Bleaching threatens positive carbonate budgets on Bahraini reefs

Abstract

Bleaching events impact coral reef functionality and carbonate budget dynamics, which is reflected in reduced reef framework accretion, hindering reef ability to keep pace with sea level rise projections. Reefs in the Arabian Gulf exist in harsh environmental conditions with seasonal temperatures ranging between 16–36◦C. Despite the high thermal thresholds (~ 35◦C) of corals in this region, extensive bleaching and high coral mortality has been reported regionally in 2017. We quantify reef carbonate budgets at a near shore and an offshore reef site in Bahrain in May 2017 and 2018 to assess the impacts of the 2017 severe bleaching event on the budgetary state. Results indicate an overall decrease in hard coral cover from 14.2 ± 5.5% to 8.5 ± 1.4% and a decline in the net carbonate budget state from 3.6 ± 2.2 to 0.3 ± 0.3 kg CaCO3 m− 2 y− 1 at the shallow nearshore Fasht Al Adhm reef indicating a shift from a positive budgetary state to nearly a collapse, whilst the deeper offshore Reef Bul Thamah has increased positively from 3.7 ± 1.2 to 4.2 ± 0.6 kg CaCO3 m− 2 y− 1. We attribute the decline in the nearshore reef to the bleaching event which took place between June – September 2017, resulting in high coral mortality rates and subsequent reduced framework carbonate production. Predicted warming trends present a threat to the structural integrity of shallow Bahraini reefs, compromising their ability to keep pace with future SLR projections.