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.
Highlights
The calcareous skeletons of zooxanthellate scleractinian corals are the basic building blocks of tropical shallow30 water reefs, the most diverse ecosystem in the ocean (Hughes et al, 2017)
Compared to corals from typical reef sites unaffected by upwelling used as a reference dataset (Hawaii and Great Barrier Reef), mean annual skeletal density is reduced by 28 %, which is in good agreement with values reported from corals of eastern Pacific upwelling zones
The intra-annual variability of skeletal density is negatively related to sea surface temperature (SST), but skeletal density remains consistently too low throughout the entire year relative to the reference data set, likely because of a 440 comparatively low aragonite saturation state at the site of calcification. cf is assumed to be kept low by the coral, but relatively constant and independent of changes in external sw between the upwelling and non-upwelling seasons
Summary
The calcareous skeletons of zooxanthellate scleractinian corals (reef corals) are the basic building blocks of tropical shallow water reefs, the most diverse ecosystem in the ocean (Hughes et al, 2017). Reef corals are highly adapted to oligotrophic waters because symbiosis with phototrophic zooxanthellae allows an efficient use of essential nutrients and to outcompete other fast-growing biota on a reef whose growth is inhibited by the undersupply of nutrients (Vermeij et al, 2010; Barrot and Rohwer, 2012). Strong eutrophication disturbs this adaptive advantage, leading to harmful algal blooms followed by reef coral mass mortality (Al Shehhi et al, 2014) and reef destruction due to the increasing abundance of bioeroders (Hallock, 1988). Increasing eutrophy is considered to explain enhanced coral calcification rates along an offshore – inshore gradient where the effect of temperature is negligible (D’Olivo et al, 2013; Risk and Sammarco, 1991; 65 Manzello et al, 2015)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
