Study: Invasive algae outcompetes natives in coastal areas affected by polluted groundwater

Human-disturbed watersheds, particularly those with urban development and conventional agriculture, and climate change have altered the amount and composition of underwater discharges of fresh or brackish groundwater from land into the sea.

A team of University of Hawaiʻi at Mānoa researchers studied how the coastal ecosystem off O’ahu has been affected by these changes. They found native macroalgae (seaweed) have adapted to the changes in nutrient and salinity levels, but that invasive algae are outcompeting the natives near submarine groundwater discharges with nutrient pollution.

The review was published by the Association for the Sciences of Limnology and Oceanography.

The study results show the critical importance of sound land, water and wastewater management policies and practices, the researchers said in a press release.

With tens of thousands of cesspools still in operation in Hawaiʻi and the Pacific Islands, wastewater management will continue to be a challenge.

Henrietta Dulai, lead author of the study and professor of Earth Sciences at the UH Mānoa School of Ocean and Earth Science and Technology, said there are multiple ways to improve the situation and prevent further deterioration of submarine groundwater discharge.

They include:

• 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.

• Upgrades should be made to wastewater infrastructure to deal with sea-level rise.

The study found that native macroalgae, also known as limu (seaweed), had faster growth and photosynthesis rates in regimes of natural leakage of groundwater along coastlines.

Numerous studies already have shown that humans have greatly affected the composition and amount of submarine groundwater discharge. The composition has changed with excess nitrogen and other contaminants including agricultural fertilizer, and common-use pharmaceuticals and pesticides.

Efforts to remove land-based sources of wastewater from submarine groundwater discharge are likely to benefit communities in short and long-term ways. In the short term, shallow reefs are likely to return to overall healthier water qualities in these now impacted coastal regions. In the longer term, regrowth of native, not invasive algae, in those same regions is likely.

“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. “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.”

To investigate the relationship between groundwater input and algal species in an area off southeast Oʻahu, the research team used a variety of methods.

In the field, they relied on thermal infrared imagery of the coastline and naturally occurring chemical tracers to assess groundwater seepage. The also documented the abundance of native and invasive algal species.

In the lab, they measured the growth response of various species of algae to conditions that simulated varying compositions of submarine groundwater.

With future studies, the team aims to determine the responses by native and invasive algae to the full range of groundwater discharge—pristine to tainted—to inform biocontrol efforts that outplant native algae and to gain insight into sea-level rise impacts on these plants.

Additional study co-authors are Veronica Gibson and Daniel Amato from UH Mānoa Department of Botany, and Leah Bremer from UH Economic Research Organization.