LAUSR

Context Primary production on coral reefs varies under changing conditions such as light and nutrient availability. This variation causes changes in basal stable isotopes as photosynthetic and nutrient pathways change. Aims This study provides a preliminary baseline of nitrogen (δ15N) and carbon (δ13C) stable isotope profiles in Symbiodinium and macroalgae at a spatial scale and along a depth gradient around an island. Methods Coral fragments and macroalgae were collected at depths from the surface to 26 m. δ15N and δ13C stable isotope values were assessed for Symbiodinium relative to cell density per surface area. Key results δ15N values showed a uniform nutrient profile across primary producers. However, chlorophyll-a and Symbiodinium density from Montipora stellata had higher concentrations on the southern side of the island. δ15N values of Symbiodinium from Stylophora pistillata and macroalgae did not change with depth. Depth was associated with a significant decrease in Symbiodinium density, and δ13C values in macroalgae. Conclusions We attribute these findings to Symbiodinium from S. pistillata as depth increases, decreasing cell density but maintaining chlorophyll-a concentration to satisfy the coral-host nutrient requirements. Implications This study sets the scene for future, more comprehensive research on detecting carbon and nitrogen stable isotope values on primary producers in coral reefs.