Seaweed may be contributing to the decline of coral in the ocean, according to a paper published in PNAS this week. A new field study that examined the effects of rampant seaweed growth near coral reefs found that between 40 and 70 percent of common seaweeds can contribute to the bleaching and death of coral. The damage often originates from chemicals that seaweed can emit, but researchers found that coral damage could be curtailed when there were herbivores like fish around to eat the seaweed and temper its influence.
Coral reefs are known for their diverse and often delicate ecosystems, and marine biologists have conducted many studies to find out what factors can cause reefs to break down. Studies that looked at the negative effects of seaweed's metabolic products have produced mixed results in the past: one study done in a lab showed that sugars and carbohydrates released by seaweed can mediate coral mortality, while another field study suggested that seaweed didn't affect coral health one way or another.
A new field study in the Caribbean and tropical Pacific placed certain varieties of common seaweed next to coral to observe any effects. They found that there were some varieties of seaweed that did negatively affect the coral, but didn't do it through shading it and hampering its photosynthetic efficiency or through physical abrasion.
Instead, the damage came from the production of lipid-soluble metabolites that originated both within and on the surface of the seaweed. The metabolites caused the coral to lose some of the symbiotic protozoan inhabitants that give the coral its color, a process called "bleaching" that often leads to death.
Fish and other herbivores can easily keep the situation in check by eating seaweed—one area in Fiji sees the weeds kept at bay by a single species of fish—meaning that this negative impact on coral can be yet another consequence of overfishing. To help preserve the coral reefs, the authors note that more information is needed on which species of seaweed are the most dangerous, and which herbivore species can contain them.