Coral bleaching induced by nutrient enrichment

16 October 2012

Mass coral bleaching, resulting from the breakdown of coral–algal symbiosis has been identified as the most severe threat to coral reef survival on a global scale. Regionally, nutrient enrichment of reef waters often caused by fertilizers running off farmland is often associated with a significant loss of coral cover and diversity.

Corals are symbiotic organisms – a cooperation for mutual benefit between two very different species. In this case, the partners are the coral animal, which builds the stony skeletons, and a population of specialized algae known as zooxanthellae. These tiny plants live inside the coral tissue that covers the skeleton, getting protection and certain vital nutrients in exchange for providing the coral animals with large amounts of the carbon compounds they produce through photosynthesis. Bleaching represents a breakdown in this symbiosis, in which the beneficial algae die off or are lost, leaving the coral animals vulnerable or even unable to survive.

Coral brown colors fading to white indicate bleaching events. It's generally agreed to be caused primarily by higher seawater temperatures, which put the corals under stress by disrupting their algae's ability to photosynthesize. In mild cases the corals can recover; in severe ones, whole reefs can bleach and die.

Recently, increased dissolved inorganic nitrogen concentrations have been linked to a reduction of the temperature threshold of coral bleaching, a phenomenon for which no mechanistic explanation is available. Scientists already knew that nutrient pollution could make the situation worse, but this is the first time anyone's identified a mechanism by which the nutrients contribute to bleaching.

The study recently published in Nature Climate Change indeed shows and models how increased levels of dissolved inorganic nitrogen in combination with limited phosphate concentrations result in an increased susceptibility of corals to temperature- and light-induced bleaching.

This understanding is urgently required to support knowledge-based management strategies to mitigate the effects of climate change on coral reefs.