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The bay experiences an annual range of wind, wave, and water quality conditions. Trade winds blow onshore at 10-25 kts 90% of the summer, generating waves 1-3 m in height with periods of 6-9 s. During the winter, trade winds blow at variable speeds 50-80% of the season, and storms in the North Pacific deliver large swells (4 m height, 10-20 s periods) that may refract into the bay. Observational data sets describing conditions are available from various sources (principally NOS buoys and weather stations) extending through recent decades. Learn more about the study area by visiting my Kailua page.

Benthic habitats

A georeferenced, orthographically-rectified, digital mosaic of airborne multi-spectral data (above) illustrates the distribution of benthic substrates in Kailua. Dark areas are consolidated substrate, living coral, and algae; light areas are typically sandy regions. The sandy shoreline grades seaward to a shallow (<5 m depth), fossil limestone surface with a veneer of sand (hardgrounds). Various benthic environments of this region are composed of sand, rubble, limestone outcrops, meadows of the calcareous alga Halimeda and fleshy algae such as Sargassum, and mounds of Porites lobata like the one below.

The shoreface is dominated by a broad (4 sq km), shallow fringing reef 5-20 m in depth. The inner reef platform lies in 5-8 m water depth and is composed largely of fossil reef outcrops with 25-50% living coral cover. The outer reef platform extends seaward to 20 m depth as spur and groove topography with 50-100% living coral cover. Along the reef margin 3 km offshore, the fore-reef slopes steeply seaward (in some places as talus and elsewhere as living coral cover) to abut a sand field at -25 m (Hampton et al. 1998). The scleractinian coral genera Porites, Montipora, and Pocillopora characterize the reef platform, and their distribution, abundance, and morphology varies with depth and hydrodynamic energy. A photo of the reef slope is shown below.

Coralline (red) algae are also prolific inhabitants of the benthic community at all depths. Branching Porolithon gardineri is most abundant in shallow waters, while encrusting Porolithon onkodes and other species are found in all depths and habitats.

Porolithon gardineri		Porolithon onkodes

Age and composition of sediments

Beach and submarine sediments of Kailua Bay are primarily composed of carbonate skeletal fragments produced by two processes: mechanical, chemical, and biological destruction of reef framework limestone (coralline algae and coral) into rubble, sand, and silt; and direct sedimentation upon the death of organisms such as Halimeda, molluscs, and foraminifera. Although coral is abundant atop the reef platform, it is a minor constituent (<15%) of most sediment assemblages. Shoreface sands are instead dominated by coralline algae (up to 60%) and Halimeda (up to 35%). Other constituents include molluscs (15-20%), benthic foraminifera (<10%), and echinoderms (<5%).

Despite an apparently healthy ecosystem engaging in active carbonate production, sediments in Kailua are dominantly fossil-aged. Of 20 accelerator-radiocarbon ages, only one dates post-1950; twelve ages are 500-1000 calendar years before present (cal yr BP); five are 2000-5000 cal yr BP. Sand composition and age across the shoreface broadly reflect a spatial relationship to carbonate production. Corals and coralline algae, principle builders of the reef framework, are younger and more abundant in sands along the channel axis and in offshore areas, while Halimeda, molluscs, and foraminifera are younger and more dominant in sediments landward of the main region of framework-building. The figures below illustrate our age data (Harney et al. in press)

Age of carbonate skeletal material in sediments of Kailua Bay		Distribution of 20 ages of bulk sand and skeletal consitutents

NEXT... Sediment storage

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