Spatial variability of δ15N and δ13C in symbiotic corals

Primary production on coral reefs varies under changing conditions such as light and nutrient availability. Coral fragments and co-occurring macroalgae were collected along a depth gradient and at same depths around Hideaway Island, Vanuatu. The primary aim was to examine if there was a nutrient input around the island that could be detected in coral and macroalgae tissues using δ15N signatures. The second aim was to examine the effect of depth over the δ15N and δ13C signatures of benthic primary producers to investigate their nutrient source and trophic dynamics. The δ15N signatures did not show any trace of a nutrient input around the island. However, the chlorophyll a and Symbiodinium density from the coral Montipora stellata were the only indicators of a potential nutrient input around the island. Similarly, no effect of depth was found in the δ15N of the macroalgae, epiphytes, and Symbiodinium from Stylophora pistillata. The only effect of depth was a decrease in Symbiodinium density and in the δ13C values of macroalgae. We attribute these findings to Symbiodinium from S. pistillata, compensating for the decrease of nutrients as depth increases by decreasing the cell density but maintaining the chlorophyll a concentrations to satisfy the coral-host nutrient requirements.