Functional roles of coral reef primary producers examined with stable isotopes

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.