Calcification response of reef corals to seasonal upwelling in the northern Arabian Sea (Masirah Island, Oman)

[摘要] Tropical shallow-water reefs are the most diverse ecosystems in the ocean. Their persistence rests upon adequate calcification rates of the reefbuilding biota, such as reef corals. Coral calcification is favoured in oligotrophic environments with high seawater saturation states of aragonite( Ω sw ), which leads to an increased vulnerability to anthropogenic ocean acidification and eutrophication. Here we present Porites calcification records from the northern Arabian Sea upwelling zone and investigate the coral calcification response to low Ω sw and high nutrient concentrations due to seasonal upwelling. The calcification rate was determined from the product of skeletalextension rate and bulk density. Skeletal Ba / Ca and Li / Mg proxy data were used to identify skeletal portions that calcified during upwelling andnon-upwelling seasons, respectively, and to reconstruct growth temperatures. With regard to sub-annual calcification patterns, our resultsdemonstrate compromised calcification rates during the upwelling season. This is due to declining extension rates, which we attribute to lightdimming caused by high primary production. Interestingly, seasonal variations in skeletal density show no relationship with temporally low Ω sw during upwelling. This suggests relatively constant, year-round saturation states of aragonite at the site of calcification( Ω cf ) independent of external variability in Ω sw . Although upwelling does not affect seasonal density variability,exceptionally low mean annual density implies permanent Ω cf adjustment to the lowest sub-annual Ω sw (e.g.upwelling). In the Arabian Sea upwelling zone, the mean annual calcification rate is similar to Porites from non-upwelling regions because lowskeletal density is compensated by high extension growth. Variable responses of reef coral extension to nutrients, which either exacerbate orcompensate negative effects of diminished skeletal density associated with ocean acidification, may therefore be critical to the maintenance ofadequate carbonate accumulation rates in coral reefs under global change.

[学科分类] 大气科学