Algae invasion of Hawaii’s coral reefs

Invasive algae blooms are damaging Hawaii’s famous coral reefs, according to Celia Smith, a marine botanist at the University of Hawaii. Smith told EarthSky that algae are a natural part of reefs. But in the past couple of decades, non-native algae species are increasingly invading the reefs. She said the algae might have gotten to Hawaii on the bottom of boats, or escaped from aquaculture experiments.

Celia Smith: In some cases they may die, we may have not ever noted that they arrived, but in the few cases where they have become dominant, it is a remarkable change in ecosystem from a place that would have had abundant fish, coral, and native plants.

The brownish algae cover the coral or float in sheets on the water’s surface, endangering both the health and beauty of the reefs. She said that overfishing is one thing that’s allowing the algae to thrive.

Celia Smith: It’s analogous to not having mowed your lawn for a long time. If you don’t have enough reef fish, the algae get to be as tall and impressive as it would be if you haven’t cut the grass all year.

Smith said that the algae are also fueled by nutrients from agricultural fertilizers and sewage that seep into the groundwater and are carried into the reef. Hawaii is battling the algae by manually removing it and vacuuming it from the reef, Smith said. Even so, Smith estimated it would take 30 years of work to improve the reefs. She said the problem is not just the algae, but changes in the ecosystem that allow it to dominate.

Celia Smith: Once something starts to change in an ecosystem – that could be over fishing, or it could be addition of nutrients so the plants grow more quickly, we enter a period where the relationships that hold the balance between plants and animals change.

She said that if the fish that eat algae disappear from the system – due to over fishing – there’s nothing to stop its growth.

Celia Smith: If they are introduced to an ecosystem, they may have a little bit of a lag, a few years, but within a short time you’ll see changes on the reef.

Smith said that for a while, scientists had suspected that additional nutrients were contributing to the huge biomass of the plants. In a 2010 paper published in the Marine Pollution Bulletin, Smith and her colleagues identified the source of the nutrient pollution: the underground waterways were carrying wastewater and agricultural fertilizers. In Maui, algae blooms only occur where there are significant human inputs, the paper said.

Celia Smith: So we’ve elevated the nutrient inputs above and beyond what the ecosystem evolved over millions of years to handle.

Smith said that so far, the algal problem is limited to the islands of Oahu and Maui. Our thanks today to NOAA Pacific Services Center – linking culture, science, and people to build resilient Pacific Island communities.