Lernen von der Klimageschichte Panamas: Kältephase führte zu Unterbrechung im Korallenwachstum

Die Klimaerwärmung tötet die Korallenriffe. Das lesen wir immer wieder in den Zeitungen. Es ist richtig, dass Korallen nach Hitzeereignissen bleichen. Genauso richtig ist jedoch, dass sich viele Korallen danach wieder erholen. Wir wollen Ihnen heute weitere wissenschaftliche Studien vorstellen, die sie in keiner Zeitung lesen können, da ihre Resultate offenbar zu unbequem sind. Erstes Beispiel ist eine Arbeit von Perry et al., die 2015 in Scientific Reports erschien. Die Autoren untersuchten Korallen im Indischen Ozean, die 1998 während eines Hitzeereignisses starke Verluste erlitten. Perry und Kollegen waren überrascht, dass sich die Korallen hiervon bestens erholt haben. Die Wachstumsraten sind sogar so hoch, dass die Korallen dem erwarteten Meeresspiegelanstieg der kommenden Jahrzehnte problemlos hinterherwachsen können. Abstract:

Remote coral reefs can sustain high growth potential and may match future sea-level trends
Climate-induced disturbances are contributing to rapid, global-scale changes in coral reef ecology. As a consequence, reef carbonate budgets are declining, threatening reef growth potential and thus capacity to track rising sea-levels. Whether disturbed reefs can recover their growth potential and how rapidly, are thus critical research questions. Here we address these questions by measuring the carbonate budgets of 28 reefs across the Chagos Archipelago (Indian Ocean) which, while geographically remote and largely isolated from compounding human impacts, experienced severe (>90%) coral mortality during the 1998 warming event. Coral communities on most reefs recovered rapidly and we show that carbonate budgets in 2015 average +3.7 G (G = kg CaCO3 m−2 yr−1). Most significantly the production rates on Acropora-dominated reefs, the corals most severely impacted in 1998, averaged +8.4 G by 2015, comparable with estimates under pre-human (Holocene) disturbance conditions. These positive budgets are reflected in high reef growth rates (4.2 mm yr−1) on Acropora-dominated reefs, demonstrating that carbonate budgets on these remote reefs have recovered rapidly from major climate-driven disturbances. Critically, these reefs retain the capacity to grow at rates exceeding measured regional mid-late Holocene and 20th century sea-level rise, and close to IPCC sea-level rise projections through to 2100.

Gute Nachrichten auch aus der Karibik. In einer Pressemitteilung vom 20. Februar 2015 gab die Griffith University bekannt:

Corals may be better equipped to tolerate climate change than previously believed, according to research led by Griffith University’s Dr Emma Kennedy

Working with scientists from the University of Exeter in the UK, Dr Kennedy says the findings — published in the journal Coral Reefs — relate to an extensive study of Caribbean corals, but could influence future analysis of Australia’s Great Barrier Reef. Using a high-resolution molecular screening technique called Real Time-PCR, the researchers confirmed that the partnership between Symbiodinium D — a symbiotic algae associated with resistance to coral bleaching — and Caribbean corals is more common than had been supposed.

„Corals rely on a relationship with algae in order to get energy via photosynthesis,“ says Dr Kennedy, a Postdoctoral Research Fellow in the School of Environment’s Australian Rivers Institute. „However, under stressful conditions such as increased temperatures, this relationship can be disrupted, resulting in a loss of the algae in an event known as bleaching. In an extreme event, this can lead to coral death. „Our study focused on populations of the Mountain Star coral, Orbicella annularis, a widespread and prominent reef species in the Caribbean. „Understanding its ability to weather the pressures of a changing climate, in particular rising sea temperatures, is a key question for conservationists.“

Symbiodinium D was found to be present in low abundances at almost every location the researchers tested, from Tobago to the Bahamas. As well as being geographically widespread, it was also more common in individuals, found on average in more than 30 per cent of the corals in each location. Dr Kennedy says previous studies have shown that if Orbicella annularis contains just a small amount of Symbiodinium D it can sometimes respond better to stress events — such as heatwaves — and is more likely to avoid coral bleaching. A 2007 research paper (Mieog et al. 2007, Coral Reefs) reported the presence of Symbiodinium D in 71 per cent of coral colonies tested on the Great Barrier Reef. Having completed her PhD at the University of Exeter, Dr Kennedy’s latest research involves assessing the responses of coralline algae to ocean acidification and warming. It aims to determine whether coralline algae can be used to track the impacts of climate change in the Great Barrier Reef.

