Coral reef carbonate budgets and ecological drivers in the naturally high temperature and high alkalinity environment of the Red Sea

Abstract

AbstractThe coral structural framework is crucial for maintaining reef ecosystem function and services. In the central Red Sea, a naturally high alkalinity is beneficial to reef growth, but rising water temperatures impair the calcification capacity of reef-building organisms. However, it is currently unknown how beneficial and detrimental factors affect the balance between calcification and erosion, and thereby the overall growth of the reef framework. To provide insight into present-day carbonate budgets and reef growth dynamics in the central Red Sea, we measuredin situnet-accretion and net-erosion rates (Gnet) by deployment of limestone blocks and estimated census-based carbonate budgets (Gbudget) in four reef sites along a cross-shelf gradient (25 km). We assessed abiotic variables (i.e., temperature, inorganic nutrients, and carbonate system variables) and biotic drivers (i.e., calcifier and bioeroder abundances). On average, total alkalinity AT(2346 - 2431 μmol kg−1), aragonite saturation state (4.5 - 5.2 Ωa), and pCO2(283 -315 μatm) were close to estimates of pre-industrial global ocean surface waters. Despite these calcification-favorable carbonate system conditions, Gnetand Gbudgetencompassed positive (offshore) and negative net-production (midshore-lagoon and exposed nearshore site) estimates. Notably, Gbudgetmaxima were lower compared to reef growth from pristine Indian Ocean sites. Yet, a comparison with historical data from the northern Red Sea suggests that overall reef growth in the Red Sea has likely remained similar since 1995. When assessing sites across the shelf gradient, ATcorrelated well with reef growth rates (ρ = 0.89), while temperature was a weaker, negative correlate (ρ = −0.71). Further, ATexplained about 65 % of Gbudgetin a best fitting distance-based linear model. Interestingly, parrotfish abundances added up to 82% of explained variation, further substantiating recent studies highlighting the importance of parrotfish to reef ecosystem function. Our study provides a baseline that will be particularly useful in assessing future trajectories of reef growth capacities in the Red Sea under continuous ocean warming and acidification.