Imbalanced coral growth between organic tissue and carbonate skeleton caused by nutrient enrichment

Abstract

Effects of moderate nutrient enrichment (NO 3 − : <5 µmol L −1 , PO 4 3‐ : <0.3 µmol L −1 ) on two carbon (C) fixation rates (photosynthesis and calcification) of the zooxanthellate coral Acropora pulchra were investigated under laboratory conditions. The coral branches were incubated in the nutrient condition for three different periods (0, 5, 10 d) to observe changes in tissue biomass and zooxanthellate chlorophyll a (Chl a ) concentration. Next, the incubated corals were simultaneously transferred to nutrient‐depleted seawater containing 13 C‐labeled dissolved inorganic carbon to assay net photosynthesis and calcification rates. Chl a concentration per unit surface area increased 2.6‐fold for the 10‐d enrichment, and net photosynthetic rates were also stimulated up to a similar level (2.8‐fold). Tissue biomass of the host coral and zooxanthellae was approximately doubled during the period. On the other hand, calcification rates only increased 1.3‐fold, suggesting that even moderate nutrient loading resulted in one‐sided enhancement of the algal photosynthetic activity. The measured C fixation ratios of organic C: skeletal C were higher than the structural ratios, and the inconsistency became greater as Chl a concentration increased. The increased photosynthetic products could be excessively stored in the organic tissue and/or released into the ambient seawater.