Calcification and organic productivity at the world's southernmost coral reef

Estimates of coral reef calcification and organic productivity provide valuable insight to community functionality and the response of an ecosystem to stress events. High-latitude coral reefs are expected to experience rapid changes in calcification rates and become refugia for tropical species following climate change and increasing bleaching events. Here, we estimate ecosystem-scale calcification and organic productivity at the world's southernmost coral reef using seawater carbon chemistry observations (Lord Howe Island, Australia). We reduce uncertainties in metabolic calculations by producing a detailed bathymetric model and deploying two current meters to refine residence time and volume estimates. Bathymetry-modelled transect depths ranged from 74% shallower to 20% deeper than depths averaged from reef crest/flat current meters, indicating that higher-resolution depth observations help to reduce uncertainties in reef metabolic calculations. Rates of ecosystem calcification were 56.6 ± 14.8 mmol m−2 d−1 in the winter and 125.3 ± 39.4 mmol m−2 d−1 in the summer. These rates are lower than most other high-latitude reefs according to our compilation of high-latitude coral ecosystem metabolism estimates. Coral cover ranged from 14.7 ± 2.3% in winter to 19.8 ± 2.1% in the summer. A concurrent bleaching event and cyclone occurred during summer sampling (February – March 2019), resulting in 47% of corals bleached at the study site and 2% mortality due to cyclonal damage. Therefore, it is likely that the summertime Gnet rates underestimate baseline calcification. Our results enable future assessments of long-term change, but do not resolve the impact of bleaching at Lord Howe Island. •High-latitude coral reefs are hotspots of ocean change and vulnerable to bleaching.•Coral ecosystem calcification in winter was lower than most studied ecosystems.•The reef was net heterotrophic in the winter and net respiratory in the summer.•Detailed bathymetric observations reduce uncertainties in metabolic calculations.•Summer calcification was not driven by temperature or aragonite saturation state.