A multi-year scientific expedition including the University of Hawaiʻi 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 Hawaiʻi Institute of Marine Biology in the UH Mānoa School of Ocean and Earth Science and Technology (SOEST). "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 Moʻorea were highest in concentration closer to the island, lower farther offshore.
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 ahupuaʻa, land use divisions that connect mauka to makai, are central to watershed management here in Hawaiʻi," 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."
