Award: OCE-1605365

Award Title: Can Coral Reefs in the Central Pacific Survive Ocean Warming? A 2015 El Nino Test
Funding Source: NSF Division of Ocean Sciences (NSF OCE)
Program Manager: David L. Garrison

Outcomes Report

Intellectual Merit: Coral reefs in the central equatorial Pacific (CEP) experience dramatic swings in sea surface temperature (SST) on inter-annual timescales, the frequency and magnitude of which far exceed that imposed on other Indo-Pacific reefs. In the last 3 decades, CEP coral communities experienced 8 El Niño-driven warm events during which the Degree Heating Weeks (DHWs) thermal stress index exceeded 10. By comparison, the northern Great Barrier Reef has experienced one such event, in 2015, leading to catastrophic bleaching and mortality. Yet CEP reefs remain highly productive, with high percent live coral cover and fish biomass. This apparent contradiction raises questions about the nature of CEP coral reef response to, and mechanisms of recovery from, repeat episodes of extreme thermal stress. In the absence of direct observations of remote CEP reefs during peak El Niño, we analyzed 3-D CT scan images of skeletal cores extracted from 40-120 year old Porites corals in the CEP. Each coral has witnessed multiple successive El Niño events in their lifetimes. We developed an automated Matlab code to identify anomalously high-density skeletal structures or "stress bands" in the CT images (Figure 1). Our initial Palau study linked Porites stress bands with the severity of coral community bleaching. Interpreting the CEP data within this framework led us to hypothesize that CEP corals are not resistant to extreme heat. Rather, they appear to experience regular episodes of bleaching and mortality with varying degrees of severity. The 2015/16 El Niño provided an opportunity to test this hypothesis. Under this award, we chartered the 80m schooner Machias, captained by Bill Austin, from Honolulu to Jarvis Island (0°S, 160°W) in November 2015, during the peak of the SST anomaly. By that time, Jarvis coral communities had been exposed to SST anomalies above 3°C for 22 consecutive weeks. Our team of four scientists and graduate students conducted photo-ecosurveys at 3 depths on the east and west sides of the island, deployed pH, oxygen, current, salinity and temperature sensors and collected discrete water samples for alkalinity, dissolved inorganic carbon and nutrients, and 15N nitrate (Figure 2). We were also permitted to remove small cores from bleached Porites colonies that we had previously tagged and sampled. 95% of the corals on Jarvis were bleached down to 30m depth and the biogeochemical environment of the reef had completely changed. pH and aragonite saturation state (?ar) were significantly higher than normal, salinity was low and surface nitrate concentrations dropped from an average of 5 µmolar to below detection. Porites tissue biomass had declined by 75%, and CT scans revealed 88% of Porites forming skeletal stress bands. These observations enabled us to link prolonged bleaching with stress band formation and played a critical role in validating stress bands as proxies for community bleaching. In collaboration with NOAA?s Coral Reef Ecosystem Division, we returned to Jarvis in May 2016. Wind-driven and topographic upwelling had recommenced. The biogeochemical environment of the reef had returned to normal, nitrate and chlorophyll concentrations, and reef fish biomass were high, sharks, dolphins and manta rays were abundant once again. However, 95% of the corals were dead and the reef covered in cyanobacteria and algae. We returned again with NOAA in April 2017. We documented remarkably rapid recovery of many Porites colonies (Figure 3), and live, albeit few, Acropora, Hydnophora and Pocillopora including some juvenile Pocillopora. Encrusting coralline algae, several millimeters thick was observed growing over the cyanobacteria and turf. The historical stress band record from Jarvis, validated by the observations made under this award, implies that Jarvis experiences regular episodes of bleaching, sometimes with catastrophic mortality. Stress bands indicate that the impacts of the 1982/83 and 1997/98 El Niño may have been close in terms of severity to the 2015 event, resulting in loss of significant live coral cover. Combining our 2017 observations with the record of productivity, coral cover and fish biomass maintained on Jarvis and other CEP reefs, we propose that CEP coral communities may have evolved uncommon resilience as a mechanism for surviving the extreme and dynamic conditions in the region. Our documentation of Jarvis before, during and soon after the 2015 El Niño provides an important and unprecedented opportunity to test this hypothesis, to study the factors that promote or prevent recovery, and the mechanisms by which reefs might renew themselves after catastrophic loss. Broader Impacts: This project supported the research and training of four PhD students, one post-doctoral investigator, and one undergraduate senior thesis. Two manuscripts are published, one submitted and two prepared for submission. Results were reported at the International Coral Reef Conference in 2016, the 98th Annual Meeting of the AAAS Pacific Division, the annual SYMBIOFEST at University of Georgia, and will be featured at the United Nations-sponsored "The Ocean" Conference in New York. Published data from this award are archived under BCO-DMO account 1605365. Last Modified: 05/06/2017 Submitted by: Anne Cohen

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Principal Investigator: Anne Cohen (Woods Hole Oceanographic Institution)