Paper: Emma V. Kennedy, Nicola L. Foster, Peter J. Mumby, Jamie R. Stevens. Widespread prevalence of cryptic Symbiodinium D in the key Caribbean reef builder, Orbicella annularis. Coral Reefs, 2015; DOI: 10.1007/s00338-015-1264-4

Weitere gute Nachrichten kamen am 24. Mai 2016 von der University of Sydney. Korallenschäden in den heißen Oberflächenwässern könnten durch Korallen aus kühleren tiefergelegenen Wasserschichten ausgeglichen werden:

Coral bleaching ‚lifeboat‘ could be just beneath the surface
A ‚lifeboat‘ for coral reefs could lie in deeper mesophotic coral ecosystems. A United Nations report edited by the University of Sydney’s UNESCO Chair in Marine Science offers a glimmer of hope to those managing the impact of bleaching on the world’s coral reefs, including the Great Barrier Reef.

Shallow coral reefs up to 40 metres deep are the tip of the iceberg that comprises the ocean’s extensive coral ecosystem. Now, a United Nations report co-authored by the University of Sydney’s UNESCO Chair in Marine Science provides a glimmer of hope for those managing the impact of bleaching on the world’s coral reefs, including the Great Barrier Reef. Coral bleaching has affected virtually the entire Great Barrier Reef and many other coral reef systems globally, a result of the continuing rise in global temperatures and exacerbated by the summer’s major El Niño event. The 35 authors of the United Nations Environmental Programme report launched today – including the University’s Professor Elaine Baker in the School of Geosciences – say the deeper, mesophotic coral ecosystems (MCEs) may act as a lifeboat for shallow coral reefs.

MCEs are intermediate depth reefs starting at about 40 metres depth and continuing to around 150 metres.  The report – Mesophotic Coral Ecosystems A lifeboat for coral reefs? – looks at the role MCEs could play in the preservation of shallower reefs. The report asks if MCEs can provide a refuge for the species under threat in shallower reef ecosystems and whether they can provide the stock to re-populate shallow reefs if they continue to decline. “More research needs to be done to firmly establish the role of MCEs in preserving our reefs,” said Professor Baker. “They aren’t a silver bullet but they may be able to resist the most immediate impacts of climate change and help replenish destroyed surface reef and fish populations. “It may be that the cooler, deeper water in MCEs could be more hospitable to many species than the warmer surface water,” she said. “They also are less prone to waves and turbulence, therefore potentially offering a more stable environment in which to replenish coral.

The review brought together information on the geology, biology, distribution and socio-economic aspects of mesophotic reefs in order to examine their potential resilience. It found some deep mesophotic coral ecosystems may be immune from the most extreme ocean warming, but other ecosystems are just as vulnerable as their shallow counterparts and cannot be relied on to act as life boats.

Weitere interessante Ergebnisse: Bay et al. 2014 beschrieben in Current Biology Korallenarten, die mit Hitzewellen besser zurecht akommen als andere. Eine andere Studie von Toth et al. (2015) in Nature Climate Change rekonstruierte das Korallenwachstum in Panama während der vergangenen 7000 Jahre. Unerwarteterweise wurden die Wachstumsunterbrechungen im Korallenriff durch Kälte- und  nicht Hitzephasen ausgelöst:

Climatic and biotic thresholds of coral-reef shutdown
Climate change is now the leading cause of coral-reef degradation and is altering the adaptive landscape of coral populations1, 2. Increasing sea temperatures and declining carbonate saturation states are inhibiting short-term rates of coral calcification, carbonate precipitation and submarine cementation3, 4, 5. A critical challenge to coral-reef conservation is understanding the mechanisms by which environmental perturbations scale up to influence long-term rates of reef-framework construction and ecosystem function6, 7. Here we reconstruct climatic and oceanographic variability using corals sampled from a 6,750-year core from Pacific Panamá. Simultaneous reconstructions of coral palaeophysiology and reef accretion allowed us to identify the climatic and biotic thresholds associated with a 2,500-year hiatus in vertical accretion beginning ~4,100 years ago8. Stronger upwelling, cooler sea temperatures and greater precipitation—indicators of La Niña-like conditions—were closely associated with abrupt reef shutdown. The physiological condition of the corals deteriorated at the onset of the hiatus, corroborating theoretical predictions that the tipping points of radical ecosystem transitions should be manifested sublethally in the biotic constituents9. Future climate change could cause similar threshold behaviours, leading to another shutdown in reef development in the tropical eastern Pacific.