Pristine Groundwater Seeps Support Native Algae on Hawai‘i’s Coasts

Native marine macroalgae, also known as limu, or as seaweed, thrive in environments created by natural groundwater seeps, specifically benefiting from the combined effects of enhanced  nutrients despite lowered salinity levels, according to a minireview published recently by a team of University of Hawai‘i at Mānoa researchers. Conversely, in areas where the seeping groundwater is tainted by excess nitrogen typical of wastewater, the study found it is invasive species that typically flourish in Hawaiʻi.

Native macroalgae studied here demonstrate faster growth and photosynthesis rates in regimes of natural leakage of groundwater along coastlines, also termed submarine groundwater discharge. But numerous studies have shown that the composition and amount of submarine groundwater discharge have been greatly affected by humans.

Human disturbed watersheds, particularly those with urban development and conventional agriculture, show a common theme of problems. Not just in Hawaiʻi but across the Pacific Islands, wastewater management can be a challenge, with tens of thousands of cesspools still in operation. Especially those along the shoreline are subject to subsurface inundation—readily delivering excess nitrogen and other contaminants to the coastline. Downstream of these areas, submarine groundwater discharge can be polluted by wastewater, agricultural fertilizer inputs and even common-use pharmaceuticals and pesticides.

Read more at University of Hawaii at Manoa

Image: Hawaiian coasts can have abundances of highly-prized, native limu, or macroalgae. This favorite site has a ready supply of limu palahalaha, or Ulva lactuca, as a reward for limu pickers who trek to this relatively unimpacted Oʻahu coastline. (Credit: Bill Thomas)