Researchers from SCRIPPS Oceanographic Institute have published an important article in the journal PLoS ONE, detailing research that confirms what has suspected for some time – that local stressors reduce the resilience of corals to bleaching events.

Jessica Carilli and colleagues set out to test the hypothesis that chronic local stress reduces coral resistance and resilience to bleaching, by investigating coral growth before and after the 1998 bleaching event in Belize. The authors took over 90 coral core samples from sites with relatively high and low chronic stress, and determined changes in growth rate over the past decade (much like the declining calcification on the GBR Science paper released earlier this year).

The results are striking – after the 1998 bleaching event, the massive star corals (Montastraea faveolata) from ‘healthy’ reefs (low chronic stressors) were able to to recover and grow normally within two to three years, whilst star corals from unhealthy (high chronic stressors) reefs showed no sign of a complete recovery in the 8 years following the bleaching event.

(A) Coral without the 1998 growth suppression, indicating resistance to bleaching in 1998. (B) Coral with the 1998 growth suppression, recognized by the bright high-density band, but with a quick return to pre-1998 extension rates, indicating resilience after bleaching. (C) Coral with the 1998 growth suppression and continuing depressed extension rates after 1998, indicating a lack of both resistance and resilience to bleaching. (D) A coral with relatively high average extension rate. (E) A coral with relatively low average extension rate. (F) A coral with a partial mortality scar on the left (noted by white arrow), coincident with the 1998 growth anomaly.

(A) Coral without the 1998 growth suppression, indicating resistance to bleaching in 1998. (B) Coral with the 1998 growth suppression, recognized by the bright high-density band, but with a quick return to pre-1998 extension rates, indicating resilience after bleaching. (C) Coral with the 1998 growth suppression and continuing depressed extension rates after 1998, indicating a lack of both resistance and resilience to bleaching. (D) A coral with relatively high average extension rate. (E) A coral with relatively low average extension rate. (F) A coral with a partial mortality scar on the left (noted by white arrow), coincident with the 1998 growth anomaly.

“You can imagine that when you are recovering from a sickness, it will take a lot longer if you don’t eat well or get enough rest,” said Jessica Carilli, Scripps graduate student and lead author on the study. “Similarly, a coral organism that must be constantly trying to clean itself from excess sediment particles will have a more difficult time recovering after a stressful condition like bleaching.”

“It is clear that Mesoamerican corals really fell off a cliff in 1998 — nearly everybody suffered mass bleaching,” said Dick Norris, Scripps professor of paleooceanography and co-author of the study. “There are no pristine reefs in the region, but the ones in the best shape clearly are more resilient than those that are long-suffering. It shows that a little improvement in growing conditions goes a long way in recovering coral health.” (Read More)

Almost as striking are the obvious ‘scars’ left by the 1998 bleaching event, as evidenced by the decline in coral growth (annual extension rate) across all four sites:

Picture 610

Means (solid lines) and 95% confidence intervals (shading) for extension rates after 1955. Extension rates at Sapodilla and Utila remain suppressed after the 1998 bleaching event.

The authors show that the fastest recovering corals were collected from the offshore site at Turneffe Atoll, whilst the more heavily polluted sites at Sapodilla Cayes and Utila in Honduras suffer from significant impacts linked to local factors such as development, sewage and runoff. Considering that the entire Mesoamerican Barrier Reef was bleached during the 1998 bleaching event, it’s great to see that ameliorating local impacts can have a significant effect on reducing the effects of regional-scale bleaching:

“… local conservation efforts that reduce stress, such as reducing runoff by replanting mangroves at the coast or protecting an area from overfishing, could have significant impacts on the ability of corals to withstand the effects of climate change. Future research could investigate whether this interaction between local and global stressors extends to other coral species.”

Citation: Carilli JE, Norris RD, Black BA, Walsh SM, McField M (2009) Local Stressors Reduce Coral Resilience to Bleaching. PLoS ONE 4(7): e6324. doi:10.1371/journal.pone.0006324

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