Coral reef carbonate budgets and ecological drivers in the central Red Sea: a naturally high temperature and high total alkalinity environment.

The structural framework provided by corals is crucial for reef ecosystem function and services, but high seawater temperatures can be detrimental to the calcification capacity of reef-building organisms. The Red Sea is very warm, but total alkalinity (TA) is naturally high and beneficial for reef accretion. To date, we know little about how such detrimental and beneficial abiotic factors affect each other and the balance between calcification and erosion on Red Sea coral reefs, i.e., overall reef growth, in this unique ocean basin. To provide estimates of present-day reef growth dynamics in the central Red Sea, we measured two metrics of reef growth, i.e., in situ net-accretion/-erosion rates (Gnet) determined by deployment of limestone blocks and ecosystem-scale carbonate budgets (Gbudget), along a crossshelf gradient (25 km, encompassing nearshore, midshore, and offshore reefs). Along this gradient, we assessed multiple abiotic (i.e., temperature, salinity, diurnal pH fluctuation, inorganic nutrients, and TA) and biotic (i.e., calcifier and epilithic bioeroder communities) variables. Both reef growth metrics revealed similar patterns from nearshore to offshore: net-erosive, neutral, and net-accretion states. The average cross-shelf Gbudget was 0.66 kg CaCO3 m−2 yr−1 , with the highest budget of 2.44 kg CaCO3 m−2 yr−1 measured in the offshore reef. These data are comparable to the contemporary Gbudgets from the western Atlantic and Indian oceans, but lie well below “optimal reef production” (5–10 kg CaCO3 m−2 yr−1 ) and below maxima recently recorded in remote high coral cover reef sites. However, the erosive forces observed in the Red Sea nearshore reef contributed less than observed elsewhere. A higher TA accompanied reef growth across the shelf gradient, whereas stronger diurnal pH fluctuations were associated with negative carbonate budgets. Noteworthy for this oligotrophic region was the positive effect of phosphate, which is a central micronutrient for reef building corals. While parrotfish contributed substantially to bioerosion, our dataset also highlights coralline algae as important local reef builders. Altogether, our study establishes a baseline for reef growth in the central Red Sea that should be useful in assessing trajectories of reef growth capacity under current and future ocean scenarios

The structural framework provided by corals is
crucial for reef ecosystem function and services, but high
seawater temperatures can be detrimental to the calcification
capacity of reef-building organisms. The Red Sea is very
warm, but total alkalinity (TA) is naturally high and beneficial for reef accretion. To date, we know little about how
such detrimental and beneficial abiotic factors affect each
other and the balance between calcification and erosion on
Red Sea coral reefs, i.e., overall reef growth, in this unique
ocean basin. To provide estimates of present-day reef growth
dynamics in the central Red Sea, we measured two metrics of reef growth, i.e., in situ net-accretion/-erosion rates
(Gnet) determined by deployment of limestone blocks and
ecosystem-scale carbonate budgets (Gbudget), along a crossshelf gradient (25 km, encompassing nearshore, midshore,
and offshore reefs). Along this gradient, we assessed multiple abiotic (i.e., temperature, salinity, diurnal pH fluctuation, inorganic nutrients, and TA) and biotic (i.e., calcifier
and epilithic bioeroder communities) variables. Both reef
growth metrics revealed similar patterns from nearshore to
offshore: net-erosive, neutral, and net-accretion states. The
average cross-shelf Gbudget was 0.66 kg CaCO3 m−2 yr−1
,
with the highest budget of 2.44 kg CaCO3 m−2 yr−1 measured in the offshore reef. These data are comparable to
the contemporary Gbudgets from the western Atlantic and
Indian oceans, but lie well below “optimal reef production” (5–10 kg CaCO3 m−2 yr−1
) and below maxima recently recorded in remote high coral cover reef sites. However, the erosive forces observed in the Red Sea nearshore
reef contributed less than observed elsewhere. A higher TA
accompanied reef growth across the shelf gradient, whereas
stronger diurnal pH fluctuations were associated with negative carbonate budgets. Noteworthy for this oligotrophic region was the positive effect of phosphate, which is a central micronutrient for reef building corals. While parrotfish
contributed substantially to bioerosion, our dataset also highlights coralline algae as important local reef builders. Altogether, our study establishes a baseline for reef growth in the
central Red Sea that should be useful in assessing trajectories of reef growth capacity under current and future ocean
scenarios

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