How did reef calcification change after bleaching at Lizard Island?

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Severe bleaching events have caused massive losses of hard coral cover on the Great Barrier Reef. Researchers used flow respirometry approaches to assess reef net ecosystem calcification (NEC) and net ecosystem production (NEP) after the 2015/2016 bleaching event at heavily impacted Lizard Island in the northern Great Barrier Reef. They used studies conducted in 2008 and 2009 by Silverman and colleagues (2014; http://10.1016/j.gca.2014.09.011) as preimpact data.

Lagrangian and Eulerian approaches produced varied results. Estimated 2016 rates of NEC (29.1 to 137.7 mmol m−2 day−1) and NEP (−876.7 to 50.5 mmol m−2 day−1) were highly sensitive to assumptions about reef water residence times and oceanic end-member concentrations. When the researchers replicated the 2008 and 2009 methodology, estimated NEC after bleaching in 2016 was 32 ± 10.8 mmol m−2 day−1, 40%–46% lower than prebleaching estimates of 61 ± 12 in 2008 and 54 ± 13 mmol m−2 day−1 in 2009.

The slope of the total alkalinity versus dissolved inorganic carbon plot fell from approximately 0.3 in 2008 and 2009 to 0.1 in 2016, indicating elevated organic production and a shift in community function. Changes in NEC relative to the previous study were not driven by changes in Ωarag. Coral cover was 8.3% in 2008 and 7.1% in 2009, compared with 3.0% in 2016.

The researchers concluded that reef NEC clearly decreased following bleaching and that subtle differences in assumptions or methods can bias interpretation. They cautioned that comparisons of reef metabolism datasets and predictions of long-term reef calcification from existing short-term datasets need care.