Quantifying the transfer of prey δ15N signatures into coral holobiont nitrogen pools

Nitrogen stable isotope (δ15N) signatures of coral and skeletal tissues are commonly used to identify spatial and temporal patterns in the source and supply of nitrogen to coral reefs. A 15N labelled particulate food source (rotifers) was used to quantify the incorporation of prey nitrogen into the nitrogen pools (coral, algal tissues and skeletal organic matter) of Porites lutea, and to estimate the time taken for the δ15N signature of source nitrogen to be reflected in the different tissue fractions of the coral holobiont. Neither coral nor algal fractions displayed the full expression of the food source δ15N over a 60 d experimental period. The response of the skeletal δ15N value to the food δ15N was slower than the coral tissue, but this may have been caused by coarse sampling resolution coupled with a short experimental period. Using a mass-balance model, we determined that the corals must have been augmenting their rotifer diets by up to 50% with dissolved nitrogen from the water column. Using the δ15N of the combined food source (i.e. dissolved and rotifer nitrogen), we calculated tissue turnover rates of 87 d for the coral tissue and 111 d for the algal symbionts. These values dictate that the duration of any change in the δ15N of a coral’s N source needs to be greater than 3 mo to register its full magnitude in the tissue and skeletal nitrogen pools. This has implications for studies in which the host, symbiont and skeletal δ15N are used as a proxy for temporal changes in the source of nitrogen to coral reefs. Our results also support the notion of a bidirectional exchange of N between the coral and algae fractions, and provide estimations of the assimilation and excretion of N during heterotrophic feeding.