Carbon turnover rates in the One Tree Island reef: A 40-year perspective

During November-December 2009 community rates of gross photosynthesis (P_g), respiration (R) and net calcification (G_net) were estimated from low-tide slack water measurements of dissolved oxygen, dissolved inorganic carbon and total alkalinity at the historical station DK13 One Tree Island reef, Great Barrier Reef, Australia. Compared to measurements made during the 1960s-1970s at DK13 in the same season, P_g increased from 833 to 914 mmol O₂m^-2d^-1 and P_g:R increased from 1.14 to 1.30, indicating that the reef has become more autotrophic. In contrast, G_net decreased from 133 mmol Cm ^-2d^-1 to 74 24 mmol Cm^-2d^-1. This decrease stems primarily from the threefold increase in nighttime CaCO ₃ dissolution from-2.5 mmolm^-2h^-1 to-7.5 mmolm^-2h^-1. Comparison of the benthic community survey results from DK13 and its vicinity conducted during this study and in studies from the 1970s, 1980s and 1990s suggest that there have been no significant changes in the live coral coverage during the past 40 years. The reduced G _net most likely reflects the almost threefold increase in dissolution rates, possibly resulting from increased bioerosion due to changes in the biota (e.g., sea cucumbers, boring organisms) and/or from greater chemical dissolution produced by changing abiotic conditions over the past 40 years associated with climate change, such as increased temperatures and ocean acidification. However, at this stage of research on One Tree Island the effects of these changes are not entirely understood.