Community-based monitoring detects catastrophic earthquake and tsunami impacts on seagrass beds in the Solomon Islands.

Seagrass beds are an important component of the marine ecosystem and play a significant role in coastal protection and maintaining fish production. Despite a global decline in seagrass, the Indo-Pacific region supports a high diversity of seagrass species with many seagrass beds still intact and healthy. Tetepare Island in the Solomon Islands is the largest uninhabited island in the South Pacific and supports seagrass beds inside fringing reefs along its southern coastline. We monitored the diversity and abundance of seagrass species on Tetepare and nearby sparsely-populated Rendova Island over a 12 year period, 4 years before and up to 8 years after a major earthquake and tsunami event in January 2010. Changes to seagrass beds were compared at sites close to and remote from the earthquake epicentre. Before the earthquake, eight seagrass species were recorded on Tetepare Island with an average cover at monitoring sites of approximately 50%. The seagrass beds within Tetepare's Marine Protected Area (MPA) on the SW side of the island supported a family of dugongs and hundreds of green and hawksbill turtles. The 2010 earthquake registered 7.1 on the Richter scale and the epicentre was only 20 km from Rendova and Tetepare Island. It created a tsunami wave that surged at least 7 m, caused permanent subsidence of up to 70 cm and caused major landslides that deposited sediment onto the seagrass beds inside the fringing reefs of the Tetepare MPA. Both seagrass cover and diversity declined after the tsunami. Seagrass cover declined the fastest at sites on Rendova, closest to the epicentre, declining from 50% to <10% cover within 12 months of the earthquake. At sites within the Tetepare MPA, seagrass cover took longer to decline and dropped from an average of 50% to <10% within 2 years of the 2010 earthquake and became dominated by Halophila ovatus. Species richness declined from 9 to 4 species and diversity significantly declined with some species such as Syringodium isoetifolium disappearing completely from monitoring sites. Anecdotal turtle sightings reduced and the dugongs left the lagoon. Sites on Tetepare East, furthest from the epicentre, remained unaffected by the earthquake until 3 years later when they began to decline, possibly due to subsidence changing water depth and light availability. The 2010 earthquake triggered a major change in seagrass diversity and cover on Tetepare and neighbouring Rendova Islands and seagrass cover and diversity had not reached pre-earthquake levels 8 years after the event. Changes are likely to be related to physical damage from the tsunami wave, changes to turbidity from landslides and changes to water depth and light penetration from subsidence.