Study: Nutrient Pollution Worsens Ocean for Coral Reefs

Zach Quinlan and Nyssa Silbiger collect samples during experiment at HIMB. PC: Henry Shiu.

A recent study published by researchers, alumni and students from the University of Hawai‘i at Mānoa’s School of Ocean and Earth Science and Technology (SOEST) shows that nutrient pollution from human activities on land may be accelerating the negative impacts of ocean acidification on coral reefs.

Coral reefs are crucial to ecosystem health, providing food security and shoreline protection for coastal marine life communities. Their function largely depends on their three-dimensional structure. For reefs to thrive, calcifying organisms like corals must build the reef faster than organisms and natural elements break them down.

“There is a long history of examining the impacts of nutrient pollution and ocean acidification on coral reefs,” said lead author of the study, Nyssa Silbiger, assistant professor at California State University, Northridge and alumna of the UH Mānoa Hawai‘i Institute of Marine Biology (HIMB) at SOEST. “However, little is known about how these two stressors interact and influence coral reef ecosystem functioning.”

Coral reef in Kaneohe Bay, Hawai‘i, where the experiment was conducted. PC: Nyssa Silbiger.

Previous studies have shown that stressors like carbon dioxide emissions that contribute to ocean acidification are shifting coral reefs toward overall destruction. The new study, published in Proceedings of the Royal Society B: Biological Sciences, shows that nutrient pollution, which includes septic systems and agricultural and stormwater runoff, may be increasing coral reefs’ vulnerability to ocean acidification and accelerating their destruction.

To conduct the study, researchers at HIMB used a state-of-the-art system to continuously add nutrients like nitrate and phosphates commonly found in fertilizers to aquarium environments including corals, seaweeds, dead reef rubble or sand. They compared these environments with an environment mimicking natural processes of coral reef communities including calcification, dissolution, photosynthesis and respiration.

“We showed that nutrient pollution decreases overall reef growth and disrupts the natural chemical dynamics on coral reefs,” said Silbiger. “In nutrient polluted seawater, calcifiers were less able to capitalize on the dissolved compounds that make up the building blocks of coral reefs. Nutrient pollution reduced calcification rates—a measure of how quickly reef builders are creating the skeletal framework—nearly tenfold in waters that would otherwise promote reef growth, and enhanced both skeletal dissolution and the growth of seaweed competitors.”

The study results reinforce the observation that nutrients from fertilizers can impact reefs indirectly—for example, by giving an advantage to weedy seaweeds that can overtake them.

“These ‘phase shifts’ to algal reefs are occurring globally, causing a major change in how reefs function,” said co-author Craig Nelson, faculty at UH Mānoa in Oceanography and Hawaiʻi Sea Grant. “But until now we never quantified how nutrient pollution can directly reduce corals’ ability to build reef structure.”

The study results also revealed that specific nutrients can alter the environment’s pH by compromising the respiration process of corals and other reef organisms, thereby altering their chemistry.

“The relationship between nutrients and reef metabolism exacerbates pH change, which can make the whole system more susceptible to global threats such as ocean acidification,” said co-author Hollie Putnam, assistant professor at University of Rhode Island and alumna of HIMB.

“Nutrient pollution negatively affects reef growth both directly and indirectly, creating a double whammy for coral reefs already stressed by ocean acidification around the world,” said co-author Megan Donahue, researcher at HIMB. “Our data indicate that both local management efforts such as reducing nutrient runoff and seepage into groundwater, and global actions, such as reducing global carbon dioxide emissions, are required to protect reefs from rapidly declining.”

In future studies, the research team will examine how other marine life such as fish interact with the main processes of coral reef and impact their functioning.