Findings make key links between land-sea nutrients, impacts

Mo'orea, French Polynesia, is surrounded by a vibrant coral reef ecosystem. Credit: Christian John

Development projects expose bare soil, which is easily eroded by rain. Credit: Christian John.

A spiky algae, seen with coral, was measured in this study. Credit: Christian John.

A multi-year scientific expedition including the University of Hawai‘i at Mānoa and led by researchers from the University of California, Santa Barbara and collaborating institutions, were able to find critical connections between land, rainwater and lagoon waters.

The researchers determined that land use on tropical islands can shape water quality in lagoons and that rainfall can be an important mediator for connections between land and lagoon waters. These findings provide vital information for ecosystem stewards facing global reef decline. Their findings were published in Limnology and Oceanography.

“The links between land and sea are dynamic and complex, so it’s a topic that has remained elusive to science,” said Mary Donovan, co-author and faculty at the Hawai‘i Institute of Marine Biology in the UH Mānoa School of Ocean and Earth Science and Technology. “It took a dream team to pierce through that complexity. We brought together a group of interdisciplinary thinkers, from students to senior investigators, across at least five major institutions to tackle this immense challenge.”

Understanding the phase shift

Scientists have long been concerned that with an increase in human-associated inputs from land to a coral reef, there is often a "phase shift"—a decline in corals accompanied by an increase in harmful algae. This ecological shift is often linked to excessive nutrients and changes in the microbial community, but the precise connection between land use and coral reef health has been poorly understood. 

Through its investigation, the team found that nutrients in the lagoons off Mo‘orea were highest in concentration closer to the island, lower farther offshore. 

“This indicates that at least some of the nutrients in Mo‘orea's lagoons are coming from land,” said Christian John, lead author of the study and postdoctoral scholar at the University of California, Santa Barbara. “Additionally, we found that nutrients are higher in lagoon habitats that are downstream of heavily impacted watersheds. This tells us that human activities on land can play a large role in lagoon water quality, which is important for corals, fishes, and all of the other creatures that live in coral reef ecosystems.”

The study also provides evidence that rain acts as a key mediator for the connections between land use and lagoon waters, potentially flushing more land-based nutrients to the lagoon and reef systems. This suggests that as climate change alters regional rainfall patterns, the impacts of terrestrial factors on coral reefs may also be affected.

Informing stewardship efforts

“Gravity is a unifying force in ecology, and islands are always uphill from the coral reefs that surround them,” said Christian John, lead author of the study and postdoctoral scholar at the University of California, Santa Barbara.

Across Pacific Island systems, the flow of nutrients from mountains to the ocean is a central focus for coastal resource management. Targeted strategies, such as reducing polluted runoff, developing buffers along rivers, or actively mitigating soil loss at development sites, can significantly dampen the adverse effects of land use on lagoon water quality.

“The ahupua‘a, land use divisions that connect mauka to makai, are central to watershed management here in Hawai‘i,” said Nyssa Silbiger, co-author and associate professor in the SOEST Department of Oceanography. “Understanding water quality is a fundamental challenge for everyone: it is key to assessing coral reef health and it is inseparable from human health.”