Tracking Coral Larvae to Understand Hawai‘i Reef Health
Scientists can predict almost to the hour when the reef-building "rice coral" (Monitpora capitata) off O‘ahu will spawn, but no one knows where the resulting floating coral larvae go.
From June 11 to 16, the U.S. Geological Survey (USGS), the University of Hawai‘i (UH) at Manoa's Kewalo Marine Laboratory, and the community group Malama Maunalua conducted an experiment along O‘ahu's south shore in an effort to better understand why certain reefs in Maunalua Bay are doing well and others are doing poorly.
Maunalua Bay is a linked watershed-reef complex ("ahupua'a" in Hawaiian) of concern to the U.S. Coral Reef Task Force's Hawai‘i Local Action Strategy (USCRTF-LAS) and is degraded by polluted runoff and sediment, invasive algae, and unsustainable harvesting. The reduced quality of the water and the seabed affect not only the corals in the bay, but also the ability of larvae from elsewhere to settle on the seabed and replenish depleted populations (the "recruitment" process). If new corals are unable to replace those that die, the reefs will eventually disappear, along with the other associated marine resources.
Community-based efforts, coupled with American Recovery and Reinvestment Act (ARRA) restoration efforts to remove invasive algae are focused on improving coral and fish resources. However, coral reef recovery requires effective new recruitment, and Maunalua Bay characteristically has a low abundance of coral recruits. There is uncertainty as to whether the lack of recruitment is due to a low supply of coral larvae in the bay or to anthropogenic stressors that impair the recruitment success.
To help resolve this question, a multipartner experiment to investigate coral spawning and recruitment in Maunalua Bay was conducted this past June. The study focused on the coral species Monitpora capitata. It is a major reef builder in Hawai‘i, and the large size of the planula (free-swimming) larvae (0.75–1.50 mm) allows optical and acoustic detection of the larvae during the spawning event. The peak spawning periods for Monitpora capitata in Hawai‘i are around the summer new moon between 9:00 and 11:00 PM Hawaiian Standard Time (HST). This year, Monitpora capitata spawning began the evening of Saturday, June 12th.
Scientists Curt Storlazzi, Josh Logan, Kathy Presto, Tom Reiss, and Pete Dal Ferro (all USGS, Santa Cruz) collected information on circulation and water-column properties of Maunalua Bay during summer spawning conditions, complementing a dataset collected during the 2008-2009 winter. They were hosted by and collaborated with Robert Richmond (UH Kewalo Marine Laboratory), his students, and other staff at the Kewalo Marine Laboratory. Team members using scuba equipment conducted nightly fixed-station monitoring of select coral colonies before, during, and after spawning to determine the proportion of coral colonies spawning during the event. Satellite-tracked global positioning system (GPS) drifters were deployed above select reefs after spawning each night to track the movement of the buoyant coral larvae to determine if they are being retained in or carried out of Maunalua Bay. The team also surveyed current speeds and directions along more than 20 km of transects and water-column properties at more than 15 locations daily by boat before, during, and following the June spawning event. These surveys will provide information on hydrography of the bay during summer conditions and will allow scientists to identify the presence of eddies and shear zones in the bay that help to retain larvae or sediment, nutrients, or contaminants.
The relatively short-duration vessel surveys are being supplemented by longer term measurements of currents and water-column properties at a number of fixed stations in the bay from June through the end of September. For the June vessel surveys and the four longer term fixed stations, the team is using 600-kHz acoustic Doppler current profilers (ADCPs) to collect acoustic backscatter data. These data allow them to image the Monitpora capitata larvae and quantify the relative intensity of spawning events. Collectively, the combination of mobile vessel surveys and fixed instrument deployments will put the intensive June measurement efforts in the context of the full June-September Monitpora capitata spawning season. Once scientists understand the circulation, larval dispersal patterns, and "connectivity" between reefs, areas where reef recovery efforts should be focused can be identified.
While on-the-reef studies were taking place, Renee Takesue (USGS, Santa Cruz) worked with staff and volunteers from Malama Maunalua to collect sediment samples from the coral reef flat and the adjacent watersheds to determine the source(s) of the terrestrial sediment on the reef flat using geochemical means. Renee was joined on the reef flat by Cheryl Hapke (USGS, Woods Hole), who was collecting ground-truthing information for her effort to use historical aerial imagery to detect and map changes on the reef flat over the past 70 years.
At a reception hosted by Malama Maunalua, USGS scientists gave a public lecture for the community. The USGS and UH researchers were also interviewed by the local CBS affiliate (a nightly news "top story"), Hawaiian Public Radio, and the Star-Advertiser, Hawai‘i's largest newspaper. See Scientists seek data as corals get frisky, Secret love lives of Oahu's reefs unveiled, and Tracking Coral Larvae to Understand Hawai‘i Reef Health.
in this issue:
Tracking Coral Larvae