USGS Pacific Coral Reefs Website

Sediment Studies

Historical patterns of sedimentation are important for determining the role of human activities and their impact on a coral reef. Sediment eroded from land as a result of both natural processes and human activities can pose a serious threat to coral reef health by (1) reducing the area of sea floor suitable for growth of new coral, (2) creating cloudy or turbid water that diminshes the amount of light available for photosynthesis by symbiotic algae that live within individual coral animals, and (3) in extreme cases, burying coral colonies. We are using a variety of methods and tools to help understand the processes that affect sedimentation on coral reefs.

We use two different types of sediment traps in our studies: rotating and stationary. Rotating sediment traps measure sediment accumulation over a short time span. Every four or five days, the system rotates to position a new bottle beneath the collection funnel, allowing scientists to see spikes of sediment input from events such as storms. Solitary, stationary tube traps made from PVC are used to measure sediment accumulation over an extended period of time.

(r) A rotating sediment trap instrument package is lowered to the seafloor from a boat using a rope and supervised by safety divers. The large cylinder covered with a red cap is the rotating sediment trap collection funnel. The smaller red-capped cylinder is an additional stationary tube trap. After the instrument package is in place, the caps are removed and the system begins collecting sediment. (l) The instrument package at the end of a three-month deployment before it is retrieved from the seafloor. Photos from Moloka‘i.

Tube trap bottles from a three-month period show the temporal variability in trap accumulation rate, along with the variation in grain size and composition (white carbonate sand and reddish-brown terrestrial mud). Several bottles have an increased amount of sediment, indicating a time period of storms or high-waves.

A single tube trap is attached to a stake on the high point of a reef. Photo from Moloka‘i.

Another method uses a camera placed directly on the reef to provide visual documentation of sedimentation on coral surfaces, which can then be correlated with measurements of waves, currents, rainfall, and stream discharge. The camera obtains a time-series of digital photographs (for example, every four hours) with the time and date recorded on each image. Companion instruments record the changes in light penetration, or turbidity, and the camera images show whether the particles actually settled on the bottom or were transported through the region. A gridded concrete block is used as a proxy for an irregular coral surface so that estimates of levels of sediment accumulation—and erosion—can be correlated with other measurements and improve understanding of the conditions that affect sedimentation on corals.

The reef camera on its first day of deployment. A small coral, broken off from a previous storm, was also placed under the camera as a reference. Photo from Hanalei Bay, Kaua‘i.

Images from the reef camera show a gradual accumulation of sediment on the gridded block. During this deployment, the small piece of coral visible in the first image was moved out of the camera's view by waves. Photo from Hanalei Bay, Kaua‘i.

On shallow reef-flat areas, we use sediment probes (graduated stainless steel rods) to measure sediment thickness. Maps of sediment thickness made using these measurements, combined with sediment ages and compositional data, provide a detailed record of the history of deposition.

Scientists measuring sediment thickness with a graduated rod. Photo from Moloka‘i.

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