Patterns of reef coral calcification in an upwelling zone (Oman)

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Skeletal calcification of corals in tropical shallow-water coral reefs is increasingly threatened by globally rising sea surface temperatures (SST), lowering of ocean pH, and anthropogenic eutrophication. Except for few culture experiments, the contribution of a single stressor such as high SST, low pH or eutrophy to net calcification of reef corals is not known from the natural environment. The coral reefs of Oman are a natural laboratory to the study of environmental controls on skeletal calcification of future reefs because anthropogenic disturbances are negligible and pronounced spatial gradients of environmental stresses exists between the Arabian Sea upwelling-zone (low SST, low pH) and the Gulf of Oman (high SST, high pH).In an ongoing project funded by the DFG we studied patterns of skeletal calcification of corals (Porites) from Oman. According to our data, corals affected by summer upwelling of the Arabian Sea have diminished skeletal bulk density due to low seawater pH, but enhanced extension rate due to high nutrient availability. On the other hand, corals from the Gulf of Oman have high density because of high aragonite super-saturation and inconspicuous extension. They are protected from bleaching because they benefit during summer from cool-water filaments derived from the upwelling zone. The findings support hypotheses predicting regions of upwelling to represent refuges for coral reefs during future and past warm periods. Important for the success of the project were improvements of the sampling method and calibration of coral-internal age models that allow us for the first time to separate the effects of different (sub-) seasonal stresses on the overall calcification balance. However, delays of the project through unexpected logistical problems with permits by Omani authorities, a laboratory conversion at Mainz University and the general shutdown of laboratory facilities due to the Corona pandemic brought us one year behind schedule. New research questions that came up during the ongoing project and the delays led us to submit a renewal proposal. Within the additional work time, we will complete all planned analytics of the materials already available to us and follow three additional research aims: (1) dating of selected corals (230Th/U) for an even better documentation of high-frequency variations of upwelling, (2) reconstructing the carbonate chemistry of the calcification fluid (B/Ca-δ10B) in order to better understand internal regulation mechanisms vs. external seawater effects on calcification performance, and (3) extracting short cores from massive Porites at an environmental logging station (pH, SST) installed during the project for calibration purposes of proxy data and calcification records.