Study of Ofu reef shows how waves may influence coastal dynamics and coral reef resilience

Pago Pago, AMERICAN SAMOA — Researchers from the USGS and the National Park Service have published the first documentation of exceptionally large low-frequency waves sweeping across the southern fringing reef of Ofu, American Samoa. This discovery, outlined in a new study, sheds light on how these waves may influence coastal dynamics and coral reef resilience on fringing reefs.

Ofu's southern fringing reef has long intrigued coral researchers due to the presence of super-heated pools on the reef flat, where corals exhibit remarkable resilience in the face of elevated sea temperatures. However, the new study examines a previously unrecognized phenomenon: the occurrence of exceptionally large low-frequency waves traversing the reef flat. 

During a large southerly swell event, researchers observed low-frequency waves with mean heights reaching 0.7 meters and periods spanning 2 to 4 minutes. These waves, characterized by periods exceeding 100 seconds, are estimated to have contributed up to 50% of the total water levels during the swell event, underscoring their significant role in coastal dynamics on the reef. 

Imagery looking oceanward over the West reef flat during calm conditions (a, b), and during the large wave event (c, d) with representative snapshots from low and high tides. Dark colors in lower portion of images are the exposed beach, and the light colors are the water. The ruler in a indicates the approximate vertical excursion up the beach face. e Four-day time series showing water levels before and during the large wave event and the timing of the images. [courtesy graphic]

One particular observation stands out amidst the data — a low-frequency wave, likely amplified by reef resonance, had a trough-to-peak vertical height of 1.5 meters, potentially representing the largest ever directly documented on a reef flat. 

While extreme waves like these can be hazardous in terms of coastal flooding and erosion, they also hold relevance to coral health and resilience. Low-frequency waves are important drivers of mass transport on reefs, which influences coral health through sediment and larvae transport, as well as exposure to nutrients and contaminants. 

The interplay between these waves and coral ecosystems remains an area needing further study, with implications for improving our understanding of reef ecology and developing effective adaptation strategies in the face of climate change.