wave energy generally has a more severe impact on southeastern to southwestern coasts, whereas North Pacific swell predominantly impacts west-to northeast-facing shores. However, longer-period swells can refract substantial energy around to coastlines facing away from the incident wave direction (e.g., Storlazzi et al 2002).…”

Calcium Carbonate Production, Coral Cover and Diversity along a Distance Gradient from Stone Town: A Case Study from Zanzibar, Tanzania

“…A number of physical parameters, such as wave exposure, currents and temperature, affect the development of coral reefs, its biodiversity (Roberts et al, 2002) and reef morphology (Mallela et al, 2004;Webb and Kench, 2010;Perry et al, 2013). For instance, platy coral morphologies are more successful on deep fore reef sites, due to their ability to harvest light (Todd, 2008), whereas more robust, boulder corals are generally found in the reef crest under moderate to high hydrodynamic regimes (Storlazzi et al, 2002).…”