MEPS 570:101-112 (2017)  -  DOI: https://doi.org/10.3354/meps12079

ABSTRACT: To study the effects of nutrient availability on scleractinian corals, fragments of Montipora digitata and Porites cylindrica were cultured in nutrient-enriched seawater (nitrate: 1.4 to 1.9 µmol l^-1 and phosphate: 0.1 µmol l^-1) for 2 mo under laboratory conditions. For both coral species, the chlorophyll a concentration of the endosymbiotic algae increased significantly in both nitrogen only (+N) and nitrogen plus phosphorus (+NP) treatments compared to the control. Endosymbiont carbon (C) content of M. digitata increased only under +NP conditions, indicating that phosphorus (P) was limiting the production of endosymbiont cells. Host C and nitrogen (N) contents were not affected by nutrient enrichment for both coral species, suggesting that the moderate nutrient enrichment did not contribute considerably to the production of host tissue. C stable isotope ratios (δ¹³C) of the endosymbionts and host gradually decreased during the experiment, and even more so in the +N treatment. This suggests that the coral host preferentially catabolized δ¹³C-enriched organic matter, such as storage lipids, and that this catabolism was enhanced when the N:P ratio of available nutrients was not balanced. Finally, the skeletal δ¹³C was also positively correlated with the host and endosymbiont δ¹³C values, which implies that the δ¹³C of host tissue was at least partially affecting the skeletal δ¹³C. Overall, moderate nutrient enrichment should not have a negative effect on coral metabolism provided that the N:P ratio of available nutrients is balanced.

KEY WORDS: Corals · Endosymbionts · Nutrient enrichment · N:P ratio · Photosynthesis · Translocation · Calcification · Stable isotope ratios