Does nutrient enrichment contribute to coral bleaching? A Fijian case study

Abstract

This study investigated the effects of nutrient enrichment and thermal stress on the cellular and photosynthetic response of a dominant, reef-building coral in Fiji. Porites cylindrica corals were collected from Votua, Fiji, placed in individual mesocosms, and enriched with one of three concentrations of labelled nitrate (15NO3-) for two weeks. Then, colonies were gradually heated above the identified bleaching threshold for Fijian corals for a final seven days. Stable isotope values confirmed that enrichment had a significant impact on symbiotic algal populations (Symbiodinium spp.) within P. cylindrica colonies, and demonstrated rapid uptake and cycling of nutrients between algae and coral host. Regression analysis showed a strong, positive relationship between algae populations and nitrate concentration exposure. While no visual bleaching events were observed in this experiment, there was a tendency for corals in highly enriched seawater to lose proportionately more symbiotic algae than control samples after exposure to heat stress, suggesting greater potential for bleaching events in nutrient-enriched waters.