Study Analyzes Native Algae, Invasive Species & Groundwater Seeps in Hawai'i

Jan. 27, 2022

Coastal groundwater-dependent ecosystems benefit from lowered salinity, nutrient-rich submarine groundwater discharge.

According to a review published by a team of University of Hawaiʻi at Mānoa researchers, native marine macroalgae (seaweed) thrive in environments created by natural groundwater seeps. 

The review found that coastal groundwater-dependent ecosystems benefit from lowered salinity, nutrient-rich submarine groundwater discharge (SGD).

And in areas where the seeping groundwater is tainted by excess nitrogen typical of wastewater, the study found an invasive species typically flourishes in Hawaiʻi. The team determined that native macroalgae have adapted to SGD nutrient and salinity gradients, yet invasive algae are outcompeting the natives near SGD with nutrient pollution.

The research team used thermal infrared imagery of the coastline and naturally occurring chemical tracers to assess groundwater seepage. Then, they documented the abundance of native and invasive algal species. Thereafter, the team measured the growth response of various species of alga to conditions that simulated varying compositions of submarine groundwater, reported University of Hawai’i News.

The native macroalgae studied showed faster growth and photosynthesis rates in systems of natural leakage of groundwater along coastlines.

Modifying groundwater sustainable yields and improving wastewater infrastructure to keep SGD reductions minimal and nitrogen inputs in range is challenging, according to the study, as global sea level rise and reductions in groundwater recharge greatly impact coastal groundwater systems and their dependent ecosystems (GDE).

“There are multiple ways to improve the situation and prevent further deterioration of submarine groundwater discharge,” said Henrietta Dulai, lead author of the study and professor of Earth Sciences at the UH Mānoa School of Ocean and Earth Science and Technology, reported University of Hawai’i News. “Recharge can be improved by land-use choices through having more permeable urban surfaces and by restoring native forests. Lower groundwater withdrawal rates can be achieved by better management of water resources and water re-use. Additionally, upgrades to our wastewater infrastructure in light of impending sea-level rise should be one of the primary goals.”

According to the study, in order to preserve native GDEs, preserving SGD flow and keep associated nutrient loads in check is essential. 

“The bottom line is if we want to sustain native macroalgae, we need to preserve submarine groundwater discharge flow and keep associated nutrient loads in check,” said Celia Smith, study co-author, botany professor at UH Mānoa School of Life Sciences, and co-director of the Marine Biology Graduate Program, reported University of Hawai’i News. “Keeping the discharged groundwater as close to pristine as possible needs to be a goal. Otherwise, we risk setting the stage for persistent, multi-year invasive algal dominance.”

The team plans to continue their studies to determine the responses by native and invasive algae to the full range of groundwater discharge to inform biocontrol efforts that outplant native algae. Sea-level rise impacts on these plants will also be studied.

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