Reef Community Productivity and Calcification – Spatial and Temporal Variability in Recovering Coral Reefs

Abstract

Coral reefs are currently under threat due to multiple anthropogenic stressors, with increasing temperatures leading to more frequent bleaching events. We assessed reef Net Community Production (NCP) and Net Community Calcification (NCC), measures of reef ecosystem functioning, using a benthic gradient-flux approach on forereef and lagoon coral reefs recovering from the major 2015–2016 bleaching event in the Chagos Archipelago, Indian Ocean. Hard coral cover at the lagoon was higher (44% vs. 21% at the forereef, 3 years post-bleaching) and increased by ∼60% at both reefs 6 years post-bleaching. Calcification, computed using Structure-from-Motion photogrammetry, increased by 34%, and rugosity by ∼10% over this period. Biogeochemical measurements show net heterotrophy at both reefs 3 years within the recovery, with the lagoon reef exhibiting particularly high rates. Six years into the recovery process, productivity and calcification rates at the forereef more than doubled compared with the rates 3 years prior. Large day-to-day variability was documented. This included a transition in the lagoon reef from net calcification to dissolution within days, despite the long-term trend of net CaCO₃ accumulation. On the forereef, heterotrophy increased by 51% on cloudy days, while a shift to autotrophy (147% increase in NCP) and 47% higher calcification were found on sunny days that directly followed cloudy days. An internal wave event presumably led to enhanced production on one of the days on the forereef. Our findings highlight the importance of combining long-term reef health indicators with short-term measurements of reef functioning to capture the dynamics of reef recovery.