A strong temperature effect on coral P/Ca nutrient proxy based on Porites aquaria culture experiments

Abstract:

Coral skeletal P/Ca is a useful indicator of seawater dissolved inorganic phosphorus (DIP), applied to infer oceanographic and climatic controls like nutrient upwelling and terrigenous runoff on marine biogeochemical cycling. We conducted temperature-controlled aquaria culture experiments using two coral colonies of Porites australiensis to evaluate the effect of seawater temperature variation (21–29 °C) on skeletal P speciation and P/Ca ratio. We identified the skeleton growth after a near half-year acclimation period and measured the skeletal P/Ca and P speciation using several spectroscopic approaches. Coral growth rate ranges from 0.6 to 3.1 g/day, displaying an overall positive correlation with temperature. Inter-colony growth difference is caused by differences in zooxanthellae density of corals. Skeletal P/Ca varies between colonies of a same genotype and is affected by temperature. Skeletal P/Ca ranges from 3.9 to 27.5 μmol/mol, negatively correlated to temperature, and substantially affect growth rate and skeletal P speciation, which are biologically mediated. With incremental temperature, skeletal P/Ca decreases, along with the increase in growth kinetics and organic-P fraction. Combined laboratory data with field observations, we infer that (i) skeletal P/Ca ratio of Porites corals is a promising proxy of seawater DIP at global scales; (ii) there are no uniform rules of skeletal P/Ca-DIP calibration for coral inter-genus comparison; (iii) additional cautions are needed for skeletal P/Ca-DIP proxy in field corals (or fossils) affected by temperature fluctuation, nutrient supply, and speciation-specificity. Analyses of coral records developed at low seawater temperatures may overestimate past seawater DIP and ocean upwelling/runoff fluxes. When compiling multi-taxa coral samples, the interpretation is further complicated by physiological differences between genotypes. The outcome of our study focusing on coral nutrient proxy offers better understandings of biomineralization processes, and is of great interest for palaeoceanographic applications.