Spatial and temporal effects of anthropogenic carbon on bioerosion
Zusammenfassung der Projektergebnisse
Coral reefs are under threat exerted by a number of interacting effects inherent to the present climate change, including rapid ocean acidification and global warming. Bioerosion drives reef degradation by recycling carbonate skeletal material and is an important but understudied factor in this context. Unlike skeletal growth, we expect bioerosion by chemical means to be facilitated in a high-CO2 world. This study focuses on two of the most detrimental bioeroders, the zooxanthellate sponge Cliona orientalis, which attacks and kills live corals on Australia’s Great Barrier Reef, and its azooxanthellate relative Cliona celata, a dominant and widely distributed species in higher latitudes. Complementing experiments were carried out at Orpheus Island Research Station (GBR) and the Helgoland Alfred Wegener Institute (North Sea). Exposure to lowered and elevated levels of pCO2 and temperature resulted in a significant enforcement of the sponges’ chemical bioerosion capacity with increasing pCO2 and under more acidic conditions, confirming our initial hypothesis that sponge bioerosion will be accelerated by ocean acidification - across species and latitudes, affecting corals and molluscs alike. Temperature, in contrast, yielded inconclusive results and did not markedly influence bioerosion rates, which were slightly reduced at both colder as well as warmer temperatures than ambient. In addition, we extended a long-term field survey using quadrats at the tropical study site in Little Pioneer Bay in order to allow a temporal analysis and prognosis with respect to an observed increase in bioeroding sponge abundance over time (repeated field assessments since 1997). While the first study informed about existing densities and the biodiversity of bioeroding sponges, the second one was performed after two major bleaching events causing massive coral mortalities. These initial two studies found a diverse bioeroding sponge fauna of 30+ species, which were not harmed but partly increased in abundance after the bleaching events. Preliminary data analysis of the 2010-11 survey suggests that the negative trend of increasing abundances of bioeroding sponges on this stressed reef continues. Sponge abundances did not return to original, low levels, and 10+ new species were found. Further observations on bioeroding sponges were made with respect to the regeneration ability as well as stress reactions of sponge tissue exposed to light after breakup of colonies. The general spectrum of bioeroders and their traces at the fringing reef in Little Pioneer Bay was studied by means of four in situ settlement platforms placed for one year in the intertidal lagoon, near the reef crest, the shallow fore reef, and the deeper reef slope. In total, 27 ichnotaxa and -forms attributed to cyanobacteria, chlorophytes, fungi, other microheterotrophs, macroborers, grazers, and attachment scars of epiliths were recorded, with the highest ichnodiversity at the reef slope in 5 and 15 m water depth, whereas bioerosion rates were found highest near the reef crest. Considering that bioeroding sponges often contribute the lion share of internal carbonate bioerosion on coral reefs, our findings imply that tropical reef ecosystems are facing the combined effects of weakened coral calcification and accelerated bioerosion, resulting in critical pressure on the dynamic balance between biogenic carbonate build-up and degradation.
Projektbezogene Publikationen (Auswahl)
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(2012) Ocean acidification accelerates reef bioerosion. PLoS ONE 7(9): e45124
Wisshak M, Schönberg CHL, Form A & Freiwald A
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(2012) The perks of being endolithic. Aquatic Biology 17:1-5
Schönberg CHL & Wisshak M
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(2013) Effects of ocean acidification and global warming on reef bioerosion - lessons from a clionaid sponge. Aquatic Biology 19: 111- 127
Wisshak M, Schönberg CHL, Form A, Freiwald A
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(2014) Sponge bioerosion accelerated by ocean acidification across species and latitudes? Helgoland Marine Research, online
Wisshak M, Schönberg CHL, Form A, Freiwald A
