Abstract
Estimates of coral reef ecosystem calcification (Gnet) and productivity (Pnet) provide insight into coral community health and functionality in response to short- and long-term stressors such as ocean warming and acidification. Here, we investigate spatial variability in calcification and organic production at One Tree Island (OTI) and compare our new observations to sporadic metabolic rates reported over the previous 50 years on the same reef flat. Gnet and Pnet estimates at the nearshore site were 50% and 166% lower than an offshore site with a shift in organic production from net productive to net respiratory. Contrary to expectations, calcification rates in 2017 (145.7 ± 20.24 mmol m-2 d-1) were comparable to the 1970’s estimate (125.0 ± 12.5 mmol m-2 d-1) and 400% greater than similar observations in 2014. Our results indicate only weak associations between Gnet and aragonite (Ωar). A local increase in coral cover from 18% in 2014 to 31% in 2017 was the likely driver of increased calcification. A steeper TA – DIC slope in 2017 demonstrates a greater control of calcification on seawater carbonate chemistry than prior years. Overall, these results highlight the importance of site selection and replication when comparing metabolic datasets, and demonstrate major short-term variability in metabolic rates. The predictive capabilities of ecosystem metabolism studies may be constrained by using the available short-term datasets to represent long-term calcification trends.
Highlights
Coral reefs are threatened by climate change and other anthropogenic activities
The inclusion of a third site allows us to assess the potential influence on ecosystem metabolism of groundwater-derived inputs of carbon, alkalinity, or nutrients that can be important in coral reefs (Cyronak et al, 2014a; McMahon and Santos, 2017)
We report relatively high calcification at the One Tree Island (OTI) reef flat in 2017 compared to earlier observations
Summary
Coral reefs are threatened by climate change and other anthropogenic activities. Conditional stressors such as ocean warming, acidification, and eutrophication are predicted to increase in severity for the foreseeable future (Borges and Gypens, 2010; Andersson and Gledhill, 2013; Manzello et al, 2013; Hughes et al, 2018). Investigations of the major metabolic processes can provide valuable information on the long-term response of coral reef ecosystems to anthropogenic stressors (Albright et al, 2013; Cyronak et al, 2018). Field results support ocean acidification and coral bleaching predictions for ecosystem metabolism in a changing ocean (Kayanne et al, 2005; Kleypas et al, 2011; Albright et al, 2016, 2018) consistent with an observed increase in coral reef carbon dioxide and decrease in the aragonite saturation state (Cyronak et al, 2014a)
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