31 Aug 2021
31 Aug 2021
Status: a revised version of this preprint is currently under review for the journal BG.

Nutrients attenuate the negative effect of ocean acidification on reef coral calcification in the Arabian Sea upwelling zone (Masirah Island, Oman)

Philipp Michael Spreter1, Markus Reuter1, Regina Mertz-Kraus2, Oliver Taylor3, and Thomas Christian Brachert1 Philipp Michael Spreter et al.
  • 1Institut für Geophysik und Geologie, Universität Leipzig, Talstraße 35, 04103 Leipzig, Germany
  • 2Institut für Geowissenschaften, Johannes Gutenberg-Universität Mainz, Johann-Joachim-Becher-Weg 21, 55128 Mainz, Germany
  • 3Five Oceans Environmental Services, Villa 1756, Way 3021; Shatti Al Qurm, Muscat, Sultanate of Oman

Abstract. Tropical shallow-water reefs are the most diverse ecosystem in the ocean. Its persistence rests upon adequate calcification rates of the reef building biota, such as reef corals. Optimum calcification rates of reef corals occur in oligotrophic environments with high seawater saturation states of aragonite (Ωsw), which leads to increased vulnerability to anthropogenic ocean acidification and eutrophication. The calcification response of reef corals to this changing environment is largely unknown, however. Here, we present annually and sub-annually resolved records of calcification rates (n = 3) of the coral Porites from the nutrient rich and low Ωsw Arabian Sea upwelling zone (Masirah Island, Oman). Calcification rates were determined from the product of skeletal extension and bulk density derived from X-ray densitometry. Compared to a reference data set of coral skeletons from typical reef environments (Great Barrier Reef, Hawaii), mean annual skeletal bulk density of Porites from Masirah Island is reduced by 28 %. This density deficit prevails over the entire year and probably reflects a year-round low saturation state of aragonite at the site of calcification (Ωcf), independent of seasonal variations in Ωsw (e.g. upwelling). Mean annual extension rate is 20 % higher than for the reference data set. In particular, extension rate is strongly enhanced during the seasons with the lowest water temperatures, presumably due to a high PO43−/NO3-ratio promoting rapid upward growth of the skeleton. Enhanced annual extension attenuates the negative effect of low density on calcification rate from −25 % to −11 %, while sub-annual calcification rates during the cool seasons even exceed those of the reference corals. We anticipate optimal nutrient environments (e.g. high PO43−/NO3-ratios) to have significant potential to compensate the negative effect of ocean acidification on reef coral calcification, thereby allowing to maintain adequate rates of carbonate accumulation, which are essential for preserving this unique ecosystem.

Philipp Michael Spreter et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Comment on bg-2021-213', Dieter Garbe-Schönberg, 07 Sep 2021
    • CC2: 'Reply on CC1', Philipp Spreter, 08 Sep 2021
  • RC1: 'Comment on bg-2021-213', Anonymous Referee #1, 10 Sep 2021
    • AC1: 'Reply on RC1', Philipp Spreter, 16 Sep 2021
    • AC3: 'Reply on RC1', Philipp Spreter, 25 Nov 2021
  • RC2: 'Comment on bg-2021-213', Anonymous Referee #2, 27 Oct 2021
    • AC2: 'Reply on RC2', Philipp Spreter, 25 Nov 2021

Philipp Michael Spreter et al.

Philipp Michael Spreter et al.


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Short summary
Progressive ocean acidification and eutrophication pose a threat to skeletal calcification of reef corals. Recent upwelling areas serve as natural laboratories to study their calcification response to low seawater pH and eutrophic conditions. We compare patterns of reef coral calcification from the Arabian Sea upwelling zone with those from typical reef environments and identify nutrient conditions that contribute to the maintenance of high calcification rates in times of global change.